Claims:We Claim:
1. Low Ni austenitic stainless steel comprising

Mechanical properties including:
(a) yield strength in the range of 350 MPa to 450 MPa, ultimate tensile strength in the range of 700 MPa to 780 MPa and elongation in the range of 60 to 70%;(b) a hardness in the range of 90-97 HRB; and (c) Charpy V-notch impact toughness properties with impact values 250 to 275 J at Room temperature and 150 to 180 J at 40 oC; and

steel composition having low Ni of upto 7 % by wt. and selectively including anyone of the following low or high Mn content based compositions:
(i) low Mn stainless steel composition in wt% comprising: C (0.06 to 0.1%), Mn (6 to 8%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (5 to 7%), Mo (2 to 3%), Cu (0.5 to 0.8%) and N (0.2 to 0.4%) and balance being Fe; and

(ii) high Mn high N stainless steel composition in wt % comprising: C (0.06 to 0.1%), Mn (16 to 20%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (1 to 3%), Mo (0 to 1.5%), Cu (1.00 to 2%) and N (0.3 to 0.5%) and balance being Fe.

2. Low Ni austenitic stainless steel as claimed in claim 1 comprising a fully austenitic microstructure with grain size in the range of about 10 to 11 μm.

3. Low Ni austenitic stainless steel as claimed in anyone of claims 1 to 2 having mechanical properties for both said low Mn and high Mn stainless steels comprising (a) a yield strength in the range of 350 MPa to 450 MPa, ultimate tensile strength in the range of 700 MPa to 780 MPa and elongation in the range of 60 to 70%;(b) a hardness in the range of 90-97 HRB; and (c) Charpy V-notch impact toughness properties with impact values 250 to 275 J at Room temperature and 150 to 180 J at 40 oC.

4. Low Ni austenitic stainless steel as claimed in anyone of claims 1 to 3 having corrosion resistance in basic environment
(i) of said low Mn steel composition comprising
(a) corrosion rate in the range of 0.1 to 0.2 mpy in 3.5% NaCl solution as determined by potentiodynamic polarisation test;
(b) pitting potential in the range of 700 to 800 mpy in 3.5% NaCl solution as determined by potentiodynamic polarisation test;
(c) charge transfer resistance in the range of 5 to 6 X105 Ω.cm2 in 3.5% NaCl solution as determined by electrochemical impedance spectroscopy test;
(ii) of said high Mn steel composition comprising
(a) corrosion rate in the range of 0.3 to 0.45 mpy in 3.5% NaCl solution as determined by potentiodynamic polarisation test;

(b) pitting potential in the range of 250 to 400 mpy in 3.5% NaCl solution as determined by potentiodynamic polarisation test;
(c) charge transfer resistance in the range of 3 to 4 X105 Ω.cm2 in 3.5% NaCl solution as determined by electrochemical impedance spectroscopy test; and
(iii) both low Mn and high Mn stainless steel composition have lower film thickness (1700 to 2300 nm) as compared to 316L stainless steel (2817 nm).

5. Low Ni austenitic stainless steel as claimed in anyone of claims 1 to 4 having corrosion resistance in acidic environment comprising
(i) Of both low Mn and high Mn stainless steels having corrosion rate in the range of 0.25 to 1.2 mpy in 0.1m H2SO4 solution as determined by potentiodynamic polarisation test;
(ii) Of both low Mn and high Mn stainless steels having Of both low Mn and high Mn stainless steels having pitting potential in the range of 950 to1050 mpy in 0.1m H2SO4 solution as determined by potentiodynamic polarisation test;
(iii) Of low Mn stainless steel composition having charge transfer resistance in the range of 5 to 6 X104 Ω.cm2 in 0.1m H2SO4 solution as determined by electrochemical impedance spectroscopy test;
(iv) Of high Mn stainless steel composition having charge transfer resistance in the range of 3 to 4 X104 Ω.cm2 in 0.1m H2SO4 solution as determined by electrochemical impedance spectroscopy test.
(v) both low Mn and high Mn stainless steels having lower film thickness (1600 to 1800 nm) as compared to 316L stainless steel (2817 nm).

6. Low Ni austenitic stainless steel as claimed in anyone of claims 1 to 5 wherein both said low Mn and high Mn stainless steels are immune to intergranular corrosion(IGC) having degree of sensitization (DoS) values of  0.05 obtained through double loop electrochemical potentiokinetic reactivation (DL-EPR) testing in 0.5 M H2SO4 + 0.01 M KSCN as per ASTM G 108.

7. Low Ni austenitic stainless steel as claimed in anyone of claims 1 to 6 wherein both said low Mn and high Mn stainless steels are free from deleterious intermetallic phases such as sigma (), chi (), Laves (), alpha prime () etc. ascertained from corrosion rates of <10 mdd in ferric chloride corrosion testing as per ASTM A 923 Method C.

8. A method of manufacture of Low Ni austenitic stainless steel as claimed in anyone of claims 1 to 7 comprising:

involving steel composition having low Ni of upto 7 % by wt. and selectively including anyone of the following low or high Mn content based compositions:
(i) low Mn stainless steel composition in wt% comprising: C (0.06 to 0.1%), Mn (6 to 8%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (5 to 7%), Mo (2 to 3%), Cu (0.5 to 0.8%) and N (0.2 to 0.4%) and balance being Fe; and

(ii) high Mn high N stainless steel composition in wt % comprising: C (0.06 to 0.1%), Mn (16 to 20%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (1 to 3%), Mo (0 to 1.5%), Cu (1.00 to 2%) and N (0.3 to 0.5%) and balance being Fe.
a. and carrying out the following:Reheating and soaking the steel ingots/slabs in a reheating furnace at 1150 to 1250 oC for 3 hours for thermal/ compositional homogenization;
b. Hot-rolling the slabs with finish rolling temperatures of 1000 to 1100 oC to avoid edge cracking and achieving final rolling reduction of about 30-35%;
c. Annealing the strips after hot rolling carried out in the temperature range of 1100 to 1200 oC. with soaking for 1 to 2 hrs ; followed by
d. rapid quenching in water for dissolution of deleterious intermetallic compounds and secondary phases and to prevent their re-precipitation in the steels.

