WO2011096454A1 - 靭性に優れた高耐食性フェライト系ステンレス冷延鋼板およびその製造方法 - Google Patents
靭性に優れた高耐食性フェライト系ステンレス冷延鋼板およびその製造方法 Download PDFInfo
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- 229910001220 stainless steel Inorganic materials 0.000 title claims abstract description 53
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- 238000005260 corrosion Methods 0.000 title abstract description 53
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- 239000010935 stainless steel Substances 0.000 title abstract description 7
- 229910000859 α-Fe Inorganic materials 0.000 title abstract 5
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 17
- 239000012535 impurity Substances 0.000 claims abstract description 7
- 229910000831 Steel Inorganic materials 0.000 claims description 64
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- 238000005098 hot rolling Methods 0.000 claims description 9
- 229910052750 molybdenum Inorganic materials 0.000 claims description 7
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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
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
-
- 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
- C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
- B21B3/02—Rolling special iron alloys, e.g. stainless steel
-
- 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
- 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
- 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
-
- 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
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- 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
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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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- 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
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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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
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- 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present invention relates to a high-corrosion resistance cold rolled steel sheet and a manufacturing method of high corrosion resistance ferritic stainless steel cold rolled steel sheet having excellent toughness or workability.
- SUS304 (% Cr-8% Ni) (Japan Industrial Standards, JIS G 4305) of austenitic stainless steel is widely used due to its excellent corrosion resistance and toughness.
- this steel type is expensive because it contains a large amount of Ni.
- SUS436L (18% Cr-1% Mo) (JIS G 4305) containing Mo is used as a steel type having excellent corrosion resistance equivalent to SUS304. is there.
- Mo is also an expensive element, even if the content is only 1%, the cost is significantly increased (increase in cost).
- SUS436L has sufficient toughness as a structural member.
- ferritic stainless steel not containing Mo there is SUS430J1L (19% Cr-0.5% Cu-0.4% Nb) (JIS G 4305), which has sufficient toughness as a structural member. It is hard to say.
- ferritic stainless cold-rolled steel sheets that have corrosion resistance equivalent to SUS430 and SUS304, which are general-purpose steel types of stainless steel, and are excellent in low-temperature toughness that can be applied to structural member applications have been demanded.
- SUS430 and SUS304 which are general-purpose steel types of stainless steel, and are excellent in low-temperature toughness that can be applied to structural member applications have been demanded.
- a hot-rolled sheet having a thickness of 6 mm or more is secured.
- a steel roll sheet will be produced.
- Ferritic stainless steel is inferior in toughness of hot rolled material and cold rolled material compared to austenitic stainless steel.
- Ferritic stainless steel hot-rolled steel sheets are usually subjected to hot-rolled sheet annealing using a continuous annealing furnace, but if the toughness of the hot-rolled steel is insufficient, the line of the continuous annealing process with tension applied to the hot-rolled steel sheet When the plate is passed, the possibility of plate breakage increases as the plate thickness increases. For this reason, the thickness of a ferritic stainless hot-rolled steel sheet as a cold-rolling material has conventionally been 4 to 5 mm. For this reason, the improvement of the toughness of a ferritic stainless steel hot rolled steel sheet having a thickness of 6 mm or more is required.
- a technique for improving deep drawability is particularly desired among the press workability of ferritic stainless steel cold-rolled steel sheets.
- an increase in the cold rolling reduction ratio is effective.
- a thickness of 6 mm or more is required.
- a hot-rolled steel sheet will be manufactured. For this reason, as described above, in order to increase the cold rolling reduction ratio in order to improve the average r value, it is required to improve the toughness of a ferritic stainless hot rolled steel sheet having a thickness of 6 mm or more.
- Patent Document 1 C: 0.020% or less, Si: 0.30 to 1.00%, Mn: 1.00% or less in mass%. , P: 0.040% or less, S: 0.010% or less, Cr: 20.0-28.0%, Ni: 0.6% or less, Al: 0.03-0.15%, N: 0 0.020% or less, O: 0.0020 to 0.0150%, Mo: 0.3 to 1.5%, Nb: 0.25 to 0.60%, Ti: 0.05% or less, the balance
- a ferritic stainless steel sheet for water heaters is disclosed, which comprises Fe and inevitable impurities, and satisfies 25 ⁇ Cr + 3.3Mo ⁇ 30 and 0.35 ⁇ Si + Al ⁇ 0.85.
- Patent Document 2 by mass%, C: 0.1% or less, N: 0.003 to 0.05%, Si: 0.03 to 1.5%, Mn: 1.0% or less, P : 0.04% or less, S: 0.03% or less, Cr: 10 to 30%, Cu: 2% or less, Ni: 2% or less, Mo: 3% or less, V: 1% or less, Ti: 0.0.
- Patent Document 1 improves the toughness of a hot-rolled sheet having a thickness of 4 mm at 0 ° C. for the purpose of ensuring productivity such as prevention of breakage of a steel strip during hot-rolled sheet annealing or cold rolling. It is a technology, and since it contains a large amount of Mo elements, it is easy to produce intermetallic compounds that reduce toughness. For this reason, it is thought that toughness is inadequate for the application to the use with thicker plate thickness which this invention aims at. Also in Patent Document 2, it is difficult to control dispersion of Ti-based inclusions, and the toughness tends to decrease due to coarsening, and sufficient toughness is not obtained.
