WO2021246208A1 - Ferritic stainless steel - Google Patents
Ferritic stainless steel Download PDFInfo
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- WO2021246208A1 WO2021246208A1 PCT/JP2021/019473 JP2021019473W WO2021246208A1 WO 2021246208 A1 WO2021246208 A1 WO 2021246208A1 JP 2021019473 W JP2021019473 W JP 2021019473W WO 2021246208 A1 WO2021246208 A1 WO 2021246208A1
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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
- C22C38/26—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
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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
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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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
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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
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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
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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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
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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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
- C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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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
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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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
- C21D2211/005—Ferrite
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Definitions
- the present invention relates to a ferritic stainless steel having excellent rust resistance.
- Stainless steel is generally not painted and is put to practical use as it is, so rusting starting from CaS exposed on the surface of the steel material becomes a problem.
- a mechanism for forming CaS a type that crystallizes in molten steel and a type that S contained in the base metal reacts with inclusions containing CaO when the slab is heated after solidification is known are known. There is.
- the former type has become less of a problem because it has become possible to achieve stable low S due to the improvement of refining ability in recent years, while the latter type is still often a problem. As an effort to suppress these, it is known to control the melting conditions.
- the CaO concentration in the slag at the end of refining is controlled to 35% or less to suppress the accumulation of S in CaO, and the MgO concentration is 30% or less to match with CaS.
- Disclosed is a method capable of producing a high-Al stainless steel having excellent rust resistance by preventing CaS from being easily deposited by forming a solid-phase MgO having a good rust resistance.
- Patent Document 2 and Patent Document 3 the value of the formula relating to the composition of inclusions represented by the X value is set to a certain value or less, and [Ca], [S], [Al], T.I. It is characterized in that CaS formation is suppressed by refining so as to satisfy the formula consisting of [O], and a ferritic stainless steel with less rust formation is provided.
- Japanese Unexamined Patent Publication No. 5-339620 Japanese Unexamined Patent Publication No. 2012-184494 Japanese Unexamined Patent Publication No. 2014-162948
- Patent Document 1 Since the technique of Patent Document 1 requires that the CaO concentration of slag is low, even in a high Al component system, careful control is required to stably perform deoxidation, and the operation is performed. The load on the top increases. In addition, the desulfurization behavior tends to be unstable, and on the contrary, CaS and other sulfides may be generated to deteriorate the rust resistance.
- Patent Document 2 there are many restrictions on the slag composition and the concentration of Ca and S in the molten steel, and the cost increase due to the increase in the refining load becomes a problem.
- the present invention has been made to solve the remaining problems as described above, and the gist thereof is as follows.
- the chemical components are C: 0.001 to 0.02%, Si: 0.02 to 1.5%, Mn: 1.5% or less, P: 0.040% or less, S in mass%. : 0.006% or less, Cr: 10 to 25%, Al: 0.01 to 0.20%, O: 0.0005 to 0.010%, N: 0.005 to 0.025%, Ca: 0 Containing .0030% or less, further containing one or both of Ti: 0.35% or less, Nb: 0.70% or less, consisting of the balance Fe and impurities, and inclusions having a maximum diameter of 2 ⁇ m or more containing CaO.
- the number ratio of inclusions having one or more types of M (C, N) on the outer peripheral portion and the area ratio of the M (C, N) portion is 40% or more is 70% or more.
- Ferritic stainless steel characterized by this.
- M (C, N) represents a carbonitride of the element M
- M is one or more elements selected from Ti, Nb, and Cr, and the total of the other elements is less than 1%. But it's okay.
- V 2.0% or less
- Zr 0.0050% or less
- B 0.0001 to 0 in mass% instead of a part of the Fe.
- Ga Contains one or more of 0.010% or less, and the element M of the M (C, N) is selected from Ti, Nb, Cr, V, Zr, B, and Ga1.
- CaS which is considered to be the starting point of rusting in stainless steel, does not exist at the slab stage, that is, at the steelmaking stage, but as described above, S in the base metal diffuses when the slab is heated before hot rolling. , It is said that it is produced by reacting with CaO in inclusions.
- Elemental shading mapping using EPMA was performed on 20 inclusions with a maximum diameter of 5 ⁇ m or more randomly selected, and the formation status of CaS was confirmed. As a result, it was found that the longer the heating condition was high temperature, the more remarkable the formation of CaS, and even at high temperature, CaS was not formed in a short time of about 5 minutes. From this, it was found that short-time heating such as annealing was excluded as a step including heating in which CaS formation could occur.
- carbides, nitrides, and carbonitrides existing on the outer periphery of the inclusions containing CaO were investigated, the elements other than C and N were Ti, Nb, Cr, V, Zr, B, and Ga. And so on. In some cases, these had a composition close to that of a pure substance, and in other cases, a plurality of carbides and nitrides were in a solid solution state. Therefore, in the following, carbides, nitrides, and carbonitrides are referred to as M (C, N) without distinction. As described above, M is not limited to one type, and may be two or more types. In some cases, two or more phases of M (C, N) existing on the outer peripheral portion of the inclusions containing CaO coexist.
- M (C, N) is accompanied by the outer peripheral portion of the inclusions at the steel plate stage. Since it may not be covered, it cannot be used as an indicator that CaS is hardly produced.
- the ratio of the M (C, N) portion 8 to the area of the inclusions 2 and the outer periphery of the oxide portion of the inclusions containing CaO was investigated.
- 50 randomly selected inclusions containing CaO were photographed using an optical microscope and evaluated by an image analyzer.
- the outer peripheral length of the oxide portion of the inclusion containing CaO and the partial length in contact with M (C, N) are measured, and the partial length is divided by the outer peripheral length and multiplied by 100 to obtain the coverage. (%) was calculated.
- the area of inclusions containing CaO total area of oxide portion and M (C, N) part
- the area occupied by M (C, N) in the inclusions are measured, and the latter is used.
- the area ratio (%) of the M (C, N) portion was calculated by dividing by the former and multiplying by 100. As shown in FIG. 1, when the ratio of M (C, N) to the area of inclusions (area ratio (%) of the M (C, N) portion) is 40% or more, the coverage is almost 100. It was found that M (C, N) covered the outer peripheral portion of the oxide portion of the inclusions containing most of CaO.
- CaS can be used by using the area ratio of the M (C, N) portion in the steel plate stage.
- an index showing rust resistance There is a possibility that it can be applied as an index showing that is hardly generated, that is, an index showing rust resistance. Therefore, after observing inclusions in the hot-rolled plate and the cold-rolled plate, they were subjected to a salt spray test-JIS-Z-2371 (hereinafter referred to as SST) to investigate the rusting property.
- SST salt spray test-JIS-Z-2371
- FIG. 3 shows a rolling direction 4, a thickness direction 5, a plate width direction 6, and a steel plate surface 7, and the steel plate surface 7 is the observation surface 1.
- the present invention among the inclusions having a maximum diameter of 2 ⁇ m or more containing CaO, one or more kinds of M (C, N) are accompanied on the outer peripheral portion, and the area ratio of the M (C, N) portion is high. It is stipulated that the number ratio of inclusions of 40% or more is 70% or more.
- the present invention relates to the control of inclusion composition and is applicable to generally manufactured ferritic stainless steels.
- the range of components that can be suitably used is shown below, but the present invention is not limited thereto.
- C 0.001 to 0.02%
- C is a component of M (C, N) that suppresses the formation of CaS, and 0.001% or more is required for the formation of M (C, N), and the higher the concentration, the more M (C, N) is generated and CaS is less likely to be generated. It is preferably contained in an amount of 0.003% or more. On the other hand, if it is contained in an excessive amount, the processability may be deteriorated, so the content should be 0.02% or less. It is preferably 0.015% or less.
- Si 0.02 to 1.5% Si is an element that promotes the formation of M (C, N) in order to reduce the solubility of N. As another effect, it is an element that is also effective for desulfurization by promoting deoxidation. For example, it is an element that is effective in suppressing CaS production because it can indirectly suppress CaS production during solidification. In order to exhibit these effects, 0.02% or more is required, and it is preferable to add 0.05% or more. However, if it is added in excess of 1.5%, the workability is deteriorated. In particular, it is preferably 1.0% or less in applications where workability is a problem.
- Mn 1.5% or less Since Mn is an element that contributes to deoxidation, it may be added as preliminary deoxidation before adding Al. When added, it is preferably 0.01% or more, preferably 0.05% or more in order to exhibit the effect. On the other hand, it is set to 1.5% or less in order to reduce workability. In particular, it is preferably 0.3% or less in applications where workability is a problem.
- P 0.040% or less
- P is an element that is particularly harmful to stainless steel, such as lowering toughness, hot workability, and corrosion resistance. Therefore, the smaller the amount, the better, and make it 0.040% or less. It is preferably 0.035% or less, and more preferably 0.030% or less. However, excessive reduction may be contained in an amount of 0.005% or more in actual operation because the load at the time of refining is high or it is necessary to use high-priced raw materials.
- CaS formation during slab heating can be suppressed by the above requirements, but if S is contained in excess of 0.006%, it is in the pre-solidification or intermediate stage of solidification, that is, M (C, Since CaS is generated before N) covers the inclusions containing CaO and leads to rusting, the upper limit is set to 0.006%.
- the preferred upper limit is 0.003%.
- Cr 10 to 25% Cr is an important element that brings corrosion resistance to stainless steel, and it is necessary to add 10% or more, preferably 15% or more. On the other hand, since a large amount of addition causes deterioration of processability, the upper limit is set to 25%, preferably 21% or less.
