Patent Publication Number: US-2002011284-A1

Title: Method for making seamless tubing with a stable elastic limit at high application temperatures

Description:
DESCRIPTION  
       [0001] The invention relates to a process for producing seamless line pipes within the quality grade range X 52 to X 90.  
       [0002] In the course of the exploration of deposits of hydrocarbons, deposits are increasingly being discovered which are difficult to convey owing to the fact that the hydrocarbons (e.g. natural gas) are at relatively high temperatures of, for example, from 100 to 200° C. The materials which can be used for line pipes under such conditions not only have to be sufficiently weldable and to have a certain resistance to corrosion, but also have to have a comparatively stable yield strength. For example, the reduction in yield strength at a temperature of 160° C. compared to the yield strength at room temperature should be as low as possible. Furthermore, an essentially constant stress-strain characteristic is required, i.e. the so-called Luders strain should be as low as possible.  
       [0003] It is generally relatively rare to use tungsten as an alloying element. As a strong carbide-forming element, it is regularly used to produce cold-work, hot-work and high-speed steels. It increases their high-temperature strength, ability to withstand tempering and, in particular, the wear resistance at high temperatures. Tungsten acts in a similar way to molybdenum, so that it can replace molybdenum in a ratio of 2:1.  
       [0004] In modern power plant construction, ferritic alloys containing 9 to 12% chromium which are able to withstand high temperatures are used for steam pipelines. For these steels, it is known to add from 1 to 2% tungsten to the alloy in order to increase the creep rupture strength. Examples of such steels are the alloys P 92 and P 122 from Japan and the European-developed material E 911.  
       [0005] Hitherto, there has been no knowledge whatsoever of using tungsten as an alkalyne element for line pipe steels.  
       [0006] The object of the invention is to propose a method for the production of seamless line pipes, in which method it is possible to reliably set a quality grade in the range from X 52 to X 90 by means of a quenching and tempering treatment and to ensure a stable yield strength combined with an essentially constant stress-strain characteristic up to temperatures of use of 200° C. 
     
    
    
     [0007] According to the invention, this object is achieved by hot-rolling of a pipe blank made from a steel of the following composition (% by weight):  
                                                      C   0.06-0.18%           Si   max. 0.40%           Mn   0.80-1.40%           P   max. 0.025%           S   max. 0.010%           Al   0.010-0.060%           Mo   max. 0.50%           V   max. 0.10%           Nb   max. 0.10%           N   max. 0.015%           W   &gt;0.30-1.00%                      
 
     [0008] remainder iron and usual impurities.  
     [0009] Following the hot rolling and cooling of the pipes, they are reheated to a temperature above AC 3  and quenched to below 100° C. at a cooling rate of at least 15° C./s. Then, the pipes are tempered within the temperature range from 500 to 700° C., depending on the quality grade desired.  
     [0010] In many cases, for fixation of the nitrogen content, it is recommended to add up to 0.050% Ti to the steel alloy used. The tungsten content expediently lies in the range from 0.35 to 0.70%, particularly preferably in the range from 0.35 to 0.40%. It is recommended to set the vanadium content at at least 0.04%. A molybdenum content in the range from 0.05 to 0.40%, preferably in the range from 0.10 to 0.25%, has proven advantageous particularly for the higher quality grades.  
     [0011] The steel alloy which is to be used for the hot rolling according to the invention may perfectly well contain further accompanying substances, such as those which are used in particular for electric-furnace steelmaking, without its properties being impaired. Examples of such accompanying substances are copper, chromium and nickel. Expediently, the steel should contain at most 0.15% of each of these accompanying substances.  
     [0012] A line pipe which has been hot-rolled and quenched after reheating according to the invention can be set at any desired quality grade within the range from X 52 to X 90 by means of quenching and tempering. The lower the tempering temperature selected, the higher the strength characteristics which can be achieved. The toughness properties are improved by increasing the tempering temperatures. A line pipe which is produced according to the invention has a stable yield strength at least up to a temperature of use of 200° C., i.e. the reduction in yield strength is very low (&lt;10%). The stress-strain characteristic is essentially constant. The weldability, which is important for line pipes, is guaranteed. The carbon equivalent according to IIW can be set at relatively low levels. The molybdenum content can be limited to very low values or may even be zero. Since tungsten is less expensive than molybdenum, the alloy which is to be used according to the invention costs less to produce.  
     [0013] The important addition of tungsten to the alloy, which is the decisive factor for the invention, has produced a positive effect which is surprising to the person skilled in the art. This is to be illustrated below using an exemplary embodiment and a comparative example. The stress-strain characteristic of specimens of the two examples is illustrated in graphs in FIG. 1 (invention) and FIG. 2 (comparison).  
     [0014] Tests were carried out on test specimens with a thickness of 35 mm in each case, which had been rolled in a pilger-rolling mill train. The alloys used for the two examples are given in the following table:  
                                                       Element   Invention   Comparison                          C   0.13%   0.13%           Mn   1.30%   1.25%           Mo   0.15%   0.30%           V   0.05%   0.05%           Cr   0.10%   0.10%           W   0.35%   —           Ti   0.018%   0.018%           N   70 ppm   70 ppm                      
 
     [0015] For the steel used according to the invention, the carbon equivalent was determined to have the value TE IIW =0.42 or CE PCM =0.23. The carbon equivalent values for the comparison steel were 0.44 and 0.24, respectively. The alloy of the steel used according to the invention differs from the comparison alloy essentially only in that the molybdenum content is 0.15% lower, and an additional content of 0.35% tungsten is added instead. During the testing of the strength properties at a test temperature of 160° C., the yield strength of the steel used according to the invention fell by only approx. 5%. As can be seen from the stress-strain characteristic in FIG. 1, surprisingly the stress-strain curves at room temperature (RT) and at the test temperature of 160° C. coincide virtually completely beyond a plastic extension of approx. 0.7%. By comparison, the similar stress-strain diagram for the molybdenum-alloyed comparison steel which is illustrated in FIG. 2 reveals a very different behavior. In this case, the stress-strain curve at the test temperature of 160° C. lies significantly below the stress-strain curve at room temperature over the entire range tested. This stress-strain performance of the line pipe steel used according to the invention, which is comparatively much more advantageous, was completely unexpected.  
     [0016] At a tempering temperature of 670° C., the tested specimen of the steel according to the invention had a yield strength of R p0.2 =594 MPa, thus achieving the level of quality grade X 85. The use of higher tempering temperatures makes it possible to reduce the strength level, while lower temperatures increase the strength level. Within the limits of the alloying ranges according to the invention, it is possible to select alloys which, by means of appropriate quenching and tempering treatment, can produce the quality grade range from X 52 to X 90. In terms of the notched-impact strength at a test temperature of −30° C. (specimen position: center of sheet, transverse), the tested steel specimen according to the invention achieved a notched-impact energy value of 92 J/cm 2 , which is regarded as extremely good for the quality grade X 85. The weldability of the steel according to the invention can be classified as entirely satisfactory, and there is no evidence of the addition of tungsten to the alloy having any adverse effect.