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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties of ferrous metals or ferrous alloys 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium

Definitions

• the invention relates to a method for producing a seamless and hot-rolled tubular semi-finished product.
• the EP 1 264 912 A1 discloses a free-cutting steel with good machinability when machining with a hard metal tool.
• This steel is made from an alloy that essentially has the following composition in percent by weight: C 0.05-0.8%, Si 0.01-2.5%, Mn 0.1-3.5%, O 0, 0005-0.01%, S 0.01-0.2%, Al 0.001-0.020%, Ca 0.0005-0.02% and can optionally Cr up to 3.5%, Mo up to 2.0%, Cu up to 2.0%, Ni up to 4% and B 0.0005-0.01%.
• Nb can be up to 0.2%, Ti up to 0.2%, V up to 0.5% and N up to 0.04%, Ta up to 0.5%, Zr up to 0.5% and Mg up to 0.02% be contained, the rest iron and impurities caused by fusion. Additions of Pb, Bi, Se, Te, Sn and Tl are also possible.
• the machinability of steels can be improved by adding lead, phosphorus and especially sulfur.
• High-sulfur steels are widely used as solid materials and are referred to as free-cutting steels, particularly because of their good machinability.
• the improvement in machinability is accompanied by a negative influence on the mechanical properties and a greatly reduced hot formability. This makes tube production using hot forming considerably more difficult.
• a higher sulfur content can also result in brittleness, such as red fracture or hot fracture. Brittleness is particularly problematic in pipe manufacturing.
• a key difference in pipe production compared to solid material is punching. This exposes material defects and material weak points to a greater extent than if only external force is applied, and leads to less welding of defects promotes the formation of cracks and the progression of cracks in the material. Compared to solid material, pipes have less material in their cross section. If cracks occur, especially under tensile loads, they are less able to be compensated for. Tearing or tearing of the material is encouraged.
• the invention is based on the object of demonstrating a process for producing a seamless and hot-rolled tubular semi-finished product with an increased sulfur content and which is particularly suitable for machining.
• the steel alloy used for the semi-finished product is intended to prevent or reduce manufacturing and quality problems resulting from poorer hot forming properties through a special alloy concept.
• a defined level of good mechanical characteristics should be achieved despite the negative influence of sulfur on the mechanical characteristics. Material weak points due to large oxidic inclusions should be reduced, which negatively influence machinability, especially tool life.
• the semi-finished product should be very suitable for machining and in particular should have very good machinability.
• the tubular semi-finished product produced using the process which is seamless and hot-rolled and is intended for machining, consists of the following steel alloy, whereby all information is in percent by weight: C 0.04 - 0.48 Si max. 0.60 Mn 1.10-2.90 S 0.10-0.40 Al 0.002 - 0.060 Approx 0.0001 - 0.02 O max. 80 ppm and optional v max. 0.5 N max. 0.15 Pb max. 0.1 P max. 0.1 b max. 0.01 N+P max. 0.2 Bi max. 0.1 Te max. 0.07 Se max. 0.2 Ni max. 2 Cu max. 0.8 Nb max. 0.3 Ti max. 0.5
• An essential feature of the invention is to bind sulfur reliably and appropriately in order to improve the hot forming properties of the semi-finished product.
• the setting takes place in particular by alloying manganese, which has a high affinity for sulfur and forms manganese sulfide.
• a minimum content of 1.1% by weight of Mn is specified.
• a ratio of Mn to S of at least 3.3:1 wt.% and max. 30:1 wt.% is specified. Too high a manganese content leads to hardening of the steel and reduces machinability.
• the manganese content is therefore limited to a maximum of 2.9% by weight of Mn, preferably a maximum of 1.8% by weight of Mn.
• the binding of the sulfur ensures that no harmful levels of low-melting iron sulfides (melting point approx. 1200°C) occur, which can cause hot fracture in the pipe manufacturing process.
• the melting point can drop to below 1000 ° C and, in particular, cause red fracture.
• Iron sulfide is deposited particularly along the grain boundaries. This further weakens the material and promotes intergranular fracture.
• high-melting manganese sulfides (melting points approx. 1600 ° C) are preferably formed.
• the proportion of iron sulfides in the sulfides present is a maximum of 6%, preferably a maximum of 3%.
