New Duplex Stainless Steel

The present invention relates to a duplex stainless steel having a high Cr-, W- and N-content.

Today the pressure of an oil well is increasing and there are plans to develop wells with pressures of 20 kpsi. At offshore, a long liner, also known as an umbilical, usually link the surface and seafloor equipment. These umbilicals contain tubing for both hydraulic and chemical injection to the subsea but also signal lines for providing electric and fiber-optics signals and electric power. The tubing of umbilicals consists of long tubes which may be welded together.

Super duplex stainless steels, i.e., an austenitic-ferritic iron chromium nickel alloy with molybdenum addition with PRE above 40, have been used as these steels will provide enough pitting resistance and mechanical strength for traditional subsea wells. However, for deeper wells, there will be a huge pressure increase, which means that the umbilicals have to withstand higher internal pressure. This in turn means that the wall thickness of the tube of the umbilicals has to be increased in order to withstand the higher pressure. Thus, these tubes will suffer from an increased weight which will increase the likelihood of failure as the duplex stainless steels of today do not have enough strength to carry their own weight if the wall thickness is increased to meet the higher pressure demands. Furthermore, in certain future applications, the duplex stainless steels of today may not possess enough corrosion resistance and may also have problems with formation of sigma phase during welding, especially in the heat affected zone. This will be very problematic when manufacturing umbilicals as these tubes may contain a lot of weld joints where cracks may be formed.

Hence, there is therefore a need for new duplex stainless steel which will have high strength and high corrosion resistance and both in the stainless steel itself and when used for welding.

The present disclosure provides a duplex stainless steel having a combination of high strength and excellent corrosion resistance as it contains a low coverage of quenched-in nitrides in the ferrite grain boundaries in its final form and will essentially not contain any sigma phase in the steel as such and in the heat affected zone when exposed to welding. The low coverage of quenched-in nitrides in the ferrite grain boundaries will increase the corrosion resistance and the impact resistance. Additionally, the low content of sigma phase will increase the corrosion resistance. Both these properties are important for the endurance of products in operation, such as when used for tubes in umbilicals. By the term “final form” is meant that the present duplex stainless steel has been exposed to the metallurgy processes necessary for being used in different applications. Hence, the present duplex stainless steel as defined hereinafter will contain low amounts of or essentially no sigma phase both in the duplex stainless steel as such and in the Heat Affected Zone (HAZ) after welding and additionally will have low coverage of quenched-in nitrides in the ferrite grain boundaries after being subjected to solution annealing and quenching.

The present disclosure therefore relates to a duplex stainless steel having the ranges of elements as disclosed hereinbelow. For the present duplex stainless steel, the inventors have carefully adapted both which elements and which ranges of these elements to be used in order to obtain a duplex stainless steel having the properties as mentioned hereinabove or hereinafter.

Thus, the present duplex stainless steel therefore comprises the following elements in weight %:

Hence, in other words, the present duplex stainless steel will have a low coverage of quenched-in nitrides in the ferrite grain boundaries after solution annealing followed by quenching. The coverage of quenched-in nitrides in the ferrite grain boundaries is measured as described in the method of the Example, Example 4. Further, the present duplex stainless steel will, due to its composition, have resistance against formation of sigma phase in the Heat Affected Zone (HAZ) and will therefore essentially not contain any sigma phase.

In the present disclosure, the term “solution annealed condition” is intended to mean the condition obtained after subjecting the present duplex stainless steel to a heat treatment, for the purpose of solution annealing, followed by cooling from the solution annealing temperature to a temperature of about 900 to about 1000° C., and thereafter quenching. Further, in the present disclosure, the term “solution annealing” is intended to include both heat treatment and cooling from the solution annealing temperature to a temperature between about 900 to about 1000° C.

In the present disclosure the term “quenched-in nitrides” refers to the nitrides which are formed at the ferrite grain boundaries when a steel is cooled fast as the elements will have no time to migrate between the phases and therefore will be precipitated in the ferrite grain boundaries. The coverage of quenched-in nitrides in the ferrite grain boundaries is measured as according to the method as described in the Example.

