Classifications

• • 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
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • 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
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations

Definitions

• the invention relates to a Fe—Al—Ti—B-based flat steel product and a method for producing such a flat steel product.
• flat steel products refers to rolled products formed as strips, sheet metal or the cuts and blanks derived therefrom.
• the flat steel products according to the invention are in particular heavy plates with a typical sheet thickness of 6-200 mm, or hot-rolled strips or strips with a typical sheet thickness of 1.5-6 mm.
• Steels of the type in question are characterised by TiB 2 precipitates embedded in a Fe 3 Al matrix. As a result of this characteristic, such steels are low in density and, consequently, low in weight. These properties, which are interesting for many applications, are balanced in known materials of the type in question here by high brittleness up to high temperatures and insufficient strength at temperatures above 500° C.
• the heat-resistant iron-based alloy presented therein is composed of the general formula Fe x Al y C z , where (in at. %) variable y is 1% ⁇ y ⁇ 28% and variable z is ⁇ 24%, and variable x is determined based on a diagram depending on the respective C and Al content of the steel. It is mentioned as an aside that the steel may contain more than forty other elements, including TiB 2 , whereby each of these elements has a content allowance of 0.1-2% by atomic weight. How steel designed in such a way can be processed into flat steel products is not specified.
• Fe—Al—Ti—B casting alloys can also be modified by adding further elements.
• elements that increase the D0 3 /B 2 transition temperature are considered here.
• Mo promotes the formation of complex borides, so that no more TiB 2 is formed.
• the object was to provide a flat steel product based on a Fe 3 Al alloy and a method that allows reliable production of such flat steel products.
• the invention has achieved this object with a flat steel product created according to claim 1 .
• the invention proposes the method specified in claim 11 .
• a flat steel product according to the invention is characterised in that it is made of a steel which consists of (in % by weight):
• the ratio % Ti/% B formed from the Ti content % Ti and the B content % B of the steel is
• the structure of the steel or the resulting flat steel product consists of 0.3-5% by volume TiB 2 precipitates that are embedded in a structural matrix with at least 80% by volume Fe 3 Al.
• the boride-strengthened Fe 3 Al alloy that a flat steel product according to the invention is made of exhibits strength above 500° C. and ductility that is markedly improved over conventional alloys of this type known from the prior art.
• the parameters of the production of the flat steel product according to the invention from such a composite steel are set according to the invention in such a way that microstructure optimisation is achieved, by which the properties of a flat steel product according to the invention are further optimised.
• the method according to the invention for producing a flat steel product according to the invention comprises the following steps:
• Aluminium is contained in the steel flat product according to the invention in levels of 12-20% by weight. At Al levels of at least 12% by weight, in particular more than 12% by weight, the intermetallic iron aluminide phase Fe 3 Al forms, which is the main component of the microstructure of a flat steel product according to the invention.
• the high Al levels lead to reduced density, accompanied by reduced weight, high resistance to corrosion and oxidation, as well as high tensile strength.
• excessive Al levels would make it difficult to cold form the steels according to the invention.
• Excessive Al levels would also result in deteriorated weldability by forming a stable weld slag in the welding process, and increased electrical resistance in resistance welding. For these reasons, the Al level of steel according to the invention is limited to a maximum of 20% by weight, in particularof 16% by weight.
• Ti and B form titanium borides in the steel according to the invention, which produce a fine microstructure, an increased yield strength, higher ductility, a higher modulus of elasticity and increased resistance to wear.
• Ti levels of at least 0.2% by weight, in particular at least 0.4% by weight, and B levels of at least 0.10% by weight, in particular at least 0.15% by weight are required.
• the Ti levels % Ti are matched to the B levels % B of the steel, so that the ratio % Ti/% B, i.e. the quotient of the Ti content % Ti as a dividend and the B content % B as a divisor, is 0.33 to 3.75, in particular 0.5-3.75 or 1.0-3.75.
• the risk of FeB formation is reduced when the ratio % Ti/% B is at least 0.33. Otherwise, the low melting phase FeB could lead to cracks during hot rolling and ductility loss (reduction in elongation at break). This can be safely avoided in particular if the ratio % Ti/% B is 0.5-3.75, in particular 1.0-3.75.
• the presence of Ti in the flat steel product according to the invention can also improve oxidation resistance and heat resistance.
