Description:TECHNICAL FIELD
[0001] The present disclosure relates to the field of duplex stainless steels. More particularly, it relates to TRIP-enhanced lean duplex stainless steel with improved mechanical and formability properties, and method for production thereof.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art
[0003] As is well known in the art, duplex stainless steels (DSS) are a family of stainless steels. These are commonly known as duplex (or austenitic-ferritic) grades because their metallurgical structure consists of two phases, austenite (face-centered cubic lattice) and ferrite (body centered cubic lattice) in roughly equal proportions. Duplex stainless steels are, generally, designed to provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than conventional austenitic stainless steels.
[0004] Duplex stainless steels with equal volume fractions of ferrite (a) and austenite phases (?) are often use in demanding applications like oil, gas, paper and pulp, desalination, chemical processing and offshore industries owing to their exceptional properties like strength, toughness, chloride stress corrosion cracking (CSCC) resistance, weldability and comparable costs as compared to the conventional austenitic stainless steels. In recent years, the rising/ fluctuating cost/prices and limited resources of nickel have compelled stainless steel manufacturers to look for cheaper alternatives, thereby fuelling the development of cost-effective and cost-competitive lean duplex stainless steels (LDSS). Commercial grades of LDSS such as UNS S32101 and UNS S32304 possess ultimate tensile strength levels of up to 600 – 700 MPa with total elongations of up to 30%. Increasing strength and ductility in these steel is challenging and will widen market usage.
[0005] There is, therefore, a need in the art to provide an improved lean duplex stainless steel with superior mechanical and formability properties, and method for production thereof.

OBJECTS OF THE PRESENT DISCLOSURE
[0006] Some of the objects of the present disclosure, which at least one embodiment herein satisfies are as listed herein below.
[0007] An object of the present disclosure is to provide an improved lean duplex stainless steel.
[0008] Another object of the present disclosure is to provide a TRIP-enhanced lean duplex stainless steel with improved mechanical and formability properties.
[0009] Another object of the present disclosure is to provide a method for production of lean duplex stainless steel with improved mechanical and formability properties.
[0010] An object of the present disclosure is to provide an improved duplex stainless steel which has high strength and excellent corrosion resistance comparable to conventional duplex stainless steels.
[0011] An object of the present disclosure is to provide a simple and cost effective method which can be easily implemented for production of TRIP-enhanced lean duplex stainless steel with improved mechanical and formability properties.
[0012] These and other objects of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.

SUMMARY
[0013] Aspects of the present disclosure relates to duplex stainless steels. More particularly, it relates to a TRIP-enhanced lean duplex stainless steel with improved mechanical and formability properties, and method for production thereof. In an aspect, the present disclosure provides an innovative thermo-mechanical processing (TMP) methodology for attainment of this seemingly unlikely combination of properties through experimental cold rolling and short annealing treatment.
[0014] In an embodiment, the present disclosure provides a lean duplex stainless steel having composition, in weight percentage (%), comprising carbon (C) in amount ranging from 0.01 to 0.12%, silicon (Si) in amount ranging from 0.10 to 0.5%, manganese (Mn) in amount ranging from 4 to 8%, sulphur (S) in amount ranging from 0 to 0.04%, phosphorus (P) in amount ranging from 0 to 0.04%, chromium (Cr) in amount ranging from 18 to 23%, nickel (Ni) in amount ranging from 0.1 to 0.60%, copper (Cu) in amount ranging from 0.1 to 0.60%, nitrogen (N) in amount ranging from 0.1 to 0.5%, and balance Iron (Fe).
[0015] In an embodiment, the lean duplex steel includes about 43 to 46 volume% of ferrite (fe).
[0016] In another embodiment, the lean duplex steel can include includes inevitable impurities.
[0017] In another embodiment, the lean duplex steel can include molybdenum (Mo) in amount ranging from 0.1 to 1.0%.
[0018] Another aspect of the present disclosure provides method for producing a lean duplex stainless steel that can include composition, in weight percentage (%), including carbon (C) in amount ranging from 0.01 to 0.12%, silicon (Si) in amount ranging from 0.10 to 0.5%, manganese (Mn) in amount ranging from 4 to 8%, sulphur (S) in amount ranging from 0 to 0.04%, phosphorus (P) in amount ranging from 0 to 0.04%, chromium (Cr) in amount ranging from 18 to 23%, nickel (Ni) in amount ranging from 0.1 to 0.60%, copper (Cu) in amount ranging from 0.1 to 0.60%, nitrogen (N) in amount ranging from 0.1 to 0.5%, and balance Iron (Fe). The method can includes step of: heating and soaking a steels ingot at a first predefined temperature for a first predefined period of time; performing hot rolling on the heated steels ingot to reduce thickness of the heated steels ingot to a first predefined thickness strip; annealing the hot rolled steel strip at a second predefined temperature for a second predefined period of time; quenching the annealed steels strip; performing cold rolling on the quenched steel strip to reduce thickness of the quenched steel strip to a second predefined thickness; and annealing the cold rolled steel strip at a third predefined temperature for a third predefined period of time.
[0019] In an embodiment, the lean duplex steel comprises about 43 to 46 volume% of ferrite (fe). In another embodiment, the lean duplex steel can include includes inevitable impurities.
[0020] In another embodiment, the lean duplex steel comprises molybdenum (Mo) in amount ranging from 0.1 to 1.0%.
[0021] In an embodiment, the first predefined temperature can be in a range of 1150 to 1250 oC and the first predefined period of time can be about 3 hours. The second predefined temperature is in a range of 1000 to 1100 oCand the second predefined period of time can be about 2 hours. The third predefined temperature can be in a range of 1000 to 1100 oC and the third predefined period of time can be about 2 to 10 minutes.
