WO2019189707A1 - 二相ステンレスクラッド鋼板及びその製造方法 - Google Patents
二相ステンレスクラッド鋼板及びその製造方法 Download PDFInfo
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- WO2019189707A1 WO2019189707A1 PCT/JP2019/013897 JP2019013897W WO2019189707A1 WO 2019189707 A1 WO2019189707 A1 WO 2019189707A1 JP 2019013897 W JP2019013897 W JP 2019013897W WO 2019189707 A1 WO2019189707 A1 WO 2019189707A1
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- clad
- duplex stainless
- stainless steel
- steel sheet
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 134
- 239000010959 steel Substances 0.000 title claims abstract description 134
- 238000004519 manufacturing process Methods 0.000 title claims description 19
- 238000000034 method Methods 0.000 title description 15
- 239000000463 material Substances 0.000 claims abstract description 83
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- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 27
- 229910001039 duplex stainless steel Inorganic materials 0.000 claims description 72
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- 238000001816 cooling Methods 0.000 claims description 37
- 229910052804 chromium Inorganic materials 0.000 claims description 15
- 238000003303 reheating Methods 0.000 claims description 15
- 229910052750 molybdenum Inorganic materials 0.000 claims description 14
- 238000005496 tempering Methods 0.000 claims description 10
- 238000010438 heat treatment Methods 0.000 claims description 9
- 239000012535 impurity Substances 0.000 claims description 9
- 229910052757 nitrogen Inorganic materials 0.000 claims description 6
- 229910052742 iron Inorganic materials 0.000 claims description 5
- 229910052748 manganese Inorganic materials 0.000 claims description 5
- 229910052759 nickel Inorganic materials 0.000 claims description 5
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- 238000005098 hot rolling Methods 0.000 claims description 4
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- 230000009467 reduction Effects 0.000 claims description 4
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- 229910052799 carbon Inorganic materials 0.000 claims description 3
- 238000010030 laminating Methods 0.000 claims description 3
- 229910052782 aluminium Inorganic materials 0.000 claims description 2
- 229910052802 copper Inorganic materials 0.000 claims description 2
- 229910052717 sulfur Inorganic materials 0.000 claims description 2
- 239000004615 ingredient Substances 0.000 claims 1
- 238000003475 lamination Methods 0.000 claims 1
- 230000007797 corrosion Effects 0.000 abstract description 74
- 238000005260 corrosion Methods 0.000 abstract description 74
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- 230000000694 effects Effects 0.000 description 27
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- 229910045601 alloy Inorganic materials 0.000 description 19
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- 229910001566 austenite Inorganic materials 0.000 description 16
- 238000001556 precipitation Methods 0.000 description 16
- 230000006866 deterioration Effects 0.000 description 10
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- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 8
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 2
- 239000002253 acid Substances 0.000 description 2
- 229910052787 antimony Inorganic materials 0.000 description 2
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- 150000002739 metals Chemical class 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- VLTRZXGMWDSKGL-UHFFFAOYSA-N perchloric acid Chemical compound OCl(=O)(=O)=O VLTRZXGMWDSKGL-UHFFFAOYSA-N 0.000 description 2
- 239000000047 product Substances 0.000 description 2
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- 238000010998 test method Methods 0.000 description 2
- 229910052718 tin Inorganic materials 0.000 description 2
- 229910052726 zirconium Inorganic materials 0.000 description 2
- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 1
- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 description 1
- VEXZGXHMUGYJMC-UHFFFAOYSA-M Chloride anion Chemical compound [Cl-] VEXZGXHMUGYJMC-UHFFFAOYSA-M 0.000 description 1
- -1 Cr 2 N is promoted Chemical class 0.000 description 1
- 229910001208 Crucible steel Inorganic materials 0.000 description 1
- 229910001209 Low-carbon steel Inorganic materials 0.000 description 1
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 description 1
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- YLRAQZINGDSCCK-UHFFFAOYSA-M methanol;tetramethylazanium;chloride Chemical compound [Cl-].OC.C[N+](C)(C)C YLRAQZINGDSCCK-UHFFFAOYSA-M 0.000 description 1
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- 229910052720 vanadium Inorganic materials 0.000 description 1
- 238000004065 wastewater treatment Methods 0.000 description 1
- 239000002023 wood Substances 0.000 description 1
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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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
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- C21D2251/00—Treating composite or clad material
- C21D2251/02—Clad material
Definitions
- the present invention relates to a duplex stainless steel clad steel plate used for, for example, a reaction vessel of a chemical plant and a manufacturing method thereof.
- duplex stainless steel has been adopted in a high chloride environment such as seawater and in a severe corrosive environment such as an oil well or a gas well.
- oil and gas well piping, flue gas desulfurization equipment, wastewater treatment facilities, seawater pumping generators and other structural members, papermaking rolls, centrifuges, pumps and valves, heat exchangers, etc. Steel is adopted.
- a duplex stainless steel is a stainless steel having a composite structure in which two phases of an austenite phase and a ferrite phase are mixed, and has both excellent corrosion resistance and excellent strength characteristics. It is known that the corrosion resistance is most excellent when the area ratio (phase fraction) to the phase is approximately 1: 1.
- the practical component composition of the duplex stainless steel is defined such that the area ratio (phase fraction) of the austenite phase and the ferrite phase is approximately in this vicinity.
- SUS329Jl, SUS329J3L, SUS329J4L, and the like are standardized as bars and plates in the Japanese Industrial Standard (JIS).
- JIS Japanese Industrial Standard
- SUS329J1FB is standardized as a forged steel product
- SCS10 is standardized as a cast steel product.
- the price of alloy elements typified by Cr, Ni and Mo, which are the main raw materials for duplex stainless steel sometimes rises and fluctuates greatly.
- the superior corrosion resistance of the duplex stainless steel can be utilized more economically when used as a clad steel having the same thickness as the solid material, rather than using the duplex stainless steel as a solid material.
- a clad steel plate is a steel plate in which two or more types of steel plates having different properties are joined, for example, a base steel plate made of a so-called ordinary steel material such as carbon steel, and a high alloy steel plate exhibiting high corrosion resistance joined together. It is.
- the clad steel plate is obtained by metallographically bonding dissimilar metals, and unlike plating, there is no fear of peeling.
- the clad steel plate has various characteristics that cannot be achieved with a single metal or alloy.
- the clad steel plate can provide the same corrosion resistance as solid wood while suppressing the amount of expensive alloy elements used, and at the same time can ensure the same strength and toughness as carbon steel and low alloy steel. Has the advantage of being able to achieve both economy and functionality.
- the clad steel plate using a high alloy steel material as a laminated material is considered to be a very useful functional steel material, and in recent years, its needs are increasing in various industrial fields.
- Patent Document 1 discloses that “in weight%, C: 0.15% or less, Si: 0.5% or less, Mn: 1.5% or less, Ni: 3.0%.
- Ti 0.008 to 0.025%
- B 0.0004 to 0.0020%
- N 0.006 to 0.015%
- SUS316L clad steel has been used as a material for reaction vessels in chemical plants.
- duplex stainless steel clad steel such as SUS329J3L clad steel, which has better corrosion resistance than SUS316L clad steel.
- the conventional duplex stainless steel clad steel sheet has insufficient corrosion resistance of the laminated material.
- Patent Document 1 since only a suitable material is disclosed for the clad steel laminate, it is impossible to grasp the characteristics of the clad steel combined with the laminate and the base material. For this reason, in the clad steel using the base material described in Patent Document 1, sufficient corrosion resistance cannot be obtained.
- clad steel is required to have higher base metal strength and toughness.
- an object of the present invention is to provide a duplex stainless steel clad steel plate excellent in all of the corrosion resistance of a laminated material and the strength and toughness of a base material, and a method for producing the same.
