Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• the present invention relates to austenite stainless steel in which the content of Ni indispensable to austenite stainless steel is reduced to the minimum quantity thereof and which is excellent in the workability, the season cracking resistance, the corrosion resistance and the stress corrosion cracking resistance, not detracting from the surface quality thereof.
• Austenite stainless steel such as typically SUS304 has been much used in various fields of western tableware and pans, kitchen utensils, building materials, household electrical appliances and others, as having excellent workability and corrosion resistance. In these applications, not only workability and corrosion resistance but also designability is often required; and the nickel silver color peculiar to stainless steel is essential for industrial product materials in point of both interior and exterior aspects.
• SUS301 having an Ni content of more than 6 % and SUS304 having an Ni content of at least 8 % could not be used in some applications owing to their costs.
• 200-series stainless steel-based steels have been being provided as a substituent for 300-series stainless steel (Patent References 1 to 6).
• Patent References 7 and 8 the cost reduction is attained, after all with sacrificing any of the corrosion resistance and the workability that are excellent characteristics of 300-series stainless steel.
• the austenite phase is excessively unstable in drawing, and after worked, season cracking may occur therein. Accordingly, the steel could not gain a sufficient degree of workability.
• the steel of Patent Reference 8 is disadvantageous in point of the corrosion resistance as its Cr content is low; and according to the present inventors' investigations, it has been known that, since the solid solution content of C and N is small therein, the steel is also disadvantageous in point of obtaining sufficient ductility.
• the present invention is to provide an Ni-reduced austenite stainless steel having excellent recyclability with no problem of producibility depression to be caused by the surface property thereof and having good workability, season cracking resistance, corrosion resistance and stress corrosion cracking resistance in point of the material characteristics thereof.
• an Ni-reduced austenite stainless steel which has a reduced Mn content of at most 2.5 % by mass and has good recyclability with no problem of producibility depression to be caused by the surface property thereof, and which has good corrosion resistance, workability and stress corrosion cracking resistance.
• this steel may be used in various applications. Accordingly, the invention may solve the problem of remarkable rise of Ni material cost both in the aspect of material expense and in the aspect of product quality.
• the present inventors have assiduously studied for the purpose of attaining the above-mentioned object of providing an austenite stainless steel having a reduced Ni content of at most 6 % by mass, and have reached the following findings. [Workability, Shapability]
• austenite stainless steel there is known a phenomenon of transformation induced plasticity (TRIP) of such that the austenite phase in the steel is transformed into a hard, work-induced martensite ( ⁇ ') phase owing to work strain whereby the strain is dispersed to be in uniform strain distribution.
• TRIP transformation induced plasticity
• the Md30 of formula (1) which is the index of the austenite stability, is at least -50, and the content of C is more than 0.05 % by mass, and when the content of (C+N) is at least 0.15 % by mass, then the ⁇ ' phase having a suitable intensity effective for ductility improvement may be suitably formed.
• austenite stainless steel such as typically SUS304 undergoes delayed fracture often referred to as season cracking, after deep drawn and left as such in an atmospheric environment at room temperature for a few hours to a few days.
• Ni-reduced austenite stainless steel also undergoes the same phenomenon.
• the present inventors have found that, when Md30 defined by the formula
• a pitting potential as one index of corrosion resistance evaluation generally depends on the Cr content. Merely for increasing the pitting potential alone, the lowermost limit of the Cr content of Ni-reduced austenite stainless steel may be increased. However, in the invention, the Ni-reduced austenite stainless steel must have good workability and season cracking resistance, and therefore, in this, the lowermost limit of the Cr content is defined to be 16 % by mass with taking a good balance to the other alloying ingredients, as will be described hereinunder.
• Austenite stainless steel is often problematic in point of the stress corrosion cracking at the worked part and the welded part thereof.
• the present inventors have made various studies how to impart stress corrosion cracking resistance to Ni-reduced austenite stainless steel. As a result, the inventors have found that the stress corrosion cracking behavior depends on the stacking fault energy of steel. As a result of further detailed experiments, the inventors have found that, when the stacking fault energy SFE of steel defined by the formula (2) is at least 5, then the steel may prevent the formation of stacking faults in its working mode to have a problem of stress corrosion cracking, and therefore the steel may have excellent stress corrosion cracking resistance.