- Al 0.01 to 0.20%
- Al is an element added to deoxidize molten steel, and is also an element necessary to reduce S to 0.006% or less. Therefore, the lower limit is set to 0.01%. The preferred lower limit is 0.05%. Since excessive addition reduces workability, the upper limit is set to 0.20%. The preferred upper limit is 0.15%.
- Ti or Nb is the main component of M (C, N), it is necessary to add at least one of Ti and Nb.
- Ti 0.35% or less Ti can be added to form M (C, N).
- M (C, N) containing Ti in order to generate M (C, N) containing Ti as a main component, it is preferable to add 0.01% or more, and the preferable addition amount is 0.05% or more. If it is added in an excessive amount, a large amount of TiN is generated before or during casting, which causes nozzle blockage and surface defects of the product. Therefore, the upper limit is set to 0.35%.
- Nb 0.70% or less Nb can be added to form M (C, N).
- M (C, N) containing Nb as a main component the effect is exhibited by adding 0.003% or more.
- the preferred addition amount is 0.2% or more.
- the content should be 0.70% or less. It is preferably 0.6% or less.
- O 0.0005-0.010%
- O forms CaO-containing inclusions that can generate CaS when the slab is heated.
- concentration of O increases, the amount of inclusions containing CaO increases, and it becomes difficult to cover with M (C, N). Therefore, the upper limit is set to 0.010%.
- the lower limit is set to 0.0005%.
- O is T.I. Means O.
- N 0.005 to 0.025%
- N is an element that forms M (C, N) that suppresses the formation of CaS, and the effect is exhibited by adding 0.005% or more.
- the content should be 0.025% or less. .. It is preferably 0.020% or less.
- CaS formation during slab heating can be suppressed by the above requirements, but if Ca is contained in excess of 0.0030%, it is in the pre-solidification or intermediate stage of solidification, that is, M (C, Since CaS is generated before N) covers the inclusions containing CaO and leads to rusting, the upper limit is set to 0.0030% or less.
- M Since CaS is generated before N
- the upper limit is set to 0.0030% or less.
- Ca may not be contained.
- the rest of the steel component is Fe and impurities.
- the impurity is a component that is mixed by various factors in the manufacturing process, including raw materials such as ore and scrap, when steel is industrially manufactured, and is a range that does not adversely affect the present invention. Means what is acceptable in.
- V 2.0% or less
- Zr 0.0050% or less
- B 0.0001 to 0.0020 in mass%.
- Ga One or more of 0.010% or less may be contained. When these elements are not contained, the lower limit of these elements is 0%.
- V 2.0% or less V itself has an effect of improving corrosion resistance and may be contained as necessary because it forms M (C, N) and suppresses the formation of CaS. Addition of 0.02% or more is preferable.
- the preferable addition amount for producing M (C, N) containing V as a main component is 0.1% or more. If V is excessively contained, the toughness decreases, so the content should be 2.0% or less. It is preferably 1.0% or less, and more preferably 0.5% or less.
- Zr 0.0050% or less Since Zr forms M (C, N) and suppresses the formation of CaS, it may be contained as necessary.
- the preferable addition amount for producing M (C, N) containing Zr as a main component is 0.0010% or more. However, if it is added in an excessive amount, sulfide is formed at the melting stage, which in turn lowers the corrosion resistance. Therefore, the upper limit is set to 0.0050%.
- B 0.0001 to 0.0020%
- B forms M (C, N) and suppresses the formation of CaS, so that it may contain 0.0001% or more as necessary.
- the content is set to 0.0020% or less. It is preferably 0.0010% or less.
- Ga 0.010% or less Ga itself has the effect of enhancing corrosion resistance, and also forms M (C, N) to suppress the formation of CaS. Therefore, in an amount of 0.010% or less as necessary. Can be contained.
- the lower limit of Ga is not particularly limited, but it is desirable that it contains 0.001% or more to obtain a stable effect.
- Mo 2.0% or less Mo has the effect of further enhancing the high corrosion resistance of stainless steel by adding it.
- the upper limit is set to 2.0%.
- the preferred lower limit is 0.5% and the preferred upper limit is 1.5%.
- Mg 0.0030% or less Since Mg is an effective element for deoxidation and desulfurization, it may be contained if necessary. However, if it is added in an excessive amount, sulfide is formed before or during casting, which in turn lowers the corrosion resistance. Therefore, the upper limit is set to 0.0030%.
- REM 0.01% or less REM (rare-earth metal) has a high affinity with O and S, and is therefore an effective element for deoxidation and desulfurization, and may be contained as necessary. However, if it is added in an excessive amount, a large amount of oxide is generated before or during casting, which causes nozzle blockage and surface defects of the product. Therefore, the upper limit is 0.01%.
- Ta 0.001 to 0.100% Since Ta is an element effective for deoxidation and desulfurization, it may be contained if necessary. In order to obtain this effect, it is preferable to contain 0.001% or more. However, if it is added in an excessive amount, it causes a decrease in ductility at room temperature and a decrease in toughness, so the upper limit is set to 0.100%.
- Ni 0.1-2.0% Since Ni has the effect of enhancing corrosion resistance, it can be added as needed. In order to obtain this effect, it is necessary to add 0.1% or more. On the other hand, since it is an expensive element and the effect corresponding to the increase in alloy cost cannot be obtained even if it is added in excess of 2.0%, the upper limit is set to 2.0%. It is preferably 1.5% or less.
- Sn 0.01 to 0.50%
- Sn 0.01 to 0.50%
- Cu 0.01-2.00% Since Cu has an effect of enhancing corrosion resistance, it can be added as needed. In order to obtain this effect, it is necessary to add 0.01% or more. However, since excessive addition leads to embrittlement, the content should be 2.00% or less.
- W 0.05 to 1.00% Since W has an effect of enhancing corrosion resistance, particularly pitting corrosion resistance, it can be added as needed. In order to obtain this effect, it is necessary to add 0.05% or more. However, since excessive addition causes a decrease in toughness, the upper limit is set to 1.00%.
- Co 0.10 to 1.00% Since Co has the effect of increasing the strength of the steel material, it can be added as needed. In order to obtain this effect, it is necessary to add 0.10% or more. However, since excessive addition causes a decrease in toughness, the upper limit is set to 1.00%.
- Sb 0.01-0.30% Since Sb has an effect of enhancing corrosion resistance, it can be added as needed. In order to obtain this effect, it is necessary to add 0.01% or more. However, since excessive addition causes a decrease in manufacturability, the upper limit is set to 0.30%.
- the steel plate surface 7 is observed as the observation surface 1.
- the position of the observation surface 1 in the depth direction of the steel sheet shall be the outermost layer as much as possible, and the minimum polishing necessary for mirror finishing for observation shall be performed.
- 100 or more inclusions 2 having a maximum diameter of 2 ⁇ m or more containing CaO are randomly selected, and this is used as a population, and inclusions 2 contained in the population are analyzed by SEM-EDS. Identify the size, composition and number of objects. At this time, the observation area is also recorded.
- JIS G0555 which is generally used as an evaluation method for inclusions, even if two or more inclusions are separated from each other, they may be regarded as one inclusion depending on the type and distance, but in the present invention.
- JIS G0555 which is generally used as an evaluation method for inclusions, even if two or more inclusions are separated from each other, they may be regarded as one inclusion depending on the type and distance, but in the present invention.
- the area of inclusions containing CaO and the area occupied by M (C, N) in the inclusions are measured, and the latter is divided by the former and multiplied by 100 to obtain the area ratio of the M (C, N) part. Calculate (%).
- Casting is performed by melting steel adjusted to have the above-mentioned predetermined components. At this time, casting is performed by controlling the cooling rate in order to cover the periphery of the inclusions containing CaO with M (C, N).
- the formation temperature of M (C, N) differs depending on the component system, but the slower the cooling rate, the higher the coverage by M (C, N).
- the average cooling rate at 1400 to 700 ° C. near the surface of the slab to 50 ° C./min or less during continuous casting, the conditions for inclusions specified above can be satisfied.
- the preferred range of the average cooling rate is 30 ° C./min or less, and the more preferable range is 15 ° C./min or less.
- the number of inclusions containing CaO having a maximum diameter of 2 ⁇ m or more increases as the O concentration increases, but by controlling the O concentration to 0.010% or less, favorable inclusion conditions (maximum diameter containing CaO).
- the number density of inclusions of 2 ⁇ m or more is less than 30 pieces / mm 2).
- the molten steel obtained by melting the steel adjusted to have the above-mentioned predetermined components was made into slabs by continuous casting.
- the average cooling rate in the temperature range of 1400 to 700 ° C. near the surface of the slab was controlled to various rates for casting.
- the average cooling rate in the temperature range of 1400 to 700 ° C. near the surface of the slab was evaluated by numerical calculation using heat transfer analysis, and the results are shown in Table 2.
- the obtained slab is heat-treated at 1200 ° C. for 2 hours to heat the slab before hot rolling, then hot-rolled, then hot-rolled and annealed and pickled, and then cold-rolled and annealed and pickled.
- a 1.0 mm thick cold-rolled plate was manufactured and used for inclusion measurement and SST test.
- a part of the slabs obtained above was heat-treated at 1200 ° C. for 2 hours in simulation of slab heating, and the state of CaS formation was confirmed.
- Table 1 shows the chemical composition
- Table 2 shows the CaS formation status of the slab heating simulated sample (the number of CaS generated in 20 inclusions with a maximum diameter of 5 ⁇ m or more) and the measurement results of cold-rolled plate inclusions (containing CaO).
