EP0694626A1

EP0694626A1 - Austenitic stainless steel with low nickel content

Info

Publication number: EP0694626A1
Application number: EP94500134A
Authority: EP
Inventors: Ignacio Fernandez De Castillo Y Valderrama; Jaime Botella Arboledas; Francisco Fernandez De La Mata
Original assignee: Acerinox SA
Current assignee: Acerinox SA
Priority date: 1994-07-26
Filing date: 1994-07-26
Publication date: 1996-01-31

Abstract

A new austenitic stainless steel featuring low nickel and high manganese contents is described. The steel has good weldability and is consisting, in percentage by weight, of:
C: <0.1%
Si: <0.5%
Mn: 9.0 - 11.0%
Ni: 1.5 - 3.5%
Fe: balance
Cu: <3.0%
Cr: 16.0 - 18.0%
Mo: <3.0%
N: 0.10 - 0.20%

Description

SCOPE OF THE INVENTION

The invention deals with a low nickel, high manganese austenitic stainless, featuring, with regard to the well-known AISI-304 type, lower nickel (about 2wt%) and higher manganese (about 10wt%) contents, remarkably lower production cost due to its low nickel content, equivalent or somewhat superior mechanical and forming properties, improved resistance to cold deformation-induced martensite transformation and good weldability.

BACKGROUND

The stainless steel term is related to both chromium and chromium/nickel steels group, having a noteworthy corrosion resistance in atmospheric and chemical environments. To get it, more than 12wt% chromium is needed. Depending on the microstructure of the steel, stainless group may be classified as follows: ferritic, martensitic and austenitic stainless steels. Austenitic stainless steels have a 18wt% chromium (Cr) / 8wt% nickel (Ni) base alloy; the former provides corrosion resistance and the latter undertakes structural stability of the austenite (together with less than 0.1%wt carbon (C)). These steels may be alloyed with different contents of several elements, such as silicon (Si), manganese (Mn), copper (Cu), molibdenum (Mo), nitrogen (N), with the aim of obtaining steels with special properties.
The ability of alloying elements to stabilize or promote phases in stainless steels has been treated a great deal. Schaeffler [A. L. Schaeffler, Metals Progress, 56, p. 680, (Nov. 1949)] and Delong [C.J. Long and W.T. Delong, "The ferrite content of austenitic stainless steel weld metal", Welding Journal, 52, pp. 281s-297s (1973)] built constitutive diagrams to determine the amount of ferrite in weld deposits of austenitic stainless steels like 304, 308, 316 grades and others, by means of chromium and nickel equivalents, that is, grouping the effects of the so-called alphagene (Cr, Si, Mo) and gammagene (Ni, C, N, Mn) elements, respectively, like this (from Delong):