[0022] In an embodiment, the hot rolling can be performed to reduce thickness of the steels ingot/slab from 100 mm square cross-sectioned to 6 mm strip. The cold rolling can be performed to further reduce thickness of the strip from 6mm to in a range of 1.22 to 0.8 mm.
[0023] In an embodiment, the lean duplex stainless steel (both without Mo and Mo added) may have yield strength in the range of 400 MPa to 450 MPa, in hot rolled annealed condition and 500 MPa to 550 MPa, in cold rolled annealed condition. Ultimate tensile strength in the range of 600 MPa to 900 MPa in hot rolled annealed condition and 900 MPa to 1100 MPa, in cold rolled annealed condition.
[0024] In an embodiment, the lean duplex stainless steel may have elongation in the range of 45 to 50% in hot rolled annealed condition, and elongation in the range of 60 to 70% in cold rolled annealed condition.
[0025] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure. The diagrams are for illustration only, which thus is not a limitation of the present disclosure.
[0027] FIG. 1 illustrates an exemplary flow diagram representation of the proposed method for producing a lean duplex stainless steel with improved mechanical and formability properties, in accordance with embodiments of the present disclosure.
[0028] FIG. 2 illustrates an exemplary thermocal thermo-chemical software thermo-chemical software analysis showing thermodynamic equilibrium and stability of various phases in LDSS-1 stainless steel as a function of temperature, in accordance with embodiments of the present disclosure.
[0029] FIG. 3 illustrates an exemplary thermocal thermo-chemical software thermo-chemical software analysis showing thermodynamic equilibrium and stability of various phases in LDSS-2 stainless steel as a function of temperature, in accordance with embodiments of the present disclosure.
[0030] FIG. 4 illustrates an exemplary thermocal thermo-chemical software thermo-chemical software analysis showing thermodynamic equilibrium and stability of various phases in LDSS-3 stainless steel as a function of temperature, in accordance with embodiments of the present disclosure.
[0031] FIGs. 5A to 5C illustrate exemplary light optical micrographs of hot Lean duplex stainless steels showing ferrite (?) manifesting as darker, continuous phase and austenite (?) as lighter, discontinuous phase: (a) LDSS-1 (b) LDSS-2 (c) LDSS-3 (Elongated grains and crystallographic texture/ anisotropy are evident in micrographs, Electrolytic etch, 20% NaOH, 3V dc, 30s), in accordance with embodiments of the present disclosure.
[0032] FIGs. 6A to 6C illustrate exemplary secondary electron images of hot Lean duplex stainless steels showing ferrite (?) as a continuous phase and austenite (?) as discrete phase with convex boundaries: (a) LDSS-1 (b) LDSS-2 (c) LDSS-3 (Elongated grains and crystallographic texture/ anisotropy are evident in micrographs, Electrolytic etch, 20% NaOH, 3V dc, 30 s), in accordance with embodiments of the present disclosure.
[0033] FIG. 7 illustrates an exemplary volume fraction of ??-martensite (SIM) as a function of cold reduction in Lean duplex stainless steels at ambient temperature of cold rolling, in accordance with embodiments of the present disclosure.
[0034] FIG. 8A to 8C illustrate exemplary light optical micrographs of cold Lean duplex stainless steels showing ferrite (?) manifesting as darker, continuous phase and austenite (?) as lighter, discontinuous phase: (a) LDSS-1 (b) LDSS-2 (c) LDSS-3 (Elongated grains and crystallographic texture/ anisotropy are evident in micrographs, Electrolytic etch, 20% NaOH, 3V dc, 30 s), in accordance with embodiments of the present disclosure.
[0035] FIG. 9 illustrates a typical polarization plots of cold rolled Lean duplex stainless steels in 3.5% NaCl superimposed with plots for AISI 304 L and AISI 316 L for comparison, in accordance with embodiments of the present disclosure.
[0036] FIG. 10 illustrates an exemplary pitting potentials of cold rolled Lean duplex stainless steels in 3.5% NaCl vis-à-vis AISI 304 L and AISI 316 L austenitic stainless steels, in accordance with embodiments of the present disclosure
[0037] FIG. 11 illustrates an exemplary Average Plastic Strain Ratio (rm) of Cold rolled annealed LDSS stainless steels and commercially produced 316L SS stainless, in accordance with embodiments of the present disclosure.
[0038] FIG. 12 illustrates an exemplary Planar Anisotropy (?r) measurements for of Cold rolled annealed LDSS stainless steels and commercially produced 316L SS stainless, in accordance with embodiments of the present disclosure.
[0039] FIGs. 13A to 13C illustrate exemplary microtectural analysis of Hot rolled annealed LDSS-1 stainless steels showing formation of 54% ? fiber in FCC phase confirming recrystallization and weak ? fiber (14%) in BCC phase suggesting partial recrystallization (a) Phase Map (b) ?1 450 section of FCC phase and (c) ?1 450 section of BCC phase, in accordance with embodiments of the present disclosure.
[0040] FIGs. 14A to 14C illustrate exemplary microtectural analysis of Hot rolled annealed LDSS-2 stainless steels showing weak formation of ? fiber in FCC phase (6%) and BCC phase (8%) (a) Phase Map (b) ?1 450 section of FCC phase and (c) ?1 450 section of BCC phase, in accordance with embodiments of the present disclosure.