- the method of containing Mo or Ti does not sufficiently improve the toughness of hot rolled and cold rolled sheets of ferritic stainless steel. Therefore, the present invention significantly improves the toughness of a ferritic stainless hot-rolled steel sheet with a thickness of 6 mm or more, so that a cold impact steel plate with a thickness of 4 mm or less has a Charpy impact value at ⁇ 50 ° C. of 100 J. It is an object to provide a highly corrosion-resistant ferritic stainless steel cold-rolled steel sheet excellent in toughness characterized by being / cm 2 or more and a method for producing the same.
- the present invention further provides a high corrosion resistance ferritic stainless steel having excellent toughness and workability, in particular, deep drawability, with a Charpy impact value at ⁇ 50 ° C. of 100 J / cm 2 or more in a cold rolled steel sheet having a thickness of 2 mm or less It aims at providing a cold-rolled steel plate and its manufacturing method.
- the inventors have conducted intensive studies on a method for obtaining an Nb-containing ferritic stainless steel cold-rolled steel sheet that does not contain expensive Ni and Mo, and is excellent in corrosion resistance and toughness or further in workability. It was.
- a hot-rolled sheet having a thickness of 7 mm was prepared at 450 ° C.
- a Charpy impact test at 0 ° C. was performed on the obtained hot-rolled sheet having a thickness of 7 mm. The result is shown in FIG.
- the value on the vertical axis in FIG. 1 is a value converted to absorbed energy per unit area by dividing the value of absorbed energy obtained by the test by the cross-sectional area of the notch portion of the impact test piece (hereinafter referred to as Charpy impact value). It is called).
- the inventors annealed the hot rolled steel sheet having a thickness of 7 mm to produce a hot rolled and annealed steel sheet, pickled, and then cold rolled to a thickness of 4 mm. Further, by subjecting the cold-rolled sheet to final annealing at 980 ° C., a Charpy impact test at ⁇ 50 ° C. of the obtained cold-rolled annealed sheet having a thickness of 4 mm was performed. The result is shown in FIG. The values on the vertical axis in FIG. 2 are the same as those in FIG.
- the hot-rolled sheet having a thickness of 7 mm was annealed to produce a hot-rolled annealed sheet, pickled, and then cold-rolled to a thickness of 4 mm. Further, the cold-rolled sheet was subjected to finish annealing at 980 ° C. to produce a cold-rolled annealed sheet having a thickness of 4 mm, and a Charpy impact test was performed at a test temperature of ⁇ 50 ° C. The result is shown in FIG.
- the horizontal axis of FIG. 3 and FIG. 4 indicates the product of Ti (%) and N (%), which is a value corresponding to the solubility product constant (hereinafter referred to as “solubility product”). From FIG. 3 and FIG. 4, in the range where Ti (%) ⁇ N (%) exceeded 8.0 ⁇ 10 ⁇ 5 , the toughness was remarkably lowered.
- both the hot-rolled sheet and the cold-rolled annealed sheet have a low toughness test 2-5 (the solubility product of Ti and N is 9.52 ⁇ 10 ⁇ 5 ). A large number of rectangular solid like Ti nitrides were observed. From this, when the solubility product of Ti and N exceeds 8.00 ⁇ 10 ⁇ 5 , Ti nitride is precipitated and coarsened from the solidification stage, and the toughness is reduced due to the notch effect. Conceivable.
- a hot-rolled sheet thickness appropriate for manufacturing a cold-rolled annealed sheet having a thickness of 4 mm was also examined.
- four types of hot-rolled plates having thicknesses of 5.0 mm, 5.7 mm, 6.8 mm, and 8.0 mm were manufactured by hot rolling.
- cold rolling and finish annealing were performed to produce a cold-rolled annealed sheet having a thickness of 4 mm.
- the Charpy impact test at ⁇ 50 ° C. of the cold-rolled annealed plate having a thickness of 4 mm was performed.
- the Cr content is set to 18.0 to 24.0% from the viewpoint of corrosion resistance and manufacturability, and after containing an appropriate amount of Al, the solubility product of Ti and N contained in the steel [ By controlling the Ti%] ⁇ [N%] to an appropriate amount, it suppresses the generation of coarse TiN precipitates that precipitate from the molten steel stage, which causes a reduction in the toughness of hot-rolled sheets and cold-rolled annealed sheets. It was found that a high corrosion resistance ferritic stainless steel cold-rolled steel sheet can be obtained. The present invention has been made based on these findings.
- a JIS No. 13 B test piece was cut out from the obtained cold-rolled annealed sheet having a thickness of 2 mm along the rolling direction, 45 ° to the rolling direction, and 90 ° to the rolling direction. After giving a tensile strain of 15%, the average r value shown in the following formula was obtained.
- Average r value (r L + 2r D + r C ) / 4, where r L , r D , and r C are r in the rolling direction, 45 ° direction with respect to the rolling direction, and 90 ° direction with respect to the rolling direction, respectively. Value. The larger the average r value, the better the deep drawability.
- the present invention has been made based on these findings.
- the configuration of the present invention is as follows. (1) By mass%, C: 0.020% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.06% or less, S: 0.01% or less, Cr: 18 0.0 to 24.0%, Mo: 0.3% or less, Ti: 0.015% or less, Al: 0.20 to 0.40%, N: 0.020% or less, and further 10 ⁇ (C + N) ⁇
- each element symbol represents the component content (% by mass) in the steel.
- the ferritic stainless steel cold-rolled steel sheet according to (1) characterized in that, by mass, Si: 0.5% or less, Mn: 0.8% or less, and Ti: 0.010% or less.