• Stainless steel is required to have designability in various applications, in addition to corrosion resistance and workability. Accordingly, it is desired to efficiently produce stainless steel having a homogeneous surface, and in its turn, it may greatly contribute toward the production cost reduction.
• AP annealing and pickling
• the homogeneousness of the scale forming in the annealing and the peelability of the scale in the pickling are important factors in determining the producibility.
• the present inventors have found that, when the Mn content is regulated to be at most 2.5 % by mass in a steel having an Ni content of at most 6 % by mass, then the scale removal' in the AP line for the steel can be attained smoothly like that for conventional SUS304 in the same working condition as that for SUS304. Accordingly, the production cost increase owing to the Mn content increase can be prevented. In case where the Mn content is more than 2.5 % by mass, the load in pickling may increase and the cost reduction may be difficult.
• the load increase mechanism in picking may be as follows: Ordinary austenite stainless steel such as typically SUS304 homogeneously forms a Cr oxide on the surface of the steel sheet during annealing, and it may be pickled efficiently.
• the steel having a reduced Ni content of at most 6 % by mass and an Mn content of more than 2.5 % by mass forms an Cr oxide and may often form a Cr-Mn composite oxide unevenly during atmospheric annealing, and the site with the formed Cr oxide having therein and the site with the formed Cr-Mn composite oxide having therein differ in the pickling operability.
• the steel has uneven pickling operability. In order to obtain a favorably-pickled surface of steel under that condition, pickling for a longer period of time is needed, therefore bringing about the load increase in pickling and after all the producibility depression.
• the present inventors have known that the problem can be evaded when the Mn content of the steel which has a reduced Ni content of at most 6 % by mass is less than 2.5 % by mass.
• the increase in the Mn content causes the producibility depression also in bright annealing. Specifically, when the steel is annealed in a reductive atmosphere mainly comprising hydrogen, it may often color in blue yellow. This may be considered as follows: In general, with the increase in the Mn content, the proportion of Mn in the steel sheet surface increases.
• Mn is more readily oxidized at an annealing temperature, and therefore, in the gas atmosphere in bright annealing having a relatively low reducing potency (in which the dew point is relatively high), an Mn oxide film layer may be readily formed on the steel sheet surface, and in many cases, the steel colors in blue yellow.
• the Mn content is defined to be at most 2.5 % by mass, then the coloration to be caused by the Mn oxide film may be evaded, and the steel may have a good surface property on the same level as that of SUS304 not detracting from the production efficiency thereof.
• C and N are elements useful for strengthening the work-induced martensite ( ⁇ ') phase with solid solution strengthening.
• the total content of C and N (hereinafter referred to as " (C+N) content”) is defined to be at least 0.15 % by mass to make sure sufficient ductility by TRIP in formation of the ⁇ ' phase. It is important to secure the C content of more than 0.05 % by mass for stably attaining remarkable ductility improvement.
• the (C+N) content is preferably defined to be at most 0.3 % by mass.
• C is preferably controlled to be at most 0.15 % by mass, more preferably at most 0.1 % by mass.
• N is defined to be at most 0.25 % by mass, but in general, it may be controlled to fall within a range of from 0.04 to 0.2 % by mass.
• Mn is a useful austenite-forming element which is more inexpensive than Ni and which may be substitutable for the function of Ni.
• the Mn content must be at least 0.5 % by mass.
• the Mn content is defined to be at most 2.5 % by mass, preferably less than 2.5 % by mass.
• Ni is an element indispensable for austenite stainless steel; however, in the invention, the steel composition is so planned as to reduce the Ni content as much as possible from the viewpoint of cost reduction. Concretely, the Ni content is reduced to be at most 6 % by mass. It may be less than 6 % by mass. However, for realizing the composition balance capable of satisfying the producibility, the workability and the corrosion resistance within the above-mentioned Mn content range, the Ni content must be at least 3 % by mass.
• Cr is an element indispensable for formation of passive film to secure the corrosion resistance of stainless steel.
• the Cr content is not more than 16 % by mass, then the corrosion resistance required for conventional austenite stainless steel, for which the invention is substitutable, could not be sufficiently secured.
• Cr is a ferrite forming element, addition of too much Cr is unfavorable as resulting in the formation of much ⁇ -ferrite phase at a high temperature and therefore detracting from the hot workability of steel.
• the Cr content in the invention may be up to 25 % by mass. Accordingly, the Cr content is defined to be from more than 16 to 25 % by mass.
• Si is an element useful for deoxidation in steelmaking; however, too much Si content may harden steel and may detract from the workability of steel.
• Si is a ferrite-forming element, too much addition thereof may result in the formation of much ⁇ -ferrite phase at a high temperature, therefore detracting from the hot workability of steel. Accordingly, the Si content is limited to be at most 1 % by mass.
• Cu is an element that inhibits work hardening to be caused by the formation of a work-induced martensite phase, therefore contributing toward softening the austenite stainless steel. Since Cu is an austenite-forming element, the latitude in planning the Ni content in accordance with the increase in the Cu content may broaden, therefore facilitating the Ni-reduced alloy composition planning. Further, Cu is an element extremely effective for increasing the value of SFE, and therefore greatly contributes toward improving the stress corrosion cracking resistance by inhibiting the formation of stacking faults. For sufficiently securing these effects, the Cu content must be at least 0.8 % by mass. However, too much Cu over 4 % by mass may detract from the hot workability of steel. Accordingly, the Cu content is defined to be from 0.8 to 4 % by mass.
• P and S may be mixed in steel as inevitable impurities; and P is allowable in an amount of approximately up to 0.045 % by mass, and S is approximately up to 0.03 % by mass.
• the steel of the invention may be prepared as a melt according to a steelmaking process for ordinary stainless steel. Next, according to a production method for ordinary austenite stainless steel sheets, for example, cold-rolled annealed steel sheets having a thickness of from 0.1 to 3.5 mm may be produced.
• the samples of the invention having a C content of more than 0.05 % by mass and a (C+N) content of at least 0.15 % by mass had a high fracture elongation of at least 40 %.
• the fracture elongation of the steel F having a (C+N) content of less than 0.15 % by mass was 35 % and was low.
• the shaped article was left in air at room temperature for 24 hours, and then this was checked for the presence or absence of cracking at the edge of the shaped cup.
• the season cracking susceptibility limit drawing ratio of each steel was determined according to the following formula (4):
• Db max means the maximum disc diameter (mm) not causing season cracking
• Dp means the punch outer diameter (mm).
• the samples having an Md30 value of at most 10 realized good season cracking resistance, as having a season cracking susceptibility limit drawing ratio of at least 2.0.
• the samples of the invention having an SFE value of at least 5 did not crack even after 24 hours from the start of the test, and showed good stress corrosion cracking resistance.
• the samples of the invention having an Mn content of at most 2.5 % by mass took the pickling time of not longer than 30 seconds, and it is judged that the samples could be well pickled in an ordinary continuous annealing/pickling line with no specific line speed depression.
• the samples of the invention having an Mn content of at most 2.5 % by mass had good coloration resistance, as well clearing the condition of "uppermost dew point temperature of not lower than -50°C", and the samples are free from the problem of producibility depression in bright annealing.

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• Chemical & Material Sciences (AREA)
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• Organic Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)

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• 2007
  • 2007-08-09 JP JP2007207482A patent/JP5014915B2/ja active Active
• 2008
  • 2008-07-25 KR KR1020080073032A patent/KR20090015817A/ko not_active Application Discontinuation
  • 2008-08-01 TW TW097129188A patent/TWI394847B/zh active
  • 2008-08-05 US US12/222,201 patent/US20090041613A1/en not_active Abandoned
  • 2008-08-06 EP EP08014087A patent/EP2025770A1/de not_active Withdrawn
  • 2008-08-07 CN CNA2008101456478A patent/CN101363103A/zh active Pending

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