9. A method as claimed in claim 8 comprising the steps of
a. making separate heats to be cast into ingots/slab of desired section having target compositions comprising
i. low Mn stainless steel composition in wt% comprising: C (0.06 to 0.1%), Mn (6 to 8%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (5 to 7%), Mo (2 to 3%), Cu (0.5 to 0.8%) and N (0.2 to 0.4%) and balance being Fe; and

ii. high Mn high N stainless steel composition in wt % comprising: C (0.06 to 0.1%), Mn (16 to 20%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (1 to 3%), Mo (0 to 1.5%), Cu (1.00 to 2%) and N (0.3 to 0.5%) and balance being Fe.

b. Reheating and soaking the steel ingots/slabs obtained in step (i) in a reheating furnace at 1150 to 1250 oC for 3 hours for thermal/ compositional homogenization;
c. Hot-rolling the slabs with selective draft schedule to plates of desired thickness involving finish rolling temperatures of 1000 to 1100 oC to avoid edge cracking and achieving final rolling reduction of about 30-35%;
d. Annealing the strips after hot rolling carried out in the temperature range of 1100 to 1200 oC. with soaking for 1 to 2 hrs ; followed by
e. rapid quenching in water for dissolution of deleterious intermetallic compounds and secondary phases and to prevent their re-precipitation in the steels.

10. A method as claimed in anyone of claims 8 or 9 wherein values of Cr- and Ni-equivalents chosen for predicting the stability of austenite and evolution of MnSS compositions wherein said values are computed using the formulae :
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)

Dated this the 25th day of November, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
, Description:FIELD OF THE INVENTION

The present invention relates to low cost low-Ni austenitic stainless steel with superior mechanical property and localized corrosion resistance and a process for producing the same. More particularly, the present invention is directed to provide low-Ni and high Mn stainless steel (MnSS) composition evolved through formulation of appropriate chromium and nickel equivalents using the well-known Schaeffler-Delong diagram to achieve stable austenite (γ) in the experimental steels. All the steels showed a fully austenitic microstructure with average grain size of about 10 to 11μm. The developed MnSS steels revealed an exceptional combination of properties: higher yield strength, higher tensile strength and higher elongation compared to that of commercial available AISI 316L SS. The hardness of Mn stainless steels was in the range of 90-96 HRB. The yield strength of Mn containing austenitic stainless steels works out to be about 1.6-1.8 times that of conventional austenitic stainless steel grade, AISI 316L SS. 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 & gas refineries, chemical processing, paper & pulp and fertilizer industries. The newly developed MnSS steels showed superior Charpy V-notch impact toughness properties and no ductile to brittle transition observed for theses steels even at -40 oC. MnSS steels show relatively nobler electrochemical corrosion potentials, improved passivities, and lower corrosion currents compared to austenitic stainless steel AISI 316L SS. MnSS steels were also found to exhibit high pitting potentials of 400-740 mV and very low corrosion rates of 0.17-0.4 mpy, underlining their superior localized as well as general corrosion resistance in the chloride environment. The general corrosion performance of MnSS was apparently superior to AISI 316L austenitic stainless steels. The corrosion rates of the experimental steels were substantially lower in the acidic environment and were found to vary between 0.29–1.1 mpy as compared to 1.55 mpy for the AISI 3i6L austenitic stainless steel. Weight loss in ferric chloride corrosion test confirm the absence of deleterious intermetallic phases such as sigma (), chi (), Laves (), alpha prime () etc. The newly developed low cost low-Ni high-Mn high-N austenitic stainless steel thus can replace existing costlier AISI 316L stainless steel.

BACKGROUND OF THE INVENTION

Nickel containing austenitic stainless steels (SS), such as AISI 304 and 316 SS has been workhorse for a variety of industrial applications. The rise in the cost of nickel has been a problem for stainless manufactures in containing the cost of stainless steels. Manganese stabilizes the austenitic phase, like nickel, and costs much lesser than nickel are the reasons for its use in place of nickel. There has been some work on the corrosion studies of manganese substituted stainless steels. Some of these studies indicate that manganese is beneficial to pitting and crevice corrosion, while a few of them show that manganese is detrimental to the pitting resistance of stainless steels, but few say it is detrimental above certain level. Examination of the studies reported in the literature suggests that manganese can be less detrimental if the environment is less aggressive. One of the beneficial effects of manganese is that it enables high nitrogen addition to stainless steels. Nitrogen being an efficient element to promote pitting resistance of stainless steels, the presence of nitrogen in Mn containing stainless steels is expected to enhance their pitting resistance.

The presently available AISI 316L austenitic stainless steel posses excellent localized corrosion resistances with pitting potential > 300 mV and excellent mechanical properties (YS of >300 MPa and elongation > 55%). But these properties are achieved by adding costlier alloying element like nickel (~8 wt%) and molybdenum (2-3 wt%).

The present invention relates to a stainless steel composition having low nickel with high localized corrosion resistance and which includes at least the main component as iron, the main alloy components as chromium, manganese, nitrogen and low nickel (1-2 wt%). These types of steel composition was achieved earlier (US Patent 9,145,598, CN102965584 (A)) through powder metallurgy route. They narrated that, processing of high manganese and nitrogen content steel is very difficult by normal steel making route.

Later these types cost effective steels were produced though various costlier production method like, duplex metallurgy route (European Patent: CN105002431 (A)) refining methods like oxygen decarbonization refining, ladle refining process (CN102965584 (A)) AOD etc.

