KR20010083939A - Cr-mn-ni-cu austenitic stainless steel - Google Patents

Abstract

The austenitic stainless steel according to the present invention is C 0.025-0.15, Mn 4-12, Si max 1.0, P max 0.2, S max 0.1, Cr 15.5-17.5, Ni 1-4, Mo based on the weight percent unit. It has a wide composition range of 0.25-1.5, Cu 1.5-4, W max 1.0, Co max 1.0, N 0.05-0.30. The balance of this alloy is iron and common impurities. Within the aforementioned range, the sum of the amount of carbon and the amount of nitrogen is about 0.19% or more, the sum of the amount of chromium and the amount of molybdenum is less than about 17.75%, and% Cr + 3.3 (% Mo) + 13 (% N) Adjust the proportions of the elements to establish a relationship> 20.5. This alloy has a unique combination of corrosion resistance, formability and resistance to work hardening.

Description

Cr-MN-Ni-Cu Austenitic Stainless Steel {CR-MN-NI-CU AUSTENITIC STAINLESS STEEL}

Known austenitic stainless steels have the following weight percent compositions.

C max 0.15

Mn 6.4-8

Si max 1.0

Cr 16.5-17.5

Ni 2.5-5

Cu 2.0-3.0

N max 0.2

C + N up to 0.30

The balance of this alloy is iron and common impurities. According to published data for this alloy, anode polarization tests have shown that this alloy has corresponding corrosion resistance to AISI Type 201L and AISI Type 430 stainless steels. In addition, the above data shows that the pitting resistance of this alloy has much room for improvement compared to AISI Type 304 stainless steel, which is one of the most widely used stainless steels. In addition, commercially available products of this known austenitic stainless steel have been found to have a fairly weak resistance to crack erosion of chlorides in the salt spray test, and undesirable rusting occurs.

Therefore, corrosion resistance to chloride is equivalent to Type 304 stainless steel, but austenitic stainless steel having a lower nickel content is required. It is well known that molybdenum improves pitting resistance to chlorides of austenitic stainless steels. However, it is also known that molybdenum has a strong tendency to form ferrite, which disturbs the phase equilibrium of austenitic stainless steel and adversely affects hot workability. Considering the above, the resistance to corrosion and crack corrosion corresponds to Type 304 stainless steel, but austenite stainless steel is required, which is hot enough to be easily formed into various product shapes.

The present invention relates to Cr-Mn austenitic stainless steel, and more particularly, to Cr-Mn-Ni-Cu auster having a unique combination of corrosion resistance, hot workability and resistance to work hardening in an environment containing chloride. Relates to a knight stainless steel.

According to the present invention, there is provided an austenitic stainless steel alloy having a wide composition range, an intermediate composition range and a preferred composition range, described below in weight percent.

element Wide composition range Medium composition range Preferred Composition Range

C 0.025-0.15 0.05-0.12 0.08-0.11

Mn 4-12 6-10 7-9

Si max 1.0 max 0.75 max 0.50

P max 2.0 max 0.1 max 0.05

S max 0.1 max 0.05 max 0.025

Cr 15.5-17.5 16.0-17.0 16.25-16.75

Ni 1-4 1.5-3.5 2.0-3.0

Mo 0.25-1.5 0.5-1.2 0.7-1.0

Cu 1.5-4 2.0-3.5 2.5-3.0

W max 1.0 max 1.0 max 0.75

Co max 1.0 max 1.0 max 0.75

N 0.05-0.30 0.10-0.25 0.12-0.20

Fe balance balance balance

By adjusting the ratio of each element within the composition range of the above-described elements it is possible to obtain a unique composition characterizing the austenitic stainless steel alloy according to the present invention. More specifically, the sum of chromium and molybdenum is kept below about 17.75% by weight, the sum of carbon and nitrogen is about 0.19% by weight or more, and in the cast state the ferrite in the alloy is below about 9% by volume. Be sure to In addition, the ratio of chromium, molybdenum and nitrogen is adjusted according to the following relationship.

