Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• the invention relates to the use of a steel alloy according to the features of patent claim 1.
• the state of the art for steel pipes with increased strength can be described by micro-alloyed fine grain steels with ferritic-pearlitic structure, for example steel StE 460.
• this steel achieves breaking strengths of 650 - 750 MPa and elongation at break of approximately 20 - 25%.
• the product of strength and elongation at break is usually about 16,000 - 18,000 [MPa *%].
• This combination of properties allows a good cold workability, eg by pulling, pressing, thread rolling.
• the properties of the StE 460 achieved by variations of steel alloy 20MnV6.
• the solid solution hardening by the alloying element manganese together with the precipitation of vanadium carbonitrides causes a comparatively high strength at a moderate cost.
• the strength is generally adjusted by varying the carbon content in the range between 0.12 and 0.22%.
• vanadium, titanium and niobium also play an important role as micro-alloying elements.
• the micro-alloying elements are generally alloyed in small proportions of up to 0.2%, the amount and choice of the micro-alloying elements being dependent on thermoforming, eg hot-rolled strip production.
• the structure of a classic StE 460 consists of a mixture of ferrite and pearlite and is generally formed by cooling in air after rolling or austenitizing.
• An advantage of these steels is the property, by a so-called normalization, generally carried out in the form of austenitization and cooling in air, restore the initial structure and the initial properties even after a complex manufacturing history.
• a further increase in the strength through additional alloying elements leads to increased costs and to a pronounced decrease in the elongation at break, so that the desired cold workability is not guaranteed.
• additional heat treatment such as soft annealing before forming, this problem can be easily overcome.
• this procedure is also associated with increased costs.
• the described ferritic-pearlitic structure of the state of the art steel tubes has in addition to the only moderate ratio of strength and ductility additional disadvantages.
• the microstructures ferrite-perlite are not evenly distributed but show a pronounced linearity, the first consequence of which is a pronounced anisotropy of the properties brings and leads in the cold forming to undesirable effects. For example, there are significant differences along and across the rolling direction.
• Welded steel pipes are often produced by pressure welding.
• the strip edges are heated by resistance heating (high-frequency or direct current) and then welded at high pressure with significant plastic deformation, without a molten phase is formed.
• Such welding methods are therefore covered by the term solid state welding methods.
• a great advantage of the welding process described is the extremely high welding speed, which is significantly higher than other methods, e.g. above that of the laser beam welding, and thus brings a superior cost-effectiveness.
• pressure welding of ferritic-pearlitic steels however, the formation of the weld bead as a result of the necessary plastic deformation results in the effect that pearlite rows are deflected and reach the surface in the region of the weld zone.
• brittle cementite lamellae of the pearlitic structure constituent form metallurgical notches, which in the worst case emerge perpendicular to the surface.
• These fins can already be used during the following processing, e.g. Calibration of pipes for roundness, lead to cracks.
• these notches mean that even with high static strength no high dynamic strength can be achieved. Consequently, pearlite-free structures are particularly suitable for producing high-strength press-welded steel tubes.
• TRIP steels usually contain over 0.2% carbon, with the silicon content often exceeding 1.5%.
• the microstructure of these steels has a ferritic-bainitic base matrix containing retained austenite constituents, which are converted to hard martensite during transformation of the steel.
• the retained austenite is stabilized by alloying elements and a special heat treatment.
• the advantage of the TRIP steel lies in the good forming properties at high strengths and high breaking strengths.
• a TRIP steel has a high solidification capacity even with large changes in shape and a high energy absorption capacity, which is maintained even under dynamic load.
• TRIP steels generally require a complex and technically difficult heat treatment to stabilize the desired amount of retained austenite to room temperature.
• the TRIP heat treatment generally consists of accelerated cooling from the austenite region to prevent perlite formation and holding for a few minutes at temperatures just above the martensite start temperature. This heat treatment requires a complex process control and is difficult to implement reliably in conventional production facilities of plants for pipe production.
• ferritic-bainitic steels (FB steels) which have strengths of 500-1,000 MPa and exhibit better properties than ferrite-pearlitic materials in terms of forming behavior Steels of equal strength.
• FB steels ferritic-bainitic steels
• the achievable plastic deformations at strengths above 700 MPa are still too low.
• the production of ferritic-bainitic steels generally requires a so-called thermomechanical treatment, ie special rolling and cooling conditions. For this reason, conventional ferritic-bainitic steels are mainly available as hot-rolled strip.
• TRIP steels and FB steels can not yet be normalized analogously to ferritic-pearlitic steels, since during normalization the necessary cooling conditions are not guaranteed.
• the first three steels shown have a much higher carbon content and also differ in the other elements of Although the presented TRIP steel (number 3) achieves comparable mechanical properties, for the processing is, however, as already explained, an expensive to implement temperature-time curve during production required.
• Material characteristics of Docol 1000 DP, TRIP steel RA-K 42/80 and FB-W 600 are only available in strip material. Therefore, the table also indicates the A80 instead of the A5 elongation for the DP / TRIP and FB steel.
