JPH0233773B2 - YUSEIYOATSUPUSETSUTOKOKANNOSEIZOHO - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) This invention relates to a method for manufacturing high-strength oil well upset steel pipes. (Prior art) High-strength oil well upset steel pipes are manufactured by subjecting the upset portion of the end of the raw pipe to high temperatures (e.g., over 1200°C).
It is manufactured by heating it to a temperature of 100°C for upset processing, and then performing heat treatments such as normalizing, normalizing-tempering, or quenching-tempering. This conventional method has the disadvantage of being expensive in terms of energy because reheating is required to reach the austenite region, and that the upset steel pipe has portions with different wall thicknesses, resulting in large distortions during heating and cooling. Further, when hardening is performed, special hardening equipment is required to uniformly harden parts of different wall thicknesses. (Objective of the Invention) The present invention avoids the drawbacks of these conventional methods and provides a manufacturing method for obtaining a high-strength oil well upset steel pipe with uniform material quality only by tempering at a low temperature that does not cause thermal distortion after upsetting. It is. (Structure of the Invention) The present invention will be described in detail below. The composition range of the steel according to the present invention ranges from an austenite region to a bainitic structure when air-cooled. Here, the composition of the steel of the present invention is shown. When C exceeds 0.30wt%, toughness deteriorates, so
Add at 0.30wt% or less. Si is added for deoxidation or strength adjustment, but 0.1wt% or more is required for deoxidation, and since embrittlement occurs if it exceeds 1wt%, it is set at 0.10 to 1.00wt%. Since Mn is air-cooled to form a bainitic structure,
1.5wt% or more is required, and if it exceeds 3.0wt%, embrittlement occurs, so it is set at 1.5 to 3.0wt%. Al is in the amount required for deoxidation, i.e. 0.005 ~
Add 0.10wt%. Nb is added to form a bainitic structure by air cooling, but for this purpose, it is necessary to use more than 0.01wt%, and even if it exceeds 0.15wt%, there is no improvement in the effect.
The content should be 0.01-0.15wt%. Cr, Ni, Mo, B, and Cu are within the effective range for bainite structure generation, that is, Cr: 1.0wt% or less, Ni;
2.0wt% or less, Mo; 0.5wt% or less, B; 0.002wt%
Hereinafter, Cu; 1.0wt% or less, and V and Ti within effective ranges for grain refinement, that is, Vt; 0.15wt% or less, Ti;
One or more types are added at 0.10wt% or less. As a result of researching the tempering resistance of bainite structure, the present inventor found that bainite structure has higher tempering resistance than martensite, that is, it does not soften easily. Utilizing this bainite structure with high tempering resistance, a homogeneous upset steel pipe can be manufactured by simply performing low-temperature tempering treatment after upsetting. Upset processing is performed by heating the tube end to a high temperature (over 1200℃), but when looking at the temperature distribution throughout the tube, the temperature ranges from 1200℃ at the tube end to room temperature. Therefore, after the upsetting process is performed, changes in material quality, especially changes in strength, occur mainly in response to temperature distribution. Therefore, in the conventional method, it is necessary to reheat the material to the austenite region after upsetting. In the steel of the present invention, which has a composition that becomes a bainite structure by air cooling from an austenite region, the structure of the raw tube before upset, that is, the structure in the hot rolled state still exhibits a bainite structure. Figure 1 shows how this changes depending on the temperature history during upsetting. Figure 1 shows the approximately 500 mm end of a steel pipe with a wall thickness of 8.0 mm and an outer diameter of 75 mm that has the chemical composition (wt%) shown in Table 1.
The pipe is heated to 1250℃ and the length of the tube end is
FIG. 3 is a diagram showing the strength distribution in the longitudinal direction of a 110 mm steel pipe with a wall thickness of 14 mm and an outer diameter of 85 mm and air-cooled, in correspondence with the maximum temperature reached during upset processing.

[Table] Parts heated from room temperature to the _A1 transformation point correspond to tempering treatment. Therefore, the tensile strength is lower than as hot rolled, but the yield strength is higher. In the part heated from the A ₁ transformation point to the A ₃ transformation point, during heating the raw tube retains its original bainite structure and partially transforms into austenite, and after cooling, the part transformed into austenite becomes concentrated. The transformation temperature is lower than that of the base material.
That is, it transforms into hard bainite or martensite. Therefore, the tensile strength is higher than that of uniform bainite. However, the yield strength decreases due to residual stress due to partial transformation. A All parts heated above _{the 3} transformation point transform into austenite during heating, and after cooling transform into uniform bainite, the same as after hot rolling. The strength changes depending on the austenite grain size and carbide solid solubility during heating, but since the structure is uniform bainite, the strength is almost the same as that of the hot-rolled pipe, that is, the raw pipe. Now, the changes when low-temperature tempering is applied to a tube having such an intensity distribution are shown in FIG. 2. FIG. 2 is a diagram showing the strength distribution in the longitudinal direction after upsetting was performed under the same conditions as in FIG. 1, and after air cooling and tempering at 45° C. for 15 minutes, the strength distribution in the longitudinal direction corresponds to the maximum temperature reached during upsetting. In the part heated during upsetting from room temperature to the _A1 transformation point, the part below this tempering temperature has undergone low-temperature tempering, and the part from the tempering temperature to the _A1 transformation point is only slightly tempered. It is. As mentioned above, due to the high tempering resistance of the bainite tube, there is no significant change in strength, but there is a slight decrease in tensile strength and an increase in yield strength. In the part heated between A ₁ and A ₃ transformation point,
Since the residual stress caused by the partial transformation is released by tempering, the yield strength increases significantly and the tensile strength decreases. When the _A3 transformation point is higher than that, the tempering behavior is the same as that of the tempering of the bainite structure after hot rolling, and the yield strength increases and the tensile strength slightly decreases. (Example) Next, an example of the present invention will be shown. Examples A, B, C, and D in Table 2 are examples of the present invention, and although the variation in yield strength is inferior to E in the conventional method, the variation range is sufficient for practical use.

【table】

[Table] Wall thickness of setup section Expansion rate =

Claims (1)

[Claims] 1 By weight, C≦0.30%, Si: 0.10-1.00%,
Mn: 1.5-3.0%, Nb: 0.01-0.15%, Al: 0.005
~0.10%, additionally Cr≦1.0%, Ni≦2.0%, Mo
≦0.5%, B≦0.002%, Cu≦1.0%, V≦0.15%,
A steel tube containing one or more Ti≦0.10%, the balance consisting of Fe and unavoidable impurities, is hot upset and then tempered in a temperature range of 500℃ or less. A manufacturing method for oil well setup steel pipes.