Production of HSLA Seamless Steel Pipes for Offshore Structures and Line Pipes by Direct-quench and

KOBAYASHI,** Katsuo
Synopsis This report is concerned with the capability of manufacturing by directquenchand tempering of seamless steel pipesfor grades HT60 (TS> 60 kg/mm2) to HT 80 (TS> 80 kg/mm2) and API 5L X60 to U80. Althoughpipes of the strength level of these grades can be produced with low alloy steels, toughness of the direct-quenchedand tempered pipes has not beensatisfactory. A series of studies on the effectof alloyingadditives on the hardenability of steels have shown that titanium addition to the boron steelyields the maximum hardenability effect. To improve toughness,controlling the amountof titanium in the range of 0.008 wt% to 0.011 wt% has beenfound e ffective for the content of nitrogen between 25 ppm and 45 ppm. This range of nitrogen is determined to retain toughnessin the weldedjoint. The direct quenching,thus, enables theproduction of highstrength seamless steel pipes with markedly improved toughness in both pipes and weldedjoints, when the content of titanium in the boron steel is chosenwithin the optimumrange.
Production Structures
of HSLA and Line
Seamless Pipes
Steel
Pipes
for and
Offshore Tempering*
by Direct-quench
By Yoshimitsu IWASAKI,** Kunihiko UENO** and Yasue KOYAMA***
** *** 1-
Based on the paper presented at the 107th ISIJ Meeting, April 1984, S485, at Chiba Institute of Technology in Chiba. received on November 21, 1984; accepted in the final form on April 11, 1985. © 1985 ISIJ Chiba Laboratory, Research Laboratories, Kawasaki Steel Corporation, Kawasaki-cho, Handa 475. Chita Works, Kawasaki Steel Corporation, Kawasaki-cho, Handa 475. Hereafter, DQ and NQ are abbreviations of direct and normal quenchings , respectively.
seamless steel pipes with high strength and toughness in both pipes and their welded joints. II. Exporimental Procedure
I.
Introduction The exploitation of energy resources is now being developed even in such severe operating conditions as in the Arctic regions or deep seas. In these exploitations seamless steel pipes are employed as structural components of jackup rigs and line pipes for transporting production oil and gas. These structural steel pipes are keenly required to have the properties of both high strength and toughness in the low temperature environment. Quenching just after hot rolling is called direct quenching.t The method proposed since the late 1950's has been recognized as an effective way of strengthening steels and thus reducing material and processing cost. Deterioration in toughness and inhomogeneity of mechanical properties involved in the direct quenching have tended to hinder the application of the method to real plants. In recent years, one has overcome such difficulties in the field of heavy plate."2~ Seamless steel pipes of grades HT60 to HT80 and API 5L X60 to U80 (Table 1) have, however, been manufactured by another quenching method. This method is a conventional heat treatment, here called normal quenching,t consisting of cooling hot-worked steels in air, repeating these at austenitizing temperature and then quenching. Decrease in notch toughness brought about by the direct quenching prevents applying the method to the production of the seamless steel pipes. The present report deals with direct quenching capability of manufacturing
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Vol.Biblioteka 25,1985in the line of the medium diameter seamless pipe mill at Chita Works, Kawasaki Steel Corp., in December 1980 (Fig. 2).3~ The quenching process enables one to quench directly pipes of 177.8 mm (7 in.) to 425.5 mm (16 3/4 in.) OD, and 5.5 m (18 ft.) to 16.5 m (54 ft.) length. The maximum production rate of the DQ process is 150 t/h. A pipe immersed into water in the tank is supported on the lower part of a casing (Fig. 3), when water is further injected onto both inside and outside of the pipe in the longitudinal direction (Fig. 4). During water injection, the upper half of the casing is closed so as to keep injected water in axial laminar flow. Effective quenching is carried out in this way. III. Effect of Alloying Elements on Hardenability The hardenability of steel depends on the chemical composition, the condition of the austenite prior to quenching and the quenching process. In this
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1. Materials Low alloy steels of 0.1 wt% C were prepared (Table 2). The first five heats PI to R2 were melted for rigs by basic oxygen converter and a ladle refining furnace. Three heats P1 to P3 were used for preliminary study of the effect of alloying elements on hardenability. The last four heats L 1 to L4 were prepared for the use of line pipes as 5 t ingots by vacuum melting. Round billets forged from the cast ingots were heated up to 1 250 °C, held for 4 h and hot-rolled to the pipe dimension by the plug mill process (Fig. 1). In the direct quenching, hot-rolled pipes were reheated at 920 °C prior to cooling below the Ar3 point and then quenched into water from a temperature above the Ar3 point. NQ pipes were cooled in air, reheated at an austenitizing temperature and quenched under the same condition as the DQ pipes. Both DQ and NQ pipes were subsequently tempered over the temperature range of 550 to 690 °C for 15 min to determine the resistance to softening during tempering. 2. Direct QuenchingProcess An equipment of direct quenching was installed Table 1. Specification.