• the levels of Ti borides are too high, this would lead to strong solidification when a flat steel product according to the invention is cold formed. Therefore, the upper limit of the Ti levels are 2% by weight, in particular at most 1.5% by weight or 1.1% by weight, and the upper limit of the B levels are 0.60% by weight, in particular at most 0.4% by weight.
• Chromium may optionally be present in the steel according to the invention at levels of up to 7% by weight, in particular at least 0.3% by weight, or at least 0.5% by weight, or at least 1.0% by weight in order to lower the brittle-ductile transition temperature and improve overall ductility.
• the presence of Cr also increases the steel's resistance to low and high temperature corrosion and improves oxidation resistance. There is no increase in these effects at levels higher than 7% by weight, while Cr levels of up to 5% by weight have been found to be particularly effective as far as weighing the cost/benefit, while in practice contents of up to 3% by weight have also proven sufficient to cause improvements of the steel according to the invention related to the addition of Cr.
• Mn manganese in levels of up to 1% by weight can also lower the brittle-ductile transition temperature.
• Mn also enters the steel as an unavoidable production-related impurity when Mn is used for deoxidation. Mn helps increase strength, but may deteriorate the corrosion resistance. This is prevented by limiting the maximum Mn levels according to the invention to 2% by weight, in particular max. 1% by weight or max. 0.3% by weight.
• Silicon can enter the steel of a flat steel product according to the invention as a deoxidiser in the steel production process, but can also be selectively added to the steel in levels of up to 5% by weight, in particular up to 2% by weight, in order to optimise strength and corrosion resistance, although excessive Si levels can lead to brittle material behaviour.
• the Si levels of a flat steel product according to the invention are typically at least 0.05% by weight, in particular at least 0.1% by weight.
• P levels are limited to max. 0.1% by weight and S levels to max. 0.03% by weight, wherein S levels of max. 0.01% by weight or P levels of max. 0.05% by weight have been found to be particularly advantageous.
• the optionally present elements niobium, tantalum, tungsten, zirconium and vanadium form strength-enhancing carbides with C in the steel according to the invention, and can contribute to improving heat resistance, at excessively high levels they degrade cold formability and weldability.
• the latter applies in particular to Nb, Ta and W, which are therefore permitted in the steel according to the invention in levels of at most 0.2% by weight, in particular at most 0.1% by weight.
• the Zr and V levels are limited toup to 1% by weight in the steel according to the invention, while Zr levels ofup to 0.1% by weight and V levels ofup to 0.5% by weight have been found to be particularly favourable. If the levels are excessive, Zr deteriorates corrosion behaviour, whereas excessive levels of V impair oxidation behaviour.
• the positive effects of Zr and V can be used especially when at least 0.02% by weight of either Zr or V are present in the steel.
• Molybdenum may optionally be added to the steel of a flat steel product according to the invention to improve tensile strength as well as creep resistance and high temperature fatigue strength. Mo can additionally contribute to a fine microstructure by forming fine carbides and complex borides. These positive effects are achieved when the Mo levels are at least 0.2% by weight. However, excessive levels of Mo lead to a deterioration of hot and cold formability. Therefore, the Mo levels of a flat steel product according to the invention are limited to a maximum of 1% by weight, in particular max. 0.7% by weight.
• Nickel may optionally be present in the flat steel product according to the invention in levels of up to 2% by weight to improve its strength and ductility, as well as to improve its corrosion resistance. At Ni levels of more than 2% by weight, these effects cease to significantly increase.
• the positive effects of Ni can be used especially when at least 0.2% by weight, in particular at least 1% by weight, of Ni is present in the steel.
• Copper may also optionally be present in the steel according to the invention to improve corrosion resistance.
• Up to 3% by weight Cu, in particular up to 1% by weight Cu, can be added to the steel for this purpose.
• the hot formability, weldability and recyclability of a flat steel product according to the invention deteriorate at higher Cu levels.
• the positive effects of Cu can be used especially when at least 0.2% by weight Cu is present in the steel.
• Calcium can be added to the steel during steel production to bind S and prevent clogging during casting of the steel.
• Optimum effects are achieved in steel compositions according to the invention when the Ca levels are up to 0.015% by weight, in particular max. 0.01% by weight, wherein Ca is safe to use when there is at least 0.001% by weight Ca in the steel.
• Rare earth metals “REM” can be added to the steel according to the invention in levels of up to 0.2% by weight, in particular up to 0.05% by weight, to improve oxidation resistance. This effect is achieved in particular if at least 0.001% by weight REM are present in the steel.
