WO2017169377A1 - フェライト系ステンレス鋼板 - Google Patents
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- WO2017169377A1 WO2017169377A1 PCT/JP2017/006973 JP2017006973W WO2017169377A1 WO 2017169377 A1 WO2017169377 A1 WO 2017169377A1 JP 2017006973 W JP2017006973 W JP 2017006973W WO 2017169377 A1 WO2017169377 A1 WO 2017169377A1
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Definitions
- the present invention relates to a ferritic stainless steel sheet. Furthermore, the present invention relates to a ferritic stainless steel sheet that is excellent in weld penetration and that is difficult to peel off the black spot of the weld bead during bending.
- Ferritic stainless steel sheets are cheaper and have better price stability than austenitic stainless steel sheets containing a lot of Ni. Furthermore, since ferritic stainless steel plates are excellent in resistance to rusting, they are used in various applications such as building materials, transportation equipment, home appliances, and kitchen equipment.
- Ti stabilized ferritic stainless steel sheet containing Ti as a stabilizing element. It contains Ti and produces Ti carbonitrides in the steel, reduces solute C and N, and promotes the development of ⁇ 111 ⁇ recrystallized texture, which is excellent in workability. Steel plate.
- Ti-stabilized ferritic stainless steel sheets tend to generate oxides called black spots on the weld bead even when sufficient gas shielding is performed during TIG welding (Tungsten Inert Gas welding).
- Patent Document 1 discloses a ferritic stainless steel in which the BI value (3Al + Ti + 0.5Si + 200Ca) of the steel composition satisfies 0.8 or less as a ferritic stainless steel with little black spot generation.
- Patent Document 2 discloses a ferritic stainless steel that satisfies the above-described BI value of 0.8 or less as a ferritic stainless steel with less black spot generation.
- Ti-stabilized ferritic stainless steel sheets as described above have been widely used in household appliances for the purpose of reducing costs.
- the Ti-stabilized ferritic stainless steel sheet may be applied to a portion exposed to a severe corrosive environment after bending the welded portion. Therefore, there is a need for a Ti-stabilized stainless steel plate that hardly causes crevice corrosion due to black spot peeling even when the weld bead is bent.
- An object of the present invention is to provide a ferritic stainless steel plate in which black spots of TIG welds are difficult to peel off during bending.
- the inventors of the present invention have made a comprehensive study on the above-described problems in order to suppress peeling at the time of black spot bending.
- the O (oxygen) content is set to a certain value or less
- the PBI value represented by “(7Al + 2Ti + Si + 10Zr + 130Ca) ⁇ O (oxygen) ⁇ 1000” is a steel composition having a certain value or less
- the black spot moves while being coarsened on the weld bead so as to be dragged by the electrode during TIG welding, and is fixed to the weld bead when it becomes coarser than a certain level. If there are few elements and oxygen with high affinity with oxygen in a ferritic stainless steel plate, the surface tension of the molten pool formed at the time of TIG welding will fall, so that temperature becomes high. As a result, a strong flow is formed on the surface of the weld pool from the center of the weld bead having a high temperature toward the saddle of the weld bead having a low temperature. This strong flow activates outward Marangoni convection. As a result, the black spot is fixed to the bead wrinkle in a state where it is not relatively coarse. Thereby, each black spot becomes thin and small, and peeling hardly occurs during bending.
- the present invention was completed through further studies based on the above findings.
- the summary composition is as follows.
- B 0.0003 to 0.0030%
- Mg 0.0005 to 0.0100%
- Y 0.001 to 0.20%
- REM rare earth metal
- the ferritic stainless steel sheet of the present invention is excellent in weld penetration of the weld bead and excellent in corrosion resistance including the bent portion.
- 3 is a diagram illustrating an appearance of a generated black spot.
- the ferritic stainless steel sheet of the present invention satisfies the following formula (1).
- PBI (7Al + 2Ti + Si + 10Zr + 130Ca) ⁇ O (oxygen) ⁇ 1000
- Al, Ti, Ti, Si, Zr, Ca, and O (oxygen) in the formula (1) are the content [% by mass] of each component in the ferritic stainless steel sheet, and the elements not contained are 0.
- Al, Ti, Si, Zr, and Ca are elements that have a particularly strong affinity with oxygen and are likely to be oxides. When the product of the amount of these elements and the amount of oxygen is large, the black spot is likely to be peeled off during bending.
- the coefficients of Al, Ti, Si, Zr, and Ca in the above equation (1) are determined based on the magnitude of the effect on the weld penetration of the weld bead and the effect of causing black spot peeling during bending. Has been.
- the PBI value exceeds 20.0, the black spot peels off during bending.
- the PBI value is set to 20.0 or less.
- the PBI value is 5.0 or less, it is possible to more effectively suppress black spot peeling during bending.
- the PBI value is 0.5 or more.
- the PBI value is 1.5 or more, the weld penetration of the weld bead is excellent.
- the PBI value is 1.5 or more, the black spot peeling is better suppressed during the bending process than when the PBI value is less than 1.5.