The properties both corrosion resistance and mechanical properties were achieved through alloy modification technique like, increasing nickel content >6% (Patent No.: KR20120050085 (A), Patent No. TW200831685 (A), Patent Publication No. MX2012012874 (A), European Patent: CN103154291 (A) and Indian Patent: 223848), lowering nickel content <1-2% and increasing Mo content to 4-5%( Patent Publication No.: KR20120050085 (A), United states Patent : US 8,877,121, and United states Patent : US 8,858,872), having duplex structure and also be alloying of various trace elements like Co, Nb, B, Ce, Nb+Ta, Ti, W and Ti+W, United states Patent : US 6,267,921, Patent Publication No. : CN101845605 (A), Japanese Patent: JP,11-092885,A, Japanese Patent: JP,57-108250, Japanese Patent: JP,2013-036090A, United states Patent: US 20080292489 A

Most of the prior art on the subject are related to the novelty in:

a. Processing of steel through powder metallurgy.
b. Production of these steel using various refining methods like oxygen decarbonization refining, ladle refining process AOD etc.,.
c. Properties were achieved with higher nickel content >6%
d. Properties were achieved with lowering nickel content <1-2% and increasing Mo content to 4-5%
e. Addition of some various alloying elements like Co, Nb, B, Ce, Nb+Ta, Ti, W and Ti+W.

Recently high Mn and high N austenitic stainless steel have been successfully developed for the first time in laboratory scale at Applicant’s research establishment at RDCIS, SAIL. The present study is focused on the detailed characterization of newly developed high Mn and high N steel and comparing with the standard AISI 316L SS.

The methodology for manufacturing low Mn and high Mn stainless steels according to present work with superior mechanical and corrosion properties than that of AISI 316L stainless steel is very different from the existing prior art, which are mentioned above.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide low cost low-Ni austenitic stainless steel with superior mechanical property and localized corrosion resistance and a process for producing the same.

A further object of the present invention is directed to provide low-Ni austenitic stainless steel having low Ni and high Mn in composition replacing costly Nickel as alloying element.

A still further object of the present invention is directed to provide low Ni austenitic stainless steel wherein use of high Mn enables high nitrogen addition to stainless steels as an efficient element to promote pitting resistance of stainless steels.

Another object of the present invention is directed to provide low cost low-Ni high-Mn stainless steel(MnSS) wherein stable fully austenitic microstructure with average grain size of about 10 to 11μm would be achieved.

Yet another object of the present invention is directed to provide low cost low-Ni austenitic stainless steel having yield strength of Mn containing austenitic stainless steels substantially higher than conventional austenitic stainless steel grade, AISI 316L SS, so that 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 & gas refineries, chemical processing, paper & pulp and fertilizer industries.

A still further object of the present invention is directed to provide low cost low-Ni austenitic stainless steel having superior Charpy V-notch impact toughness properties with higher impact values both in longitudinal and transverse direction almost for all test temperature (Room Temperature to -400C), having no ductile to brittle transition observed for theses steels even at -40 oC.

A still further object of the present invention is directed to provide low cost austenitic stainless steel wherein experimental stainless steels having tendency for formation of more protective, tenacious passive films in the chloride environment than AISI 316L SS, reflecting superior localized as well as general corrosion resistance in the chloride environment.

A still further object of the present invention is directed to provide low cost austenitic stainless steel wherein oxide thickness of Mn containing steels is higher as compared to AISI 316L SS in salt solution and smaller in acidic environment and are not vulnerable to intergranular corrosion (IGC).

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to low Ni austenitic stainless steel comprising

Mechanical properties including:
(a) yield strength in the range of 350 MPa to 450 MPa, ultimate tensile strength in the range of 700 MPa to 780 MPa and elongation in the range of 60 to 70%;(b) a hardness in the range of 90-97 HRB; and (c) Charpy V-notch impact toughness properties with impact values 250 to 275 J at Room temperature and 150 to 180 J at 40 oC; and

steel composition having low Ni of upto 7 % by wt. and selectively including anyone of the following low or high Mn content based compositions:
(i) low Mn stainless steel composition in wt% comprising: C (0.06 to 0.1%), Mn (6 to 8%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (5 to 7%), Mo (2 to 3%), Cu (0.5 to 0.8%) and N (0.2 to 0.4%) and balance being Fe; and

(ii) high Mn high N stainless steel composition in wt % comprising: C (0.06 to 0.1%), Mn (16 to 20%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (1 to 3%), Mo (0 to 1.5%), Cu (1.00 to 2%) and N (0.3 to 0.5%) and balance being Fe.

A further aspect of the present invention is directed to low Ni austenitic stainless steel comprising a fully austenitic microstructure with grain size in the range of about 10 to 11 μm.

A still further aspect of the present invention is directed to low Ni austenitic stainless steel having mechanical properties for both said low Mn and high Mn stainless steels comprising (a) a yield strength in the range of 350 MPa to 450 MPa, ultimate tensile strength in the range of 700 MPa to 780 MPa and elongation in the range of 60 to 70%;(b) a hardness in the range of 90-97 HRB; and (c) Charpy V-notch impact toughness properties with impact values 250 to 275 J at Room temperature and 150 to 180 J at 40 oC.

A still further aspect of the present invention is directed to low Ni austenitic stainless steel having corrosion resistance in basic environment
(i) of said low Mn steel composition comprising
(a) corrosion rate in the range of 0.1 to 0.2 mpy in 3.5% NaCl solution as determined by potentiodynamic polarisation test;
(b) pitting potential in the range of 700 to 800 mpy in 3.5% NaCl solution as determined by potentiodynamic polarisation test;
(c) charge transfer resistance in the range of 5 to 6 X105 Ω.cm2 in 3.5% NaCl solution as determined by electrochemical impedance spectroscopy test;
(ii) of said high Mn steel composition comprising
(a) corrosion rate in the range of 0.3 to 0.45 mpy in 3.5% NaCl solution as determined by potentiodynamic polarisation test;

(b) pitting potential in the range of 250 to 400 mpy in 3.5% NaCl solution as determined by potentiodynamic polarisation test;
(c) charge transfer resistance in the range of 3 to 4 X105 Ω.cm2 in 3.5% NaCl solution as determined by electrochemical impedance spectroscopy test; and
(iii) both low Mn and high Mn stainless steel composition have lower film thickness (1700 to 2300 nm) as compared to 316L stainless steel (2817 nm).

