EP1204772B1 - Method for producing welded steel pipes with a high degree of strength, ductility and deformability - Google Patents

Description

The invention relates to a method for the production of welded steel pipes of high strength, toughness and deformation properties, in particular large pipes according to the UOE method, in which, starting from a hot-rolled sheet, cold formed a tube, welded and calibrated to nominal diameter and after welding and calibration a heat treatment at a temperature in the range of 100-400 ° C is subjected.

By cold forming, z. B. produced by the UOE process tubes require yield strengths in the amount of the specified minimum value in order to reliably meet the required safety against flow on the finished tube.

For pipes made of high-strength steels with a yield strength R _{t 0.5} ≥ 550 MPa (X80 according to API-5L) these requirements due to the simultaneously required toughness and deformation properties in practice only with comparatively high Ausgangssstreckgrenzenverhältnis represented so that compliance with current regulations maximum permissible yield strength ratios eg max. 0.93 to API5L as a result of work hardening during molding and calibration of the tubes in mass production is difficult or can be accomplished only with increased technical complexity and correspondingly high production costs. In addition, the integral deformation reserve by the cold forming decreases as a result of high Ausgangssstreckgrenzenverhältnisse with increasing quality, so that in practice the required on the component integral deformation reserve ε _up ≥ 2% in the context of the usual scattering of steel pipes with a yield strength R _{t 0 , 5} ≥ 550 MPa (X80) was barely achievable and could not be reached on tubes made of steel with a yield strength R _{t 0.5} ≥ 620 Mpa (X90). By "integral deformation reserve ε _up " is meant the mean circumferential plastic elongation of the pipe before the wall constriction commences analogously to the uniform elongation in the laboratory tensile test ( Hohl, GA and Vogt, GH: Allowable strains for high strength line pipe. 3R international, 31st Century, Issue 12/92, pp. 696-700 ).

To overcome this problem, it has been considered in the past by changing the alloy composition and / or the rolling technique to achieve the required higher deformation characteristics. However, these possibilities are limited in practice because, on the one hand, certain additions, such as e.g. Nickel significantly increase the cost of the product or the addition of which poses deformation problems such as e.g. Boron and on the other the technology of thermomechanical rolling with respect to the temperature window to be set, the cooling rates and the degree of deformation is limited changeable.

From 196 10 675 C1 a known under the name "bake hardening" method for increasing the component strength is known. This is understood to mean artificial aging as a result of baking varnishing. The coating is preferably carried out in a zinc bath which is run through by the previously cold-rolled strip. The zinc bath temperatures are in the range of 450-470 ° C. So that the surface refinement of conventional DP (dual-phase) steels is reliably possible, a steel of the following composition is proposed in% by weight
0.05 to 0.3% carbon
0.8 to 3.0% manganese
0.4 to 2.5% aluminum
0.01 to 0.2% silicon

Remaining iron with impurities caused by melting. After the cold rolling, a heat treatment preferably follows in a hot-dip galvanizing plant or in a continuous annealing furnace.

The structure consists of a ferritic matrix in which martensite is embedded in the form of a honeycomb. The minimum parameters that can be achieved with the known method _Yield strength (R _p0.2 ) ≥ 200 MPa Tensile strength (R _m ) ≥ 550 MPa Elongation at break (A ₈₀ ) ≥ 25% Yield strength (R _p0.2 / R _m) ≤ 0.7

The main elements favoring the proposed process are aluminum and silicon. The latter element Si is kept low to suppress the formation of red scale during hot rolling. Red tinder carries the danger of scale rolling, which leads to surface inhomogeneities when the strip is pickled. High Al contents promote the formation of the fenite during annealing between the transition temperatures A _C1 and A _C3 . The formation of pearlite is postponed to significantly longer times, so that it can be suppressed at the realizable cooling rates. The adhesion conditions of both the zinc layer and the zinc-iron alloy layer are improved by Al.

The known method is for welded pipes of high-strength steels, e.g. the grade X80 with a minimum yield strength of 550 MPa is not applicable, because a heat treatment in the temperature range of 450 - 470 ° C is uneconomical because of the long warm-up and hold times. The yield strength of these high-strength steels is, for example,> 0.70 for a grade X65, otherwise in the range between 0.80 and 0.93.

From the JP-B 61-44123 and JP-B 60-26809 For example, a method of producing a high-strength X80 grade steel (API standard) having excellent low-temperature toughness is known. In this known method, a steel with the elements C, Si, Mn, P, S, Nb and Al, remainder iron and process-related impurities is melted and cast a slab in the strand. By a TM-rolling the slab is transformed into a hot-rolled sheet and this too molded a slot tube. After welding and calibration, the tube thus produced is subjected to a heat treatment in the range of 100-400 ° C with a holding time of between 0.5-120 minutes.

