FERRITIC STAINLESS STEEL WITH SUPERIOR FORMABILITY PROPERTIES AND A PROCESS THEREFOR

FIELD OF THE INVENTION

This invention relates to ferritic stainless steel with superior corrosion resistance, and formability properties. This invention particularly relates to ferritic stainless steel with superior corrosion resistance, and formability properties at par or superior to AISI 304 austenitic stainless steel. The invented steel has uses in selective kitchen wares, consumer durables, buildings, equipments, automobiles, and in different structural applications.
The invention also relates to a process for producing the said steel.

BACKGROUND OF THE INVENTION AND PRIOR ART

Austenitic stainless steel AISI 304 is one of the most frequently used grades of stainless steel owing to its excellent corrosion resistance and formability properties. For most stainless steel makers, the type 304 is the principal stainless steel product and incidentally, its production accounts for about half the total production of stainless steels in the world. However, the principal drawback of this steel is its unstable price owing to price volatility of nickel (Ni), one of the key alloy components of type 304. On the other hand, the most common grade of ferritic stainless steel produced without any Ni addition is type 430, which has inferior corrosion resistance and formability compared with type 304 and is thus relegated to use in limited areas of application.

In the recent years, a slew of ferritic stainless grades have been developed with Ti/ Nb alloying such as types 439, 441 for improved formability properties and with Mo additions such as types 434, 436, 444 for enhanced localized corrosion resistance. However, the shortcoming of these steels is their inferior corrosion resistance compared to AISI 304/ AISI 316 or their high cost. Incidentally, stainless producers such as JFE Steel Corporation of Japan have discontinued the production of AISI 304 since 2004 and have since specialized in the production of ferritic stainless steels.

The novelty of the existing prior art is mostly related to-

(a) Processing of steel through costlier route AOD (Argon-Oxygen-Decarburization) VOD (Vacuum-Oxygen-Decarburization) producing technology.
(b) Properties were achieved with Lean austenitic stainless steel
(c) Properties were achieved with dual phase stainless steel containing austenite and ferrite
(d) Properties were achieved with ultra-low-carbon less than 0,01 % carbon,
(e) Properties were achieved with dual titanium and niobium stabilized ferritic stainless steel
(f) Properties were achieved with dual titanium and vanadium stabilized ferritic stainless steel
(g) Properties were achieved with dual Mo, Ni and N combination stabilized ferritic stainless steel
(h) Properties were achieved with combination of Mo, Cu and N combination stabilized ferritic stainless steel
(i) Lowering C and Cr and increasing Si and Mn to higher end in AISI 430SS
(j) Properties were achieved with Zr or Sn

References of a few published prior patents are given herein;

1. Ferritic stainless steel sheet having excellent corrosion resistance and method of manufacturing the same – US patent : US 8.465,604 B2
Abstract
A ferritic stainless steel sheet having excellent corrosion resistance and a method of manufacturing the steel sheet are provided. Specifically, the ferritic stainless steel sheet of the invention contains C of 0.03% or less, Si of 1.0% or less, Mn of 0.5% or less, P of 0.04% or less, S of 0.02% or less, Al of 0.1% or less, Cr of 20.5% to 22.5%, Cu of 0.3% to 0.8%, Ni of 1.0% or less, Ti of 4×(C %+N %) to 0.35%, Nb of less than 0.01%, N of 0.03% or less, and C+N of 0.05% or less, and has the remainder including Fe and inevitable impurities, wherein 240+35×(Cr %-20.5)+280×{Ti %-4×(C %+N %)}?280 is satisfied.

2. Process for manufacturing ferritic stainless steel sheet having good formability, surface appearance and corrosion resistance – US patent : 4,374,683
Abstract
A ferritic stainless steel containing 12.00-25.00% of Cr, 0.1-2.0% of Cu and 0.2-2.0% of Nb with sulfur being restricted to not greater than 0.02%, preferably not greater than 0.005% is hot rolled with a finishing temperature of 850° C. or less, which is lower than the usual finishing temperature, and the resulting hot rolled steel strip is annealed at a relatively high temperature of 950°-1050° C. The resulting cold rolled steel sheet, though it is less expensive, possesses good formability, ridging resistance, surface appearance and corrosion resistance, and is particularly suitable for making ornamental articles, such as automotive mouldings.

3. Ferrite-based stainless steel and method for manufacturing same – Korean patent: WO2016105092A1
Abstract
The present invention relates to a ferrite-based stainless steel and a method for manufacturing the same, wherein the ferrite-based stainless steel has excellent moldability by controlling the contents of ingredients constituting molten steel and the size of crystal grains. The ferrite-based stainless steel according to an embodiment of the present invention contains C (0.0005-0.01 wt%), N (0.005-0.015 wt%), Si ( 0.01-0.20 wt%), Mn ( 0.01-0.20 wt%), P (0.001-0.03 wt%), S (0.0001-0.005 wt%), Cr (10-20 wt%), Ni (0.001-0.15 wt%), Al (0.05-0.30 wt%), the balance Fe, and other inevitable impurities, the ratio of Nb/Ti being 0.1-0.6, and satisfies expression (1) below. 0.1 < 400C + 85.7N + 55.6P + 7.7Si + 7.3Nb < 5 ----------- (1), wherein in expression (1), C, N, P, Si, and Nb mean contents (wt%) of respective ingredients.