A still further aspect of the present invention is directed to low Ni austenitic stainless steel having corrosion resistance in acidic environment comprising
(i) Of both low Mn and high Mn stainless steels having corrosion rate in the range of 0.25 to 1.2 mpy in 0.1m H2SO4 solution as determined by potentiodynamic polarisation test;
(ii) Of both low Mn and high Mn stainless steels having Of both low Mn and high Mn stainless steels having pitting potential in the range of 950 to1050 mpy in 0.1m H2SO4 solution as determined by potentiodynamic polarisation test;
(iii) Of low Mn stainless steel composition having charge transfer resistance in the range of 5 to 6 X104 Ω.cm2 in 0.1m H2SO4 solution as determined by electrochemical impedance spectroscopy test;
(iv) Of high Mn stainless steel composition having charge transfer resistance in the range of 3 to 4 X104 Ω.cm2 in 0.1m H2SO4 solution as determined by electrochemical impedance spectroscopy test.
(v) both low Mn and high Mn stainless steels having lower film thickness (1600 to 1800 nm) as compared to 316L stainless steel (2817 nm).

Another aspect of the present invention is directed to low Ni austenitic stainless steel wherein both said low Mn and high Mn stainless steels are immune to intergranular corrosion(IGC) having degree of sensitization (DoS) values of  0.05 obtained through double loop electrochemical potentiokinetic reactivation (DL-EPR) testing in 0.5 M H2SO4 + 0.01 M KSCN as per ASTM G 108.

Yet another aspect of the present invention is directed to low Ni austenitic stainless steel wherein both said low Mn and high Mn stainless steels are free from deleterious intermetallic phases such as sigma (), chi (), Laves (), alpha prime () etc. ascertained from corrosion rates of <10 mdd in ferric chloride corrosion testing as per ASTM A 923 Method C.

A further aspect of the present invention is directed to a method of manufacture of Low Ni austenitic stainless steel as described above comprising:
involving steel composition having low Ni of upto 7 % by wt. and selectively including anyone of the following low or high Mn content based compositions:
(i) low Mn stainless steel composition in wt% comprising: C (0.06 to 0.1%), Mn (6 to 8%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (5 to 7%), Mo (2 to 3%), Cu (0.5 to 0.8%) and N (0.2 to 0.4%) and balance being Fe; and

(ii) high Mn high N stainless steel composition in wt % comprising: C (0.06 to 0.1%), Mn (16 to 20%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (1 to 3%), Mo (0 to 1.5%), Cu (1.00 to 2%) and N (0.3 to 0.5%) and balance being Fe.
a. and carrying out the following:Reheating and soaking the steel ingots/slabs in a reheating furnace at 1150 to 1250 oC for 3 hours for thermal/ compositional homogenization;
b. Hot-rolling the slabs with finish rolling temperatures of 1000 to 1100 oC to avoid edge cracking and achieving final rolling reduction of about 30-35%;
c. Annealing the strips after hot rolling carried out in the temperature range of 1100 to 1200 oC. with soaking for 1 to 2 hrs ; followed by
d. rapid quenching in water for dissolution of deleterious intermetallic compounds and secondary phases and to prevent their re-precipitation in the steels.

A still further aspect of the present invention is directed to a method comprising the steps of
a. making separate heats to be cast into ingots/slab of desired section having target compositions comprising
i. low Mn stainless steel composition in wt% comprising: C (0.06 to 0.1%), Mn (6 to 8%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (5 to 7%), Mo (2 to 3%), Cu (0.5 to 0.8%) and N (0.2 to 0.4%) and balance being Fe; and

ii. high Mn high N stainless steel composition in wt % comprising: C (0.06 to 0.1%), Mn (16 to 20%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (1 to 3%), Mo (0 to 1.5%), Cu (1.00 to 2%) and N (0.3 to 0.5%) and balance being Fe.

b. Reheating and soaking the steel ingots/slabs obtained in step (i) in a reheating furnace at 1150 to 1250 oC for 3 hours for thermal/ compositional homogenization;
c. Hot-rolling the slabs with selective draft schedule to plates of desired thickness involving finish rolling temperatures of 1000 to 1100 oC to avoid edge cracking and achieving final rolling reduction of about 30-35%;
d. Annealing the strips after hot rolling carried out in the temperature range of 1100 to 1200 oC. with soaking for 1 to 2 hrs ; followed by
e. rapid quenching in water for dissolution of deleterious intermetallic compounds and secondary phases and to prevent their re-precipitation in the steels.

A still further aspect of the present invention is directed to a method wherein values of Cr- and Ni-equivalents chosen for predicting the stability of austenite and evolution of MnSS compositions wherein said values are computed using the formulae :
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)