• the billet material is deoxidized in a suitable manner.
• Oxygen can improve machinability, particularly by modifying the manganese sulfides.
• the oxygen content is therefore limited to a maximum of 80 ppm and preferably a maximum of 60 ppm.
• aluminum is used as the central element and is added to the melt for deoxidation. Silicon and manganese also have a deoxidizing effect, but with a significantly lower affinity for oxygen than aluminum.
• the melt is subjected to a calcium treatment, which modifies the oxides, especially the aluminum oxides.
• the compounds are partially flushed out with a gas flush and an Al content of 0.002 - 0.060% by weight is set.
• a minimum Al content remains in the material due to the process.
• Limiting the Al content is necessary because an Al value that is too high can lead to the introduction of material defects during casting, which in turn is disadvantageous for hot forming during pipe production. In addition, this would promote the formation of larger clusters of aluminum oxides, which are also disadvantageous and, for example, increase tool wear during machining due to high hardness.
• the Al content can therefore be limited to a preferred range of 0.002 to 0.020% by weight in all alloys according to the invention.
• oxides in the material are modified.
• calcium aluminates are formed.
• the calcium can be added in various ways, for example as calcium-silicon wire.
• Oxides containing calcium have the positive property that they improve chip breaking. The negative influence on tool wear is reduced because the number of hard aluminum oxides that promote wear is greatly reduced. A certain proportion of calcium-treated oxides can therefore remain in the material. When the alloy is flushed with gas, a large proportion of the inclusions are flushed out. Calcium aluminates are easier to rinse out compared to untreated aluminum oxides. Overall, this also enables, among other things, improved control of the Al content. Calcium also influences the formation of manganese sulfides.
• a combination of manganese sulfides with Ca leads to an increase in the size of the inclusions, which also has a positive effect on machinability. In addition, elongation of the sulfides during the hot rolling process is reduced for these inclusions.
• a combination of sulfur with calcium aluminates or with calcium is also possible.
• the calcium content remaining in the material is in the range of 0.0001 - 0.02% by weight. Since high calcium contents make raw material production difficult, especially in continuous casting, the Ca content is preferably 0.0005 - 0.01% by weight. .
• Lead also contributes to improving machinability. Lead improves chip breaking and has a lubricating effect between the workpiece and Tool. Due to its toxic properties, the content of lead is limited to a maximum of 0.1% by weight and is preferably a maximum of 0.035% by weight.
• Carbon is a key element that increases strength in steel.
• the minimum content is therefore specified as 0.04% by weight.
• a C content that is too high increases tool wear in particular, which is why an upper limit of 0.48% by weight is specified.
• Silicon increases strength and especially the yield strength. At the same time, the weldability is hardly affected. However, silicon reduces ductility and machinability.
• the content is limited to a maximum of 0.60% by weight.
• the Si content can be specified as 0.20 to 0.60% by weight. This is particularly advantageous for free-cutting steels with higher demands on the mechanical characteristics that are welded after machining.
• Phosphorus and nitrogen improve machinability, especially chip breaking.
• the elements have a brittle effect on the material and worsen the mechanical technological properties and the hot forming properties.
• the content is therefore limited to P max. 0.1% by weight and N max. 0.15% by weight. Particularly in the case of pipes with special requirements for mechanical characteristics, the content is preferably P max. 0.025% by weight and N max. 0.05% by weight.
• the total P and N contents must not exceed 0.2% by weight.
• boron can be alloyed, which also has an embrittling effect. Among other things, boron increases strength, which can lead to increased tool wear and has a negative effect on hot formability.
• the boron content is therefore limited to a maximum of 0.01% by weight and preferably a maximum of 0.002% by weight. A general producibility with boron contents above these ranges is possible with the method according to the invention, but is not advantageous.
• bismuth, tellurium and/or selenium can be alloyed with the steel.
• the effect is based, among other things, on the enrichment in manganese sulfides or in the vicinity of manganese sulfides.
• the elements reduce the hot formability at higher proportions.
• the contents are therefore limited to bismuth max. 0.1% by weight, tellurium max. 0.07% by weight and selenium max. 0.2% by weight.
• Higher tellurium levels can, among other things, increase the likelihood of Increase surface defects.