4. Ferritic stainless steel – US patent : 10047419
Abstract
The invention relates to a ferritic stainless steel having enhanced high temperature strength and good resistance to high cycle fatigue, creep and oxidation for use in high temperature service, for components such as automotive exhaust manifolds. The steel contains in weight % less than 0.03% carbon, 0.05-2% silicon, 0.5-2% manganese, 17-20% chromium, 0.5-2% molybdenum, less than 0.2% titanium, 0.3-1% niobium, 1-2% copper, less than 0.03% nitrogen, 0.001-0.005% boron, the rest of the chemical composition being iron and inevitable impurities occurring in stainless steels.

5. FERRITIC STAINLESS STEEL – US patent: 20160281184
Abstract
The invention relates to a ferritic stainless steel having excellent corrosion and sheet forming properties. The steel consists of in weight percentages 0.003-0.035% carbon, 0.05-1.0% silicon, 0.1-0.8% manganese, 20-24% chromium, 0.05-0.8% nickel, 0.003-0.5% molybdenum, 0.2-0.8% copper, 0.003-0.05% nitrogen, 0.05-0.8% titanium, 0.05-0.8% niobium, 0.03-0.5% vanadium, less than 0.04% aluminium, and the sum C+N less than 0.06%, the remainder being iron and inevitable impurities in such conditions, that the ratio (Ti+Nb/(C+N) is higher or equal to 8, and less than 40, and the ratio Tieq/Ceq=(Ti+0.515*Nb+0.940*V)/(C+0.858*N) is higher or equal to 6, and less than 40

6. Ferritic stainless steel with execellent corrosion resistnace and excellent discoloration resistance - WO2008082096A1
Abstract
The present invention relates to a high Cr ferritic stainless steel substituting Cr, Cu, Ti, Nb, etc, for an austenitic steel containing expensive Ni high corrosion resistance ferritic stainless steel and to a high corrosion resistance ferritic stainless steel with the same or more corrosion resistance as compared to the type 304 steel. The ferritic stainless steel is made of C of 0.01% or less, Si of 0.2 to 1.0%, Mn of 0.3% or less, Cr of 20 to 23%, Ni of 0.2 to 0.4%, N of 0.01% or less, Al of 0.03 to 0.10%, S 0.002% or less, Cu of 0.3 to 0.5%, Zr of 0.02 to 0.06%, Ti of 0.2 to 0.4% (Ti/(C+N) > 20), and Nb of 0.06 to 0.45% (Nb/(C+N)>28)), and Fe and inevitable impurities as the remaining components, wherein a unit is mass %. Also, a manufacturing method of performing a cold rolling and a surface polishing on the ferritic stainless steel is provided.
7. Ferritic stainless steel sheet and method for manufacturing the same – Japanese patent : JP2011214060A
Abstract
PROBLEM TO BE SOLVED: To provide a ferritic stainless steel sheet which is superior in toughness, has adequate corrosion resistance and is superior in productivity and economical efficiency.SOLUTION: The ferritic stainless steel sheet includes, by mass%, 0.03% or less C, 0.03% or less N, 0.05% or less C+N, 0.70% or less Si, 0.50% or less Mn, 0.04% or less P, 0.02% or less S, 16-25% Cr, 1.0% or less Ni, 4×(C+N) to 0.40% Ti, 0.1% or less V, 0.1% or less Nb, 0.01-0.05% Al, 0.02-0.25 Zr and the balance Fe with unavoidable impurities; and contains a nitride in the steel sheet, which is substantially ZrN.

8. Ferritic stainless steel sheet and method for manufacturing the same – Japanese patent : JP2011214063A
Abstract
PROBLEM TO BE SOLVED: To provide a ferritic stainless steel sheet which is superior in toughness, has adequate corrosion resistance and is superior in productivity and economical efficiency.
SOLUTION: The ferritic stainless steel sheet includes, by mass%, 0.03% or less C, 0.03% or less N, 0.05% or less C+N, 0.70% or less Si, 0.50% or less Mn, 0.04% or less P, 0.02% or less S, 16-25% Cr, 1.0% or less Ni, less than 4×(C+N)% Ti, 0.1% or less V, 0.1% or less Nb, 0.01-0.05% Al, 0.02-0.40% Zr and the balance Fe with unavoidable impurities; and contains a nitride in the steel sheet, which is substantially ZrN.

9. Ferrite-based stainless steel and production method therefor – WO2015141145A1
Abstract
Provided is a ferrite-based stainless steel having excellent corrosion resistance and showing good brazing performance when brazing at high temperature using a Ni-containing brazing material, as a result of generating a nitrogen-concentrated layer having a peak value for nitrogen concentration between the surface and a depth of 0.05 µm of 0.50%-0.30% by mass and as a result of having a composition containing 0.003%-0.020% C, 0.05%-1.00% Si, 0.10%-0.50% Mn, no more than 0.05% P, no more than 0.01% S, 16.0%-25.0% Cr, 0.05%-0.35% Ti, 0.005%-0.05% Al, and 0.005%-0.025% N, with the remainder being Fe and unavoidable impurities.

