US20180037970A1 - Metastable austenite stainless steel strip or steel sheet and manufacturing method thereof - Google Patents

Metastable austenite stainless steel strip or steel sheet and manufacturing method thereof Download PDF

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US20180037970A1
US20180037970A1 US15/788,310 US201715788310A US2018037970A1 US 20180037970 A1 US20180037970 A1 US 20180037970A1 US 201715788310 A US201715788310 A US 201715788310A US 2018037970 A1 US2018037970 A1 US 2018037970A1
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stainless steel
steel
steel strip
steel sheet
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Yuta Matsumura
Kyouhei OGAWA
Shinichi Tanaka
Yoshihiro Hosoya
Tatsumi HIRATA
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TOKUSHU KINZOKU EXCEL CO Ltd
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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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/18—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
    • 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
    • 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
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a metastable austenite stainless steel strip or steel sheet having well- ⁇ balanced strength and ductility and a manufacturing method of the same.
  • Functioning members of devices such as smartphones, clamshell computers, and cameras and highly durable frame structure members of automobiles and airplanes are required to be thinned and lightened through a reinforcement process while their workability and size accuracy are maintained satisfactorily. Furthermore, when the devices and machines are miniaturized, a workload of these members increases, and thus, an excellent durability is required to be used repeatedly in a harsh environment.
  • ⁇ -SUS steel and a twinning induced plasticity (TWIP) steel which contain more than 20 mass % of Mn and/or Ni have been developed.
  • TWIP twinning induced plasticity
  • Such steels have a conventional strength to ductility balance of a transformation induced plasticity (TRIP) steel.
  • TRIP transformation induced plasticity
  • such a high strength and high ductility steel requires expensive cost of additional elements and has difficulty to be subjected to a cold rolling process to manufacture a steel strip and a steel sheet.
  • many conventional steels do not contain Cr, corrosion resistance is insufficient, and thus, an antirust process is required.
  • Non Patent Literature 1 With a low alloy TRIP dual phase steel which is a center of attention recently, a result of TS:980 MPa-EL:30% and TS:1180 MPa-EL:25% is obtained (cf. Non Patent Literature 1). However, even the steel with the above property is insufficient. A steel strip or a steel sheet having both a yield strength (YP) of 1400 MPa or more which is required as a structural member and a high ductility has not been realized yet.
  • YiP yield strength
  • Patent Literature 1 JP2002-173742 A disclose, as a method to improve flatness of steel, a manufacturing method of a high strength austenite stainless steel strip with a good flatness, having a Vickers hardness of 400 or more by performing a solution treatment of a stainless steel, then generating a strain induced transformation martensite phase ( ⁇ ′ phase) by cold rolling, and performing a reverse transformation process which generates a ⁇ T phase (reverse transformation austenite phase) of 3 volume % or more in a ⁇ ′ phase.
  • ⁇ ′ phase strain induced transformation martensite phase
  • the amount of ⁇ T phase greatly depends on a temperature, and although it may vary with chemical components, the amount of ⁇ T phase exceeds approximately 60% through a reverse transformation process in a temperature of 500° C. or more, and a strength of 1400 N/mm 2 or more is difficult to achieve. Furthermore, when the reverse transformation process is performed for a short period of time (for example, one to five minutes), ductility is improved to some extent, and when the reverse transformation process is performed for a longer period of time (for example, five to fifteen minutes), the ductility decreases rapidly. That is, the reverse transformation process is a very unstable process, and a steel strip or a steel sheet with a stable mechanical characteristics is difficult to achieve. Furthermore, a gain of 0.2% yield strength is few since carbide precipitation of Cr—C and Mo—C does not progress well. Therefore, with the manufacturing method of Patent Literature 1, a steel of both high strength and high ductility is practically impossible.
  • Patent Literature 2 JP54-120223 A discloses a stainless steel containing components similar to those of the stainless steel strip or steel sheet of the present invention, and it discloses a solution treatment and low temperature annealing in a temperature of 400° C.
