EP0615551B1 - Weldable high-strength structural steel with 13 % chromium - Google Patents

Abstract

PCT No. PCT/DE92/00987 Sec. 371 Date Jul. 25, 1994 Sec. 102(e) Date Jul. 25, 1994 PCT Filed Nov. 23, 1992 PCT Pub. No. WO93/11270 PCT Pub. Date Jun. 10, 1993.Disclosed is a process for producing seamless steel pipes or flat products (strip or sheet) for pipes or vessels which are intended for the conveyance, transport or processing of gaseous or liquid hydrocarbons containing CO2 and water and possibly small proportions of H2S and are resistant to stress crack corrosion and have good welding properties at the same time and a 0.2-percent elongation limit of at least 450 N/mm2. The process uses a nickel-containing steel of the following composition (percent by weight): min. 0.015% C, 0.15-0.50% Si, max. 2.00% Mn, max. 0.020% P, max. 0.003% S, 12.0-13.8% Cr, 0.002-0.02% N, 0.01-0.05% Nb, remainder iron and usual impurities. It is presently suggested that the nickel content is limited to a maximum of 0.25%, the manganese content amounts to at least 1.0%, the carbon content is limited to 0.035%, and 0.01 to 1.2% molybdenum is contained as additional alloying component.

Description

The invention relates to a process for the production of seamless steel tubes or flat products (strip and sheet) for tubes or containers which are intended for the conveyance, transport or processing of hydrocarbons. In the presence of CO ₂ and water and possibly small amounts of H ₂ S, there are corrosive conditions in the media to be transported or processed.

For the extraction of hydrocarbons under corrosive conditions, pipes made from low-alloy steels with passive corrosion protection (inhibition) or from high-alloy corrosion-resistant steels are usually used in order to meet the high requirements with regard to corrosion resistance, in particular resistance to stress corrosion cracking. A suitable steel can be found, for example, in DE 26 16 599 C2. Because of the high proportions of expensive alloying elements (eg 22% Cr, 5% Ni, 3% Mo), tubes and containers made from such steels are extremely cost-intensive for the above-mentioned applications. These relatively high-strength duplex steels usually have low C contents and are therefore easy to weld.

Steels containing 0.18 - 0.22% C and 12.5 - 14% Cr (material AISI 420) are also known for use in oil fields. This material has very good corrosion resistance in a humid CO ₂ environment. Since pipes made of this material are practically not weldable under construction site conditions, only screw connections are used to connect the pipes. Pipes made of this steel are therefore only used as delivery pipes, but not as line pipes. If traces of H ₂ S are also present in the hydrocarbons to be conveyed through the pipes, stress corrosion cracking can result, since this material has only a comparatively low resistance to this type of corrosion.

Furthermore, 13% chromium steels are known for the production of steel pipes, which are weldable. An example of this is the material AISI 410 (material no. 1.4006), which 0.08 - 0.12% C, max. Contains 1.0% Mn and 12.0 - 14.0% Cr. The weldability of this steel is guaranteed due to the lower carbon content. However, the heat treatment of the rolled products produced from them is often problematic, since this often leads to an inhomogeneous structure, which is responsible for the very poor resistance of these steels to stress corrosion cracking in the presence of H ₂ S. For this reason, this material, which is known as and is acid-resistant, used for pump pipes, heat exchangers and the like, but not used for the production of hydrocarbons; it is only used as a cast or forged product for fittings in the area of the wellhead. Its low corrosion resistance is adequately documented by reports of damage cases in the literature.

Finally, a steel for the production of seamless steel tubes with the following composition is known from JP 57-5849: Max. 0.015% C. 0.10 - 0.80% Si 0.10 - 2.00% Mn Max. 0.025% P Max. 0.010% S 11.0 - 17.0% Cr 0.10 - 3.00% Ni Max. 0.015% N 0.01 - 0.05% Nb 0.01 - 0.10% Al

Balance iron and usual impurities.

This steel is described as weldable, tensile, tough and corrosion-resistant. The seamless steel tubes produced therefrom had a yield strength in the range of 428-502 N / mm ² after heat treatment. Compliance with the upper limits for C with a max. 0.015% and N with max. 0.015% viewed. Mo is not provided for in this steel.