- a Charpy impact value at ⁇ 50 ° C. at a plate thickness of 4 mm is 100 J / cm 2 or more, and more preferably 150 J / cm 2 or more.
- a rolled steel sheet can be obtained.
- a high corrosion resistance ferritic stainless steel cold-rolled steel sheet excellent in workability can be obtained.
- C 0.020% or less C is easy to form Cr carbide by combining with Cr, and when Cr carbide is formed in a heat-affected zone during welding, it causes intergranular corrosion. Since it becomes a cause, C is so preferable that it is low. Therefore, C is 0.020% or less, more preferably 0.015% or less. In addition, it is preferable to set the content to 0.003 to 0.010% from the viewpoint of particularly high prevention of intergranular corrosion and from the viewpoint of refining cost.
- Si 1.0% or less
- Si is an element useful as a deoxidizing agent. In order to obtain this effect, 0.05% or more is preferable. However, if contained in a large amount, the toughness is lowered. Therefore, Si is 1.0% or less. More preferably, it is 0.5% or less, and further preferably 0.05 to 0.3% or less.
- Mn 1.0% or less Mn combines with S present in steel to form MnS, which is a soluble sulfide, and lowers the corrosion resistance. Therefore, Mn is 1.0% or less. More preferably, it is 0.8% or less. In addition, it is more preferably 0.05 to 0.6% or less from the viewpoint of particularly high corrosion resistance and from the viewpoint of refining cost.
- P 0.06% or less Since P is an element harmful to corrosion resistance, it is preferably reduced as much as possible. Moreover, when it exceeds 0.06%, workability (workability) falls by the solid solution strengthening (solid solution strength). Therefore, P is set to 0.06% or less. Furthermore, considering workability and toughness, it is preferably 0.04% or less.
- S 0.01% or less Since S is an element harmful to corrosion resistance, it is preferably reduced as much as possible, and is 0.01% or less. Furthermore, in order to obtain high corrosion resistance, it is preferably 0.006% or less.
- Cr 18.0 to 24.0%
- Cr is an element that improves the corrosion resistance by forming a passive film on the surface. If the Cr content is less than 18.0%, sufficient corrosion resistance cannot be obtained. On the other hand, if it exceeds 24.0%, sigma phase embrittlement and 475 ° C brittleness tend to occur, and the toughness tends to decrease. Therefore, Cr is made 18.0 to 24.0%. Further, from the viewpoint of high corrosion resistance, it is preferably 20.0 to 24.0%.
- Mo 0.3% or less If Mo is contained, it has the effect of increasing the corrosion resistance of stainless steel. When excessively contained, coarse intermetallic compounds such as Laves phase are generated, and toughness is reduced. Therefore, Mo is 0.3% or less.
- Nb 10 ⁇ (C + N) to 0.40%
- Nb has the effect of detoxifying C and N, which are harmful to corrosion resistance, as Nb carbide, Nb nitride, or a composite of these, and improving corrosion resistance.
- the amount of Nb is less than 10 times the amount of (C + N)
- Nb is 10 ⁇ (C + N) to 0.40%.
- it is 12 ⁇ (C + N) to 0.30%.
- C and N represent the content of each component in mass%.
- Ti 0.015% Ti forms coarse nitrides and reduces toughness. Therefore, Ti is made 0.015% or less. More preferably, it is 0.010% or less. In addition, when especially high toughness is requested
- N 0.020% or less N reduces toughness by forming a nitride with Ti or Nb.
- Ti coexists, coarse TiN precipitates are generated from the solidification stage of the molten steel, and the toughness is significantly lowered due to the notch effect. Therefore, N is set to 0.020% or less.
- particularly high corrosion resistance it is 0.015% or less, more preferably 0.010% or less.
- Al 0.20 to 0.40%
- Al is an important element in the present invention, and has an effect of improving toughness. With respect to the toughness targeted by the present invention, the effect is insufficient if it is less than 0.20%. Moreover, when it exceeds 0.40%, hot workability (hot-workability) falls. Therefore, Al is made 0.20 to 0.40%. Further, when particularly high toughness is required, the content is preferably 0.20 to 0.30%.
- Ti ⁇ N ⁇ 8.00 ⁇ 10 ⁇ 5 As described above, coarse TiN precipitates that cause a reduction in the toughness of ferritic stainless steel are generated from the solidification stage.
- Ti and N are preferably as small as possible, and the solubility product of Ti ⁇ N is limited to 8.00 ⁇ 10 ⁇ 5 or less. Preferably, it is 5.00 ⁇ 10 ⁇ 5 or less.
- Ti and N represent the content of each component in mass%.
- Fe The balance other than the chemical components described above is Fe and inevitable impurities.
- Inevitable impurities include, for example, Mg: 0.0020% or less, Ca: 0.0020% or less, and V: 0.10% or less, but are not limited to these elements.
- the ferritic stainless steel of the present invention can provide the desired characteristics with the above-mentioned essential elements, but can contain the following elements according to the desired characteristics. However, as long as the effects of the present invention are not impaired, the inclusion of components other than the following is not rejected.
- Cu 0.3 to 0.8%
- it is an element useful for improving the corrosion resistance, and is an element particularly effective in reducing crevice corrosion. In order to exhibit this effect, a content of 0.3% or more is necessary. On the other hand, when it contains exceeding 0.8%, hot workability will fall. Therefore, Cu is 0.3 to 0.8%. Preferably it is 0.3 to 0.5%.