  • Patent Literature 2 discloses adding 2.0% or less of Mo (in its specification, only 1.15% of Example 9) as an effective element to increase the corrosion resistance, and Mo is not added as a precipitation strengthening component. Furthermore, with such a small amount of Mo, a precipitation strengthening function in a low temperature heat treatment is difficult to achieve.
  • Patent Literature 3 JP2012-201924 A discloses a stainless steel sheet manufactured through annealing in a temperature of 700 to 1100° C., cold rolling of 10% or more, and aging treatment in 300° C. However, the stainless steel sheet does not contain Mo and a precipitation strengthening function in a low temperature heat treatment with addition of Mo is not achieved.
  • Non-Patent Literature 2 discloses a target steel which aims a balanced tensile strength (TS) and elongation (EL) relationship within a range of 300 to 500° C., and therein, the tensile strength (TS) increases to approximately 1750 N/mm 2 while 0.2% yield strength is approximately 1250 N/mm 2 .
  • the target steel of Non-Patent Literature 2 is a Fe—Cr—C steel with a ⁇ phase as its parent phase, and it is out of the category of metastable austenite stainless steels as in the present invention.
  • SUS304 and SUS301 are metastable austenite stainless steels such as SUS304 and SUS301.
  • SUS301 is a steel which can reduce Ni content to perform strain induced transformation from an austenite ( ⁇ phase) to a martensite ( ⁇ ′ phase) through cold rolling in order to acquire more strength.
  • These types of stainless steels are advantageous in individual characteristic such as strength and workability; however, to achieve 0.2% yield strength (YS) over 1400 N/mm 2 , the elongation (EL) becomes 10% or less and a YS-EL balance (an indexing value by YS ⁇ EL) is approximately 14000.
  • YS-EL balance an indexing value by YS ⁇ EL
  • Inventors of the present application focused on a potential of an ⁇ ′ phase generated by strain induced transformation and tried to raise 0.2% yield strength (YS) of a metastable austenite stainless steel to approximately 1400 N/mm 2 .
  • the inventors have found that by transforming a metallic structure of the stainless steel to an ⁇ ′ phase by cold rolling of 1 to 80% and subjecting the steel to low temperature heat treatment of 250 to 480° C., a diffusion concentration of supersaturated solid solution carbon into a ⁇ phase of a few % in a volume ratio is obtained by using a strain energy accumulated in the ⁇ ′ phase as a driving force, and the adjacent ⁇ ′ phase can be reversely transformed into a ⁇ T phase with the ⁇ phase as a nucleus.
  • the inventors have found that carbides of Cr and Mo are finely precipitated in the ⁇ ′ phase by the heat treatment, and therefore, due to the process strain induced transformation (TRIP) effect by dispersing the ⁇ T phase simultaneously with the further increase in strength, the carbide of 0.2% yield strength (YS) of 1400 N/mm 2 or more and elongation (EL) of 15% or more can be realized. Furthermore, the inventors have found that it is possible to achieve both 0.2% yield strength (YS) of 1550 N/mm 2 or more and elongation (EL) of 23% or more under suitable conditions within the scope of the present invention, it is possible to realize a characteristic in which the value of the YS-EL balance obtained by the following formula (1) exceeds 35,000.
  • TRIP process strain induced transformation
  • ⁇ ′ phase is strain induced martensite phase.
  • ⁇ R phase is retained austenite phase.
  • ⁇ T phase is reversely transformed austenite phase.
  • the present invention provides a steel strip or a steel sheet which has both high strength, high ductility, and high corrosion resistance, and a manufacturing method thereof.
