Method for Producing a Wear-Resistant Steel Pipe, Wear-Resistant Steel Pipe, and Use of Such a Steel Pipe

Abstract

A process for the industrial production of wear-resistant steel pipes having an optimized life. The process includes providing a wear-resistant, hardenable steel sheet in an unhardened or tempered state, shaping the steel sheet into a tubular preform in which two longitudinal edges of the steel sheet are positioned opposite one another with a welding gap extending between the two edges, welding the longitudinal edges by forming a welded seam which closes the welding gap, thereby forming a steel pipe, and heat treating the steel pipe. The heat treatment of the steel pipe includes heating the steel pipe at an average heating rate of 5-400 K/s to a hold temperature which is the Ac3 temperature of the steel and 1100 C., holding the steel pipe at the hold temperature for 1-120 s, and cooling the steel pipe at an average cooling rate of 10-600 K/s to room temperature.

Claims

1.-15. (canceled)

16. A steel pipe having a diameter of at least 200 mm, a wall thickness of at least 15 mm and having a welded seam which extends linearly in the longitudinal direction of the steel pipe or runs helically around the longitudinal axis of the steel pipe, wherein the steel pipe is formed from a steel sheet comprising (in % by weight): C: 0.2-0.4%, Si: 0.6-0.9%, Mn: 1.0-2.0% optionally, one element or a plurality of elements selected from the group consisting of Cr, Mo, Ti, Ni, B, where Cr: 1.0-2.0%, Ti: up to 0.04%, Mo: 0.3-0.7%, Ni: up to 2.0%, and B: up to 0.004%, and iron and unavoidable impurities as the remainder and the difference between a hardness of a heat affected zone (HAZ) adjoining the welded seam of the steel pipe and a hardness of the steel sheet outside of the heat affected zone is not more than 30 HV10.

17. The steel pipe according to claim 16, wherein the wall thickness is at least 40 mm.

18. A process for producing a steel pipe according to claim 16, comprising the following working steps: a) providing an at least 15 mm thick steel sheet which comprises a wear-resistant, hardenable steel, wherein the steel sheet is provided in an unhardened or tempered state and wherein the steel sheet comprises (in % by weight): C: 0.2-0.4%, Si: 0.1-0.9%, Mn: 1.0-2.0%, S: up to 0.03%, P: up to 0.04%, optionally, one element or a plurality of elements selected from the group consisting of Cr, Mo, Ni, Ti, B, wherein Cr: 0.1-2.0%, Mo: 0.3-0.7%, Ti: up to 0.04%, Ni: up to 2.0%, and B: up to 0.004%, and iron and unavoidable impurities as the remainder; b) shaping of the steel sheet into a tubular preform in which two longitudinal edges of the steel sheet are positioned opposite one another with a welding gap extending between the two edges; c) welding of the longitudinal edges which are arranged opposite one another by forming a welded seam which closes the welding gap, thereby forming a steel pipe; d) heat treating the steel pipe, wherein the heat treatment comprises the following working steps: d.1) heating the steel pipe at an average heating rate of 5-400 K/s to a hold temperature which is at least equal to the Ac3 temperature of the steel and is not more than 1100 C.; d.2) holding the steel pipe at the hold temperature for 1-120 s; and d.3) cooling the steel pipe at an average cooling rate of 10-600 K/s to room temperature.

19. The process according to claim 18, wherein, in working step b), the steel pipe is formed in at least two working substeps.

20. The process according to claim 18, wherein the steel sheet is provided in working step a) as a cut-to-size sheet with a width that corresponds to a circumferential length of the steel pipe to be produced and a length that corresponds to a length of the steel pipe to be produced.

21. The process according to claim 20, wherein a preform having a U-shaped cross section is formed from the steel sheet in the first working substep and a preform having a circular or ellipsoidal cross section is formed from the U-shaped preform in the second working substep.