Hence, the inventors have found that it is important, in order to obtain the properties of the present duplex stainless steel, to balance the elements having an impact on the sigma phase formation during welding, as the sigma phase has a negative impact on the strength. The inventors have thus found that if the following condition is fulfilled:

wherein the numerical values should be in weight % the sigma phase content of the present duplex stainless steel will be low, i.e., will be less than 0.2 vol % when it is cooled with a cooling rate of 40° C./min from the solution annealing temperature to room temperature as shown in the Examples, Table 2. This is turn implies that content of sigma phase will be low or essentially zero in the HAZ after welding. Thus, this low content of sigma phase means that the strength and the impact toughness of the duplex stainless steel will be high when it is used as a base metal (e.g., in the form of a tube or a pipe) or as a welding material. Furthermore, due to the reduction of brittleness due to the low sigma phase content of the heat affected zone, the impact toughness and the strength will be increased.

According to the present disclosure, equation 1, may also fulfill the following: 40<[Cr]+4.0*[Mo]+2.0*[W]<42.

The ferrite content of the present duplex stainless steel is between 40-60 vol %, the reminder is essentially composed of austenite. If the ferrite content is too low, the mechanical strength will be too low and if the ferrite content is too high, the corrosion properties will deteriorate. For example, the resistance to hydrogen induced cracking will deteriorate if the ferrite content is too high. According to embodiments, the ferrite content is at least 44 vol %, the reminder being essentially austenite. The phase balance is important in duplex stainless steels as it helps to obtain the optimum mechanical properties and corrosion resistance. Additionally, the ferrite content of the present duplex stainless steel has been adapted so that phase balance is still within acceptable ranges in the heat affected zone. This is important as the ferrite content in the heat affected zone is often increase, which will have a negative impact on the mechanical properties and corrosion resistance.

In solution annealing with subsequent quenching, the quenched-in nitrides formed in the ferrite grain boundaries, will as, mentioned above, have a negative impact on both corrosion resistance and mechanical properties. It is therefore important to avoid formation of these quenched-in nitrides. The present inventors have found that the present duplex stainless steel having the specified element ranges as defined hereinabove or hereinafter will have a low coverage of quenched-in nitrides in the ferrite grain boundaries (less than 10%) in a solution annealed condition. Hence, according to the present disclosure, by a low coverage of quenched-in nitrides in the ferrite grain boundaries is meant that the percentage of ferrite grain boundaries covered with quenched-in nitrides is less than 10%. With typical expected grain sizes in finished products of the present duplex stainless steel, a 10% coverage of ferrite grain boundaries with quenched-in nitrides will correspond to a very low fraction of quenched-in nitrides in the material as a whole. Therefore, less than 10% of ferrite grain boundaries covered with quenched-in nitrides is considered to be a low coverage of quenched-in nitrides.

The terms “weight %” and “wt %” are used interchangeably herein.

The terms “HAZ” or “heat affected zone” are used interchangeably and is the non-melted area of metal that has undergone changes in material properties or microstructure as a result of being exposed to the heat generated by welding. The HAZ is thus the area between the weld and the unaffected base metal. In the present duplex steel, the selected ranges of elements together with the condition that balance certain element will significantly reduce sigma phase formation in HAZ.

The advantages of the present duplex stainless steel and selection of the ranges of the elements of the duplex stainless steel which render the unexpected superiority of the present duplex stainless steel are described in a more detail below. It should be noted that the inventors have adapted both the composition both in regard to the selected elements and to the ranges of these in order to obtain a duplex stainless steel having the desired properties as discussed herein. The present disclosure is however not limited to the exemplifying embodiments discussed but may be varied within the scope of the appended claims. Upper and lower limits of the individual elements of the composition can be freely combined within the broadest limits set out in the claims, unless explicitly disclosed otherwise. Additionally, when ranges are disclosed in the present disclosure, such ranges include the respective end values of the range, unless explicitly disclosed otherwise. Similarly, when an open range is disclosed, the open range also include the single end value of the open range, unless explicitly disclosed otherwise.