- the position of the observation surface 1 in the depth direction of the steel sheet shall be the outermost layer as much as possible, and the minimum polishing necessary for mirror finishing for observation shall be performed.
- 100 or more inclusions 2 having a maximum diameter of 2 ⁇ m or more and containing CaO were randomly selected, and the areas of the oxide portion and the M (C, N) portion were measured to measure M (C, N).
- the ratio of the N) part to the area of the inclusions (the area ratio (%) of the M (C, N) part) is calculated, and the number ratio of the inclusions in which the area ratio of the M (C, N) part is 40% or more.
- the number of pieces per unit area was calculated by recording the measured area.
- SST test a neutral salt spray test was used as a salt solution based on JIS Z 2371, and a continuous spray test was conducted for 2 hours, and the number of rusting points per 100 cm 2 was measured. It was considered good if the number of rusting points was 5 or less.
- the S concentration was out of the upper limit of the range of the present invention, and as a result, the inclusion form of the steel sheet satisfied the condition of the present invention, but CaS was present as is clear from the observation result of the slab heating simulated sample. Therefore, a large number of rusts were observed in the SST test. It is presumed that the S concentration is high and CaS is generated before or during the coagulation stage.
- the Ca concentration was out of the upper limit of the range of the present invention, and as a result, the inclusion form of the steel plate satisfied the condition of the present invention, but CaS was present as is clear from the observation result of the slab heating simulated sample. Therefore, a large number of scabs were observed in the SST test. It is presumed that the Ca concentration is high and CaS is produced before or during the coagulation stage.
- Reference numeral b4 indicates that the O concentration is out of the upper limit of the range of the present invention, and as a result, the total area ratio of the M (C, N) portions of the inclusions in the steel sheet is 40% or more, and the number ratio of inclusions is low. Rust was observed. The number density of inclusions containing CaO having a maximum diameter of 2 ⁇ m or more was high.
- the Ti concentration of the symbol b5 was too high, and a large amount of TiN was generated during casting, so that the nozzle was blocked and casting was stopped.
- the workability was very poor and a large amount of surface defects caused by TiN occurred.
- Reference numeral b6 is due to the fact that the average cooling rate in the temperature range of 1400 to 700 ° C. near the surface of the slab was high, so that M (C, N) was not sufficiently generated and the coverage was low. The ratio of the number of inclusions having an area ratio of 40% or more in the N) portion was low. Therefore, CaS was generated and a large number of rusts were observed.
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Abstract
Description
本発明は、耐発銹性に優れるフェライト系ステンレス鋼に関するものである。 The present invention relates to a ferritic stainless steel having excellent rust resistance.
ステンレス鋼は一般に塗装等を行わず、無垢のまま実用に供されるものであるため、鋼材の表面に露出したCaSを起点とする発銹が問題になる。CaSの生成機構としては、溶鋼中で晶出するタイプと、凝固完了後の鋳片加熱時などにCaOを含む介在物と母材に含まれるSが反応して生成するタイプとが知られている。前者のタイプは近年の精錬能力の向上により安定的に低S化が達成できるようになったことから問題となることが少なくなってきた一方、後者のタイプは現在でも問題となることが多く、これらを抑制するための取り組みとして、溶製条件の制御によるものが知られている。 Stainless steel is generally not painted and is put to practical use as it is, so rusting starting from CaS exposed on the surface of the steel material becomes a problem. As a mechanism for forming CaS, a type that crystallizes in molten steel and a type that S contained in the base metal reacts with inclusions containing CaO when the slab is heated after solidification is known are known. There is. The former type has become less of a problem because it has become possible to achieve stable low S due to the improvement of refining ability in recent years, while the latter type is still often a problem. As an effort to suppress these, it is known to control the melting conditions.
例えば、特許文献1には精錬終了時のスラグ中CaO濃度を35%以下に制御することでCaOへのSの集積を抑制し、またMgO濃度を30%以下にすることでCaSと格子整合性の良い固相MgOが生成してCaSが容易に析出するのを防止し、耐銹性に優れた高Alステンレス鋼を製造可能な方法が開示されている。
For example, in
特許文献2および特許文献3ではX値で表される介在物の組成に関する式の値を一定以下にするとともに、[Ca]、[S]、[Al]、T.[O]からなる式を満たすように精錬を行うことでCaS生成を抑制し、発銹の少ないフェライト系ステンレス鋼を提供することを特徴とする。
In
しかしながら、上記技術では解決できない課題が存在していた。
特許文献1の技術はスラグのCaO濃度を低位にすることが必要なため、高Alの成分系であっても、脱酸を安定的に行うためには慎重な制御が必要になる等、操業上の負荷が大きくなる。また脱硫挙動も不安定になりやすく、却ってCaSやその他の硫化物が生成して耐銹性を悪化させることもある。
However, there are problems that cannot be solved by the above techniques.
Since the technique of
特許文献2および特許文献3の技術では、スラグ組成や溶鋼中CaやSの濃度に関する制約が多く、精錬負荷の増大によるコストアップが問題となる。
In the techniques of
そこで本発明は上記現状の問題点に鑑み、CaSの少ない耐発銹性に優れるフェライト系ステンレス鋼を提供することを課題とする。 Therefore, in view of the above-mentioned problems of the present situation, it is an object of the present invention to provide a ferritic stainless steel having a small amount of CaS and excellent rust resistance.
本発明は、上記のように残存する課題を解決するためになされたものであって、その要旨は以下の通りである。
[1]化学成分が、質量%で、C:0.001~0.02%、Si:0.02~1.5%、Mn:1.5%以下、P:0.040%以下、S:0.006%以下、Cr:10~25%、Al:0.01~0.20%、O:0.0005~0.010%、N:0.005~0.025%、Ca:0.0030%以下を含有し、更にTi:0.35%以下、Nb:0.70%以下の一方又は両方を含有し、残部Feおよび不純物からなり、CaOを含有する最大径2μm以上の介在物のうち、外周部に1種または2種以上のM(C,N)を伴い、かつM(C,N)部の面積率が40%以上である介在物の個数割合が70%以上であることを特徴とするフェライト系ステンレス鋼。
ここで、M(C,N)は元素Mの炭窒化物を表し、MはTi、Nb、Crから選ばれる1種または2種以上の元素であり、その他元素の合計として1%未満を含んでも良い。
[2][1]に記載の化学成分に加え、前記Feの一部に代えて、質量%で、V:2.0%以下、Zr:0.0050%以下、B:0.0001~0.0020%、Ga:0.010%以下の1種または2種以上を含有し、前記M(C,N)の元素MがTi、Nb、Cr、V、Zr、B、Gaから選ばれる1種または2種以上の元素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鋼。
[3]更に、前記Feの一部に代えて、質量%で、Mo:2.0%以下、Mg:0.0030%以下、REM:0.01%以下、Ta:0.001~0.100%、Ni:0.1~2.0%、Sn:0.01~0.50%、Cu:0.01~2.00%、W:0.05~1.00%、Co:0.10~1.00%、Sb:0.01~0.30%の1種または2種以上を含有する[1]または[2]に記載のフェライト系ステンレス鋼。
The present invention has been made to solve the remaining problems as described above, and the gist thereof is as follows.
[1] The chemical components are C: 0.001 to 0.02%, Si: 0.02 to 1.5%, Mn: 1.5% or less, P: 0.040% or less, S in mass%. : 0.006% or less, Cr: 10 to 25%, Al: 0.01 to 0.20%, O: 0.0005 to 0.010%, N: 0.005 to 0.025%, Ca: 0 Containing .0030% or less, further containing one or both of Ti: 0.35% or less, Nb: 0.70% or less, consisting of the balance Fe and impurities, and inclusions having a maximum diameter of 2 μm or more containing CaO. Among them, the number ratio of inclusions having one or more types of M (C, N) on the outer peripheral portion and the area ratio of the M (C, N) portion is 40% or more is 70% or more. Ferritic stainless steel characterized by this.
Here, M (C, N) represents a carbonitride of the element M, and M is one or more elements selected from Ti, Nb, and Cr, and the total of the other elements is less than 1%. But it's okay.
[2] In addition to the chemical components described in [1], V: 2.0% or less, Zr: 0.0050% or less, B: 0.0001 to 0 in mass% instead of a part of the Fe. 0020%, Ga: Contains one or more of 0.010% or less, and the element M of the M (C, N) is selected from Ti, Nb, Cr, V, Zr, B, and Ga1. The ferritic stainless steel according to [1], which is a species or two or more kinds of elements.
[3] Further, instead of a part of the Fe, in mass%, Mo: 2.0% or less, Mg: 0.0030% or less, REM: 0.01% or less, Ta: 0.001 to 0. 100%, Ni: 0.1 to 2.0%, Sn: 0.01 to 0.50%, Cu: 0.01 to 2.00%, W: 0.05 to 1.00%, Co: 0 The ferritic stainless steel according to [1] or [2], which contains one or more of 10 to 1.00% and Sb: 0.01 to 0.30%.
CaSを含有する介在物を起点とした発銹の少ないフェライト系ステンレス鋼を提供することができる。 It is possible to provide a ferritic stainless steel with less rusting starting from inclusions containing CaS.
以下、本発明の内容を詳細に説明する。
<CaSの形成(鋳片段階)>
まず、本発明を着想するに至った実験について述べる。
Hereinafter, the contents of the present invention will be described in detail.
<Formation of CaS (shard stage)>
First, the experiment that led to the idea of the present invention will be described.