10. FERRITIC STAINLESS STEEL PLATE WHICH HAS EXCELLENT RIDGING RESISTANCE AND METHOD OF PRODUCTION OF SAME – US patent: 20170349988
Abstract
The present invention focuses on Sn and has as its problem to not only improve the corrosion resistance and rust resistance of Cr-containing ferritic stainless steel but also improve the ridging resistance. The present invention derives the relationship between Ap, which shows the ?-phase rate at 1100° C. due to a predetermined ingredient, and Sn in ferritic stainless steel which becomes a dual phase structure of a+? in the hot rolling temperature region, applies and adds Sn, and hot rolls the steel to give a total rolling rate of 15% or more in 1100° C. or higher hot rolling to thereby obtain ferritic stainless steel sheet which has good ridging resistance, which also has excellent corrosion resistance and rust resistance, and which can be applied to general durable consumer goods: 0.060?Sn?0.634-0.0082 Ap 10?Ap?70

11. Ferritic stainless steel plate which has excellent ridging resistance and method of production of same – US patent: 9,771,640
Abstract
A ferritic stainless steel sheet having ridging resistance contains, by mass, 0.025 to 0.30% C, 0.01 to 1.00% Si, 0.01 to 2.00% Mn, 0.050% or less P, 0.020% or less S, 11.0 to 22.0% Cr, and 0.022 to 0.10% N. In addition, Ap, which is defined as 420C+470N+23Ni+9Cu+7Mn-11.5(Cr+Si)-12Mo-52Al-47Nb-49Ti+189 wherein each of Sn, C, N, Ni, Cu, Mn, Cr, Si, Mo, Al, Nb, and Ti denotes the content of the element, satisfies 10?Ap?70. Furthermore, a content of Sn satisfies 0.060?Sn?0.634-0.0082Ap. Residual ingredients are Fe and unavoidable impurities, and a metal structure of the steel sheet is a ferrite single phase. The ferritic stainless steel sheet has a ridging height of less than 6 µm. This ferritic stainless steel sheet improves the corrosion resistance and rust resistance of Cr-containing ferritic stainless steel as well as the ridging resistance.

12. Ferritic stainless steel having good corrosion resistance – US patent: US 4360381
Abstract
A 10-35% Cr ferritic stainless steel stabilized with 0.20-1.00% of Nb wherein Nb%.gtoreq.(8.times.C%+2.0%) with impurities, such as carbon, nitrogen, phosphorous, oxygen and sulfur reduced to given levels is disclosed. By reducing the sulfur content to a level of not greater than 0.002%, preferably less than 0.001% or in cases where copper and/or nickel is added, to a level of not greater than 0.005%, the corrosion resistance of the resulting steel can markedly be improved. Since the ferritic stainless steel of this invention also exhibits a combination of good surface appearance and formability, it can be substituted for certain austenitic stainless steels not only as a general corrosion resistant material, but also as a material for making external automotive trims and so on.

13. Ferritic stainless steel sheet excellent in press formability and workability and method for production thereof – US patent: US 7341637
Abstract
A ferritic stainless steel sheet excellent in press formability and operability, characterized by: containing appropriate amounts of C, N, Cr, Si, Mn, P, S, Al, Ti and V, with the balance consisting of Fe and unavoidable impurities; having a solid lubricating film or films on one or both of the surfaces; and having a ratio Z, defined as Z=Z1/Z2, of less than 0.5, a tensile strength of 450 MPa or less and an average r-value of 1.7 or more, wherein Z1 is a friction coefficient of the surface of a solid lubricating film and Z2 that of the surface of a reference material coated with neither a coating nor lubricating oil. In the ferritic stainless steel sheet, the amounts of Sol-Ti and Insol-V may be regulated to appropriate ranges, wherein Sol-Ti means the amount of Ti existing in the state of solid solution in steel and Insol-V means the amount of V existing in the state of precipitation in steel.

14. Ferritic stainless steel welded pipe superior in expandability – US patent: US 7754344
Abstract
A ferritic stainless steel welded pipe is ferritic stainless steel welded pipe contains, by wt %, C: 0.001 to 0.015%, N: 0.001 to 0.020%, Cr: 11 to 25%, Mo: 0.01 to 2.0%, one or both of Ti and Nb in 0.05 to 0.5%, and B: 0.0003 to 0.0030%, having an elongation of the welded pipe material in the direction becoming the circumferential direction of 30% or more, and having an average Lankford value of 1.5 or more, which is formed, welded, and sized by 0.5 to 2.0% in terms of circumferential length, then annealed at 700 to 850° C., and has the hardness difference between the weld zone and the matrix is 10 to 40 in range and a ratio between the bead thickness of the weld zone and the thickness of the matrix is 1.05 to 1.3.

15. Ferritic stainless steel sheet superior in shapeability and method of production of the same – US patent: US 20090000703
Abstract
The present invention provides a ferritic stainless steel sheet superior in shapeability containing, by wt %, C: 0.001 to 0.010%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, P: 0.01 to 0.04%, Cr: 10 to 20%, N: 0.001 to 0.020%, Nb: 0.3 to 1.0%, and Mo: 0.5 to 2.0%, wherein the total precipitates are, by wt %, 0.05 to 0.60%. A method of production of a ferritic stainless steel sheet superior in shapeability comprising producing a cold rolling material in the production process so that the Nb-based precipitates become, by vol %, 0.15% to 0.6% and have a diameter of 0.1 ?m to 1 ?m and/or so that the recrystallized grain size becomes 1 ?m to 40 ?m and the recrystallization rate becomes 10 to 90%, then cold rolling and annealing it at 1010 to 1080° C.