- the ferrite phase increases at a high temperature range from the melting point to 1200 ° C. In the middle temperature range of 600 to 900 ° C., different phases such as intermetallic compounds such as sigma phase and carbonitride precipitate. In a low temperature range of 450 to 500 ° C., a reaction considered to be decomposition of the ferrite phase occurs. Thus, the metal structure changes in each temperature range, and the corrosion resistance and strength characteristics change accordingly.
- the problem is the precipitation of intermetallic compounds such as sigma phase, carbides such as Cr 23 C 6 , and nitrides such as Cr 2 N.
- carbides such as Cr 23 C 6
- nitrides such as Cr 2 N.
- a layer lacking corrosion-resistant elements such as Cr and Mo is formed around the sigma-phase, carbide, nitride, and carbonitride, and the corrosion resistance is significantly lowered.
- the present inventors investigated the relationship between precipitates and corrosion resistance using various test materials made of duplex stainless steel. As a result, it was found that the deterioration of corrosion resistance was caused by precipitates such as sigma phase, carbide, nitride and carbonitride. Furthermore, regarding the duplex stainless steel of the laminated material, the knowledge that there is a correlation between the amount of Cr and Mo contained in these precipitates (that is, the amount of Cr and Mo present as precipitates) and the corrosion resistance. Obtained. And even if it was a case where Cr of the duplex stainless steel as a laminated material was comparatively high, the manufacturing condition which reduces the amount of Cr which exists as a precipitate was discovered.
- the base steel sheet is, by mass, C: 0.06 to 0.25%, Si: 0.05 to 0.50%, Mn: 0.70 to 1.60%, P: 0.030% or less.
- Ni: 5.00 to 8.00%, Cr: 24.0% to 28.0%, Mo: 2.5 to 4.0%, and N: 0.08 to 0.30%, A second component composition containing PI defined by the formula (2) under the range of 34.0 to 43.0, the balance consisting of Fe and inevitable impurities; PI Cr + 3.3Mo + 16N (2) (In the above formula (2), the element symbol indicates the content (% by mass) of each element) A structure containing 35 to 65% of the ferrite phase by area fraction, A duplex stainless steel clad steel sheet, wherein the amount of precipitated Cr in the structure is 2.00% or less and the amount of precipitated Mo is 0.50% or less.
- the second component composition of the duplex stainless steel sheet is, by mass, Cu: 1.50% or less, W: 1.50% or less, Co: 1.50% or less, Ti: 0.25%
- the first component composition of the base steel sheet is, by mass, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.40% or less, and V: 0.050% or less.
- [4] A method for producing a duplex stainless steel clad steel sheet in which a duplex stainless steel sheet as a laminated material is bonded to one or both surfaces of a base steel sheet,
- the first material plate having the first component composition described in [1] or [3]
- the duplex stainless steel plate as a laminated material Laminating the second component composition described above and a second material plate having a structure containing 35 to 65% of the ferrite phase by area fraction to obtain a clad slab; Heating the clad slab to 1050-1250 ° C .;
- the step of cooling the clad rolled body so that
- duplex stainless steel clad steel sheet of the present invention According to the method for producing a duplex stainless steel clad steel sheet of the present invention, it is possible to produce a duplex stainless steel clad steel sheet excellent in all of the corrosion resistance of the laminated material and the strength and toughness of the base material.
- the duplex stainless steel clad steel sheet of the present invention is excellent in the corrosion resistance of the laminated material and the strength and toughness of the base material.
- duplex stainless steel clad steel plate in which a duplex stainless steel plate as a bonding material is bonded to one side or both sides of a base steel plate.
- the thickness of the duplex stainless steel clad steel plate is not particularly limited, but is preferably 6 to 45 mm.
- the thicknesses of the base steel plate and the laminated material are preferably about 5 to 40 mm and 1 to 5 mm, respectively. In this embodiment, it is possible to achieve both improvement of pitting corrosion resistance and improvement of strength and toughness by using a duplex stainless clad steel in which a specific base material and a laminated material are combined.
- the component composition of the base steel plate and the laminated material, which are constituent elements of the duplex stainless steel clad steel, will be described in detail.
- the unit of element content in the component composition is “mass%”, but hereinafter, it is simply indicated by “%” unless otherwise specified.
- Component composition of base steel sheet By using a low carbon steel having the following composition as a base steel plate, a duplex stainless steel clad having excellent mechanical properties such as strength and toughness can be provided.
- C 0.06 to 0.25% C is an element that improves the strength of the steel, and by containing 0.06% or more, sufficient strength is exhibited. Therefore, the C content is 0.06% or more, and preferably 0.08% or more. However, if the amount of C exceeds 0.25%, weldability and toughness are deteriorated. Therefore, the C content is 0.25% or less, preferably 0.20% or less.
- Si 0.05 to 0.50% Si is effective for deoxidation, and is contained at 0.05% or more in order to improve the strength of the steel. Further, Si is an element that inevitably enters steel from iron ore and other materials, and suppressing the Si content to less than 0.05% also causes an increase in cost in the steelmaking process. Accordingly, the Si content is 0.05% or more, and preferably 0.10% or more. However, if the amount of Si exceeds 0.50%, the surface properties and toughness of the steel are deteriorated. Therefore, the Si content is 0.50% or less, preferably 0.45% or less.
- Mn 0.70 to 1.60%
- Mn is an element that increases the strength of steel, and its effect is manifested at 0.70% or more. Therefore, the amount of Mn is 0.70% or more, and preferably 1.00% or more. However, if the amount of Mn exceeds 1.60%, weldability is impaired and alloy costs also increase. Therefore, the Mn content is 1.60% or less.
- P 0.030% or less
- P is an unavoidable impurity in steel, and when the amount of P exceeds 0.030%, toughness deteriorates. Therefore, the P content is 0.030% or less, preferably 0.020% or less, more preferably 0.015% or less. However, the amount of P is preferably 0.0001% or more from the viewpoint of dephosphorization cost.
- S 0.010% or less S, like P, is an unavoidable impurity in steel. If the amount of S exceeds 0.010%, the toughness deteriorates. Therefore, the S content is 0.010% or less, preferably 0.005% or less, more preferably 0.003% or less. However, from the viewpoint of desulfurization cost, the amount of S is preferably 0.0001% or more, and more preferably 0.0003% or more.
- Al 0.005 to 0.100% Al is added as a deoxidizer. Deoxidizing effect is exhibited at 0.005% or more. Therefore, the Al content is 0.005% or more, and preferably 0.010% or more. However, if the Al content exceeds 0.100%, the toughness of the welded portion is deteriorated. Therefore, the Al content is 0.100% or less, and preferably 0.070% or less.
- Mo 0.01 to 0.15%
- Mo is an element that improves the hardenability of steel, and improves the strength and toughness of the steel after rolling. The effect is manifested at a content of 0.01% or more. Therefore, the Mo amount is 0.01% or more, and preferably 0.05% or more. However, if the amount of Mo exceeds 0.15%, weldability is deteriorated. Therefore, the Mo amount is 0.15% or less.
- Nb 0.010 to 0.040%
- Nb precipitates as Nb nitride has the effect of suppressing the coarsening of austenite grains and improving the strength and toughness of the steel. Further, in the austenite region rolling, the recrystallization temperature region is expanded to a low temperature, crystal grains can be refined, and toughness is improved. These effects are obtained when the content is 0.010% or more. Therefore, the Nb content is 0.010% or more, preferably 0.013% or more, and more preferably 0.015% or more. However, if the Nb content exceeds 0.040%, coarse Nb nitrides are formed and the toughness deteriorates.
- the Nb content is 0.040% or less, preferably 0.035% or less, more preferably 0.030% or less.
- the inhibitory effect of the austenite grain coarsening can be exhibited more by making a ratio with the below-mentioned nitrogen atom or more into a predetermined value or more.
- Ti Less than 0.005%
- Ti forms a composite carbide and / or composite nitride with Nb.
