MXPA98005287A - Austenoferritic stainless steel with very low nickel content and presenting a considerable carrier by tracc - Google Patents

Abstract

Austenoferritic stainless steel with very low nickel content and showing considerable tensile elongation, characterized by the following composition by weight: carbon <0.04%; 0.4%

Description

STAINLESS STEEL AUSTENOFERRITIC WITH VERY LOW CONTENT - NICKEL AND PRESENTING A CONSIDERABLE EXTENSION BY TRACTION n ^ CRIPTION OF THE INVENTION Stainless steels are classified by large families according to their metallurgical structures, after a thermal treatment. Ferritic stainless steels, stenotic, austenitic and austenoferritic steels are known. This last family includes steels that are generally rich in chromium and nickel, that is to say they contain some contents in chromium and ñique! respectively, greater than 20% and greater than 4%. The structure of these steels, after a tratamifint? at a temperature comprised between 950QC and 1150 ° C it is constituted by ferrite and austenite in a propojrtion generally superior to 30% for one and the other of the two phases. These steels have many interesting practical features, in particular, they have in the acceding state, for example at 1.0509C, some mechanical characteristics, especially in the elastic limit, higher than the ferritic or austenitic stainless steels in the annealed state. Conversely, the ductility of these steels is of the same order of magnitude as that of ferritic steels and less than that of austenitic steels, REF. 27741 One of the advantages of austenoferritic steels refers to the welding properties. After a welding operation, the structure of these stainless steels, in a molten zone and in a zone affected by heat, remains largely polyphasic with ferrite and austenite, on the contrary that the austenitic steels, whose welding remains mainly austenitic. This results in high mechanical characteristics of the welds, whose characteristics are sought when the welded assemblies must resist the mechanical stresses due to the operation. Finally, certain austenoferritic steels with a finely divided a ^ s tenite may exhibit an elevated plasticity, called superplasticity, when performing hot slow conformations. These austoferric steels also have some disadvantages, such as, for example, their high price because of their composition with a high nickel content or also because of manufacturing difficulties, especially linked to their considerable crotam content, They are, for example, the formation of a fragile sigma phase or the decomposition of the mixtures in a ferrite rich in iron and a ferrite rich in chromium, with embrittlement of the steels during the cooling after a lamination in hot. Its ductility, measured by the elongation by traction at room temperature, does not exceed 35%, which makes it easy to employ by embossing, stamping or any other - procedure. Felling is also carried out within the sea with the use of steel at a temperature higher than -300 ° C, when the maintenance at temperature exceeds the duration of a few hours. The purpose of the invention is to make a steel - austenitic glass having a very small content in nickel and having the advantageous characteristics of the austenoferritic family, associated with improved general characteristics. The object of the invention is an austene-ferritic stainless steel having a very low nickel content and which has a considerable tensile elongation, characterized by the following composition by weight: Carbides 0.0% 0.4% < sackcloth < 1.2% 2% ¿. manganese < 4% 0.1% ^ nickel 1% 18% < chrome < 22% 0.05% copper 4% azüf / re 0.03% phosphorus < 0.1% 0.1% < nitrogen < , 3% molybdenum 3%, presented the steel a bifa-ucidad that comprises between 30 and 70% of auc.tenita, such that: Creq = Cr% + Mo% + 1,5 Si% ieq = Ni% + 0,33 Cu% + 0.5 Mn% + 30 C% + 30 N%, the Creq / Nieq ratio being between 2.3 and 2.75, the stability of the aust.enita of said steel being regulated by the IM index. , defined from the weight composition of • - steel by IM = 551 - 805 (C * •)% - 8,52 Si% - 8,57 Mn% - 12,51 Cr% -36 Ni% - 34, 5 Cu% - 14 Mol, with IM to be between 40 and 115. The other characteristics of the invention are that: - the composition satisfies the ratio: Creq / Nieq between 2.4 and 2.65. - the sulfur content is less than or equal to 0.0pf >;%. - the steel also comprises, in its composition by weight, from 0.010% to 0.030% aluminum. - the steel also comprises, in its composition by weight, from 0.0005% to 0.0020% calcium. - the steel also comprises, in its composition by weight, from 0.0005% to 0.0030% boron. - the carbon content is less than or equal to 0.03% - the nitrogen content is between 0.12% and - the chromium content is between 19% and 2%. - the silicon content is between 0.5% and 1%. - the copper content is less than 3%. - the phosphorus content is less than or equal to 0.04%. The description given below, completed by the A single attached figure, all of which being given as a non-limiting example, will make the invention well understood. The single figure presents a curve showing the dependence of the elongation characteristic with respect to the IM index. The invention concerns an austenoferritic steel having reduced contents in alloying elements, and especially a nickel content of less than 1% and a lower chromium content which 22% The small content in n_ quel is imposed, for economic and ecological reasons, allowing the reduction of the chromium content, on the one hand, - - assuring an easy processing of the steel and, on the other hand, evi- hot both when making the steel and during its use. The invention is the result of an investigation, as a result of which it has been found that a specific sector of composition allows, in the family of steel considered, the obtaining of a particular improvement in the elongation by -traction associated with a high elastic limit.

Steel can be produced in the form of molded or forged products, hot rolled or cold rolled sheets, or bars, tubes or wires. Different castings have been made, whose compositions are presented in the following table I: Composition in weight of steels: D C XS A A E F C G C i S tu i (S low) (S. B children) c 0 02. 0 023 0031 0 033 0 03 0.03 0.032 0.033 0.036 0.033 Yes 0 538 0 52J 0 485 1 055 1 06 1.10 0.J75 0.494 0.947 0.538 Mn) 718 3 747 3 786 4 073 3 89 3.99 3,847 3,825 5,018 3,758 Ni 0 0 * 7 0 809 0 81 1 0 817 0 824 0 821 0.327 0.839 0432 0.840 C r 18 9 19 89 20 71 21 19 20.2 19 01 19.86 18.96 19.86 Mo U 035 0 0J6 0 036 0 037 0 211 0.212 0.211 0.206 EYE 0.209 (or 0 044 0 392 0 391 0 395 0 4 0 402 1,023 0.384 3,048 0.333 0 35-37 ppm 17-19 ppm 33-37 ppm 37-38 ppm 32-32 26-28 ppm S 34 ppm 35 ppm 35 ppm 37 ppm * ppm 4 ppm 10 ppm 12 ppm ppm 10 ppm B 9 ppm 14 ppm P 0 017 0018 0017 0018 0017 0.017 0.018 0.016 0.016? l0 0 0 010 0 007 0.007 0.019 0.007 V 0 132 0 15 0 136 0 17 0 167 0 166 0.155 0.143 0.011 0.136 \ 0 091 0 094 0 097 0 103 - 0.072 0078 0.081 0.104 0.086 0.088 Table 2 below shows the characteristics of steels in the IM index sector and the ratio of chromium equivalent to nickel equivalent.

D C B A A E F C G C (S low) (S low) (S, B low) IM 144 81 78 35 38 51 68 78 12 85 Creq / Nieq 2.92 2.57 2.74 2.51 2.61 2.50 2.39 2.55 2.41 2.64 In a short processing range, the steel is subjected to a forging from the temperature of 1,200 ° C and then to a hot transformation from 1.2409 C to obtain, for example, a hot rolled strip with a thickness of 2.2 mm. -The band is treated at 1050dC and then tempered with water. In a so-called long range, after the short range, the hot-rolled strip can then be cold rolled and again treated at 1040 ° C for one minute, and then quenched with water. All steels presented are composed of ferrite and austenite with the exception of steel D, which also contains mar - tensite formed during the cooling of austenite. structure d * - steels are always free of carbides and nitrates. It is verified that, three steels, B, C and F, have, on the one hand, an elongation at break greater than or equal to 40% -when these are made with the long range, and on the other hand, greater elastic limits than 450 MPa as well as - loads at break greater than 700 MPa. In addition, the steel presents both a high elastic limit and a lengthening - particularly high. Using a stability index of austenite - such that: IM = 551 - 805 (C + N)% - 8.52 Si% - 8.57 Mp% - 12, 51 Cr% -36.02 N.% - 34 , 52 Cu% - 13.95 Mo%, it is verified, as shown in the single figure, that the elongation at break of these austenoferritic steels goes through a maximum when the IM index defined above, linked to the composition of the steel according to the invention, is comprised between 40 and 115, which defines a steel according to the invention that has an elongation of more than 35%. The characteristics of the sheet obtained according to the invention are grouped in Table 3, which shows the percentages of austenite for four steels in the different phases of transformation, raw of hot rolling, processed in short range and in long range. Table 3: Contents in austenite expressed in% Steel DCBA Raw hot lamination 37 42 33 35 Short range 41 49 39 40 Long range 42 52 41 43 These contents in austenite are included within the ranges of 30% to 70% Investigate in austenoferritic steels. The steels presented comprise respectively a Creq / Nieq ratio as recommended according to the invention. The following Table 4 presents the mechanical characteristics for the steels B and C according to the invention, subjected to the two preparation ranges, for the steels E and F according to the invention, subjected to the long preparation range, whose characteristics are compared with those of steels A and D that are outside the invention.