• the tellurium content can therefore preferably be limited to a maximum of 0.03%. Due to the reduced hot forming properties, selenium contents of up to 0.12% are preferred.
• Alloys that are particularly suitable for the invention are listed in Table 1, with all alloys listed in the table (all data in wt. %, except O) being Al 0.002-0.060; Ca 0.001- 0.02 as well as O max. 80 ppm and optionally P max. 0.1; Pb max. 0.1; N max. 0.15; Bi max. 0.1; Te max. 0.07; Se max. 0.2; B max. 0.01; Ni max. 2; Cu max. 0.8, Nb max. 0.3; Ti max. 0.5; N+P max. 0.2; The remainder contains iron and melting-related impurities and accompanying elements, the ratio of Mn/S being in the range from 3.3:1 to 30:1.
• the semi-finished product produced using the process preferably has the following mechanical characteristics: Re min. 190 MPa; Rm min. 310 MPa, A min. 7%.
• tubular semi-finished products begins with the provision of continuously cast or pre-formed billets with different cross-sectional geometries, with round billets being preferred, made from one of the steel alloys mentioned above.
• a stick is divided into smaller blocks. The division into blocks can be done in a cold or warm state.
• the blocks are heated to a temperature of 1100°C - 1400°C.
• the blocks are punched using a hole press. This creates a hollow block.
• This hollow block is elongated by means of diagonal rolling while reducing the wall thickness (WD) and the outer diameter.
• the block is punched in a cross-rolling process using an internal tool.
• Different roller geometries and numbers of diagonal rollers (2 or 3) can be used.
• Various guide devices such as guide rulers, guide disks or guide rollers, can also be used.
• the material has a minimum temperature of 1,000°C.
• the elongated hollow block is then elongated again while warm at at least 750°C.
• the push bench method is suitable for this, in which the perforated hollow block is placed on a mandrel rod as an internal tool through scaffolding arranged in a row, with non-driven rollers, or a linear rolling mill with an internal tool and driven stands, also known as the pipe continuous process, or its further developments such as the Multi-stand Plug Mill or Multi-stand Pipe Mill (MPM) in which the internal tool controls to be led.
• MPM Multi-stand Plug Mill
• a different number of rollers can be used per stand. It is possible to adjust the rolls during the rolling process depending on the process. Due to the material concept, especially since there is almost no FeS, lower temperatures can also be used. Brittleness, especially red brittleness, does not occur. Temperatures immediately after the hollow block has been lengthened in the push bench, tube continuous rolling mill and the described further developments in the range 900 - 1130 °C are preferred. However, the rolling process must never fall below
• the plug rolling process can also be used.
• the hollow block is rolled using work rolls onto a plug rod with rolling plugs, which is supported against abutment.
• Other alternatives to elongation are a cross-rolling mill, e.g. Assel rolling mill, Diescher rolling mill or planetary cross-rolling mill with 3 or 4 rolls and a controlled inner tool. Even with these alternatives, the minimum temperature mentioned must not fall below 750°C.
• the final geometry is set in a final rolling process without an internal tool at a material temperature at the start of the process of 850 °C - 1200 °C and preferably 900 - 1100 °C.
• a stretch-reducing mill, a reducing mill or a sizing mill can be used for this.
• the previous intermediate product can optionally be reheated in advance. Inductive reheating within the process is also possible.
• the final rolling temperature after final forming is in a range of 740°C - 1,150°C and preferably 800 - 1070°C.
• the tubular semi-finished products are cooled, for example on a cooling bed. The cooling can take place down to room temperature; alternatively, it can be provided that an intermediate step such as heat treatment and/or straightening is carried out before final cooling to room temperature.
• the combination of material analysis, hot forming and temperature control results in a fine distribution of manganese sulfides in the material.
• elongation occurs in the rolling direction during rolling.
• Smaller pipe dimensions have, on average, a greater elongation of the manganese sulfides.
• a certain curvature of the elongated manganese sulfides remains. The curvature decreases with the degree of elongation of the manganese sulfides and the pipe.
• Manganese sulfides can have reduced elongation or maintain a spherical shape when the manganese sulfides are present in combination with other elements such as calcium.
• the process control according to the invention with the alloy concept used enables production at comparatively lower temperatures and thereby enables energy to be saved and thus CO2 emissions to be reduced. At the same time, the process becomes more resistant to process disruptions, as cooler hollow blocks or intermediate products can be manufactured without red breakage occurring.
• the mechanical properties of the semi-finished product are good in the process according to the invention despite the high sulfur content.
• Vanadium can be alloyed to further improve the mechanical characteristics. Vanadium refines the grain. It leads to an improvement in strength and toughness, while at the same time reducing machinability. Vanadium contents of 0.5% should not be exceeded. Optionally, a V-span of 0.06 - 0.17% can be used, which combines improved mechanical characteristics with very high machinability.
• the mechanical characteristics based on tensile tests at room temperature (RT) of particularly suitable alloys are given in Table 1 and in accordance with the subclaims.
• the tensile tests can be carried out according to European Standard EN 10002-1 or International Standard ISO 6892-1.
• the top and bottom sides of the samples can be processed or unprocessed, ie corresponding to the pipe surfaces. Flat samples are usually used.
• the alloys can optionally be manufactured with specified impact energy in Joules.
• the impact energy can be determined for the alloys specified in the claims using the method according to European Standard EN 10045-1 or International Standard ISO 148-1. Longitudinal Charpys and a V-Notch (KV) were used to determine this.
• WD wall thickness
• the steel alloy used according to the invention for the semi-finished product and the manufacturing process result in improved chip formation, improved tool life and an overall improvement in machinability.
• the lubricating effect of the sulfur is retained in the steel alloy used according to the invention for the semi-finished product.
• the steel alloy used according to the invention for the semi-finished product or the method for producing seamless, hot-rolled tubular semi-finished products makes it possible to adapt the cutting parameters, e.g. the cutting speed, the feed and the cutting depth, to higher values. Faster processing is possible, reducing manufacturing costs. The proportion of scrap parts, e.g. B. due to system or process disruptions is reduced.
• the tubular semi-finished product is cold-drawn.
• the pipe surface is prepared, for example by pickling, and a lubricant is applied to improve the sliding properties. So that the tubular semi-finished product can be inserted into a drawing die, the outer diameter at one end of the pre-pipe is reduced to a dimension below the drawing die. Depending on the dimensions, this forming at one end of the pre-pipe is optionally carried out with preheating.
• the pre-pipe is then pulled through the drawing die with or without an internal tool/drawing mandrel, with the pipe dimensions being changed over the entire length of the tubular semi-finished product.
• the lubricant reduces friction with the drawing die.
• the cross-sectional reduction can be chosen to be the same as that of non-sulphurized variants.
• the desired cross-sectional reductions are in the range of 15-45% per move.
• the hot-rolled or hot-rolled and drawn tubular semi-finished product is usually straightened after the manufacturing process.
• the shape of the hot-rolled or hot-rolled and drawn tubular semi-finished product can be changed by at least one cold forming or cold deformation.
• this process can also be carried out in a heated state or after previous heat treatment.
• the long product can be divided into semi-finished products of the required length by cutting, especially by sawing.
• these semi-finished products are machined at least partially with respect to the surface of the semi-finished products.
• Cutting manufacturing processes include, in particular, the separating processes, usually with a geometrically defined cutting edge, in particular processes such as turning, milling or drilling. As a rule, processing is carried out in order to join the products with other components.
• the tubular semi-finished product is essentially rotationally symmetrical due to the machining manufacturing process. Essentially this means that there is rotational symmetry at least in a length section of a surface, preferably in a predominant part of the length of the semi-finished product. Therefore, a tubular semi-finished product that has, for example, a transverse bore is essentially rotationally symmetrical in the sense of the invention.
• An example of a machined pipe product made from this semi-finished product are sleeves and spacer rings. These are short cylindrical, possibly with Stepped, essentially rotationally symmetrical, machined components with different wall thicknesses.
• Coupling sleeves are cylindrical components in different lengths and wall thicknesses with an internal thread. The inner diameter and thread dimensions are matched to a counterpart (rod, anchor, etc.). Externally, such sleeves are either processed or remain unprocessed. Some coupling sleeves are provided with tapers, shoulders and holes.

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Citations (4)

• 2022
  • 2022-08-19 EP EP22191218.1A patent/EP4324941A1/fr active Pending

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