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1: show graphically Thermocal thermo-chemical software analysis showing thermodynamic equilibrium and stability of various phases in MnSS-2 stainless steel as a function of temperature.
Fig. 2: show graphically Thermocal thermo-chemical software analysis showing thermodynamic equilibrium and stability of various phases in MnSS-3 stainless steel as a function of temperature.
Fig. 3 : show graphically Thermocal thermo-chemical software analysis showing thermodynamic equilibrium and stability of various phases in MnSS-4 stainless steel as a function of temperature
Fig. 4: show Light optical micrographs of (a) 316L (b) MnSS-2 stainless steel showing fully austenitic stainless steel with annealing twins; and Light optical micrographs of (c) MnSS-3 (d) MnSS-4 stainless steel showing fully austenitic stainless steel with annealing twins.
Fig. 5: show Three dimensional light optical micrographs of (a) MnSS-2 (b) MnSS-3 (c) MnSS-4 stainless steel showing fully austenitic stainless steel with annealing twins in all the directions.
Fig.6: show Charpy V-notch impact toughness of Mn stainless steels as a function of test temperature in longitudinal direction.
Fig.7: show Charpy V-notch impact toughness of Mn stainless steels as a function of test temperature in transverse direction.
Fig. 8: illustrates Polarization plots of Mn stainless steels in 3.5% NaCl superimposed with plots for AISI 316L SS for comparison.
Fig. 9: illustrates Pitting potentials of Mn stainless steels in 3.5% NaCl vis-à-vis AISI 316 L austenitic stainless steels.
Fig. 10: illustrates Polarization plots of Mn stainless steels in 0.1m H2SO4 solutions superimposed with plots for AISI 316L SS for comparison.
Fig. 11: illustrates Pitting potentials of Mn stainless steels in 0.1m H2SO4 solutions vis-à-vis AISI 316L austenitic stainless steels.
Fig. 12: illustrates Electrochemical impedance spectra of Mn stainless steels in 3.5% NaCl solution superimposed with typical spectra with AISI 316L SS for comparing the relative impedances of their passive films: (a) Nyquist plots (b) Bode magnitude & phase plots.
Fig. 13: illustrates Electrochemical impedance spectra of Mn stainless steels in 0.1m H2SO4 solutions superimposed with typical spectra with AISI 316L SS for comparing the relative impedances of their passive films: (a) Nyquist plots (b) Bode magnitude & phase plots.
Fig.14: shows Equivalent electrical circuit model used to regress EIS spectra for computing impedances of stainless steels.
Fig. 15: illustrates Charge transfer resistance/ Polarisation resistance of Mn stainless steels in 3.5% NaCl vis-à-vis AISI 316 L austenitic stainless steels.
Fig. 16: illustrates Charge transfer resistance/Polarisation resistance of Mn stainless steels in 0.1m H2SO4 solutions vis-à-vis AISI 316L austenitic stainless steels.
Fig. 17: show Oxide film thickness of Mn stainless steels in 3.5% NaCl vis-à-vis AISI 316 L austenitic stainless steels.
Fig. 18: illustrates Oxide film thickness of Mn stainless steels in 0.1m H2SO4 solutions vis-à-vis AISI 316 L austenitic stainless steels.
Fig. 19: show Typical double-loop electrochemical potentiokinetic reactivation (DL-EPR) plots of Mn stainless steels and AISI 316 L austenitic stainless steel showing their relative insusceptibility to intergranular corrosion (Test solution: 0.5 M H2SO4 + 0.01 M KSCN; Test temperature: 30±1 oC; ASTM G 108).

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to provide low cost austenitic stainless steel with superior mechanical property and localized corrosion resistance and a process for producing the same. The present invention provides low cost austenitic stainless steel having low Ni and high Mn in composition replacing costly Nickel as alloying element as well as enabling more N addition leading to fully austenitic microstructure with coarse grains. The austenitic stainless steel is having superior strength, impact toughness and corrosion resistance properties as compared to conventional austenitic stainless steel grade, AISI 316L SS, so that 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 & gas refineries, chemical processing, paper & pulp and fertilizer industries.

The alloy design for low-Ni and high Mn stainless steels (MnSS) was evolved through formulation of appropriate chromium and nickel equivalents using the well-known Schaeffler-Delong diagram to achieve austenite (γ) in the experimental steels. The 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)

The values of Cr- and Ni-equivalents chosen for evolution of MnSS compositions and the designed chemical compositions of the steels have presented in Table 1. The comparison commercial available 316L SS was chosen, its chemical composition is given in Table 1.

Additionally, for developing an understanding of the phase transformations in designed Mn containing austenitic stainless steels, the thermodynamic equilibrium and stability of various phases in Mn containing austenitic stainless steels were analyzed as a function of temperature using Thermocal thermo-chemical software (version Thermo3.0) for different temperature ranges (Figs.1 – 3):

The following salient observations on phase transformation and stability can be deduced from these thermodynamic plots:
1. The liquidus temperature for the steel is ~1400 oC.
2. The primary nucleating phase in liquid steel is ferrite ( or ) i.e. the primary solidification mode of the steel is ferritic.
3. Austenite phase begins to nucleate at around 1330 oC.
4. Secondary phases such as carbides of M23C6 type start nucleating at ~900-950 oC
5. Intermetallic phases such as sigma () and chi () begin to nucleate in steel at around ~650 oC.
6. Nucleation of other intermetallic compounds and secondary phases such as Alpha prime (), Laves (), nitrides (Cr2N) is thermodynamic feasible in the lower temperature range of 600-300 oC.

Heats of designed MnSS compositions were melted in an Inductotherm-USA make 100 kg air induction furnace in separate campaigns using low nickel stainless steel SSLNA slab pieces and precisely weighed alloy additions of nitrided Mn, nitrided FeCr, low carbon FeCr & FeMo lumps, Mn metal chips, shredded copper scrap, and nickel briquettes. In all, four laboratory heats were made and target chemistries were effectively achieved in three heats i.e. one AISI 216L and two high Mn stainless steel 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 1250 oC for 3 hours for thermal/ compositional homogenization and then hot-rolled in Hillé-UK make experimental rolling mill in 2 rolling campaigns to 17 strips with finish rolling temperatures of 1000 - 1050 oC 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 17 mm strips using a 3-pass draft schedule, 50372517 mm, after reheating the plates at 1250 oC for 1 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 1150 oC 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.

The newly developed Mn containing stainless steels (Table -1) compositions were met as per the designed chemistry. Topographical and three dimensional microstructural analysis were carried out for all the steels at hot rolled solution annealed condition and the microstructure were given in Fig. 4 and 5 respectively. All the steels showed a fully austenitic microstructure with average grain size of about 10 to 11μm. The inclusion rating and volume fraction of the inclusion were carried out on the Mn stainless steel (MnSS) according to ASTM E45A and JIS standard respectively and tabulated in Table 2. It is clear from the table that the steels under investigation were found to be clean and has very small oxide inclusions.