[0041] FIGs. 15A to 15C illustrate exemplary microtectural analysis of Hot rolled annealed LDSS-3 stainless steels showing formation of 46% ? fiber in FCC phase confirming recrystallization and weak ? fiber (6%) in BCC phase suggesting partial recrystallization (a) Phase Map (b) ?1 450 section of FCC phase and (c) ?1 450 section of BCC phase, in accordance with embodiments of the present disclosure.
[0042] FIGs. 16A to 16C illustrate typical microtectural analysis of 80% cold rolled annealed LDSS-1 stainless steels showing 36% ? in fiber FCC phase confirming full recrystallization and 22% ? fiber in BCC confirming partial recrystallization (a) Phase Map (b) ?1 450 section of FCC phase and (c) ?1 450 section of BCC phase, in accordance with embodiments of the present disclosure.
[0043] FIGs. 17A to 17C illustrate typical microtectural analysis of 80% cold rolled annealed LDSS-2 stainless steels showing 16% ? in fiber FCC phase confirming full recrystallization and 8% ? fiber in BCC confirming partial recrystallization (a) Phase Map (b) ?1 450 section of FCC phase and (c) ?1 450 section of BCC phase, in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION
[0044] In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be apparent to one skilled in the art that embodiments of the present invention may be practiced without some of these specific details.
[0045] Various methods described herein may be practiced by combining one or more machine-readable storage media containing the code according to the present invention with appropriate standard computer hardware to execute the code contained therein. An apparatus for practicing various embodiments of the present invention may involve one or more computers (or one or more processors within a single computer) and storage systems containing or having network access to computer program(s) coded in accordance with various methods described herein, and the method steps of the invention could be accomplished by engine s, routines, subroutines, or subparts of a computer program product.
[0046] If the specification states a component or feature “may”, “can”, “could”, or “might” be included or have a characteristic, that particular component or feature is not required to be included or have the characteristic.
[0047] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0048] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0049] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all groups used in the appended claims.
[0050] Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. These embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those of ordinary skill in the art. Moreover, all statements herein reciting embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future (i.e., any elements developed that perform the same function, regardless of structure).
[0051] Embodiments explained herein relate to duplex stainless steels. More particularly, it relates to a TRIP-enhanced lean duplex stainless steel with improved mechanical and formability properties, and method for production thereof. The present disclosure provides an innovative thermo-mechanical processing (TMP) methodology for attainment of this seemingly unlikely combination of properties through experimental cold rolling and short annealing treatment.
[0052] In an embodiment, the present disclosure provides a lean duplex stainless steel. The lean duplex stainless steel may have composition, in weight percentage (%), comprising carbon (C) in amount ranging from 0.01 to 0.12%, silicon (Si) in amount ranging from 0.10 to 0.5%, manganese (Mn) in amount ranging from 4 to 8%, sulphur (S) in amount ranging from 0 to 0.04%, phosphorus (P) in amount ranging from 0 to 0.04%, chromium (Cr) in amount ranging from 18 to 23%, nickel (Ni) in amount ranging from 0.1 to 0.60%, copper (Cu) in amount ranging from 0.1 to 0.60%, nitrogen (N) in amount ranging from 0.1 to 0.5%, and balance Iron (Fe).
[0053] In an embodiment, the lean duplex steel includes about 43 to 46 volume% of ferrite (fe). In another embodiment, the lean duplex steel can include includes inevitable impurities.
[0054] In another embodiment, the lean duplex steel can include molybdenum (Mo) in amount ranging from 0.1 to 1.0% by weight of the entire composition.
[0055] In an embodiment, the proposed lean duplex steel poses enhance the mechanical characteristics, such as yield strength, ultimate tensile strength, elongation, corrosion rate, pitting resistance, formability with plastic strain ratio, charpy impact value, and the likes.
[0056] Referring to FIG. 1, where a flow diagram of the proposed method for producing a lean duplex stainless steelis shown, where the lean duplex stainless steel may have composition, in weight percentage (%), including carbon (C) in amount ranging from 0.01 to 0.12%, silicon (Si) in amount ranging from 0.10 to 0.5%, manganese (Mn) in amount ranging from 4 to 8%, sulphur (S) in amount ranging from 0 to 0.04%, phosphorus (P) in amount ranging from 0 to 0.04%, chromium (Cr) in amount ranging from 18 to 23%, nickel (Ni) in amount ranging from 0.1 to 0.60%, copper (Cu) in amount ranging from 0.1 to 0.60%, nitrogen (N) in amount ranging from 0.1 to 0.5%, and balance Iron (Fe).
[0057] In an embodiment, the lean duplex steel comprises about 43 to 46 volume% of ferrite (fe), and other inevitable inevitable impurities.
[0058] In another embodiment, the lean duplex steel comprises molybdenum (Mo) in amount ranging from 0.1 to 1.0%.
[0059] As shown in FIG. 1, the proposed method 100 can include at a step 102,heating and soaking a steels ingot at a first predefined temperature for a first predefined period of time. In an embodiment, heating can be performed in a furnace. The first predefined temperature can be in a range of 1150 to 1250 oC and the first predefined period of time can be about 3 hours.
[0060] In an embodiment, the method 100 can include at a step 104, performing hot rolling on the heated steels ingot to reduce thickness of the heated steels ingot to a first predefined thickness, which can be 6mm. In an embodiment, the hot rolling can be carried out in 2 to 3 rolling campaigns to 16 and 6 mm strip with finish rolling temperatures of 950-980 oC to avoid edge cracking. In an exemplary embodiment, a 100 mm square cross-sectioned ingot can be initially hot-rolled to 50 mm plate using 4-pass draft schedule including 100?95?78?65?50 mm. The 50 mm plate can be reheated to 1150 to 1250 oC for 2 hours and hot rolled to 16mm strips using a 3-pass draft schedule, 50?37?25?16 mm. The 16 mm stripcan be further hot-rolled down to 5 to 6 mm strip using 3-pass draft schedule, 16?12?8?6 mm, after re-soaking at 1150 oC for 2 hr. Hot-rolled to 5 to 6 mm plate with finish rolling temperatures of 900 to 800 oC. Hot rolling reduction can be about 80-90% (~6mm strip).