where $Creq= Cr + 1.5·Si + Mo + 0.5·Nb$

$Nieq= Ni + 30·(C + N) + 0.5·Mn$

Pryce and Andrews [L. Pryce and K. W. Andrews, JISI, 195, pp. 415-417 (1960)] created a diagram for stainless steels at high temperature (1150 °C) and added the effects of Nb and Ti, obtaining Si coefficient of 3.
Irvine et al [K.J. Irvine, D.T. Lewellyn and F.B. Pickering, JISI, 192, pp. 227-228 (1959)] carried out a work on a 0.1%C-17%Cr-bal Fe basis stainless steel, yielding no effect for Mn > 4%; neither for Cu from 2.5 %Ni.
Guiraldenq [P. Guiraldenq, "Action alphagène and gammagène des principaux élements d'addition dans les aciers inoxydables nickel-chrome dérive du type 18-10", Memoires Scientifiques Rev. Metallurg. LXIV, N° 11 (1967)] worked with Mn contents up to 8% but not having low Ni levels; individual variations on a base alloy led to determine equivalent coeffiecients for each element.
The effect of chemical composition on delta-ferrite and martensite formation was studied by Hull [F.C. Hull, "Delta ferrite and martensite formation in stainless steel", Welding Journal, 51, pp. 193s-203s (1972)] from small casts of 25 gr. He used alloys including Mn up to 20% and low Ni contents, and came to the conclusion that equivalent coefficients depend on chemical composition. Concerning the effect of Mn, there has not been complete agreement for different authors, anyway its effect depending on base composition and Mn content itself.
Austenitic stainless steel type 304 is used for general purposes comprising architecture, transportation systems, furniture, power generation, and petrochemical plants among others. However, specific cases within these application examples represent extreme conditions for type 304 steel (and any other of the austenitic grade), that can not be overcome with guarantee. Among them, those in which non-magnetic condition would be lost by required increasing deformation. Moreover, type 304 is a high cost steel due to its Ni content, what contributes to increase dependence on the strategic value of a scanty element in european mining resources.
Therefore, it would be useful to have an austenitic stainless steel suitable both for the above mentioned specific purposes, and for general applications, those in which type 304 is commonly used. Taking this into account, mechanical properties similar to those of AISI-304 steel, higher resistance to cold-induced martensite transformation, good weldability and low production cost should be the characteristics of a high-performance new austenitic stainless steel.
These features can be found in this invention, from a low nickel, high manganese austenitic stainless steel, as will be described in detail hereafter.

DETAILED DESCRIPTION OF THE INVENTION

The invention has been carried out by the applicant through the framework of a ECSC R&D program (7210.MA/934), and deals with a new austenitic stainless steel developed with the aim of providing lower cost and similar mechanical, corrosion and welding properties, in relation to those of AISI-304 austenitic stainless steel. These objectives can be achieved by lowering the Ni content down to 1.5-3.5wt% and balancing the Ni effect on the new austenitic structure by those of Mn, C, N, and Cu.
From now on, the new austenitic stainless steel provided by this invention will be called low-Ni alloy or steel, and has the following chemical composition (I), in percentage by weight:
C: <0.1%
Si: <0.5%
Mn: 9.0 - 11.0%
Ni: 1.5 - 3.5%
Cu: <3.0%
Cr: 16 - 18%
Mo: <3.0%
N: 0.10 - 0.20%
Fe: balance,
which should be balanced to meet proper production process and application conditions by adjusting elements and ferrite content from the next experimental formula [1]: $Ferrita (vol%) = 5.602·(-10.166+Creq-Nieq/1.73)-3.29$
where $Creq= Cr + 0.62·Si + 1.12·Mo$

$Nieq= Ni + 36.4·C + 27.5·N + 0.54·Cu + 0.01·Mn$

To achieve the final low-Ni composition, this would have to account for some requirements (see them later), the first one being to meet the following limits:

Ni Mn C N Mo Si Cu Cr

Maxim. 3.0 13.0 0.120 0.2 3.0 1.0 3.0 18.0

Mimim. 1.5 7.0 0.015 0.1 --- --- --- 15.0
For this invention, predicting the structures to be obtained within the chemical composition range proposed, and the relation between the element contents and ferrite content should be known.
For this, predictability of Delong diagram for low-Ni/high-Mn compositions was studied by means of magnetic measurements on 40 gr cast buttons vacuum melted in a centrifugal furnace Lecomelt Vac 3.3 µ. The spectrometric buttons solidify and cool inside a copper mold, providing a rapid cooling that can be compared to that of welds. The ferrite content of low-Ni/high-Mn buttons did not fit the prediction of Delong diagram, so next steps were determining individual coefficients valid within the new range and to find the relation to predict percentage ferrite properly.
By using spectrometric button technique, base compositions having known ferrite contents were used to vary elements increasingly and determine chromium and nickel equivalents. A population of 450 spectrometric buttons were manufactured following the above mentioned ranges. One hundred buttons were also cast having individual variations for element contents lying out of the proposed limits.
The coefficients were deduced from the slope of Δferrite vs Δ(X) plots -X being the element at issue-. From the complete population of buttons the following conclusions were obtained.

a) Mn contents ranging from 7% and 13.5% do not affect phase content.
b) For Mn<6%, martensitic transformation occurs. The lower the %Mn, the higher the magnetic phase percentage.
c) Gammagene effect of Cu decreases to great extent for Cu>2.5%
d) Gammagene coefficient of C is higher than that of N.

Alphagene power of Mo exceeds that of Cr.
A multiple linear regression on all the buttons manufactured and their respective ferrite contents results in the next formula [2]:

where $Creq= Cr + 0.62·Si + 1.12·Mo$

$Nieq= Ni + 36.4·C + 27.5·N + 0.54·Cu + 0.01·Mn$

The button population is concentrated between 0% and 11% ferrite, for which the above mentioned multiple linear regression has a correlation coefficient r=0.946.
Short influence of manganese on austenite content has been verified, in spite of the great Mn dependence of austenite stability. Low-Ni austenite has to comprise requirements of phase stability and morphology; in this sense, the following conclusions were confirmed on the spectrometric buttons.

a) Ni≈0.65% / Mn<4% compositions lead to mainly martensitic structures, with small quantities of retained austenite and residual ferrite. For Mn=6%, magnetic reading amounts 17% Phase identification was performed by electrochemical (NaOH) and chemical (Vilella) etchings, and X-ray diffractrometry, for austenite and ferrite. The X-ray macrodiffraction is unable to distinguish between B.C.C. martensite and ferrite, so they are differentiated by optical microscopy and hardness measurements.
b) Ni>1.5% / Mn≈10.5% structures feature ferrite dendrites in a high stability austenite matrix.

Predictability of button correlation has been analyzed for a series of 40 kg vacuum induction melted ingots, manufactured to study the effects of a half-way cooling rate on low-Ni austenite structure. The ingots met the following conditions: Mn>10%; Ni≈1.5%; ferrite contents of <10% and ≈2% for as cast and soaked conditions, respectively; and Cr high enough for corrosion resistance purposes.
Experimental ingots were 300 mm height, truncated-pyramid shaped, having square bases, 140 mm side up and 120 mm side down.
Ferrite formation, phase stability and response of structure to soaking and freezing treatments have been studied on ingot samples.
Ingot structure consisted of randomly distributed ferrite in austenite matrix. The structure was studied by metallographic techniques on lengthwise and crosswise sections of the ingots, comprising electrochemical etching with oxalic acid or 10% NaOH for observation by light microscopy, and magnetic measurement (ferritoscope) of ferrite.
In spite of button correlation predicts higher ferrite contents than those magnetic measured in the ingots, data could be easily adjusted by a constant term, leading to the above mentioned formula [1].
Soaking treatments at 1050, 1200 and 1275 °C with holding times ranging from 1 to 8 hours, and subsequent freezing cycles at -30, -80 and -196 °C performed on ingot samples gave rise to the fact that ferrite percentage does not decrease in low-Ni alloys at 1200 °C and holding time similar to that of re-heating cycles. This is consistent with the Fe-Cr-Mn phase diagram for 8%Mn and 16%Mn, from Kinzel and Franks [A. Kinzel and B. Franks, Alloys of iron and chromium, II, Mc Graw-Hill, p. 277 (1940)]. So ferrite content has to be fixed in advance, by chemical composition, to the required amount, since it will not decrease during stay in reheating furnace prior hot rolling. In other hand, after freezing low-Ni soaked samples, there is no evidence of martensite formation or magnetic reading increase.
Hot deformability of low-Ni austenitic stainless steel was studied by tensile tests and plane compression tests, and compared with that of type-304. As it is shown in the example no. 2, hot tensile tests to fracture were performed at 900 through 1250 °C, and strain rates of 1 and 5 per second. Deformability results, as reduction of area, show lower average values for low-Ni alloys with regard to those of 304 steel at any strain rate (25% vs 45% at 1000 °C and 5 /s), although the difference decreases as temperature rises (50% vs 65% at 1200 °C and 5 /s). Results of 3-stage plane compression tests at 1180 °C, having 15% reduction and 39 mm/s each pass, exhibit higher stress values for low-Ni austenitic steel, but also a greater restoration rate between passes.
Accounting for austenite stability, ferrite content, hot deformability and some other criteria, like %C as low as possible and %Cr to the utmost among the admissible values to get a suitable corrosion resistance properties, the following feasible low-Ni composition range is selected (I'):
C: 0.075 - 0.085%
Si: 0.20 - 0.35%
Mn: 10.2 - 10.4%
Ni: 1.65 - 2.6%
Cu: 2.0%
Cr: 16.4 - 17.0%
Mo: <3.0%
N: 0.135 - 0.175%
Fe: balance,
which has to be balanced by adjusting the elements and ferrite content from the above mentioned experimental formula [1].
Forty-kilogram ingots of several low-Ni compositions were vacuum induction melted to be subsequently hot and cold rolled, while a similar quantity of type-304 material originating in Acerinox' daily production was chosen. Samples from finished cold products were taken to characterize their mechanical, welding and corrosion-resistance properties (examples no. 3 through 5).
Mechanical properties comprise tensile strength, yield stress and elongation of samples having Θ degrees with rolling direction. Similar tensile strength and elongation for both materials (low-Ni and type 304 alloys), and a +10% difference for the former concerning yield stress were achieved. In general, the low-Ni steel appear to be a little more isotropic in the sheet plane (lower -Δr -planar anisotropy-) than the other type 304, as can be seen in Table 1 (example no. 3.1).
Forming-limit diagrams (FLD's) were determined, and it has been found that they do not present significant differences for the two grades of stainless steels (example no. 3.2).
Hardness and strain-induced martensite were also measured on samples from low-Ni and 304 cold rolled sheets. Hardness data showed that structural hardening causes on low-Ni steel the same effect than martensite-precipitation hardening on 304 steel, that is, hardness increase (example no. 3.3.2). On the other hand, no ε martensite has been detected by X-ray diffraction in the cold strained Low-Ni steel (example no. 3.3.1).
Weldability of the low-Ni steel of this invention has been studied, establishing that Low-Ni steel can be welded without significant operational problems, attaining full penetration in the different procedures and showing good visual appearance, that is, porosity and cracking free welds (example no. 4).
Comparison of corrosion resistance on low-Ni and AISI-304 (example no. 5) showed that even at room temperature, 304 steel exhibited higher pitting and repassivation potentials than low-Ni alloy (example no. 5.1), as well as the rather limited behaviour of this steel to crevice corrosion (example no. 5.2). Also the results on the resistance to stress corrosion cracking in CaCl₂ pointed out the similar response of both steels, cracking evidence not being observed below 270 Mpa (example no. 5.3). Intergranular corrosion tests (TTT curves) and microstructural analysis showed the sensitization of low-Ni steel, at temperatures within 700-800 °C interval (example no. 5.4).
A number of trials on the corrosion resistance behaviour of the experimental low-Ni alloys pointed out that their performances can be improved by the means normally used for the rest of austenitic stainless grade, that is, lowering %C and increasing %Cr or %Mo lead to fit results. Re-adjusting the rest of elements for proper ferrite content and austenite stability is then required, according to the specific use.
In relation to manufacturing experimental vacuum melted ingots, differences between low-Ni stainless steel and type 304 are concerned the efficence of the alloy elements.
This is due to the fact that desired chemical composition has to be achieved mainly from the weight of ferroalloys, previously calculated at the beginning of the melt. Deviations of element composition because chemical and thermodynamic processes taking place inside the melted steel, are balanced subsequently for the next melt by adjusting the efficiency of the element at issue.