• the A80 elongation is used for strip material due to sample geometry, as opposed to strip tensile.
• the invention is based on the object of demonstrating how steel pipes with high strength and good formability can be produced without costly heat treatment and without costly alloying concepts, wherein the elongation at break should at least equal the steel StE 460 and wherein the steel pipes have a breaking strength above 700 MPa should.
• the solution of the problem of the invention is achieved by a new structure concept and its alloy implementation.
• the new alloy concept is based on the avoidance of perlite and on the setting of a ferritic-bainitic structure with small amounts of lamellar retained austenite. As a result, favorable, low yield strength ratios are achieved for cold forming.
• the product of breaking strength and elongation at break reaches very good values of more than 20,000 [MPa *%].
• This microstructure is achieved by adapting the chemical composition to predefined cooling conditions of the steel tubes from the austenite region. The cooling conditions are described by a continuous cooling with cooling rates between 0.5 K / sec and 5 K / sec.
• the alloy concept prevents the formation of perlite in this cooling zone.
• ferrite or bainitic ferrite and a Residual phase or several residual phases which, depending on the cooling conditions, consist of lower bainite and martensite with lamellar retained austenite.
• the steel is characterized by excellent formability in the cold state, as well as by a high breaking strength at high elongation at break, which is caused by the strong solidification due to the multi-phase character.
• the pipes are intended to be cold formed in further processing.
• the alloys show a basic ferritic structure with bainite, martensite and partially retained austenite, the grain sizes being 10-20 ⁇ m for the rolling conditions not optimized here. Occasionally it comes to the formation of fine and small pearlite nests, which are not arranged in a row. By improving the hot rolling conditions, the microstructures can be significantly improved and thus also the properties of the materials.
• alloys 2 and 3 after improved hot rolling conditions, ie from the standard production of seamless tubes measuring 36 ⁇ 3.6 mm with final rolling temperature 860 ° C.
• Alloy 2 was chosen by way of example because it has a high fracture toughness.
• Alloy 3 was chosen as an example because it has a high strength.
• alloys 2 and 3 after hot rolling of seamless tubes Rp0.2 [MPa] Rm [MPa] A5 [%] Fracture Z [%] True Breaking Voltage [MPa] Alloy 2 375 677 32 68 1310 Alloy 3 545 960 24 55 1610
• Tubes made of such a steel have a pearlite-free multi-phase structure and open up a variety of applications and uses, some of which are exemplified below.
• hot-rolled steel alloy pipes Due to the excellent relationship between strength and ductility, hot-rolled steel alloy pipes have particular advantages in subsequent cold forming processes, eg drawing, rotary kneading, spinning, thread rolling, extrusion, compression, autofretting, bending.
• the steel alloy can be used to produce high-strength and cost-effective cold-drawn steel tubes, eg drill pipes, line pipes, diesel injection lines, cylinder tubes, tubes for airbag generators, and pipes for side impact beams for motor vehicles produce.
• work hardening is used to achieve high strength.
• Soft annealing before cold drawing is not required. Tempering is optionally possible after cold drawing, depending on the desired strength. Stresses in the range of well over 1,000 MPa up to 1,600 MPa are possible.
• the alloy is distinguished by the fact that no pearlite line appears, so that the tubes react less sensitively to internal defects caused by pleats.
• the tubes produced from the steel alloy are also particularly suitable for further processing by hydroforming.
• the excellent deformation behavior of the steel alloy results in hydroforming advantages, since high component strengths can be achieved using the steel alloy.
• the production of welded pipes from the alloy is also advantageously possible.
• the alloy concept allows the production of hot strip and cold strip. Compared to conventional DP steels and TRIP steels, a comparatively simple temperature-time control is required.
• the alloy can be normalized up to a plate thickness of 4 mm, ie Develops the target structure in case of air cooling.
• the low carbon content results in only comparatively low hardness peaks in the welded seam of welded pipes. this applies especially in comparison to TRIP steels, which show a high degree of hardening with twice the carbon content. Due to the lack of pearlite brittleness, advantages arise in classical and very economical pressure welding.
• the alloy concept also advantageously allows beam welding by means of laser beam or electron beam.
• the advantage of the alloy concept is also the low carbon content and the normalization capability of the tubes.
• the steel alloy is suitable for the production of tubes for chassis applications in the automotive industry. Due to the good breaking strength-Umform zucchinis ratio more complex components are conceivable, which could not be produced with the previous steel grades or only with great technical and therefore benefited insomniaßem effort. In addition, the low carbon content in combination with the other alloying elements ensures good weldability.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacturing & Machinery (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Steel (AREA)

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• 2007
  • 2007-06-27 DE DE200710030207 patent/DE102007030207A1/de not_active Withdrawn
• 2008
  • 2008-06-27 EP EP08011681.7A patent/EP2009120B1/fr not_active Not-in-force

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