EP2009120B1 - Use of an extremely resistant steel alloy for producing steel pipes with high resistance and good plasticity - Google Patents

Description

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. At yield strengths of 460 - 490 MPa, 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. Classic become the properties of the StE 460 achieved by variations of 20MnV6 steel alloy. Here, 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. In the above-mentioned high-strength micro-alloyed fine-grained steels, the strength is adjusted by varying the carbon content generally in the range between 0.12 and 0.22%. Besides 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, for example in the form of solid solution hardening, leads to increased costs and to a pronounced decrease in the elongation at break, so that the desired cold workability is not guaranteed. With an additional heat treatment, such as soft annealing before forming, this problem can be easily overcome. However, 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 lineiness, which as a first consequence brings about a pronounced anisotropy of the properties and leads to undesirable effects during cold forming. For example, there are significant differences along and across the rolling direction.

Normally, the pearlite lines are parallel to the surface and do not disturb most applications. In the production of steel pipes, however, there are some disadvantages.

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. In 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. Here, 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.

Especially with dynamically loaded components, 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.

A similar problem due to deflected pearlite lines can arise in the production of seamless steel tubes. This often leads to the formation of so-called pleats during thermoforming. These pleats generally weld during the manufacturing process and then present macroscopically small defects. However, brittle cementite lamellae emerging perpendicularly to the surface also have a negative effect on fatigue strength. Consequently, for the production of seamless pipes is the Use of pearlite-free structures advantageous if local crinkles can not be excluded.

It is known that the product of elongation at break and breaking strength can be improved by the targeted adjustment of retained austenite in the microstructure. The so-called TRIP effect (TRANSformation Induced Plasticity) enables comparatively high strains at high strengths. 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. However, 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.

From the field of strip production, ferritic-bainitic steels (FB steels) are known which have strengths of 500-1,000 MPa and exhibit better properties in relation to the forming behavior than ferritic-pearlitic steels of the same strength. However, the achievable plastic deformations at strengths above 700 MPa are still too low. Also, 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.

For the reasons mentioned above, it follows that 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.

For comparison, the chemical analysis and the associated mechanical characteristics for a steel StE 460 (Benteler Stahl / Rohr GmbH), a high-strength Dulphasen (DP) steel (Docol 1000 DP, SSAB Swedish Steel GmbH), a TRIP steel (RA-K 42/80, Fa. Thyssen Krupp GmbH), a FB steel (FB-W 600, Fa. Thyssen Krupp GmbH) listed. C Si Mn Cr al V Nb N Ti B StE460 0.18 0.23 1.37 0.15 0.03 0.09 0.037 0.01 0.00 0.00 Docol 1000 DP 0.15 0.2 1.6 0.04 0,015 RA-K 42/80 0.22 1.5 2 0.5 0.7 FB-W 600 0.09 0.03 1.46 0.02 0.031 0.001 0.032 0,008 0.001 0.0001

Material characteristics:

Rp0.2 [MPa] Rm [MPa] A5 [%] StE 460 473 709 19.47 Docol 1000 DP 700 - (950) 1000 - 1200 5 * RA-K 42/80 453 832 23.1 * FB-W 600 534 592 19.8 * *) Elongation A80

The first three steels shown have a significantly higher carbon content and also differ in the other elements from the composition of the below presented and inventive alloys 1 to 4. Although the presented TRIP steel (number 3) achieves comparable mechanical properties, for processing However, as already explained, a consuming to be realized temperature-time course during production is 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.

From the US 2003/0047258 A1 For example, a steel alloy is known for high strength cold rolled sheets. The steel alloy contains 0.05% to 0.30% carbon, 0.4% to 2.0% silicon, 0.7% to 3.0% manganese, maximum 0.8% phosphorus, maximum 0.02% aluminum and 0.0050% to 0.0250% nitrogen, balance iron, wherein the ratio N / Al <= 0.3. The thin cold-rolled steel sheet is heated to a temperature between the Ac1 transformation point and the austenitizing temperature + 50 ° C, then cooled at a cooling rate between 5 and 150 ° C / s to a temperature range of at least 600 to 500 ° C. Subsequently, the temperature is maintained in a range of 350 to 500 ° C. Such a steel sheet has good ductility properties and exhibits good values in terms of breaking strength and elongation at break, so that it has good crash properties when used in the automotive field.