AISI-304 Acero bajo-Ni

Nitrogen 90.0% 100.0%

Carbon 80.0% 77.0%

Silicon 88.0% 85.0%

Manganese 85.0% 97.5%
On the contrary, the characteristics of A.O.D. manufacturing process allow to intervene as it progresses depending on the partial results of chemical composition (decarburation, deoxidation, sulphur removal, etc.). For the industrial low-Ni melts, two significant matters were established. In one hand, oxigen efficiency to decarburate was about 35% lower than normal; on the other, nitrogen content adjustment by argon had the difficulty derived from the high manganese content, that makes the N to dissolve into the melted steel.
In all the cases the chemical composition of ingots and intermediate samples of industrial melts are determined by means of X-ray fluorescence equipments and carbon, sulphur and nitrogen determinators.
Hot rolling of low-Ni slabs can be performed by using the program of common austenitic stainless steels, featuring a stress level at half way between AISI-316 and AISI-304 steels, for each pass. Black coil is subsequently annealed in the same way than AISI-304. For pickling, the strip goes through two acid bath (HNO₃ and HF). Low-Ni strips can be cold rolled, reducing thickness from 3.0 to 0.7 mm, without problems. Final annealing of low-Ni steel was performed under the usual conditions of temperature and speed, with final air cooling.
The application field of the new low nickel austenitic stainless steel from this invention, comprises cold deformation requiring suitable drawability and strength, and use under atmospheric conditions (i.e. cold-storage plants, bar counters, cookware and cutlery).
Industrial low-Ni heats were produced to get cold sheets in several thicknesses which were supplied to different manufacturers. Finished products as spoons, knives, beer barrels, pans, cooker among others have been manufactured with good results, only requiring setting machine parameters for the somewhat higher strength of the new alloy.
Additionaly, the steels provided by this invention can be used in non-magnetic applications, such as nuclear industry, computer hardware and electronics, as well as in some applications of ferritic stainless steels (those involving excessive high manufacture costs, since they can not be welded or deep-drawn).
These low-Ni, high-Mn austenitic stainless steels feature the following main advantages, among others:

a. In relation to AISI-304 austenitic stainless steel, the low-Ni alloy has lower cost owing to its lower nickel content.
b. Dependence on the great strategic value of Ni is markedly reduced as this element do not take part to great extent in the chemical composition of the new alloy.
c. For somewhat higher values of mechanical strength and similar ductility, with regard to type 304, the low-Ni stainless steel offers significantly higher resistance to cold-induced martensite transformation, what makes it fit for non-magnetic applications (nuclear industry, computer hardware and electronics).
d. Having similar corrosion resistance, for some applications, to that of 304, exceeds in this matter the properties of ferritic stainless steels, being also possible its use within specific field of the latter.
e. Copper (<3%) in the low-Ni alloy allows achieving a fine polishing quality (brightness and colour), suitable for manufacturing special cutlery.
f. Unlike nickel, manganese is a common element of the european mining resources.
g. This low-Ni alloy is a complete family of austenitic stainless steels itself, as its composition range allows adaptation to specific uses by means of modifying the element contents such as C, Mo, N and Cr.

Following is a description of some examples which will show several forms of performing the present invention. They should not be considered as limits of the scope of the invention.

EXAMPLES

Example no. 1: Manufacturing low-Ni steels.

Low-Ni alloys have been manufactured as 40 kg vacuum melted ingots (300 mm height, truncated-pyramids shaped, having square bases, 140 mm side up and 120 mm side down), and 100 Tn melts subsequently continuous cast (1000 mm width). From the former, samples and specimens were obtained to perform the tests appearing described from example no. 2 through 5. Industrial heats were produced to assess the characteristics of the new alloy along the production process. Moreover, cold sheets of those heats were to be manufactured as final products (cutlery, beer barrels, etc.) and check their fabricability. Below are two examples of chemical compositions of low-Ni ingots (the variations of element contents correspond to the usual tolerances in steelmaking process).