The mechanical properties of the developed MnSS were given in the Table 3. For comparison the mechanical properties of commercial available AISI 316L SS are also given in the table. The developed MnSS steels revealed an exceptional combination of properties: higher yield strength, higher tensile strength and higher elongation compared to that of commercial available AISI 316L SS. The hardness of Mn stainless steels was in the range of 90-96 HRB. The yield strength of Mn containing austenitic stainless steels works out to be about 1.6-1.8 times that of conventional austenitic stainless steel grade, AISI 316L SS. 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 & gas refineries, chemical processing, paper & pulp and fertilizer industries. Since the Mn stainless steels are composed of only fully austenite in their microstructures with higher grain size, their overall elongations were found to be marginally inferior to conventional austenitic stainless steel AISI 316LSS. Table 4 gives the Charpy impact energy (CIE) values for Mn containing austenitic stainless steels for test temperatures of room, 0 oC, -20 oC and -40 oC both in longitudinal and transverse direction. The same results are depicted graphically in Fig. 6 and 7. The newly developed MnSS steels showed superior Charpy V-notch impact toughness properties with impact values almost for test temperature (Room Temperature to -400C).

The Mn stainless steels were found to exhibit Charpy V-notch impact energies in the range of 260- 270 193 J at room temperature in longitudinal direction and were found to retain their excellent toughness properties at -40 oC with CIE values (210 to 240 54 J), respectively. Similar behavior was observed for all temperature at transverse direction. It can also be confirmed from Fig. 6 and 7 that there is no ductile to brittle transition observed for theses steels even at -40 oC.

Figs. 8 and 10 show the typical potentiodynamic polarization plots for Mn stainless steels in 3.5% NaCl and 0.1 m H2SO4 solutions. Tables 5 and 6 give the electrochemical corrosion properties deduced from these potentiodynamic polarization scans, respectively. The figures and tables also depict and present correspondingly the electrochemical corrosion behaviour and properties of austenitic stainless steel AISI 316L SS for comparison and reveal the corrosion, passivation and pitting characteristics of the investigated steels in the corrosive aqueous media.

The Mn containing stainless steels (MnSS) were found to exhibit spontaneous and stable passivities in the aggressive test environment of 3.5% NaCl solution. It can be discerned from Table 5 that the MnSS steels show relatively nobler electrochemical corrosion potentials, improved passivities, and lower corrosion currents compared to austenitic stainless steel AISI 316L SS. The experimental stainless steels revealed the tendency for formation of more protective, tenacious passive films in the chloride environment than AISI 316L SS. This observation was concluded from the lower passive current density values of 1.0-1.7 µA/cm2 exhibited by the steels at 300 mV as compared to the corresponding value of 2.00 µA/cm2 measured for AISI 316L SS. MnSS steels were also found to exhibit high pitting potentials of 400-740 mV and very low corrosion rates of 0.17-0.4 mpy, underlining their superior localized as well as general corrosion resistance in the chloride environment. The general corrosion performance of MnSS was apparently superior to AISI 316L austenitic stainless steels which were found to exhibit corrosion rate of 0.84 mpy. The pitting potentials and the corrosion rates of MnSS steels in 3.5% NaCl vis-à-vis AISI 316 L austenitic stainless steels have been graphically depicted in Fig. 9 for comparative evaluation.

Even in the strongly reducing environment of 0.1 N H2SO4, the MnSS steels were found to exhibit the proclivity for passive film formation. The experimental steels revealed marginally lower passive current densities of 2.3-3.4 µA/cm2 and higher passive film breakdown potentials of 990-1000 mV in comparison to AISI 316 L stainless steels, which correspondingly exhibited passive current densities of 4.2 µA/cm2 and breakdown potentials of 980 mV (Fig.11). The corrosion rates of the experimental steels were substantially lower in the acidic environment and were found to vary between 0.29–1.1 mpy as compared to 1.55 mpy for the AISI 3i6L austenitic stainless steel.

Figs. 12 and 13 show the electrochemical impedance spectra for MnSS steels including AISI 316L austenitic stainless steels in 3.5% NaCl and 0.1 N H2SO4 solutions, respectively. The impedance spectra have been depicted in the form of superimposed complex plane Nyquist, Bode magnitude and Bode phase plots for the investigated stainless steels. Since the chemical composition, physical and electrical properties of passive films are largely governed by the composition of stainless steels, the measured electrochemical impedances are characteristic of protective properties of the films on stainless steels, per se. The electrochemical impedances of the passive films were computed by regressing the complex plane Nyquist impedance plots with an equivalent electrical circuit of the type, Rs(QRct), where Rs is solution resistance, Rct is the charge transfer resistance of passive film and Q is passive film capacitance (constant phase element (CPE) or non-ideal capacitor). Tables 7 and 8 show the electrochemical impedances of the passive films on investigated stainless steels in 3.5% NaCl and 0.1 N H2SO4 solutions, respectively, deduced from electrochemical impedance spectra depicted in Figs. 12 and 13. The equivalent electrical circuit model used to regress the EIS spectra is depicted in Fig. 14. In 3.5% NaCl solution, the total charge transfer resistance/polarization resistances (Rct) of the passive films on MnSS steels were found to be in the order of 105 Ω.cm2, and it is one order greater than that of AISI 316L(104 Ω.cm2) (Fig.15). This high magnitude of impedance is clearly indicative of relative impermeability of passive films on Mn stainless steels to chloride ions. The total charge transfer resistances/polarization resistances (Rct) of the passive films in 0.1 N H2SO4 were also found to be comparable to AISI 316L austenitic stainless steel with resistance in order of 104 Ω.cm2 (Fig.16). The oxide thickness was calculated from the capacitance value from Table 7 and 8 for 3.5% NaCl and 0.1 N H2SO4solutions respectively and represented in Fig.17 and 18. From Fig 17 and 18 it is clear that oxide thickness of Mn containing steels is higher as compared to 316L SS in salt solution and smaller in acidic environment.

