GB2392919A

GB2392919A - A corrosion resistant steel for marine applications

Info

Publication number: GB2392919A
Application number: GB0221129A
Authority: GB
Inventors: Edward Marsh; David G Lapwood; Edmund Marsh
Original assignee: Corus UK Ltd
Current assignee: Corus UK Ltd
Priority date: 2002-09-12
Filing date: 2002-09-12
Publication date: 2004-03-17
Anticipated expiration: 2022-09-12
Also published as: GB0221129D0; GB2392919B

Abstract

A steel comprising by weight percent: carbon 0.05-0.25, manganese 0.80-1.70, chromium 0.75-1.50, molybdenum 0.20-0.50, aluminium 0.40-0.80 with the balance being Fe and other incidental or residual impurities. Other alloying additions, in weight percent, may include: silicon 0.00-0.06; titanium 0.00-0.05; phosphorous 0.00-0.045 and sulphur 0.00-0.045.

Description

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CORROSION RESISTANT STEELS

This invention relates to corrosion resistant steels and products of such steels. The invention relates especially, but not exclusively, to corrosion resistant steels for products for use in marine applications.

These products include inter alla sheet piling, bearing piles, combiwalls, walings and tie bars which, in use, are immersed in sea water. The invention does, however, have applications for products which are not for use in marine applications.

Corrosion resistant steels are, of course, well known. However, many of these steels may be susceptible to unsatisfactory corrosion behaviour when immersed permanently or predominantly in seawater.

Much of the information available in the published literature on the effects of alloying elements on corrosion performance is contradictory and illustrates a need to generate reliable long-term corrosion performance data. For example, an early statistical analysis of a limited amount of corrosion data indicated that chromium, molybdenum and aluminium are beneficial for enhancing general corrosion resistance of low alloy steels but that the beneficial effect of chromium was reversed after about four years. The analysis applied to steels having chromium contents in the range 2% to 5% and indicated that molybdenum showed a trend opposite to that for chromium. Data subsequently published for specimens immersed at depths of 45m and 90m in the North Sea contradicted some aspects of this earlier work indicating that chromium and aluminium were beneficial to corrosion resistance even after 7.2 years exposure, albeit in steels containing only up to 1.6% chromium. These data also provided evidence that small amounts of molybdenum reduced localised corrosion. The improved corrosion performance in seawater of steels containing chromium, aluminium and molybdenum was attributed to the formation on the steel surface of a continuous magnetite layer

having reduced porosity and improved adherence because of the incorporation of the alloying elements.

Applicants have found that seawater corrosion resistance is significantly increased when prescribed amounts of chromium, aluminium and molybdenum are added to a steel which also contains specific amounts of carbon, silicon, manganese and other elements to ensure adequate strength and toughness in the finished product. Typical mechanical properties are a minimum yield stress of about 355 MPa, a minimum tensile strength of about 480 MPa and a minimum Charpy absorbed impact energy of 273 at a test temperature of 0 C or lower.

The present invention sets out to provide a weldable corrosion resistant steel with enhanced properties or strength, toughness and, more especially, increased corrosion resistance in the presence of seawater.

According to the present invention in one aspect, there is provided a corrosion resistant steel having the following composition by weight percent; Carbon 0.05 - 0.25 Silicon up to 0.60 Manganese 0.80- 1.70 Chromium 0.75- 1.50 Molybdenum 0.20 - 0.50 Aluminium 0.40 - 0.80 Titanium up to 0.05 Phosphorous up to 0.045 Sulphur up to 0.045 Balance iron and incidental and/or residual impurities Steel and steel products in accordance with the invention may be manufactured using conventional steel making (e.g. basic oxygen, electric arc) and processing (e.g. hot rolling, cold forming) techniques.

A preferred steel includes by weight per cent: carbon 0.08-0.10; silicon 0.30-0.50; manganese 1.00-1.10; chromium 0.95-1.05; molybdenum 0.28-0.33; aluminium 0.57-0.63; titanium 0.015-0.025; phosphorous up to 0.012; and sulphur up to 0.015, balance iron and incidental and/or residual properties.

In formulating the composition ranges specified above, a carbon content of at least 0.05% by weight was selected to ensure adequate strength. Contents greater than 0.25% by weight have been found to have an adverse effect on the toughness and weldability of the steel.

As is well known, silicon contributes to the strength of the steel.

However, contents greater than 0.6% have been found to have an adverse effect on weld heat affected zone (HAZ) toughness. A maximum silicon content of 0.60% by weight was therefore selected.

Manganese is known to be an effective solid solution strengthening element. It was decided that a content of at least 0.8% by weight is required to ensure adequate strength. It was also appreciated that manganese also increases hardenability and a content greater than about 1. 7% by weight has been found to lead to the formation of upper bainite in the microstructure of the steel which has a deleterious effect on toughness. A range of from 0.8 to 1.7% by weight was therefore selected. Chromium is also known to contribute to the strength of the steel.

However, in steels in accordance with this invention the prime role of the chromium content is to contribute to the required enhanced seawater corrosion resistance. Higher levels have been found to lead to the reversal of this beneficial effect. These criteria were considered carefully.

A range of from 0.75 to 1.50% by weight was accordingly selected.

Molybdenum is also known to contribute to the strength of the

steel. However, in steels in accordance with the invention the prime role of the molybdenum content is to contribute to the required enhanced seawater corrosion resistance. It was also appreciated that higher levels than 0.5% by weight can lead to an unacceptable increase in hardenability and an adverse effect on toughness. Consequently, a range of 0.20 to 0. 50% by weight was selected.

Conventionally, only a small amount (0.005-0.05%) of aluminium is required for deoxidation purposes. A higher than normal range starting at 0.4% by weight was selected to provide enhanced seawater corrosion resistance. It was appreciated, however, that aluminium can have other less desirable effects. Above about 0.2% by weight, it promotes decomposition of iron carbides and graphite films can form at the ferrite grain boundaries which are detrimental to toughness and strength. The presence of carbide forming elements such as manganese, chromium and molybdenum oppose graphitisation. When the aluminium content is above 1%, the ferrite to austenite transformation is suppressed and the steel is essentially ferritic at all temperatures. Austenite farmers such as manganese are required to counter balance this effect. These criteria were considered carefully and a range of 0.4 to 0.8% by weight was selected for steels in accordance with the invention.

An addition of up to 0.05% titanium counteracts problems which can occur during continuous casting and subsequent rolling of the steel due to its high aluminium content. Titanium combines with nitrogen to form titanium nitride particles which, being stable at high temperatures, have the additional benefit of restricting austenite grain growth during reheating prior to rolling thus promoting the formation of finer grained ferrite in the microstructure of the final product. It can also promote the formation of a finer microstructure and hence improved toughness in the weld HAZ. The preferred titanium content is between 0.015-0.025% by weight. Titanium contents above 0.05% by weight have been found to have an adverse effect on toughness.

Phosphorous and sulphur are naturally occurring impurities in the steel; the latter combines with manganese to form manganese sulphide inclusions. In excessive amounts, both elements can have an adverse effect on toughness and therefore need to be restricted to a maximum of 0.045%, preferably lower.

The invention will now be described with reference to the following Examples of steels exemplary of the invention.