Ni Mn C N Si Cu Cr

1.69 10.45 0.079 0.141 0.36 1.97 16.4

1.61 10.29 0.079 0.138 0.37 1.98 16.3
The chemical compositions of industrial heats differ from those of ingots in Ni (≈2.5%), Si (≈0.20%), N (≈0.170%) and Cr (≈17.0%).
The chemical composition of ingots and intermediate samples of industrial melts were determined by means of X-ray fluorescence equipments and carbon, sulphur and nitrogen determinators.
Hot rolling low-Ni slabs could be performed by using the program of common austenitic stainless steels, featuring a stress level at half way between AISI-316 and AISI-304 steels, for each pass. Black coil was subsequently annealed in the same way than AISI-304, that is, at 1130 °C and 25 m/min. For pickling, the strip went through two acid bath (15-20% HNO₃ and 4-5% HF). Low-Ni strips were cold rolled, reducing thickness from 3.0 to 0.7 mm, without problems. Final annealing of low-Ni steel was performed under the usual conditions of temperature (1160 °C) and speed (45 m/min), with final air cooling.

Example no. 2: Hot deformability.

Hot deformability of low-Ni austenitic stainless steel was studied by tensile tests and plane compression tests, and compared with that of type-304.

2.1 Hot tensile tests.

Hot tensile test procedure met the so-called "Gleeble test". Test conditions were: temperature from 900 to 1250 °C (100 °C intervals); and strain rates of 1 and 5 s⁻¹.
From the low-Ni and type 304 ingots (example no. 1), round, threaded ends, tensile specimens were prepared, having the following dimesions: 10 mm x 120 mm (diameter x length). They were electric resistance heated by low fercuency AC current, at 40 °C/s up to 1250 °C (reference temperature, Rt). The heated length was 11 mm (L₀= 11 mm). Later, specimens were kept at Rt for 1 minute, varying temperature at 5 °C/s until test temperature was reached. After 20 seconds the specimen was tensioned to fracture.
Deformability results, assessed by means of reduction of area percentage, show lower mean values for low-Ni steel, in relation to AISI-304 at any strain rate (25% vs 45% at 1000 °C and 5 s⁻¹), although difference decreases as temperature rises (50% vs 65% at 1200 °C and 5 s⁻¹).

2.2 Hot plane-strain compression tests.

Three-pass plane compression tests were performed in the Gleeble machine, at 1180 °C, 39 mm/s and 15% reduction for each pass (time between passes, 15 seconds). Round specimens having dimensions of 15 mm x 20 mm x 10 mm (X Z Y), were horizontally compressed (Y-direction), in such a way that two tungsten carbide anvils, between which the specimen is located, reduce the Y-dimension of the this one and increase the X-, whereas Z- remains without change, thus plane-strain conditions are met (the width of anvil contact face is X= 5 mm, having two bevel edges at 45 º). The process is repeated so many times as steps of the program (three in this case).
Results exhibit higher stress values for low-Ni austenitic steel, but also a greater restoration rate between one pass and the following.

Example no. 3: Mechanical and forming properties.

Concerning mechanical properties and forming-limit curves (FLC's), time and test conditions were set in order to equal the strain rates of all tests ( ${ε/t=3.10}^{-3} /s$
) by means of von Misses' equations for plane strain. Strain hardening exponent (n) and plastic anisotropy factor (r) have been determined from uniaxial tensile test. In both material grades the n-value changes with deformation and has been taken from the 10%<ε<30% interval.

3.1 Uniaxial tensile test.

These tests were performed following ASTM A 370 standard. Mechanical properties include tensile strength, yield strength, uniform elongation and total elongation of specimens having Θ degrees in relation to rolling direction, as shown in table 1 (average of low-Ni alloys in italics).
From the results shown in table 1, similar behaviours concerning tensile strength and elongation have been obtained for low-Ni and 304 austenitic stainless steels, with the exception of a +10% difference in elastic limit for the former. The n-coefficients of low-Ni alloys seem to be close to those of type-304 steel, and the same happens with mean-r values (average normal anisotropy). In general, the low-Ni steel appears to be a little more isotropic in the sheet plane (lower -Δr -planar anisotropy-).