ステンレス鋼で発銹の起点になるとされるCaSは鋳片の段階、つまり製鋼段階では存在していなくても、前述のように熱間圧延前の鋳片加熱時に母材中のSが拡散し、介在物中のCaOと反応して生成するとされている。 CaS, which is considered to be the starting point of rusting in stainless steel, does not exist at the slab stage, that is, at the steelmaking stage, but as described above, S in the base metal diffuses when the slab is heated before hot rolling. , It is said that it is produced by reacting with CaO in inclusions.
そこで、CaSの生成に及ぼす鋳片加熱条件の影響を調査した。種々の成分を持つフェライト系ステンレス鋼鋳片から試料を切り出し、大気雰囲気下、1000~1300℃の条件で5分~3時間加熱した後に空冷を行い、適当な断面を切り出して鏡面仕上げで研磨を行った。 Therefore, the effect of slab heating conditions on the formation of CaS was investigated. A sample is cut out from a ferritic stainless steel slab having various components, heated in an air atmosphere at 1000 to 1300 ° C. for 5 minutes to 3 hours, then air-cooled, an appropriate cross section is cut out, and polished with a mirror finish. gone.
無作為に選んだ最大径が5μm以上の介在物20個についてEPMAを用いた元素濃淡マッピングを行い、CaSの生成状況を確認した。その結果、加熱条件が高温で長時間であるほどCaSの生成が顕著であり、また高温でも5分程度の短時間ではCaSは生成しないことが分かった。このことから、CaSの生成が起こり得る加熱を含む工程として、焼鈍等の短時間加熱は除外されることが分かった。 Elemental shading mapping using EPMA was performed on 20 inclusions with a maximum diameter of 5 μm or more randomly selected, and the formation status of CaS was confirmed. As a result, it was found that the longer the heating condition was high temperature, the more remarkable the formation of CaS, and even at high temperature, CaS was not formed in a short time of about 5 minutes. From this, it was found that short-time heating such as annealing was excluded as a step including heating in which CaS formation could occur.
また長時間加熱の条件では[S]≦5ppmのような極低Sの試料ではCaSの生成が少なかったが、そうではない場合([S]>5ppmの場合)ではCaSが顕著に生成している介在物とCaSが全く生成していない介在物が同一の試料中に存在している場合があることが分かった。 Further, under the condition of long-time heating, CaS was less generated in the sample having extremely low S such as [S] ≤ 5 ppm, but in the other case (when [S]> 5 ppm), CaS was remarkably generated. It was found that inclusions that are present and inclusions that do not produce CaS at all may be present in the same sample.
更に詳細に調査すると、CaSが全く生成していない介在物は、CaOを含有する介在物の周囲を炭化物や窒化物、炭窒化物等が覆っていることが分かった。前述の通りCaSは母材に含まれるSが拡散し、介在物中のCaOと反応して生成するので、介在物の周囲を覆う炭化物や窒化物、炭窒化物によってSの拡散が物理的に遮断された結果、CaSの生成が抑制されたものと考えられる。 Further detailed investigation revealed that the inclusions in which CaS was not generated at all were covered with carbides, nitrides, carbonitrides, etc. around the inclusions containing CaO. As described above, CaS is generated by the diffusion of S contained in the base metal and reacting with CaO in the inclusions. Therefore, the diffusion of S is physically caused by the carbides, nitrides, and carbonitrides surrounding the inclusions. As a result of the blockage, it is considered that the production of CaS was suppressed.
次に、CaOを含有する介在物の外周部に存在する炭化物や窒化物、炭窒化物について調査を行ったところ、C,N以外の元素はTi、Nb、Cr、V、Zr、B、Ga等であることが分かった。これらは純物質に近い組成の場合もあれば、複数の炭化物や窒化物が固溶した状態の場合もあった。そのため以下では炭化物、窒化物、炭窒化物を区別せずにM(C,N)と表す。前述のようにMは1種とは限らず、2種以上である場合もある。またCaOを含有する介在物の外周部に存在するM(C,N)は、2相以上が共存している場合もあった。 Next, when the carbides, nitrides, and carbonitrides existing on the outer periphery of the inclusions containing CaO were investigated, the elements other than C and N were Ti, Nb, Cr, V, Zr, B, and Ga. And so on. In some cases, these had a composition close to that of a pure substance, and in other cases, a plurality of carbides and nitrides were in a solid solution state. Therefore, in the following, carbides, nitrides, and carbonitrides are referred to as M (C, N) without distinction. As described above, M is not limited to one type, and may be two or more types. In some cases, two or more phases of M (C, N) existing on the outer peripheral portion of the inclusions containing CaO coexist.
<介在物の形態(加工後の段階(鋼板段階)と鋳片段階の比較)>
次に介在物の形態について、圧延等の加工後の段階と鋳片段階との関係について調査を行った。
<Form of inclusions (comparison between post-machining stage (steel plate stage) and slab stage)>
Next, regarding the morphology of inclusions, the relationship between the post-processing stage such as rolling and the slab stage was investigated.
熱延板や冷延板(以下総称して「鋼板」という。)の介在物を観察すると、CaOを含有する介在物の外周部にM(C,N)が伴っているものの、覆われてはいない場合もあった。このような介在物の中にもCaSが顕著に生成している場合とほとんど生成していない場合があることが分かった。前述の通り、CaSは鋳片加熱時に生成するため、鋳片段階でCaOを含有する介在物の酸化物部分の外周部をM(C,N)が覆っていればCaSは生成しない。このことから、熱延板や冷延板等、加工を受けて変形した後のM(C,N)を伴ったCaOを含有する介在物について、CaSが顕著に生成している場合は鋳片段階でM(C,N)に覆われておらず、CaSがほとんど生成していない場合は鋳片段階でM(C,N)に覆われていたものと推定される。しかし、鋳片段階でM(C,N)に覆われていたものと推定される場合であっても、鋼板段階では、介在物の外周部にM(C,N)が伴っているものの、覆われてはいない場合があるため、CaSがほとんど生成していないことを示す指標として用いることができない。 When observing inclusions in hot-rolled plates and cold-rolled plates (hereinafter collectively referred to as "steel plates"), the outer periphery of the inclusions containing CaO is covered with M (C, N). In some cases, it wasn't. It was found that CaS may be remarkably generated or hardly generated in such inclusions. As described above, since CaS is generated when the slab is heated, CaS is not generated if M (C, N) covers the outer peripheral portion of the oxide portion of the inclusions containing CaO at the slab stage. From this, when CaS is remarkably generated in inclusions containing CaO with M (C, N) after being deformed by processing, such as hot-rolled plates and cold-rolled plates, slabs are formed. If it was not covered with M (C, N) at the stage and almost no CaS was produced, it is presumed that it was covered with M (C, N) at the slab stage. However, even if it is presumed that the material was covered with M (C, N) at the slab stage, M (C, N) is accompanied by the outer peripheral portion of the inclusions at the steel plate stage. Since it may not be covered, it cannot be used as an indicator that CaS is hardly produced.
そこで、鋼板段階での介在物観察によって、CaSがほとんど生成していないことを示す指標を見いだすべく、検討を行った。 Therefore, we conducted a study to find an index showing that CaS was hardly generated by observing inclusions at the steel sheet stage.
まず、鋳片段階におけるCaOを含有する介在物(図4参照)について、M(C,N)部8が介在物2の面積に占める割合と、CaOを含有する介在物の酸化物部分の外周部をM(C,N)が覆っている割合(被覆率)との関係について調査した。調査は光学顕微鏡を用いて無作為に選択したCaOを含有する介在物50個について写真撮影を行い、画像解析装置により評価を行った。第1に、CaOを含有する介在物の酸化物部分の外周長およびM(C,N)と接している部分長を測定し、部分長を外周長で除して100倍することにより被覆率(%)を算出した。第2に、CaOを含有する介在物の面積(酸化物部分とM(C,N)部の合計面積)と、当該介在物においてM(C,N)が占める面積とを測定し、後者を前者で除して100倍することでM(C,N)部の面積率(%)を算出した。図1に示すように、M(C,N)が介在物の面積に占める割合(M(C,N)部の面積率(%))が40%以上の場合には、被覆率がほぼ100%であり、ほとんどのCaOを含有する介在物の酸化物部分の外周部をM(C,N)が覆っていることが判明した。
First, regarding the inclusions containing CaO in the slab stage (see FIG. 4), the ratio of the M (C, N)
介在物において、上記M(C,N)部の面積率が、鋳片段階から鋼板段階に受け継がれるとすれば、鋼板段階でのM(C,N)部の面積率を用いることにより、CaSがほとんど生成していないことを示す指標、即ち耐発銹性を示す指標として適用できる可能性がある。そこで次に熱延板および冷延板について、介在物の観察を行った後、塩水噴霧試験-JIS-Z-2371(以下、SST)に供して発銹性の調査を行った。 If the area ratio of the M (C, N) portion is inherited from the slab stage to the steel plate stage in the inclusions, CaS can be used by using the area ratio of the M (C, N) portion in the steel plate stage. There is a possibility that it can be applied as an index showing that is hardly generated, that is, an index showing rust resistance. Therefore, after observing inclusions in the hot-rolled plate and the cold-rolled plate, they were subjected to a salt spray test-JIS-Z-2371 (hereinafter referred to as SST) to investigate the rusting property.
