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| Steel for steel pipes |
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After casting, the round billets were formed into seamless steel pipe with pipe-forming by a piercing mill, hot rolling and size-adjusting by a mandrel mill and a stretch reducer. The chemical compositions of the obtained steel pipes were analyzed and, after polishing a cross section perpendicular to the longitudinal direction, the (Ca %)/(Al %) and the (Ca %)/(Ti %) in the inclusions were measured by an energy dispersiveX-ray spectrometer (EDX), and the mean value therefor was determined based on the analytical values of the inclusions by the number of 20. The chemical compositions of the steel pipes, the (Ca %)/(Al %) and the (Ca %)/(Al %) in the inclusions were shown in Table 1. After heating at 920 degrees C., the steel pipes were quenched, and then, they were prepared into the steel pipes having a yield strength of 758 MPa or more corresponding to "110 ksi class" and the steel pipes having a yield strength of 861 MPaor more corresponding to "125 ksi class" by controlling a tempering temperature. For the steel pipes confirmed for strength and hardness after applying the heat treatment, a SSC resistance test was conducted by sampling the tensile test pieces, each being a round bar of 6.35 mm diameter in parallel with the longitudinaldirection of the steel pipe. That is to say, the "110 ksi class" (having a yield strength of 758 to 861 MPa) was evaluated in 0.5% acetic acid 5% saline at 25 degrees C. saturated with hydrogen sulfide at 101325 Pa (1 atm), and the "125 ksi class"(having a yield strength of 861 to 965 MPa) was evaluated in 0.5% acetic acid 5% saline at 25 degrees C saturated with gas at 101325 Pa (1 atm) comprising gaseous carbon dioxide and a residue of 10132.5 Pa (0.1 atm) of hydrogen sulfide, according to themethod of NACE-TM-0177-A-96 method, applying a 90% load for the actual yield strength and keeping for 720 hours respectively, in order to test the absence or presence of fracture. For the HIC resistance, a steel pipe controlled to a strength of "110 ksi class" was used, from which test pieces each having 10 mm thickness, 20 mm width and 100 mm length were sampled in parallel with the longitudinal direction. The testpieces were dipped in 0.5% acetic acid 5% saline at 25 degrees C saturated with hydrogen sulfide at 101325 Pa (1 atm), with no stress for 96 hours, and the occurrence of hydrogen induced cracking was investigated. Table 3 shows the result of evaluation for the SSC resistance and the HIC resistance of the steel pipes using the steels shown in Table 1. As apparent from the results, it can be seen that steels A to E and G to K according to the presentinvention cause no crackings in the SSC test and the HIC test and have excellent corrosion resistance. On the other hand, in the steels M, N, P to R and T to X, the (Ca %)/(Al %) in the inclusions is less than 0.55 or more than 1.72, and those steelpipes are poor in the SSC resistance and the HIC resistance because of the out of appropriate compositions of the inclusions. Furthermore, in the steels O, Q, S and U to W, the (Ca %)I(Ti %) in the inclusions is less than 0.7 or more than 19, and so agreat amount of TiN inclusions were formed and therefore those steel pipes are poor in the SSC resistance. |