- high corrosion resistance is not particularly required, it is not necessary to contain Cu since the manufacturing cost is increased and the economy is impaired.
- Ni 1.0% or less
- Ni 1.0% or less
- it is 0.05 to 0.4%.
- Co 1.0% or less
- Co is an element that contributes to improvement of low-temperature toughness when it is contained. In order to obtain this effect, 0.05% or more is preferable. However, excessive inclusion reduces ductility. Therefore, Co is set to 1.0% or less.
- B 0.0002 to 0.0020%
- B it is an effective element for improving the resistance to cold-work embrittlement during deep drawing (resistance to cold-work embrittlement). The effect cannot be obtained at less than 0.0002%. On the other hand, excessive inclusion reduces hot workability and deep drawability. Therefore, when B is contained, the range of 0.0002 to 0.0020% is preferable.
- the temperature of ⁇ 50 ° C. was selected in the Charpy impact test because the present invention steel was cold cold. This is because the environment used as a structure in the building material field was taken into consideration, and after finishing annealing of the cold-rolled steel sheet, a Charpy impact value in a Charpy impact test at ⁇ 50 ° C. was evaluated as 100 J / cm 2 or more as good. Moreover, 150 J / cm 2 or more was evaluated as extremely good.
- the corrosion resistance of the steel sheet was evaluated by measuring the pitting potential as described later in accordance with JIS G 0577, with 180 mV vs. SCE or better being good and less than 180 mV vs. SCE being bad.
- the manufacturing method of the ferritic stainless steel of this invention is demonstrated.
- continuous casting is performed on a slab, heating is performed in a range of 1100 to 1300 ° C., and hot rolling is performed to obtain a hot rolled coil.
- the coil winding temperature in the hot rolling exceeds 650 ° C., carbides and intermetallic compounds are precipitated after winding and the toughness is lowered.
- the coiling temperature is preferably 650 ° C. or lower, and when high toughness is required, the coiling temperature is preferably 450 ° C. or lower.
- the thickness of the rolled steel sheet (also simply referred to as “hot rolled sheet”) is 6 mm or more. More preferably, it is 7 mm or more.
- the obtained hot-rolled sheet is subjected to continuous annealing and pickling in the range of 900 to 1150 ° C. and pickling. Depending on the continuous annealing line of the hot-rolled sheet, the line is passed in a state where tension is applied to the hot-rolled sheet. If the toughness of the hot-rolled sheet is insufficient, the plate may break, so at 0 ° C
- the hot rolled sheet preferably has a Charpy impact value of 50 J / cm 2 or more.
- cold rolling and finish annealing are performed on the hot-rolled annealed plate (the steel plate obtained by annealing the hot-rolled steel plate) to obtain a cold-rolled annealed plate.
- the hot-rolled annealed plate the steel plate obtained by annealing the hot-rolled steel plate
- a cold-rolled annealed sheet having high toughness and high workability it is preferable to secure a cold rolling reduction ratio of 65% or more and to sufficiently accumulate strain energy.
- the finish annealing is preferably performed at a temperature of 950 ° C. or higher.
- the manufacturing method of the ferritic stainless steel hot-rolled steel sheet of the present invention is not limited to the method described here, and a known method can be applied.
- Ferritic stainless steels having chemical components shown in Tables 4-1 and 4-2 were melted to form slabs having a thickness of 250 mm by a continuous casting method. These slabs are heated to 1200 ° C., then roughly rolled to a thickness of 35 mm, finish rolling is started at 1050 ° C., finished at 900 ° C., wound into a coil at 500 ° C. and cooled, and has a thickness of 5 to 8 mm. Samples of No.
- the hot-rolled sheet was held at 1050 ° C. or higher for 80 seconds (maximum temperature: 1100 ° C.), and then subjected to annealing for cooling to obtain a hot-rolled annealed sheet. After pickling the obtained hot-rolled annealed plate, it was cold-rolled to the plate thickness shown in Table 5 and subjected to finish annealing at 980 ° C. From these cold-rolled annealed plates, No.
- pitting corrosion potential (pitting potential) was measured in a 3.5% NaCl solution at 30 ° C. in accordance with JIS G 0577, and the corrosion resistance was evaluated on the test piece collected from the cold-rolled annealed plate. 180 mV vs SCE or higher was considered good, and 180 mV vs SCE or less was considered bad.
- the steel plate No. 4 to 7, 10 to 16, 18 to 21, 23 to 24, 26 to 34, and 36 each have a Charpy impact value of ⁇ 50 ° C. of 100 J / cm 2 or more, good toughness, corrosion resistance, and pitting potential.
- No. 35 had a pitting potential of less than 180 mV vs SCE and poor corrosion resistance.
- Steel plate No. 1, 2, 8, 9, 17, 22, and 25 have a Charpy impact value at 0 ° C.
- the steel plate No. 1 produced by the manufacturing method in which the solubility product of Ti and N was large, the thickness of the hot-rolled steel plate was 5 mm, and the reduction ratio of cold rolling was 20%.
- No. 37 had a Charpy impact value at ⁇ 50 ° C. of less than 100 J / cm 2 .
- Example 2 is the same as Example 1 except for the cold rolling reduction of the cold rolled steel sheet of Example 1 and the thickness of the cold rolled annealed sheet. Therefore, the steel No. in Table 6 of Example 2 was used.