- the PBI value is more preferably 1.5 or more and 5.0 or less.
- % which shows the component of steel means the mass% unless there is particular notice.
- C 0.020% or less C is an element effective for increasing the strength of steel. Therefore, the C content is preferably 0.001% or more. On the other hand, when the C content exceeds 0.020%, the corrosion resistance and workability are significantly lowered. Therefore, the C content is 0.020% or less. The C content is preferably 0.015% or less, more preferably 0.010% or less.
- Si 0.05 to 0.50% Si is an element useful as a deoxidizer. This effect can be obtained by setting the Si content to 0.05% or more.
- the Si content is preferably 0.08% or more. If the Si content exceeds 0.50%, the steel becomes hard and the workability decreases. Even in the composition satisfying the formula (1), black spots generated in TIG welding of a ferritic stainless steel sheet are likely to be peeled off during bending, and the peeled portion may be a starting point for crevice corrosion. Therefore, the Si content is 0.50% or less.
- the Si content is preferably 0.30% or less, and more preferably 0.15% or less.
- Mn 0.05 to 0.50% Mn has a deoxidizing action. This effect can be obtained by setting the Mn content to 0.05% or more.
- the Mn content is preferably 0.10% or more, more preferably 0.15% or more, and further preferably 0.17% or more. If the Mn content exceeds 0.50%, precipitation and coarsening of MnS are promoted, resulting in a decrease in corrosion resistance. Therefore, the Mn content is 0.50% or less.
- the Mn content is preferably less than 0.30%, more preferably 0.20% or less.
- P 0.040% or less
- P is an element that lowers corrosion resistance. Moreover, P reduces hot workability by segregating at the grain boundaries. Therefore, the P content is desirably as low as possible, and is set to 0.040% or less. The P content is preferably 0.030% or less. In addition, the minimum of P content is not prescribed
- S 0.030% or less S forms Mn and precipitate MnS. This MnS becomes a starting point of corrosion and causes a decrease in corrosion resistance. Therefore, the lower S content is desirable, and it is 0.030% or less.
- the S content is preferably 0.020% or less.
- the S content is more preferably 0.010% or less, and still more preferably 0.005% or less.
- the minimum of S content is not prescribed
- Al is an element effective for deoxidation. This effect is obtained when the Al content is 0.001% or more.
- the Al content is preferably 0.005% or more, and more preferably 0.010% or more. If the Al content exceeds 0.150%, generation of a surface scale on the slab having a lubricating effect during hot rolling is suppressed, surface defects are likely to be generated, and productivity is reduced. Further, when the Al content exceeds 0.150%, even in the composition satisfying the formula (1), the black spots generated in the TIG welding of the steel sheet are easily peeled during bending, and the peeled portion is crevice corrosion. May be the starting point. Therefore, the Al content is 0.150% or less.
- the Al content is preferably 0.100% or less, and more preferably 0.050% or less.
- Cr 18.0-25.0%
- Cr is an element that increases 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. Therefore, the Cr content is 18.0 or more, preferably 20.0% or more, and more preferably 20.5% or more. If the Cr content exceeds 25.0%, the toughness tends to decrease due to the influence of the ⁇ phase and 475 ° C brittleness. Therefore, the Cr content is 25.0% or less. The Cr content is preferably 23.0% or less, and more preferably 21.5% or less.
- Ti 0.01 to 0.50% Ti is an element effective for deoxidation. Ti is an element that fixes C and N, suppresses the formation of Cr carbonitride and a Cr-free layer, prevents sensitization, and improves corrosion resistance. Furthermore, Ti improves workability by promoting the development of ⁇ 111 ⁇ recrystallized texture. This effect is obtained when the Ti content is 0.01% or more.
- the Ti content is preferably 0.05% or more, more preferably 0.20% or more. If the Ti content exceeds 0.50%, the ferritic stainless steel sheet becomes hard, bending workability is lowered, and TiN becomes a starting point of corrosion, thereby lowering the corrosion resistance.
- the Ti content exceeds 0.50%, even in the composition satisfying the formula (1), the black spots generated in the TIG welding of the steel sheet are easily peeled during bending, and the peeled portion is crevice corrosion. May be the starting point. Accordingly, the Ti content is 0.50% or less.
- the Ti content is preferably 0.40% or less, more preferably 0.30% or less.
- Ca 0.0001 to 0.0015%
- Ca is an element effective for deoxidation. This effect is obtained when the Ca content is 0.0001% or more.
- the Ca content is preferably 0.0002% or more, more preferably 0.0003% or more.
- the Ca content is 0.0015% or less.
- the Ca content is preferably 0.0010% or less, and more preferably 0.0005% or less.
- O (oxygen) is an element that improves weld penetration in the thickness direction of the weld bead in TIG welding. This effect is obtained when the O (oxygen) content is 0.0015% or more.
- the O (oxygen) content is preferably 0.0020% or more, more preferably 0.0025% or more.
