CS216241B2 - Ferritic non-corroding steel - Google Patents

Abstract

A ferritic stainless steel characterized by superior toughness both prior to and after welding, and by superior crevice and integranular corrosion resistance consists of, by weight, up to 0.08% carbon, up to 0.06% nitrogen, from 25.000 to 35.00% chromium, from 3.60 to 5.60% molybdenum, up to 2.00% manganese, from 2.00 to 5.00% nickel, up to 2.00% silicon, up to 0.5% aluminum, up to 2.00% titanium, zirconium and/or niobium, balance essentially iron. The sum of carbon plus nitrogen is in excess of 0.0275%. Titanium, zirconium and niobium are in accordance with the following equation: %Ti/6+%Zr/7+%Nb/8>/=(%C+%N)

Description

The invention relates to ferritic stainless steel, characterized by a higher toughness both before and after welding and a higher resistance to cracking and intergranular corrosion. The steel comprises essentially in weight percent from 0.005 to 0.08 carbon, from 0.010 to 0.06 nitrogen, from 25 to 35 chromium, from 3.6 to 5.6 molybdenum, from 0.001 to 2 manganese, between 2 to 5 nickel, from 0.001 to 2 silicon, from 0.001 to 0.5 aluminum, from 0.001 to 2 element groups consisting of titanium, zirconium and niobium; the equilibrium factor is essentially iron. The total of carbon and nitrogen is more than 0.0275%. Titanium, zirconium, niobium correspond to the equation: percent titanium —Ti / 6 -] - percent zirconium —Zr / 7 ψ percent niobium —N'b / 8 percent carbon —C — j- percent nitrogen —N.

218241

The invention relates to ferritic stainless steels ^. increased corrosion resistance in cracks - and intercrystalline corrosion.

This steel differs from known steels in that it has a maximum of 2 percent of the weight of the elements of the titanium, zirconium and niobium groups according to the following equation: percent - titanium - Ti / 6 - i - percent zirconium - Zr / 7 - - Nb / 8> proc% carbon - C + nitrogen - N, with a carbon plus nitrogen content greater than 275 ppm. Owing to the higher carbon and nitrogen content, the steel according to the invention is fusible and refinable in less expensive processes than in the known steels.

The invention provides a steel having a higher toughness. In addition to the stabilizers of the group consisting of titanium, zirconium and niobium having a carbon and nitrogen content of more than 275 ppm, the steel according to the invention has 2 to 5 percent nickel, a range of 3 to 4.5 percent being particularly preferred. has a maximum of 2 percent, and typically less than 1 percent nickel. Nickel has been found to increase the toughness of the alloy of the present invention.

For the above reasons, the alloy of the invention is clearly distinguished from known alloys having a maximum molybdenum content lower than that of the invention.

SUMMARY OF THE INVENTION It is therefore an object of the invention to provide stainless steel.

The principle of the invention consists in that the ferritic stainless steel consists essentially of a weight percentage of from 0.005 to 0.08 carbon, from 0.010 to 0.06 nitrogen, from 25 to 35 chromium, from 3.6 to 5.6 molybdenum, from 0.001 to 2 manganese, from 2 to 5 nickel, from 0.001 to 2 silicon, from 0.001 to 0.5 aluminum, -from 0.001 to 2 elements of the group containing titanium, zirconium and niobium, the basic element being essentially iron and titanium, zirconium and niobium corresponds to the following equation: percent titanium - Ti / 6 -J- percent zirconium - Zr / 7 -f + percent niobium - Nb / 8 š percent coal k - C ψ percent nitrogen N, where the total carbon -a nitrogen is greater than 0.027 percent. It is a feature of the invention that the total of carbon and nitrogen is greater than -0.03 percent by weight, and that ferritic steel contains, by weight, from 0.001 to 1 percent by weight of elements of the group consisting of titanium, zirconium and niobium according to - Ti / 6 + percent zirconium - Zr / / 7 -J- percent niobium - Nb / 8 = 1 to 4 [percent carbon - C + percent nitrogen - N], from 0.15 percent to 2 percent titanium, 0.15 to 2 percent niobium, from 0.005 to 0.08 percent carbon, from 0.010 to 0.06 percent nitrogen, from 28.5 to 30.5 chromium, from 3.75 to 4.75 percent molybdenum, from 3 up to 4.5 percent nickel and from 0.001 to 1 percent elements of the group consisting of titanium, zirconium and niobium according to :; equation percent titanium - - Ti / 6 · + percent zirconium —Zr / 7 --- percent niobium Nb / 8 = ^: = 1 to 4 (percent - carbon + percent nitrogen], with total carbon and nitrogen greater than 0.03 percent.

