FR2473069A1 - FERRITIC STAINLESS STEEL - Google Patents

Abstract

STEEL ACCORDING TO THE INVENTION CONTAINS UP TO 0.08 BY WEIGHT OF CARBON, UP TO 0.06 BY WEIGHT OF NITROGEN, FROM 25.00 TO 35.00 BY WEIGHT OF CHROME, FROM 3.60 TO 5 , 60 BY WEIGHT OF MOLYBDENE, UP TO 2.00 BY WEIGHT OF MANGANESE, UP TO 2.00 BY WEIGHT OF NICKEL, UP TO 2.00 BY WEIGHT OF SILICON, UP TO 0.5 BY WEIGHT OF ALUMINUM, UP TO 2.00 BY WEIGHT OF ELEMENTS OF THE TITANIUM, ZIRCONIUM AND COLOMBIUM GROUP, THE COMPLEMENT BEING ESSENTIALLY IRON, THE CONTENTS OF SAID TITANIUM, ZIRCONIUM AND COLOMBIUM MEET THE FOLLOWING EQUATION: TI6 ZR7 CB8 C NET THE SUM OF THE SAID CARBON AND NITROGEN BEING GREATER THAN 0.0275 BY WEIGHT.

Description

The present invention relates to ferritic stainless steel. Licences

  US 3,932,174 and 3,929,473 describe ferritic stainless acids having superior resistance to crack and intergranular corrosion. The steels described in these patents contain 29% chromium and 4% molybdenum. They also have a maximum carbon + nitrogen content of 250 parts per million. Carbon and nitrogen are limited because the corrosion resistance of

steels is altered when their grades increase.

  The condition of low carbon and nitrogen contents

  for the alloys of patents Nos. 3,932,174 and 3,929,473 has the disadvantage

negate the need for more expensive fusion techniques, such as

than vacuum induction melting.

The present invention relates to an alloy having properties comparable to those of patents Nos. 3,929,174 and 3,929,473,

  but which does not require the expensive fusion techniques mentioned-

  born above. The alloy of the invention can, for example, be melted

  and refined using argon-oxy- decarburization processes

gene (AOD).

The alloy of the invention contains up to 2.00% by weight of elements from the group of titanium, zirconium and colombium, according to the following equation:% / Ti / 6 + 7 / Zr / 7%% Cb / 8>,% C +% N

and a carbon + nitrogen content of more than 275 parts per million.

It is characterized by resistance to corrosion by cracks and

superior intergranular corrosion, good resilience

dage and a satisfactory resistance, both before and after welding.

  For the reasons indicated above, the alloy according to the invention is clearly distinguished from those of the patents of United States of America Nos. 3,932,174 and 3,929,473. It is also distinguished from two other alloys, that of the patent no 3 957 544 and

  that of patent No. 4,119,765. These two alloys have maximum contents

  molybdenum males lower than that indicated for the invention.

  Another reference of interest is the work by Remus A. Lula entitled "Ferritic Stainless Steel Corrosion Resistance and Economy", published in the July 1976 edition of Metal Progress, pages 24-29. It does not describe the ferritic stainless steel according to the invention. The invention therefore relates to a ferritic stainless steel. The ferritic stainless steel of the invention is characterized by superior resistance before and after welding, superior resistance to cracks and intergranular corrosion and good weldability. It essentially contains: up to 0.08% by weight of carbon, up to 0.06% by weight of nitrogen, from 25.00 to 35.00% by weight of chromium, from 3.60 to 5, 60% by weight of molybdenum, up to 2.00% by weight of manganese, up to 2.00% by weight of nickel, up to 2.00% by weight of silicon, up to 0.5% by weight of aluminum, up to 2.00% by weight of elements chosen from titanium, zirconium and colombium, the balance being essentially iron. The carbon + nitrogen sum is greater than 0.0275%. The titanium, zirconium and colombium contents satisfy the following equation:% .Ti / 6 + 7.Zr / 7 +% Cb / 8> 7 /% C + 7N

The amounts of carbon and nitrogen are ordinary-

  at least 0.005 and 0.010% 7. by weight, respectively, the sum being greater than 0.0300%. The amounts of chromium and molybdenum are preferably from 28.50 to 30.50% by weight and 3.75 to 4.75% by weight respectively. The amounts of manganese and silicon are usually each less than 1.00% by weight. The aluminum which may be present for its deoxidizing effect is usually

  in an amount of less than 0.1% by weight.