16. Ferritic stainless steels with improved drawability and resistance to ridging – US patent: US 3713812A
Abstract
A method for increasing the deep drawability, as represented by r value, of ferritic stainless steels. A complex equation shows the interrelation of the various alloying elements. Within the compositional range similar to that of type 430 steel, the r value may be increased by employing C and Cr at the lower end of the range and employing Si at the higher end of the range. Increasing the amount of Mn will increase r value for a type 430 steel, but will have just the opposite effect if Cb is present to any appreciable degree.

17. Ferritic stainless steel – European patent: EP2922978B1

Abstract
Ferritic stainless steel having excellent corrosion and sheet forming properties, characterized in that the steel consists of in weight percentages 0,003 - 0,035 % carbon, 0,05 - 1,0 % silicon, 0,1 - 0,8 % manganese, 20 - 21,5 % chromium, 0,05 - 0,8 % nickel, 0,003 -0,5 % molybdenum, 0,2 - 0,8 % copper, 0,003 - 0,05 % nitrogen, 0,05 - 0.15 % titanium, 0.25% - 0,8 % niobium, 0,03 - 0,5 % vanadium, 0.010- 0,04 % aluminium, and the sum C+N less than 0,06 %, the remainder being iron and inevitable impurities, wherein the ratio (Ti+Nb)/(C+N) is higher or equal to 8, and less than 40, and the ratio Tieq/Ceq = (Ti + 0,515*Nb +0,940*V)/(C+0,858*N) is higher or equal to 6, and less than 40, and the steel is produced using AOD (Argon-Oxygen-Decarburization) technology.

18. Method for manufacturing hot-rolled ferritic stainless steel sheet excellent in toughness – Japanese patent: JP2010100877A
Abstract
ROBLEM TO BE SOLVED: To provide a method for manufacturing a hot-rolled ferritic stainless steel sheet excellent in the toughness, with high efficiency and at a low cost.
SOLUTION: A steel material containing, by mass, =0.03% C, =0.03% N, =0.05% C+N, =0.70% Si, =0.50% Mn, =0.04% P, =0.02% S, 20.5-25% Cr, 0.3-0.8% Cu, =1.0% Ni, 4×(C+N) to 0.4% Ti, =0.1% V, =0.5% Nb, =0.1% Mo, 0.02-0.08% Al and the balance Fe with inevitable impurities, is hot-rolled to form the steel sheet, and then reheated to the temperature of =550°C and a water-toughening treatment is applied in the method for manufacturing the hot-rolled ferritic stanless steel sheet excellent in toughness.

19. Lean austenitic stainless steel containing stabilizing elements – US patent: US 9,133,538
Abstract
An austenitic stainless steel composition including relatively low nickel and molybdenum levels, and exhibiting corrosion resistance, resistance to elevated temperature deformation, and formability properties comparable to certain alloys including higher nickel and molybdenum levels. Embodiments of the austenitic stainless steel include, in weight %, up to 0.20 C: 2.0 to 9.0 Mn, up to 2.0 Si, 16.0 to 23.0 Cr, 1.0 to 7.0 Ni, up to 3.0 Mo, up to 3.0 Cu, 0.05 to 0.35 N, up to 4.0 W, (7.5(C)).ltoreq.(Nb+Ti+V+Ta+Zr).ltoreq.1.5, up to 0.01 B, up to 1.0 Co, iron and impurities. Additionally, embodiments of the steel may include 0.5.ltoreq.(Mo+W/2).ltoreq.5.0 and/or 1.0.ltoreq.(Ni+Co).ltoreq.8.0.

20. Austenitic-ferritic stainless steel – European Patent No. : EP1715073B1
Abstract
An austenitic-ferritic stainless steel sheet showing excellent deep drawability, the stainless steel sheet having austenite and ferrite two-phase structure, and comprising 0.2% or less C, 4% or less Si, 7% or less Mn, 0.1% or less P, 0.03% or less S, 15 to 35% Cr, 1 to 3% Ni, 0.05 to 0.6% N, optionally 4% or less Mo, optionally 4% or less Cu, optionally 0.5% or less V, optionally 0.1% or less Al, optionally 0.01% or less B, optionally 0.01% or less Ca, optionally 0.01% or less Mg, optionally 0.1% or less REM, optionally 0.1% or less Ti, and optionally 2.0% or less Nb, by mass, and balance of Fe and inevitable impurities, the amount of (C + N) in the austenite phase being in a range from 0.16 to 2% by mass, and the volume percentage of the austenite phase being in a range from 10 to 85%, wherein a high ductile characteristic of 48% or larger total elongation for a sheet thickness of 0.8mm can be attained by controlling the strain-induced martensite index Md(?) of austenite phase to a range from -30 to 90.
The present invention for the development of Ferritic stainless steel with superior mechanical and corrosion resistant properties than that of AISI 304 and 304L stainless steel is very different from the above said prior art and a research program was conceived to develop a suitable alloy design and process technology for manufacture of low-cost ferritic stainless steel with good corrosion resistance and formability properties on par or superior to AISI 304 austenitic stainless steel.

OBJECTS OF THE INVENTION

The object of the invention is to provide for a suitable alloy design for the development of a Ferritic stainless steel with superior mechanical and corrosion resistant properties on par or superior to AISI 304 austenitic stainless steel.

The other object is to provide for a process to produce the same.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: Shows the phase diagram constructed to design steel using Thermo-Calc software.

Figure 2: Depicts the corresponding superimposed stress-strain plots for the hot rolled and annealed steels.