% Cr + 3.3 (% Mo) + 13 (% N)> 20.5

This relation defines the pitting resistance equivalent number (PRE) of the austenitic stainless steel alloy according to the present invention. This PRE is preferably approximately greater than 21.0.

The above table of composition ranges is presented for the sake of convenience and is not intended to limit the upper and lower limits of the range of elements used in combination with each other, or to limit the range of elements used only in combination. Thus, one or more of the ranges may be used with one or more other ranges of the remaining elements. In addition, a maximum or minimum value of a wide composition range, an intermediate composition range or a preferred composition range of one element can be used together with the maximum or minimum value of another intermediate composition range or a preferred composition range of the same element. The terms "percent" and "%" symbols throughout this specification mean weight percent unless otherwise indicated.

In the austenitic stainless steel alloy according to the invention, chromium is present at least about 15.5%, more preferably at least about 16.0%, preferably at least about 16.25%, thereby improving the overall corrosion resistance of the alloy. Chromium also contributes to the corrosion resistance of the chloride of the alloy. If the chromium is excessive, the hot workability of the alloy is lowered, so the amount of chromium in the alloy is limited to about 17.5% or less, more preferably about 17.0% or less, and preferably about 16.75% or less.

Molybdenum improves the alloy's resistance to chlorides, so that the alloy contains molybdenum at least about 0.25%, more preferably at least about 0.5%, preferably at least about 0.7%. However, when the amount of molybdenum becomes excessive, the hot workability of the alloy is lowered. Thus, the amount of molybdenum in the alloy is limited to about 1.5% or less, more preferably about 1.2% or less, preferably about 1.0% or less.

Both chromium and molybdenum are elements that form ferrites in alloys. When the amount of ferrite in the alloy is excessive, the hot workability of the alloy is lowered. Therefore, the amount of chromium and molybdenum should be limited so that ferrite does not form to a harmful degree. As described above, the sum of the amount of chromium and the amount of molybdenum in the alloy (% Cr +% Mo) is about 17.75% or less, preferably about 17.5% or less.

Nitrogen also contributes to the corrosion resistance of the alloy to chlorides and also aids in the austenite phase equilibrium of the alloy. For this reason, nitrogen is contained in the alloy at least about 0.05%, more preferably at least about 0.10%, preferably at least about 0.12%. Excess nitrogen increases the work hardening rate of the alloy and thus adversely affects the cold formability of the alloy, thus limiting the amount of nitrogen in the alloy. Thus, the content of nitrogen in the alloy is about 0.30% or less, more preferably about 0.25% or less, preferably about 0.20% or less.

Carbon also aids in the austenite phase equilibrium of alloys. Accordingly, carbon is present in the alloy at least about 0.025%, more preferably at least about 0.05%, preferably at least about 0.08%. When the amount of carbon in the alloy is excessive, carbides are precipitated, which lowers the corrosion resistance and cold formability of the alloy. Therefore, the amount of carbon in the alloy is limited to approximately 0.15% or less, more preferably 0.12% or less, preferably 0.11% or less.

As described above, chromium and molybdenum are present in the alloy, so that the amount of carbon in the alloy and the amount of nitrogen in the alloy (% C +% N) is approximately 0.19% or less, in order to avoid the formation of harmful levels of ferrite. Limited to approximately 0.25% or less.

Nickel, which improves the austenite phase equilibrium of the alloy, is included at least about 1%, more preferably at least 1.5%, preferably at least about 2.0%. Nickel also lowers the work hardening rate of the alloy to improve the cold formability of the alloy. The content of nickel in the alloy is limited to about 4% or less, more preferably about 3.5% or less, preferably about 3.0% or less.

Copper is included in the alloy because it significantly lowers the work hardening rate of the alloy than nickel. Copper is also very advantageous for the cold formability of alloys. In addition, copper lowers the strength at which the alloy is annealed, thus improving cold formability. To attain these desirable properties, the alloy comprises at least about 1.5% copper, for example at least about 1.6 or 1.7%. The content of copper is more preferably about 2.0% or more, preferably about 2.5% or more. Excessive copper content results in high temperature brittleness, particularly when the amount of ferrite in the alloyed state is at least about 9% by volume, thus limiting the amount of copper in the alloy. High temperature brittleness causes severe cracking and / or tearing of the alloy during hot working. Therefore, the content of copper is about 4% or less, more preferably about 3.5% or less, preferably about 3.0% or less.