- O (oxygen) is contained in excess of 0.0040%, even in a composition satisfying the formula (1), black spots generated in TIG welding of a steel sheet are easily peeled off during bending, and the peeled portion May be the starting point of crevice corrosion. Therefore, the O (oxygen) content is set to 0.0040% or less.
- the O (oxygen) content is preferably 0.0035% or less, more preferably 0.0030% or less.
- N 0.025% or less
- the N content is 0.025% or less.
- N is preferably reduced as much as possible, and the N content is preferably 0.020% or less, more preferably 0.015% or less.
- the lower limit of N content is not specified in particular.
- Zr 0.01 to 0.80% Zr, like Ti, is an element effective for deoxidation.
- Zr is an element that fixes C and N, suppresses the formation of Cr carbonitride and a Cr-free layer, prevents sensitization, and improves corrosion resistance.
- the Zr content is preferably 0.01% or more.
- the Zr content is more preferably 0.02% or more, and further preferably 0.03% or more.
- the ferritic stainless steel sheet may be hardened and the bending workability may be reduced.
- the Zr content exceeds 0.80%, even in the composition satisfying the formula (1), the black spots generated in the TIG welding of the steel sheet are easily peeled at the time of bending, which becomes a starting point for crevice corrosion. There is a case. Therefore, the Zr content is 0.80% or less.
- the Zr content is more preferably 0.30% or less, and still more preferably 0.10% or less.
- Nb 0.01% or more and less than 0.40%
- Nb is an element that fixes C and N, suppresses the formation of Cr carbonitride and a Cr-free layer, prevents sensitization, and improves corrosion resistance.
- the Nb content is preferably 0.01% or more.
- the Nb content is more preferably 0.03% or more, still more preferably 0.05% or more. If the Nb content is 0.40% or more, the ferritic stainless steel sheet may be hardened and the workability may be lowered, and further, the recrystallization temperature is increased and the productivity is lowered. Therefore, the Nb content is preferably less than 0.40%.
- the Nb content is more preferably 0.30% or less, and further preferably 0.15% or less.
- V 0.01 to 0.50%
- V is an element that improves the crevice corrosion resistance of the ferritic stainless steel sheet.
- the V content is preferably 0.01% or more.
- the V content is more preferably 0.03% or more, and even more preferably 0.05% or more. If the V content exceeds 0.50%, workability may be reduced. Therefore, the V content is preferably 0.50% or less.
- the V content is more preferably 0.30% or less, and still more preferably 0.10% or less.
- Cu 0.30 to 0.80%
- Cu is an element that strengthens the passive film and improves the corrosion resistance.
- the Cu content is preferably 0.30 to 0.80%.
- the lower limit side is more preferably 0.35% or more, and further preferably 0.40% or more.
- the upper limit side is more preferably 0.50% or less, and further preferably 0.45% or less.
- Ni 0.01-2.50%
- Ni is an element that suppresses the anodic reaction due to acid and enables the passive state to be maintained even at a lower pH. That is, Ni is highly effective in improving crevice corrosion resistance, and significantly suppresses the progress of corrosion in the state of active dissolution, thereby improving the corrosion resistance. From the viewpoint of obtaining this effect, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.05% or more, and further preferably 0.10% or more. If the Ni content exceeds 2.50%, hydrogen embrittlement cracks are likely to occur in the processed part. Therefore, the Ni content is preferably 2.50% or less. The Ni content is more preferably 0.80% or less, and even more preferably 0.25% or less.
- Co 0.01 to 0.50%
- Co is an element that improves the crevice corrosion resistance of ferritic stainless steel. From the viewpoint of obtaining this effect, the Co content is preferably 0.01% or more.
- the Co content is more preferably 0.10% or more. If the Co content exceeds 0.50%, workability may be reduced. Therefore, the Co content is preferably 0.50% or less.
- the Co content is more preferably 0.30% or less, and still more preferably 0.15% or less.
- Mo 0.01-2.00% Mo has the effect of improving the crevice corrosion resistance of the ferritic stainless steel sheet. From the viewpoint of obtaining this effect, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.10 or more, and further preferably 0.30% or more. If the Mo content exceeds 2.00%, a coarse intermetallic compound may be generated and the toughness may be reduced. Therefore, the Mo content is preferably 2.00% or less. The Mo content is more preferably 1.00% or less, and even more preferably 0.60% or less.
- W 0.01 to 0.50% W is an element that improves the crevice corrosion resistance of the ferritic stainless steel sheet. From the viewpoint of obtaining this effect, the W content is preferably 0.01% or more. The W content is more preferably 0.10% or more. If the W content exceeds 0.50%, workability may be reduced. Therefore, the W content is preferably 0.50% or less. The W content is more preferably 0.30% or less.
- B 0.0003 to 0.0030%
- B is an element that improves hot workability and secondary workability, and it is known that B addition to Ti-added steel is effective. From the viewpoint of obtaining this effect, the B content is preferably 0.0003% or more. The B content is more preferably 0.0010% or more. If the B content exceeds 0.0030%, the toughness may decrease. Therefore, the B content is preferably 0.0030% or less. The B content is more preferably 0.0025% or less.
