DE2457562A1 - SEAWATER-RESISTANT STEEL HIGH STRENGTH - Google Patents

Description

OENTRO SPERIMENTALE METALLURGICO S.p.A. Rome, Italy

Seawater resistant steel of high strength

The invention "relates to a steel with very good mechanical properties, which at the same time has good weldability and resistance to corrosion in the various applications in the presence of sea water. In particular, the invention relates to the technical problem of producing a steel with a completely bainitic structure that , WJB known, has a satisfactory compromise between toughness and strength due to a combination of alloying elements which impart _m steel together de even the favorable properties with regard to corrosion resistance and weldability.

Have the great performance required of steels for marine related applications

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it made it necessary to find new compositions which meet the demand for increasingly higher quality satisfy. Many so-called seawater-resistant steels that are in normal production have been known for a long time and use. However, their strength is not excellent and their resistance to corrosion is not excellent is only a little better than ordinary steel; they are therefore uneconomical to use.

From the point of view of corrosion, many complex phenomena in the marine field are due to chemical To name components of sea water and the products of biological activity contained therein.

Since the definition and development of new steels suitable for use in sea water is a possibility are linked to assess their corrosion behavior in this environment, investigation methods have been developed, which enable the most dangerous types of corrosive attack to be identified and interpreted of their mechanisms allow.

The most serious reasons for a

Steels of high strength in sea water are the following:

a) Pickling brittleness and stress corrosion. Hydrogen is in the atomic state, is generated by corrosive attack and penetrates the metal and causes deterioration in its mechanical properties.

b) Crevice corrosion.

This type of corrosion starts at points or surfaces

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where the metal comes into contact with non-conductive parts, "which are in the marine field general about paints (decorative and protective kind) and / or encrustations, whose origin in Sea are to look for, acts.

c) Corrosion due to pitting. This type of corrosion starts in the parts the metal surface and develops from these, the weakest from the point of view of passivity are. In the sea water breaks "with a certain Potential of the passivation film on because the CT ions of the film is replaced by Gl "~ ions from sea water will. Resistance to pitting corrosion is a measure of the stability of the Passivation film: the higher the substitution potential. the greater the resistance to pitting corrosion.

According to the invention it should now be possible to use components to propose for use in marine environment whose tensile strength and corrosion resistance are better than are those of the steels currently used for such applications.

A steel according to the invention that meets these requirements is fair, is surprisingly characterized by a composition with

low carbon, high manganese and Maximum content of ferritizing elements (such as Al,

Ti, Si and Mo), which are compatible with the ^ > X

Phase transition between 950 and 105O ⁰ C are. Preferably a steel according to the invention has the following

B 0-9 8 2.4 / 067.8 - 4 -

- 4 basic composition (weight percent): - 0.1 Gj 1.5 - 3.0 Mn} - 0.5 Ou; - 2.5 Cr; - 0.2 Wi;

^ 1.5 Si; ^ 1.5 Al; - 1.5 TiJ - 1.5 Mo; - 1.0 lib; S and P at the impurity level and the rest essentially

It has been found that, within the above-mentioned compositional limits, the high Mn content does not adversely affect the toughness, machinability and weldability of the steel. In addition, exercise within the stated composition limits the Mn also has an austenitic effect, which is due to the ferritizing influence of the Al, Ii, Si and Mo counteracts /. Another function of the Mn is to have the graphical influence to neutralize Al and Si due to their special chemical affinity for carbon.

50 can alloy aluminum and silicon in Sufficiently large amounts are added to their positive influence on the corrosion resistance of the To take full advantage of steel.

* and regulates

Cr, Cu and Ni are also provided in order to reduce the positive influence of the ferritizing elements on the To increase corrosion resistance characteristics. In particular, Ni also prevents rolling defects from forming in steel during rolling caused by the presence of Cu.

51 is always present in these steels wherever it is The role of the deoxidant. It was also found that within the specified limits the other

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pungent or different properties for applications in the navy is not adversely affected will.