Furthermore, the state of the art is the JP 2003 342687 A to name a steel pipe with very good strength / toughness ratios.

The steel tube consists of an alloy of 0.02% carbon, 0.3% to 2.5% silicon, 0.10% to 3.0% manganese, 0.005% sulfur maximum, 0.15% phosphorus maximum, 0.005% bis 0.100% aluminum, at most 0.0050% nitrogen and at least one element from the following group with the following proportions: 0.001% to 0.200% titanium, 0.001% to 0.200% niobium, 0.001% to 0.200% vanadium and 0.0005% to 0.0030% boron, balance iron and unavoidable impurities. The structure has a complex structure containing 5 to 20% retained austenite, with the remainder of the structure consisting of a structure of the following group: bainite, ferrite and perlite.

In the manufacturing process of such a tube, the steel tube is heated above the Ac1 transformation point to the Ac3 transformation point. The temperature is kept for 30 minutes or less. Subsequently, it is cooled at a cooling rate of at least 0.5 ° C / sec to a temperature between the MS transformation point + 100 ° C to the MS transformation point. Then, the temperature is maintained for 30 to 300 seconds. This is followed by air cooling to room temperature.

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.

This object is achieved by the use of a steel alloy having the features of patent claim 1.

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, on the other hand, 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. Instead, ferrite or bainitic ferrite and one or more residual phases are formed 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.

• Absenkung des C-Gehalts auf 0,06 bis 0,15 % um nur ein geringes Potential zur Bildung von Zementit zu bilden und Härtespitzen bei einer schweißtechnischen Verarbeitung zu verringern.
• Erhöhung des Si-Gehalts auf 1,0 % oder mehr, um eine Zementitbildung zu unterdrücken.
• Einstellung eines Mischkristall-Gehaltes (MK-Gehalt) von mehr als 3 % um die kritische Abkühlungsgeschwindigkeit zur Ferrit-/Perlit-Bildung zu verringern und weniger als 4 % um eine hinreichende Duktilität zu erhalten. Dabei besteht der MK-Gehalt aus der Summe der Elemente, die mit Eisen im Austenit Substitutions-MK bilden. Vorteilhaft aus Kostengründen ist hierbei die Verwendung von Si, Mn, Cr und Cu. Kobalt, Nickel, Molybdän können ebenfalls einen entsprechenden Beitrag leisten, haben jedoch Nachteile bezüglich der Kosten. Durch Variation von Cr, Mn, Si innerhalb der beschriebenen Grenzen können zusätzlich die Umwandlungstemperaturen entsprechend der vorgesehenen Verarbeitungsprozesse eingestellt werden, sowie der gewünschte Grad an Mischkristallverfestigung eingestellt werden.
• Sicherstellung von wenigstens 0,001 und höchstens 0,004 gelöstes freies Bor im Austenit, um im Wesentlichen die Ferritkeimbildung an den Austenitkorngrenzen zu unterdrücken. Hierfür ist das Abbinden des Stickstoffs im Stahl durch Elemente wie Titan, Zirkonium und/oder Hafnium notwendig. Die Menge der Zugabe des oder der Refraktärmetalls/-metalle ergibt sich aus der Stöchiometrie der entsprechenden Nitride. Aus Kostengründen ist dabei die Verwendung von Titan sinnvoll.
• Sicherstellung eines Stickstoffgehalts, der eine vorteilhafte Keimbildung durch Refraktär-Nitride erlaubt. Der gewünschte Phasenanteil hängt von den angestrebten Warmformstufen ab. Dabei muss beachtet werden, dass Refraktärnitride als harte Phasenanteile auch metallurgische Kerben bilden, die eine Dauerfestigkeit bei höchstfesten Stählen negativ beeinträchtigen können. Stickstoffgehalte in einem Bereich von 0,005% und 0,015% sind vorteilhaft.
• Gezielter Einsatz der Mikrolegierungselemente Niob und Vanadium für die Bildung von Keimen und die Beeinflussung der Rekristallisation beim Warmwalzen. Niob in Gehalten von 0,02 - 0,05 hat sich als vorteilhaft herausgestellt.
• Absenkung des Gehalts teurer Legierungselemente (Mo, V, Ni, W) auf maximal 0,3 %, aus Kostengründen vorzugsweise maximal 0,15 %.
• Verwendung eines geringen Aluminium-Gehalts (maximal 0,1 %), der sonst zur Bildung von harten Aluminiumoxiden führen würde, um die Dauerfestigkeit zu verbessern.