In contrast, it was found in the context of the present invention that a steel with the composition specified in claim 1 not only also has excellent properties with regard to corrosion resistance and is easy to weld and very tough, but also even allows a 0.2% proof stress, which significantly exceeds the values known from JP 57-5849. This is due in particular to the surprising finding that a limitation of the Ni content, which is up to 3.0% in the known steel must be at a maximum value of 0.25%. Under this condition, within the framework of the values mentioned in patent claim 1, contents of C in the range from 0.015% to 0.035% and of N in the range from 0.002 to 0.02% can be permitted for the other alloy elements; This opens up new possibilities with regard to the mechanical properties. In contrast to the known steel, the steel used according to the invention also contains Mo, in the range from 0.01% to 1.2%; this content is advantageously limited to values of at most 0.2 to 0.3%. The minimum content of Mn is 1.0%, while in the known steel, much lower contents of up to 0.1% are permissible for Mn; there is a limit of 2.0%. The Cr content is said to be in the range of 12.0 to 13.8%. Values in the range 0.02-0.04% have proven to be particularly favorable for the addition of Nb; however, a range of 0.01-0.05% is permissible. Since the C content is limited to 0.015 - 0.035%, these steels have good welding properties. A content of 0.15 - 0.50% is prescribed for Si and a content of 1.0 - 2.0% for Mn. The impurities on P and S must be max. 0.020% and 0.003% respectively.

A steel used in JP 57-5849 as a comparative example with the following composition, with the following composition, shows just how important it is to precisely observe the content limits of the individual alloy elements specified according to the invention:

Balance iron and usual impurities.

This steel, which differs from the steel of the present invention in the contents of Mn, Mo and Ni by at most about half a percentage point, was not found to be corrosion-resistant there.

With regard to the rolling processing of the steel to be used according to the invention, there are several possibilities. In the production e.g. of sheets for containers or welded pipes, the primary material should be heated to 1100 - 1250 ° C, then pre-rolled in a first rolling phase at temperatures above 1000 ° C and then in a second rolling phase at temperatures in the range of 850 - 750 ° C with a minimum deformation of 30% are final rolled.

The second rolling phase is preferably carried out in such a way that, when accelerated from a final rolling temperature of greater than or equal to 850 ° C., the cooling rate is at least 5 K / s to below 200 ° C. Further cooling can take place in air. Subsequent starting is recommended, but is not absolutely necessary.

In another advantageous variant of the method of the invention, the cooling takes place from a final rolling temperature greater than or equal to 850 ° C. with a cooling rate of 0.5 to 2 K / s to ambient temperature.

In order to selectively set narrow ranges in the strength values of the products (for example 15 ksi 103.4 N / mm ² ), these can be heat-treated in a separate process step in a manner known per se.

The invention is explained in more detail below on the basis of comparative examples and test results. Figures 1 and 2 show measurement results with regard to the erosive corrosion for different steels under different conditions.

Table 1 shows the chemical compositions of three different 13% chromium steels with the designations 410, 411 and 413. Steel 410 corresponds to the present invention, while the other two steels are to be regarded as comparative examples. The steel 411 differs from the invention by one. Ni content of 2.09% and the steel 413 due to a too low Mn content of 0.57% and an excessively high Ni content of 4.19%. Table 2 shows the mechanical-technological properties for flat products and pipes manufactured under different rolling and heat treatment conditions. A TM-rolled sheet, which was used at 1140 ° C and finally rolled at 800 ° C, achieved the excellent mechanical properties shown in the first line under work number 410A without any tempering treatment. By lowering the final rolling temperature to 750 ° C (work number 410B), the strength values could be increased even further, although the toughness properties deteriorated slightly. From the test results shown in the lower part of Table 2 (working numbers 410.1 to 410.5), the influence of heat treatment by hardening and tempering under different conditions is shown under the same rolling conditions. One can clearly see the considerable increases in the values achieved in terms of strength and Toughness properties.

Table 3 shows that the steel 410 according to the invention is clearly superior to the known steels 411 and 413 in terms of its resistance to stress corrosion cracking.
Only under very extreme test conditions (0.01 bar H ₂ S and 5% NaCl) did the steel 410 fail the round tensile test after 1000 hours with a load of 90% R _{p 0.2} . The comparative steels showed sample failures even under much milder test conditions.

Tabelle 3: Ergebnisse von Spannungsrißkorrosionsversuchen Prüfbedingungen Befund p(H₂S) bar % c(NaCl) % 410 411 413 0,001 0 O O O 5 O X X 0,0035 0 O O O 5 O X X 0,01 0 O n.g. n.g. 0,5 O X n.g. 5 X X n.g. Rundzugproben unter konstanter Last Belastung: 90% R_P0.2
Versuchsdauer: 1000 h
Trägergas: CO₂ unter Normaldruck
Symbole: O: ohne Befund; X: Probenausfall n.g.: nicht geprüft FIGS. 1 and 2 show the resistance of the steel according to the invention to abrasive corrosion under different conditions in comparison to the steels 411 and 413 and a steel X20Cr13. Taking into account the analysis values from Table 1, it can be seen that increased levels of Ni and in particular Mo reduce the rate of corrosion in the event of abrasive corrosion. The durability of the steel 410 according to the invention is, however, as the comparison with the steel X20Cr13 shows, still quite good. Despite their better resistance to abrasive corrosion, the comparison steels 411 and 413, as can be seen from Table 3, with their increased Ni and Mo contents are clearly inferior to the steel according to the invention in terms of resistance to stress corrosion cracking. Surprisingly, the reason for the success according to the invention can be seen in the drastic limitation of the Ni and Mo contents. In the event that the stress corrosion cracking resistance compared to the resistance to abrasive corrosion is given a much greater importance, the Mo content should even be limited to values below 0.2%.

Table 3: Results of stress corrosion cracking tests Test conditions Finding p (H ₂ S) bar% c (NaCl)% 410 411 413 0.001 0 O O O 5 O X X 0.0035 0 O O O 5 O X X 0.01 0 O ng ng 0.5 O X ng 5 X X ng Round tensile specimens under constant load load: 90% R _P0.2
Test duration: 1000 h
Carrier gas: CO ₂ under normal pressure
Symbols: O: no findings; X: Sample failure ng: not checked

Claims (8)

1. A method for the production of seamless steel pipes or flat products (band or sheet metal) for pipes or containers which are intended for the conveying, transporting or processing of gaseous or fluid hydrocarbons which contain CO₂ and water and also possibly small components of H₂S, and are resistant to stress corrosion and at the same time are able to be welded well and have a 0.2 % yield strength of at least 450 N/mm², in which a steel containing Ni is used which, furthermore, has the following composition (% by weight): 0.015 - 0.035 % C 0.15 - 0.50 % Si 1.0 - 2.00 % Mn max. - 0.020 % P max. - 0.003 % S 12.0 - 13.8 % Cr 0.002 - 0.02 % N 0.01 - 0.05 % Nb >0 - 0.25 % Ni 0.01 - 1.2 % Mo Remainder iron and usual impurities.
2. A method according to Claim 1,
   characterised in that
   the Mo content is limited to a maximum 0.20%.
3. A method according to one of claims 1 to 2,
   characterised in that
   the Nb content is set to a value between 0.02% and 0.04%.
4. A method for the production of flat products according to one of Claims 1 to 3,
   characterised in that
   the initial material is heated from 1100 to 1250°C, then is pre-rolled in a first rolling phase at temperatures down to a maximum of 1000°C and is then finish-rolled in a second rolling phase at temperatures in the range of 850 - 700°C with a minimum deformation of 30%.
5. A method according to one of Claims 1 to 3,
   characterised in that
   starting from a finish-rolling temperature of at least 850°C accelerated with a cooling rate of at least 5 K/s, cooling takes place to below 200°C.
6. A method according to Claim 5,
   characterised in that
   after the accelerated cooling, tempering is carried out separately.
7. A method according to one of Claims 1 to 3,
   characterised in that
   from a finish-rolling temperature of at least 850°C with a cooling rate of 0.5 to 2 K/s, cooling takes place to ambient temperature.
8. A method according to one of Claims 1 to 4 or 7,
   characterised in that
   the rolling products are subjected to a separate heat treatment to set the desired degree of stability.

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