- Mg 0.0005 to 0.0100% Mg forms Mg oxide together with Al in molten steel and acts as a deoxidizer. From the viewpoint of obtaining this effect, the Mg content is preferably 0.0005% or more. The Mg content is more preferably 0.0010% or more. If the Mg content exceeds 0.0100%, the toughness of the steel is lowered and the productivity may be lowered. Therefore, the Mg content is preferably 0.0100% or less. The Mg content is more preferably 0.0050% or less, and still more preferably 0.0030% or less.
- Y 0.001 to 0.20%
- Y is an element that suppresses the decrease in viscosity of the molten steel and improves the cleanliness of the molten steel. From the viewpoint of obtaining this effect, the Y content is preferably 0.001% or more. If the Y content exceeds 0.20%, workability may be reduced. Therefore, the Y content is preferably 0.20% or less. The Y content is more preferably 0.10% or less.
- REM rare earth metal
- REM rare earth metal: elements having atomic numbers 57 to 71 such as La, Ce, and Nd
- the REM content is preferably 0.001% or more.
- the REM content is more preferably 0.005% or more. If the REM content exceeds 0.10%, surface defects may occur during hot rolling. Therefore, the REM content is preferably 0.10% or less.
- the REM content is more preferably 0.05% or less.
- Sn 0.01 to 0.50% Sn is effective for suppressing roughening of the processed skin by promoting the generation of deformation bands during rolling. From the viewpoint of obtaining this effect, the Sn content is preferably 0.01% or more. The Sn content is more preferably 0.03% or more. If the Sn content exceeds 0.50%, the workability may decrease. Therefore, the Sn content is preferably 0.50% or less. The Sn content is more preferably 0.20% or less.
- Sb 0.01 to 0.50% Similar to Sn, Sb is effective for suppressing roughening of the processed skin by promoting the generation of deformation bands during rolling. From the viewpoint of obtaining this effect, the Sb content is preferably 0.01% or more. The Sb content is more preferably 0.03% or more. If the Sb content exceeds 0.50%, the workability may decrease. Therefore, the Sb content is preferably 0.50% or less. The Sb content is more preferably 0.20% or less.
- the balance other than the above components is Fe and inevitable impurities.
- the steel having the above component composition is melted by a known method such as a converter, electric furnace, vacuum melting furnace or the like, and further subjected to secondary refining by a VOD (Vacuum Oxygen Decarburization) method or the like to control the oxygen concentration. .
- a steel material (slab) is obtained by a continuous casting method or an ingot-bundling method.
- the steel material is heated to 1000 ° C. to 1200 ° C., and then hot-rolled to a plate thickness of 2.0 mm to 5.0 mm under a finishing temperature of 700 ° C. to 1000 ° C.
- the hot-rolled sheet thus produced is annealed at a temperature of 850 ° C. to 1100 ° C. and pickled, and then cold-rolled and cold-rolled sheet is annealed at a temperature of 800 ° C. to 1000 ° C.
- pickling is performed to remove scale. Skin pass rolling may be performed on the cold-rolled sheet from which the scale has been removed.
- the ferritic stainless steel sheet of the present invention is effective not only for the cold rolled sheet product as described above but also for a hot rolled sheet product. Moreover, the ferritic stainless steel sheet of the present invention is suitable for bending. Furthermore, the ferritic stainless steel sheet of the present invention is also suitable for applications in which a weld is bent.
- the welding method for forming the weld is not particularly limited, and the weld is preferably formed by TIG welding.
- a ferritic stainless steel having the composition shown in Tables 1 to 5 (the balance is Fe and inevitable impurities) is melted in a 100 kg steel ingot, heated to 1200 ° C. and hot-rolled to obtain a thickness of 3 A hot-rolled sheet of 0.0 mm was obtained. Then, after performing annealing at 1050 ° C. and pickling by a normal method, cold rolling to a plate thickness of 1.0 mm, further annealing at 900 ° C., pickling by a normal method was performed. .
- the obtained cold-rolled annealed plate was cut into a size of 35 mm ⁇ 200 mm, both surfaces were dry-polished with # 600 emery paper, and then the end surface was processed smoothly to obtain a test piece.
- I-shaped groove TIG welding was implemented and the welding member was produced.
- TIG welding conditions were a welding current of 70 A, a welding voltage of 11 V, and a welding speed of 40 cm / min.
- Argon was used as the shielding gas, and the torch side was 15 L / min and the back side was 10 L / min.
- the bead width between the front bead and the back bead was measured for the weld member.
- the value obtained by dividing the bead width of the front bead by the bead width of the back bead was 2 or less, “ ⁇ ” (pass: excellent), 2 Those larger than 3 were evaluated as “ ⁇ ” (passed), and those larger than 3 were evaluated as “ ⁇ ” (failed).
- the evaluation results are shown in Tables 1 to 5 “Welability”.
- the results obtained are shown in Tables 1-5.