Figure 3: Shows the typical optical micrographs of hot rolled and annealed ferritic stainless steels designated FSS-1 and FSS-2, each for two different annealing temperatures.

Figure 4: Depicts the corresponding superimposed potentiodynamic polarisation plots for hot rolled and annealed ferritic stainless steels designated FSS-1 and FSS-2 in 3.5% NaCl solution including 304 and 304L austenitic stainless steels.

Figure 5: Depicts graphically pitting potentials of ferritic stailess steels FSS-1 and FSS-2 in 3.5% NaCl solution as a function of annealing temperature vis-a-vis 304 and 304 L austenitic stainless steels.

Figure 6: Depicts graphically the formability properties for cold rolled and annealed stainless steels FSS-1 and FSS-2 in terms of average Plastic Strain Ratio (r m) and Anisotropy (Delta r) values.

Figure 7 to 10: Show the results of EBSD analysis including textural aspects for cold reductions followed by annealing at 850 deg. C.

DESCRIPTION OF THE INVENTION

According to the invention there is provided a high corrosion resistant and high formable Ferritic stainless steel having composition comprising in percent by wt.: C : 0.01 to 0.08, Si : 0.10 to 0.5, Mn : 0.1 to 1, S : up to 0.03, P : up to 0.03, Cu : 0.2 to 0.5 , Cr : 16 to 22, Ti : 0.1 to 0.5, N : 0.01 to 0.1 and the balance being Fe.

Ferritic stainless steel of the invention as stated above has the yield strength in the range of 250 MPa to 350 MPa, ulimate tensile strength in the range of 350 MPa to 460 MPa and elongation in the range of 20 to 40 % in hot rolled annealed condition.

Said Ferritic stainless steel has the yield strength in the range of 300 MPa to 400 MPa, ultimate tensile strength in the range of 400 MPa to 500 MPa and elongation in the range of 20 to 30 % in cold rolled annealed condition.

The said Ferritic stainless steel has the corrosion rate in the range of 0.1 to 1 mpy in 3.5 % NaCl solution and has Pitting resistance with pitting potential in the range of 220 to 450 mpy in 3.5 % NaCl solution and corrosion resistance corrosion current density ranging from 0.1 to 1 micro A /cm2 in 3.5 % NaCl solution.

Ferritic steel of the invention has superior formability with Plastic Strain Ratio (r m) ranging from 0.5 to 2 and lower "Earing" tendency (Delta-r) : -0.05 to 0.1 and partial recrystallisation of ferritic grains with formable texture component " gamma fibre" of about 20 to 30 %.

The invention also relates to a method for producing high corrosion resistant and high formable Ferritic stainless steel which comprises of the following steps:

a) The steels ingots/slabs are reheated and soaked in a reheating furnace to 1150 to 1250o C for 3 hours;

b) Hot-rolled to 5 to 6 mm plate with finish rolling temperatures of 900 to 800o C.;

c) Hot rolling reduction is about 80-90% (~6mm strip).
d) Solution annealing at 750 to 900o C for 2 hours and subsequent water quenching;

e) The final cold rolling reduction is about 60-95% and final thickness 1.22 to 0.2mm.

f) Annealing the cold strips after cold rolling in the range of 800 to 900o C. for 2 to 10 min. and subsequent water quenching.

The development of this new steel is intended to provide a practical substitute for AISI 304 in selective kitchenware, consumer durable, building, and equipment, automobile and structural applications.

Production process and evaluation of the properties of the invented steel :

Examples:

Invented Ferritic stainless steel with improved corrosion resistance, and formability properties viz. pitting potential > 300 mV in 3.5% NaCl and Lankford coefficient (r-value) > 1.0, which are comparable to austenitic stainless grade AISI 304, was made on laboratory scale.

The alloy chemistry was designed incorporating low interstitial content (C+N: 400 ppm) and high Cr content (Cr~ 21 wt. %) with controlled additions of Cu (0.4 wt. %) and Ti. Thermodynamic phase equilibria were examined from 1700 to 300o C for steel composition using ThermoCalc?.

Two heats were made with varying Ti levels [Ti stoichiometry: 4*(C+N) & 5*(C+N)] in 100-kg air induction furnace. 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 1250o C for 3 hours for thermal/ compositional homogenization and then hot-rolled in Hillé-UK make experimental rolling mill in 2 rolling campaigns to 5-6 strips with finish rolling temperatures of 850o C to avoid edge cracking. Hot rolled strips were annealed between 750 to 900o C for 15 min and water quenched post heat treatment. Tensile properties of hot rolled and annealed steels were evaluated using 50 mm standard gauge length samples machined as per ASTM A370-15 and tested at cross-head speed of 2 mm/min in Dak System UTM-7200 series universal testing machine. Electrochemical corrosion properties of the hot rolled and annealed steels were investigated through potentiodynamic polarization technique in 3.5% NaCl solution using EG&G PAR Model 273A potentiostat annexed with a PAR FRD-100 frequency response detector. Stainless strips solution-annealed at 850oC were further subjected to scale removal and cold rolled in laboratory rolling mill with overall cold reductions to the tune of 64-93% through multipass cold rolling to final thickness of 1.22 to 0.20 mm. Finish thickness as low as 0.20-0.24 mm could be achieved in these novel stainless materials, which demonstrated their excellent cold reducibility. Cold rolled strips were eventually annealed at 850oC to evolve favourable recrystallisation texture for achieving superior and formability properties. Tensile properties of cold rolled and annealed steels were evaluated using 25 mm subsize gauge length samples machined as per ASTM A370-15 and tested at cross-head speed of 2 mm/min in universal testing machine. Formability parameters in terms of average plastic strain ratio (rm) and planar anisotropy values (?r) were determined using Tinius Olsen MODUL-r drawability tester based on resonant frequency technique. Textural aspects related to microstructures of cold rolled and annealed steel samples were studied using electron backscattered diffraction (EBSD) analysis in Carl Zeiss EVO MA10 scanning electron microscope (SEM-EBSD).