3.2 Forming-limit diagrams (FLD's).

Forming-limit diagrams were determined by four kind of tests:

a) hydraulic bulge test with elliptical die (100 mm x 190 mm);
b) hydraulic bulge test with circular die (⌀ = 100 mm);
c) uniaxial tension test, to ASTM A 370; and
d) tension of notched specimens (plane strain):
Length= 160 mm
Width= 30 mm
Two semicircle (r= 5 mm) notches in the middle of the specimen. Width without notches = 30 mm - 2 x 5 mm = 20 mm

3.3 Hardness measurements and cold-induced martensite on low-Ni and AISI-304 steels samples.

Samples from hot and cold rolled sheets (both low-Ni -example no. 1- and AISI-304 steels) were subsequently cold rolled in a laboratory mill with the aim of providing successive reductions of thickness and assessing the acquired hardness and the amount of cold-induced martensite. The initial dimensions of samples were: 50 mm x 350 mm x 2 mm (width, length, thickness).

3.3.1 Martensite measurements

After each cold reduction, a 50 mm x 25 mm x 2 mm specimen was cut from the initial sample, and magnetic measured by means of a Förster-1053 ferrite content meter, calibrated with secondary standards. Results are the average value of 15 measurements per specimen.
Martensite magnetic measurements vs amount of deformation data were fitted by ${VFM/(1-VFM) = A.E}^{B},$
where "VFM" is the martensite formed and "E" is the cold reduction, and "A" and "B" constants are concerning with the transformation trend of the material. This relation corresponds to single power law like ${y = A·e}^{B}$
, and therefore "A" and "B" terms can be computed from the respective power regression.
Low-Ni steel: A= 0.32 and B= 2.52
AISI-304 alloy: A= 2.83 and B= 3.16
The presence of non-magnetic, close-packed hexagonal ε martensite has been widely reported in high-Mn/high-N stainless steels. However, no ε has been detected by X-ray diffraction in the cold strained low-Ni steel.

3.3.2 Hardness

Hardness measurements were performed according to ASTM E 92 standard. Hardness vs deformation results have been fitted by ${HRC=A.e}^{n},$
where "HRC" is the Rockwell-C hardness, and "e" is the reduction percentage. "A" deals with cold hardening trend, and "n" with the catalytic character of the cold hardening itself. This relation also corresponds to single power law like ${y = A·e}^{B}$
, and therefore "A" and "n" terms could be computed from the respective power regression.
Low-Ni steel: A= 11.4 and n= 0.341
AISI-304 alloy: A= 11.2 and n= 0.307
These results show that structural hardening causes on low-Ni steel the same effect than martensite-precipitation hardening on 304 steel, that is, hardness increase. A number of trials on other austenitic stainless steels (metastable type 301 and deep drawing quality grade) yielded the following "A" and "n" parameters.
AISI-301: A= 14.85 and n= 0.307
D.D.Q.: A= 6.79 and n= 0.430
Alpha prime martensite (α') appears to be concentrated along shear bands on AISI-301. In the rest of austenitic stainless steels shear bands have been observed with difficulty, although they seem to be present to more extent in type-304 than in low-Ni steel.

Example no. 4: Welding tests.