The alloy concept is based on the following fundamental considerations:

• Reduction of the C content to 0.06 to 0.15% in order to form only a small potential for the formation of cementite and to reduce hardness peaks during a welding process.
• Increasing the Si content to 1.0% or more to suppress cementite formation.
• Adjustment of a mixed crystal content (MK content) of more than 3% to reduce the critical cooling rate for ferrite / pearlite formation and less than 4% to obtain a sufficient ductility. The MK content consists of the sum of the elements that form substitution MK with iron in austenite. Advantageous for reasons of cost here is the use of Si, Mn, Cr and Cu. Cobalt, nickel, molybdenum can also contribute, but have disadvantages in terms of cost. In addition, by varying Cr, Mn, Si within the described limits, the transformation temperatures can be adjusted according to the intended processing, and the desired degree of solid solution hardening can be set.
• Ensuring at least 0.001 and at most 0.004 dissolved free boron in austenite to substantially suppress ferrite nucleation at the austenite grain boundaries. For this purpose, the bonding of nitrogen in the steel by elements such as titanium, zirconium and / or hafnium is necessary. The amount of addition of the refractory metal (s) results from the stoichiometry of the corresponding nitrides. For cost reasons, the use of titanium makes sense.
• Ensuring a nitrogen content that allows for beneficial nucleation by refractory nitrides. The desired phase proportion depends on the desired thermoforming stages. It should be noted that refractory nitrides form as hard phase fractions also metallurgical notches, which can adversely affect fatigue strength in high-strength steels. Nitrogen contents in the range of 0.005% and 0.015% are advantageous.
• Targeted use of the micro-alloying elements niobium and vanadium for the formation of nuclei and the influence of recrystallization during hot rolling. Niobium at levels of 0.02-0.05 has been found to be advantageous.
• Reduction of the content of expensive alloying elements (Mo, V, Ni, W) to a maximum of 0.3%, for cost reasons, preferably a maximum of 0.15%.
• Use of a low aluminum content (maximum 0.1%), which would otherwise lead to the formation of hard aluminas to improve fatigue strength.

In the following tables, by way of example, chemical compositions of selected experimental alloys that have been prepared in accordance with the material concept given above, together with their material characteristics, are indicated. The investigated material was hot rolled without special process control in a laboratory mill and then normalized, i. austenitized at 950 ° C and cooled in air at a sheet thickness of 5 mm. As a result, the specified values for non-optimized processing steps are determined.

All data in the chemical analysis are in mass percent Rp02 indicates the technical elastic limit, Rm the breaking strength and A5 the elongation at break. experimental alloys C Si Mn Cr al W Nb N Ti B number 1 009 1.49 1.52 0.15 0.01 <0.01 0.04 0.01 0.05 0,002 No. 3 0.08 1.51 2.02 0.16 0.01 <0.01 0.05 0.01 0.04 0,002 Mo, Cu, Ni each less than 0.15%, V each less than 0.03%

The following table shows the material characteristics of the four alloys after normalizing. Rp0.2 [MPa] Rm [MPa] A5 [%] Alloy 1 343 710 30 Alloy 3 419 831 25

The alloys show a basic ferritic structure with bainite, martensite and partially retained austenite, the grain sizes being 10-20 μm 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.