As essential to the invention it is emphasized that in order to increase the low-temperature toughness, the total residence time between the first rolling sequence and the second rolling sequence should be in the range of ≦ 60 seconds.

The object of the invention is to provide a method for producing welded steel pipes of high strength, toughness and deformation properties in particular large pipes according to the UOE method, with the qualities ≥ X90 with a minimum yield strength of 620 MPa and sour gas-resistant grades economically and process-safe in compliance with the rules fixed upper limit for the yield ratio can be represented.

This object is achieved with the features in claim 1. Advantageous developments are the subject of dependent claims.

According to the proposed solution is based on a sheet of a steel of the composition in wt.%
0.02 to 0.20% carbon
0.05 to 0.50% silicon
0.50 to 2.50% manganese
0.003 to 0.06% aluminum

Remaining iron with impurities caused by melting subjected to the tube after welding and calibration of a post-heat treatment in the temperature range of 100-300 degrees Celsius and a holding time adapted to the pipe wall thickness with subsequent cooling in air or forced cooling. The holding time is mainly dependent on the product wall thickness to be heated and depends on the type of heat input. This means that the holding times can be only seconds in one extreme case and several hours in the other extreme case. The tube produced in this way has more than twice as high deformation reserves with the same high strength compared to conventionally manufactured products, without exceeding the upper limit for the yield ratio determined by the current regulations. Optimum results are achieved if the minimum yield strength limit at the sheet corresponds to the minimum yield strength at the pipe which is reduced by the yield strength increase due to cold forming and heat effect. A pipe produced in this way is characterized by resistance to aging and particularly high homogeneity of the properties at the circumference of the pipe, whereby the steel analysis given with regard to the main elements covers the range of high-strength large-diameter steel steels. According to a further feature of the invention, it is optionally possible to add further elements up to the specified maximum limit meet special requirements with regard to the mechanical characteristics as a function of the product wall thickness.

Investigations have shown that with the proposed heat treatment, the mechanical material properties, in particular the yield strength, are increased to an extent so that the required minimum values are reliably achieved. By process is meant that the increase means a reserve, which allows the usual variations in alloy composition, wall thickness, rolling parameters, etc., without running the risk to fall below the required minimum value even at the meeting of several unfavorable parameters. The otherwise customary special measures can be omitted.

Another advantage is the fact that conditioned by such a heat treatment tubes at operating temperature below the heat treatment temperature, for. B. 200 degrees Celsius, behave as aging resistant, so that no further changes in the mechanical properties are expected for lines from such pipes during the operational life. Naturally, this statement also applies to tubes made of steel grades <X80 whose properties on the circumference and in the production series can be adjusted by means of such a heat treatment with greater process reliability and smaller scattering.

The heat treatment can be carried out in a continuous furnace or during the passage of an induction coil. The latter method is preferably integrated into a pipe external insulation system. This means that the heating of the tube required for the application of the single-or multi-layer insulation can be used simultaneously to increase the strength properties to the required level, since the temperature required for the insulation is in the proposed range of 100-300 degrees Celsius ,

The advantage is that the determined in the acceptance test after isolation strength and deformation properties thus for the entire useful life of a Piping are authoritative. The use of sheets and tapes with low output yield strength also appears to be advantageous in the way in that smaller forming forces are required for forming the slot pipe. This advantage is particularly important in thick-walled pipes.

Another advantage of the proposed heat treatment is the fact that it contributes to the reproducible representation of the yield ratio at a low level and a homogenization of the strength properties in the production series, so that compared to conventionally produced pipes on the component higher deformation reserves against ductile breakage can be achieved.

The effect of a homogenization of the strength properties can be further increased if, in the case of the large pipes produced by the UOE process before the heat treatment, a conditioning of the tubes according to the in the DE 195 22 790 A1 proposed method. The tube properties which can be represented thereby quite purposefully depending on the application for internal or external pressure loading bring in conjunction with the proposed here after heat treatment in terms of dispersion of the values at the tube circumference and from tube to tube and with respect to the potentionell representable on the component deformation reserve the best results.

The proposed method is applicable to longitudinally welded and helically welded tubes (also called spiral tubes) according to the HFI and after the UOE method.