To assess weldability of the low-Ni stainless steel of this invention, welding tests following ASME Part-QW standard were performed. For that, sheets of the new alloy and others of type 304, in 1 and 2mm thicknesses, were welded in two positions (butt and fillet), with and without filler material, using TIC (GTAW), synergic MIG (GMAW) and LASER processes.
The results showed that welding of the Low-Ni steel can be carried out without significant operational problems, attaining full penetration in the different procedures. The main welding parameters for this steel have been the following.
GTAW without filler material / butt joint / 1mm thick:
Voltage... 9.3 to 9.5 V
Current... 49 to 53 A
Welding speed... 23 to 29 cm/min
GTAW without filler material / butt joint / 2mm thick:
Voltage... 9.3 to 9.8 V
Current... 88 to 98 A
Welding speed... 24 cm/min
GTAW with filler material (parent material) / butt joint / 2mm thick:
Voltage... 9.1 to 11 V
Current... 80 to 112 A
Welding speed... 14 to 17 cm/min
GTAW with filler material (parent material) / fillet joint / 2mm thick:
Voltage... 10.3 to 10.7 V
Current... 100 to 114 A
Welding speed... 12 to 18 cm/min
GMAW with ASME AWS ER 3082 filler material / butt joint / 2mm thick:
Voltage... 18.7 to 19.8 V
Current... 68 to 85 A
Welding speed... 29 to 35 cm/min
GMAW with ASME AWS ER 3082 filler material / fillet joint / 2mm thick:
Voltage... 20.4 to 22 V
Current... 96 to 112 A
Welding speed... 29 to 39 cm/min
LASER / butt joint / 2mm thick:
Power... 1160 to 1460 W
Axial gas... He or Ar
Welding speed... 500 mm/min
Welding runs are visually with good appearance, without porous or cracks. Solidification structure of deposit and ferrite distribution are, in general, of good appearance in both materials, having the usual structure of type 304 austenitic stainless steels. The major difference lies in the ferrite content of the low-Ni welds, that only equals that of the 304 steel (about 5%) when AISI-308-L-Si is used.
ASTM A370 tension specimens with longitudinal axis being normal to weld bead were tested. Bending tests were performed up to 180º in face and root. Yield and tensile strength values are similar in both materials, and all the bending tests had fair results. Low-Ni weld's elongation is quite higher than that of AISI-304.

Example no. 5: Corrosion resistance tests.

Corrosion resistance of low-Ni steel was evaluated, in comparison with that of 304 grade, in relation to its response to pitting and crevice and SCC. Intergranular corrosion tests were also performed.

5.1 Pitting corrosion resistance.

Pitting behaviour was determined by potentiodinamic means on 30x40 mm samples, ranging thicknesses between 1 and 2 mm and testing temperatures of 25 and 65 °C. It can be found that even at room temperature, 304 steel exhibits higher pitting and re-passivation potentials than low-Ni material. The difference between the two grades increases for 65 °C.

5.2 Crevice corrosion resistance.

Resistance to crevice corrosion was evaluated by means of Anderson-type multicrevice device. The results are concerning with weight loss and penetration depth caused by the attack and show the limited behaviour of low-Ni stainless steel.

5.3 Stress corrosion cracking in CaCl₂.

Resistance to Stress Corrosion Cracking in CaCl₂ was performed with samples prepared according to ASTM G39 standard (Three-point Loaded Specimens). Tests conditions are as follows:
applied stress: 40-20 MPa;
test duration: 500 horas;
solution: CaCl₂ 400 gr/l; and
temperature: 100°C.
Results show the proper behaviour of both steels; cracking evidence not being observed below 270 Mpa.

5.4 Intergranular corrosion tests and microstructural analysis.

Intergranular corrosion tests (TTT curves) and microstructural analysis were carried out according to practice ASTM-A.262-A, on samples previously treated at 1100 °C during 30 minutes for carbide disolution, that gave rise to 6-sized grains. Samples (30x20x2mm) underwent heat treatments at 700-900 °C, with holding time in the 10 min to 6 h range and water-quenching. Low-Ni steel shows sensitization to intergranular corrosion at temperatures within 700-800 °C interval. This phenomenon, due to chromium carbide precipitation, can be observed also in AISI-304 in the same temperature range, although corrosion rate is slightly lower in this latter steel.

Claims

A low-Ni/high-Mn austenitic stainless steel, fit to general applications, and modifiable to meet specific uses, having a chemical composition [I] consisting, in percentage by weight, of: C: <0.1% Cu: <3.0% Si: <0.5% Cr: 16.0 - 18.0% Mn: 9.0 - 11.0% Mo: <3.0% Ni: 1.5 - 3.5% N: 0.10 - 0.20% Fe: balance

The chemical composition required should be balanced to meet proper production process and application conditions by adjusting elements and ferrite content from the following formula [1]: $Ferrite (vol%) = 5.602·(-10.166+Creq-Nieq/1.73)-3.29$
where $Creq= Cr + 0.62·Si + 1.12·Mo$

$Nieq= Ni + 36.4·C + 27.5·N + 0.54·Cu + 0.01·Mn$