  • a metastable austenite stainless steel strip or steel sheet includes: a mass percent composition of C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5 to 3.5%, Cu: 0.4 to 1.0% with a remaining part of Fe and unavoidable impurities; a dual phase structure of an ⁇ ′ phase and a ⁇ phase where the ⁇ phase is composed of a ⁇ T phase and a ⁇ R phase, a total of the ⁇ T phase and the ⁇ R phase is 15 to 50 volume %, and a ⁇ T phase area ratio defined by the following formula (2) is between 1% and 20% inclusive; and 0.2% yield strength (YS) of 1400 N/mm 2 to 1900 N/mm 2 where a value of YS-EL balance derived from the formula (1) satisfies at least 21,000 to 48,000.
  • a manufacturing method of a metastable austenite stainless steel strip or steel sheet includes the steps of: subjecting cold rolling to a stainless steel strip or steel sheet of the above composition to form a strain induced martensite phase ( ⁇ ′ phase) from an austenite phase ( ⁇ phase); and subjecting a low temperature heat treatment in a temperature of 250 to 480° C. to the stainless steel strip or steel sheet, thereby growing an austenite phase ( ⁇ T phase) from the strain induced martensite phase ( ⁇ ′ phase).
  • ⁇ T phase area ratio (%) 100 ⁇ (total area ratio of ⁇ T phase of entire observation area) (2)
  • the ⁇ ′ phase is a strain induced martensite phase
  • the ⁇ phase is combined the ⁇ T phase and the ⁇ R phase
  • the ⁇ R phase is a reverse transformation austenite phase in which an area per crystal grain is between 5 ⁇ m 2 and 20 ⁇ m 2 inclusive
  • the ⁇ R phase is an austenite phase other than the ⁇ T phase
  • YS is 0.2% yield strength and EL is elongation.
  • the structure including the above phases involves both characteristics of 0.2% yield strength (YS) of 1400 N/mm 2 or more and elongation (EL) of 15% or more.
  • YS 0.2% yield strength
  • EL elongation
  • the inventors consider that the former characteristic is achieved by the ⁇ ′ phase hardened by carbide precipitation of Cr and/or Mo and the latter characteristic is achieved from the TRIP effect of the ⁇ T phase diffused in the ⁇ ′ phase.
  • the stainless steel strip or steel sheet of the present invention is a metastable austenite stainless steel including a mass percent composition of C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5 to 3.5%, Cu: 0.4 to 1.0%.
  • C is added 0.05% or more to achieve a strength required for the strain induced transformation during cold rolling and the ⁇ ′ phase after the transformation.
  • addition of C above 0.15% stabilize the austenite phase and the strain induced transformation during the cold rolling is blocked and secondary workability for punching or the like is deteriorated.
  • the upper limit of C is set to 0.15%.
  • Si is an element necessary in the steel manufacturing as a deoxidizer, and is added 0.05% or more. However, Si above 1% lowers ductility and toughness, and thus, the upper limit of Si is set to 1%.
  • Mn is an element which stabilize the austenite phase as with Ni, and if the large amount thereof is added, a structure having 50% or more strain induced ⁇ ′ phase cannot be achieved through an ordinary cold rolling process.
  • the upper limit of Mn is set to 2%.
  • the lower limit of Mn is not set specifically; however, 0.1% is preferable in order to deal with cracks during a hot rolling process.
  • Cr is added 16% or more in order to achieve the corrosion resistance as a stainless steel.
  • 18% or more Cr stabilizes the austenite phase and a sufficient amount of strain induced transformation ⁇ ′ phase cannot be achieved through an routinely carried out cold rolling process.
  • the upper limit of Cr is set to 18%.
  • Ni is an austenite stabilizing element and a certain amount thereof must be added to maintain the structure before the cold rolling in a metastable austenite state.
  • the lower limit of Ni is 4% in order to achieve the metastable austenite phase after the solution treatment process.
  • Ni above 11 stabilize the austenite phase and 50% or more volume ratio of strain induced transformation ⁇ ′ phase cannot be achieved through an ordinary cold rolling.
  • the upper limit of Ni is set to 11%.
  • Mo is a significant element of the present invention.
  • Mo is known as an element effective to increase pitting corrosion resistance of stainless steels, and in the present invention, is also a precipitation strengthening element which is significant in the low temperature heat treatment.
  • the lower limit of Mo carbide to achieve the precipitation strengthening of ⁇ ′ phase is 2.5% and the upper limit of Mo is set to 3.5% since the amount of Mo exceeding the upper limit value saturates the precipitation strengthening property and the costs of alloy becomes disadvantage.
  • one or more elements such as Ti and/or Al may be added for precipitation strengthening.
  • the amount of additional elements is approximately 0.1 to 3.5% and is set in consideration of a balance with the other elements.
  • Cu of 0.4 to 1.0% in mass % is preferably added. Cu below 0.4% does not show a remarkable corrosion resistance improvement effect, and Cu above 1.0% may cause a problem during the manufacturing process such as cracks in the hot rolling process.
  • P, N, S, and O may be contained in the steel strip and the steel sheet of the present invention.
  • Such impurities do not block the purpose of the present invention as long as the amount thereof falls within a range acceptable in an ordinary manufacturing process.
  • YS yield strength
  • the metastable austenite stainless steel strip or steel sheet with the above metallic structure and characteristics can be manufactured by subjecting cold rolling to the stainless steel strip or steel sheet of the above composition to form a strain induced martensite phase ( ⁇ ′ phase) from an austenite phase ( ⁇ phase) and subjecting a low temperature heat treatment in a temperature of 250 to 480° C. to the stainless steel strip or steel sheet in which the strain induced martensite phase ( ⁇ ′ phase) is formed in order to grow an austenite phase ( ⁇ T phase) from the martensite phase ( ⁇ ′ phase) formed in the strain induced martensite phase formation process.
  • the inventors consider that the above characteristics is achieved in the metastable austenite stainless steel strip or steel sheet through the following mechanism.
  • a low temperature heat treatment is subjected in the metallic structure such that supersaturated solid solution carbon stored in the ⁇ ′ phase is diffused and concentrated into a micro ⁇ R phase which is a core of the reverse transformation using a strain energy accumulated in the ⁇ ′ phase which is transformed from the ⁇ phase during the cold rolling through the strain induced transformation as a driving force and the ⁇ phase grows.
  • the precipitation hardening phenomenon of the ⁇ ′ phase proceeds.
  • the strength of the ⁇ ′ phase and the high ductility of ⁇ phase by the strain induced transformation can be achieved. That is, a value of the YS-EL balance derived from formula (1) satisfies 21,000 or more.
  • the ratio of the ⁇ ′ phase after the cold rolling is less than 50%, the strain energy accumulated in the ⁇ ′ phase is low and insufficient, and thus, the diffusion and concentration of C in the ⁇ phase from the ⁇ ′ phase do not occur. Thus, the cold rolling rate becomes low, and the dislocation density in the ⁇ ′ phase becomes low. Thus, a balance between strength and elongation, that is, a value of the YS-EL balance does not exceed that of conventional materials.
  • Evaluation of the martensite phase ( ⁇ ′ phase) and the austenite phase ( ⁇ phase) of the present invention is performed through an electron backscatter diffraction (EBSD) method.
  • EBSD electron backscatter diffraction
  • the number of crystal grains included in the observation area is at least 1,000, an area of 0.05 ⁇ 0.05 mm with respect to a surface vertical to the rolling direction of the steel material (that is, an RD surface) is observed.
  • An area ratio calculated from a measurement result of the phase where misorientation of 5° or more is defined as a grain boundary is converted into a volume ratio. The same applied to volume %.
  • the metastable austenite stainless steel strip or steel sheet with the above composition and metal structure can exert the characteristics of 0.2% yield strength (YS) of 1400 N/mm 2 and elongate (EL) of 15% or more.
  • YS-EL balance is at least 21,000.