  Titanium, columbium and / or zirconium are added to improve resistance to crack corrosion and corrosion

  intergranular of the alloy, which is in a sense a ver-

  high carbon + nitrogen content of United States patents 3,932,174 and 3,929,473. Applicants have determined that stabilizers can be added to the high carbon and / or nitrogen versions of the United States patent. United States of America No. 3,929,473 without destroying resistance and / or ability to weld

  of the alloy. Although it is preferred to add at least 0.15% of ti-

  tane, insofar as the mere presence of colombium can be harmful

  detrimental to the suitability of the alloy for welding,. according to the invention,

  tion the necessary amount of stabilizer in the form of either titanium or columbium. Colombium has a beneficial effect, compared to titanium, that of colombium. Colombium has a beneficial effect, compared to titanium, on the resistance of the alloy. According to a particular embodiment of the invention, the alloy contains at least 0.15% of columbium and at least 0.15% of titanium. The titaniums, columbium and zirconium are preferably present in amounts of up to 1.00%, according to the following equation:% Ti / 6 +% Zr / 7 +% .Cb / 8 - 1.0 to 4, 0 (% C +) The ferritic stainless steel according to the invention is particularly suitable for use as a welded article having a thickness of not more than 1,778 mm (ordinarily not more than 1,219 mmn) and, in particular, in the form of 'a tube

welded condenser from 0.660 to 0.939 mm.

The following examples illustrate the invention without

however limit its scope.

  Ingots cast from 15 heaters are heated to 1,121 ° C (heaters A to 0), hot rolled to 3,175 mm, annealed at temperatures from 1,066 to 1,121 ° C, cold rolled to about 1 , 57 to 1.65 mm and annealing at temperatures from 1 066 to 1 121 C. The resistance to corrosion by cracks of the samples is then evaluated. Other samples are welded ("TIG welded ') and their resistance to corrosion by cracks and to intergranular corrosion is then determined. The chemical composition

  heaters is shown in Table I below.

Other data are shown in Table II below. Note that the heaters A and B are not part of the invention. They do not satisfy the following equation:% Ti / 6 + '/.Zr/7 +% Cb / 8 y,% C +%' N The resistance to corrosion corrosion is evaluated by immersing polished test pieces on a surface 2.54 x 5.08 cm

in a 10% ferric chloride solution for 72 h. We do

kills the test at temperatures of 35 & 50 C. Cracks are created

on edges and surfaces using polytetra-

  fluoroethylene forwards and backwards, held in position by pairs of rubber bands stretched at 320C towards each other in the longitudinal and transverse directions. The test is described under the name G 48-76 of the American Society for Testing and Materials. The results obtained are shown in the table

III below.

  It can be seen from Table III that the resistance to crack corrosion of the heaters C to G and I to 0 is greater than that of the heaters A and B. The loss of base metal of the heater B reaches 0, 8519 g. The loss of welded metal from heaters A and B reached

0.4195 and 0.5783 g, respectively.

  Typically, the heaters A and B are not part of the invention. On the other hand, the heaters C to G

and I to 0 are in accordance with the invention.

  The resistance & the intergranular corrosion are evaluated.

glass by immersing polished test specimens on a surface of

2.54 x 5.08 cm in a boiling solution of cupric sulfate-

  50% sulfuric acid for 120 h. The usual criteria for this test are a corrosion rate of 60.96 P / year (5.08 / p / month) and a satisfactory microscopic examination. This test is recommended for

  ferritic stainless steels & high stable chromium content

  read. The results of the evaluation appear in the

Table IV below.