- the steel Nos. Shown in Table 4-1 and Table 4-2 and Table 5 of Example 1 Is the same. That is, after pickling the hot-rolled annealed plate of Example 1, it was cold-rolled to the plate thickness shown in Table 6 and subjected to finish annealing at 980 ° C. JIS No. 13 B specimens were cut out along the rolling direction of these cold-rolled annealed plates, 45 ° to the rolling direction, and 90 ° to the rolling direction, and 15% tensile strain was applied to each specimen. Then, the average r value was determined. An average r value of 1.30 or more was judged good, and a value less than 1.30 was judged as poor.
- JIS Z 2202 In addition, five No. 4 test pieces specified in JIS Z 2202 were sampled each (the rolling direction is the sampling direction, the impact direction is the rolling width direction, and the width of the test piece is the plate thickness), and JIS Z 2242 is specified.
- the Charpy impact test was conducted under the conditions of the test temperature: -50 ° C. Five Charpy impact values were averaged, and a case of 100 J / cm 2 or more was judged good and a case of less than 100 J / cm 2 was judged as bad. Further, the case of 150 J / cm 2 or more was considered extremely good.
- the pitting corrosion potential was measured in a 3.5% NaCl solution at 30 ° C. according to JIS G 0577 on the test piece collected from the cold-rolled annealed plate to evaluate the corrosion resistance. 180 mV vs SCE or higher was considered good, and 180 mV vs SCE or less was considered bad.
- the steel plate No. 1 having a low Nb content was used.
- No. 75 had a pitting potential of less than 180 mV vs SCE and poor corrosion resistance.
- the steel plate No. 1 was obtained by a manufacturing method in which the solubility product of Ti ⁇ N was large, the thickness of the hot-rolled steel plate was 5 mm, and the reduction ratio of cold rolling was 20%.
- No. 77 had a Charpy impact value at ⁇ 50 ° C. of less than 100 J / cm 2 .
- the ferritic stainless steel cold-rolled steel sheet provided by the present invention is excellent in corrosion resistance and toughness, and is a civil engineering such as a truck of the truck, a grating, various floor materials, and a metal clasp. It is promising as a material for structural members, such as for civil engineering and construction applications. Or, furthermore, the ferritic stainless steel cold-rolled steel sheet provided by the present invention is excellent in high toughness and corrosion resistance in addition to workability, so in addition to deep drawing applications such as kitchen equipment, home appliances, trucks It is also promising for materials for transportation equipment such as cargo beds, various flooring materials such as gratings, civil engineering such as metal fittings, and structural members such as architectural applications.
Abstract
Description
特に、板厚2mm超え~4mmの板厚の厚い低温靱性に優れた冷延鋼板を得る上で、冷延圧下率(rolling reduction of cold rolling)の確保の観点から、板厚6mm以上の熱延鋼板(hot rolled steel sheet)を製造することになる。フェライト系ステンレス鋼は、オーステナイト系ステンレス鋼に比べて熱延材および冷延材の靱性が劣る。フェライト系ステンレス熱延鋼板は、通常、連続焼鈍炉を用いて熱延板焼鈍が行われるが、熱延材の靱性が不十分な場合、熱延鋼板に張力付加した状態で連続焼鈍工程のラインを通板すると、板厚の厚いほど板破断の可能性が高くなる。このため従来から冷延用素材としてのフェライト系ステンレス熱延鋼板の板厚は4~5mmが主流である。このため、板厚6mm以上のフェライト系ステンレス熱延鋼板の靱性向上が求められている。
また、フェライト系ステンレス鋼は、オーステナイト系ステンレス鋼に比べ安価でかつ耐食性に優れていることから、板厚2mm以下の冷延鋼板は、厨房機器(kitchen instruments)や家電機器(household electrical appliance)などに広く使用されている。近年、製鋼工程(steel making process)での脱炭および脱窒技術(decarburizing and denitrogenation technology)の向上により、加工性と耐食性が一段と改善されたCおよびNを低減した高純度のフェライト系ステンレス冷延鋼板が開発され、より広範囲な用途で複雑な形状に加工されて使用される機会が多くなってきた。
平均r値改善には、冷延圧下率の増大が効果的であるが、板厚2mm以下の冷延鋼板を得る上で、より高い冷延圧下率の確保の観点から、板厚6mm以上の熱延鋼板を製造することになる。
このため、前述したように平均r値を改善するために冷延圧下率の増大を図るためには、板厚6mm以上のフェライト系ステンレス熱延鋼板の靱性向上が求められている。
また、特許文献2では、質量%で、C:0.1%以下、 N:0.003~0.05%、Si:0.03~1.5%、Mn:1.0%以下、 P:0.04%以下、 S:0.03%以下、Cr:10~30%、 Cu:2%以下、 Ni:2%以下、Mo:3%以下、V:1%以下、Ti:0.02~0.5%、O(酸素):0.001~0.005%、Nb:0.8%以下、Al:0.001~0.15%、Zr:0.3%以下、B:0.1%以下、Ca:0.003%以下、Mg:0.0005%未満、Ti×N:0.0005以上を満足し、残部はFe及び不可避不純物の化学組成で、鋼中にMgとAlの含有量の比が0.3~0.5のAl及びMgを含有する介在物(inclusion)とTi系介在物との複合介在物が分散した加工性と靱性に優れたフェライト系ステンレス鋼が開示されている。
また、特許文献2においても、Ti系介在物の分散制御(dispersion control)は困難であり、粗大化により靱性が低下しやすく、十分な靭性が得られていない。
そこで、本発明は、フェライト系ステンレス熱延鋼板の板厚6mm以上における靱性を大幅に改善することにより、板厚4mm以下の冷延鋼板において−50℃のシャルピー衝撃値(charpy impact value)が100J/cm2以上であることを特徴とする靱性に優れた高耐食性フェライト系ステンレス冷延鋼板およびその製造方法を提供することを目的とする。
また、本発明は、さらに板厚2mm以下の冷延鋼板における−50℃のシャルピー衝撃値が100J/cm2以上の靱性とさらに加工性、特に、深絞り性にも優れた高耐食性フェライト系ステンレス冷延鋼板およびその製造方法を提供することを目的とする。
まず、フェライト系ステンレス鋼の安定化元素として、Ti含有鋼では凝固段階から生成する粗大なTiN析出物が鋼中に存在し、この析出物の切り欠き効果により靱性が大幅に低下することを突き止めた。そこで、Tiを極力混入させずに安定化元素としてNbを選択し、さらにNb含有フェライト系ステンレス鋼の靱性に及ぼす種々の合金の影響について検討した結果、Alに着目した。21%Cr−0.25%Nb−低C、N鋼にAlを0.03~0.50%含有した鋼を溶製し、1200℃に加熱して熱間圧延を行い、巻取温度を450℃として、板厚7mmの熱延板を作製した。得られた板厚7mmの熱延板の0℃におけるシャルピー衝撃試験(Charpy impact test)を行った。その結果を図1に示す。図1の縦軸の値は、試験により得られた吸収エネルギーの値を衝撃試験片のノッチ部の断面積にて除することにより、単位面積当たりの吸収エネルギーに換算した値(以下シャルピー衝撃値と称す)である。
さらに、発明者らは、上記板厚7mmの熱延鋼板を焼鈍して、熱延焼鈍板(hot rolled and annealed steel sheet)を製造し、酸洗したのち、板厚4mmまで冷間圧延した。さらに冷延板を980℃で仕上焼鈍することにより、得られた板厚4mmの冷延焼鈍板の−50℃におけるシャルピー衝撃試験を行った。その結果を図2に示す。
図2の縦軸の値は、図1と同じである。
図1および図2からAlを0.20~0.40%の範囲で含有させることにより、板厚7mmの熱延鋼板および板厚4mmの冷延焼鈍板の靭性が著しく向上することが分かる。
この理由は、はっきりしないが、Alの含有により鋼中のO量が低下し、介在物が減少したことなどによるものと推察される。また、Alの0.40%を超えた範囲における靭性の低下については、固溶Alの増加に起因するものと思われる。
(1)質量%で、C:0.020%以下、Si:1.0%以下、Mn:1.0%以下、P:0.06%以下、S:0.01%以下、Cr:18.0~24.0%、Mo:0.3%以下、Ti:0.015%以下、Al:0.20~0.40%、N:0.020%以下、さらに10×(C+N)≦ Nb ≦ 0.40%、かつ、成分含有量が下記式(A)を満足し、残部がFeおよび不可避的不純物からなることを特徴とする靱性に優れた高耐食性フェライト系ステンレス冷延鋼板。
Ti× N ≦ 8.00×10−5 ・・・・(A)
ここで、各元素記号は鋼中の成分含有量(質量%)を表す。
(2)質量%で、Si:0.5%以下、Mn:0.8%以下、Ti:0.010%以下であることを特徴とする(1)に記載のフェライト系ステンレス冷延鋼板。
(3)質量%で、C:0.015%以下、N:0.015%以下を含有する(1)に記載の靱性に優れた高耐食性フェライト系ステンレス冷延鋼板。
(4)さらに、質量%で、下記のグループAまたは、Bのグループの内、少なくとも1つを含有する(1)~(3)のいずれかに記載の靱性に優れた高耐食性フェライト系ステンレス冷延鋼板。
グループA:Cu:0.3~0.8%、Ni:1.0%以下およびCo:1.0%以下のうちから選んだ1種以上
グループB:B:0.0002~0.0020%
(6)前記(1)~(4)のいずれかに記載の組成を有するスラブを用い、加熱、熱間圧延、熱延鋼板焼鈍、酸洗、冷間圧延、仕上焼鈍を行いフェライト系ステンレス冷延鋼板を製造する方法であって、前記熱間圧延後であって熱延鋼板焼鈍前の熱延鋼板の厚みを6mm以上とする靱性に優れた高耐食性フェライト系ステンレス冷延鋼板の製造方法。
(7)前記熱延鋼板のシャルピー衝撃特性が0℃におけるシャルピー衝撃値で50J/cm2以上であることを特徴とする(6)に記載の靱性に優れた高耐食性フェライト系ステンレス冷延鋼板の製造方法。
(8)前記冷間圧延における圧延の圧下率を30%以上とすることを特徴とする(6)または(7)に記載の靱性に優れた高耐食性フェライト系ステンレス冷延鋼板の製造方法。
さらに、上記靭性の改善に加えて、加工性にも優れた高耐食性フェライト系ステンレス冷延鋼板を得ることができる。
Cは、Crと結合してCr炭化物を形成しやすく、溶接時、熱影響部(heat−affected zone)にCr炭化物が形成されると粒界腐食(intergranular corrosion)の原因となるので、Cは低いほど望ましい。よって、Cは0.020%以下、より好ましくは、0.015%以下とする。なお、さらに特に高い粒界腐食防止が要求される場合と、精錬コスト(refining cost)の観点から、0.003~0.010%とすることが好ましい。
Siは、脱酸剤(deoxidizing agent)として有用な元素である。この効果を得るためには、0.05%以上が好ましい。しかし、多量に含有させると靭性を低下させる。よって、Siは1.0%以下とする。より好ましくは、0.5%以下、さらに好ましくは、0.05~0.3%以下である。
Mnは、鋼中に存在するSと結合して、可溶性硫化物(fusible sulfide)であるMnSを形成し、耐食性を低下させる。よって、Mnは1.0%以下とする。より好ましくは、0.8%以下である。なお、特に高い耐食性が要求される場合と精錬コストの観点から、より好ましくは、0.05~0.6%以下である。
Pは、耐食性に有害な元素であるので可能な限り低減することが好ましい。また、0.06%を超えると固溶強化(solid solution strengthening)により加工性(workability)が低下する。よって、Pは0.06%以下とする。さらに、加工性および靭性を考慮すると、好ましくは、0.04%以下である。
Sは、耐食性に有害な元素であるので可能な限り低減することが好ましく、0.01%以下とする。さらに、高い耐食性を得るには、好ましくは0.006%以下である。
Crは表面に不働態皮膜(passive film)を形成して耐食性を高める元素である。Cr含有量が18.0%未満では十分な耐食性が得られない。一方、24.0%を越えるとσ相脆化(sigma phase embrittlement)や475℃脆性が生じやすくなり、靱性が低下しやすくなる。よって、Crは18.0~24.0%とする。さらに、高耐食性の観点から、好ましくは20.0~24.0%である。
Moは、含有させれば、ステンレス鋼の耐食性を高める効果を有する。過剰に含有させるとラーベス相(Laves phase)等の粗大な金属間化合物を生成させ、靱性を低下させる。よって、Moは0.3%以下とする。
Nbは、耐食性に有害なCやNをNb炭化物、Nb窒化物またはこれらが複合した析出物として無害化し、耐食性を向上させる効果を有する。しかし、Nb量が(C+N)量の10倍を下回ると、Nb炭化物、Nb窒化物または、これらが複合した析出物の析出が不十分となり、Cr炭化物、Cr窒化物または、これらが複合した析出物が析出し、耐食性が低下する。よって、Nbは10×(C+N)%以上に限定する。一方、過剰に含有させるとラーベス相等の粗大な金属間化合物を生成させ、靱性が低下する。よって、Nbは10×(C+N)~0.40%とする。好ましくは12×(C+N)~0.30%である。
ここで、C、Nはそれぞれの成分の含有量を質量%で表している。
Tiは、粗大な窒化物を形成し、靱性を低下させる。よって、Tiは0.015%以下とする。より好ましくは、0.010%以下とする。なお、さらに特に高い靭性が要求される場合には、0.005%以下とすることが好ましい。
Nは、Tiあるいは、Nbと窒化物を形成することにより、靱性を低下させる。特に、Tiが共存する場合は、溶鋼の凝固段階から粗大なTiN析出物が生成し、その切り欠き効果により著しく靭性が低下する。よって、Nは0.020%以下とする。なお、さらに特に高い耐食性が要求される場合には、0.015%%以下、より好ましくは、0.010%以下である。
Alは、本発明で重要な元素であり、靭性を向上させる効果を有する。本発明の目的とする靭性に対して、0.20%未満ではその効果が不十分である。また、0.40%を超えた場合、熱間加工性(hot−workability)が低下する。よって、Alは、0.20~0.40%とする。また、さらに特に高い靭性が要求される場合には、0.20~0.30%とすることが好ましい。
上述のとおり、フェライト系ステンレス鋼の靭性低下の要因となる粗大なTiN析出物は、凝固段階から生成する。このTiNの析出を抑制するためには、Ti、Nは少ないほど好ましく、Ti×N の溶解度積を8.00×10−5以下に限定する。好ましくは5.00×10−5以下とする。ここで、Ti、Nはそれぞれの成分の含有量を質量%で表している。
Cuは、含有すれば、耐食性を向上させるために有用な元素であり、特に隙間腐食(crevice corrosion)を低減させる上で有効な元素である。この効果が発揮されるためには、0.3%以上の含有量が必要である。一方、0.8%を超えて含有させると、熱間加工性が低下する。よって、Cuは0.3~0.8%とする。好ましくは0.3~0.5%である。ただし、特に高い耐食性を必要としない場合には、製造コストを上昇させ経済性を損なうのでCuを含有させることを要しない。
Niは、含有させれば、Cu含有による熱間加工性の低下を防ぐ効果がある。また、隙間腐食を低減させる効果を有する。この効果を得るためには、0.05%以上が好ましい。しかし、高価な元素であることに加え、1.0%を超えて含有してもそれらの効果は飽和し、かえって熱間加工性を低下させる。よって、Niは1.0%以下とする。好ましくは0.05~0.4%である。
Coは、含有させれば、低温靭性の改善に寄与する元素である。この効果を得るためには、0.05%以上が好ましい。ただし、過剰な含有は延性(ductility)を低下させる。よって、Coは1.0%以下とする。
Bは、含有させれば、深絞り成形(deep drawing)時の耐二次加工脆性(resistance to cold−work embrittlement)を改善するために有効な元素である。その効果は、0.0002%未満では得られない。一方、過剰な含有は熱間加工性と深絞り性(deep drawability)を低下させる。よって、Bは含有させる場合は、0.0002~0.0020%の範囲が好ましい。
本発明の効率的な製造方法は、スラブ(slab)に連続鋳造(continuous casting)し、1100~1300℃の範囲に加熱して、熱間圧延を行い、熱延コイルとする。熱間圧延におけるコイル巻取温度が650℃を超えると、巻取り後に炭化物や金属間化合物が析出して靭性が低下する。このため、巻取温度は650℃以下とすることが好ましく、高い靭性が要求される場合は、巻取温度を450℃以下とすることが好ましい。また、板厚4mm以下の靱性に優れた冷延焼鈍板、あるいは、板厚2mm以下の靭性および加工性に優れた冷延焼鈍板を製造するためには、冷延圧下率確保の観点から熱延鋼板(単に「熱延板」とも称す)の板厚を6mm以上とする。さらに好ましくは、7mm以上である。得られた熱延板を連続焼鈍、酸洗ラインにより900~1150℃の範囲で焼鈍、酸洗を行う。熱延板の連続焼鈍ラインによっては、熱延板に張力を付与した状態でライン通板するため、熱延板の靱性が不十分な場合、板破断が生じることもあるため、0℃での熱延板のシャルピー衝撃値は50J/cm2以上であることが好ましい。
高靱性の冷延焼鈍板を得るためには、仕上焼鈍で圧延方向に伸展した粗大な回復組織が残存しないよう、冷間圧延における冷延圧下率を合計で30%以上確保することが好ましい。
さらに、高靭性および高加工性の冷延焼鈍板を得るためには、冷延圧下率を65%以上確保して、歪エネルギーを十分に蓄積することが好ましい。
また、仕上焼鈍は、950℃以上の温度で焼鈍することが好ましい。
本発明のフェライト系ステンレス熱延鋼板の製造方法は、ここに述べた方法に限られず、公知の方法を適用することができる。
一方、冷延圧下率が65%未満である鋼板No.50~51は、平均r値が1.30未満であった。また、鋼板No.41,42,48,49,57,62,65は、熱延板の0℃のシャルピー衝撃値が50J/cm2未満であるため、張力付加での連続焼鈍ライン通板における板破断の可能性があり、以降の評価は行っていない。また、Nbの含有量が少ない鋼板No.43およびCrの含有量が少ない鋼板No.75は、孔食電位が180mV vs SCE未満で、耐食性が不良であった。また、Ti×Nの溶解度積が大きく、熱延鋼板の板厚が5mmで冷間圧延の圧下率が20%であった製造方法に依った鋼板No.77は、−50℃のシャルピー衝撃値が100J/cm2未満となった。
あるいは、さらに、本発明が提供するフェライト系ステンレス冷延鋼板は、加工性に加え、高靭性および耐食性にも優れていることから、厨房機器等の深絞り用途に加え、家庭電化製品、トラックの荷台などの輸送機器、グレーチングなどの各種床材、金具といった土木、建築用途などの構造部材用などの素材にも有望である。
Claims (8)
- 質量%で、C:0.020%以下、Si:1.0%以下、Mn:1.0%以下、P:0.06%以下、S:0.01%以下、Cr:18.0~24.0%、Mo:0.3%以下、Ti:0.015%以下、N:0.020%以下、さらにAl:0.20~0.40%、さらに10×(C+N)≦ Nb ≦ 0.40%かつ、成分含有量が下記式(A)を満足し、残部がFeおよび不可避的不純物からなるフェライト系ステンレス冷延鋼板。
Ti×N≦8.0×10−5 ・・・・(A)
ここで、各元素記号は鋼中の成分含有量(質量%)を表す。 - さらに、請求項1において、質量%で、Si:0.5%以下、Mn:0.8%以下、Ti:0.010%以下を含有するフェライト系ステンレス冷延鋼板。
- さらに、請求項1において、質量%で、C:0.015%以下、N:0.015%以下を含有するフェライト系ステンレス冷延鋼板。
- さらに、質量%で、下記のグループAまたは、Bのグループの内、少なくとも1つを含有する請求項1~3のいずれかに記載のフェライト系ステンレス冷延鋼板。
グループA:Cu:0.3~0.8%、Ni:1.0%以下およびCo:1.0%以下のうちから選んだ1種以上
グループB:B:0.0002~0.0020% - 前記冷延鋼板の仕上焼鈍後のシャルピー衝撃特性が−50℃におけるシャルピー衝撃値で100J/cm2以上である請求項1~請求項4のいずれかに記載のフェライト系ステンレス冷延鋼板。
- 請求項1~請求項4のいずれかに記載の組成を有するスラブを、加熱、熱間圧延、熱延鋼板焼鈍、酸洗、冷間圧延、仕上焼鈍を行いフェライト系ステンレス冷延鋼板を製造する方法であって、前記熱間圧延後であって熱延鋼板焼鈍前の熱延鋼板の厚みを6mm以上とするフェライト系ステンレス冷延鋼板の製造方法。
- 前記熱延鋼板のシャルピー衝撃特性が0℃におけるシャルピー衝撃値で50J/cm2以上である請求項6に記載のフェライト系ステンレス冷延鋼板の製造方法。
- 前記冷間圧延における圧延の圧下率を30%以上とする請求項6または請求項7に記載のフェライト系ステンレス冷延鋼板の製造方法。
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UA111115C2 (uk) | 2012-04-02 | 2016-03-25 | Ейкей Стіл Пропертіс, Інк. | Рентабельна феритна нержавіюча сталь |
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