METHOD OF MANUFACTURING A TUBULAR PRODUCT AND TUBULAR PRODUCT

Abstract

The present invention relates to a method for manufacturing a tubular product, characterized in that the tubular product is manufactured from steel comprising chromium in the range of 2.5 to 9.5 wt. % and silicon in an amount of more than 1.0 wt. %, and the method comprises the steps of austenitizing, quenching and tempering at a tempering temperature in the range of 300 C. to 550 C. Furthermore, the invention concerns a tubular product produced by this method.

Claims

1. Method of manufacturing a tubular product, characterized in that the tubular product is manufactured from steel comprising chromium in the range of 2.5 to 9.5 wt. % and silicon in an amount of more than 1.0 wt. %, and the method comprises the steps of austenitizing, quenching and tempering at a tempering temperature in the range of 300 C. to 550 C.

2. Method according to claim 1, characterized in that the tempering temperature is in the range of 350 C. to 450 C. and preferably at 400 C.

3. Method according to claim 1, characterized in that the steel consists, besides iron and unavoidable impurities caused by melting, of the following alloying elements in wt. %: TABLE-US-00005 C 0.05-0.3 Si 1.1-4 Mn 0.5-2.0 Cr 2.5-9.5 Al 0.01-0.1 and at least one of the following alloying elements in the specified ranges in wt. %: TABLE-US-00006 Nb 0.001-0.1 V 0.001-0.2 Ti 0.001-0.1 Mo 0.001-0.7.

4. Method according to claim 3, characterized in that the manganese content is in the range of 0.5-1.0 wt. %.

5. Method according to claim 1, characterized in that the silicon content is in the range of 1-4 wt. %, preferably 1.1-3 wt. % and further preferably in the range of 1.5-2 wt. %.

6. Method according to claim 1, characterized in that the chromium content is in the range of 2-8 wt. %, in particular in the range of 3-7 wt. %.

7. Method according to claim 3, characterized in that the aluminum content is about 0.02 wt. %.

8. Method according to claim 3, characterized in that the niobium content is 0.0175 wt. %.

9. Method according to claim 1, characterized in that the tubular product is tempered for less than 120 minutes, for less than 60 minutes, for less than 30 minutes or for more than 5 minutes.

10. Method according to claim 1, characterized in that the quenching is carried out with water or oil.

11. Tubular product, characterized in that it is manufactured from steel comprising chromium in the range of 2.5 to 9.5 wt. % and silicon in an amount of more than 1.0 wt. %, and the method of manufacturing comprises the steps of austenitizing, quenching and tempering at a tempering temperature in the range of 300 C. to 550 C.

12. Tubular product according to claim 11, characterized in that the ablation of the surface is uniform in the event of corrosion, in particular in the event of sweet gas corrosion.

13. Tubular product according to claim 11, characterized in that the tubular product has a yield strength of at least 550 MPa, preferably a maximum of 1,100 MPa.

14. Tubular product according to claim 11, characterized in that the tubular product is a seamless tubular product.

15. Tubular product according to claim 11, characterized in that the tubular product has a structure of tempered martensite with a maximum retained austenite content of 20%.

16. Tubular product according to claim 11, characterized in that the density of the carbides in the structure of the tubular product is below 10.sup.22 m.sup.3, preferably below 10.sup.21 m.sup.3.

17. Tubular product according to claim 11, characterized in that the mean size of the carbides in the structure of the tubular product is <20 nm and preferably >50% of the carbides have a size of less than 15 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] This invention will be explained in more detail below with reference to the enclosed figures, wherein:

[0037] FIG. 1: shows a schematic illustration of the process steps of an embodiment of the method according to the invention;

[0038] FIGS. 2 to 7: show TEM images of the structure of a tubular product according to the invention; and

[0039] FIG. 8: shows a schematic diagram showing the distribution of the size of carbides in the structure of a tubular product according to invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] As shown in FIG. 1, the first step is to heat the formed tubular product to a temperature above the Ac3 temperature of the steel alloy. Thereby, the structure is transformed into austenite. After austenitizing, the tubular product is quenched with water to a temperature below the martensite finish temperature (Mf). The tubular product is then heated to a temperature between 300 and 550 C. and tempered. The tempering duration can, for example, be 5 minutes.

[0041] As can be derived from the above, the silicon content in the steel from which the tubular product is made is adjusted so that the precipitation of cementite is effectively suppressed. The tempering of the steel is preferably carried out by the steps of austenitizing, quenching with water and tempering to a temperature below the formation temperature for special carbides. The alloy composition and the special heat treatment effectively suppress the formation of carbides.

[0042] With the present invention it is therefore possible to reduce the material ablation caused by corrosion, in particular highly localized corrosion in the form of pitting, without requiring the excessive use of expensive alloying elements, in particular chromium.

[0043] This invention has a number of advantages. By suppressing the carbides, the local chromium depletion of the steel can be effectively prevented. Pitting, in contrast to conventional, low-alloyed chromium steels, is only observed to a greatly reduced extent in this invention.

[0044] The carbide distribution in the structure of the tubular product according to the invention is characterized by an evenly distributed structure of very small carbides. By the alloy used according to the invention, the quantity and size of the carbides (special carbides, transition carbides, cementite) can be limited to a minimum. Relevant for the corrosion resistance are particularly chromium carbides, i.e. carbides that bind chromium, while niobium carbides, for example, do not significantly worsen the corrosion resistance.

[0045] As can be derived from FIGS. 2 to 5, which show transmission electron microscopy images, only smaller precipitates (see arrow) are present in the structure, which also exhibit low size-heterogeneity. These images also show the low density of the precipitates and their uniform size distribution.

[0046] FIG. 6, which is also a TEM image, shows a detailed image of the structure of a tube according to the invention. This image shows the martensite structure in the form of martensite lancets and the very small precipitates within the martensite lancets. At the boundaries of the lancets there are only a few precipitates. The precipitate marked with the arrow in FIG. 6 was identified as M.sub.2C carbide by electron diffraction.

[0047] FIG. 7 shows a further detail image of small precipitates of different size within the martensite lancets. The particle marked with the arrow in FIG. 7 was identified as M.sub.3C carbide by electrode diffraction.

[0048] The average particle sizes and the number of carbides determined from the images of FIGS. 2 to 7 are shown in Table 1 below:

TABLE-US-00004 TABLE 1 Statistical Parameters Figure analyzed D (10.sup.9 m) N.sub.V (10.sup.20 m.sup.3) FIG. 2 6 3 3.667 FIG. 3 7 4 5.905 FIG. 4 7 3 5.524 FIG. 5 8 4 4.000 FIG. 6 8 4 3.143 FIG. 7 6 3 12.381 Mean value 7 3 5.770 3.116

[0049] The relative size distribution is shown schematically in a diagram in FIG. 8. This diagram shows that the largest portion (>30%) has a particle size in the range of 6-7 nm. Particle sizes of more than 20 nm are only present for less than 5% of the particles.