Table 1 shows the designed chemistry for ferritic stainless steel and the chemical composition of the steels, designated FSS-1 and FSS-2, produced in the laboratory.

Table 1: Chemical composition of ferritic stainless steel heats made in laboratory including the designed composition (in wt %)

Steel C Si Mn P S Cr Cu Ti N Fe
Design 0.025 max 0.10-0.20 0.20-0.30 0.03 max 0.01 max 21.00-22.00 0.30-0.40 0.25-0.30 0.02-5 max Bal
FSS-1 0.043 0.46 0.32 0.021 0.015 20.34 0.44 0.29 0.02-6 Bal
FSS-2 0.052 0.40 0.39 0.028 0.018 20.32 0.38 0.42 0.02-8 Bal

The steels were produced with Cr content (~20 wt %) higher than AISI 430, the common grade of ferritic stainless steel, and also contain Cu to the tune of 0.4 wt % for improved corrosion resistance. Ti is also incorporated in the chemical composition of steels for:

1. Stabilization against sensitization and
2. Tying up of interstitials (C,N) for improving formability and intergranular corrosion (IGC) resistance

Also, Ti levels in the steels were varied to investigate its specific influence on resultant properties. Ti stoichiometry of steels designated FSS-1 and FSS-2 was 4 and 5 times C+N, respectively.

Accompanying Figure 1 shows the phase diagram constructed for designed composition of steel using Thermo-Calc software. The phase diagram clearly shows the stability of ferrite phase with no major transformation over a wide temperature range. This implies that grain refinement would pose a challenge in the steel and may only be achieved through recrystallization annealing after sufficient cold deformation. It also means high temperature exposures, such as during hot rolling, would lead to irrevocable coarsening of the ferrite in steel leading to considerable diminution in mechanical properties. The phase diagram also indicates thermodynamic stability of secondary phases (or, precipitates) such as carbides, nitrides, intermetallic compounds (IMCs, Sigma, Laves) between 650-300oC, and it is understood that these precipitates could potentially affect the dynamic softening process and recrystallisation kinetics during hot deformation and annealing treatments as well as have marked influence on resultant mechanical and corrosion properties.

Tables 2 and 3 show the room temperature tensile properties and hardness of hot rolled and annealed ferritic stainless steels designated FSS-1 and FSS-2.

Table 2: Tensile properties and hardness of as-hot rolled and annealed ferritic stainless steel FSS-1 with lower Ti content vis-à-vis 304 and 304L austenitic stainless steels (Ti stoichiometry = 4*(C+N))
Heat No. FSS-1
YS (MPa) UTS (MPa) % Elongation Strain hardening exponent (n) Hardness (HRB)
Uniform Total
As-rolled 396.21 480.00 9.47 17.38 0.12 89.8
Annealed at 750oC 348.31 456.34 10.83 21.16 0.15 86.2
Annealed at 800oC 327.19 446.32 14.98 24.95 0.17 85.0
Annealed at 850oC 322.25 423.31 15.23 26.00 0.17 83.3
Annealed at 900oC 268.96 397.31 17.54 28.32 0.20 78.2
AISI 304 (No.1 Finish) 275.46 719.77 60.18 65.62 0.60 89.6
AISI 304L (No.1 Finish) 297.54 645.63 41.64 47.03 0.31 92.0

Table 3: Tensile properties and hardness of as-hot rolled and annealed ferritic stainless steel FSS-2 with higher Ti content (Ti stoichiometry = 5*(C+N))

Heat No. FSS-2
YS (MPa) UTS (MPa) % Elongation Strain hardening exponent (n) Hardness (HRB)
Uniform Total
As-rolled 392.81 487.07 10.45 18.13 0.13 89.6
Annealed at 750o C 332.46 438.78 13.65 27.41 0.14 86.8
Annealed at 800o C 307.86 424.85 15.43 28.74 0.17 83.0
Annealed at 850o C 268.05 412.33 20.15 35.47 0.21 79.3
Annealed at 900o C 273.36 416.92 19.91 32.50 0.21 78.7

Figure 2 depicts the corresponding superimposed stress-strain plots for the hot rolled and annealed steels. Table 2 also gives the tensile properties of AISI 304 and 304L austenitic stainless steels in No.1 finish (hot rolled, annealed and pickled condition) for comparison.

The following major inferences can be derived from these results:
1. In hot rolled and annealed condition, the steels FSS-1 and FSS-2 are found to exhibit YS of 268-348 MPa, UTS of 397-456 MPa and total elongation of 21-35%.
2. Steel with higher Ti level viz. FSS-2 is found to exhibit better elongations, in general, owing to effective tying up of residual interstitials (C & N).
3. As-rolled elongations are quite poor for both FSS-1 and FSS-2 steels regardless of Ti content and this could be attributed variously to coarse grain size, non-uniform Cr distribution, internal stress and precipitates.
4. Elongations are found to improve with annealing in temperature range of 750-900oC, although at higher temperature this improvement is at the expense of considerably loss in strength and is evidently due to ferrite grain coarsening.
5. In comparison, the typical YS, UTS and total elongation for AISI 304 austenitic stainless steel are 275 MPa, 720 MPa and 66% respectively, while the same for 304L are 297 MPa, 646 MPa and 47%, correspondingly.

