US 3926685 A
Description (OCR text may contain errors)
United States Patent Gueussier et al.
SEMI-FERRITIC STAINLESS MANGANESE STEEL Inventors: Andre Gueussier, 6 rue Claude Genoux; Roland Tricot, 14 rue du Val des Roses; Jean Letevre, 3 bis rue de Ripaille, all of Albertville (Savoie), France Filed: Dec. 1, 1972 Appl. No.: 311,111
Related US. Application Data Continuation of Ser. No. 42,037, June 1, 1970,
Foreign Application Priority Data June 3, 1969 France 69.18142 US. Cl. 148/12 EA; 148/37; 148/135 Int. Cl C2ld 9/48 Field of Search 75/123; 148/136, 135
References Cited UNITED STATES PATENTS 12/1941 Becket 75/123 2,380,821 7/1945 Brecler 148/136 2,518,715 8/1950 Payson 148/136 3,152,934 10/1964 Lula 148/136 3,401,035 9/1968 Moskowitz 148/135 3,473,973 10/1969 Maekawa 148/135 Primary ExaminerHyland Hizot Attorney, Agent, or Firm-Webb, Burden, Robinson & Webb [5 7] ABSTRACT A semi-ferritic stainless steel having a austenite con tent while hot of from 10 to 50%, and a composition of 0.2% max. C, 1 10% Mn, 3% max. Si, 5% max. Ni, 14 25% Cr, 3% max. Mo, 5% max. Cu, 0.3% max. N 10% max. C0, 1% max. Ti, 1.0% max. Cb, 0.005% max. B, 0.5% max. Al, the balance Fe and incidental impurities. The composition is characterized by particular nickel and chromium equivalents. The steels are treated to retain the austenite and ferrite at ambient temperatures, and then the austenite is transformed into martensite and subsequently the martensite is transformed into ferrite and carbides.
11 Claims, 1 Drawing Figure US. Patent Dec. 16, 1975 3,926,685
l4 l6 I8 20 22 24 CHROMIUM EQUIVALENT steels. which are also shown in Table l. are generally used in a full hardened state or cold-worked state, or sometimes after an appropriate low temperature stress relief.
5 These austenitic steels have the advantage of good weldability and satisfactory resistance to corrosion, but the disadvantage of involving a marked increase in the cost of the alloy due to the addition of nickel or a combination of the elements nickel and molybdenum. of
exhibiting a special susceptibility to corrosion under tension which limits their use for certain applications (water heating), and of having a low elastic limit.
TABLE 1 Stainless Steels Chemical Analysis (Weight Percent) Type Desig. C Mn S! P S Ni Cr Mo Ti (b Cu AFNOR Semi- Z 8 C 17 .1 max. 1. max. 1. max .04 max. .03 max. $5 16-18 Ferritic Semi- Z 8 CD 17 .1 max. 1. max. I. max .04 max. .03 max. s .5 16-18 .9/1.3 Ferritic Semi- Z 10 CF 17 .12 max. 1.5 max. I. max .06 max. 2.15 s .5 16-18 Fcrritic Ferritic Werkstoff .1 max. 1. max. 1. max. 17 7(C) No. 4510 Ferritic Werkstoff .1 max. 1. max. 1.5 max. 17.5 121C) No. 451 1 Austenitic AFNOR Z 10 CN .12 max. 2.0 max 1.0 max. .04 max. .03 max. 8-10 17-19 18-09 Austenitic AFNOR 7. 6 (ND .07 max. 2.0 max 1.0 max. .04 max. .03 max. 10-12 16-18 .0-2.5
17-1 1 Austcnitic A150 2 O1 .15 max. 5.5-7.5 1.0 max. .06 max. .03 max. 3.5-5.5 16-18 Austeno- Typical .05 10 2.0 18 .4 Ferritic Austeno- Typical .05 .5 .5 8 20 2 5 l 5 Ferritic Austeno- Typical .05 .5 .5 6.5 26 .2 Ferritic The semi-ferritic stainless steels are characterized by a mixed structure of delta ferrite and austenite when hot, the austenite transforming to martensite on cooling, and the martensite transforming to alpha ferrite and carbides after tempering. The transformation from austenite to martensite takes place by rapid cooling to below the critical temperature (A1) of about 850C and transformation of martensite to alpha-ferrite takes place by tempering below the Al. The final structure of alpha and delta ferrite and carbides is the traditional structure employed for applications of rods, sheets and wires, into which the semi-ferritic steels are easily transformed.