- the invention examples pass all of “peeling during processing”, “corrosion resistance” and “weldability”. Further, among the inventive examples, those having a PBI value of 1.5 or more and 5.0 or less are evaluated for peeling during processing of the black spot, corrosion resistance after bending of the black spot generating portion, and in the thickness direction of the weld bead.
- the weld penetration properties are both excellent and good. That is, it can be seen that the black spot does not peel off during bending of the weld bead and has excellent corrosion resistance, and further the weld bead is likely to be melted.
- Test No. In the comparative examples 116, 118, 120 to 123, and 127, the content of Al, Ti, Si, Ca, O, and Zr is higher than the range of the present invention. The corrosion resistance of is poor.
- the ferritic stainless steel sheet of the present invention is excellent in weld penetration of the weld bead, and black spots generated during welding are difficult to peel off during bending, and crevice corrosion due to black spot peeling is unlikely to occur.
- the ferritic stainless steel sheet of the present invention is suitable for containers, containers, kitchen equipment, building interior / exterior materials, automobile parts, escalators, railway vehicles, electrical equipment casing outer plates and the like.
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Abstract
Description
一方、O(酸素)含有量やPBI値が極度に小さくなると、溶接ビードが板厚方向に溶け込みにくくなり溶接溶け込み性が低下することを知見した。そこで、O(酸素)含有量を一定の範囲内とし、かつ、PBI値が一定の範囲内である鋼組成とすることで、良好な溶接溶け込み性と良好なブラックスポット剥離抑制を並立可能であることを本発明者らは見出した。その機構は以下によると推察される。
0.5≦PBI≦20.0 ・・・(1)
(ただし、PBI=(7Al+2Ti+Si+10Zr+130Ca)×O(酸素)×1000であり、式中のAl、Ti、Si、Zr、Ca、およびO(酸素)は、フェライト系ステンレス鋼板中の各成分の含有量[質量%]であり、含有しない元素は0とする。)
[2]さらに、質量%で、Zr:0.01~0.80%、Nb:0.01%以上0.40%未満、およびV:0.01~0.50%から選ばれる1種以上を含有する、[1]に記載のフェライト系ステンレス鋼板。
0.5≦PBI≦20.0・・・(1)
ただし、PBI=(7Al+2Ti+Si+10Zr+130Ca)×O(酸素)×1000
(なお、(1)式中のAl、Ti、Si、Zr、Ca、およびO(酸素)は、フェライト系ステンレス鋼板中の各成分の含有量[質量%]であり、含有しない元素は0とする。)
Al、Ti、Si、Zr、およびCaは、酸素との親和力が特に強い元素であり、酸化物となりやすい。これらの元素の量の値と、酸素量の値との積が大きいと、ブラックスポットが曲げ加工時に剥離しやすい。上記(1)式におけるAl、Ti、Si、Zr、およびCaの係数は、溶接ビードの溶接溶け込み性へ与える影響、および、曲げ加工時にブラックスポットの剥離を引き起こす影響、の大きさに基づいて決定されている。
また、PBI値が1.5以上の場合は、PBI値が1.5未満の場合よりも曲げ加工時にブラックスポットの剥離抑制が良好となる。これは、上述したようにPBI値が1.5以上の場合は1.5未満の場合よりも溶接溶け込み性が良いことが影響していると考えられる。よって、本発明では、PBI値は1.5以上5.0以下が更に好ましい。