  • 0.2% yield strength (YS) of 1550 N/mm 2 or more and elongation (EL) of 23% or more can be achieved, and thus, a characteristics of the value of YS-EL balance of more than 35,000 can be achieved.
  • the characteristics involve both strength and ductility higher than those of conventional stainless steel strip and steel sheet.
  • the above metallic structure and characteristics of the present invention is achieved through the following manufacturing method, for example.
  • the manufacturing method will be explained in comparison with a routinely carried out manufacturing method of stainless steel.
  • a conventional manufacturing method of a precipitation strengthening metastable austenite stainless steel strip includes rolling a skin pass-finished stainless steel strip obtained through an ordinary method with 85% rolling reduction ratio, for example, and a solid solution heat treatment.
  • the solid solution heat treatment includes a solid solution treatment of the steel strip in a temperature of 1100° C. and water cooling. Then, a martensite transformation treatment is performed. Specifically, the steel strip is rolled with 60% rolling reduction ratio. Then, in order to use precipitation strengthening of intermetallic compounds, a precipitation hardening treatment is performed in a temperature of 475° C., for example.
  • First step In the first step, a cold rolling is subjected to a stainless steel strip having the composition of the present invention (for example, SUS631(17-7PH)) obtained through an ordinary method.
  • the cold rolling is intended to increase a ratio of ⁇ ′ phase by the strain induced transformation.
  • the work ratio is set between 20 and 90%, or preferably, 30% or more.
  • Second step In the second step, a solid solution heat treatment is subjected to the stainless steel strip after the rolling.
  • the heat treatment is intended to perform the reverse transformation of the ⁇ ′ phase after the strain induced transformation by the cold rolling into a ⁇ T phase such that C oversaturated in the ⁇ ′ phase is dispersed evenly in the ⁇ phase, and to uniform the metallic structure in a martensite transformation treatment performed next.
  • a temperature of the solid solution heat treatment differs depending on the composition of the stainless steel strip, the temperature is set between 900° C. and 1150° C., and is preferably 1000° C. or more.
  • a rapid cooling for example, water cooling
  • a martensite transformation treatment is performed.
  • a rolling reduction ratio (work ratio) in this treatment differs depending on the required characteristics, composition of steel strip, and thickness of steel sheet, etc., the ratio is set between 0% and 60% with respect to the steel material or steel strip before the treatment, or preferably between 5% and 40%.
  • the rolling reduction ratio exceeds 60%, the ⁇ phase serving as a nuclear of the reverse transformation becomes insufficient, and the structure of the scope of the present invention can not be obtained by the subsequent reverse transformation treatment.
  • a low temperature heat treatment is subjected to the steel strip or steel sheet subjected to the martensite transformation treatment corresponding to the required characteristics in the third step where the treatment temperature is set between 250° C. and 480° C., or preferably between 300° C. and 450° C. If the temperature is below 250° C., the diffusion and concentration of the supersaturated solid solution carbon in the ⁇ ′ phase do not occur sufficiently, and the ⁇ phase does not grow. Thus, improvement of a balance of strength and ductility cannot be expected. Furthermore, the temperature above 480° C. is close to a temperature where the solid solution begins, and thus, the diffusion of the supersaturated solid solution carbon in the ⁇ ′ phase is promoted, and stabilized ⁇ phase grows excessively.
  • the steel strip or steel sheet manufactured through the first to fourth steps can exert an improved balance of strength (YS) and elongation (EL) with a change of the ratio of ⁇ ′ phase to ⁇ phase, and the characteristics of the present invention can be achieved.
  • a heat treatment is performed in a lower temperature (for example, 250 to 300° C.) as compared to the temperature used for a metastable austenite stainless steel.
  • the inventors have found that a high strength and a high ductility can be achieved at the same time by using increase of ⁇ T phase by the low temperature heat treatment and carbide precipitation by low temperature heat treatment.
