STEEL

STEEL FOR WEIGHTED DRILL PIPE AND KELLYS OF STRENGTH GROUP "K"

Yu. V. Yakubovich, V. M. Khokhlov, and I. G. Pogorelova UDC 669.15'14'24-194.621.774

The Taganrog Metallurgical Plant makes weighted drill pipe (WDP) and kellys (KY) of strength group "K" out of silicon-manganese steel 36G2S. This steel contains 0.37-0.42% C, 1.7-1.9% Mn, 0.5-0.7% Si, and no more than 0.040% P and S.

In order to obtain a fine-grained structure and the required mechanical properties, WDP and KY, after rolling on a Pilger mill, are cooled on a rack until the inside surface no longer glows. They are then heated in a furnace to 890-920~ (WDP) and 900-950~ (KY). Uni- form heating of the pipes is assured by turning them at the proper times and observing the prescribed heating regime.

However, according to statistical data, 19% of samples of the resulting WDP and KY fail mechanical tests. Of these failures, 1.7% are for ultimate strength, the same percentage are for reduction of area, 8.6% are for impact toughness, and 6.9% are for reduction of area and impact toughness simultaneously.

The quality of steel 36G2S is affected by its content of manganese. In the present technology, the metal is deoxidized and alloyed in the ladle in order to reduce the consump- tion of scarce ferromanganese. As a result of poor averaging of the chemical composition, the manganese content of the ingots on the last stools (about 30 tons) often exceeds 1.9%o This is above the allowable maximum, and the ingot must be rejected~ When manganese content is higher than Io8~ there is a sudden change in the structure of the metal - a bainite component appears. This results in a sharp deterioration in the ductility and toughness properties. The yield point is reduced considerably when the metal has a manganese content below 1.6-1.65% and a carbon content below 0.37%, which necessitates reassignment of the metal to strength group "Do" Also, rejections and waste are high when pipes made of steel 36G2S are rolled: the level of rejection reaches 32% in some heats.

To determine the reasons for the low quality of the ppes and its dependence on chemical composition, we analyzed the mechanical properties of WDP and KY of strength group "K" by the method of mathematical statistics. Here, we analyzed data from 58 tests.

The analysis showed that the ranges of values for the contents of C, Mn, and Si exceed the prescribed composition range for this steel~ A large scatter was also seen for values of tensile strength, yield point, reduction of area, and impact toughness~

Study of the dependence of the mechanical properties on chemical composition showed that tensile strength and yield point depend only on the manganese content. The effect of chemi- cal composition on ductility characteristics and impact toughness is negligible.

The mechanical properties of pipes made of steel 36G2S is affected not so much by the deviation from the prescribed chemical composition of the steel as by productin factors - the steelmaking process, adherence to the prescribed rolling practice, and proper heating of the pipes before hot finishing.

Taking the above into consideration, we studied the feasibility of replacing steel 36G2S by chromium steel 40KhF, which contains 0.37%-0.42% C; 0.6-0.9% M_n; 0.17-0.37% Si; 0~ Cr; 0.08-0.16% V; not more than 0.040% P and S. Laboratory studies of the kinetics of grain growth showed that steel 40KhF is inherently more fine=grained and that grain growth occurs more smoothly in this steel than in steel 36G2S as temperature increases~ We determined the dependence of the mechanical properties of these steels on normalization temperature (Fig. I). It is evident from the graphs that steel 40KhF retains the required mechanical proper- ties within a broader range of normalization temperatures than does steel 36G2S. The duc-

Taganrog Metallurgical Plant. Translated from Metallurg, No. I0, pp. 28-29, October, 1987.

336 0026-0894/87/0910-0336512.50 �9 Plenum Publishing Corporation

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tility and toughness properties of steel 40KhF are also higher than those of steel 36G2S, while the strength properties are roughly the same.

The mechanical properties of pipes roiled from heats of steel 40KhF meet the specifica- tions for strength group "K". Also, rejects and waste are reduced by 15%, while the consump- tion coefficient is reduced by 0.2 and 0oli for WDP and KY, respectively.

The savings realized from the replacement of steel 36G2S by steel 40KhF have amounted to more than 53,000 rubles. Moreover, ferromanganese consumption has been cut by a factor of 2.5.

Thus, steel 40KhF can be recommended for the production of weighted drill pipe and kellys at other metallurgical plants and pipe mills.

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