The advantage of the ferritic stainless steel according to the invention lies in its higher toughness both before and after welding. Titanium, zirconium and niobium increase its corrosion resistance, - a considerable amount of nickel increases its toughness. Tough-weldable steel is particularly resistant to corrosion in a chloride-containing environment, as found in condenser conduits.

The following examples illustrate several variants of the invention.

Twenty-four test batches (batches A to X) were heated to 1121. And hot rolled into 3.175 mm strip, annealed at 1065 ° C or 1121 ° C, cold rolled to 1.575 mm strip and annealed at 1065 ° or 1121 ° C. The hot-rolled or cold-rolled samples were gradually evaluated for toughness. Other samples were welded in tungsten inert gas - and then evaluated for toughness.

The chemistry of the test batches is shown below in Table I.

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what ID CD LD LD CM CD O O O O 00 what LD © CD © ~ © ID CD b ^ r- ^ rH Q CD oo d oo oo oo d CD © © oo CD d © © oo CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM

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It should be noted that test batches A to F are outside the scope of the invention. They do not have a content by weight between 2 and 5 'percent nickel. The invention is dependent on a nickel content of more than 2%.

Further data on the chemistry of the test batches are given in Table II.

% Carbon test charge - C j percent titanium Ti / 6 -J- percent '+% nitrogen —N percent zirconium —Zr / 7 -J- percent niobium —Nb / 8

Table II

% Ti / 6 J% Zr / 7 +% Nb / 8 test charge

AND 0,055 0,083 0,073 (B) 0,056 C 0,056 0.071 D 0.CKJ.1 0,083 E 0,061 0,086 F 0,056 0,107 G 0,031 0.052 H 0,0,45 0.052 AND 0,047 0.100 J 0,045 0,046 TO 0.050 0,063 L 0.050 0,081 M 0,043 0,054 N 0,042 0,068 O 0,042 0,069 P 0,045 0,086 Q 0,042 0,054 R 0,043 0,080 WITH 0,048 0.056 T 0.040 0,068 AT 0,037 0,074 IN 0.040 0,084 W 0,043 0,055 X 0,046 0,080

The toughness was evaluated by a transient finding. temperature at which Charpy transverse notch samples were used - hot-rolled and annealed (samples 3.175 X 10.0 / 8 mm, for cold-rolled and annealed samples (samples 1.575-X 10.008 mm) as well as for welded material (samples 1,575 X 10,008 mm), for welded and annealed materials (samples

1.575 X 10.008 mm).

The transition temperatures for the hot-rolled and cold-rolled samples are given in Table III. Test batches A and L were annealed at 1065 ° C. Additional test batches were calcined at 1121 ° C.

Transition temperature ° C

Table III

Hot rolling and annealing Cold rolling and annealing Test batch Hardening with water Noc: 2 ° C Air cooling ° C Hardening with water Noc: 2 ° C Air cooling ° C

AND 54.4 148.9 1.7 46.1 В 48.9 126.7 - 28.9 18.3 C 43.3 110.0 - 12,2 10.0 D 57.2 160.0 + 4,4 29.4 E 60.0 160.0 - 12,2 29.4 F 28.9 98.9 + 4,4 32.2 G —37.2 —117.8 - 73.3 H —43.3 —115.0 - 73.3 AND -15.0 —117.8 - 67.8 J —84.4 —123.3 —112.2 К —56.7 —128.9 - 76.1 L —40.0 —115.0 - 90,0 M +10.0 37.8 - 90,0 N + 1.7 18.3 - 84.4 0 —15.0 12.8 - 87,2 P - 1,1 21.1 - 92.8 Q —37.2 15.6 —101.1 R - 9,4 15.6 —109.4 WITH —37.2 - 9,4 —120.6 T —31.7 - 9,4 —117.8 at —53.9 - 23,3 —115.0 in —56.7 - 31.7 -117.8 w —67.8 - 31.7 - 128.9 X —73.3 - 31.7 —142.8