However, the rapid cooling of semi-ferritic steels while hot causes grain enlargement of the delta ferrite, as well as the transformation of the austenite into martensite. This is particularly troublesome in weld areas where a fragile martensitic phase forms about the ferritic grains. Further, the semi-ferritic steels, which are reactant to corrosion under tension in chloride solution, have other corrosion resistance inadequacies.
Attempts to improve the weldability problems economically have been tried by using compositions which result in a complete ferritic structure. This is accomplished by adding titanium or columbium as shown for the Werkstoff alloys in Table I. However, a complete ferritic structure is very prone to grain growth, and further, processing problems while hot are acute because of the inherent softness of the ferritic structure.
These 17% chromium steels can be transformed entirely into an austenitic structure by adding nickel, manganese or a suitable combination of the two. These The so-called austeno-ferritic steels form an intermediate group between semi-ferritic and austenitic steels. Their analysis, as given in Table I, is so selected as to 40 produce a biphased structure, i.e., austenite ferrite.
The fundamental difference between this family of steels and semi-ferritic steels, which also contain a relatively substantial quantity of austenite while hot, is due to the special stability of this biphase structure compared to semi-ferritic steels whose structure after tempering is decomposed into ferrite carbide, as shown above.
The standard thermal treatment for the austenoferritic steels before use, is a heat treatment around as resistance to corrosion under tension is concerned as compared to austenitic steels. The disadvantage of the austeno-ferritic steels is the difficulty in processing when hot, and in addition, due to the composition, a significant increase in the price is necessary.
Our invention provides a semi-ferritic steel in which the fragile martensite phase is not formed and. therefore, the weldability is greatly improved. Further, the corrosion resistance is substantially improved over semi-ferritic steels known heretofore.
The composition limits are such that the alloy can be economically produced and because of certain critical heat treatments, can be economically processed. In fact, a substantial gain in productivity in the manufac- 3 ture of sheets, rods and wires is obtained because of the notable reduction in the total time that heat treating furnaces are used.
The steels of the invention have a austenite content while hot of from 10 to 50% by weight. and a composition comprising by weight 0.2% max. C, 1 10% Mn, 3% max. Si, 5% max. Ni, 14 25% Cr, 3% max. Mo. 5% max. Cu, 0.3% max. N max. Co. 1% max. Ti. 1.0% max. Cb, 0.005% max. B. 0.5% max. A1. and the balance Fe and incidental impurities. The composition is also characterized by the fact that it has representative points in a system of coordinates (equivalent Cr equivalent Ni) which are included in the interior of a pentagon which has the following points as its corners, A 15, 4.5), B 18.5, 8). C (24, 8), D (24, 6) and E (20, 2). While hot, these steels have a structure identical to that of semi-ferritic steels (ferrite austenite but they preserve this biphase structure when cooled without the formation of a fragile martensitic phase. In other words, this austenite is retained and is later transformed to martensite and subsequently to ferrite and carbides by treatments which form a part of this invention.
FIG. 1 is a graph showing the composition limits of our invention in terms of nickel and chromium equivalents.
The broad composition limits of our invention and three preferred ranges (shown as A, B and C) within the broad range, are set forth in Table 11.
TABLE 11 Analysis of Stainless Steels By Weight) Broad Preferred Preferred Preferred A B C C .2 max. .1 max. .1 max. .1 maxv Mn l-l0 3-6 3-6 3-6 Si 3 max. 1 max. 1 max. 1 2 Ni 5 max. 1 max. 1 max. 1 max. Cr 14-25 18-22 -18 15-18 Mo 3 max. 1.5 max. 1.5 3.0 .5 2.0 Cu 5 max. 1 max. 1 max. 1 max. N .3 max. .1 max. .1 max. .1 max. Fe Balance Balance Balance Balance Co 10 max. Ti 1 max. Ch 1.0 max. B .005 max. Al .5 max.
The Preferred A" composition is economically advantageous because of the limited amounts of expensive additive elements. The Preferred B composition, in addition to economy, exhibits outstanding resistance to corrosion. Preferred C composition is similar to Preferred B composition in properties and differs only in that a portion of the molybdenum is replaced by silicon. When it is desired to improve the machinability of the steels of the invention, one may add to them in The various analyses are further limited by their chromium and nickel equivalents. For the chromium equivalent we use Cr) Si) Mo) +4 Ti Nb). For the nickel equivalent weuse Ni) +0.5 Mn) +0.5 Cu) Co) 20 C Ni). As shown in FIG. 1, the limitation in' terms of the chromium and nickel equivalents is represented by a pentagon in which the chromium equivalent is the abscissa and the nickel equivalent is the ordinate and the pentagon can then be defined by the following ordinates. A (15, 4.5), B (18.5, 8), C (24, 8), D (24, 6) and E (20, 2).
The steels of the invention which are delivered in the form of sheets, rods. or wires receive the usual manufacturing operations of semi-ferritic steel of the Z 8 C 17 type, namely blooming, hot rolling, heat treating and cleaning. cold rolling or wire drawing, and final heat treating. The heat treatment which follows the final pass of hot rolling is normally a prolonged annealing at a temperature on the order of 800C which leads to the maximum softening favorable to the subsequent operations of deformation while cold.
According to the new method of carrying out this heat treatment which forms part of the invention. one operates in two stages, the first being a transformation into martensite of the austenite retained at ambient temperature. and the second being a transformation of this martensite into ferrite and carbides. This latter stage is referred to as tempering, i.e., the conversion of martensite to ferrite and cementite after the steel has been hardened by the prior heat treatment.
The first stage can be accomplished by any of the following four alternatives:
1. Annealing between 700 and 900C, preferably between 750 and 800C, for instance for about 4 hours, followed by a slow cooling on the order of 25+C per hour down to 650C, then cooling in the air;
2. Cryogenic treatment, for instance 3 hours at 3. Cold reducing at ambient'temperature, for instance rolling or wire drawing while cold with reduction of the section of at least 30%;
4. Slow cooling from the exit temperature from the last hot roll pass (for instance cooling of 25C/hr. from 850C to 650C) then cooling in the air.
The second stage is tempering at a temperature lower than 850C that is continued until the martensite disappears. Its duration, which may be an hour or even less. is substantially reduced compared to that of the usual single annealing of 10 to 20 hours, and this improves the profitableness of the treating furnaces.
By way of example 5 heats of steel according to the invention were made. Their'analyses appear in the following Table 111.
Experimental Heats Composition (Weight Per Cent) known manner sulfur and/or selenium, and/or tellurium in amounts not exceeding 0.4%, in all.
Their mechanical characteristicswere determined after treatment in accordance with our invention. Cy
TABLE IV Test Results of Experimental Heats Casting R (h bar) E (h bar) A 7: (L 50 mm) Ff/r (l) 56 33 30 56 (2) 58 34 29 58 (3) 59 35 26 5S (4) 63 39 25 5X (5) 68 44 31 77 TABLE V Micro-Hardness Results Vickers Micro-Hardness (Load Applied 50 g) inside the at the joints of ferritic grains the ferritic grams Conventional semiferritic steel (Martensitic phase) Steel of the invention 260 r 320 (Austenitic phase) The poor resilience of the fusionweld is characteristic of semi-ferritic sta-inless steels. This. resilience is greatly increased in the steel of the invention as shown by Table VI below:
'TABLE VI Weld Resilience Steels Resilience:
Charpy' V in daJ/cm Semi-ferritic (Z 8 C 17) 0.5 Ferritics (Werkstoff 45 I and 451]) 0.5 to L Austeno-ferritics L5 to 3 Austenitics (A181 201 and 202) 4 Steels of the invention 2 to 4 From the point of view of corrosion, the two principal modes to which stainless steels can be subjected in practice are corrosion by pitting in the presence of Cl'ions and general corrosion in a diluted acid and unchlorinated medium. The following tests were made on sheets of 10 mm thickness. The steels of our invention, after hot rolling, were subjected to treatment also in accordance with our invention.
1. Corrosion by pitting in the presence of Clions in a solution that was non-oxidizing in itself, but aerated.
This type of corrosion simulates certain atmospheric corrosion (Clions always present, even far from the sea) and corrosion of saline solutions, e.g., food products, etc. These very chlorinated or freely oxidizing solutions often warrant steels having at least 10 18 Mo.
The resistance to corrosion in this field has been estimated by the potential for pitting in sodium chloride solutions by means of an anodic polarization curve. It has been shown. in fact. that the cathodic reaction of oxygen reduction was insensitive to the gradation. and the determination of the potential for pitting (anodic characteristic) therefore constitutes a good scale for classifying the gradations.
In the form of intervals the following Table Vll gives the results of comparative tests on the various steels indicated.
TABLE VI] Potential for Pitting Potential of Fitting (in a V) in a Na Cl. 0.02 M Medium Semi-fcrritic steels (Z 8 C 17) 530-580 Ferritic Steels T 540-590 (Werkstoff 4510 and 4511) Austeno-ferritic steels 650-730 Austenitic manganese steels 6l0-660 (AlSl 20] and 202) Austenitic nickel steels 640-700 (7 I0 CN 18 U9) Steels of the invention 620-680 2. General corrosion in a dilutedand non-chlorinated acid medium. In the great majority of cases, a stainless steel should be found in the passive state to resist corrosion in an acid medium, otherwise it will be corroded in the active state.
In a given acidsolution a given stainless steel will be passive if the "cathodic reaction of reduction of the oxidizer in solution (dissolved oxygen, ions Fe, Cu, etc.) brings the potential of the metal into the zone of passivity. This will be the case if the speed of reduction of the oxidizer, expressed in A/cm is greater than the critical intensity of passivation i per cm measured on the anodic polarization curve of themetal traced in the absence of an oxidizer. lnversely, for a given speed of reduction of the oxidizer the steels will be passive that have an i smaller than this speed, and the steels will be active that have an i that is greater. Accumulated experience has shown that at least in a very large area it is indeed i which is the deciding parameter. The smaller the i is, the better the steel will be. The results of thetests in this area are presented in Table Vlll.
:TABLE viii Corrosion Passivation C rfitical Course of passivation Steels In a general manner, the preceding results make it possible to locate the alloys of our invention at a level that is intermediate between austenitic nickel steels and traditional semi-ferritic steels.
The applications of the steels of the invention in the form of rods, sheets, and wires are very varied. Besides the usual usage of semi-lerritic steels. austenitic manganese or low content nickel steels. such as the coppersmiths trade. architecture. sinks. hubcaps. bumpers. moulding for automobile decoration. they have new markets due to their price which is lower than that of austenitic steels and austenoferritic steels and also due to the improved quality that they present compared to semi-ferritic steels. Moreover. they are particularly suitable for the manufacture of hot water flasks. automobile radiators (because of their resistance to corrosion under tension. their weldability and their price). mufflers. welded pipes. stove grills (because of their weldability). hooks for attaching the slate for roofs (because of their resistance to corrosion).
1. A process for treating a semi-ferritic stainless steel having an in-use structure of alpha and delta ferrite and carbides. characterized by high strength properties. good bending characteristics. good micro-hardness. good fusion weld resilience and good corrosion resistance. both in the presence and absence of chlorine and acid media. comprising:
A. Preparing a steel having a composition selected from the group consisting of by weight 0.1% max. C. 3-6% Mn. 1% max. Si. 1% max. Ni, 18-22% Cr, 1.5% max. Mo. 1% max. Cu. 0.1% max. N and the balance Fe and incidental impurities. .1% mac. C. 3-6%- Mn. 1% max. Si, 1% max. Ni. -18% Cr, l.5-3.0% Mo. 1% max. Cu. .1% max. N and the balance Fe and other incidental impurities and 0.1% max. C, 36% Mn. 12% Si. 1% max. Ni. 15-18% Cr, 0.52.0% Mo. 1% max. Cu. 0.1% max. N and the balance Fe and other incidental impurities, said steel characterized by chromium and nickel equivalents falling within the pentagon on the graph of the sole FIGURE. said pentagon having as its ordinates, A (15, 4.5) B (18.5, 8). C (24. 8). D (24. 6), E (20. 2);
B. Hot working said steel so as to maintain an austenite content while hot of from 10 to 50%. the balance being delta ferrite;
C. Cooling said steel to retain said austenite at room temperature;
D. Treating said steel to transform the austenite into martensite; and
E. Tempering the martensite to transform the martensite into alpha ferrite and carbides.
2. A process for treating a semi-ferritic stainless steel having an in-use structure of alpha and delta ferrite and carbides, characterized by high strength properties, good bending characteristics, good micro-hardness, good fusion weld resilience and good corrosion resistance. both in the presence and absence ofchlorine and acid media. comprising:
A. Preparing a steel having a composition selected from the group consisting of by weight 0.1% max. C. 3-6% Mn. 1% max. Si. 1% max. Ni, 18-22% Cr. 1.5% max. Mo. 1% max. Cu. .01% max. N and the balance Fe and incidental impurities. 0.1% max. C, 36% Mn. 1% max. Si, 1% max. Ni, 1518% Cr. l.53.0% Mo. 1% max. Cu, 0.1% max. N and the balance Fe and other incidental impurities and 0.1% max. C, 36% Mn. l2% Si. 1% max. Ni, 15-1 8% Cr. 05-20% Mo, 1% max. Cu. .1% max. N and the balance Fe and other incidental impurities. said steel characterized by chromium and nickel equivalents falling within the pentagon on the graph of the sole FlGURE. said pentagon having as its ordinates, A 15. 4.5), B (18.5, 8), C (24, 8). D (24, 6), E (20, 2);
B. Hot working said steel so as to maintain an austenite content while hot of from 10 to 50%, the balance being delta ferrite;
C. Annealing the steel for a prolonged period at about 800C so as to transform the austenite to alpha ferrite and carbides; and
D. Cold forming the steel.
3. The process of claim 1 wherein the treating of step D includes annealing between 700 and 900C for about 4 hours, slow cooling at a rate of about 25C per hour down to 650C, and then air cooling.
4. The process of claim 1 wherein the treating of step D includes cryogenically treating at about C for about 3 hours.
5. The process of claim 1 wherein the treating of step D includes cold working at ambient temperature by a cold reduction of at least 30%.
6. The process of claim 1 wherein the treating of step D includes slow cooling from a last processing hot rolling pass at a rate of about 25C per hour to 650C and then air cooling.
7. The process of claim 1 wherein the tempering of step E includes annealing at a temperature less than 850"C until the martensite disappears. v v
8. The process of claim 1 wherein the annealing is between 750 and 800C.
9. The process of claim 1 wherein the annealing is for a duration of less than 1 hour.
10. The product made in accordance with the process of claim 1.
11. The product made in accordance with the process of claim 10.
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,926,685
DATED December 16, 1975 INVENTOR( 2 Andre Gueussier et al.
It is certified that error appears in the above-identified patent and that said Letters Patent I are hereby corrected as shown below:
Page 1 of the Patent After the line listing the inventors insert:
-v-Assignee: Ugine Aciers Paris, France--.
In Table I Under the heading entitled Desig. "AISO" should read --AISI--.
Column 4 Line 37 "25+C" should read --25C--.
Claim 1 Column 7 Line 27 "mac. C," should read --max. C,
Claim 2 Column 8 Line 6 ".0l%" should read l%--.
Claim 11 Column 8 Line 51 "10'' should read --2--.
Signed and Scaled this Twenty-ninth Day of November I977 [SEAL] A ttest:
RUTH C. MASON LUTRELLE F. PARKER Attesting Officer Acting Commissioner of Patents and Trademarks