The inventors of the present application have found that even if the content of the ferrite is only 9% by volume, the alloy cannot be hot worked and serious cracking or tearing occurs. In order to minimize the high temperature brittleness of the alloy, the composition of the alloy should be adjusted such that the content of ferrite in the casted state of the alloy is less than approximately 9 vol%. The composition of the alloy is adjusted so that the content of ferrite in the cast state is about 8.5 vol% or less, more preferably about 7 vol% or less, and about 6 vol% or less for best results.

Manganese contributes to austenite stability, that is to say that manganese acts to stabilize the austenite state so that it does not transfer to the martensite state upon mechanical deformation, so that it is at least about 4%, more preferably at least about 6% in the alloy. Preferably it is contained in about 7% or more. Manganese also increases the limit of nitrogen solubility of solid solutions to help preserve nitrogen in the alloy. If the amount of manganese is excessive, harmful ferrite is formed in the alloy, so that the content of manganese in the alloy is about 12% or less, more preferably about 10% or less, preferably 9% or less.

In the austenitic stainless steel alloy according to the present invention there may be a small amount of silicon preserved from the deoxygenation additives upon melting. Since silicon is an element that forms ferrite in this alloy, the content of silicon is limited to about 1.0% or less, more preferably about 0.75% or less, preferably about 0.50% or less.

As an alternative, up to approximately 1.0% tungsten may be included to improve the corrosion resistance of the alloy to chlorides. However, tungsten also forms ferrite in the alloy, and if the amount of tungsten is excessive, the hot workability of the alloy is lowered, so that the content of tungsten in the alloy is preferably limited to about 0.75% or less.

And, cobalt may be present in minor amounts in the alloy up to approximately 1.0% or 0.75%. If so, there are advantages similar to those of nickel and copper.

If it is necessary to improve the machinability of the alloy, small amounts of phosphorus may be present in the alloy by up to approximately 0.2% or 0.1% and / or up to approximately 0.1% or 0.05% sulfur. However, since phosphorus and sulfur adversely affect the corrosion resistance and formability of the alloy, it is desirable to limit their amount. Therefore, in order to obtain good moldability, it is preferable to limit the content of phosphorus to about 0.05% or less and the content of sulfur to about 0.025% or less. In order to optimize formability, the content of sulfur in the alloy is limited to about 0.01% or less, preferably about 0.005% or less.

The balance of the alloy of the present invention is generally made of iron, with the exception of the common impurities found in commercially available steels of grades used for similar purposes.

No special technique is required for melting, casting or processing the alloy of the present invention. Although argon-oxygen decarburization after arc melting is the preferred melting and purification method of the alloy of the invention, other methods such as vacuum melting may be employed. The alloy of the present invention may also be produced by powder metallurgy, such as powder injection molding and metal injection molding. And a continuous casting method can also be used.

The alloy of the present invention can be molded into various shapes to suit a wide range of applications. Preferred product forms include billets, bars, rods, wires, strips, plates and sheets, and the like. The alloy of the present invention has molding properties that make it easy to manufacture cold formed products such as cold drawn wires or rods. In addition, useful products can be easily manufactured by cold bending or flattening wires or rods made of the alloy of the present invention.

Example

In order to illustrate the uniquely combined properties of the alloy of the present invention, Examples 1 and 2 having compositions shown in weight percent in Table 1 were prepared and tested. For comparison, Heat A-L having a composition outside the scope of the present invention was also prepared and tested. Their weight percent compositions are also listed in Table 1 below.