介在物の観察については、図3に示すように、鋼板表面を鏡面研磨して検鏡面とした。図3には、圧延方向4、厚み方向5、板幅方向6、鋼板表面7が記載され、鋼板表面7が観察面1になる。まず発銹点の介在物径を調べたところ、最大径が2μm未満の介在物ではほとんど発銹が起きていないことが判明した。そこで本発明では、CaOを含有する最大径3が2μm以上の介在物2を評価対象とする。続いて、前記鋳片段階と同様に、M(C,N)部が介在物の面積(酸化物部分とM(C,N)部の合計面積)に占める割合(M(C,N)部の面積率(%))を評価した。さらに、M(C,N)部の面積率(%)が40%以上の介在物の個数割合と発銹性(SST)調査結果との関係を評価した。図2に示すように、M(C,N)部が介在物の面積に占める割合(M(C,N)部の面積率(%))が40%以上の介在物の個数割合が70%以上であればSSTの評価が5以下となり良好で、85%以上であれば更に良好となることが分かった。
Regarding the observation of inclusions, as shown in FIG. 3, the surface of the steel plate was mirror-polished to obtain a mirror surface. FIG. 3 shows a
以上の通り、鋳片段階で、CaOを含有する介在物の酸化物部分の外周部をM(C,N)が覆っていることで鋳片加熱時のCaS生成を抑制できること、および鋳片段階でM(C,N)部の面積率が40%以上であれば鋳片段階でCaOを含有する介在物の酸化物部分の外周部をM(C,N)が覆っていた率が高いことが判明した。また圧延等の加工後の鋼表面の介在物観察において、CaOを含有する最大径2μm以上の介在物のうち、外周部に1種または2種以上のM(C,N)を伴い、かつM(C,N)部の面積率が40%以上である介在物の個数割合が70%以上であれば、SSTの成績が良いことが分かった。 As described above, in the slab stage, CaS generation during slab heating can be suppressed by covering the outer peripheral portion of the oxide portion of the inclusions containing CaO with M (C, N), and in the slab stage. If the area ratio of the M (C, N) portion is 40% or more, the ratio of M (C, N) covering the outer peripheral portion of the oxide portion of the inclusions containing CaO at the slab stage is high. There was found. Further, in the observation of inclusions on the steel surface after processing such as rolling, among the inclusions having a maximum diameter of 2 μm or more containing CaO, one or more kinds of M (C, N) are accompanied on the outer peripheral portion, and M (C, N) is accompanied. It was found that the SST results were good when the number ratio of inclusions having an area ratio of 40% or more in the (C, N) portion was 70% or more.
そこで本発明では、CaOを含有する最大径2μm以上の介在物のうち、外周部に1種または2種以上のM(C,N)を伴い、かつM(C,N)部の面積率が40%以上である介在物の個数割合が70%以上であることと規定した。 Therefore, in the present invention, among the inclusions having a maximum diameter of 2 μm or more containing CaO, one or more kinds of M (C, N) are accompanied on the outer peripheral portion, and the area ratio of the M (C, N) portion is high. It is stipulated that the number ratio of inclusions of 40% or more is 70% or more.
<鋼成分>
上述したように本発明は介在物組成制御に関するもので、一般的に製造されているフェライト系ステンレス鋼に適用可能なものである。以下に好適に用いることができる成分範囲を示すが、これに限定されるものではない。
<Steel component>
As described above, the present invention relates to the control of inclusion composition and is applicable to generally manufactured ferritic stainless steels. The range of components that can be suitably used is shown below, but the present invention is not limited thereto.
C:0.001~0.02%
CはCaSの生成を抑制するM(C,N)の成分であり、M(C,N)の形成のために0.001%以上が必要であり、高濃度に含有するほどM(C,N)が生成してCaSが生成しにくくなる。好ましくは0.003%以上含有すると良い。その一方で過剰に含有すると加工性を低下させたりするため、0.02%以下とする。好ましくは0.015%以下である。
C: 0.001 to 0.02%
C is a component of M (C, N) that suppresses the formation of CaS, and 0.001% or more is required for the formation of M (C, N), and the higher the concentration, the more M (C, N) is generated and CaS is less likely to be generated. It is preferably contained in an amount of 0.003% or more. On the other hand, if it is contained in an excessive amount, the processability may be deteriorated, so the content should be 0.02% or less. It is preferably 0.015% or less.
Si:0.02~1.5%
SiはNの溶解度を下げるため、M(C,N)の生成を促進させる元素である。その他の効果として、脱酸促進による脱硫にも有効な元素であり、例えば凝固中のCaS生成を間接的に抑制可能であるため、CaS生成抑制に有効な元素である。これらの効果を発現させるためには0.02%以上が必要であり、0.05%以上添加することが好ましい。ただし、1.5%を超えて添加すると加工性が低下する。特に加工性が問題になる用途では1.0%以下とすることが好ましい。
Si: 0.02 to 1.5%
Si is an element that promotes the formation of M (C, N) in order to reduce the solubility of N. As another effect, it is an element that is also effective for desulfurization by promoting deoxidation. For example, it is an element that is effective in suppressing CaS production because it can indirectly suppress CaS production during solidification. In order to exhibit these effects, 0.02% or more is required, and it is preferable to add 0.05% or more. However, if it is added in excess of 1.5%, the workability is deteriorated. In particular, it is preferably 1.0% or less in applications where workability is a problem.
Mn:1.5%以下
Mnは脱酸に寄与する元素であるため、Alを添加する前に予備脱酸として添加しても良い。添加する場合はその効果を発現させるためには0.01%以上にするとよく、好ましくは0.05%以上にするとよい。一方、加工性を低下させるため、1.5%以下とする。特に加工性が問題になる用途では0.3%以下とすることが好ましい。
Mn: 1.5% or less Since Mn is an element that contributes to deoxidation, it may be added as preliminary deoxidation before adding Al. When added, it is preferably 0.01% or more, preferably 0.05% or more in order to exhibit the effect. On the other hand, it is set to 1.5% or less in order to reduce workability. In particular, it is preferably 0.3% or less in applications where workability is a problem.
P:0.040%以下
Pは靱性や熱間加工性、耐食性を低下させる等、ステンレス鋼にとっては特に有害な元素であるため、少ないほど良く、0.040%以下にする。好ましくは0.035%以下であり、より好ましくは0.030%以下である。但し、過剰な低下は精錬時の負荷が高いか、または高価格の原料を用いる必要があるため、実操業としては0.005%以上含有しても良い。
P: 0.040% or less P is an element that is particularly harmful to stainless steel, such as lowering toughness, hot workability, and corrosion resistance. Therefore, the smaller the amount, the better, and make it 0.040% or less. It is preferably 0.035% or less, and more preferably 0.030% or less. However, excessive reduction may be contained in an amount of 0.005% or more in actual operation because the load at the time of refining is high or it is necessary to use high-priced raw materials.
S:0.006%以下
前述の要件によって鋳片加熱時のCaS生成は抑制できるが、Sが0.006%を超えて含まれていると、凝固前あるいは凝固中段階、すなわちM(C,N)がCaOを含有する介在物を覆う前にCaSが生成してしまい、発銹に繋がるため、上限を0.006%とする。好ましい上限は0.003%である。
S: 0.006% or less CaS formation during slab heating can be suppressed by the above requirements, but if S is contained in excess of 0.006%, it is in the pre-solidification or intermediate stage of solidification, that is, M (C, Since CaS is generated before N) covers the inclusions containing CaO and leads to rusting, the upper limit is set to 0.006%. The preferred upper limit is 0.003%.
Cr:10~25%
Crはステンレス鋼に耐食性をもたらす重要な元素で、10%以上の添加が必要であり、好ましくは15%以上にするとよい。その一方で多量の添加は加工性の低下を招くため、上限を25%とし、好ましくは21%以下にするとよい。
Cr: 10 to 25%
Cr is an important element that brings corrosion resistance to stainless steel, and it is necessary to add 10% or more, preferably 15% or more. On the other hand, since a large amount of addition causes deterioration of processability, the upper limit is set to 25%, preferably 21% or less.
Al:0.01~0.20%
Alは溶鋼を脱酸するために添加する元素であり、Sを0.006%以下にするためにも必要な元素である。そのため下限を0.01%とする。好ましい下限は0.05%である。過剰な添加は加工性を低下させるため、その上限を0.20%とする。好ましい上限は0.15%である。
Al: 0.01 to 0.20%
Al is an element added to deoxidize molten steel, and is also an element necessary to reduce S to 0.006% or less. Therefore, the lower limit is set to 0.01%. The preferred lower limit is 0.05%. Since excessive addition reduces workability, the upper limit is set to 0.20%. The preferred upper limit is 0.15%.
TiまたはNbは、M(C,N)の主成分となるため、Ti、Nbの少なくとも一方は添加する必要がある。 Since Ti or Nb is the main component of M (C, N), it is necessary to add at least one of Ti and Nb.
Ti:0.35%以下
Tiは添加することでM(C,N)を形成することができる。Tiを主成分としたM(C,N)を生成させるため、0.01%以上添加することが良く、好ましい添加量は0.05%以上である。過剰に添加すると、鋳造前あるいは鋳造中にTiNが多量に生成し、ノズル閉塞や製品の表面欠陥を招くため、その上限を0.35%とする。
Ti: 0.35% or less Ti can be added to form M (C, N). In order to generate M (C, N) containing Ti as a main component, it is preferable to add 0.01% or more, and the preferable addition amount is 0.05% or more. If it is added in an excessive amount, a large amount of TiN is generated before or during casting, which causes nozzle blockage and surface defects of the product. Therefore, the upper limit is set to 0.35%.