  • the inventors have found that when the precipitation hardening heat treatment is subjected at a temperature (for example, 500° C.) which is routinely carried out after forming into a desired shape, the diffusion of solute atoms is promoted to precipitate intermetallic compounds is accelerated, and further strength increase can be expected.
  • a temperature for example, 500° C.
  • PH stainless steels such as SUS631 described as a metastable austenite stainless steel strip or steel sheet excellent in balance between strength and ductility.
  • the metastable austenite stainless steel strip or steel sheet in which a value of the YS-EL balance exceeds at least 21,000 can be manufactured.
  • the manufacturing method of the present invention without largely deviating from the scope of the ordinary secondary work processing step and without significantly increasing the production costs and environmental load, it is possible to obtain a stainless steel strip or steel sheet simultaneously having two characteristics which can not be attained by the conventional method can be produced. Furthermore, according to the manufacturing method of the present invention, the manufacturing steps shown in the first step and the second step may be repeated depending on the state of the raw material and thereafter the martensite transformation treatment of the third step is performed.
  • the stainless steel strip or steel sheet of the present invention can be applied to a component which requires extremely high strength in terms of structure which can not be realized by a conventional high strength material and which enables design of components of more complicated shape.
  • the metastable austenite stainless steel strip which is used as a base largely contains Cr and Ni and has superior corrosion resistance to high strength and high ductility materials such as steel plates for automobile, and thus, an antirust surface treatment after the steel manufacturing process may be unnecessary. In this way, it can be expected to be applied not only to strength and ductility but also to applications of the technical field requiring corrosion resistance.
  • the metastable austenitic stainless steel strip of the present invention not only has a high 0.2% proof stress (YS) exceeding 1400 N/mm 2 but also obtains elongation (EL) exceeding 15% at the same time.
  • FIG. 1 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 1 described in Table 2 below.
  • FIG. 2 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 2 described in Table 2 below.
  • FIG. 3 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 3 described in Table 2 below.
  • FIG. 4 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 4 described in Table 2 below.
  • FIG. 5 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 5 described in Table 2 below.
  • FIG. 6 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 6 described in Table 2 below.
  • FIG. 7 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 7 described in Table 2 below.
  • FIG. 8 shows chronological changes of YS ⁇ EL values on the basis of temperatures of a low temperature heat treatment where a sample of steel 1 described in the Table 1 is used. Note that a dotted line indicates a case where a low temperature heat treatment time is 15 minutes, a solid line indicates a case where the time is 60 minutes, and a single dashed line indicates a case where the time is 360 minutes.
  • FIG. 9 shows changes of YS ⁇ EL values in temperatures on the basis of periods of a low temperature heat treatment where a sample of steel 1 described in the Table 1 is used. Note that a dotted line indicates a case where a temperature of a low temperature heat treatment is 300° C., a solid line indicates a case where the temperature is 400° C., and a single dashed line indicates a case where the temperature is 500° C.
  • Example steel 1 and comparative steels 2 to 4 with different amounts of Mo were prepared. Chemical compositions thereof are shown in Table 1.
  • steels samples of Identification 1 to 5 having the metallic structure within the scope of the present invention and steels having metallic structures outside the scope of the present invention (samples of Identification 6 and 7) were manufactured.
  • the metallic structures of the steels are shown in Table 2.
  • manufacturing conditions of the steels are shown in Table 3.
  • HV hardness
  • Ts tensile strength
  • YS 0.2% yield strength
  • EL elongation
  • step 1 Order of step 1 2 6 First Second 3 4 5 Low temperature 0 step step Third Fourth Fifth heat treatment Material Cold Heat step step step step ⁇ 3 thickness rolling treatment Cold Heat Cold Low temperature Identification Acceptance ⁇ 1 ⁇ 2 rolling treatment rolling heat treatment 1 1 mm 0.45 mm 1050 ⁇ 1150° C. 0.2 mm 1050 ⁇ 1150° C. 0.15 mm 250° C. 2 1 mm 0.45 mm 1050 ⁇ 1150° C. 0.2 mm 1050 ⁇ 1150° C. 0.15 mm 300° C. 3 1 mm 0.45 mm 1050 ⁇ 1150° C. 0.2 mm 1050 ⁇ 1150° C. 0.15 mm 300° C. 4 1 mm 0.45 mm 1050 ⁇ 1150° C.