- the Ti content is less than 0.005%, preferably 0.003% or less, more preferably 0.001% or less.
- the amount of Ti is preferably reduced as much as possible, but may be, for example, 0.0001% or more, or 0.0003% or more.
- N 0.0010 to 0.0100%
- N is an element indispensable for formation of Nb nitride, and Nb nitride is formed with a content of 0.0010% or more. Therefore, the N content is 0.0010% or more, preferably 0.0020% or more, more preferably 0.0025% or more. However, when the N amount exceeds 0.0100%, deterioration of weldability and toughness is caused. Therefore, the N content is 0.0100% or less, preferably 0.0070% or less, more preferably 0.0050% or less. Moreover, it is thought that the inhibitory effect of the coarsening of (gamma) grain (austenite grain) can be exhibited more by making ratio with Nb mentioned later into a predetermined value or more.
- Nb / N 3.0 or more
- Nb / N 3.0 or more
- the precipitation of Nb nitride and the effect of solid solution Nb are sufficiently exhibited.
- Nb / N is 3.0 or more, preferably 3.5 or more.
- Nb / N can be 20.0 or less.
- Ceq 0.35 to 0.45 Ceq is an index of the hardenability of steel and is represented by the following formula (1).
- Ceq C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
- an element symbol shows content (mass%) of each element, and when not containing the said element, it calculates as zero.
- Ceq is set to 0.35 or more, preferably 0.38 or more.
- Ceq exceeds 0.45, weldability is impaired. For this reason, Ceq is set to 0.45 or less.
- optional components are selected from the group consisting of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.40% or less, and V: 0.050% or less. You may further contain 1 type, or 2 or more types.
- Cu 0.50% or less
- Cu is an element that improves the hardenability of the steel, and improves the strength and toughness of the steel after rolling. The effect is manifested at a content of 0.01% or more. Therefore, when the improvement of the hardenability by Cu is expected, the Cu content is preferably 0.01% or more, and more preferably 0.05% or more. However, when the amount of Cu exceeds 0.50%, deterioration of weldability and toughness is caused. Therefore, when adding Cu, the amount of Cu shall be 0.50% or less.
- Ni 0.50% or less
- Ni is an element that improves the hardenability of steel and is particularly effective in improving toughness. The effect is manifested at a content of 0.01% or more. Therefore, when the improvement of the hardenability by Ni is expected, the Ni content is preferably 0.01% or more, and more preferably 0.05% or more. However, if the Ni content exceeds 0.50%, the weldability is impaired and the alloy cost also increases. Therefore, when adding Ni, the amount of Ni is made 0.50% or less.
- Cr 0.40% or less Cr, like Cu, is an element that improves the hardenability of steel, and improves the strength and toughness of the steel after rolling. The effect is manifested at a content of 0.01% or more. Therefore, when this effect by Cr is expected, the Cr content is preferably 0.01% or more, and more preferably 0.05% or more. However, if the Cr content exceeds 0.40%, deterioration of weldability and toughness is caused. Therefore, when adding Cr, the Cr content is 0.40% or less.
- V 0.050% or less
- V is an element that improves the strength of steel by forming carbonitride. The effect is manifested with a content of 0.001% or more. Therefore, when this effect by V is expected, the V amount is preferably 0.001% or more, and more preferably 0.005% or more. However, when the V content exceeds 0.050%, the toughness deteriorates. Therefore, when adding V, the amount of V is made into 0.050% or less.
- the remainder other than the above is Fe and inevitable impurities.
- the component composition of the base material is as follows: Ca: 0.010% or less, B: 0.0050% or less, Sn: 0.050% or less, Sb: 0.050% or less, Zr: 0.050% or less, W : 0.050% or less, Co: 0.050% or less, Mg: 0.020% or less, REM: 0.010% or less, O: 0.0100% or less within this range However, no significant change occurs in the characteristics of the base material.
- C 0.030% or less C is inevitably one of the elements present in steel.
- the C content is 0.030% or less, preferably 0.025% or less, more preferably 0.020% or less.
- C amount shall be 0.001% or more from a viewpoint of manufacturing cost.
- Si 1.00% or less
- Si is an element that remarkably accelerates the precipitation of intermetallic compounds such as a sigma phase.
- the amount of Si needs to be 1.00% or less. Therefore, the Si amount is 1.00% or less, preferably 0.50% or less, more preferably 0.40% or less. However, from the viewpoint of manufacturing cost, the Si amount is preferably 0.01% or more.
- Mn 2.00% or less Mn is an element useful for deoxidation, preferably 0.01% or more.
- the Mn content is 2.00% or less, preferably 1.70% or less, more preferably 1.50% or less, and still more preferably 1.00% or less.
- the P content is 0.050% or less, preferably 0.040% or less, more preferably 0.020% or less.
- the amount of P is preferably 0.0001% or more from the viewpoint of dephosphorization cost.
- the S content is 0.0100% or less, preferably 0.0050% or less, more preferably 0.0020% or less.
- the S amount is preferably 0.0001% or more.
- Ni 5.00 to 8.00%
- Ni is an essential element as an element for stabilizing the austenite phase which is one phase of the duplex stainless steel. The effect is exhibited by containing 5.00% or more. Therefore, the Ni content is 5.00% or more, preferably 5.50% or more, more preferably 5.70% or more, and still more preferably 6.00% or more. However, since Ni is an expensive metal, if it is contained in a large amount, the price of the alloy itself is increased. Therefore, the Ni content is 8.00% or less, preferably 7.50% or less, more preferably 7.00% or less.
- the duplex stainless steel has excellent corrosion resistance when the ratio of the austenite phase to the ferrite phase is 35:65 to 65:35, preferably about 1: 1, so this phase ratio is satisfied. Therefore, the Ni content is 5.00 to 8.00%.
- Cr 24.0% to 28.0% or less
- Cr is an indispensable element for ensuring the corrosion resistance of the alloy and stabilizing the ferrite phase, which is the other phase of the duplex stainless steel.
- a content exceeding 24.0% is necessary. Therefore, the Cr content is over 24.0%.
- the Cr content is 28.0% or less, preferably 27.0% or less, more preferably 26.0% or less.
- the duplex stainless steel has excellent corrosion resistance when the ratio of the austenite phase to the ferrite phase is 35:65 to 65:35, preferably about 1: 1, so this phase ratio is satisfied.
- the Cr content is made 24.0% to 28.0% or less.
- Mo 2.5-4.0%
- Mo is also an important element for improving the corrosion resistance of the alloy. In order to exert the effect, it is necessary to contain 2.5% or more. Therefore, the Mo amount is 2.5% or more, preferably 3.0% or more. However, if the Mo content exceeds 4.0%, precipitation of the sigma phase is remarkably promoted, which adversely affects ductility and toughness. Therefore, the Mo amount is 4.0% or less.
- the duplex stainless steel has excellent corrosion resistance when the ratio of the austenite phase to the ferrite phase is 35:65 to 65:35, preferably about 1: 1, so this phase ratio is satisfied. Therefore, the Mo content is set to 2.5 to 4.0%.
- N 0.08 to 0.30%
- N is important as an element for improving the corrosion resistance of the alloy, and is also effective as an element for improving the strength. In order to exhibit the effect, the content of 0.08% or more is necessary. Therefore, the N content is 0.08% or more, preferably 0.10% or more, more preferably 0.14% or more. However, if the N content exceeds 0.30%, precipitation of nitrides such as Cr 2 N is promoted, which adversely affects the corrosion resistance. Therefore, the N content is 0.30% or less, preferably 0.25% or less.
- the duplex stainless steel has excellent corrosion resistance when the ratio of the austenite phase to the ferrite phase is 35:65 to 65:35, preferably about 1: 1, so this phase ratio is satisfied. Therefore, the N amount is set to 0.08 to 0.30%.
- PI 34.0-43.0
- PI is Pitting Index (pitting corrosion resistance index) and is defined by the following formula (2).
- PI Cr + 3.3Mo + 16N (2)
- an element symbol shows content (mass%) of each element.
- the PI value calculated by the above formula (1) is an index value of the pitting corrosion resistance of the solution material having a fraction of sigma phase, carbide and nitride of 0%.
- the pitting corrosion resistance is determined by the balance between the PI value and the amount of Cr and Mo contained in these precipitates. Is done.
- the PI value increases, the pitting corrosion resistance of the base material improves.
- the PI value is high, the content of Cr, Mo, or N naturally increases, so that the precipitate is likely to precipitate.
- the amount of precipitated Cr or the amount of precipitated Mo described later increases, and as a result, the pitting corrosion resistance decreases. Therefore, in the present invention, the range of the PI value is 34.0 to 43.0.
- Cu 1.50% or less
- W 1.50% or less
- Co 1.50% or less
- Ti 0.25% or less
- Nb 0.25%
- Cu 1.50% or less Cu is an element that improves the corrosion resistance, and the effect is manifested when the content is 0.01% or more. For this reason, when improving corrosion resistance with Cu, it is preferable to make Cu amount 0.01% or more. However, if the amount of Cu exceeds 1.50%, the hot workability is significantly deteriorated. Therefore, when Cu is contained, the Cu content is 1.50% or less, preferably 1.00% or less.
- W 1.50% or less W is an element that improves the corrosion resistance of the alloy, and the effect is manifested by inclusion of 0.01% or more. For this reason, when improving corrosion resistance by W, it is preferable to make the amount of W 0.01% or more. However, when the amount of W exceeds 1.50%, sigma phase precipitation is promoted. Therefore, when W is contained, the W amount is set to 1.50% or less, preferably 1.00% or less.
- Co 1.50% or less Co is also an element that improves the corrosion resistance, and the effect is manifested by inclusion of 0.01% or more. For this reason, when improving corrosion resistance by Co, it is preferable to make Co amount 0.01% or more. However, when the Co content exceeds 1.50%, the alloy cost increases. Therefore, when Co is contained, the Co content is 1.50% or less, preferably 1.00% or less.
- Ti 0.25% or less Ti has the property of being easily bonded to C, and when contained in the alloy, precipitation of carbides such as Cr 23 C 6 that are harmful to corrosion resistance can be delayed. The effect is manifested at 0.01% or more. Therefore, when this effect by Ti is expected, the Ti content is preferably set to 0.01% or more. However, even if the content exceeds 0.25%, the effect is not improved and the alloy cost increases. Therefore, when Ti is contained, the Ti amount is 0.25% or less, preferably 0.20% or less.
- Nb 0.25% or less
- Nb has the property of being easily bonded to C like Ti, and when contained in the alloy, precipitation of carbides such as Cr 23 C 6 that are harmful to corrosion resistance can be delayed. The effect is manifested at 0.01% or more. Therefore, when this effect by Nb is expected, the Nb content is preferably 0.01% or more. However, even if the content exceeds 0.25%, the effect is not improved and the alloy cost increases. Therefore, when Nb is contained, the Nb content is 0.25% or less, preferably 0.20% or less.
- the remainder other than the above is Fe and inevitable impurities.
- the component composition of the laminated material is Al: 0.05% or less, V: 0.2% or less, Ca: 0.02% or less, B: 0.01% or less, O: 0.02% or less, Sn : 0.2% or less, Sb: 0.2% or less, Zr: 0.2% or less, Mg: 0.02% or less, REM: 0.2% or less within this range
- Ferrite phase area fraction 35-65%
- the “area fraction of the ferrite phase” is a value estimated from the component composition of the duplex stainless steel plate, calculated according to the following formulas (3) to (5), or a value calculated from a microscope image described later. is there.
- Ferrite phase area fraction (%) 4.01 Creq ⁇ 5.6 Nieq ⁇ 4.13
- Creq Cr + 1.73Si + 0.88Mo
- Nieq Ni + 24.55C + 21.75N + 0.4Cu (5)
- an element symbol shows content (mass%) of each element, and when not containing the said element, it calculates as zero.
- the duplex stainless steel sheet as a laminated material exhibits corrosion resistance when the phase fraction of the ferrite phase and the austenite phase is in the range of 35:65 to 65:35. If the value of the area fraction of the ferrite phase is 35 to 65%, the phase fraction of the ferrite phase and the austenite phase is approximately 35:65 to 65:35, and excellent corrosion resistance is exhibited. From this viewpoint, the area fraction of the ferrite phase is 35% or more, preferably 40% or more, more preferably 45% or more, 65% or less, preferably 60% or less, more preferably 55% or less. To do.
- the area fraction of the ferrite phase can be calculated by a known method other than the method calculated from the above formula (3), and is not particularly limited.
- it can be obtained by the following method. .
- Ferrite phase, austenite phase, and precipitates (sigma phase, carbide, nitride, and carbonitride) by performing electrolytic etching on duplex stainless steel sheet and processing color photographs taken with an optical microscope with image processing software ), And the area fraction of each can be calculated. It has been confirmed that the value calculated by this method is correlated with the value calculated according to the above equation (3).
- the area fraction is 100% in total of ferrite phase + austenite phase + precipitate (sigma phase, carbide, nitride, and carbonitride). When the precipitate is zero, ferrite phase + austenite phase 100%.
- the structure of the laminated material contains a predetermined amount or more of Cr-based precipitates and Mo-based precipitates, corrosion resistance is deteriorated. Therefore, in the present invention, it is important to contain a predetermined amount or less of the amount of precipitated Cr and the amount of precipitated Mo (that is, the amount of Cr and Mo present as precipitates).
- precipitate refers to one or more selected from the group consisting of sigma phase, carbide, nitride and carbonitride present in the structure of the laminated material.
- Cr and Mo are generally widely known as elements that form a passive film.
- Cr and / or Mo in the matrix collects in the precipitate, the concentration of Cr and / or Mo in the periphery of the precipitate is lowered, and in a corrosive environment, this low Cr and / or low Mo region is preferential. Corroded. This phenomenon is called sensitization.
- sensitization When the amount of precipitated Cr exceeds 2.00% by mass%, sensitization proceeds and corrosion resistance deteriorates. Therefore, the amount of precipitated Cr is set to 2.00% or less.
- the amount of precipitated Mo exceeds 0.50% by mass%, sensitization proceeds and corrosion resistance deteriorates. Therefore, the amount of precipitated Mo is set to 0.50% or less.
- the amount of precipitated Cr and the amount of precipitated Mo can each be 0.00% or more.
- the 1st material board used as a base material steel plate is melted so that it may become a component composition of said base material steel plate, and can be manufactured by a conventional method.
- the 2nd raw material board used as the duplex stainless steel plate as a bonding material can be melted so that it may become a component composition of said bonding material, and can be manufactured by a conventional method.
- the first material plate and the second material plate are laminated to assemble a clad slab. For example, as shown in FIG.
- a clad slab 10 is formed by stacking two sets of laminates in which a first material plate 1 and a second material plate 2 are laminated so that the second material plates face each other. can do.
- the release agent 3 can be applied between the two second material plates 2.
- the release agent 3 is not particularly limited, relatively inexpensive as Al 2 O 3, preferably has a sufficient peelability.
- reference numeral 4 is a spacer, and 5 is a welded portion. In consideration of warping during cooling, it is desirable that the two first material plates and the two second material plates have the same thickness. Of course, it goes without saying that it is not necessary to limit to the assembly system shown in FIG.
- the clad slab thus obtained is heated and further hot-rolled to obtain a clad rolled body in which the base steel plate and the duplex stainless steel plate are joined.
- Heating temperature 1050-1250 ° C
- the reason why the heating temperature is set to 1050 ° C. or higher is to ensure the bondability between the base material steel plate and the laminated material and the toughness of the base material steel plate.