Figure 3 shows the typical optical micrographs of hot rolled and annealed ferritic stainless steels designated FSS-1 and FSS-2, each for two different annealing temperatures. Steel FSS-1 was found to exhibit rather coarse, elongated but recovered ferrite grains replete with copiously dispersed precipitates, presumably comprising Cr23C6 and TiN particles. In contrast, steel FSS-2 revealed more equiaxed ferrite grain structure, which was even so equally coarse as FSS-1. Further, steel FSS-2 was characterised by an overall heterogeneity in grain size and was found to exhibit multimodal grain size distribution, albeit with a more equiaxed grain aspect ratio compared to FSS-1. In comparison, this steel also did not reveal as profuse a dispersion of precipitate particles as FSS-1.

Table 4 shows the electrochemical parameters deduced from potentiodynamic polarisation of ferritic stainless steels in 3.5% NaCl depicting their corrosion, passivation and pitting characteristics in aqueous chloride medium vis-à-vis 304 and 304L austenitic stainless steels.

Table 4: Electrochemical parameters deduced from potentiodynamic polarisation of ferritic stainless steels in 3.5% NaCl depicting their corrosion, passivation and pitting characteristics in aqueous chloride medium vis-à-vis 304 and 304L austenitic stainless steels

Steel Condition (As-rolled/ Annealed) Ecorr
(mV) Icorr
(mA/cm2) Ipass at 100 mV (mA/cm2) Epit
(mV) Corrosion rate (mpy)
FSS-1 As rolled -56 1.8 1.2 223 0.8
7500 C -104 0.7 0.8 312 0.3
8000 C -81 0.9 1.3 355 0.4
8500 C -48 1.1 1.8 330 0.5
9000 C -55 0.9 1.0 318 0.4
FSS-2 As rolled -63 1.5 1.7 325 0.7
7500 C -62 0.7 1.0 346 0.3
8000 C -58 0.5 0.8 338 0.2
8500 C -55 0.5 0.8 354 0.2
9000 C -55 0.3 0.6 432 0.1
304 No.1 finish -87 1.9 2.0 308 0.9
304L No.1 finish -108 1.8 2.0 266 0.8

Figure 4 depicts the corresponding superimposed potentiodynamic polarisation plots for hot rolled and annealed ferritic stainless steels designated FSS-1 and FSS-2 in 3.5% NaCl solution including 304 and 304L austenitic stainless steels.

The results clearly reveal nobler corrosion potentials, lower corrosion & passive currents and higher pitting potentials for annealed ferritic stainless steels compared to 304 and 304L stainless steels indicating formation of more stable, tenacious and protective passive films on these steels. It is also discernible that the pitting potentials for FSS-1 and FSS-2 improve significantly with annealing between 750-900oC. Incidentally, the steel with higher Ti content viz. FSS-2 was found to reveal extremely low passive currents and high pitting potentials in chloride medium underlining its superior corrosion performance over other steels. Steels FSS-1 and FSS-2 were found to reveal corrosion rates as low as 0.1-0.5 mpy and pitting potentials in excess of 300 mV consistently in annealed condition as compared to 0.9 mpy & 308 mV and 0.8 mpy & 266 mV for 304 and 304L stainless steels, respectively.

Figure 5 graphically depicts pitting potentials of ferritic stainless steels FSS-1 & FSS-2 in 3.5% NaCl as a function of annealing temperature vis-à-vis 304 and 304L austenitic stainless steels.

Tables 5 and 6 show the tensile properties of cold rolled and annealed ferritic stainless steels designated FSS-1 and FSS-2.

Table 5: Tensile properties of cold rolled and annealed ferritic stainless steel FSS-1 with lower Ti content (Ti stoichiometry = 4*(C+N))
Sl. No. Condition Strip thickness (mm) YS (MPa) UTS (MPa) Uniform elongation (%) Total elongation to fracture (%)
1. 64% cold reduction + Annealing at 850oC for 137 sec 1.10 354.15 509.98 19.62 30.96
2. 83% cold reduction + Annealing at 850oC for 69 sec 0.53 330.42 499.84 19.68 30.77
3. 89% cold reduction + Annealing at 850oC for 60 sec 0.35 353.64 528.35 20.00 31.06
4. 91% cold reduction + Annealing at 850oC for 56 sec 0.28 365.52 524.81 21.48 28.14
5. 93% cold reduction + Annealing at 850oC for 56 sec 0.20 327.15 481.83 18.98 19.14

Table 6: Tensile properties of cold rolled and annealed ferritic stainless steel FSS-2 with higher Ti content (Ti stoichiometry = 5*(C+N))
Sl. No. Condition Strip thickness (mm) YS (MPa) UTS (MPa) Uniform elongation (%) Total elongation to fracture (%)
1. 65% cold reduction + Annealing at 850oC for 137 sec 1.22 367.80 533.83 19.31 31.04
2. 81% cold reduction + Annealing at 850oC for 77 sec 0.65 356.91 530.29 19.79 33.66
3. 83% cold reduction + Annealing at 850oC for 69 sec 0.57 342.25 501.49 20.36 34.17
4. 91% cold reduction + Annealing at 850oC for 60 sec 0.31 359.71 523.29 20.91 32.20
5. 93% cold reduction + Annealing at 850oC for 56 sec 0.24 379.19 555.99 18.04 22.56