The transition temperatures for welding and the welded and annealed samples are given in Table IV below. Test batches A to F were annealed at 1063 ° C prior to welding. Additional test batches were calcined at 1121 ° C. All test batches were quenched with water. After welding, the material is annealed at test batches A to F at 1065 ° C and at 1121 ° C for other test batches. All test batches were quenched after annealing with water.

Table IV

Test batch

Transition temperature in ° C for welding ° C welded and annealed by _C

AND 43.3 - 1,1 В 15.6 + 1,7 C 32.2 + 4,4 D 40.6 - 3,9 E 68.3 54.4 + 4,4 F + io, o G —76.1 - 76.1 H —62.2 - 70.6 AND —42.8 - 70.6 J —78.9 -137.2 к —67.8 —103.9 L —51.1 - 84.4 M —51.1 - 53,9 N —17.8 - 40.0 O —28.9 - 65.0 P —23.3 - 59,4 Q —51.1 - 78,9 R -28.9 - 59,4 WITH —40.0 - 92.8 T —51.1 - 73.3 at —78.9 - 81.7 in —81.7 - 84.4 w —73.3 —106.7 X —95.6 —129.9

The benefits of the presence of nickel are clearly apparent from Tables III and IV. Test batches G and X have considerably lower transition temperatures and are therefore substantially tougher than test batches A to F. Only test batches G to F are within the scope of the invention, whereas test batches A to F are not the subject of the invention. Test batches G to X contain more than 2 percent nickel.

Examples of lower transition temperatures of test batches G to X are given below in Table V, which is a combination of Tables III and IV.

Table V

Transition temperature in ° C

Test Charges A — F Test Charges

G — X

Hot rolled and annealed (water hardened)

Hot rolled and annealed (air-cooled) material

Cold rolled and annealed material (water cooled)

Cold rolled and annealed M-aterial (air-cooled)

Material for welding

Welded and annealed material

43.3 to 98.9 ° C

98.9 to 160.0 ° C —28.9 to -J-4.4 ° C

10.0 to 46.1 ° C

15.6 to 68.3 ° C — 3.9 to +10.0 ° C —84.4 to +10.0 ° C —31.7 to + 37.8 ° C —142.8 to —84, 4 ° C —112.2 to —67.8 ° C —95.6 to —17.8 ° C —137.2 to —40.0 ° C

The table below shows, for each example, that the maximum transition temperature for test batches G to X is lower than the minimum transition temperature for test batches A to F. The figures clearly show that test batches G to X are tougher than test batches A to F. F.

Additional samples of the test batches G to X were evaluated for crack and intergranular corrosion resistance. These samples were prepared as before.

Crack corrosion resistance was evaluated by immersing 25.4 mm by 30.8 mm lower surface of the samples in 10 percent ferric chloride solution for 72 hours. The test was performed at 50 ° C. The cracks were formed by using polytetrafluoropolyethylene blocks at the front and rear, held in the desired relative position by pairs of rubber bands joined at 33 ° C in the longitudinal and transverse directions.

Table VI

Crack corrosion test with 10% ferric chloride

Weight loss (in grams)

Test batch Cold and annealed when welding welded and annealed at 1121 ° C G - 0.0001 0.0008 H - 0.1588 0.0005 AND - 0.0 0.0004 J - 0.0 0.0001 К -. 0.0 0.0015 L - 0.0001 0.0001 M 0.0 0.0004 0.0003 N 0,000'9 0,0027 0.0009 O 0.0 0.0007 0.0001 P 0.0001 0.0004 0.0004 Q 0.0 0.0005 0.0039 R 0.0007 0.0032 0.0068 WITH 0.0056 0.0007 0.0 T 0.0 0.0001 0.0056 at 0.0002 0.0001 0.0 in 0.0001 0.0078 0.0002 w 0.0001 0.0 0,0063 X 0.0 0.0003 0.0060

From Table VI, it can be seen that the crack corrosion resistance of the test batches G to X is excellent. Indeed, the alloy of the invention is characterized by high resistance to crack corrosion.

