Metastable Austenitic Stainless Steel

Abstract

A metastable austenitic stainless steel containing 0.07 to 0.18 percent carbon, 0.9 to 6.2 percent manganese, 4.1 to 7.7 percent nickel, 14.1 to 17.9 percent chromium and 0.01 to 0.14 percent nitrogen, the balance essentially iron. The steel has an Instability Function (IF) of 0.0 to 2.9 as determined by the following equation: If = + 37.193 - 51.248 (% c) - 1.0174 (% mn) - 2,5884 (% Ni) - 0.46770 (% Cr) - 34.396 (% N).

Description

This invention relates to a metastable austenitic stainless steel sheet and strip with improved forming characteristics.

Metastable austenitic stainless steels are those stainless steels that have an austenitic structure (face-centered arrangement of atoms) in the annealed condition, but are capable of undergoing a progressive solid-state transformation to a martensitic structure (body-centered arrangement of atoms) during mechanical deformation. The good ductility usually exhibited by such steel accounts for their widespread use in the production of formed parts. A typical metastable austenitic stainless steel is AISI Type 301.

Manufacturers of formed parts from metastable austenitic stainless steels desire that the steel possess great ductility but limited ultimate strength. Great ductility permits the steel to be formed into the intricate shapes aesthetically pleasing objects that so often require severe forming strains. Limited ultimate strength assures low forming stresses, or minimum resistance to forming, low wear on forming dies and equipment, low expenditure of energy during forming, and minimum springback in severely formed articles. Great ductility and limited ultimate strength are of particular importance in annealed sheet and strip, from which a majority of parts are formed. However, these qualities are also important in strip cold rolled to a high-yield strength level. Such strip is used when the strength level desired in the finished part must exceed some minimum value. But cold rolling, to achieve a high yield strength, reduces ductility and increases ultimate strength. In cold-rolled strip, manufacturers desire the greatest ductility and the lowest ultimate strength available at a given yield strength level.

The present invention provides a metastable austenitic stainless steel sheet and strip that exhibits superior combinations of high ductility and low ultimate strength. Moreover, these combinations of properties, for both annealed sheet and strip and cold-rolled sheet and strip, exist in a steel of the same composition so that fewer steels may be useful for different end applications.

The steel of the present invention is essentially an iron-base alloy that contains carbon, manganese, nickel, chromium and nitrogen as major alloying elements. It has been discovered that excellent formability is achieved in such a steel when two conditions are met: (1) the steel has an instability function (a measure of a steel's propensity to transform from austenite to martensite upon straining) within a restricted range; (2) the concentration of each major alloying element is within a specified range.

The instability function (IF) is defined by the following equation:

If =

+37.193

-51.248 (% c)

-1.0174 (% mn)

-2.5884 (% Ni)

-0.46770 (% Cr)

-34.396 (% N)

The principal feature, and absolute requirement, of the invention is a composition such that the IF has a value between zero and 2.9.

Steels that exhibit IF values between zero and 2.9 are defined as "slightly metastable" austenitic stainless steels. Such steels exhibit levels of ductility appreciably better than those of "wholly stable" austenitic stainless steels, i.e. those that exhibit negative IF values. Also, such steels exhibit levels of ductility appreciably better than those of "moderately metastable" austenitic stainless steels which are defined as those that exhibit IF values greater than 2.9.

Among stainless steels so constituted that the IF is within the aforementioned critical limits, are those having 0.07 to 0.18 percent carbon. Among such steels, the ranges for the other major alloying elements are rather wide: manganese, 0.9 to 6.2 percent nickel, 4.1 to 7.7 percent; chromium, 14.1 to 17.9 percent; and nitrogen, 0.01 to 0.14 percent. The IF is applicable within these broad ranges of composition and the excellent formability observed in steels with IF values inside the critical limits is available within these broad ranges of composition.

The critical limits specified for the IF are highly restrictive but include groups of steels with identical manganese, nickel, chromium and nitrogen contents. IN this group of steels, the maximum carbon content range permitted by the limits on the IF is a part of the overall range within the invention for this element, i.e. 0.07 to 0.18 percent. To utilize a wider portion of the overall carbon range and still achieve the excellent formability available within the limits set on the IF, compensatory adjustments in one or more of the other major alloying elements will keep the IF within the critical limits. When such adjustments are made, however, excellent formability is obtained within the overall range for carbon. Likewise, when all major alloying elements are considered, only a very small fraction of all possible combinations of these elements within their overall ranges satisfy the limits imposed by the IF (equation). Nevertheless, the excellent formability achieved when the IF is between zero and 2.9 is available at least within the overall ranges specified.