  • ⁇ 1 Temperature of cold rolling is within range of temperature of routinely carried out cold rolling, and cold rolling is performed below transformation point of various materials.
  • ⁇ 2 Heating time in heat treatment process is set using time reaching a predetermined temperature as reference based on characteristics of heat treatment equipment.
  • ⁇ 3 Heating time of low temperature treatment is set in order to obtain intended metallic structure and characteristics.
  • FIGS. 1 to 7 show metallic structure images of these samples of Identifications 1 to 7, respectively.
  • steel 1 having the composition within the scope of the present invention and steels 2 to 4 having the composition out of the scope of the present invention were prepared as in Table 1, and stainless steel strips were manufactured on the basis of various manufacturing conditions of Table 6.
  • the metallic structures thereof are shown in Table 5, and their characteristics are shown in Table 7.
  • a mark * indicates that the value therewith is out of the scope of the present invention.
  • Temperature of cold rolling is within range of temperature of routinely carried out cold rolling, and cold rolling is performed below transformation point of various materials.
  • ⁇ 2 Heating time in heat treatment process is set using time reaching a predetermined temperature as reference based on characteristics of heat treatment equipment.
  • ⁇ 3 Heating time of low temperature treatment is set in order to obtain intended metallic metal structure and characteristics.
  • FIG. 8 shows chronological changes of YS ⁇ EL values on the basis of a low temperature heat treatment temperature where the steps shown in Table 6 were performed using the sample of the example steel 1.
  • FIG. 9 is a graph showing the change in YS ⁇ EL value for each temperature according to the low temperature heat treatment time when the step shown in Table 6 was performed using the sample of the example steel 1.
  • the YS ⁇ EL value is stable at a low level at a value of 22,000 or more at 300° C. and the YS ⁇ EL value at a high level at a value of 29,000 or more at 400° C. In contrast, at 500° C., the YS ⁇ EL value sharply decreases in the range of about 37,000 to 20,000 as the low-temperature heat treatment time becomes longer.
  • the base of the present invention is a metastable austenite stainless steel including a mass percent composition of C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5 to 3.5%, Cu: 0.4 to 1.0%.
  • 50% or more of strain induced martensite phase ( ⁇ ′ phase) as a parent phase is obtained by cold rolling, and the strain induced ⁇ ′ phase obtained, preferably, through a low temperature treatment performed in a temperature between 250° C. and 480° C.
  • ⁇ T phase+ ⁇ R phase form a dual phase structure of the stainless steel strip or steel sheet where a ⁇ T phase area ratio defined by the following formula (2) is between 1% and 20% inclusive, and a remaining phase is a metal structure composed of ⁇ ′ and ⁇ R .
  • a method of performing the reverse transformation of a metal structure of a conventional steel in which Ni and Mn are 11% or less through a low temperature heat treatment performed in a temperature of 480° C. or less is a novel technique. Furthermore, the structure obtained from the above method can achieve 0.2% yield strength (YS) of 1400 N/mm 2 or more and exerted elongation (EL) of 15% or more in the ⁇ phase.
  • the metastable austenite stainless steel strip which is used as a base largely contains Cr and Ni and has superior corrosion resistance to conventional iron-based high strength and high ductility materials. That is, the present invention can be expected to not only achieve high strength and ductility but also be used in the technical field where the corrosion resistance is required. Furthermore, if hardness is required, a stainless steel strip or steel sheet having the above characteristics and 450 or more Vickers hardness can be achieved.

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