Table 4: Mechanical characteristics Limit at breakage Elongation IM Martensite Steel Elastic limit after tension Rp 0.2% (Pal R (MPa) A%% 1 4 D 804 32 Short range 406 31 Long range 433 855 24 81 C 757 46 Short range 476 81 7 43 27 Long range 501 78 B 668 34 Short range 450 14 40 Long range 471 7 51 E Short range Long range 484 737 36 68 F Short range Long range 492 819 44 35 A 36 Short range 496 718 Long range 520 773 33 It is found that the steels B, C and F, whose index IM is respectively 78, 81 and 68, that is to say between 40 and 115, have a particularly high elongation in relation to steels A and D , which are part of the invention Table 5 below shows the percentage of cold-hardening martensite formation under the effect of traction in steels subjected to hypertext-to 1.04 ° C. STEEL ABCD% of austenite 43 41 52 42 Elongation distributed 25 33 37 22% austenite after 43 36 25 9 traction Appearance of 27 31 martensite (%) Fraction of the auteite 0.12 0.52 0.74 transformed into martensite when traction is performed. For steels B and C respectively, they are transformed into martensite by traction by 12% and 52% respectively of the initial austenite, which gives them a good ductility; on the other hand, steel A does not have any - austenite transformation in martensite by the traction and steel D has a too high percentage of austenite transformation, of 74%, which gives it an insufficient ductility. Tables 6 and 7 present the characteristics of - hot extraction of different steels. The mechanical properties have been evaluated in a welded steel. The welding is done by forging from 1,200 Q C. The steel is then subjected to annealing at a temperature of 1.1 () 0 ° C for 30 min. The traction pro-betas used are specimens that have a -tronco or shaft with a circular section with a diameter of 8 mm and a length of 5 mm. These specimens are subjected to a pre-heating for 5 min at 1.2009C or 1.2809C, and then to a cold at a rate of 2sC / s up to the test temperature at which the traction is carried out, this traction being carried out with a speed of 73 mm / s.

Table 6: Reduction of the diameter in% in hot tensile tests with initial maintenance at 1,200 ° C STEEL C E F C G C S low (S; B low) TEST TEMPERATURE 900 ° C 34 42 50 46 22 39 950 ° C 33 43 - 45 46 13 47 1,000 ° C 36 44 42 49 24 53 1 .050 ° C 48 - 40 49 24 53 1 .100 ° C 52 - 43 54 35 59 1 .150 ° C 65 - 51 58 42 62 1 .200 ° C 69. 61 68 42 65 Table 7: Reduction of the diameter in% in hot tensile tests with initial maintenance at 1,280 ° C STEEL A E F C (S low) C (S, B low) TEMPERATURE TEST 900 ° C 3 333 3 333 37 39 950 ° C 3 344 3 311 37 38 1 .000 ° C 3 355 3 355 38 38 1 .050 ° C 4 422 3 388 43 44 1,100 ° C 4477 4433 50 54 1,150 ° C 5500 4488 55 53 1,200 ° C 6622 5544 63 64 1 .250 ° C 6677 6677 77 70 1 .280 ° C 8811 7777 85 76 The hot ductility is generally weak, but an improvement is found for the steels contain in its composition less than 15.10% sulfur, a diametral stricture higher than 45% at 1,000SC is considered necessary for hot rolling of steels: steel C (with low S) and steel C (with S; B low) which contains boron in its composition, reach this characteristic if the superheat is -elevated to 1,200 ° C. The characteristics of high ductilities in a cartridge are obtained according to the invention in the presence of a very low content of sulfur. Steel C, which comprises 35.10% sulfur, does not have sufficient hot ductility, the carbon content should not exceed 0.04%, otherwise, upon cooling after a heat treatment, chromium carbides precipitate at the interfaces of ferrite and austenite and degrade the corrosion resistance. A carbon content less than 0, 03% allows to avoid this -precipitation at the smallest cooling speeds. The silicon content must necessarily be higher than 0.4% to avoid excessive oxidation during the reheating of slabs or slabs. This content is limited to 1.2% to avoid favoring the precipitations - f agi 1 izantes of phases intermetálicas or phase sigma during the transformation in hot. Preferably, the silicon content is between 0.5% and 1%.