- the heating temperature is 1050 ° C. or higher, preferably 1100 ° C. or higher.
- the heating temperature exceeds 1250 ° C., the crystal grains are extremely coarse and the toughness of the base steel plate is deteriorated. Therefore, heating temperature shall be 1250 degrees C or less.
- Rolling ratio 2.0 or more
- the rolling ratio refers to the thickness of the clad slab before rolling / the thickness of the rolled clad body after rolling.
- the reduction ratio is 2.0 or more, preferably 3.0 or more. Thereby, good bondability is obtained.
- the crystal grain of a base material steel plate is refined
- the reduction ratio can be 20.0 or less.
- the rolled clad body is allowed to cool in the air and then reheated to 1000 to 1100 ° C.
- Cooling means that the clad rolled body is exposed to the atmosphere without forced cooling by water injection or the like, and means air cooling without active cooling.
- the positive cooling referred to here means “actively cooling with gas, liquid or a mixture thereof”.
- the cooling stop temperature in the cooling is preferably 400 ° C. or lower.
- Reheating temperature 1000-1100 ° C
- the reason for reheating after hot rolling is to ensure the corrosion resistance of the laminated material. By performing reheating after hot rolling, the precipitate can be remelted and the corrosion resistance of the laminated material can be ensured.
- the reheating temperature is set to 1000 ° C. or higher.
- the reheating temperature is 1100 ° C. or lower, preferably 1050 ° C. or lower.
- Cooling rate of the laminated material after reheating 0.8 ° C./s or more
- the cooling rate of the laminated material is set to 0.8 ° C./s or more.
- the cooling rate of the laminated material can be 100 ° C./s or less.
- Cooling rate of base steel after reheating 1.0 ° C./s or more
- the cooling rate of the base steel is set to 1.0 ° C./s or more, preferably 2.0 ° C./s or more.
- the cooling rate of the base steel sheet can be 100 ° C./s or less.
- the control of the cooling rate as described above can be realized by setting the cooling method and the cooling conditions in consideration of the thickness of the base steel plate and the laminated material, the form of the clad slab, and the like.
- the cooling rate of the base steel plate becomes larger than the cooling rate of the laminated material.
- the cooling rate of the base steel plate and the combined material are changed by changing the water cooling conditions on both sides of the clad rolled material.
- the cooling rate can be individually controlled.
- both the cooling stop temperature of the laminated material and base material steel plate after reheating shall be less than 200 degreeC.
- a so-called tempering treatment is performed in which the clad rolled body thus cooled is heated at 700 ° C. or lower.
- Tempering temperature 700 ° C. or less
- the purpose of tempering is to adjust the strength of the base steel sheet. It can adjust to desired intensity
- a known process may be further added before and after each process described above.
- the obtained rolled clad body can be made into a final product plate by peeling between the laminated material and the laminated material coated with a release agent.
- first material plates base material materials
- second material plates materials for laminated materials
- area fraction of the ferrite phase shown in Table 2 was calculated according to the above formula (3).
- the joint strength between the laminated material and the base material was evaluated by a JIS G0601 shear strength test.
- the shear strength test is a method in which the laminated material is peeled from the base material in parallel with the joining surface, and the joining property is evaluated from the maximum shear strength required for the peeling. When the shear strength was 200 MPa or more, it was judged that the bondability was good.
- the toughness of the base steel plate was evaluated by a Charpy impact test.
- a 10 ⁇ 10 mm size V-notch Charpy impact test piece prescribed in JIS Z2242 was taken from the base steel plate and subjected to a Charpy impact test.
- the Charpy impact absorption energy value (vE ⁇ 20 ) at ⁇ 20 ° C. exceeds 100 J, it was judged that the toughness was good.
- the clad steel sheets according to the invention examples 1 to 18 exhibited good corrosion resistance and toughness.
- the component composition of the laminated material is outside the scope of the present invention.
- the 19 to 29 clad steel sheets were inferior in corrosion resistance.
- the 30 to 38 clad steel plate was inferior in toughness.
- the composition of the laminated material and the composition of the base material are outside the scope of the present invention.
- the 39 to 46 clad steel plates were inferior in both corrosion resistance and toughness. Among these, No. whose heating temperature is lower than the range of the present invention. No. 39 clad steel sheet and No. 39 whose rolling ratio is lower than the range of the present invention.
- the shear strength was less than 200 MPa, and the bondability was poor.
- the production conditions are outside the scope of the present invention.
- the amount of precipitated Cr was outside the range of the present invention, and as a result, the corrosion resistance was poor.
Abstract
Description
前記母材鋼板は、質量%で、C:0.06~0.25%、Si:0.05~0.50%、Mn:0.70~1.60%、P:0.030%以下、S:0.010%以下、Al:0.005~0.100%、Mo:0.01~0.15%、Nb:0.010~0.040%、Ti:0.005%未満、及びN:0.0010~0.0100%を、Nb/Nが3.0以上、かつ下記式(1)で表わされるCeqが0.35~0.45の範囲の下に含有し、残部がFe及び不可避的不純物からなる第1の成分組成を有し、
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ・・・(1)
(上記式(1)中、元素記号は各元素の含有量(質量%)を示し、当該元素を含有しない場合はゼロとして算出する)
前記二相ステンレス鋼板は、質量%で、C:0.030%以下、Si:1.00%以下、Mn:2.00%以下、P:0.050%以下、S:0.0100%以下、Ni:5.00~8.00%、Cr:24.0%超え28.0%以下、Mo:2.5~4.0%、及びN:0.08~0.30%を、下記式(2)で定義されるPIが34.0~43.0の範囲の下に含有し、残部がFe及び不可避的不純物からなる第2の成分組成と、
PI=Cr+3.3Mo+16N ・・・(2)
(上記式(2)中、元素記号は各元素の含有量(質量%)を示す)
フェライト相を面積分率で35~65%含む組織と、を有し、
前記組織における析出Cr量が2.00%以下、かつ、析出Mo量が0.50%以下であることを特徴とする二相ステンレスクラッド鋼板。
母材鋼板となる、上記[1]又は[3]に記載の第1の成分組成を有する第1素材板と、合せ材としての二相ステンレス鋼板となる、上記[1]又は[2]に記載の第2の成分組成、及びフェライト相を面積分率で35~65%含む組織を有する第2素材板とを積層させて、クラッドスラブを得る工程と、
前記クラッドスラブを1050~1250℃に加熱する工程と、
その後、前記クラッドスラブに圧下比が2.0以上となる熱間圧延を施して、前記母材鋼板と前記二相ステンレス鋼板とが接合されたクラッド圧延体を得る工程と、
前記クラッド圧延体を放冷する工程と、
その後、前記クラッド圧延体を1000~1100℃に再加熱する工程と、
その後、前記クラッド圧延体を、前記二相ステンレス鋼板における冷却速度が0.8℃/s以上、かつ前記母材鋼板における冷却速度が1.0℃/s以上となるように、冷却する工程と、
その後、前記クラッド圧延体を700℃以下で焼き戻しする工程と、
を有することを特徴とする二相ステンレスクラッド鋼板の製造方法。
本発明の一実施形態は、母材鋼板の片面又は両面に合せ材としての二相ステンレス鋼板が接合されている二相ステンレスクラッド鋼板に関する。二相ステンレスクラッド鋼板の板厚は特に限定されないが、6~45mmが好適である。また、母材鋼板及び合せ材の板厚はそれぞれ、5~40mm程度及び1~5mmが好適である。本実施形態では、特定の母材と合せ材とを組み合わせた二相ステンレスクラッド鋼により、耐孔食性の向上と、強度及び靱性の向上との両立を図ることができる。
下記の成分組成の低炭素鋼を母材鋼板として用いることで、強度や靱性等の機械的特性に優れた二相ステンレスクラッド鋼を提供することができる。
Cは鋼の強度を向上させる元素であり、0.06%以上含有させることで十分な強度を発現する。よって、C量は0.06%以上とし、0.08%以上が好ましい。しかし、C量が0.25%を超えると溶接性及び靱性の劣化を招く。したがって、C量は0.25%以下とし、0.20%以下が好ましい。
Siは脱酸に有効であり、また鋼の強度を向上させるために0.05%以上で含有させる。また、Siは鉄鉱石などの原料から鋼中へ不可避的に入る元素であり、Si量を0.05%未満に抑えることは製鋼過程でのコスト増を招くことにもなる。したがって、Si量は0.05%以上とし、0.10%以上が好ましい。しかしながら、Si量が0.50%を超えると鋼の表面性状及び靱性の劣化を招く。よって、Si量は0.50%以下とし、0.45%以下が好ましい。
Mnは鋼の強度を上昇させる元素であり、0.70%以上でその効果を発現する。よって、Mn量は0.70%以上とし、1.00%以上が好ましい。しかしながら、Mn量が1.60%を超えると、溶接性が損なわれ、合金コストも増大する。したがって、Mn量は1.60%以下とする。
Pは鋼中の不可避的不純物であり、P量が0.030%を超えると靱性が劣化する。したがって、P量は0.030%以下とし、好ましくは0.020%以下、より好ましくは0.015%以下である。ただし、脱燐コストの観点から、P量は0.0001%以上であることが好ましい。
SもPと同様に、鋼中の不可避的不純物である。S量が0.010%を超えると靱性が劣化する。したがって、S量は0.010%以下とし、好ましくは0.005%以下、より好ましくは0.003%以下である。ただし、脱硫コストの観点から、S量は0.0001%以上であることが好ましく、0.0003%以上であることがより好ましい。
Alは脱酸剤として添加する。0.005%以上で脱酸効果を発揮する。よって、Al量は0.005%以上とし、0.010%以上が好ましい。しかしながら、Al量が0.100%を超えると溶接部の靱性劣化を招く。したがって、Al量は0.100%以下とし、0.070%以下が好ましい。
Moは鋼の焼入れ性を向上させる元素であり、圧延後の鋼の強度及び靱性を向上させる。その効果は0.01%以上の含有で発現する。よって、Mo量は0.01%以上とし、0.05%以上が好ましい。しかしながら、Mo量が0.15%を超えると溶接性の劣化を引き起こす。したがって、Mo量は0.15%以下とする。
NbはNb窒化物として析出し、オーステナイト粒の粗大化を抑制して、鋼の強度及び靱性を改善させる効果がある。また、オーステナイト域の圧延において再結晶温度域を低温まで拡大させ、結晶粒の微細化が可能となり、靱性が改善する。これらの効果は0.010%以上の含有により得られる。よって、Nb量は0.010%以上とし、0.013%以上が好ましく、0.015%以上がより好ましい。しかしながら、Nb量が0.040%を超えると、粗大なNb窒化物が形成されて靱性が劣化する。したがって、Nb量は0.040%以下とし、好ましくは0.035%以下、より好ましくは0.030%以下である。また、後述の窒素原子との比を所定値以上にすることで、オーステナイト粒の粗大化の抑制効果をより発揮することができる。
Nbを必須元素として含む本実施形態の場合、TiはNbとの複合炭化物及び/又は複合窒化物を形成する。本実施形態のNb量では、Ti量が0.005%以上になると、粗大なTiとNbの複合炭化物及び/又は複合窒化物が形成されて、靱性が劣化することが確認された。よって、Ti量は、0.005%未満とし、好ましくは0.003%以下、より好ましくは0.001%以下とする。Ti量はできるだけ低減することが好ましいが、例えば0.0001%以上、あるいは0.0003%以上となり得る。
NはNb窒化物の形成に不可欠な元素であり、0.0010%以上の含有でNb窒化物が形成される。よって、N量は0.0010%以上とし、好ましくは0.0020%以上、より好ましくは0.0025%以上である。しかしながら、N量が0.0100%を超えると、溶接性及び靱性の劣化を引き起こす。したがって、N量は0.0100%以下とし、好ましくは0.0070%以下、より好ましくは0.0050%以下とする。また、後述のNbとの比を所定値以上にすることで、γ粒(オーステナイト粒)の粗大化の抑制効果をより発揮することができると考えられる。