The following table exemplifies material characteristics for Alloy 3 after improved hot rolling conditions, i. from the standard production of seamless tubes measuring 36 x 3.6 mm with a final rolling temperature of 860 ° C. Alloy 3 was chosen as an example because it has a high strength.

As an example, the material characteristics of the alloy after hot rolling of seamless tubes: Rp0.2 [MPa] Rm [MPa] A5 [%] Fracture Z [%] True Breaking Voltage [MPa] Alloy 3 545 960 24 55 1610

By lowering the final rolling temperatures, the grain size of alloy 3 was significantly reduced to about 5 μm and the microstructure was developed to be more homogeneous. The properties could be significantly improved. Noteworthy is the increase in yield strength and tensile strength with practically constant elongation at break and high uniform elongation. It is also noteworthy that a true fracture stress of 1200 - 1.600 MPa is achieved, which can be considered as untypical for steels with less than 0.1% carbon content. Alloy 3 achieves the strength and ductility of TRIP steels with significantly reduced C content and significantly simplified temperature-time guidance.

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.

Due to the excellent relationship between strength and ductility, hot-rolled steel alloy pipes have particular advantages in subsequent, predominantly cold, deformation processes, eg drawing, swaging, flow-forming, thread rolling, extrusion, compression, autofretting, bending. In principle, the steel alloy can be used to produce ultra-high-strength and cost-effective cold-drawn steel tubes, eg, drill pipes, line pipes, diesel injection lines, cylinder tubes, airbag generator tubes, and side impact carbody tubes. In the drawing process, work hardening is used to achieve high strength. Soft annealing before cold drawing is not required. Starting after cold drawing is optional depending on the desired Strength possible. Stresses in the range of well over 1,000 MPa up to 1,600 MPa are possible.

In addition, with this material, the production of pipes with cold-rolled thread is possible, as is required, for example, scaffold tubes or anchor tubes for rock anchors. Starting from an already high initial breaking strength, the yield strength and the breaking strength are further increased by work hardening. Due to only comparatively low plasticization, there is a residual elongation of the finished component of more than 15% after the thread rolling and a breaking strength of considerably more than 850 MPa.

Particularly in the case of components subjected to high internal pressure, 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.

Consequently, 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.

In addition to the production of seamless pipes, 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. In addition, 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 is especially true 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 Alloy concept is also here in the low carbon content and in the normalization capability of the pipes.

In particular, the steel alloy is suitable for the production of tubes for chassis applications in the automotive industry. Due to the good breaking strength-Umformbarkeits ratio more complex components are conceivable, which could not be produced with the previous steel grades or only with great technical and therefore kostenmäßem effort. In addition, the low carbon content in combination with the other alloying elements ensures good weldability.

Claims (2)

1. Use of a steel alloy, which [consists] in mass fractions of Carbon (C) 0.06-0.10 Silicon (Si) 1.2-1.8 Manganese (Mn) 1.4-2.2 Chromium (Cr) 0.1-0.4 Molybdenum (Mo) 0.15 or less Aluminium (Al) 0.05 or less Vanadium (V) 0.15 or less Nitrogen (N) 0.02 or less Niobium (Nb) 0.02-0.06 Copper (Cu) 0.2 or less Nickel (Ni) 0.2 or less Boron (B) 0.001-0.004 Titanium (Ti) 0.001-0.05 Tungsten (W) 0.15 or less and iron together with impurities arising from smelting as the remainder, wherein the sum of silicon (Si) + manganese (Mn) + chromium (Cr) + copper (Cu) is in a range from 3 to 3.8% and wherein the steel alloy has a fine, largely pearlite-free multiphasic grain structure consisting of ferrite with incorporated bainite together with martensite with retained austenite, wherein no pearlite banding is present, wherein Lhe product of breaking strength and elongation at break exceeds 20,000 [MPa*%], wherein the strength Rm in the undeformed state amount to more than 600 MPa for producing welded steel tubes by fusion welding and solid-state welding, wherein the steel tubes have high strength and good ductility.
2. Use of a steel alloy according to claim 1 for producing high-strength, cold-drawn Steel tubes.