The manganese content can not exceed 4% to avoid processing difficulties. However, a minimum content of 2% is necessary to make the steel austenitic, allowing the introduction of 0.1% nitrogen, without exceeding the nitrogen solubility limit when solidifying. The nickel content is voluntarily limited to 1% for economic reasons and also to limit corrosion under tension in chlorinated media. In addition, international directives are directed towards a decrease in saline precipitation with nickel from materials, especially in the water and skin contact sectors. An addition of molybdenum may optionally be made to improve the corrosion resistance; In addition, molybdenum tends to increase its aggression by forming a sigma phase and its addition must be limited. A copper addition is particularly effective for increasing the austenite content. Beyond 4%, defects appear when carrying out hot rolling in relation to solidification segregations rich in copper. This addition also makes it possible to harden the ferrite phase by thermal treatment at temperatures between 400 ° C and 600 ° C, and to have, in use, a bactericidal and fungicidal effect. The sulfur content must be limited to 0.030% so that the steel is weldable without generating any hot cracking. A sulfur content of less than 0.0015% especially improves hot ductility and hot rolling quality. This low sulfur content can be obtained by means of the controlled utilization of calcium and aluminum in order to obtain the ranges of Ca, Al and S contents sought. A boron content of 5 to 30 * 10% also improves hot ductility. The phosphorus content is less than 0.1% and with - 0.04% preference, to avoid hot cracking when welding. The nitrogen content is naturally limited to 0.3% due to its solubility in the steel in the course of its production. For manganese contents less than -0.3% the nitrogen content should preferably be less than 0.2%. A minimum of 0.1% nitrogen is necessary to obtain an amount of austenite greater than 30%. The chromium content is sufficiently low to avoid the fragi uations due to the sigma phase and the decomposition of ferrite-ferrite mixtures when performing the hot sn transformation. The chromium contents according to the invention also allow the superlastic conformation at moderate temperatures between 700 ° C and 1,030 ° C without the formation of any sigma fragi 1 izante phase, unlike the usual qualities - austenitic enzymes used for super plastic conformation. A content in austenite of 30 to 70% is necessary - to obtain the high mechanical characteristics, that is to say - an elastic limit superior to 400 MPa, in an elaborated steel and in a welding, having the welding to be hard and resilient with a percentage of austenite greater than 20%. For this, the Creq / Nieq ratio will be respected in such a way that it is comprised between 2.30 and 2.75 and preferably between 2.4 and 2.65. The tensile elongation greater than 35% is obtained if the IM index is between 40 and 115, and the steel according to the invention has good drawing characteristics under these conditions. The steel according to the invention is particularly intended for the use of embedded parts and then assembled by welding, such as propellant tanks or containing other pyrotechnic reagents usable especially for inflatable safety cushion devices (known as airbags) in automobiles, applications They require a steel that has a high ductility for the shaping, as well as an equally high elastic limit of the base metal and the necessary solder in the use that is considered.

Steel is also used in particular for the manufacture of tubes from rolled and then welded steel strips, which can be used especially in the construction industry - of fixed mechanical structures or incorporated in mobile vehicles. These tubes can be formed with the help of high-pressure forming shaping, so-called hydroforming. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention is the conventional one for the manufacture of the objects or products to which it refers. Having described the invention as above, property is claimed as contained in the following:

Claims (5)

R E I V I D I C A C I O N S

1. - Austenoferritic stainless steel with very low nickel content and having a considerable tensile elongation, characterized by the following composition by weight: carbon 0, 04% 0.4% silicon < - 1, 2% 2% _. manganese C 4% 0.1%. nickel < 1% 18% chrome - 22% 0.05% < copper . 4% sulfur < , 0.03"phosphorus <0.1% 0.1% - nitrogen <, 3% mol ibdeno 3%, the steel having a bifasicity comprised between 30 and 70% of austenite, such that: Creq = Cr% + Mo% + 1, 5 Si% Nieq = Ni% + 0.33 Cu% + 0.5 Mn% + 30 C% + 30 N%, the Creq / Nieq ratio being between 2.3 and 2.75, being the austenite stability of said steel is regulated by the IM index defined from the weight composition of the steel by IM = 551-805 (C + N)% - 8.52 Si * - 8.57 Mn * - 12, 51 Cr -36 Ni% - 34.5 Cu% - 14 Mo%, with IM to be comprised between 40 and 115.

2. Steel according to claim 1, characterized in that the composition satisfies the ratio: Creq / Nieq comprised between 2, 4 and 2.65,

3. Steel according to the indications 1 and 2, characterized in that the sulfur content is less than or equal to 0.0015%

4. Steel according to claims 1 to 3, characterized because Steel also comprises, in its composition by weight, 0.010% to 0.030% aluminum. zero according to claims 1 to 4, characterized in that the steel also comprises, in its composition by weight, from 0.0005% to 0.0020% calcium. 6, - Steel according to claims 1 to 5, characterized in that the steel also comprises, in its composition by weight, 0.0005% 0.0030% boron. 1 . - Steel according to claims 1 to 6, characterized in that the carbon content is less than or equal to 0.03%. 8. Steel according to claims 1 to 7, characterized in that the nitrogen content is between 0.12% and 0.2%. 9, - Steel according to claims 1 to 8, characterized in that the chromium content is between 19% and 21%. 10. - Steel according to claims 1 to 9, characterized in that the silicon content is between - 0.5% and 1%. 11. Steel according to claims 1 to 10, characterized in that the copper content is less than 3%. 12. Steel according to claims 1 to 11, characterized in that the phosphorus content is less than or equal to -0.04%.