The addition of Fb in the specified proportions to steels with very low carbon content (- 0,07 $ 6) and quite hohejn manganese content (- 1.5 i ") improves the mechanical properties which can be obtained by simple cooling in air, even for Thicknesses above 20 mm. This result stems from the surprising ability of niobium to delay the minimum time required for the

Initiation of austenite \ polygonal ferrite forming

is required for continuous cooling. This means that in practice it is completely bainitic Structure is obtained even at thicknesses - 20 mm.

The steel according to the invention can be used for the production of sheets, plates, profiles, rods, Tubes and various components for marine applications (in sea water or an atmosphere, as is peculiar to the Navy).

The following examples illustrate the invention without limiting it. The superiority of the invention will proven by comparison with other steels currently considered suitable for the marine area.

example

Steels according to the invention (designated 1 to 8 in X'af el 1 ) were compared with steels currently available for use in seawater.

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BLACKBOARD

composition
StähT "" * \ ^^ 0 Mn Si Al Cr Cu - Ii Mon Nl) P. S. 1 0.06 2.0 0.20 1.0 0.50 _ 0.20 - - - 0.02 0.02 2 0.06 2.0 0.20 1.0 2.0 0.40 - - - - 0.02 0.02 3 0.03 2.5 0.20 1.0 - - 0.10 - - 0.05 0.02 0.02 4th 0.07 2.0 0.20 - 0.30 0.20 0.20 0.70 - - 0.01 0.01 5 0.06 2.0 0.20 0.20 0.50 0.40 - - 0.50 - 0.01 0.02 6th 0.06 1.5 1.5 - 0.30 - - '- 0.20 - 0.02 0.02 7th 0.07 2.5 0.20 0.50 0.50 - 0.20 0.50 - - 0.01 0.02 8th 0.04 2.0 0.20 - 0.30 0.40 2.45 0.50 0.30 - 0.01 0.01 Hy 80 0.14 0.30 0.20 - 1.45 - - - 0.45 - 0.01 0.01 ΚΈΡ-ΪΕΪΤ 60 0.18 0.50 0.30 0.50 0.50 0.60 - - - 0.08 0.01 AISI 430 0.10 0.80 0.50 16.0 ■ · - 0.03 0.02

Table II gives the results of comparison tests with regard to tensile strength, stress corrosion, resistance to pitting and especially crevice corrosion for the steels according to Table I.

TAi ¹ EIi II

stole Tensile strength
ability ₂
(kg / mm) Tension
corrosion
resistance
(D Pitting
resistant-
speed
(2) - Inclination to
Crevice corro
sion
(3) 1 80 70 35 - 2 2 105 35 30th 100 50 ■ 3 65 ' 450 5 - 80 4th 60 160 55 5 VJl 65 230 30th 15th 6th 80 300 2 120 7th 65 100 63 2 8th 70 75 41 4th Hy80 69 20th - ΝΕΡ-ΤΕΕΓβΟ 65 - - AISI 430 - - - usually
cher or
unalloyed
ter steel - - 100

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- 8 ORIGINAL INSPECTED

To the table:

The values of the stress corrosion resistance, the resistance to pitting corrosion and the tendency to Crevice corrosion after. Table II are given as multiples of one hundred, with the value 100 being usually each was assigned:

(1) Resistance to stress corrosion of steel

(2) Resistance to pitting corrosion of the steel AISI 430

(3) The tendency towards crevice corrosion of mild steel

Claims - 9 -

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Claims (3)

1. Seawater-resistant steel, characterized by the following composition in percent by weight: - 0.1 0, 1.5 - 3.0 Mn, ^ 0.5 Cu, - ^ 2.5 Cr, - 0.2 Hi, * 1.5 Si, -1.5 Al, ^ 1.5 Ti, -1.5 Mo, - 1.0 Nb, S and P at the impurity level and the kest essentially Pe. 2. Seawater-resistant steel components, marked by the following composition in percent by weight: - 0.1 C, 1.5 - 3.0 Mn, - 0.5 Cu, ^ 2.5 Cr, - 0.2 M, £ 1.5 Si, £ 1.5 Al, ^ 1.5 ϊί, -1.5 Mo, -1.0 lib, S and P at impurity level and the remainder essentially Fe. 3. Seawater-resistant steels and components made of steel, as described, shown in the examples and claimed " 5 09824/0678