Nb/Nが3.0以上の場合、Nb窒化物の析出と固溶Nbの効果が十分に発現する。しかしながら、Nb/Nが3.0未満の場合、鋼中に固溶Nが存在するため、靱性の顕著な劣化が生じる。したがって、Nb/Nは3.0以上とし、好ましくは3.5以上とする。また、Nb/Nは20.0以下となり得る。
Ceqは、鋼の焼き入れ性の指標であり、以下の式(1)で表わされる。
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ・・・(1)
上記式(1)中、元素記号は各元素の含有量(質量%)を示し、当該元素を含有しない場合はゼロとして算出する。
Cuは鋼の焼入れ性を向上させる元素であり、圧延後の鋼の強度及び靱性を向上させる。その効果は0.01%以上の含有で発現する。よって、Cuによる焼き入れ性の向上を期待する場合は、Cu量を0.01%以上とすることが好ましく、0.05%以上がより好ましい。しかしながら、Cu量が0.50%を超えると、溶接性及び靱性の劣化を引き起こす。したがって、Cuを添加する場合、Cu量は0.50%以下とする。
Niは鋼の焼き入れ性を向上させ、特に靱性の改善に効果的な元素である。その効果は0.01%以上の含有で発現する。よって、Niによる焼き入れ性の向上を期待する場合は、Ni量を0.01%以上とすることが好ましく、0.05%以上がより好ましい。しかしながら、Ni量が0.50%を超えると、溶接性を損ない、合金コストも増大する。したがって、Niを添加する場合、Ni量は0.50%以下とする。
CrもCuと同様に、鋼の焼入れ性を向上させる元素であり、圧延後の鋼の強度及び靱性を向上させる。その効果は0.01%以上の含有で発現する。よって、Crによるこの効果を期待する場合は、Cr量を0.01%以上とすることが好ましく、0.05%以上がより好ましい。しかしながら、Cr量が0.40%を超えると、溶接性及び靱性の劣化を引き起こす。したがって、Crを添加する場合、Cr量は0.40%以下とする。
Vは炭窒化物を形成することで、鋼の強度を向上させる元素である。その効果は0.001%以上の含有で発現する。よって、Vによるこの効果を期待する場合は、V量を0.001%以上とすることが好ましく、0.005%以上がより好ましい。しかしながら、V量が0.050%を超えると靱性が劣化する。したがって、Vを添加する場合、V量は0.050%以下とする。
次に、合せ材としての二相ステンレス鋼板の成分組成を説明する。
Cは不可避的に鋼材中に存在する元素の一つである。C量が0.030%を超えると炭化物の析出が顕著に生じ、耐食性の劣化を引き起こす。したがって、C量は0.030%以下とし、好ましくは0.025%以下、より好ましくは0.020%以下とする。なお、C量は、製造コストの観点から0.001%以上とすることが好ましい。
Siはシグマ相など金属間化合物の析出を著しく促進する元素であり、シグマ相などの析出を抑えるには、Si量は1.00%以下とする必要がある。したがって、Si量は1.00%以下とし、好ましくは0.50%以下、より好ましくは0.40%以下とする。ただし、製造コストの観点から、Si量は0.01%以上が好ましい。
Mnは脱酸に有用な元素であり、好ましくは0.01%以上で含有させる。一方、Mn量が2.00%を超えると、MnSを形成し耐食性を劣化させる。したがって、Mn量は2.00%以下とし、好ましくは1.70%以下、より好ましくは1.50%以下、さらに好ましくは1.00%以下とする。
P量が0.050%を超えると靭性が劣化することに加え、耐食性が劣化する。したがって、P量は0.050%以下とし、好ましくは0.040%以下、より好ましくは0.020%以下とする。ただし、脱リンコストの観点から、P量は0.0001%以上であることが好ましい。
S量が0.0100%を超えると熱間加工性が劣化することに加え、耐食性が劣化する。したがって、S量は0.0100%以下とし、好ましくは0.0050%以下、より好ましくは0.0020%以下である。ただし、脱硫コストの観点から、S量は0.0001%以上であることが好ましい。
Niは二相ステンレス鋼の一方の相であるオーステナイト相を安定化させる元素として必須の元素である。5.00%以上の含有により、その効果を発揮する。よって、Ni量は5.00%以上とし、好ましくは5.50%以上、より好ましくは5.70%以上、さらに好ましくは6.00%以上とする。しかしながら、Niは高価な金属であるために、多量に含有させると合金自体の高価格化を招く。したがって、Ni量は8.00%以下とし、好ましくは7.50%以下、より好ましくは7.00%以下とする。また、前述のように二相ステンレス鋼では、オーステナイト相とフェライト相との比率が35:65~65:35、好ましくはほぼ1:1の場合に耐食性が優れているので、この相比率を満足するためにも、Ni量は5.00~8.00%とする。
Crは合金の耐食性を保証し、かつ二相ステンレス鋼の他方の相であるフェライト相を安定化するために必要不可欠な元素である。その効果を発揮させるためには、24.0%超えの含有量が必要である。よって、Cr量は24.0%超えとする。しかしながら、Cr量が28.0%を超えると、シグマ相の析出が促進され、延性や靭性に悪影響を及ぼす。したがって、Cr量は28.0%以下とし、好ましくは27.0%以下とし、より好ましくは26.0%以下とする。また、前述のように二相ステンレス鋼では、オーステナイト相とフェライト相との比率が35:65~65:35、好ましくはほぼ1:1の場合に耐食性が優れているので、この相比率を満足するためにも、Cr量は24.0%超え28.0%以下とする。
Moも合金の耐食性を向上させる元素として重要である。その効果を発揮させるためには、2.5%以上の含有が必要である。よって、Mo量は2.5%以上とし、好ましくは3.0%以上とする。しかしながら、Mo量が4.0%を超えると、シグマ相の析出が著しく促進され、延性や靭性に悪影響を及ぼす。したがって、Mo量は4.0%以下とする。また、前述のように二相ステンレス鋼では、オーステナイト相とフェライト相との比率が35:65~65:35、好ましくはほぼ1:1の場合に耐食性が優れているので、この相比率を満足するためにも、Mo量は2.5~4.0%とする。
Nは合金の耐食性を向上させる元素として重要であり、同時に、強度を向上させる元素としても有効である。その効果を発揮させるためには、0.08%以上の含有が必要である。よって、N量は0.08%以上とし、好ましくは0.10%以上、より好ましくは0.14%以上とする。しかしながら、N量が0.30%を超えると、Cr2Nなどの窒化物の析出が促進され、耐食性に悪影響を及ぼす。したがって、N量は0.30%以下とし、好ましくは0.25%以下とする。また、前述のように二相ステンレス鋼では、オーステナイト相とフェライト相との比率が35:65~65:35、好ましくはほぼ1:1の場合に耐食性が優れているので、この相比率を満足するためにも、N量は0.08~0.30%とする。
PIはPitting Index(耐孔食性指数)であり、下記式(2)で定義される。
PI=Cr+3.3Mo+16N ・・・(2)
上記式(2)中、元素記号は各元素の含有量(質量%)を示す。
Cuは耐食性を向上させる元素であり、その効果は0.01%以上の含有で発現する。このため、Cuにより耐食性を向上させる場合には、Cu量を0.01%以上とすることが好ましい。しかし、Cu量が1.50%を超えると熱間加工性の著しい劣化を招く。したがって、Cuを含有する場合、Cu量は1.50%以下とし、好ましくは1.00%以下とする。
Wは合金の耐食性を向上させる元素であり、その効果は0.01%以上の含有により発現する。このため、Wにより耐食性を向上させる場合には、W量を0.01%以上とすることが好ましい。しかしながら、W量が1.50%を超えるとシグマ相析出が促進される。したがって、Wを含有する場合、W量は1.50%以下とし、好ましくは1.00%以下とする。
Coも耐食性を向上させる元素であり、その効果は0.01%以上の含有により発現する。このため、Coにより耐食性を向上させる場合には、Co量を0.01%以上とすることが好ましい。しかしながら、Co量が1.50%を超えると合金コストが上昇する。したがって、Coを含有する場合、Co量は1.50%以下とし、好ましくは1.00%以下とする。
TiはCと結合しやすい性質を有しており、合金中に含有すると耐食性に有害なCr23C6などの炭化物の析出を遅延させることができる。その効果は0.01%以上で発現する。したがって、Tiによるこの効果を期待する場合は、Ti量を0.01%以上とすることが好ましい。しかし、0.25%を超えて含有しても効果は向上せず、合金コストが増大する。したがって、Tiを含有する場合、Ti量は0.25%以下とし、好ましくは0.20%以下とする。
NbもTiと同様にCと結合しやすい性質を有しており、合金中に含有すると耐食性に有害なCr23C6などの炭化物の析出を遅延させることができる。その効果は0.01%以上で発現する。したがって、Nbによるこの効果を期待する場合は、Nb量を0.01%以上とすることが好ましい。しかし、0.25%を超えて含有しても効果は向上せず、合金コストが増大する。したがって、Nbを含有する場合、Nb量は0.25%以下とし、好ましくは0.20%以下とする。
フェライト相の面積分率:35~65%
本明細書において「フェライト相の面積分率」は、下記式(3)~(5)に従って算出される、二相ステンレス鋼板の成分組成から推定した値、又は後述の顕微鏡画像から算出した値である。