[0061] In an embodiment, the method 100 can include at a step 106, annealing the hot rolled steel strip at a second predefined temperature for a second predefined period of time. In an exemplary embodiment, the annealing can be a solution annealing. In an embodiment, the second predefined temperature can be in a range of 1000 to 1100 oCand the second predefined period of time can be about 2 hours.
[0062] In an embodiment, the method 100 can include at a step 108, quenching the annealed steel strip. The quenching can be water quenching.
[0063] In an embodiment, the method 100 can include at a step 110, performing cold rolling on the quenched steel strip to reduce thickness of the quenched steel strip to a second predefined thickness, which can be in a range of 1.22 to 0.8mm. Final cold rolling reduction can be about 60-95% and final thickness can be 1.22 to 0.8 mm.
[0064] In an embodiment, the method 100 can include at a step 112, annealing the cold rolled steel strip at a third predefined temperature for a third predefined period of time. In an embodiment, the third predefined temperature can be in a range of 1000 to 1100 oC and the third predefined period of time can be about 2 to 10 minutes.
[0065] In an embodiment, the disclosed lean duplex stainless steel without Mo may have yield strength in the range of 400 MPa to 450 MPa, ultimate tensile strength in the range of 600 MPa to 700 MPa, and elongation in the range of 45 to 50% in hot rolled annealed condition.
[0066] In an embodiment, the disclosed lean duplex stainless steel without Mo may have yield strength in the range of 500 MPa to 550 MPa, ultimate tensile strength in the range of 1000 MPa to 1100 MPa, and elongation in the range of 60 to 70% in cold rolled annealed condition.
[0067] In another embodiment, the disclosed lean duplex stainless steel with Mo may have yield strength in the range of 400 MPa to 450 MPa, ultimate tensile strength in the range of 800 MPa to 900 MPa and elongation in the range of 45 to 50% in hot rolled annealed condition.
[0068] In another embodiment, the disclosed lean duplex stainless steel with Mo may have yield strength in the range of 500 MPa to 550 MPa, ultimate tensile strength in the range of 900 MPa to 1000 MPa, and elongation in the range of 60 to 65% in cold rolled annealed condition.
[0069] In an embodiment, the disclosed lean duplex stainless steel without Mo may have corrosion rate in the range of 0.1 to 0.6 mpy in 3.5% a NaCl solution. The disclosed lean duplex stainless steel without Mo may have better Pitting resistance with pitting potential in the range of 700 to 750 mV in 3.5% the NaCl solution.
[0070] In an embodiment, the disclosed lean duplex stainless steel with Mo may have corrosion rate in the range of 0.1 to 0.4 mpy in 3.5% the NaCl solution. The disclosed lean duplex stainless steel with Mo may have better pitting resistance with pitting potential in the range of 800 to 900 mV in 3.5% NaCl solution.
[0071] In an embodiment, the disclosed lean duplex stainless steel without Mo may have superior formability with Plastic Strain Ratio (rm) ranging from 0.8 to 1.5 and lower ‘Earing’ tendency (Dr) (-0.01 to 0.05). The disclosed lean duplex stainless steel without Mo may have partial recrystallization of 36% ‘?’ fiber in face centered cubic (FCC) phase confirming full recrystallization, and 22% ‘?’ fiber in body centered cubic (BCC) confirming partial recrystallization.
[0072] In an embodiment, the disclosed lean duplex stainless steel with Mo may have superior formability with Plastic Strain Ratio(rm) ranging from 0.5 to 1 and lower ‘Earing’ tendency (Dr) (-0.1 to 0.3). The disclosed lean duplex stainless steel with Mo may have partial recrystallization of 16% ‘?’ fiber in FCC phase confirming full recrystallization and 8% ‘?’ fiber in BCC confirming partial recrystallization.
[0073] In an exemplary embodiment, the alloy design for the low-Ni duplex stainless steels (DSS) was evolved through formulation of appropriate chromium and nickel equivalents using the well-known Schaeffler-Delong diagram to achieve an equilibrium phase balance of ~45 vol% ferrite (a) and ~55 vol% austenite (?) in the experimental steels. A Schaeffler-Delong diagram was commonly used for predicting the stability of austenite, ferrite and martensite phases in stainless steels as a function of Cr- and Ni-equivalents. The following formulae were used for computing the Cr- and Ni-equivalents:
Creq = % Cr + 2 (% Si) + 1.5 (% Mo) + 5 (% V) + 5.5 (% Al) + 1.75 (% Nb) + 1.5 (% Ti) + 0.75 (% W)
Nieq = % Ni + % Co + 30 (% C) + 25 (% N) + 0.5 (% Mn) + 0.3 (% Cu)
[0074] In an exemplary embodiment, the finished chemical compositions of lean duplex stainless steels (LDSS) are presented in Table 1 of the three heats made in the laboratory, the target chemistries were closely achieved in three heats including LDSS-1, LDSS-2, LDSS-3 as shown in Tabe 1. The ferrite contents in the steels may be estimated by ferritoscope reading ( shown in Table 2) and found to vary between 43-46 vol%.