Among steels so constituted that their IF is between zero and 2.9, excellent formability is achieved within the overall ranges described above. However, optimum formability is approached among such steels as the overall range of one or more of the major alloying elements is in the following preferred range limits as follows:

0.10 to 0.12 percent C

0.90 to 1.10 percent Mn

6.75 to 7.25 percent Ni

16.6 to 17.8 percent Cr

0.07 to 0.09 percent N

The invention will be more fully described by the following examples:

Three series of austenitic stainless steels were prepared having the compositions given in tables I-A, B and C. ##SPC1##

Each steel described in table I was melted in a 100-pound induction furnace, cast into slab-type ingot molds, hot rolled and cold rolled to 0.035-inch-thick strip, and annealed for 60 seconds in molten salt at 2,000.degree. F. In addition, annealed strip from the first series of heats was reduced 10, 20, 30, 40 and 50 percent in thickness by cold rolling. Product equivalent to that produced in the laboratory can be produced commercially by conventional stainless steelmaking practices.

Sheet specimens from each steel were tested in tension and the results are shown in tables II-A, B, C and D. The elongation values so obtained are used to measure forming ductility. The tensile strength values obtained are used as a measure of forming resistance. Yield strength values are used as a measure of initial resistance to deformation. ##SPC2##

In addition, three discs of annealed strip from each steel in the first series of heats, with diameters of 3, 31/8 and 3 inches were drawn into flanged, cylindrical, flat-bottom cups of 1.250-inch internal diameter. The average cup depth at fracture for the three discs of each steel, given in table III, was used as another measure of forming ductility. Likewise, the ultimate stress (the average drawing load divided by the average cross-sectional area of the cup sidewall supporting this load) is also given in table III. --------------------------------------------------------------------------- --------------------------------------------------------------------------- table iii

results of Deep-Drawing Tests on the --------------------------------------------------------------------------- First Series of Steels

Ultimate Cup Depth, Stress, Heat No. inch K s.i. __________________________________________________________________________ P8206-1 0.709 140.8 P8186-1 0.728 147.9 P8209-1 0.713 145.1 P8199-1 0.848 127.2 P8204-1 0.735 141.3 P8183-1 0.859 132.9 P8203-1 0.870 124.8 P8182-1 0.901 131.9 P8198-1 0.742 133.9 P8207-1 0.936 112.3 P8189-1 0.910 115.8 P8200-1 1.000 117.4 P8196-2 0.819 113.9 P8202-2 0.977 118.0 P8185-1 0.869 121.8 P8181-1 0.893 105.4 P8193-1 0.696 127.8 P8190-1 0.932 120.5 P8194-1 0.831 133.3 P8197-1 0.929 121.0 P8201-1 0.710 138.0 P8184-1 0.856 131.7 P8192-1 0.736 155.0 P8210-1 0.924 108.1 P8208-2 0.809 141.2 P8205-1 0.910 119.0 P8195-2 0.870 133.6 P8191-1 0.892 130.0 P8188-1 0.873 135.0 P8187-1 0.898 131.9 __________________________________________________________________________

by employing a magnetic method, a measure of the propensity of each experimental steel in the first series of steels to undergo the strain-induced transformation is obtained. Values of this measure of --------------------------------------------------------------------------- instability, IF, are shown in table IV. --------------------------------------------------------------------------- TABLE IV

Measured Instability Functions for the --------------------------------------------------------------------------- First Series of Steels

Heat No. Instability Function __________________________________________________________________________ P8206-1 6.491 P8186-1 3.729 P8209-1 5.620 P8199-1 1.775 P8204-1 6.622 P8183-1 3.235 P8203-1 3.519 P8182-1 1.562 P8198-1 4.652 P8207-1 0.542 P8189-1 1.816 P8200-1 0.706 P8196-2 4.109 P8202-1 1.322 P81854-1 3.174 P8181-1 -0.179 P8193-1 6.505 P8190-1 0.531 P8194-1 3.515 P8197-1 0.320 P8201-1 3.884 P8184-1 1.205 P8192-1 4.828 P8210-1 -0.112 P8208-2 4.630 P8205-1 1.822 P8195-2 3.492 P8191-1 2.788 P8188-1 3.458 P8187-1 2.997 __________________________________________________________________________

after analyzing the data referred to above, an equation was developed relating the instability function to the composition. This equation is as follows:

If =

+37.193

-51.248 (% c)

-1.0174 (% mn)

-2.5884 (% Ni)

-0.46770 (% Cr)

-34.396 (% N)

Using the above equation, IF values for the second and third series of heats are shown in tables V-A and B. --------------------------------------------------------------------------- --------------------------------------------------------------------------- table v-a

calculated Instability Functions (IF) --------------------------------------------------------------------------- for the Second Series of Steels

Heat No. Calculated IF __________________________________________________________________________ P8063-1A 1.14 T5551-1 1.69 T5553-1 2.29 R9593-2 2.62 P8058-B 2.91 R9588-1 2.97 P8061-1A 3.62 __________________________________________________________________________ --------------------------------------------------------------------------- table v-b

calculated Instability Functions (IF) --------------------------------------------------------------------------- for the Third Series of Steels

Heat No. Calculated IF __________________________________________________________________________ T5320-1 -2.30 T5321-1 -0.23 T5322-1 -3.67 T5328-1 -4.82 T5330-1 -6.80 T5341-1 -3.92 T5343-1 -8.15 T5349-1 -7.17 __________________________________________________________________________

as discussed above, the principal feature of our invention is the critical relation between IF and forming ductility. It is observed that for both annealed and cold-rolled strip, the elongation decreases as the IF increases from zero. (A similar relation exists between cup depth and IF.) The graph in the drawing indicates that stable austenitic stainless steels have less ductility than slightly metastable austenitic stainless steels.

Also as shown in the drawing, optimum elongation is achieved only in austenitic stainless steels with an IF inside a rather narrow range. In particular, the greatest elongations are achieved in steels with IF values between zero and 2.9. In such steels, the strain-induced transformation of austenite to martensite, with its attendant strengthening of the structure, progresses at such a rate that the load-bearing capacity of the steel continues to increase until exceptionally great strains are achieved. Accordingly, elongations that approach 75 percent can be achieved in such steels. Such great elongations minimize the probability of fracture during the forming of intricate shapes.

It may also be seen from the figure that the elongation of annealed strip drops precipitously to a plateau level of 55 percent when the IF falls below zero. This plateau level, however, is maintained over a broad range of negative IF values. In stable austenitic stainless steels, virtually no martensite is formed during straining. Accordingly, these steels do not strengthen at a rate rapid enough to sustain their load-bearing capacity to exceptionally high strains. Thus, the forming operations that can be sustained by stable austenitic stainless steels before fracture are less severe than those tolerated by the metastable steels.

It may also be seen from the drawing that elongation drops continuously as the IF increases from zero. As the instability of the steel increases, the rate of strain-induced transformation increases. Consequently, the austenite remaining in the structure is quickly depleted. Thereafter, further strengthening of the structure by transformation is very limited. Thus, in moderately metastable steels, the required increase in load-bearing capacity with increasing strain does not extend to exceptionally high strains, and severe forming operations cannot be tolerated.

The practical upper limit of the IF is 2.9. The moderately metastable austenitic stainless steels with IF values greater than 2.9 generally exhibit no advantage in elongation over the stable austenitic stainless steels with negative IF values. The lower limit of the IF is zero.

Calculations of IF values and ductility of hypothetical steels show that both elongation and cup depth increase as the carbon and/or nitrogen content increases, or as the IF decreases toward zero. Accordingly, the greatest forming ductility is achieved just before the IF reaches zero. However, in such steels, appreciably better ductility is obtained at the higher nitrogen levels than at the higher carbon levels. For example, the two hypothetical steels composed in part of 0.14 percent carbon with 0.06 percent nitrogen, and 0.12 percent carbon with 0.09 percent nitrogen, both exhibit IF values of 0.77. The latter composition, however, offers 4.3 percent improvement in elongation with an 0.031-inch improvement in cup depth. This improvement occurs because, for identical IF values, 0.0149 percent nitrogen is the equivalent of 0.01 percent carbon. But 0.0149 percent nitrogen is more effective than 0.01 percent carbon in improving elongation and cup depth.

Although there is advantage to achieving a low IF value at the higher nitrogen contents, there is a practical maximum limit to the nitrogen content. Increases in nitrogen are more effective than equal increases in carbon in raising the yield strength. Therefore, to avoid yield strengths greater than those normally exhibited by metastable austenitic stainless steels, about 33 to 50 k.s.i., maximum limits on carbon and nitrogen should preferably be about 0.12 and 0.09 percent, respectively. Minimum limits for these elements are then controlled only by the requirement that the IF be within the desired limits. Carbon and nitrogen contents of 0.10 and 0.07 percent, respectively, meet this condition.

Increased amounts of chromium slightly increase the elongation of annealed strip. However, formability reaches a maximum at 17.2 percent chromium. The decreasing tensile strength is offset by an increasing drawing stress as the chromium content increases. The properties of cold rolled steel are not highly sensitive to chromium content. It has been found that optimum formability is achieved at about 17.2 percent chromium, but that formability is quite satisfactory if the chromium content decreases to 16.6 percent or increases to 17.8 percent.

Manganese and nickel are interchangeable in the proportion that 0.4 percent nickel has about the same effect on the IF as 1.0 percent manganese. Thus, a steel that contains 3.0 percent manganese and 6.2 percent nickel should exhibit properties similar to one that contains 1.0 percent manganese and 7.0 percent nickel. The latter proportioning of these two elements is preferable, however, because high manganese-low nickel alloys have poorer hot workability than their low manganese-high nickel counterparts.

The substitution of manganese for nickel reduces forming stresses in both annealed and cold-rolled strip. Also, this substitution improves the ductility of annealed strip; however, it reduces the ductility of cold-rolled strip.

The ductility of annealed strip increases, the tensile strength of both annealed and cold-rolled strip decreases, and the drawing stress of annealed strip decreases as the manganese content increases. The ductility of cold-rolled strip decreases with increasing manganese content. Very similar effects are observed when nickel content is increased. Accordingly, ductility in cold-rolled strip is sacrificed when either manganese or nickel content is increased. Accordingly, ductility in cold-rolled strip is sacrificed when either manganese or nickel content is increased. However, the interstitial elements have only desirable effects on properties and the levels of the interstitial elements are kept high. The levels of manganese and nickel, 1.0 and 7.0 percent respectively, are only high enough to provide low positive values of IF. To minimize the risk of exceeding the limits on the IF, or of upsetting previously discussed interaction-dependent results, the ranges of these elements should preferably be narrow: 0.9 to 1.0 percent manganese; 6.75 to 7.25 percent nickel.

Claims

We claim:

1. A method for producing for applications involving severe forming, a metastable austenitic stainless steel sheet product, within the range, consisting essentially of 0.07 to 0.18 percent C, 0.9 to 6.2 percent Mn, 4.1 to 7.7 percent Ni, 14.1 to 17.9 percent Cr and 0.01 to 0.14 percent N, balance iron, which comprises,

a. combining the aforesaid elements in proportions in accord with the equation

37.193-51.248(% C)- 1.0174(% Mn)- 2.5884(% Ni)

-0.4677(% Cr)- 3.396(% N)= 0 to 2.9 to provide a steel product,

b. rolling said steel product to produce a sheet which exhibits a superior combination of low ultimate strength and high ductility as evidenced by an elongation (in 2 inches of 0.035-inch-thick strip) greater than 55 percent, when annealed for 60 seconds at 2,000.degree. F.

2. The method of claim 1, in which said range consists essentially of 0.1 to 0.12 percent C, to 0.9 to 1.10 percent Mn, 6.75 to 7.25 percent Ni, 16.6 to 17.8 percent Cr, and 0.07 to 0.09 percent N.

3. A metastable austenitic stainless steel consisting essentially of 0.1 to 0.12 percent carbon, 0.9 to 1.10 percent manganese, 6.75 to 7.25 percent nickel, 16.6 to 17.8 percent chromium and 0.07 to 0.09 percent nitrogen, the balance essentially iron; said steel having an Instability Function (IF) of 0.0 to 2.9 as determined by the following equation:

If = +37.193

-51.248(% c)

-1.0174(% mn)

-2.5884(% Ni)

-0.46770(% Cr)

-34.396(% N).

4. A metastable austenitic stainless steel according to claim 3 having nominally 1.0 percent manganese, 7.0 percent nickel and 17.2 percent chromium.