Table 9 shows the intergranular corrosion (IGC) susceptibility of Mn containing austenitic stainless steel including AISI 316L austenitic stainless steels evaluated using double-loop electrochemical potentiokinetic reactivation (DL-EPR) procedure in 0.5 M H2SO4 + 0.01 M KSCN test solution. Fig. 19 depicts the superimposed DL-EPR plots for investigated stainless steels. The plots exhibit marked differences between the peak current densities of the anodic (forward) and reactivation (reverse) polarization scans. Also, the peak anodic current density (Ia) of the forward potential scan is greater than the peak reactivation current density (Ir) corresponding to the reverse potential sweep. The ratio of peak reactivation current density (Ir) and the peak anodic current density (Ia) is taken as a measure of degree of sensitization (DoS) and is termed the “current amplitude ratio (Ir/Ia)”. The lower the ratio or lesser the magnitude of peak reactivation current (Ir) with respect to the peak anodic current (Ia) or, in other words, the greater the area under the curve for the DL-EPR plot, the lower is the apparent susceptibility of the stainless steel to IGC. In the present case, the current amplitude ratios for MnSS steels were evaluated to be in the range of 0.09–0.4 x 10-4. Since the AISI 316L austenitic stainless steel have been stabilized against IGC through lowering of carbon content to 0.03 wt.%, these stainless steels were found to exhibit extremely low current amplitude ratio of 2.42 x 10-4, revealing their insusceptibility to intergranular corrosion. It has been shown previously that a sensitized stainless steel showing ditched structure (one or more grains completely surrounded by ditches) in an oxalic acid etch test (as per ASTM A 262 Practice A) is characterized by current amplitude ratio, Ir/Ia > 0.05 . The results thus show that the Mn containing austentic stainless steels are not vulnerable to intergranular corrosion (IGC). The DL-EPR plots presented in Fig. 19 are essentially cyclic polarization plots and can therefore be used to evaluate the pitting tendencies of investigated stainless steels in the test medium. The MnSS steel including AISI 316L austenitic stainless steel were found to exhibit “negative” electrochemical hysteresis loops during the reverse scan, indicating their invulnerability to pitting up to the threshold potential of 700 mV.

Table 10 gives the performance of Mn stainless steels in terms of corrosion rates calculated from weight loss in ferric chloride corrosion test for detecting detrimental intermetallic phases as per ASTM A 923 Method C. The MnSS-2 (0.02 mdd) and MnSS-4 (5.76 mdd) showed very less corrosion compared that of AISI 316L SS (6.24 mdd). MnSS-3 stainless steel showed slightly higher corrosion rates of around 7.67mdd. These values are significantly lower than the maximum acceptable corrosion rate of 10 mdd set as an acceptance criterion by the ASTM test standard and thereby, confirm the absence of deleterious intermetallic phases such as sigma (), chi (), Laves (), alpha prime () etc. in the laboratory-made low-Ni and Ni-free duplex stainless steels.

Table 1: Chemical Composition of Mn stainless steels along with AISI316L stainless steel in wt%
Alloys C Si Mn P S Cr Ni Mo Cu N
AISI 316L stainless steel 0.01 0.53 1.36 0.07 0.007 16.63 11.5 2.06 0.36 0.005
Low Mn stainless steel (MnSS-2) 0.089 0.26 7.66 0.045 0.008 18.98 6.30 2.07 0.71 0.23
High Mn stainless steel (MnSS-3) 0.08 0.10 17.38 0.048 0.001 19.36 2.03 - 1.57 0.46
High Mn stainless steel (MnSS-4) 0.08 0.15 18.26 0.060 0.001 19.17 2.12 1.04 1.32 0.40

Table 2: Inclusion analysis of stainless steels
Steels Inclusion rating (out of 5 scale)
(According to ASTM E45A) Volume fraction (%) of inclusion
(According to JIS)
Sulphide Alumina Globular Sulphide Alumina Globular Total
Thin Heavy Thin Heavy Thin Heavy
316L SS --- --- 1 1 2 4 0.0001 0.0045 0.0953 0.0999
MnSS-2 --- --- 5 2 2 5 0.0002 0.0185 0.1761 0.1948
MnSS-3 --- --- 5 2 5 4 0.0001 0.0295 0.1656 0.1961
MnSS-4 --- --- 1 1 2 1 0.0001 0.0358 0.1560 0.1919

Table 3: Room temperature tensile properties and bulk hardness of Mn stainless steels in comparison with 316 L austenitic stainless steels

Table 4: Charpy V-notch impact toughness of Mn stainless steels at various test temperatures

Table 5: Corrosion properties of Mn stainless steels in comparison with AISI 316L austenitic stainless steels in 3.5% NaCl determined using potentiodynamic polarization technique

Table 6: Corrosion properties of Mn stainless steels in comparison with AISI 316L austenitic stainless steels in 0.1m H2SO4 determined using potentiodynamic polarization technique

Table 7: Electrochemical impedances of passive films on Mn stainless steels in comparison with those on AISI 316L austenitic stainless steels in 3.5% NaCl determined using electrochemical impedance spectroscopy technique

Alloys Passive film capacitance
Q (F/cm2) x 10-5 Charge transfer Resistance
Rct(Ω.cm2) x 105
316L SS 3.55 0.35
MnSS-2 5.87 5.87
MnSS-3 4.21 3.65
MnSS-4 4.53 3.97

Table 8: Electrochemical impedances of passive films on Mn stainless steels in comparison with those on AISI 316L austenitic stainless steels in 0.1m H2SO4 determined using electrochemical impedance spectroscopy technique

Alloys Passive film capacitance
Q (F/cm2) x 10-5 Charge transfer Resistance
Rct(Ω.cm2) x 104
316L SS 4.24 2.61
MnSS-2 5.43 5.87
MnSS-3 5.5 3.58
MnSS-4 6.0 3.75

Table 9: Intergranular corrosion (IGC) susceptibility of Mn stainless steels and AISI 316L austenitic stainless steels determined using double-loop electrochemical potentiokinetic reactivation (DL-EPR) technique (Test solution: 0.5 M H2SO4 + 0.01 M KSCN, Test temperature: 30±10C, ASTM G 108)

 A current ratio of Ir/Ia > 0.05 indicates a greater susceptibility of stainless steel to sensitization and probable intergranular ditching in the test.
Table 10: Performance of Mn stainless steels in terms of corrosion rates calculated from weight loss in ferric chloride corrosion test for detecting detrimental intermetallic phases as per ASTM A 923 Method C

Steels Corrosion rate (in mdd)
316LSS 6.26
MnSS-2 0.02
MnSS-3 7.67
MnSS-4 5.46

From the above results it can be concluded that the newly developed low cost high Mn stainless steel can replace existing costlier AISI 316L stainless steel.