Creq = %Cr + 1.73 (%Si) + 0.88 (%Mo)
Nieq = %Ni + 24.55 (%C) + 21.75 (%N) + 0.4 (%Cu)
% Ferrite = –20.93 + 4.01 (Creq) – 5.6 (Nieq) + 0.016 T
where, T is the solution annealing temperature.
[0075] Table 1: Designed and achieved chemical compositions of Lean duplex stainless steels (LDSS) in wt%
C Mn Si S P Cr Ni Mo Cu N
LDSS-1 0.05 4.80 0.35 0.03 0.04 20.0 0.40 Nil 0.50 0.20
Achieved 0.056 5.49 0.16 0.02 0.03 19.8 0.38 --- 0.49 0.25
LDSS-2 0.05 4.80 0.35 0.03 0.04 20.0 0.40 0.50 0.50 0.20
Achieved 0.06 5.6 0.39 0.02 0.03 19.9 0.38 0.68 0.48 0.25
LDSS-3 0.11 4.79 0.35 0.03 0.04 19.9 0.42 Nil 0.46 0.16
Achieved 0.12 5.59 0.37 0.02 0.03 19.4 0.4 --- 0.44 0.24

Table 1
[0076] Table 2: Ferritescope measurements of Lean duplex stainless steels produced in laboratory

Alloys Average ferrite content (vol%)
LDSS-1 37.9
LDSS-2 44.6
LDSS-3 45.6
Table 2
[0077] Additionally, for developing an understanding of the phase transformations in designed duplex stainless steels, the thermodynamic equilibrium and stability of various phases in low-Ni duplex stainless steel (Composition: 0.05C - 4.5Mn - 0.4Si - 22Cr - 1.5Ni - 0.05Mo - 0.5Cu - 0.2N - 0.05Al - 70.7Fe) was analyzed as a function of temperature using thermocal thermo-chemical software (For example, version Thermo3.0) for different temperature ranges (shown in FIGs. 2 to 4).
[0078] The following salient observations on phase transformation and stability can be deduced from these thermodynamic plots:
a) The liquidus temperature for the steel is about ~1470 oC.
b) The primary nucleating phase in liquid steel is ferrite (?or ?) i.e. the primary solidification mode of the steel is ferritic.
c) Austenite phase begins to nucleate at about ~1350 oC.
d) Secondary phases such as carbides of M23C6 type start nucleating at about ~800-850 oC
e) Intermetallic phases such as sigma (?) and chi (?) begin to nucleate in steel at about ~700 oC.
f) Nucleation of other intermetallic compounds and secondary phases such as Alpha prime (??), Laves (?), nitrides (Cr2N) is thermodynamic feasible in the lower temperature range of about 600-300 oC.
[0079] In an exemplary embodiment, heats of designed low-Ni and Ni-free DSS compositions were melted in an Inductotherm-USA make 100 kg air induction furnace in separate campaigns using ferritic stainless steel billet pieces and precisely weighed alloy additions of nitridedMn, nitridedFeCr, low carbon FeCr and FeMo lumps, Mn metal chips, shredded copper scrap, and nickel briquettes. In all, six laboratory heats were made and target chemistries were effectively achieved in three heats i.e. one Ni-free and two low-Ni heats. The molten steel from each heat was cast into 100 mm square cross-sectioned 25 kg ingots. Two ingots were obtained for each heat. The top and bottom end of the ingots were cropped to exclude the pipe and other solidification defects. The ingots were subsequently reheated and soaked in a furnace at 1150 oC for 3 hours for thermal/ compositional homogenization and then hot-rolled in Hillé-UK make experimental rolling mill in 2 to 3 rolling campaigns to 16 and 6 mm strips with finish rolling temperatures of 950-980oC to avoid edge cracking. The 100 mm square cross-sectioned ingots were initially hot-rolled to 50 mm plates using 4-pass draft schedule, 100?95?78?65?50 mm and then further down to 16 mm strips using a 3-pass draft schedule, 50?37?25?16 mm, after reheating the plates at 1150oC for 2 hr. A part of 16 mm strips were further hot-rolled down to 6 mm strips using 3-pass draft schedule, 16?12?8?6 mm, after re-soaking at 1150oC for 2 hr. After each rolling campaign, the plates were allowed to air cool. The hot rolled strips were subsequently conferred a solution annealing treatment by soaking them at 1060oC for 2 hours followed by rapid quenching in water for dissolution of deleterious intermetallic compounds and secondary phases and to prevent their re-precipitation in the steels.
[0080] In an exemplary embodiment, LDSS strips solution-annealed at 1060oC were further subjected to scale removal and cold rolled in laboratory rolling mill with overall cold reductions to the tune of 75-80% through multi-pass cold rolling to final thickness of 4.0 to 1 mm. Finish thickness as low as 1.09-0.0.89 mm could be achieved in these materials, which demonstrated their excellent cold reducibility. Cold rolled strips were eventually annealed at 1050oC to evolve favorable recrystallization texture for achieving superior tensile and formability properties.
[0081] In an exemplary embodiment, metallographic specimens were sectioned in longitudinal transverse direction from annealed stainless steel strips and prepared using conventional grinding-polishing procedures and electrolytically etched in 20% sodium hydroxide (NaOH) solution at 3 V dc for 30 s for microscopic observation of austenite (?) and ferrite (?) phases in low-Ni and Ni-free duplex stainless steels.
[0082] FIGs.5 and 6 show the light optical and secondary electron images of the hot rolled LDSS duplex stainless steels that have been produced on a laboratory scale. The micrographs reveal near-equal proportions of austenite (?) and ferrite (?) phases in the microstructure of the steels. It can be discerned from the micrographs that the ferrite phase manifests as a darker, continuous phase and austenite appears as lighter, discontinuous phase. The convexity of austenite grain boundaries and the concomitant concavity of ferrite grains clearly indicate that the austenite nucleates and grows into the antecedent ferrite phase. The elongated appearance of grains, crystallographic texture and resulting anisotropy, evident in the micrographs, are very typical features of duplex stainless steels. The micrographs in FIGs. 5 and 6 also confirm the absence of any intermetallic precipitation or secondary phases, which usually nucleate at the ferrite-ferrite and ferrite-austenite grain boundaries in the duplex stainless steels.
[0083] In an exemplary embodiment, the mechanical properties of the hot rolled LDSS were evaluated by means of hardness testing, tensile testing, and Charpy V-notch impact testing at test temperatures of room, 0oC, -20oC and -40oC. For tensile testing, steel specimens were machined and prepared to dimensions from annealed strips with 50 mm gauge length as per ASTM A370-07a. Table 3 summarizes the ambient temperature tensile properties and hardness of the annealed LDSS including AISI 304 L and AISI 316 L austenitic stainless steels for comparison. The LDSS found to exhibit yield strength (YS) of 422-432 MPa, ultimate tensile strength (UTS) of 670-834 MPa with overall elongations of 45-50%. The hardness of hot rolled LDSS stainless steels was close to 94 HRB. In comparison, the annealed austenitic stainless steels AISI 304 L and AISI 316 L showed relatively lower YS of 280-312 MPa and UTS of 600-623 MPa. However, the total elongation to fracture was significantly higher for the austenitic stainless steels and was found to vary between 56-61%. The yield strength of hot rolled LDSS steels works out to be about 1.4-1.5 times that of conventional austenitic stainless steel grades, AISI 304 L and AISI 316 L. The high strength of these steels can be effectively used to an engineering advantage for reducing wall thickness of piping/ tubing and weight of structures in corrosion-prone offshore oil platforms, oil and gas refineries, chemical processing, paper and pulp and fertilizer industries. Since the LDSS are composed of only 50% austenite in their microstructures, their overall elongations were found to be marginally inferior to conventional austenitic stainless steels AISI 304 L and AISI 316 L.
[0084] Table 3: Room temperature tensile properties and bulk hardness of Lean duplex stainless steels in comparison with AISI 304 L and 316 L austenitic stainless steels:

Alloys YS (MPa) UTS (MPa) % Elongation Hardness (HRB)
LDSS-1 430 668 50 93.8
LDSS-2 419 810 48 94.2
LDSS-3 429 634 45 94.1
304L SS 312 623 56 88.2
316L SS 280 606 61 95.2

Table 3
[0085] Table 4 gives the Charpy impact energy (CIE) values of hot rolled LDSS for test temperatures of room, 0 oC, -20 oC and -40 oC and the same results are depicted graphically in FIG.15. The LDSS found to exhibit Charpy V-notch impact energies of 88 and 58 J at room temperature and were found to deterioration in toughness properties at -40 oC with CIE values of 24-9 J, respectively. It can also be observed from FIG.15 that the ductile to brittle transition for duplex stainless steels is more gradual and not abrupt like ferritic stainless steels.
[0086] Table 4: Charpy V-notch impact toughness of Lean duplex stainless steels at various test temperatures:

Alloys Charpy V-notch Impact energy (CIE, J)
Transverse
RT 00C -200C -400C
LDSS-1 95.3 73.2 28 16.6
LDSS-2 78.2 58.4 26.4 9.2
LDSS-3 97.1 88.2 58.6 24.1

Table 4
[0087] FIG. 7 depicts the volume fraction of ??-martensite (SIM) formed as a function of cold reduction in LDSS stainless steels at ambient temperature of cold rolling. The plots in FIG. 7 have the typical sigmoidal shape and the Olson-Cohen one-dimensional nucleation-controlled model is effectively used by several researchers [43, 44] to describe the kinetics of strain-induced martensitic formation during cold rolling of metastable austenitic stainless steels. In particular, the Olson-Cohen equation is used to explain the relationship between the volume fraction of ??-martensite ( ) and true strain ( ) as given below:

where and are temperature-dependent parameters and n is a fixed exponent equal to 4.5. The parameter describes the path of shear band formation, and it mainly depends on the SFE of the steel. The parameter is proportional to the possibility of the nucleation of an embryo of ??-martensite at a shear band intersection.
[0088] In an exemplary embodiment, the austenite phase was almost completely transformed into ??-martensite (~90 vol%) by applying 75% cold reduction in LDSS-1 and LDSS-2 stainless steel under ambient conditions of cold rolling. In case of LDSS-2 stainless steel even after 80% cold reduction only ~70 vol.% if ??-martensite has been transformed from austenitic. The transformed ??-martensite phase was additionally deformed during further cold rolling. It was found that the strain energy is mainly consumed for production of ??-martensite up to 70-75% cold reduction in LDSS-1 and LDSS-2 and 75-53% cold reduction for 304L austenitic stainless steel, while deformation of ??-martensite takes place at the higher cold reductions.
[0089] In an exemplary embodiment, the plots also reveal a sudden increase in formation of SIM after about 42%, 45% and 30% cold reductions for LDSS-1, LDSS-2 and LDSS-3 stainless steels, respectively. This means that a minimum build-up of strain energy through strain hardening is required for the onset of formation of strain-induced martensite (SIM, ??-martensite) in these steels.