Nb:0.70%以下
Nbは添加することでM(C,N)を形成することができる。Nbを主成分としたM(C,N)を生成させるには0.003%以上の添加で効果を発現する。好ましい添加量は0.2%以上である。一方、0.70%を超えて添加すると再結晶化しにくくなって組織が粗大化するため、0.70%以下とする。好ましくは0.6%以下にするとよい。
Nb: 0.70% or less Nb can be added to form M (C, N). In order to generate M (C, N) containing Nb as a main component, the effect is exhibited by adding 0.003% or more. The preferred addition amount is 0.2% or more. On the other hand, if it is added in excess of 0.70%, it becomes difficult to recrystallize and the structure becomes coarse, so the content should be 0.70% or less. It is preferably 0.6% or less.
O:0.0005~0.010%
Oは鋳片加熱時にCaSを生成し得るCaOを含有する介在物を形成する。Oが高濃度であるほど、CaOを含有する介在物の量が増えるため、M(C,N)で覆うのが困難になるため、上限を0.010%とする。但し、過剰な脱酸は精錬負荷が増加してコストアップを招くため、下限を0.0005%とする。OはT.Oを意味する。
O: 0.0005-0.010%
O forms CaO-containing inclusions that can generate CaS when the slab is heated. As the concentration of O increases, the amount of inclusions containing CaO increases, and it becomes difficult to cover with M (C, N). Therefore, the upper limit is set to 0.010%. However, excessive deoxidation increases the refining load and causes an increase in cost, so the lower limit is set to 0.0005%. O is T.I. Means O.
N:0.005~0.025%
NはCaSの生成を抑制するM(C,N)を形成する元素であり、0.005%以上の添加で効果を発現する。高濃度に含有するほどM(C,N)が生成してCaSが生成しにくくなる。その一方で過剰に含有するとCrの窒化物が多量に生成して粒界のCr欠乏を引き起こして却って耐食性を低下させたり、顕著に加工性を低下させたりするため、0.025%以下とする。好ましくは0.020%以下である。
N: 0.005 to 0.025%
N is an element that forms M (C, N) that suppresses the formation of CaS, and the effect is exhibited by adding 0.005% or more. The higher the concentration, the more M (C, N) is generated and the more difficult it is to generate CaS. On the other hand, if it is contained in an excessive amount, a large amount of Cr nitride is generated, which causes a Cr deficiency at the grain boundaries, which in turn lowers the corrosion resistance and significantly lowers the workability. Therefore, the content should be 0.025% or less. .. It is preferably 0.020% or less.
Ca:0.0030%以下
前述の要件によって鋳片加熱時のCaS生成は抑制できるが、Caが0.0030%を超えて含まれていると、凝固前あるいは凝固中段階、すなわちM(C,N)がCaOを含有する介在物を覆う前にCaSが生成してしまい、発銹に繋がるため、上限を0.0030%以下とする。Caが高いほどCaOを含有する介在物の量が多くなり、覆うのに必要なM(C,N)も多くなるため少ないほど良く、好ましくは0.0020%以下である。更に好ましくは0.0010%以下である。Caは含有しなくても良い。
Ca: 0.0030% or less CaS formation during slab heating can be suppressed by the above requirements, but if Ca is contained in excess of 0.0030%, it is in the pre-solidification or intermediate stage of solidification, that is, M (C, Since CaS is generated before N) covers the inclusions containing CaO and leads to rusting, the upper limit is set to 0.0030% or less. The higher the Ca, the larger the amount of inclusions containing CaO, and the larger the amount of M (C, N) required for covering, so the smaller the amount, the better, preferably 0.0020% or less. More preferably, it is 0.0010% or less. Ca may not be contained.
上記鋼成分の残部はFe及び不純物である。ここで不純物とは、鋼を工業的に製造する際に、鉱石やスクラップ等のような原料をはじめとして、製造工程の種々の要因によって混入する成分であって、本発明に悪影響を与えない範囲で許容されるものを意味する。 The rest of the steel component is Fe and impurities. Here, the impurity is a component that is mixed by various factors in the manufacturing process, including raw materials such as ore and scrap, when steel is industrially manufactured, and is a range that does not adversely affect the present invention. Means what is acceptable in.
また、本実施形態のフェライト系ステンレス鋼は、Feの一部に代えて、更に質量%で、V:2.0%以下、Zr:0.0050%以下、B:0.0001~0.0020%、Ga:0.010%以下のうちの1種または2種以上を含んでも良い。これらの元素を含まない場合のこれらの元素の下限値は0%である。 Further, in the ferrite-based stainless steel of the present embodiment, instead of a part of Fe, V: 2.0% or less, Zr: 0.0050% or less, B: 0.0001 to 0.0020 in mass%. %, Ga: One or more of 0.010% or less may be contained. When these elements are not contained, the lower limit of these elements is 0%.
V:2.0%以下
Vはそれ自体が耐食性を向上させる効果を有する他、M(C,N)を形成してCaSの生成を抑制するため、必要に応じて含有させても良い。0.02%以上の添加が好ましい。Vを主成分としたM(C,N)を生成させるための好ましい添加量は0.1%以上である。Vを過剰に含有させると、靱性が低下するため、2.0%以下とする。1.0%以下とするのが好ましく、0.5%以下にするのがより好ましい。
V: 2.0% or less V itself has an effect of improving corrosion resistance and may be contained as necessary because it forms M (C, N) and suppresses the formation of CaS. Addition of 0.02% or more is preferable. The preferable addition amount for producing M (C, N) containing V as a main component is 0.1% or more. If V is excessively contained, the toughness decreases, so the content should be 2.0% or less. It is preferably 1.0% or less, and more preferably 0.5% or less.
Zr:0.0050%以下
ZrはM(C,N)を形成してCaSの生成を抑制するため、必要に応じて含有させても良い。Zrを主成分とするM(C,N)を生成させるための好ましい添加量は0.0010%以上である。ただし、過剰に添加すると溶製段階で硫化物を形成し、却って耐食性を低下させる。そのため上限を0.0050%とする。
Zr: 0.0050% or less Since Zr forms M (C, N) and suppresses the formation of CaS, it may be contained as necessary. The preferable addition amount for producing M (C, N) containing Zr as a main component is 0.0010% or more. However, if it is added in an excessive amount, sulfide is formed at the melting stage, which in turn lowers the corrosion resistance. Therefore, the upper limit is set to 0.0050%.
B:0.0001~0.0020%
Bは粒界の強度を高める効果を有する他、M(C,N)を形成してCaSの生成を抑制するため、必要に応じて0.0001%以上を含有させても良い。しかしながら、Bを過剰に含有させると伸びの低下による加工性低下を招くため、含有量を0.0020%以下にする。好ましくは0.0010%以下である。
B: 0.0001 to 0.0020%
In addition to having the effect of increasing the strength of the grain boundaries, B forms M (C, N) and suppresses the formation of CaS, so that it may contain 0.0001% or more as necessary. However, if B is excessively contained, the processability is deteriorated due to the decrease in elongation, so the content is set to 0.0020% or less. It is preferably 0.0010% or less.
Ga:0.010%以下
Gaはそれ自体が耐食性を高める効果を持つ他、M(C,N)を形成してCaSの生成を抑制するため、必要に応じて0.010%以下の量で含有させることができる。Gaの下限は特に限定しないが、安定した効果が得られる0.001%以上含有することが望ましい。
Ga: 0.010% or less Ga itself has the effect of enhancing corrosion resistance, and also forms M (C, N) to suppress the formation of CaS. Therefore, in an amount of 0.010% or less as necessary. Can be contained. The lower limit of Ga is not particularly limited, but it is desirable that it contains 0.001% or more to obtain a stable effect.
更にFeの一部に代えて、Mo:2.0%以下、Mg:0.0030%以下、REM:0.01%以下、Ta:0.001~0.100%、Ni:0.1~2.0%、Sn:0.01~0.50%、Cu:0.01~2.00%、W:0.05~1.00%、Co:0.10~1.00%、Sb:0.01~0.30%のうちの1種または2種以上を含んでも良い。これらの元素を含まない場合のこれらの元素の下限値は0%である。 Further, instead of a part of Fe, Mo: 2.0% or less, Mg: 0.0030% or less, REM: 0.01% or less, Ta: 0.001 to 0.100%, Ni: 0.1 to 2.0%, Sn: 0.01 to 0.50%, Cu: 0.01 to 2.00%, W: 0.05 to 1.00%, Co: 0.10 to 1.00%, Sb : One or more of 0.01 to 0.30% may be contained. When these elements are not contained, the lower limit of these elements is 0%.
Mo:2.0%以下
Moは添加することでステンレス鋼の高い耐食性をさらに高める作用がある。しかし、非常に高価であるため2.0%を超えて添加しても合金コストの増大に見合う効果が得られないばかりか、シグマ相を形成して脆化と耐食性の低下を招く。そのため上限を2.0%とする。好ましい下限は0.5%、好ましい上限は1.5%である。
Mo: 2.0% or less Mo has the effect of further enhancing the high corrosion resistance of stainless steel by adding it. However, since it is very expensive, even if it is added in excess of 2.0%, not only the effect corresponding to the increase in alloy cost cannot be obtained, but also the formation of a sigma phase causes embrittlement and deterioration of corrosion resistance. Therefore, the upper limit is set to 2.0%. The preferred lower limit is 0.5% and the preferred upper limit is 1.5%.
Mg:0.0030%以下
Mgは脱酸・脱硫に有効な元素であることから、必要に応じて含有させても良い。但し、過剰に添加すると鋳造前あるいは鋳造中に硫化物を形成し、却って耐食性を低下させる。そのため上限を0.0030%とする。
Mg: 0.0030% or less Since Mg is an effective element for deoxidation and desulfurization, it may be contained if necessary. However, if it is added in an excessive amount, sulfide is formed before or during casting, which in turn lowers the corrosion resistance. Therefore, the upper limit is set to 0.0030%.
REM:0.01%以下
REM(希土類金属:Rare-Earth Metal)は、OやSと親和性が高いため、脱酸・脱硫に有効な元素であり、必要に応じて含有させても良い。但し、過剰に添加すると鋳造前あるいは鋳造中に酸化物が多量に生成し、ノズル閉塞や製品の表面欠陥を招くため、0.01%を上限とする。
REM: 0.01% or less REM (rare-earth metal) has a high affinity with O and S, and is therefore an effective element for deoxidation and desulfurization, and may be contained as necessary. However, if it is added in an excessive amount, a large amount of oxide is generated before or during casting, which causes nozzle blockage and surface defects of the product. Therefore, the upper limit is 0.01%.
Ta:0.001~0.100%
Taは脱酸・脱硫に有効な元素であることから、必要に応じて含有させても良い。この効果を得るためには0.001%以上含有すると良い。但し、過剰に添加すると常温延性の低下や靱性の低下を招くため、上限を0.100%とする。
Ta: 0.001 to 0.100%
Since Ta is an element effective for deoxidation and desulfurization, it may be contained if necessary. In order to obtain this effect, it is preferable to contain 0.001% or more. However, if it is added in an excessive amount, it causes a decrease in ductility at room temperature and a decrease in toughness, so the upper limit is set to 0.100%.
Ni:0.1~2.0%
Niは耐食性を高める作用があるため、必要に応じて添加できる。この効果を得るためには0.1%以上の添加が必要である。一方、高価な元素であり2.0%を超えて添加しても合金コストの増大に見合う効果が得られないため、その上限を2.0%とする。好ましくは1.5%以下にすると良い。
Ni: 0.1-2.0%
Since Ni has the effect of enhancing corrosion resistance, it can be added as needed. In order to obtain this effect, it is necessary to add 0.1% or more. On the other hand, since it is an expensive element and the effect corresponding to the increase in alloy cost cannot be obtained even if it is added in excess of 2.0%, the upper limit is set to 2.0%. It is preferably 1.5% or less.
Sn:0.01~0.50%
Snは添加することでステンレス鋼の高い耐食性を更に高める効果がある。含有する場合、この効果を得るためには0.01%以上含有すると良く、好ましくは0.02%以上にするとよい。一方で過剰な添加は加工性の低下に繋がるため、0.50%以下にするとよく、好ましくは0.30%以下にするとよい。
Sn: 0.01 to 0.50%
By adding Sn, there is an effect of further enhancing the high corrosion resistance of stainless steel. When it is contained, it is preferable to contain it in an amount of 0.01% or more, preferably 0.02% or more in order to obtain this effect. On the other hand, since excessive addition leads to a decrease in processability, it is preferably 0.50% or less, preferably 0.30% or less.
Cu:0.01~2.00%
Cuは耐食性を高める作用があるため、必要に応じて添加できる。この効果を得るためには0.01%以上の添加が必要である。但し、過剰な添加は脆化に繋がるため、2.00%以下とする。
Cu: 0.01-2.00%
Since Cu has an effect of enhancing corrosion resistance, it can be added as needed. In order to obtain this effect, it is necessary to add 0.01% or more. However, since excessive addition leads to embrittlement, the content should be 2.00% or less.
W:0.05~1.00%
Wは耐食性、特に耐孔食性を高める作用があるため、必要に応じて添加できる。この効果を得るためには0.05%以上の添加が必要である。但し、過剰な添加は靱性の低下を招くため、その上限を1.00%とする。
W: 0.05 to 1.00%
Since W has an effect of enhancing corrosion resistance, particularly pitting corrosion resistance, it can be added as needed. In order to obtain this effect, it is necessary to add 0.05% or more. However, since excessive addition causes a decrease in toughness, the upper limit is set to 1.00%.
Co:0.10~1.00%
Coは鋼材の強度を高める作用があるため、必要に応じて添加できる。この効果を得るためには0.10%以上の添加が必要である。但し、過剰な添加は靱性の低下を招くため、その上限を1.00%とする。
Co: 0.10 to 1.00%
Since Co has the effect of increasing the strength of the steel material, it can be added as needed. In order to obtain this effect, it is necessary to add 0.10% or more. However, since excessive addition causes a decrease in toughness, the upper limit is set to 1.00%.
Sb:0.01~0.30%
Sbは耐食性を高める作用があるため、必要に応じて添加できる。この効果を得るためには0.01%以上の添加が必要である。但し、過剰な添加は製造性の低下を招くため、その上限を0.30%とする。
Sb: 0.01-0.30%
Since Sb has an effect of enhancing corrosion resistance, it can be added as needed. In order to obtain this effect, it is necessary to add 0.01% or more. However, since excessive addition causes a decrease in manufacturability, the upper limit is set to 0.30%.
<介在物の測定方法>
以下、介在物の測定方法について説明する。図3に示すように、鋼板表面7を観察面1として観察する。観察面1の鋼板深さ方向の位置は可能な限り最表層とし、観察のための鏡面仕上げに必要な最小限の研磨を行う。観察面1において、CaOを含む最大径が2μm以上の介在物2を無作為に100個以上選び、これを母集団とし、母集団に含まれる介在物2をSEM-EDSで分析することで介在物の大きさ及び組成と個数を同定する。この際、観察面積も記録しておく。なお、介在物の評価方法として一般に用いられるJIS G0555では2つ以上の介在物が離れて存在している場合でも、種類と距離によっては一つの介在物とみなす場合があるが、本発明においては個別の介在物とみなす。CaOを含有する介在物の面積と、当該介在物においてM(C,N)が占める面積とを測定し、後者を前者で除して100倍することでM(C,N)部の面積率(%)を算出する。
<Measurement method of inclusions>
Hereinafter, a method for measuring inclusions will be described. As shown in FIG. 3, the
<製造方法>
本実施形態のフェライト系ステンレス鋼の製造方法について説明する。
<Manufacturing method>
A method for manufacturing the ferritic stainless steel of the present embodiment will be described.
上記した所定の成分になるよう調整した鋼を溶製して鋳造を行う。このときCaOを含有する介在物の周囲をM(C,N)で覆うために冷却速度を制御して鋳造を行う。成分系によりM(C,N)の生成温度が異なるが、冷却速度が緩やかであるほどM(C,N)による被覆率は上昇する。連続鋳造中において、鋳片表面近傍の1400~700℃での平均冷却速度を50℃/分以下に制御することで、上記で規定した介在物の条件を満たすことができる。平均冷却速度の好ましい範囲は30℃/分以下、より好ましい範囲は15℃/分以下である。また、CaOを含有する最大径2μm以上の介在物個数はO濃度が高いほど多くなるが、O濃度を0.010%以下に制御することで、好ましい介在物の条件(CaOを含有する最大径2μm以上の介在物の個数密度が30個/mm2未満)を満たすことができる。鋳造後は熱間圧延を行い、その後は適宜焼鈍や酸洗・冷間圧延等を行って所定のステンレス鋼を得る。種々の条件で製造した試料をSST試験に供したところ、成分や介在物の形態が上記本発明で規定する条件を満たす試料は発銹が少ないことが分かった。以上説明した要件を備えることによって、本発明の効果を得ることが可能になる。 Casting is performed by melting steel adjusted to have the above-mentioned predetermined components. At this time, casting is performed by controlling the cooling rate in order to cover the periphery of the inclusions containing CaO with M (C, N). The formation temperature of M (C, N) differs depending on the component system, but the slower the cooling rate, the higher the coverage by M (C, N). By controlling the average cooling rate at 1400 to 700 ° C. near the surface of the slab to 50 ° C./min or less during continuous casting, the conditions for inclusions specified above can be satisfied. The preferred range of the average cooling rate is 30 ° C./min or less, and the more preferable range is 15 ° C./min or less. Further, the number of inclusions containing CaO having a maximum diameter of 2 μm or more increases as the O concentration increases, but by controlling the O concentration to 0.010% or less, favorable inclusion conditions (maximum diameter containing CaO). The number density of inclusions of 2 μm or more is less than 30 pieces / mm 2). After casting, hot rolling is performed, and then annealing, pickling, cold rolling, etc. are appropriately performed to obtain a predetermined stainless steel. When the samples prepared under various conditions were subjected to the SST test, it was found that the samples in which the morphology of the components and inclusions satisfied the conditions specified in the present invention had less rusting. By providing the requirements described above, the effect of the present invention can be obtained.
上記した所定の成分になるよう調整した鋼を溶製した溶鋼を連続鋳造によって鋳片とした。連続鋳造中において、鋳片表面近傍の1400~700℃の温度範囲における平均冷却速度を種々の速度に制御して鋳造した。鋳片表面近傍の1400~700℃の温度範囲における平均冷却速度は、伝熱解析を用いた数値計算により評価し、結果を表2に記載した。得られた鋳片を熱間圧延前の鋳片加熱として1200℃×2時間の熱処理を行い、熱間圧延し、更に熱延板焼鈍・酸洗を行い、冷間圧延、焼鈍・酸洗を行うことで、1.0mm厚の冷延板を製造し、介在物測定とSST試験に供した。また、前記で得られた鋳片の一部は鋳片加熱を模擬して1200℃×2時間の熱処理を行い、CaSの生成状況を確認した。
表1に化学成分を示し、表2に鋳片加熱模擬試料のCaS生成状況(最大径5μm以上の介在物20個中のCaS生成個数)、冷延板介在物の測定結果(CaOを含有する最大径≧2μmの介在物個数密度(個/mm2)、M(C,N)部の面積率が40%以上である介在物の個数割合(%))、SST試験結果を示す。表1、表2において、本発明範囲から外れる数値及び本発明の好適な製造条件から外れる数値に下線を付している。
The molten steel obtained by melting the steel adjusted to have the above-mentioned predetermined components was made into slabs by continuous casting. During continuous casting, the average cooling rate in the temperature range of 1400 to 700 ° C. near the surface of the slab was controlled to various rates for casting. The average cooling rate in the temperature range of 1400 to 700 ° C. near the surface of the slab was evaluated by numerical calculation using heat transfer analysis, and the results are shown in Table 2. The obtained slab is heat-treated at 1200 ° C. for 2 hours to heat the slab before hot rolling, then hot-rolled, then hot-rolled and annealed and pickled, and then cold-rolled and annealed and pickled. By doing so, a 1.0 mm thick cold-rolled plate was manufactured and used for inclusion measurement and SST test. In addition, a part of the slabs obtained above was heat-treated at 1200 ° C. for 2 hours in simulation of slab heating, and the state of CaS formation was confirmed.
Table 1 shows the chemical composition, and Table 2 shows the CaS formation status of the slab heating simulated sample (the number of CaS generated in 20 inclusions with a maximum diameter of 5 μm or more) and the measurement results of cold-rolled plate inclusions (containing CaO). The number of inclusions having a maximum diameter ≧ 2 μm (pieces / mm 2 ), the number ratio (%) of inclusions in which the area ratio of the M (C, N) portion is 40% or more), and the SST test results are shown. In Tables 1 and 2, the numerical values outside the scope of the present invention and the numerical values outside the suitable manufacturing conditions of the present invention are underlined.
鋳片加熱模擬試料のCaS生成状況は、適当な断面を切り出して鏡面仕上げで研磨を行い、最大径が5μm以上の介在物20個を無作為に選んでEPMAを用いた元素濃淡マッピングを行って確認し、CaSが生成している例が1個以下であれば良好とした。 For the CaS generation status of the slab heating simulated sample, an appropriate cross section is cut out and polished with a mirror finish, and 20 inclusions with a maximum diameter of 5 μm or more are randomly selected and elemental shading mapping using EPMA is performed. After confirmation, it was considered good if the number of cases in which CaS was generated was 1 or less.
冷延板の介在物測定は図3と同様、鋼板表面を観察する。観察面1の鋼板深さ方向の位置は可能な限り最表層とし、観察のための鏡面仕上げに必要な最小限の研磨を行う。観察面1において、最大径3が2μm以上でCaOを含有する介在物2を無作為に100個以上選択し、酸化物部分とM(C,N)部分の面積を測定し、M(C,N)部が介在物の面積に占める割合(M(C,N)部の面積率(%))を算出し、M(C,N)部の面積率が40%以上の介在物の個数割合を算出した。この際、測定面積を記録することで単位面積当たりの個数を算出した。
SST試験はJIS Z 2371に基づいて、塩溶液として中性塩水噴霧試験を用い、2時間の連続噴霧試験を行い、100cm2あたりの発銹点の個数を計測した。発銹点の個数が5個以下であれば良好とした。
For the measurement of inclusions in the cold-rolled plate, observe the surface of the steel plate in the same manner as in FIG. The position of the
In the SST test, a neutral salt spray test was used as a salt solution based on JIS Z 2371, and a continuous spray test was conducted for 2 hours, and the number of rusting points per 100 cm 2 was measured. It was considered good if the number of rusting points was 5 or less.
表2に示すように、符号B1~B13は鋼成分及び鋼板での介在物形態が本発明の条件を満たしていたため、鋳片でのCaSの生成も少なく、鋼板のSST試験における耐発銹性が良好だった。 As shown in Table 2, since the steel components and inclusion morphology in the steel sheet satisfy the conditions of the present invention, CaS is less generated in the slabs, and the rust resistance in the SST test of the steel sheet is small. Was good.
符号b1はS濃度が本発明範囲を上限に外れ、結果として鋼板の介在物形態は本発明の条件を満たしていたが、鋳片加熱模擬試料の観察結果から明らかなようにCaSが存在していたため、SST試験で多数の発銹が観察された。S濃度が高く、凝固前あるいは凝固中段階でCaSが生成したと推定される。 In reference numeral b1, the S concentration was out of the upper limit of the range of the present invention, and as a result, the inclusion form of the steel sheet satisfied the condition of the present invention, but CaS was present as is clear from the observation result of the slab heating simulated sample. Therefore, a large number of rusts were observed in the SST test. It is presumed that the S concentration is high and CaS is generated before or during the coagulation stage.
符号b2はN濃度が低かったため、介在物のM(C,N)部の合計面積割合が40%以上である介在物の個数割合が低かった。そのためCaS生成を抑制できず、SST試験で多数の発銹が観察された。 Since the N concentration of the symbol b2 was low, the number ratio of inclusions in which the total area ratio of the M (C, N) part of the inclusions was 40% or more was low. Therefore, CaS production could not be suppressed, and a large number of rusting was observed in the SST test.
符号b3はCa濃度が本発明範囲を上限に外れ、結果として鋼板の介在物形態は本発明の条件を満たしていたが、鋳片加熱模擬試料の観察結果から明らかなようにCaSが存在していたため、SST試験で多数の発銹が観察された。Ca濃度が高く、凝固前あるいは凝固中段階でCaSが生成したと推定される。 In reference numeral b3, the Ca concentration was out of the upper limit of the range of the present invention, and as a result, the inclusion form of the steel plate satisfied the condition of the present invention, but CaS was present as is clear from the observation result of the slab heating simulated sample. Therefore, a large number of scabs were observed in the SST test. It is presumed that the Ca concentration is high and CaS is produced before or during the coagulation stage.
符号b4はO濃度が本発明範囲を上限に外れ、結果として鋼板の介在物のM(C,N)部の合計面積割合が40%以上である介在物の個数割合が低かったため、多数の発銹が観察された。なお、CaOを含有する最大径2μm以上の介在物の個数密度が高かった。 Reference numeral b4 indicates that the O concentration is out of the upper limit of the range of the present invention, and as a result, the total area ratio of the M (C, N) portions of the inclusions in the steel sheet is 40% or more, and the number ratio of inclusions is low. Rust was observed. The number density of inclusions containing CaO having a maximum diameter of 2 μm or more was high.
符号b5はTi濃度が高すぎたため、鋳造中に多量のTiNが生成したためノズルが閉塞して鋳造を中止した。なお、途中まで得られた鋳片を加工したところ、加工性が非常に悪く、またTiN起因の表面疵が多量に生じた。 The Ti concentration of the symbol b5 was too high, and a large amount of TiN was generated during casting, so that the nozzle was blocked and casting was stopped. When the slab obtained halfway was processed, the workability was very poor and a large amount of surface defects caused by TiN occurred.
符号b6は鋳片表面近傍の1400~700℃の温度範囲における平均冷却速度が速かったため、M(C,N)が十分に生成せずに被覆率が低かったことに起因してM(C,N)部の面積率が40%以上の介在物の個数割合が低かった。そのためCaSが生成して多数の発銹が観察された。 Reference numeral b6 is due to the fact that the average cooling rate in the temperature range of 1400 to 700 ° C. near the surface of the slab was high, so that M (C, N) was not sufficiently generated and the coverage was low. The ratio of the number of inclusions having an area ratio of 40% or more in the N) portion was low. Therefore, CaS was generated and a large number of rusts were observed.
1 観察面
2 介在物
3 最大径
4 圧延方向
5 厚み方向
6 板幅方向
7 鋼板表面
8 M(C,N)部
Claims (3)
C:0.001~0.02%、
Si:0.02~1.5%、
Mn:1.5%以下、
P:0.040%以下、
S:0.006%以下、
Cr:10~25%、
Al:0.01~0.20%、
O:0.0005~0.010%、
N:0.005~0.025%、
Ca:0.0030%以下
を含有し、更に
Ti:0.35%以下、
Nb:0.70%以下
の一方又は両方を含有し、残部Feおよび不純物からなり、
鋼表面において、CaOを含有する最大径2μm以上の介在物のうち、外周部に1種または2種以上のM(C,N)を伴い、かつM(C,N)部の面積率が40%以上である介在物の個数割合が70%以上である
ことを特徴とするフェライト系ステンレス鋼。
ここで、M(C,N)は元素Mの炭窒化物を表し、MはTi、Nb、Crから選ばれる1種または2種以上の元素であり、その他元素の合計として1%未満を含んでも良い。 The chemical composition is mass%,
C: 0.001 to 0.02%,
Si: 0.02 to 1.5%,
Mn: 1.5% or less,
P: 0.040% or less,
S: 0.006% or less,
Cr: 10 to 25%,
Al: 0.01-0.20%,
O: 0.0005-0.010%,
N: 0.005 to 0.025%,
Ca: contains 0.0030% or less, and Ti: 0.35% or less,
Nb: Contains one or both of 0.70% or less, and consists of the balance Fe and impurities.
On the steel surface, among inclusions having a maximum diameter of 2 μm or more containing CaO, one or more types of M (C, N) are accompanied on the outer peripheral portion, and the area ratio of the M (C, N) portion is 40. Ferritic stainless steel characterized in that the number ratio of inclusions of% or more is 70% or more.
Here, M (C, N) represents a carbonitride of the element M, and M is one or more elements selected from Ti, Nb, and Cr, and the total of the other elements is less than 1%. But it's okay.
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