It can be seen from Table IV that only the heater B does not pass the test. The heater B has a corrosion rate of 3.58 mm per year. As indicated above, it is one of the two heaters not part of the invention, the other heater being heater A. It is also more outside the scope of the invention

that the heater A in that it has a lower Ti / C + N ratio.

  Resistance is assessed by determining the temperature

Transition ture using Charpy transverse V notch test pieces of lower size for the hot rolled and annealed product (test pieces of 3.17 x 10.00 mm) and for the welded product (test pieces of 1.57 to 1.65 x 10.00 mm). The transition temperature

  is based on an aspect of the break 50% ductile - 50% brittle.

  The transitbn temperatures are indicated in table V below.

after. The transition temperatures indicate that the steel of the invention can be cold rolled, shaped and welded, although a

  some preheating may sometimes be desirable. The tests

Colombian vettes have lower transition temperatures than titanium specimens. Test pieces containing both titanium and columbium have transition temperatures included

  between those of the colombium test tubes and the ti-

  tane. It is understood that the invention is not limited

  to the preferred embodiments described above by way of illustration.

  tration and that the skilled person can make various modifications

and various changes without however departing from the framework and the

took from the invention.

TA B LEAU I

Composition (% by weight) Ti 0.31 0.34 0.26 0.40 0.61 0.66 0.20 0.50 0.20 0.09 O, 19 0.20 0.31 Cb Fe - complement - complement - complement complement - complement - complement 0.38 complement 0.53 complement 0.39 complement - complement 0.32 complement 0.45 complement 0.41 complement 0.42 complement 0.44 complement Heater ABCDEFGHIJKLMN o C 0.042 o 0.064 0.020 0.037 0.039 0.064 0.015 0.030 0.029 0.030 0.030 0.031 0.034 0.035 0.032 N 0.022 0.022 0.021

O, 019

0.014 0.013 o 015 0.016

O, 019

  0.025 0.026 0.025 0.027 0.026 0.024 Cr 29.09 28.98 29.08 29.05 28.88 28.91 29.10 29.10 28.92 28.96 29.05 28.96 28.95 28.75 29 , 52 MB 4.00 4.01 4.00 4.02 4.02 4.01 4.02 4.04 4.04 4.20 4.18 4.06 4.20 4.20 4.10 Mn 0 , 24 0, 24 0.24 0.24 0.24 0.24 0.35 0.36 0.35 0.34 0.34 0.36 0.43 0.40 0.37 Ni 0.31 0, 29 0.29 0.29 0.30 0.29 0.41 0.45 0.54 0.45 0.46 0.45 0.46 0.47 0.51 Si 0.34 0.34 0.33 0.34 0.33 0.32 0.38 0.40 0.39 0.36 0.37 0.29 0.37 o 0.45 0.28 A1 0.039

0.050

0.023 0.053 0.055 0.055

O, 010

0.014 0.016 0.029 O, 029

0.027

0.040

0.025 0.030 O- NI o oN o-

T A B L E A U II

% Ti / 6 + 7Zr / 7 +% Cb / 8 0.052 0.057 0.043 0.067 0.102

O, 110

0.048 0.066 0.082 0.083 0.073 0.071 0.083 0.086 0.107 DC test Heater A B Cd D G H I J K L M N o

TABLE III

> corrosion cracking in 10% ferric chloride Weight loss (g) Base metal, at 50 C Welded, & 35 C Welded, at a 50 0C

00 0.0 0.4195

0.8519 0.0198 0.5783

0 0>, 0001 0.0004

00o - 0.0 -

0 30 0.0 Qo0

0.0 0.0001 0.0

O, O O. 0001 'O, O

- Oo0 0.0

0.0003

0.0 0.0 0.0 0.0

0.0013

  Heats A B C D E - F G H I J K L M N o

% C +% N

0.064 0.086 0.041 0.056 0.053 0 077 0.030 0.046 0.048 0.055 0.056 0.056 0.061 0.061 0.056