22. The process according to claim 18, wherein the steel sheet is provided in working step a) as strip section having a width which is smaller than a circumferential length of the steel pipe to be produced and having a length which is greater than a length of the steel pipe to be produced and wherein the steel sheet is wound following a screw line in working step b) into the tubular preform.

23. The process according to claim 18, wherein the shaping of the steel sheet in the working step b) into the preform is carried out as hotforming in at least one working substep.

24. The process according to claim 18, wherein, in working substep d.1), the steel pipe is heated by inductive heating.

25. The process according to claim 18, wherein, in working substep d.1), the steel pipe is heated by conductive heating

26. The process according to claim 18, wherein in working substep d.1), heating is carried out with continuous passage.

27. The process according to claim 18, wherein, in working substep d.1), the steel pipe is introduced into a furnace and heated to the hold temperature in.

28. A transportation passage for bulk material, fluids, or mixtures thereof comprising a steel pipe according to claim 16.

Description

[0096] The invention will be illustrated below with the aid of a working example. In the figures:

[0097] FIG. 1 shows a frontal view of a longitudinal-seam-welded steel pipe produced according to the invention;

[0098] FIG. 2a shows the course of the hardness in the region of the longitudinal welded seam of the steel pipe of FIG. 1 after welding and before the heat treatment;

[0099] FIG. 2b shows the course of the hardness in the region of the longitudinal welded seam of the steel pipe of FIG. 1 after the heat treatment;

[0100] FIG. 2c shows a section of FIG. 1.

[0101] The steel pipe 1 having a circular cross section and an external diameter D of 800 mm which is shown in FIG. 1 has been produced from a cut-to-size sheet having a thickness d of 20 mm, the width of which corresponds to the circumferential length of the steel pipe 1 and the length of which corresponds to the length of the steel pipe 1 to be produced.

[0102] The steel sheet 2 consisted of a steel having the composition shown in Table 1.

TABLE-US-00001 TABLE 1 Figures in % by weight: remainder iron and unavoidable impurities C Si Mn P S Cr B Ti Mo Ni 0.3 0.25 1.3 0.02 0.01 0.3 0.0025 0.032 0.05 0.05

[0103] The steel sheet 2 which has this composition and is provided in the unhardened delivery state has been formed in a manner known per se in individual manufacture firstly into a preform configured as a slit pipe, in which preform the longitudinal edges of the steel sheet 2 were arranged opposite one another and between them bound a welding gap extending over the length of the steel pipe 1.

[0104] The preform was subsequently welded by the welding gap being closed in a manner known per se by means of underpowder welding method to form a longitudinal welded seam 3 extending over the length of the steel pipe 1. As a result of the welding and the introduction of heat associated therewith, hardening effects have occurred in the heat influence zones HAZ extending over the longitudinal edge regions of the steel sheet 2 which laterally adjoin the welded seam 3, due to which hardening effects the hardness of the steel sheet 2 in the heat influence zones HAZ was higher than in the regions 4 of the steel sheet 2 which were located outside these zones HAZ and were uninfluenced by the welding heat introduced (FIG. 2a).

[0105] After welding, the steel pipe 1 was heated by means of inductive heating at a heating rate of 9 K/s to a hold temperature of 930 C. at which it was held for 20 seconds in order to achieve reliable heating all through.

[0106] After the hold time, the steel pipe 1 was cooled at a cooling rate of 30 K/s to room temperature (25 C.).

[0107] The hardness HV10 of the thus heat-treated steel pipe 1 was measured in accordance with DIN EN ISO 6507-1:2006-03 in agreement with the procedure set down in DIN EN ISO 3183:2012 in the heat influence zones HAZ and the regions 4 of the steel pipe located outside. The hardness indentations were arranged 1.5 mm below the surface. The hardness profile determined in this way is depicted in FIG. 2b. It is found that the absolute value of the difference between the hardness in the outer regions 4 and the hardness in the heat influence zones HAZ was not more than 20 HV10.