Companhia Siderúrgica Paulista - COSIPA, Brazil
The Brazilian demand for API linepipes with relative small diameter (300 to 500 mm) is becoming stronger. Skelp used to forming these tubes must be narrow, that is, its width range varies from 950 to 1,600 mm. The use of plate rolling to make this skelp is unfeasible, as these width values make their production economically unfeasible; normally this application requires material produced in hot strip mills.
There is a chance to produce such skelp in plate mills using "multi-width" plates; that is, the width of the rolling stock processed in the mill is a multiple (double or triple) of the final product width. After rolling, the plate is longitudinally cut and two or three skelps with the requested width are got.
However, there are some metallurgical questions to be previously answered before routinely using this processing route. Normally plates are produced from continuously cast slabs, which solidification process produces segregation of some elements normally present in steel, mainly C, Mn, P and S, which concentrate in the middle portion of the slab. This segregation is inherited by the subsequent plate. So, in a double width plate, the longitudinal cut will be done exactly in the maximum segregation region. The higher C and Mn content in this portion of the skelp can be potentially harmful to the longitudinal welding process used to make the pipes, as steel with increased equivalent carbon is more prone to cracking and other welding problems. Another problem to be considered is the difference of plate crown values between the middle and the border of the skelp. In the case of triple width plates, these problems are somewhat less critical, as the longitudinal cuts did not coincide with the most critical region in terms of segregation and maximum plate crown.
No literature references were found about the metallurgical characterisation of double or triple width plates. This fact motivated the development of a research project at COSIPA with the objective to characterise quantitatively chemical segregation, microstructure and mechanical properties differences between skelps extracted from triple width plates, in order to check product consistency.
The material studied in this project was an API 5L X-60, Nb-Ti microalloyed steel. The mother plate had width of 3000 mm; it was cut longitudinally, yielding three skelps with width equal to 980 mm. About 126 skelps, produced from 42 plates, were analysed in terms of chemical composition (C, Mn, Si, P, S, Nb, Ti, Al and Ca), microstructure (global, ferritic and perlitic grain size, pearlite volumetric fraction and ferritic mean free path) and mechanical properties (yield and tensile strength, yield ratio, 5 kg Vickers hardness and energy absorbed during Charpy impact test carried out at -40, -30, -20, -10, 0 and +10oC).
The results of the Variance Analysis (ANOVA) and Test of Means (Student) revealed that no significant statistical differences were observed in the studied skelps, considering all metallurgical parameters determined here. All skelps satisfied the requirements of the API 5L-X60 standard and performed very well at the customer.
Hence, the use of this production route was approved, as it does not imply in quality restrictions in the product. Besides that, the use of triple width plates multiplied the productivity of the plate mill by 2.5 fold and increased metallic yield by 1.6% in relation to the traditional processing route of single width plates. These results make the use of plate mill feasible for this narrow product.
|Last Update: 14 August 1997|
|© Antonio Augusto Gorni|