[0090] FIGs. 8A to 8C show the optical micrographs of LDSS stainless steels after controlled reversion annealing treatments. The optical microstructures are found to reveal very fine, recrystallized austenite grains generated through controlled reversion of strain-induced martensite (SIM, ??-matensite) obtained through cold rolling.
[0091] Table 5 summaries the properties achieved in LDSS stainless steels after experimental cold rolling and Table 6 shows for Cold rolled annealed LDSS stainless steels. The results show a significant improvement in strength-ductility properties for all stainless steels in terms of YS, UTS and elongation values, when compared with hot rolled properties and also from cold rolled material. The LDSS-1 exhibits YS of 513 MPa, UTS of 1050 MPa, overall elongation of 66%, YS/UTS ratio of 0.49 for cold reduction of 80%, annealing temperature of 1050 oC for 3 min. Whereas, other LDSS stainless steel showed YS: 513-527 MPa, UTS: 950-970 MPa and Elongation around 62% at same condition.
[0092] Table 5: Tensile properties Lean duplex stainless steels after 80% cold rolling

Alloys YS (MPa) UTS (MPa) % Elongation
LDSS-1 518 1194 26
LDSS-2 535 993 21
LDSS-3 525 936 25
Table 5
[0093] Table 6: Tensile properties TRIP enhanced Lean duplex stainless steels after 80% cold rolling and annealing

Alloys YS (MPa) UTS (MPa) % Elongation
LDSS-1 513 1050 66
LDSS-2 513 951 61
LDSS-3 527 976 62
Table 6
[0094] In an exemplary embodiment, the electrochemical corrosion behavior of stainless steels including AISI 304 L and AISI 316 L was investigated using potentiodynamic polarization techniques in 3.5% sodium chloride (NaCl). Test specimens of experimental steels were sectioned in the form of 25 mm square coupons and prepared using metallographic polishing procedures. The electrochemical corrosion experiments were carried out using a standard flat corrosion testing cell (supplied by Princeton Applied Research (PAR), USA) in freely-aerated test solutions using silver-silver chloride reference electrode (Eo = 197 mV versus SHE) and platinum wire mesh as an auxiliary electrode. FIG. 9 shows the typical potentiodynamic polarization plots for cold rolled LDSS stainless steels in 3.5% NaCl solution.
[0095] Table 7 give the electrochemical corrosion properties deduced from these for both potentiodynamic polarization. The pitting potentials and the corrosion rates of cold LDSS stainless steels in 3.5% NaCl vis-à-vis AISI 304 L and AISI 316 L austenitic stainless steels have been graphically depicted in FIG. 10 for comparative evaluation. The cold rolled LDSS also showed proclivity for passive film formation. The cold rolled annealed steels also revealed lower passive current densities of 0.8-1.9 µA/cm2 and higher passive film breakdown potentials of 800 mV in comparison to hot rolled AISI 304 L and AISI 316 L. Hence it can be concluded that TRIP enhancement in LDSS stainless steel has not affected in corrosion and passive film behavior.
[0096] Table 7: Corrosion properties of cold Lean duplex stainless steels in comparison with AISI 304 L and AISI 316 L austenitic stainless steels in 3.5% NaCl determined using potentiodynamic polarization technique
Alloys Open circuit potential, OCP (mV) Passive current density at 300 mV, Ipassive (mA/cm2) Pitting potential, Epit (mV) Corrosion rate (mpy)
LDSS1 -75 1.89 730 0.48
LDSS2 -0.6 1.02 810 0.29
LDSS3 20 0.8 350 0.56
304L -151 10 200 1.42
316L -252 2.19 309 0.94
Table 7
[0097] FIGs. 11 and 12 graphically depicts the formability properties for cold rolled and annealed modified LDSS stainless steel, commercially produced 304L SS and 316L SS stainless steel in terms of average Plastic Strain Ratio (rm) and Planar Anisotropy (?r) values. It is clear that superior Plastic Strain Ratio (rm ~1.0) and lower ‘Earing’ tendency (Dr) can be achieved in modified LDSS stainless steels with 80% cold reduction followed by annealing at 1050oC.
[0098] FIGs. 13A to 13C, 14A to 14C and 15A to 15 C show the result of EBSD analysis including textural aspects for hot rolled annealed (1050oC for 3 min) LDSS-1, LDSS-2 and LDSS-3 stainless steel respectively. The EBSD images of hot rolled annealed LDSS-1 and LDSS-3 steels showed recrystallized grains with (?) fibre texture plane (111) in FCC phase around 54% and 46% respectively and weak (?) fibre texture plane (111) in BCC phase of about 15% and 6% respectively.
[0099] FIGs. 16A to 16C and 17A to 17C show the result of typical EBSD analysis including textural aspects for ~80% cold rolled and annealed (1050oC for 3 min) LDSS-1 and LDSS-2 stainless steel. The EBSD images of cold rolled annealed LDSS-1 also shows both fcc and bcc recrystallized grains by having formable component of (?) fibre texture of about 36% and 22% respectively. Whereas, LDSS-2 showing 16% ? in fiber FCC phase confirming full recrystallization and 8% ? fiber in BCC confirming partial recrystallization.
[00100] Cold rolled annealed LDSS steels EBSD images (Figure 16A to 16C and 17A to 17C) clearly delineate the high angle grain boundaries (HAGBs) and low angle grain boundaries (LAGBs) and the associated grain boundary misorientation distribution clearly confirm the sub-grain formation indicative of a dynamic recovery process. It is to be noted that (?) fibre texture plane (111) of the cold rolled all LDSS steel shows shift in 100 from their respective angle of 550 caused mainly due to high strain due to cold rolling. Hence it can be concluded that recrystallized grains formed more in FCC than in BCC that favors good formability.
[00101] Thus, it will be appreciated by those of ordinary skill in the art that the diagrams, schematics, illustrations, and the like represent conceptual views or processes illustrating systems and methods embodying this invention. The functions of the various elements shown in the figures may be provided through the use of dedicated hardware as well as hardware capable of executing associated software. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the entity implementing this invention. Those of ordinary skill in the art further understand that the exemplary hardware, software, processes, methods, and/or operating systems described herein are for illustrative purposes and, thus, are not intended to be limited to any particular named.
[00102] While embodiments of the present invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions, and equivalents will be apparent to those skilled in the art, without departing from the spirit and scope of the invention, as described in the claim.
[00103] In the foregoing description, numerous details are set forth. It will be apparent, however, to one of ordinary skill in the art having the benefit of this disclosure, that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, to avoid obscuring the present invention.
[00104] As used herein, and unless the context dictates otherwise, the term "coupled to" is intended to include both direct coupling (in which two elements that are coupled to each other contact each other)and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms "coupled to" and "coupled with" are used synonymously. Within the context of this document terms "coupled to" and "coupled with" are also used euphemistically to mean “communicatively coupled with” over a network, where two or more devices are able to exchange data with each other over the network, possibly via one or more intermediary device.
[00105] It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refers to at least one of something selected from the group consisting of A, B, C …. N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc.
[00106] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF THE PRESENT DISCLOSURE
[00107] The present disclosure provides an improved lean duplex stainless steel.
[00108] The present disclosure provides a TRIP-enhanced lean duplex stainless steel with improved mechanical and formability properties.
[00109] The present disclosure provides a method for production of lean duplex stainless steel with improved mechanical and formability properties.
[00110] The present disclosure provides an improved duplex stainless steel which has high strength and excellent corrosion resistance comparable to conventional duplex stainless steels.
[00111] The present disclosure provides a simple and cost effective method which can be easily implemented for production of TRIP-enhanced lean duplex stainless steel with improved mechanical and formability properties.

Claims:1. A lean duplex stainless steel having composition, in weight percentage (%),comprising carbon (C) in amount ranging from 0.01 to 0.12%, silicon (Si) in amount ranging from 0.10 to 0.5%, manganese (Mn) in amount ranging from 4 to 8%, sulphur (S) in amount ranging from0 to 0.04%, phosphorus (P) in amount ranging from 0 to 0.04%, chromium (Cr) in amount ranging from 18 to 23%, nickel (Ni) in amount ranging from 0.1 to 0.60%, copper (Cu) in amount ranging from 0.1 to 0.60%, nitrogen (N) in amount ranging from 0.1 to 0.5%, and ferrite (Fe).
2. The lean duplex stainless steel as claimed in claim 1, wherein the lean duplex steel comprises about 43 to 46 volume% of ferrite (fe).
3. The lean duplex stainless steel as claimed in claim 1, wherein the lean duplex steel comprises molybdenum (Mo) in amount ranging from 0.1 to 1.0% .
4. A method for producing a lean duplex stainless steel comprising:
heating and soaking a steels ingot at a first predefined temperature for a first predefined period of time;
performing hot rolling on the heated steels ingot to reduce thickness of the heated steels ingot to a first predefined thickness strip;
annealing the hot rolled steel strip at a second predefined temperature for a second predefined period of time;
quenching the annealed steels strip;
performing cold rolling on the quenched steel strip to reduce thickness of the quenched steel strip to a second predefined thickness; and
annealing the cold rolled steel strip at a third predefined temperature for a third predefined period of time.
5. The method as claimed in claim 6, wherein the lean duplex stainless steel comprises composition, percentage comprising, in weight percentage (%), comprising carbon (C) in amount ranging from 0.01 to 0.12%, silicon (Si) in amount ranging from 0.10 to 0.5%, manganese (Mn) in amount ranging from 4 to 8%, sulphur (S) in amount ranging from 0 to 0.04%, phosphorus (P) in amount ranging from 0 to 0.04%, chromium (Cr) in amount ranging from 18 to 23%, nickel (Ni) in amount ranging from 0.1 to 0.60%, copper (Cu) in amount ranging from 0.1 to 0.60%, nitrogen (N) in amount ranging from 0.1 to 0.5%, and ferrite (Fe).
6. The method as claimed in claim 6, wherein the lean duplex steel comprises about 43 to 46 volume% of ferrite (fe).
7. The method as claimed in claim 6, wherein the lean duplex steel comprises molybdenum (Mo) in amount ranging from 0.1 to 1.0%.
8. The method as claimed in claim 6, wherein the first predefined temperature is in a range of 1150 to 1250 oC and the first predefined period of time is about 3 hours, wherein the second predefined temperature is in a range of 1000 to 1100 oC and the second predefined period of time is about 2 hours, and wherein the third predefined temperature is in a range of 1000 to 1100 oC and the first predefined period of time is about 2 to 10 minutes.
9. The method as claimed in claim 6, wherein the hot rolling is performed to reduce thickness of the steels ingot from 100 mm square cross-sectioned to 6 mm strip, and wherein the cold rolling is performed to further reduce thickness of the strip from 6mm to in a range of 1.22 to 0.8 mm.
10. The method as claimed in claim 6, wherein the lean duplex stainless steel has a yield strength in the range of 450 MPa to 550 MPa, ultimate tensile strength in the range of 600 MPa to 1100 MPa, elongation in the range of 45 to 50% in hot rolled annealed condition, and elongation in the range of 60 to 70% in cold rolled annealed condition.