Example 1 Example 2 Ht. A Ht. B Ht. C Ht. D Ht. E Ht. F Ht. G Ht. H Ht. I Ht. J Ht. K Ht. L C .092 .099 .087 .091 .089 .097 .096 .050 .079 .091 .079 .095 .095 .020 Mn 7.83 7.84 7.94 7.82 7.78 7.81 7.78 7.90 6.90 7.82 6.97 8.03 8.02 1.70 Si .66 .34 .66 .63 .62 .63 .63 .40 .35 .63 .36 .66 .66 .56 P .073 .017 .019 .034 .034 .033 .034 .022 .028 .034 .029 .034 .034 .027 S 0.02 .004 .002 .003 .002 .002 .002 .003 .005 .002 .005 .003 .034 .027 Cr 16.19 16.70 16.06 16.02 15.95 16.00 15.97 17.02 17.08 15.96 16.19 16.50 16.52 18.37 Ni 2.32 2.76 2.20 2.28 2.29 2.28 2.28 2.98 2.42 4.26 2.45 2.35 2.35 8.18 Mo .70 .75 .27 .25 .25 .25 .25 <.01 .01 .25 .75 .26 .26 .68 Cu 2.04 2.98 2.98 2.01 2.98 2.01 3.01 2.64 2.38 3.06 2.40 .22 1.01 .39 Co .69 .10 .36 W .66 .09 N .20 .16 .17 .21 .20 .12 .11 .14 .111 .11 .105 .23 .22 .081 Fe Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance C + N .292 .259 .257 .301 .289 .217 .206 .19 .19 .201 .184 .325 .315 .101 Cr + Mo 16.89 17.45 16.33 16.27 16.2 16.25 16.22 17.02 17.09 16.21 16.94 16.76 16.78 19.05 PRE 21.30 21.26 19.16 16.57 19.38 18.39 18.36 18.84 18.55 18.22 19.85 20.35 20.24 21.70

Heat A, F and G represent the alloys described in US Pat. No. 5,268,510. Heat B-E and H-K are similar to Heat A, F and G, but differ in the content of carbon, copper, nickel, molybdenum and / or nitrogen. Heat L represents commercial grade Type 304 stainless steel.

Example 1 and Heat B to K were induction melted under the shield of argon gas and cast into 2 3/4 inch square ingots weighing 17 pounds. The ingot was forged from 1 inch square bar at 2200 ° F. From this bar a 4 "long member was machined and milled to 7/8" x 1 ". All of these members were hot rolled into strips approximately 0.200" thick at 2200 ° F. The strip was annealed at 1950 ° F. for 30 minutes, water cooled and washed. The test piece for the crack erosion test was prepared from this annealed material.

Heat A was prepared from continuous cast product sized heat cast into 5 "square billets. Cut 8" length pieces from this billet, press forged to 2 1/2 "at 2200 ° F, and reheat. After it was forged straight 1-1 / 16 "square. A 4 "long member was cut from this 1-1 / 16" bar, milled to 7/8 "x 1-1 / 16", and hot rolled into a 0.250 "thick strip. After annealed for 1 hour, water cooled, washed and cold rolled into 0.140 "thick strips. The material was annealed at 1950 ° F. for 15 minutes and then water cooled. From this annealed material, test pieces for crack erosion tests were prepared.

Example 2 was induction melted under shielding of argon gas and cast into a 7 1/2 inch square ingot. The ingots were press forged into 5 inch square billets at 2200 ° F. Sections were cut from one end of the billet and hot rolled into 0.200 "thick strips. The strips were annealed at 1950 ° F for 45 minutes, water cooled and pickled. After pickling, the annealed strip material was 0.140" thick. It was cold rolled, annealed again at 1950 ° F. for 15 minutes and then water cooled. Heat L was obtained from a product heat of Type 304 stainless steel as a 4 "1" round member, and treated with an annealed strip in a manner similar to Example 2. Test pieces for crack erosion tests were prepared from this 0.140 "annealed strip material.

Double specimens of Heat AC, EI and L for comparison with Examples 1 and 2 were tested in a 5% FeCl ₃ and 1% NaNO ₃ solution at 10 ° C. for 24 hours. If no pitting attack was observed during the first 24 hours, the specimens were ground and tested again at a temperature of 5 ° C. above the previous test temperature. In this way the test was continued until a pointed attack was observed on the specimen. Critical corrosion temperature (CCT) is the highest temperature at which the specimen does not corrode.

