US2684298A

US2684298A - Austenitic stainless steel

Info

Publication number: US2684298A
Application number: US321731A
Authority: US
Inventors: Robert H Henke
Original assignee: Allegheny Ludlum Steel Corp
Current assignee: Allegheny Ludlum Steel Corp
Priority date: 1952-11-20
Filing date: 1952-11-20
Publication date: 1954-07-20
Anticipated expiration: 1971-07-20

Description

Patented July 20, 1954 UNITED STATES ATENT OFFICE AUSTENITIC STAINLESS STEEL Robert H. Henke, Pittsburgh, Pa., assignor to N Drawing.

8 Claims.

This invention relates to steel and in particular to austenitic chromium manganese steel.

Austenitic chromium manganese steel has been known for a number of years. Thus it has been known that steels containing up to about 13% chromium are completely austenitic if they .contain 14% or higher manganese. However, with higher chromium contents it is found that ferrite is present in the structure and that it is not possible to prevent the formation of ferrite by increasing the manganese content. Instead steel containing 15% chromium, 12% manganese and up to 1.5% nickel has been produced as having an austenitic structure, it being noted that where the chromium content was increased, the manganese content was decreased and nickel was employed to maintain the austenitic structure.

In view of the present shortage of nickel, the chromium manganese steel has become attractive as a substitute for the well-known 18% chromium-8% nickel stainless steel. Prior attempts to produce such substitutes; for example, 13% chromium-14% manganese, or 16% chromium- 14% manganese-1% nickel, or 17% chromium- 12% manganese-2% nickel, or 17% chromium- 9% manganese-3% nickel, have resulted in the production of steel having mechanical properties similar to type 301 steel, but such steel cannot be satisfactorily produced and rolled directly from ingots larger than 9 inches square. fore desirable while producing a substitute for the well-known 18% chromium-8% nickel stainless steel to improve the corrosion resistance characteristics, the mechanical properties and the weldability and bendability of the known chromium manganese steels, while at the same time produce a steel that has an austenitic structure and is capable of being hot worked directly from large ingot size to slab form and to thereafter be capable of being cold worked.

An object of this invention is to provide a cold workable austenitic chromium manganese steel.

Another object of this inv'entionis to provide an austenitic steel containing 15 to 17.25% chromium, 15 to 19% manganese and not more than 2% nickel that is workable by a single conversion heat treatment from large ingot size to slab form and is thereafter capable of being cold worked to stripform havinggoodphysical properties and resistance tocorrosioni;

It is there- Application November 20, 1952, Serial No. 321,731

Other objects of this invention will become apparent from the following description.

The steel of this invention is an austenitic steel capable of being directly converted by hot rolling from large ingot size into slab form. Fundamentally the steel comprises a high manganese, high chromium steel containing from a small but essential amount up to 2% nickel, the nickel cooperating therein to maintain an austenitic structure. The alloying elements in the broad aspect of the invention comprise 15 to 19% manganese, '15 to 17.25% chromium, not more than 2% nickel, not over .20% carbon, not over 1% silicon, not over .04% of each of phosphorus and sulphur, .05 to 25% nitrogen, .003 to 15% of rare earth element and the balance iron. All of the compositions given herein and in the claims are by weight.

A preferred embodiment of the invention is a steel comprising 16 to 18% manganese, 15 to 16% chromium, not more than about 1.25% nickel, not over 20% carbon, not over 1% silicon, not over 030% of each of phosphorus and sulphur, .05 to 25% nitrogen, .003 to 15% rare earth element and the balance iron.

Where the chromium content is in the indicated range of 15 to 17.25% with the manganese in the high range of 15 to 19%, it is found that unless nickel is present in amounts up to 2%, the resulting steels have ferrite present in the as cast structure in amounts which render the steel undesirable for hot working directly from ingot size to slab form. Thus where from 10 to 25% ferrite with the balance austenite is formed in the steel, as is the case where nickel is not present, the austenite is usually surrounded by the ferrite with the result that as the temperature decreases during the hot rolling processing,the austenite becomes hard and the plastic deformation of the steel ingot is such that it exceeds the ductility limits of the ferrite resulting in ruptures.

Even where nickel in small amounts of up to 2% is included as an essential element to cooperate in producing a predominantly austenitic structure, it has been found substantially impossible to directly hot roll the ingot to the slab form unless from .003 to .15% of rare earth element of the lanthanide series such as cerium,

lanthanum, neodymium and similar rare earth elements is present. In practice, the rare earth element is usually added to the melt in the form of misch metal in which cerium and lanthanum predominate. While the term rare earth element is employed in this description and in the claims in the singular form, it is to be understood that such expression includes either one of the rare earth elements such as cerium or mixtures thereof such as cerium and/or lanthanum with the other known rare earth elements, the latter usually being the case since the rare earth elements are now commonly available in the open market as packaged compounds identified under the trade names of T-Conipound, L-Metal, and Lan- Ger-Amp.

As examples of the eifect of the presence of the rare earth element as an essential alloying element on the austenitic chromium manganese steel of this invention, reference may be had to the following table of analysis of the composition of diiferent heats made while developing the present steel.

4 austenite appeared to resist plastic deformation. The other ingots listed in the foregoing table and containing the rare earth element as indicated were successfully directly hot worked from Composition, Percent by Weight Heat No.

C Mn P S Si Cr Ni N2 large ingot size, as for example, 16 by 31", to slabs 3 inches or less in thickness without any accompanying detrimental cracking. The best hot rolled results are obtained Where the steel has a chromium content of from 15 to 17% and manganese content of 16 to 18% with up to 1.25%

nickel, the higher manganese and lower chromium contents rendering the steel more completely austenitic in the as cast condition. However, in all cases there is some ferrite in the structure as cast" and usually in the slab, such ferrite however tending to disappear during the annealing of the cold rolled strip. The steel as thus finally processed can therefore be termed an austenitic steel since the ferrite disappears 20 in the steel as worked into strip.

As an example of the physical properties obtained with the steel of this invention, reference Total Ce Rare Earths None None None None None None None None The heats 88880, 88884, 98070, and 98080 of the foregoing table and having a composition within the range given hereinbefore, except that such heats are devoid of the presence of the rare earth element, are not commercial since it has been found that they cannot be successfully hot rolled directly from large ingot size to slab form. Such heats can only be formed into slab form by expensive double and triple conversion heat treatments resulting in yields of only about of the metal.

Where such steels are processed in an effort to'hot roll them directly from the ingot to slab form, it is found that the slabs, are ruined as by edge checking and cracking long before the ingot is processed to a desired slab thickness of about 3 inches or less. Thus, for example, in rolling an ingot 16" by 31" of heat 88880 from a temperature of 2250 to 2300 F. after soaking at 2200 F. for 2 hours, rolling had to be stopped at approximately 6 inches thickness because of edge cracking. Similarly, hot rolling of a 16" by 31 ingot of heat 8888 1 had to be discontinued when rolled to an 8" by 22 slab size because of cracking. Similar results were obtained in rolling ingots of heats 98070 and 98080. In all cases hot rolling failure took place in an unusual manner in that in attempting to roll the ingots, they appeared to be entirely satisfactory until reduced to approximately 8" in thickness. At this point, that is in the vicinity of the 8 inch slabbing pass, the metal tore along the edges of the surface all at once as if the metal were hot short. However, a micro-examination of the metal so treated indicated that instead of the metal being hot short,

a temperature had been reached at which the may be had to the steel of heat 77834 identified in the foregoing table. A slab rolled directly from a 16" by 31" ingot to 3 inch slab as described hereinbefore, when further hot rolled from a temperature of 2225 to 2300 F. to a strip .090 inch thick and annealed at a temperature of 1850 to 2000 F. had the following properties:

Yield strength (02% ofiset) 39,830 p. s. i. Tensile strength 100,100 p. s. i Percent elongation 64.0 Rockwell B Hardness 82-84 When thereafter cold rolled to .078 inch, the strip had the following properties:

Yield strength (02% offset) 65,960 p. s. i. Tensile strength 110,750 p. s. i. Per cent elongation 53.0

Rockwell B Hardness 93-95 When such strip was thereafter annealed at 1850 to 2000 F. and then cold rolled to a thickness of .062", the resulting strip had the following properties:

Yield strength (02% offset) 113,150 10. s. i. Tensile strength 142,600 p. s. i. Per cent elongation 21.0

Rockwell C Hardness 29-31 Thereafter when again annealed at 1850 to 2000 F., further cold rolled to a thickness of .040 inch and annealed at 1850 to 2000 F., the following properties were obtained:

Yield strength (0.02% offset) 43,000 p. s. i. Tensile strength 99,950 p. s. i. Per cent elongation in 2" 58.0

Rockwell B Hardness 83-85 The mechanical properties for the steel of this invention as exemplified by heat 77834 are thus seen to closely duplicate the mechanical properties of the well-known type 301 stainless steel very closely with the strength and rate of work hardening tending to be on the low side of the type 301 average values. However, its ability to be fabricated by bending, forming and welding appears to be equal to that of the type 301 stainless steel.

As a further example of the tensile strength and physical properties obtained with a similar steel, reference may be had to heat 98027 which when processed directly from ingot to slab form as described hereinbefore and then annealed and cold worked to a thickness of .044 inch, had the following physical properties:

Yield strength (2% ofiset) 124,500 p. s. i. Tensile strength 148,500 p. s. 1. Per cent elongation in 2" 18.5 Rockwell C Hardness 35.5 Bend 135-3/16" R, 0K

On the other hand, heat 98021 having the lower manganese content of 16.06%, but a higher chromium content of 17.25% and a higher nickel content of 1.83%, when cold rolled to .078 inch and annealed at 1850-2000 F., had the following tensile and hardness properties:

Yield strength (.2% offset) 47,700 p. s. i. Tensile strength 99,760 p. s. i. Per cent elongation in 2" Rockwell B Hardness 83-84 All of the steels of this invention have adequate corrosion resistance so that they can be used in applications where the well-known type 430 stainless steel has been used and further can be employed in other applications where the type 430 stainless steel is unsatisfactory because of mechanical properties or physical properties. As an example of the corrosion resistance of the steels of this invention containing up to 1% nickel as exemplified by heat 77834 in unwelded annealed cold rolled strip, heat 77834 was subjected to the Huey test in boiling nitric acid and the weight losses of five 48 hour periods were converted to inches penetration per month and averaged with the result that the tests show a rate of attack in the range of .005-.006 inch I penetration per month. While these results are not quite as good as the rate of attack of type 301 stainless steel, nevertheless, they compare quite favorably with the rate of attack of type 430 stainless steel. On the other hand, when the steels having the higher nickel content, as for example heat 98021, were subjected to the same test, the rate of attack was in the range of 003-0035 inch penetration per month. It is thus seen that with the higher nickel content, the resulting steel has a better corrosion resistance than the steels containing up to 1% nickel. Such rate of attack for the steels containing from 1 to 2% nickel is better than the rate of attack shown by type 430 stainless steel and compares favorably with the rate of attack of less than .002 inch penetration per month for the well-known types 301 and 302.

The steels of this invention and in particular the steels containing up to 1% nickel and with from .08 to .12% carbon can be readily welded by hand arc welding using a filler rod of the same composition or a filler rod of type 308 steel as well as by the heliarc method or resistance welding, such welding results being equivalent to the .6 welding results obtained on type 301 or type 302 stainless steel. Further, in order to determine the resistance to intergranular corrosion, fusion welded light gauge sheet and strip formed of the steel of this invention have been subjected to the well-known Krupp test for 48' hours and have been found to bend satisfactorily after such test without any evidence of intergranular corrosion around the weld. Likewise, the steel of this invention containing from 1 to 2% nickel has also been welded and subjected to the Krupp tests for a 48 hourperiod. The steel containing the higher nickel content has been found to be satisfactory in that it shows a resistance to intergranular corrosion similar to type 302.

The steel of this invention has proven to be an austenitic steel of acceptable corrosion resistance with a minimum nickel content. From the weldability, bendability, corrosion resistance and physical and tensile properties, it can readily be substituted for the well-known 18% chromium- 8% nickel stainless steels. A yield of 75% or higher is obtained in working the steel from the large ingot size to slab form. Further the composition is readily controlled and can therefore be readily reproduced by anyone skilled in the art.

I claim:

1. An austenitic steel consisting essentially of 15% to 19% manganese, 15% to 17.25% chromium, not more than 2% nickel, not over 20% carbon, not over .040% of each of phosphorus and sulphur, .05% to .25% nitrogen, 003% to .15% rare earth element of the lanthanide series, and the balance iron, the steel having the characteristic of being hot workable directly from large ingot size to slab form.

2. An austenitic steel consisting essentially of 16% to 18% manganese, 15% to 16% chromium, not more than 1.25% nickel, not over .12% carbon, not over 1% silicon, not over .030% of each of phosphorus and sulphur, .05% to .25% nitrogen, .003% to .15% rare earth element of the lanthanide series, and, the balance iron, the steel having the characteristic of being hot workable directly from large ingot size to slab form.

3. An austenitic steel consisting essentially of 16% to 18% manganese, 15% to 16% chromium, 1.25% to 2% nickel, not over .20% carbon, not over 1% silicon, not over 030% of each of phosphorus and sulphur, .05% to .25% nitrogen, .003% to .15% rare earth element of the lanthanide series, and the balance iron, the steel having the characteristic of being hot workable directly from large ingot size to slab form.

4. An austenitic steel consisting of about 16.00% manganese, about 17.25% chromium, about 1.83% nickel, about .095% carbon, about .35% silicon, about .024% phosphorus, about 01% sulphur, about .11% nitrogen, about .023% rare earth element of the lanthanide series, and the balance iron.

5. An austenitic steel consisting of about 17.59% manganese, about 15.32% chromium, about .95% nickel, about .10% carbon, about 38% silicon, about .022% phosphorus, about 008% sulphur, about .13% nitrogen, about .022% rare earth element of the lanthanide series, and the balance iron.

6. An austenitic steel composed of 15% to 19% manganese, 15% to 17.25% chromium, not more than 2% nickel, not over .20% carbon, not over 040% of each of phosphorus and sulphur, .05% to .25% nitrogen, .003% to .15% of rare Z earth element selected from the lanthanide series, and the balance ircn.

'7'. An austeni-tic steel composed of 15% to 19% manganese, 15% to 17.25% chromium, not more than 2% nickel, not over .20.% carbon, not over 1% silicon, net over .040% of each of phosphorus and sulphur, 05% to 25% nitrogen, .003% to .15% of rare earth element residue from misch metal addition, and the balance iron.

8. An austenitic steel composed of 15% to 19% manganese, 15% to 17.25% chromium, not more than 2% nickel, not over 20% carbon, not over References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,553,330 Post et a1. May 15, 1951 2,616,798 Zeigler et a1 Nov. 14, 1952

Claims

1. AN AUSTENITIC STEEL CONSISTING ESSENTIALLY OF 15% TO 19% MAGANESE, 15% TO 17.25% CHROMIUM, NOT MORE THAN 2% NICKEL, NOT OVER .20% CARBON, NOT OVER .040% OF EACH OF PHOSPHOROUS AND SULPHUR, .05% TO .25% NITROGEN, .003% TO .15% RATE EARTH ELEMENT OF THE LANTHANIDE SERIES, AND THE BALANCE IRON, THE STEEL HAVING THE CHARACTERISTIC OF BEING HOT WORKABLE DIRECTLY FROM LARGE INGOT SIZE TO SLAB FORM.