The intergranular corrosion resistance was evaluated by immersing 25.4 mm X 50.8 millimeters of the bottom surface of the sample in boiling copper sulfate - 50 percent sulfuric acid solution for 120 hours.

The usual measure of loss in this test is corrosion of 0.6 mm per year, i.e. 0.05 mm per month, and thorough microscopic examination. This test is recommended for stabilized high chrome ferritic stainless steels.

The evaluation results are shown in Table VII. The samples were in the welding state and in the welding and annealing state.

Table VII

Corrosion test in cupric sulphate - 50% sulfuric acid Corrosion rate mm per month Microscopic test (at 30X magnification) Welding charge Welded Welded and annealed and annealed

G 0.01257 0.01608 ON** ON H 0.01641 0.01478 ON ON r 0.01290 0.01717 ΝΆ ON J 0.01105 0.01603 ON ON К 0.00935 0.01867 ON ON L 0.00960 0.01511 ON ON WITH 0.01273 0,01'580 ON ON T 0.01191 0.01265 ON ON no 0.01019 0.01003 ON ON in 0.01222 0.01232 ON ON w 0.01222 0.01283 ON ON X 0.01290 0.01384 ON ON

Annealing was performed at 11.21 ° C, water quenching "NA" in the table indicates that no intergranular corrosion or crystal release was detected.

Table VII shows that test batches G to L and S to X exhibit higher intergranular corrosion resistance. Each sample passed the test.

The new principles of the invention, as described above in connection with specific examples, allow for various other modifications and applications. Accordingly, the wording of the appended definition of the subject invention is not limited to the specific examples as described above.

Claims (6)

What is claimed is: 1. A ferritic stainless steel composition, characterized in that it consists essentially of a percentage by weight of from 0.005 to 0.08 carbon, from 0, 010 to 0.06 nitrogen, from 25 to 35 chromium, from -3.6 to 5.6 molybdenum, from 0.001 to 2 manganese, from 2 to 5 nickel, from 0.001 to 2 silicon, from 0.001 to 0.5 aluminum, from 0.001 to 2 elements of the titanium, zirconium and niobium group, the basic element being essentially the iron and the titanium, zirconium and niobium content by weight corresponds to the equation: percent titanium -Ti / 6 + percent zirconium -Zr / 7 + percent niobium -Nb / 8 & percent carbon-C + percent nitrogen, with total carbon - and -The nitrogen is greater than 0.0275 percent. 2. Ferritic stainless steel according to claim 1, characterized in that the total of carbon and nitrogen is greater than 0.03 percent by weight. 3. Ferritic stainless steel according to claim 1, characterized in that it contains, by weight, from 0.001 to 1 per cent of the elements of the group consisting of titanium, zirconium and niobium according to the equation niobium —Nb / 8 = 1 to 4, percent carbon —C + percent nitrogen N. 4. Ferritic stainless steel according to claim 1, characterized in that it contains from 0.15 to 2 percent of titanium by weight. 5. Ferritic stainless steel according to claim 4, characterized in that it contains from 0.15 to 2 percent niobium in a concentration by weight. 6. Ferritic. Stainless steel according to claim 1, characterized in that it has a concentration by weight of from 0.005 to 0.005. -0.08 percent carbon, from -0.10 to 0.06 percent nitrogen, -from 28.5 to 30.5 percent chromium, from 3.75 to 4.75 percent molybdenum, 3 to 4.5 percent nickel, and -from 0.001 to 1 per cent of the elements of the group consisting of titanium, zirconium and niobium according to the equation: the percentage of titanium —Ti / 6 — the percentage of zirconium — Zr / 7- — — the percentage of niobium —Nb / 8 = 1 to 4, the percentage of carbon j is the percentage of nitrogen, wherein the total of -a and nitrogen is greater than - 0.03%.