With the increases of pressure of the gas pipeline transportation, the mechanical property requirement of the pipeline is also improved. The high-performance pipeline which is needed in the API 5l steel pipe can ensure security in high pressure transfer, reducing the construction cost of the pipeline significantly. And at the same time, this can also improve the pipelines' economic benefits of operation. According to the statistics, once the grade is improved by one level, the construction cost will be reduced by 7%. But to improve the pipeline performance, we should know first what influences the performance of the line—that's the properties of the steel, which shapes different kinds of pipelines.
The alloy design of the API steel pipe is based on the alloyage of the C—Mn—Si, which has already been maturely applied to the production of steel API5LB and X42. Usually, we use single microalloy elements less than 0.065% or composite micro alloying method, and then, add a handful of alloy elements (Cu, Ni, Cr) according to the thickness of a steel plate and its rolling capacity. The major microalloying used in the production of API steel pipe is Nb. In order to get an even higher intensity, the V microalloying, as a booster action, is also widely used. Taking no account of the rolling technology, the organization pattern of the micro alloyed steel, the C—Mn—Si, is ferrite. This alloyage/organization results in the lowest manufacturing cost.
The high intensity steel X70 and the much higher ones of the alloying design are all based on the microalloying steel C—Mn—Si. And it's the same with the X65, which is designed to make up for the lack of the mill capacity. But at the same time, a small quantity of elements should be also added into it. Generally, the elements refers to Cu, Ni, Cr and so on (they can be used singly or in combination, but the total amount should be less than 0.6%). Then, in the process of design, we also need to add a spot of Mo (the maximum should be 0.3%). The alloyage of Mo, in combination with the proper rolling and cooling technology, can get the ferrite. In case of the lack of Mo, we can also get this element through adding Nb, which would not exceed the amount of 0.11%. This is called the HTP Technology for this kind of steel can be produced under a comparatively higher rolling temperature.