Cは、鋼の強度を高めるのに有効な元素である。よって、C含有量を0.001%以上にすることが好ましい。一方、C含有量が0.020%を超えると、耐食性および加工性が著しく低下する。よって、C含有量は0.020%以下とする。C含有量は好ましくは0.015%以下であり、より好ましくは0.010%以下である。
Siは、脱酸剤として有用な元素である。この効果はSi含有量を0.05%以上にすることで得られる。Si含有量は、好ましくは0.08%以上である。Si含有量が0.50%を超えると鋼が硬質化して加工性が低下する。また、式(1)を満足する組成においても、フェライト系ステンレス鋼板のTIG溶接において生成するブラックスポットが曲げ加工時に剥離しやすくなり、この剥離した部分が隙間腐食の起点となる場合がある。従って、Si含有量は0.50%以下とする。Si含有量は、0.30%以下が好ましく、0.15%以下がより好ましい。
Mnには、脱酸作用がある。この効果はMn含有量を0.05%以上にすることで得られる。Mn含有量は、好ましくは0.10%以上であり、より好ましくは0.15%以上であり、さらに好ましくは0.17%以上である。Mn含有量が0.50%を超えるとMnSの析出および粗大化を促して耐食性の低下を招く。よって、Mn含有量は0.50%以下とする。Mn含有量は、好ましくは0.30%未満であり、より好ましくは0.20%以下である。
Pは耐食性を低下させる元素である。また、Pは結晶粒界に偏析することで熱間加工性を低下させる。そのため、P含有量は可能な限り低いほうが望ましく、0.040%以下とする。P含有量は、好ましくは0.030%以下である。なお、P含有量の下限は特に規定しない。
SはMnと析出物MnSを形成する。このMnSは腐食の起点となり、耐食性の低下を招く。よって、S含有量は低いほうが望ましく、0.030%以下とする。S含有量は、好ましくは0.020%以下である。S含有量は、より好ましくは0.010%以下であり、さらに好ましくは0.005%以下である。なお、S含有量の下限は特に規定しない。
Alは、脱酸に有効な元素である。この効果はAl含有量が0.001%以上で得られる。Al含有量は、好ましくは0.005%以上であり、より好ましくは0.010%以上である。Al含有量が0.150%を超えると熱間圧延時に潤滑効果を有するスラブ上表面スケールの生成が抑制され、表面欠陥が生成しやすくなり、製造性が低下する。また、Al含有量が0.150%を超えると、式(1)を満足する組成においても、鋼板のTIG溶接において生成するブラックスポットが曲げ加工時に剥離しやすくなり、この剥離した部分が隙間腐食の起点となる場合がある。従って、Al含有量は0.150%以下とする。Al含有量は、好ましくは0.100%以下であり、より好ましくは0.050%以下である。
Crは表面に不動態皮膜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。よって、Cr含有量は18.0以上とし、好ましくは20.0%以上であり、より好ましくは20.5%以上である。Cr含有量が25.0%を超えるとσ相や475℃脆性の影響で靱性が低下しやすくなる。よってCr含有量は25.0%以下とする。Cr含有量は、好ましくは23.0%以下であり、より好ましくは21.5%以下である。
Tiは、脱酸に有効な元素である。また、TiはCおよびNを固定してCr炭窒化物および脱Cr層の生成を抑えて鋭敏化を防ぎ、耐食性を向上させる元素である。さらに{111}再結晶集合組織の発達を促進させることで、Tiは加工性を向上させる。この効果はTi含有量が0.01%以上で得られる。Ti含有量は、好ましくは0.05%以上であり、より好ましくは0.20%以上である。Ti含有量が0.50%を超えるとフェライト系ステンレス鋼板が硬質化し、曲げ加工性が低下し、さらにTiNが腐食の起点となり、耐食性が低下する。また、Ti含有量が0.50%を超えると、式(1)を満足する組成においても、鋼板のTIG溶接において生成するブラックスポットが曲げ加工時に剥離しやすくなり、この剥離した部分が隙間腐食の起点となる場合がある。以上より、Ti含有量は0.50%以下とする。Ti含有量は、好ましくは0.40%以下であり、より好ましくは0.30%以下である。
Caは、脱酸に有効な元素である。この効果はCa含有量が0.0001%以上で得られる。Ca含有量は、好ましくは0.0002%以上であり、より好ましくは0.0003%以上である。Caが0.0015%を超えて含有されると、式(1)を満足する組成においても、鋼板のTIG溶接において生成するブラックスポットが曲げ加工時に剥離しやすくなり、この剥離した部分が隙間腐食の起点となる場合がある。従って、Ca含有量は0.0015%以下とする。Ca含有量は、好ましくは0.0010%以下であり、より好ましくは0.0005%以下である。
O(酸素)は、TIG溶接における溶接ビードの板厚方向への溶接溶け込み性を向上させる元素である。この効果はO(酸素)含有量が0.0015%以上で得られる。O(酸素)含有量は、好ましくは0.0020%以上であり、より好ましくは0.0025%以上である。O(酸素)が0.0040%を超えて含有されると、式(1)を満足する組成においても、鋼板のTIG溶接において生成するブラックスポットが曲げ加工時に剥離しやすくなり、この剥離した部分が隙間腐食の起点となる場合がある。従って、O(酸素)含有量は0.0040%以下とする。O(酸素)含有量は、好ましくは、0.0035%以下であり、より好ましくは0.0030%以下である。
Nが0.025%を超えて含有されると耐食性と加工性が著しく低下する。従って、N含有量は0.025%以下とする。Nは極力低減することが好ましく、N含有量は、好ましくは0.020%以下であり、より好ましくは0.015%以下である。なお、N含有量の下限は特に規定しない。
ZrはTi同様に、脱酸に有効な元素である。また、ZrはCおよびNを固定してCr炭窒化物および脱Cr層の生成を抑えて鋭敏化を防ぎ、耐食性を向上させる元素である。この効果を得る観点からZr含有量は0.01%以上が好ましい。Zr含有量はより好ましくは0.02%以上であり、さらに好ましくは0.03%以上である。一方、Zr含有量が0.80%を超えるとフェライト系ステンレス鋼板を硬質化させて曲げ加工性を低下させるおそれがある。また、Zr含有量が0.80%を超えると、式(1)を満足する組成においても、鋼板のTIG溶接において生成するブラックスポットが曲げ加工時に剥離しやすくなり、これが隙間腐食の起点となる場合がある。従って、Zr含有量は0.80%以下とする。Zr含有量はより好ましくは0.30%以下であり、さらに好ましくは0.10%以下である。
NbはTi同様に、CおよびNを固定してCr炭窒化物および脱Cr層の生成を抑えて鋭敏化を防ぎ、耐食性を向上させる元素である。この効果を得る観点から、Nb含有量は0.01%以上が好ましい。Nb含有量はより好ましくは0.03%以上であり、さらに好ましくは0.05%以上である。Nb含有量が0.40%以上となるとフェライト系ステンレス鋼板を硬質化させて加工性を低下させるおそれがあり、さらに再結晶温度の上昇を招き製造性を低下させる。従って、Nb含有量は0.40%未満が好ましい。Nb含有量は、より好ましくは0.30%以下であり、さらに好ましくは0.15%以下である。
Vは、フェライト系ステンレス鋼板の耐隙間腐食性を向上させる元素である。この効果を得る観点からV含有量は0.01%以上が好ましい。V含有量は、より好ましくは0.03%以上であり、さらに好ましくは0.05%以上である。V含有量が0.50%を超えると、加工性が低下するおそれがある。そのため、V含有量は0.50%以下が好ましい。V含有量は、より好ましくは0.30%以下であり、さらに好ましくは0.10%以下である。
Cuは不動態皮膜を強化し、耐食性を向上させる元素である。一方、Cuが過剰に含有されるとε-Cuが析出しやすくなり、耐食性を低下させるおそれがある。そのため、Cu含有量は0.30~0.80%が好ましい。Cu含有量について、下限側は0.35%以上がより好ましく、0.40%以上がさらに好ましい。Cu含有量について、上限側は0.50%以下がより好ましく、0.45%以下がさらに好ましい。
Niは、酸によるアノード反応を抑制し、より低いpHでも不動態の維持を可能にする元素である。すなわちNiは、耐隙間腐食性の向上に効果が高く、活性溶解の状態における腐食の進行を顕著に抑制して耐食性を向上させる。この効果を得る観点から、Ni含有量は0.01%以上が好ましい。Ni含有量はより好ましくは0.05%以上であり、さらに好ましくは0.10%以上である。Ni含有量が2.50%を超えると加工部に水素脆化割れが発生しやすくなる。従って、Ni含有量は2.50%以下が好ましい。Ni含有量は、より好ましくは0.80%以下であり、さらに好ましくは0.25%以下である。
Coは、フェライト系ステンレス鋼の耐隙間腐食性を向上させる元素である。この効果を得る観点から、Co含有量は0.01%以上が好ましい。Co含有量はより好ましくは0.10%以上である。Co含有量が0.50%を超えると、加工性が低下するおそれがある。そのため、Co含有量は0.50%以下が好ましい。Co含有量は、より好ましくは0.30%以下であり、さらに好ましくは0.15%以下である。
Moには、フェライト系ステンレス鋼板の耐隙間腐食性を向上させる効果がある。この効果を得る観点からMo含有量は0.01%以上が好ましい。Mo含有量は、より好ましくは0.10以上であり、さらに好ましくは0.30%以上である。Mo含有量が2.00%を超えると、粗大な金属間化合物を生成させ、靱性を低下させるおそれがある。そのため、Mo含有量は2.00%以下が好ましい。Mo含有量は、より好ましくは1.00%以下であり、さらに好ましくは0.60%以下である。
Wは、フェライト系ステンレス鋼板の耐隙間腐食性を向上させる元素である。この効果を得る観点から、W含有量は0.01%以上が好ましい。W含有量は、より好ましくは0.10%以上である。W含有量が0.50%を超えると、加工性が低下するおそれがある。そのため、W含有量は0.50%以下が好ましい。W含有量はより好ましくは0.30%以下である。
Bは、熱間加工性や2次加工性を向上させる元素であり、Ti添加鋼へのB添加が有効であることが知られている。この効果を得る観点から、B含有量は0.0003%以上が好ましい。B含有量はより好ましくは0.0010%以上である。B含有量が0.0030%を超えると靱性が低下するおそれがある。従って、B含有量は0.0030%以下が好ましい。B含有量はより好ましくは0.0025%以下である。
Mgは、溶鋼中でAlとともにMg酸化物を形成し脱酸剤として作用する。この効果を得る観点からMg含有量は0.0005%以上が好ましい。Mg含有量はより好ましくは0.0010%以上である。Mg含有量が0.0100%を超えると鋼の靱性が低下して製造性が低下するおそれがある。従って、Mg含有量は0.0100%以下が好ましい。Mg含有量は、より好ましくは0.0050%以下であり、さらに好ましくは0.0030%以下である。
Yは、溶鋼の粘度減少を抑制し、溶鋼の清浄度を向上させる元素である。この効果を得る観点からY含有量は0.001%以上が好ましい。Y含有量が0.20%を超えると、加工性が低下するおそれがある。よって、Y含有量は0.20%以下が好ましい。Y含有量はより好ましくは0.10%以下である。
REM(希土類金属:La、Ce、Ndなどの原子番号57~71の元素)は、耐高温酸化性を向上させる元素である。この効果を得る観点からREM含有量は0.001%以上が好ましい。REM含有量はより好ましくは0.005%以上である。REM含有量が0.10%を超えると、熱間圧延の際に表面欠陥が生じるおそれがある。よって、REM含有量は0.10%以下が好ましい。REM含有量はより好ましくは0.05%以下である。
Snは、圧延時における変形帯の生成の促進による加工肌荒れの抑制に効果的である。この効果を得る観点から、Snの含有量は0.01%以上が好ましい。Snの含有量はより好ましくは0.03%以上である。Snの含有量が0.50%を超えると加工性が低下するおそれがある。よって、Sn含有量は0.50%以下が好ましい。Sn含有量はより好ましくは0.20%以下である。
Sbは、Snと同様に、圧延時における変形帯の生成の促進による加工肌荒れの抑制に効果的である。この効果を得る観点から、Sb含有量は0.01%以上が好ましい。Sb含有量はより好ましくは0.03%以上である。Sbの含有量が0.50%を超えると加工性が低下するおそれがある。よって、Sb含有量は0.50%以下が好ましい。Sb含有量はより好ましくは0.20%以下である。
ブラックスポットの加工時の剥離を評価するため、得られた溶接部材から、溶接ビードを含む30mm×200mmの曲げ試験片を切り出し、ブラックスポット生成部が曲げ中心となるように180°密着曲げを実施した。曲げ部先端部付近のみを切り出し、曲げ部最先端部について光学顕微鏡および走査型電子顕微鏡を用いて、それぞれ120倍および3000倍にて観察し、両者にて剥離が認められなかったものを「○」(合格:優れている)、光学顕微鏡にて剥離が認められず走査型電子顕微鏡にて剥離が認められたものを「□」(合格)、両者で剥離が認められたものを「▲」(不合格)として評価した。評価結果を表1~5「加工時剥離」欄に示す。
ブラックスポット加工部の耐食性を評価するため、上記曲げ加工を施した曲げ試験片について、複合サイクル腐食試験を実施した。試験片は、端部をビニールテープで覆った後、曲げ部先端が鉛直上方向となるように試験装置内へ設置した。試験環境はJASO M609-91に準拠し、1サイクルを塩水噴霧(5%NaCl)2h→乾燥(60℃)4h→湿潤(50℃)2hとした。10サイクルの試験の後、流れさびが認められなかったものを「○」(合格:優れている)、10サイクルの試験の後は流れさびが認められたが5サイクルの試験の後の時点では流れさびが認められなかったものを「□」(合格)、5サイクルの試験の後の時点で流れさびが認められたものを「▲」(不合格)として評価した。評価結果を表1~5「耐食性」欄に示す。
溶接ビードの板厚方向への溶接溶け込み性を評価するため、上記溶接部材について表ビードと裏ビードとのビード幅を計測した。表ビードのビード幅を裏ビードのビード幅で除した値(表ビードのビード幅÷裏ビードのビード幅の値)が2以下であったものを「○」(合格:優れている)、2より大きく3以下であったものを「□」(合格)、3より大きかったものを「▲」(不合格)として評価した。評価結果を表1~5「溶接性」欄に示す。
Claims (5)
- 質量%で、
C:0.020%以下、
Si:0.05~0.50%、
Mn:0.05~0.50%、
P:0.040%以下、
S:0.030%以下、
Al:0.001~0.150%、
Cr:18.0~25.0%、
Ti:0.01~0.50%、
Ca:0.0001~0.0015%、
O(酸素):0.0015~0.0040%、および
N:0.025%以下を含有し、残部がFeおよび不可避的不純物からなり、
さらに下記(1)式を満たす、フェライト系ステンレス鋼板。
0.5≦PBI≦20.0 ・・・(1)
(ただし、PBI=(7Al+2Ti+Si+10Zr+130Ca)×O(酸素)×1000であり、式中のAl、Ti、Si、Zr、Ca、およびO(酸素)は、フェライト系ステンレス鋼板中の各成分の含有量[質量%]であり、含有しない元素は0とする。) - さらに、質量%で、
Zr:0.01~0.80%、
Nb:0.01%以上0.40%未満、および
V:0.01~0.50%から選ばれる1種以上を含有する、請求項1に記載のフェライト系ステンレス鋼板。 - さらに、質量%で、
Cu:0.30~0.80%、
Ni:0.01~2.50%、
Co:0.01~0.50%、
Mo:0.01~2.00%、および
W:0.01~0.50%から選ばれる1種以上を含有する、請求項1または2に記載のフェライト系ステンレス鋼板。 - さらに、質量%で、
B:0.0003~0.0030%、
Mg:0.0005~0.0100%、
Y:0.001~0.20%、
REM(希土類金属):0.001~0.10%、
Sn:0.01~0.50%、および
Sb:0.01~0.50%から選ばれる1種以上を含有する、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鋼板。 - 前記フェライト系ステンレス鋼板が、溶接部のブラックスポットが曲げ加工時に剥離しにくいものである、請求項1~4のいずれかに記載のフェライト系ステンレス鋼板。
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