The novelty of present invention thus lies in formulation of a comprehensive methodology for manufacturing low cost high Mn stainless steel having following salient features:

(i) Alloy composition:

• Low Mn stainless steel composition-1 of C (0.06 to 0.1%), Mn (6 to 8%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (5 to 7%), Mo (2 to 3%), Cu (0.5 to 0.8%) and N (0.2 to 0.4%) and balance being Fe.

• High Mn high N stainless steel composition-2 of C (0.06 to 0.1%), Mn (16 to 20%), S (up to 0.01%), P (up to 0.01%), Si (0.10 to 0.45%), Cr (17 to 20%), Ni (1 to 3%), Mo (0 to 1.5%), Cu (1.00 to 2%) and N (0.3 to 0.5%) and balance being Fe.

(ii) Processing: Manufacturing method of claimed Mn containing stainless steel, comprising following steps:
• The steels ingots/slabs are reheated and soaked in a reheating furnace to 1150 to 1250 oC for 3 hours.
• Hot-rolled to 16 mm plate with finish rolling temperatures of 1000 to 1100 oC.
• The final rolling reduction is about 30-35%.
• Annealing the strips after hot rolling was carried out in the range of 1100 to 1200 oC. and subsequent water quenching is done

(iii) Microstructure: The claimed steel composition has a fully austenitic microstructure with grain size in the range of about 10 to 11 μm

(iv) Mechanical properties:

 Tensile properties
• The claimed steel compositions (both low Mn and high Mn stainless steels) has a yield strength in the range of 350 MPa to 450 MPa, ultimate tensile strength in the range of 700 MPa to 780 MPa and elongation in the range of 60 to 70%
 Hardness
• The claimed steel compositions (both low Mn and high Mn stainless steels) has a hardness in the range of 90-97 HRB
 Charpy Impact toughness
• The claimed steel compositions (both low Mn and high Mn stainless steels) has superior Charpy V-notch impact toughness properties with impact values 250 to 275 J at Room temperature and 150 to 180 J at 40 oC

(v) Corrosion resistance:
 In basic environment
• The claimed low Mn stainless steel composition has corrosion rate in the range of 0.1 to 0.2 mpy in 3.5% NaCl solution as determined by potentiodynamic polarisation test.
• The claimed low Mn stainless steel composition has pitting potential in the range of 700 to 800 mpy in 3.5% NaCl solution as determined by potentiodynamic polarisation test.
• The claimed high Mn stainless steel composition has corrosion rate in the range of 0.3 to 0.45 mpy in 3.5% NaCl solution as determined by potentiodynamic polarisation test.
• The claimed high Mn stainless steel composition has pitting potential in the range of 250 to 400 mpy in 3.5% NaCl solution as determined by potentiodynamic polarisation test.
• The claimed low Mn stainless steel composition has superior charge transfer resistance in the range of 5 to 6 X105 Ω.cm2 in 3.5% NaCl solution as determined by electrochemical impedance spectroscopy test.
• The claimed high Mn stainless steel composition has superior charge transfer resistance in the range of 3 to 4 X105 Ω.cm2 in 3.5% NaCl solution as determined by electrochemical impedance spectroscopy test.
• The claimed steel compositions (both low Mn and high Mn stainless steels) have lower film thickness (1700 to 2300 nm) as compared to 316L stainless steel (2817 nm).

 In acidic environment
• The claimed steel compositions (both low Mn and high Mn stainless steels) has corrosion rate in the range of 0.25 to 1.2 mpy in 0.1m H2SO4 solution as determined by potentiodynamic polarisation test.
• The claimed steel compositions (both low Mn and high Mn stainless steels) has pitting potential in the range of 950 to1050 mpy in 0.1m H2SO4 solution as determined by potentiodynamic polarisation test.
• The claimed low Mn stainless steel composition has superior charge transfer resistance in the range of 5 to 6 X104 Ω.cm2 in 0.1m H2SO4 solution as determined by electrochemical impedance spectroscopy test.
• The claimed high Mn stainless steel composition has superior charge transfer resistance in the range of 3 to 4 X104 Ω.cm2 in 0.1m H2SO4 solution as determined by electrochemical impedance spectroscopy test.
• The claimed steel compositions (both low Mn and high Mn stainless steels) have lower film thickness (1600 to 1800 nm) as compared to 316L stainless steel (2817 nm).

(vi) Intergranular corrosion resistance:
• The claimed steel compositions (both low Mn and high Mn stainless steels) were virtually immune to intergranular corrosion (IGC), and this was discerned from the quantification of degree of sensitization (DoS) through double loop electrochemical potentiokinetic reactivation (DL-EPR) testing in 0.5 M H2SO4 + 0.01 M KSCN as per ASTM G 108. The DoS values of  0.05 for all the steels

(vii) Intermetallic phases:
• The claimed steel compositions (both low Mn and high Mn stainless steels) have been found to be free from deleterious intermetallic phases such as sigma (), chi (), Laves (), alpha prime () etc. and this has been ascertained from corrosion rates of <10 mdd in ferric chloride corrosion testing as per ASTM A 923 Method C.

It is thus possible by way of the present invention to provide low cost low Ni austenitic stainless steel with superior mechanical property and localized corrosion resistance and a process for producing the same. More specifically the invention is directed to provide low-Ni and high Mn stainless steel (MnSS) composition evolved through formulation of appropriate chromium and nickel equivalents to achieve a fully austenitic microstructure with average grain size of about 10 to 11μm. The developed MnSS steels revealed an exceptional combination of properties: higher yield strength, higher tensile strength and higher elongation along with higher corrosion resistance in both basic and acidic environment, compared to that of commercial available AISI 316L SS making it suitable for effective use with engineering advantage for reducing wall thickness of piping/ tubing and weight of structures in corrosion-prone offshore oil platforms, oil & gas refineries, chemical processing, paper & pulp and fertilizer industries.