EXAMPLE 1

A steel cast was produced by a conventional steel making process having the following composition by weight % percent Carbon 0.079 Silicon 0.28 Manganese 1.04 Chromium 1.05 Molybdenum 0.32 Aluminium 0.59 Phosphorous 0. 039 Sulphur 0.023 Balance iron and incidental and/or residual impurities The steel was hot rolled under normal processing conditions in a commercial section mill to 229 x 89mm channels. Continuous and butt welded lengths of the rolled section were exposed to a marine harbour environment and their corrosion performance monitored. The butt welds were made using low alloy electrodes to prevent bi-metallic corrosion of the weld. For comparison purposes, a channel section of the same size with similar mechanical properties but produced in a steel without the specified chromium, aluminium and molybdenum additions was exposed to the same conditions. After 11.5 years exposure the channel section with the alloy additions exhibited up to a fourfold increase in corrosion

resistance in the low water and immersion zones and no preferential weld corrosion had occurred.

Hot rolled material from this cast was also subject to a weldability assessment. A butt weld was made using the manual metal arc process.

The weld was found to be sound and free of cracks. Charpy V-notch impact tests were carried out at +20 C and 0 C on specimens prepared from the weld metal and heat affected zone. All the specimens exhibited fully ductile fractures at both temperatures. Subsequent work has shown that the steel can also be laser welded.

EXAMPLE 2

A further steel cast was produced by a conventional steel making process having the following composition by weight percent: Carbon 0.09 Silicon 0. 40 Manganese 1.04 Chromium 1.00 Molybdenum 0.31 Aluminium 0.60 Phosphorous 0.012 Sulphur 0.002 Balance iron and incidental and/or residual impurities The steel was hot rolled under normal processing conditions in a commercial section mill to a Larssen sheet piling section and the tensile and Charpy V-notch impact properties evaluated.

Yield stress (MPa) 366 Tensile strength (MPa) 577 Impact energy at 0 C (a) 79

The properties of this steel were found generally to replicate those of the steel of Example 1.

It will be appreciated that the foregoing is merely exemplary of corrosion resistant steels in accordance with the invention and that modifications can readily be made thereto without departing from the invention disclosed in this Application.

Claims

1. A steel comprising by weight percent: Carbon 0.05 - 0.25 Silicon up to 0.60 Manganese 0.80- 1.70 Chromium 0.75- 1.50 Molybdenum 0.20 - 0.50 Aluminium 0.40 - 0.80 Titanium up to 0.05 Phosphorous up to 0.045 Sulphur up to 0.045 Balance iron and incidental and/or residual impurities

2. A steel as claimed in claim 1 wherein the carbon content by weight percent is from 0.08 to 0.10.

A steel as claimed in claim 1 or claim 2 wherein the silicon content by weight percent is from 0.30 to 0.50.

4. A steel as claimed in any one of claims 1 to 3 wherein the manganese content by weight percent is from 1.00 to 1.10.

5. A steel as claimed in any one of the preceding claims wherein the chromium content by weight percent is from 0.95 to 1.05.

6. A steel as claimed in any one of the preceding claims wherein the molybdenum content by weight percent is from 0.28 to 0.33.

7. A steel as claimed in any one of the preceding claims wherein the aluminium content by weight percent is from 0.57 to 0.63.

8. A steel as claimed in any one of the preceding claims wherein the

titanium content by weight percent is from 0.015 to 0.025.

9. A steel as claimed in any one of the preceding claims wherein the phosphorous content by weight percent is up to 0.012.

10. A steel as claimed in any one of the preceding claims wherein the sulphur content by weight percent is up to 0.015.

11. Products of steel having a composition as claimed in any one of the preceding claims manufactured using accepted steel making and processing techniques.

12. Steel and steel products as claimed in any one of the preceding claims having improved corrosion resistance particularly in the low water and immersion zones in marine environments when compared to conventional steels.

13. Steel and steel products as claimed in any one of the preceding claims being resistant to biological action (ALWC - accelerated low water corrosion) in marine environments when compared to conventional steels.

14. Steel products as claimed in any one of claims 11 to 13 which are weldable and possess adequate strength and toughness for the applications for which they are intended; typically a minimum yield stress of about 355 MPa, a minimum tensile strength of about 480 MPa and a minimum Charpy absorbed impact energy of 273 at a test temperature of 0 C or lower.