It can be observed that a marked improvement in the tensile properties in terms of strength-ductility combination is achieved in both ferritic stainless steels, which has been rendered possible through cold reduction of 64-93% followed by annealing at 850o C. The results show that these novel stainless steels can be effectively produced with improved yield strength to the level of 350-380 MPa with concomitant ultimate tensile strength of 520-550 MPa and total elongation of 30-34% by employing cold reductions to the tune of 80-90% followed by controlled annealing at 850o C, thereby placing them closer to the realm of high strength steels.

Figure 6 graphically depicts the formability properties for cold rolled and annealed ferritic stainless steels FSS-1 and FSS-2 in terms of average Plastic Strain Ratio (rm) and Planar Anisotropy (?r) values.

It is clear that superior Plastic Strain Ratio (rm > 1.2) and lower ‘Earing’ tendency (Dr) can be achieved in ferritic stainless steels FSS-1 and FSS-2 with 80-90% cold reduction followed by annealing at 850oC.

Figures 7-10 show the results of EBSD analysis including textural aspects for cold rolled and annealed ferritic stainless steels FSS-1 and FSS-2 after ~65% and 90% cold reductions followed by annealing at 850oC.

The EBSD images clearly show the formation of finer recrystallized ferrite grains with higher amount of cold reduction, thus corroborating the previous observations. Interestingly, pancaking of recrystallized ferrite grains could be observed in the steel with higher Ti content (FSS-2). This observation does, however, merit further investigation.

While there has been shown and described herein some preferred embodiments of the present invention many minor variations and changes apparent to those skilled in the art may be made without departing from the purport and scope of the invention which is defined in the appended claims.

WE CLAIM:

1. High corrosion resistant and high formable Ferritic stainless steel having composition comprising in percent by wt.: C: 0.01 to 0.08, Si: 0.10 to 0.5, Mn: 0.1 to 1, S: up to 0.03, P: up to 0.03, Cu: 0.2 to 0.5 , Cr: 16 to 22, Ti: 0.1 to 0.5, N: 0.01 to 0.1 and the balance being Fe.

2. Ferritic stainless steel as claimed in claim 1, wherein the steel is having the composition in percent by wt. C - 0.043, Si - 0.46, Mn - 0.32, P - 0.021, Cu - 0.44, Cr - 20.34, TI - 0.29, N - 0.026 and the balance being Fe.

3. Ferritic stainless steel as claimed in claim 1, wherein the steel has the composition in percent by wt. C - 0.052, Si - 0.40, Mn - 0.39, P - 0.028, S - 0.018, Cu - 0.38, Cr - 20.32, TI - 0.42, N - 0.028 and the balance being Fe.

4. Ferritic stainless steel as claimed in claim 1, has the yield strength in the range of 250 MPa to 350 MPa, ulimate tensile strength in the range of 350 MPa to 460 MPa and elongation in the range of 20 to 40 % in hot rolled annealed condition.

5. Ferritic stainless steel as claimed in claim 1, has the yield strength in the range of 300 MPa to 400 MPa, ultimate tensile strength in the range of 400 MPa to 500 MPa and elongation in the range of 20 to 30 % in cold roll annealed condition.

6. Ferritic stainless steel as claimed in claim 1, has corrosion rate in the range of 0.1 to 1 mpy in 3.5 % NaCl solution and has Pitting resistance with pitting potential in the range of 220 to 450 mpy in 3.5 % NaCl solution and corrosion resistance corrosion current density ranging from 0.1 to 1 micro A /cm2 in 3.5 % NaCl solution.

7. Ferritic stainless steel as claimed in claim 1, has formability with Plastic Strain Ratio ( r m) ranging from 0.5 to 2 and lower "Earing" tendency (Delta-r) : -0.05 to 0.1 and partial recrystallisation of ferritic grains with formable texture component " gamma fibre" of about 20 to 30 %.

8. A process for producing high corrosion resistant and high formable Ferritic stainless steel as claimed in claims 1 to 7, comprising the following steps:

a) The steels ingots/slabs are reheated and soaked in a reheating furnace to 1150 to 1250o C for 3 hours;

b) Hot-rolled to 5 to 6 mm plate with finish rolling temperatures of 900 to 800o C.;

c) Hot rolling reduction is about 80-90% (~6mm strip).

d) Solution annealing at 750 to 900o C for 2 hours and subsequent water quenching;

e) The final cold rolling reduction is about 60-95% and final thickness 1.22 to 0.2mm.

f) Annealing the cold strips after cold rolling in the range of 800 to 900 oC. for 2 to 10 min and subsequent water quenching.

9. A process as claimed in claim 8, wherein the steel ingots have a composition in percent by wt. C - 0.01 to 0.08 ; Si - 0.01 to 0.5 ; Mn - 0.1 to 1 ; S – up to 0.03 ; P – up to 0.03 ; Cu : 0.2 to 0.5 ,Cr - 16 to 22 ; Ti - 0.1 to 0.5 ; N - 0.01 to 0.1 and the balance being Fe.