フェライト相の面積分率(%)=4.01Creq-5.6Nieq-4.13 ・・・(3)
Creq=Cr+1.73Si+0.88Mo ・・・(4)
Nieq=Ni+24.55C+21.75N+0.4Cu ・・・(5)
上記式(4)及び(5)中、元素記号は各元素の含有量(質量%)を示し、当該元素を含有しない場合はゼロとして算出する。
合せ材の組織が所定量以上のCr系の析出物及びMo系の析出物を含むと、耐食性の劣化が生じる。そのため、本発明では、析出Cr量及び析出Mo量(すなわち、析出物として存在するCr及びMoの量)を所定量以下含有することが肝要である。なお、本明細書における「析出物」とは、合せ材の組織中に存在するシグマ相、炭化物、窒化物及び炭窒化物からなる群から選択される1種又は2種以上をいう。
本発明の一実施形態による二相ステンレスクラッド鋼板の製造方法を以下に説明する。母材鋼板となる第1素材板は、上記の母材鋼板の成分組成となるように溶製し、常法による製造することができる。合せ材としての二相ステンレス鋼板となる第2素材板は、上記の合せ材の成分組成となるように溶製し、常法による製造することができる。これらの第1素材板及び第2素材板を積層して、クラッドスラブを組み立てる。例えば、図1に示すように、第1素材板1と第2素材板2を積層した2組の積層体を、第2素材板同士が対向するように重ね合せることにより、クラッドスラブ10を形成することができる。この際、2つの第2素材板2の間には剥離剤3を塗布することができる。剥離剤3としては、特に限定されないが、Al2O3のように比較的安価であり、十分な剥離性を有していることが好ましい。なお、図1中、符号4はスペーサー、5は溶接部である。冷却時の反りを考慮すると、2枚の第1素材板同士と、2枚の第2素材板同士は、等厚であることが望ましい。もちろん、図1に示す組立方式に限定する必要が無いことは言うまでも無い。
加熱温度を1050℃以上とするのは、母材鋼板と合せ材との接合性及び母材鋼板の靱性を確保するためである。1050℃を下回る加熱温度では、高温域での圧延量が十分に確保できず、接合性が劣化する。したがって、加熱温度は1050℃以上とし、好ましくは1100℃以上とする。一方、加熱温度が1250℃を超えると、結晶粒の粗大が著しく、母材鋼板の靱性の劣化が生じる。そのため、加熱温度は1250℃以下とする。
圧下比とは、圧延前のクラッドスラブの厚さ/圧延後のクラッド圧延体の厚さをいう。クラッドスラブを高温で圧下することにより、金属相互の結合力が生じ、良好な接合が得られる。圧下比は2.0以上とし、好ましくは3.0以上とする。これにより、良好な接合性が得られる。また、母材鋼板の結晶粒が細粒化され、母材鋼板の靱性が向上する。圧下比は20.0以下とすることができる。
熱間圧延後に再加熱を行うのは、合せ材の耐食性確保のためである。熱間圧延後に再加熱を行うことで、析出物の再溶解が可能であり、合せ材の耐食性を担保できる。再加熱温度が1000℃未満となると、二相ステンレス鋼のシグマ相及び/又は炭窒化物の析出が著しくなるため、耐食性の劣化が生じる。よって、再加熱温度は1000℃以上とする。また、再加熱温度が1100℃を超えると、母材鋼板の結晶粒が粗大となって、母材鋼板の靱性の劣化が顕著となる。したがって、再加熱温度は1100℃以下とし、好ましくは1050℃以下とする。
合せ材の冷却速度が0.8℃/s未満では、合せ材中にシグマ相及び/又は炭窒化物の析出が生じて、合せ材の耐食性の劣化を引き起こす。したがって、合せ材の冷却速度は0.8℃/s以上とする。合せ材の冷却速度は100℃/s以下とすることができる。
母材鋼板の冷却速度が1.0℃/s未満では、母材の焼き入れ性が十分でなく強度及び/又は靱性の劣化が生じる。したがって、再加熱後の母材鋼板の冷却速度は1.0℃/s以上とし、好ましくは2.0℃/s以上とする。母材鋼板の冷却速度は100℃/s以下とすることができる。
焼き戻し処理を実施するのは、母材鋼板の強度調整が目的である。焼き戻し処理を実施することで所望の強度に調整することができる。また、炭化物の形態が変化して、靱性が改善する効果も期待できる。しかしながら、700℃を超える温度で焼き戻し処理を実施した場合、合せ材中に炭化物及び/又は窒化物が析出して、耐食性の劣化が生じる。したがって、焼き戻し温度は700℃以下とし、好ましくは650℃以下とする。焼き戻し温度は200℃以上とすることができる。
合せ材中の析出物の抽出には、10vol%アセチルアセトン-1mass%塩化テトラメチルアンモニウム-メタノール混合液(通称10%AA液と呼ぶ)中での電解抽出(通称SPEED法と呼ぶ)を適用した。ろ過によりフィルター上に捕集した抽出残渣を混酸(混酸成分比 硫酸10ml:硝酸10ml:過塩素酸5ml:水10ml)で溶解し、誘導結合プラズマ(ICP)発光分光分析することで析出Cr量と析出Mo量を求めた。
耐食性は、ASTM G48-試験方法(E)により評価した。試験方法は20±2℃に加熱した6%FeCl3水溶液中に試験片を24時間浸漬させ、試験後の合せ材表面に25μm以上の深さの孔食が発生していない場合に耐食性が良好であると判断した。
合せ材と母材の接合強度は、JIS G0601 せん断強さ試験によって評価した。せん断強さ試験は、合せ材を母材から接合面と平行に剥離し、その剥離に要した最大せん断強度から接合性を評価する方法である。せん断強度が200MPa以上の場合に接合性が良好であると判断した。
母材鋼板の靭性は、シャルピー衝撃試験によって評価した。母材鋼板からJIS Z2242に規定の10×10mmサイズVノッチシャルピー衝撃試験片を採取し、シャルピー衝撃試験を行った。-20℃におけるシャルピー衝撃吸収エネルギー値(vE-20)が100Jを超える場合に靭性が良好であると判断した。
母材の強度は引張試験によって評価した。クラッド鋼板の合せ材を機械加工によって取り除いた、母材のみの領域からJIS 1A号の引張試験片を採取し、引張試験を行った。引張強度が550MPa程度となるように焼き戻し温度を調整した。
2 第2素材板(合せ材の素材)
3 剥離剤
4 スペーサー
5 溶接部
10 クラッドスラブ
Claims (4)
- 母材鋼板の片面又は両面に合せ材としての二相ステンレス鋼板が接合されている二相ステンレスクラッド鋼板であって、
前記母材鋼板は、質量%で、C:0.06~0.25%、Si:0.05~0.50%、Mn:0.70~1.60%、P:0.030%以下、S:0.010%以下、Al:0.005~0.100%、Mo:0.01~0.15%、Nb:0.010~0.040%、Ti:0.005%未満、及びN:0.0010~0.0100%を、Nb/Nが3.0以上、かつ下記式(1)で表わされるCeqが0.35~0.45の範囲の下に含有し、残部がFe及び不可避的不純物からなる第1の成分組成を有し、
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ・・・(1)
(上記式(1)中、元素記号は各元素の含有量(質量%)を示し、当該元素を含有しない場合はゼロとして算出する)
前記二相ステンレス鋼板は、質量%で、C:0.030%以下、Si:1.00%以下、Mn:2.00%以下、P:0.050%以下、S:0.0100%以下、Ni:5.00~8.00%、Cr:24.0%超え28.0%以下、Mo:2.5~4.0%、及びN:0.08~0.30%を、下記式(2)で定義されるPIが34.0~43.0の範囲の下に含有し、残部がFe及び不可避的不純物からなる第2の成分組成と、
PI=Cr+3.3Mo+16N ・・・(2)
(上記式(2)中、元素記号は各元素の含有量(質量%)を示す)
フェライト相を面積分率で35~65%含む組織と、を有し、
前記組織における析出Cr量が2.00%以下、かつ、析出Mo量が0.50%以下であることを特徴とする二相ステンレスクラッド鋼板。 - 前記二相ステンレス鋼板の第2の成分組成が、質量%で、Cu:1.50%以下、W:1.50%以下、Co:1.50%以下、Ti:0.25%以下及びNb:0.25%以下からなる群から選択される1種又は2種以上をさらに含有する、請求項1に記載の二相ステンレスクラッド鋼板。
- 前記母材鋼板の第1の成分組成が、質量%で、Cu:0.50%以下、Ni:0.50%以下、Cr:0.40%以下及びV:0.050%以下からなる群から選択される1種又は2種以上をさらに含有する、請求項1又は2に記載の二相ステンレスクラッド鋼板。
- 母材鋼板の片面又は両面に合せ材としての二相ステンレス鋼板が接合されている二相ステンレスクラッド鋼板の製造方法であって、
母材鋼板となる、請求項1又は3に記載の第1の成分組成を有する第1素材板と、合せ材としての二相ステンレス鋼板となる、請求項1又は2に記載の第2の成分組成、及びフェライト相を面積分率で35~65%含む組織を有する第2素材板とを積層させて、クラッドスラブを得る工程と、
前記クラッドスラブを1050~1250℃に加熱する工程と、
その後、前記クラッドスラブに圧下比が2.0以上となる熱間圧延を施して、前記母材鋼板と前記二相ステンレス鋼板とが接合されたクラッド圧延体を得る工程と、
前記クラッド圧延体を放冷する工程と、
その後、前記クラッド圧延体を1000~1100℃に再加熱する工程と、
その後、前記クラッド圧延体を、前記二相ステンレス鋼板における冷却速度が0.8℃/s以上、かつ前記母材鋼板における冷却速度が1.0℃/s以上となるように、冷却する工程と、
その後、前記クラッド圧延体を700℃以下で焼き戻しする工程と、
を有することを特徴とする二相ステンレスクラッド鋼板の製造方法。
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