Chromium is an essential element, and the range has been selected in order to have good strength and corrosion resistance of the present duplex stainless steel. In order to have good strength properties and resistance to corrosion, the content of chromium should be at least 28.5 weight %.

However, even though chromium is a beneficial element, the content should not exceed 30.5 weight % as high contents of Cr will increase the risk of formation of sigma phase both during cooling in manufacturing processes and in HAZ. Additionally, the formation of quenched-in nitrides during solution annealing will be increased.

Further, it has been found that it is important that the contents of chromium, molybdenum, and tungsten, respectively, are balanced in order to avoid formation of sigma phase and to obtain the maximum strength. More specifically, the contents of chromium, molybdenum and tungsten are balanced such that the condition according to Equation 1, discussed above, is fulfilled.

Hence, the chromium content of the present duplex stainless steel is therefore 28.5 to 30.5 weight %. In order to even further ensure that the above-mentioned properties are obtained, according to the present disclosure, the chromium content may be 29.0 to 30.5 wt %, such as 29.5 to 30.5 wt %. According to the present disclosure, the chromium content may be from 29.1, 29.2, 29.3, 29.4 or 29.5 to 29.6, 29.7, 29.8, 29.9, 30.0, 30.1, 30.2, 30.3, 30.4 or 30.5 wt %.

Ni is used as an austenite-stabilizing element. In order to obtain a ferrite content of between 40-60 vol %, the content of nickel should be at least 6.0 weight %. Additionally, if you have a too high Ni content, the austenite content will be increase which will lead to lowers strength. However, as nickel is an expensive element, it is desirable to limit its content. According to the present disclosure, the highest content of nickel is 8.0 weight %.

According to the present disclosure, the content of Ni may be less than 7.9, 7.8, 7.7 or 7.6 weight %. According to the present disclosure, the lowest amount of Ni may be 6.0 weight %.

According to the present disclosure, the range may be between 6.0 to 7.6 wt %.

Mo is an active element which improves the strength and resistance to corrosion in chloride environments as well as in reducing acids.

However, an excessive Mo content in combination with a high Cr-content will increase the risk of the formation of sigma phase. Molybdenum may sometimes be replaced with tungsten. It is generally accepted in the art that replacement of molybdenum with tungsten may be performed in such a ratio that 1 weight % of Mo would be replaced by 2 weight % of W. However, if the present duplex stainless steel contains too low amounts of Mo, too much quenched-in nitrides in the ferrite grain boundaries will be formed during solution annealing with subsequent quenching.

Thus, it is important that the content of chromium, molybdenum and tungsten in the duplex stainless steel is optimized in order to avoid formation of sigma phase and to obtain the maximum strength. Hence, the content of Mo should be in the range of at least 0.70 weight % and the content of Mo should be no more than 3.00 wt %. According to the present disclosure, the content of Mo may be from at least 0.70 to 2.00 wt %., such as from at least 0.70 to 1.70 wt %, such as from at least 0.90 to 1.70 wt %.

W will improve the resistance to corrosion in chloride environments as well the resistance to pitting and crevice corrosion. However, a too high W content in combination with a high Cr content increase the risk of precipitation of sigma phase. Hence, it is very important that the combined amount of chromium, molybdenum and tungsten is balanced to avoid formation of sigma phase. More specifically, the contents of chromium, molybdenum and tungsten are balanced such that the condition according to Equation 1, discussed above, is fulfilled.

The W-content is therefore 2.00 to 4.40 wt %. According to ethe present disclosure, the content of W may be 2.50 to 4.40 wt %, such as 3.00 to 4.00 wt %.

N is a very active element which increases the resistance to corrosion as well as the strength of the duplex stainless steel. In order to obtain a good effect at least 0.30 weight % N should be added.