For example, to produce a tube with 56 "outer diameter and 19.1 mm wall of X100 steel according to the usual procedure, a 2.0% proof stress of Rp2.0 ≥ 710 MPa and a tensile strength of Rm ≥ 770 MPa are used on the plate Strength properties are determined by the initial values on the sheet and the work hardening when forming and calibrating the tubes to nominal diameter, yield strength ratios are achieved on the finished pipe, which is a constraint for the deformation capacity of the internal pressure-loaded component represent. As a result, the integral elongation usually required at ε _up ≥ 2% on high-strength pipes by conventional methods has hitherto scarcely been achieved in practice or can not be achieved with sufficient confidence.

• C 0,096; Si 0,383; Mn 1,95; Al 0,035; P 0,015;Ti 0,019; Cr 0,062;
• Mo 0,011; Ni 0,045; Nb 0,042; V 0,005; Cu 0,045; N 0,005; B 0,001.

In order to produce a pipe of the same grade and dimension according to a new process, only a 2.0% proof stress of Rp2.0 ≥ 640 MPa instead of ≥ 710 MPa and a tensile strength of Rm ≥ 770 MPa are required, in particular the yield strength as a function of the analysis of the steel grade used and the degree of deformation in the transformation from the sheet to the tube by the specified value varies. For example, the steel grade used has the following analysis in% by weight:

• C 0.096; Si 0.383; Mn 1.95; Al 0.035; P 0.015; Ti 0.019; Cr 0.062;
• Mo 0.011; Ni 0.045; Nb 0.042; V 0.005; Cu 0.045; N 0.005; B 0.001.

Since the strength properties required in the circumferential direction are simultaneously achieved by the heat after-treatment of the tube, lower initial values of the yield strengths and yield strength ratios are sufficient to produce the specified tube quality, thereby increasing the uniform elongation to values of Ag ≥ 8.5% on the sheet and to values of Ag ≥ 6.5 % on the pipe is possible. In comparison with conventionally produced tubes, a twice as high deformation capacity can be achieved, so that the necessary conditions for a production-reliable representation of the integral component reserve ε _up ≥ 2% within the production-related scattering can be reliably fulfilled even for tube grades of X 100.

The extent of the achievable by the heat post-treatment in the pipe circumferential direction increases in Rt0.5 Dehngrenzen depends on the steel composition, the C and N shares in forced solution and the parameters of the tube manufacturing process and is based on the current state of knowledge up to 18% of the expanded Tube to round tensile specimens proved Rt0.5 proof stress. For unexpanded pipes such. B. HFI pipes will be increases of up to 12% according to previous experience. The tensile strengths Rm increase by the heat aftertreatment by about 20 MPa.

Claims (5)

1. Method of manufacturing welded steel pipes with a high degree of strength, toughness and deformability, in particular large-diameter pipes according to the UOE method, in which starting from a hot-rolled plate a pipe is cold-formed, welded and calibrated to the desired diameter and is subjected, after welding and calibrating, to heat treatment at a temperature in the region of 100-400°C, wherein starting from a TM-rolled plate composed of steel comprising (in % by weight):

   0.02 to 0.20 % C

   0.05 to 0.50 % Si

   0.50 to 2.50 % Mn

   0.003 to 0.06 % Al

   and optionally

   up to 0.02 % P

   up to 0.06 % Ti

   up to 0.20 % Cr

   up to 0.50 % Mo

   up to 0.30 % Ni

   up to 0.10 Nb

   up to 0.08 % V

   up to 0.50 % Cu

   up to 0.030 % N

   up to 0.005 % B

   remainder iron with melt-dependent impurities, heat treatment is carried out for the pipe with the quality ≥ X90 (API standard) at a temperature in the range of 100-300°C and with a holding time adapted to the pipe-wall thickness followed by cooling in air or by forced cooling, and the pipe so produced is ageing-resistant and whilst having the same degree of strength has a sufficiently integral deformation reserve against rupture, without exceeding the upper limit for the yield point ratio fixed according to current regulations for conventional steels, wherein the minimum starting yield point in the plate corresponds to the minimum yield point on the pipe reduced by the rise in yield point by cold forming and heat treatment, heat treatment being carried out within the context of applying a single- or multiple-ply outer insulation.
2. Method according to claim 1, characterised in that the heat treatment takes place in a conveyor furnace.
3. Method according to claims 1 to 2, characterised in that the heat treatment takes place upon passing through an induction coil.
4. Method according to one of claims 1 to 3, characterised in that during the manufacture of large-diameter pipes according to the UOE method, the pipes having a longitudinal welded seam are pre-conditioned before the heat treatment by a combined application of cold expansion and cold reduction.
5. Method according to claim 4, characterised in that the sequence and degree of expansion or reduction is established according to the requirement profile.