TABLE IV

  Copper sulphate corrosion test - sulfuric acid at 50.

in the welded state Speed mmm / year 0.208 3.58 0.173 0.252 0.141 0.279 0.146 0.159 0.167 0.141 0.133 0.146 0.134 0.161 no attack

of grain.

corrosion / / month 17.37

299.36

  14.42 21.03 11.83 23.21 12.19 13.30 13.99 11.83 21.09 12.24 11.17 13.43 intergranular or microscopic examination, welded (magnification 30 x) PA *. PA PA PA PA PA PA PA decrease in size

TABLE V

  Transition temperature (C) Heater Welded specimen Hot rolled and annealed specimen

A -3.89 (1) 74 (3)

B 15.6 (1) 85 (3)

C 26.7 (1) 68 (3)

D 46 (1) 85 (3)

E 118.5 (1) 90 (3)

F 104 (1) 88 (3)

G - 37.2 (2) 35 (4)

H - 49 (4)

I 35 (2) 71 (4)

J 43 (2) 54 (4)

K 15.6 (2) 49 (4)

L 32.2 (2) 43 (4)

M 40 (2) 57 (4)

N 68 (2) 60 (4)

0 54 (2) 99 (4)

(1) ribbon annealed before welding at 1121 C - air cooled (2) ribbon annealed before welding at 1066 C - water quenched (3) annealed at 1121 C water quenched; transverse test (4) annealed at 1066 C - water quenched; transverse test Heating A B C D E F G H I J K L M N o * PA:

Claims (8)

  RE v ENDICATIONS 1. Ferritic stainless steel, characterized in that it contains up to 0.08% by weight of carbon, Up to 0.06% by weight of nitrogen, from 25.00 to 35.00% by weight of chromium, from 3.60 to 5.60% by weight of molybdenum, up to 2.00% by weight of manganese, up to 2.00% by weight of nickel, up to 2.00% by weight of silicon, up to 0.5% by weight of aluminum, up to 2.00% by weight of elements from the group of titanium, zirconium and colombium, the balance being essentially iron, the contents in said titanium, zirconium and colombium satisfying the following equation:% Ti / 6 +% Zr / 7 +% VCb / 8 /% C + N and the sum of said carbon and nitrogen being greater than 0.0275% by weight. 2. Ferritic stainless steel according to claim 1, characterized in that it contains at least 0.005% by weight of carbon and at least 0.010% by weight of nitrogen, the sum carbon + nitrogen being greater than 0.0300% by weight. 3. Ferritic stainless steel according to claim 1,   characterized in that it contains from 28.50 to 30.50% by weight of chromium. 4. Ferritic stainless steel according to claim 1,   characterized in that it contains from 3.75 to 4.75% by weight of molyb-   not. 5. Ferritic stainless steel according to claim 1, characterized in that it contains up to 1.00% by weight of elements from the group of titanium, zirconium and colombium, according to the following equation:% Ti / 6 +% Zr / 7 +% Cb / 8 = 1.0 to 4.0 (% OC + 7N) 6. Ferritic stainless steel according to claim 1,   characterized in that it contains at least 0.15% by weight of titanium.   7. Ferritic stainless steel according to claim 7,   characterized in that it contains at least 0.15% by weight of colombium.   8. Ferritic stainless steel according to claim 1, characterized in that it contains at least 0.005% 7. by weight of carbon, at least 0.010% by weight of nitrogen, from 28.50 to 30.50% by weight of chromium, from 3.75 to 4.75% by weight of molybdenum, between 3.00 and 4, 50% by weight of nickel and up to 1.00% by weight of elements from the titanium, zirconium and colombium group according to the following equation:% / Ti / 6 + 7 / r / 7 + Cb / 8 - 10 to 4.0 (% C + 7N), the carbon + nitrogen sum being greater than 0.0300% by weight.