However, as added N must be solved in the duplex stainless steel in order to provide the increase in strength and corrosion resistance, the range of N must be carefully selected as too high content of N will increase the risk of precipitation of chromium nitrides, especially when the content of chromium is also high. The N-content should therefore be limited to maximum 0.55 weight %. Hence, the content of nitrogen is 0.30 to 0.55 weight %.

According to the present disclosure, the content of N may be from 0.35 to 0.55 wt %; such as 0.38 to 0.55 wt %, such as 0.38 to 0.50 wt %.

Mn is added in order to increase the solubility of N in the steel. However, Mn may also form manganese sulphides, which act as initiation points for pitting corrosion. The content of Mn is therefore greater than or equal to 0.20 weight % but equal to or less than 2.50 weight %. According to the present disclosure, the content of Mn may be 0.30 to 2.00 weight %.

Si is frequently utilized as a deoxidizer during steel production. However, it is known that high silicon content stabilizes the sigma phase. The content of silicon should therefore be limited to max 0.30 weight %. According to the present disclosure, the content of silicon may be 0.10 to 0.30 weight %.

Carbon is an element which is very difficult to completely avoid in a duplex stainless steel. Striving towards very low carbon contents would unduly increase the manufacturing costs. Therefore, carbon may be present in an amount of at least 0.005 weight % at least for cost reasons.

C strengthens stainless steel but also promotes the formation of chromium carbides which are harmful to corrosion. Carbon has also a limited solubility in both ferrite and austenite. The carbon content should therefore be limited to max 0.030 weight %, such as max 0.025 weight %.

Cu may be added in order to improve resistance to certain corrosive environments such as acid environments and it also decreases susceptibility to stress corrosion cracking. Furthermore, Cu will increase the strength and also reduce the formation of sigma phase.

However, Cu will have a negative effect on nitrogen solubility as less nitrogen is solved in the stainless steel and formation of chromium nitrides will therefore be increase. Hence, if copper is added, it is very important to carefully adjust its content. The content of Cu is therefore limited max 0.50 wt %, such as less than 0.50 wt %. According to an embodiment, Cu is added, and the content of Cu is between 0.15 to ≤0.50 wt %, According embodiments, the Cu content may be 0.25 to 0.45 wt %.

Sulfur is an impurity element normally contained in duplex stainless steel. The sulfur content should not be more than 0.030 wt % as it will have an impact on the hot workability above said range.

Phosphorus is also an impurity element normally contained in duplex stainless steel. The phosphorus content should not be more than 0.030 wt % as it will have an impact on the hot workability above said range.

The balance of present duplex stainless steel is iron (Fe) and unavoidable impurities. The unavoidable impurities are elements which are not added on purpose but may be in the present steel due to the scrap and/or manufacturing process used for providing the duplex stainless steel. Examples but not limited to such elements are Co, V, Ti, Nb, Pb, Sn and Ce. The combined content of these elements is less than 1.0 wt %

Further, the present duplex stainless steel may optionally comprise one or more. elements which may have been added in order to improve the manufacturing process. Examples but not limiting to such manufacturing process improving elements are Aluminium (Al), Magnesium (Mg), Calcium (Ca) and Boron (B). The combined content of these improving elements is less than 0.50 weight %, such as less than 0.30 weight % with the condition that content of Al cannot be more the 0.050 weight % and with the condition that the content of B cannot be more than 0.050 weight %.

Additionally, the present duplex stainless steel may comprise or consist of the elements mentioned herein in any of the ranges of the specific elements mentioned herein and fulfilling the requirements mentioned herein.

According to embodiments, the present disclosure related to an object comprising or consisting of the duplex stainless steel as defined hereinabove or hereinafter. According to embodiments, the object is made of the duplex stainless steel as defined hereinabove or hereinafter. According to embodiment, the object is in solution annealed condition.

Hence, the present disclosure relates to an object comprising a duplex stainless steel wherein said duplex stainless steel comprising the following elements in weight %: