1. Field of the Invention
The invention relates to an austenitic stainless steel, and in particular, relates to an austenitic stainless steel which has a low nickel content and desirable metallographic, mechanical and corrosion resistance properties.
2. Description of the Invention Background
Certain iron and chromium alloys are highly resistant to corrosion and oxidation at high temperatures and also maintain considerable strength at these temperatures. These alloys are known as the stainless steels. The three major groups of stainless steels are the austenitic steels, the ferritic steels and the martensitic steels. The austenitic stainless steels have a microstructure at room temperature substantially comprised of a single austenite phase. Because of their desirable properties, the austenitic steels have received greater acceptance than the ferritic and martensitic types.
Chromium promotes the formation of delta ferrite microstructure in the stainless steels. This is usually undesirable in austenitic stainless steels. For example, in most conventional size ingots, if more than 10% delta ferrite is present during hot rolling, the resultant product will have slivers, hot tears and be prone to cracking unless costly treatments and procedures are employed. Nickel is therefore added to the austenitic stainless steels because it prevents the formation of delta ferrite and stabilizes the austenite microstructure at room temperature. Favorable mechanical properties, enhanced formability and increased corrosion resistance in reducing environments result. At present, the most widely produced austenitic stainless steel is AISI type 304, having 8.00-12.00% nickel.
Nickel is not abundant and the demand for the element has steadily increased. As such, the cost of nickel is projected to escalate, causing the price of nickel-containing austenitic steels to rise and, perhaps, become non-competitive with other materials. Because of the probability of fluctuations in the price of nickel and its increasing scarcity, it has been an object of researchers to develop an alternative austenitic stainless steel alloy which contains relatively lesser amounts of nickel, but which has corrosion resistance and mechanical properties comparable to existing nickel-containing austenitic alloys.
Lowering the nickel content of an austenitic stainless alloy promotes delta ferrite formation and the austenite phase becomes unstable. Therefore, as the nickel content is lowered in an unstable austenitic steel, the austenite phase must be stabilized by the addition of other austenite-promoting, or "austenitizing", elements. These elements include, for example, carbon, nitrogen, manganese, copper and cobalt. None of these elements as a single addition is entirely satisfactory. Cobalt is only slightly effective as an austenitizer and is quite expensive. Addition of carbon in an amount necessary to form a completely austenitic microstructure detrimentally affects ductility and corrosion resistance. Nitrogen cannot be added in quantities sufficient to achieve the desired effect, while additions of both carbon and nitrogen, due to interstitial solid solution hardening, undesirably increase the strength of the alloy. Manganese and copper are relatively weak austenitizers.
Although commercially available austenitic stainless steels exhibit predominantly the austenite phase in their asprocessed condition, certain austenitic alloy compositions become unstable by forming appreciable amounts of martensite when they are deformed during cold working. The amount of martensite formed during deformation is the most important cause of work hardening. An austenitic stainless steel may be considered "stable" if it forms less than about 10% martensite upon heavy cold deformation and "unstable" if it forms 10% or more martensite. The 10% limit is significant because deep drawing operations are less desirable above that percentage as cracking or excessive die wear tends to occur. The propensity of an austenitic steel to form martensite upon cold working may be reduced or eliminated by increasing the alloy content, especially the nickel content. However, as explained above, a high nickel content is economically undesirable. Manganese and copper, although relatively weak austenite stabilizers, have a beneficial side effect as they decrease the work hardening rate of austenitic steels by suppressing the transformation of austenite to martensite during plastic deformation. Thus, by alloying with austenite-promoting elements, a low-nickel austenitic stainless steel may be developed having a low delta ferrite content, acceptable corrosion resistance and mechanical properties, and satisfactory resistance to martensite formation upon plastic deformation.
A number of prior art stainless steels have some similarities to that of the instant application. Attention is directed to U.S. Pat. Nos. 4,568,387, 4,533,391 and 3,615,365. These prior art references neither disclose the alloy of the instant application nor suggest the combination of elements that imparts the instant alloy with its unique combination of properties.
An object of the present invention is therefore to provide a nickel-manganese-copper-nitrogen austenitic stainless steel alloy having a reduced nickel content and acceptable metallographic structure, mechanical properties, corrosion resistance and workability. More specifically, an object of the invention is to provide a nickel-manganese-copper-nitrogen austenitic stainless steel alloy which has the following properties:
a. nickel content less than about 5% by weight and preferably less than 4% by weight; PA0 b. low delta ferrite content of hot rolled and cold rolled sheet product; PA0 c. satisfactory workability; PA0 d. acceptable mechanical properties, e.g., yield strength, tensile strength and tensile elongation; PA0 e. acceptable corrosion and pitting resistance; and PA0 f. satisfactory resistance to martensite formation upon deformation.