The results of the critical corrosion temperature test for each of the double test pieces of Heat for comparison with each example of Table 1 are shown in Table 2 below. The results show the critical corrosion temperature in degrees Celsius for each test piece. In addition, the contents of Cr, Mo, and N of the heat and the equivalent number of resistance to pitting (pitting resistance equivalence number, PRE) for comparison with each Example are also described in Table 2.

% Cr % Mo % N PRE CCT Example 1 16.19 0.70 0.20 21.30 15,15 Example 2 16.70 0.75 0.16 21.26 15,20 Ht. A 16.06 0.27 0.17 19.16 *, * Ht. B 16.02 0.25 0.21 19.57 *, * Ht. C 15.95 0.25 0.20 19.38 *, * Ht. E 15.97 0.25 0.12 18.36 *, * Ht. F 17.02 <0.01 0.14 18.84 *, * Ht. G 17.08 0.01 0.11 18.55 *, * Ht. H 15.96 0.26 0.11 18.22 *, * Ht. I 16.06 0.74 0.10 19.85 *, 10 Ht. L 18.37 0.68 0.08 21.70 20,20

* Viscosity occurs below initial test temperature (10 ℃)

The data presented in Table 2 show that Examples 1 and 2 of the alloys according to the invention have better resistance to chloride induced corrosion than Heat (Heat A-C and E-I) for all comparisons. Type 304 stainless steel (Heat L) had slightly better corrosion resistance to chlorides than Examples 1 and 2.

A salt spray test was conducted on a limited number of wire test specimens of Example 2 and Heat A. A 0.264 "diameter specimen was obtained from a product material round hot rolled to 0.306", annealed and washed and then cold drawn to 0.264 ". The wire test piece was exposed to 5% NaCl solution at 95 ° F for 466 hours. After the end of the test period, the specimens of Heat A rusted up to 20% of the surface area, but hardly rusted on the specimens of Example 1. These salt spray test data also show excellent corrosion resistance to chlorides of the alloys according to the invention. It is evidence.

Table 3 below shows the results of work hardening studies conducted on the annealed strips of some embodiments of Table 1, percent cold reduction per pass (% CR), and Rockwell hardness measurements after each cold rolling. Is described. In each case, the values in parentheses are given on the Rockwell B scale (HRB) and the remaining values on the Rockwell C scale (HRC). The maximum value of the HRB hardness value is 100, which corresponds to HRC 22. The content of Cu, Mo, and N in each Example and Heat was also described for convenience of comparison.

% C.R. Example 1 (2% Cu, 70% Mo, 2% N) Ht.J (0.2% Cu, .25% Mo, .2% N) Ht.K (1% Cu, .25% Mo, .2% N) Ht.B (2% Cu, .25% Mo, 2% N) Ht.C (3% Cu, .25% Mo, .1% N) Ht.E (3% Cu, .25% Mo, .1% N) Ht.L (Type304) 0 (91.5) (95) (91) (88) (87) (80) - 5 (93) (98) (97) (93) (93) (90) (91) 10 (98.5) 28 23.5 (98.5) (98) (96.5) (99) 20 28.5 34.5 31.5 29 27.5 25.5 26.5 30 24.5 39 27 35 33 31 31.5 40 37.5 43 41 37.5 35.5 34 35 50 40 45.5 42.5 40 38 37 37

The data in Table 3 show that the alloys according to the invention have very good resistance to work hardening upon cold deformation. The data also show that copper contributes to lowering the work hardening rate of Cr-Mn-Ni austenitic stainless steel containing molybdenum. Heat C and E are very similar to work hardening rate for Type 304 (Heat L) containing 8% nickel. However, comparing Heat C and E, it was found that even with 3% copper, increasing nitrogen would adversely affect the work hardening rate of Cr-Mn-Ni austenitic stainless steel containing copper and nitrogen. .

The terms and expressions used herein are for the purpose of description and not of limitation. The use of such terms and expressions is not intended to exclude the equivalents of the elements, the features illustrated and described, or any part thereof. However, it should be understood that various modifications are possible within the scope of the present invention.

Claims (20)

It has a unique combination of corrosion resistance, formability, and resistance to work hardening, and is based on the following elements in weight percent: weight% C 0.025-0.15 Mn 4-12 Si max 1.0 P max 0.2 S max 0.1 Cr 15.5-17.5 Ni 1-4 Mo 0.25-1.5 Cu 1.5-4 W max 1.0 Co max 1.0 N 0.05-0.30 The main components of the remainder are iron and ordinary impurities, the sum of carbon and nitrogen is approximately 0.19% or more, the sum of chromium and molybdenum is less than approximately 17.75%, and% Cr + 3.3 (% Mo ) Austenitic stainless steel alloy with a relationship of + 13 (% N)> 20.5. The austenitic stainless steel alloy of claim 1, containing at least about 6% manganese. The austenitic stainless steel alloy of claim 1, containing at least about 1.5% nickel. The austenitic stainless steel alloy of claim 1, containing at least about 0.5% molybdenum. The austenitic stainless steel alloy of claim 1, containing at least about 16.0% chromium. The austenitic stainless steel alloy of claim 1, wherein the austenitic stainless steel alloy contains less than 9 volume percent ferrite in the cast state. The austenitic stainless steel alloy of claim 1, wherein the austenitic stainless steel alloy contains up to approximately 8.5 volume percent ferrite in the cast state. The austenitic stainless steel alloy of claim 1, wherein a relationship of% Cr + 3.3 (% Mo) + 13 (% N)> 21.0 is established. The austenitic stainless steel alloy of claim 1, wherein the sum of the amount of chromium and the amount of molybdenum is about 17.5% or less. It has a unique combination of corrosion resistance, formability, and resistance to work hardening, and is based on the following elements in weight percent: weight% C 0.05-0.12 Mn 6-10 Si up to 0.75 P max 0.1 S max 0.05 Cr 16.0-17.0 Ni 1.5-3.5 Mo 0.5-1.2 Cu 2.0-3.5 W max 1.0 Co max 1.0 N 0.10-0.25 The main components of the remainder are iron and ordinary impurities, the sum of carbon and nitrogen is approximately 0.19% or more, the sum of chromium and molybdenum is less than approximately 17.75%, and% Cr + 3.3 (% Mo ) Austenitic stainless steel alloy, with a relationship of + 13 (% N)> 20.5. The austenitic stainless steel alloy of claim 10, containing at least about 7% manganese. The austenitic stainless steel alloy of claim 10, containing at least about 2.0% nickel. The austenitic stainless steel alloy of claim 10, wherein the austenitic stainless steel alloy contains at least about 0.7% molybdenum. The austenitic stainless steel alloy of claim 10, containing at least about 16.25% chromium. The austenitic stainless steel alloy of claim 10, containing less than 9 volume percent ferrite in the cast state. The austenitic stainless steel alloy of claim 10, containing up to about 8.5 volume percent ferrite in the cast state. The austenitic stainless steel alloy of claim 1, wherein a relationship of% Cr + 3.3 (% Mo) + 13 (% N)> 21.0 is established. The austenitic stainless steel alloy according to claim 10, wherein the sum of the amount of chromium and the amount of molybdenum is about 17.5% or less. It has a unique combination of corrosion resistance, formability, and resistance to work hardening, and is based on the following elements in weight percent: weight% C 0.08-0.11 Mn 7-9 Si max. 0.50 P max 0.05 S max 0.025 Cr 16.25-16.75 Ni 2.0-3.0 Mo 0.7-1.0 Cu 2.5-3.0 W max 0.75 Co max 0.75 N 0.12-0.20 The main components of the balance are iron and ordinary impurities, and the sum of the amount of chromium and molybdenum is approximately 17.75% or less, and a relationship of% Cr + 3.3 (% Mo) + 13 (% N)> 21.0 is established. Austenitic stainless steel alloy. 20. The austenitic stainless steel alloy of claim 19, wherein the sum of the amount of carbon and the amount of nitrogen is at least about 0.25%.