• Nitrogen is present in the steel according to the invention as an undesirable but usually inevitable production-related impurity.
• the N levels should be kept as low as possible. Limiting the levels of N to max. 0.1% by weight, in particular max. 0.03% by weight, minimises the formation of detrimental Al nitrides, which could degrade mechanical properties and ductility.
• Cobalt may optionally be present in the steel according to the invention at levels of up to 1% by weight to increase its heat resistance. This effect is achieved in particular if at least 0.2% by weight Co is present in the steel.
• the proportion of TiB 2 in the microstructure of a flat steel product according to the invention is 0.3 to 5% by volume.
• the presence of such amounts of TiB 2 causes a ductilisation of the Fe 3 Al matrix as a result of a significantly increased dislocation density in the vicinity of the TiB 2 particles, and promotes the recrystallisation of the microstructure.
• grain boundary pinning prevents grain coarsening.
• at least 0.3% by volume TiB 2 is required in the microstructure, while the effects are particularly reliable when the levels of TiB 2 in the microstructure of the steel according to the invention are at least 0.5% by volume, in particular at least 0.8% by volume. Harmful effects of excessive levels of Ti boride can be reliably prevented by limiting the level of TiB 2 in the microstructure of the flat steel product according to the invention to max. 3% by volume.
• the average grain size of the Fe 3 Al of the structural matrix should be 20-100 ⁇ m to ensure sufficient ductility and good creep resistance of the steel at room temperature, wherein average grain sizes of 50 ⁇ m have been found in practice to be particularly advantageous.
• the effect of the TiB 2 precipitates in the structural matrix of the flat steel product according to the invention can be further optimised if at least 70% of the TiB 2 precipitates in the structural matrix have an average particle diameter of 0.5-10 ⁇ m, in particular 0.7-3 ⁇ m.
• the structural matrix of a flat steel product according to the invention consists of at least 80% by volume of the intermetallic phase Fe 3 Al, the aim here being for the matrix to consist as completely as possible, optimally up to 100% by volume, of Fe 3 Al.
• the structural matrix may also contain optional levels of the mixed crystal Fe(Al) or the intermetallic phase FeAl. High levels of at least 80% by volume Fe 3 Al are required to set high corrosion resistance, heat resistance, hardness and wear resistance.
• a molten steel composed according to the invention in the manner explained above is melted in step a) of the process according to the invention and cast into a precursor in the form of a slab, thin slab or a cast strip in step b).
• the electric furnace route better suits the operational melting production of a high-alloy steel according to the invention than the classic blast furnace converter route of an integrated smelting plants due to its suitability for liquefaction of high amounts of alloy.
• the melt can be cast in conventional continuous casting if a suitable casting powder is used.
• the respective precursor is brought to the preheating temperature of 1200-1300° C. This can be carried out in a separate heating process or by holding at the relevant temperature from the casting heat. If a separate heating process occurs, it should last for a period of 15-1500 minutes to ensure homogeneous heating. If the temperature or hold time is too low, this is not achieved with the required certainty due to the low thermal conductivity of the steel, which can lead to cracks in the hot strip.
• a suitable hot rolling start temperature ensures formability, especially in the final passes, and thus avoids high loads on the rolls.
• the danger of rolling damage as a result of excessive rolling forces can be prevented by choosing a hot rolling start temperature predetermined according to the invention in the range of 1000-1200° C., in particular 1100-1170° C.
• a hot rolling start temperature predetermined according to the invention in the range of 1000-1200° C., in particular 1100-1170° C.
• an excessive hot rolling start temperature would result in the material being too weak for hot rolling. This can lead to unwanted deformations during processing and the rolled material sticking to the rollers.
• the hot rolling end temperature must be at least 850° C. in order to avoid excessive roll forces and to achieve high degrees of formability. At even lower hot rolling temperatures, the required hot strip flatness cannot be guaranteed with the necessary certainty from an operational point of view.
• the hot strip is coiled in step d) at a coiling temperature between room temperature and 750° C.
• Water or aqueous solutions that ensure a homogeneous cooling over the strip cross-section are particularly suitable as cooling media.
• Coiling temperatures of at least 400° C., in particular at least 450° C. have proven particularly useful in practical application, wherein the upper limit of the coiling temperature range can be limited to max. 700° C., in particular max 550° C., to prevent excessive scale formation on the hot strip.
• the hot strip obtained after hot rolling has an elongation at break of 2-4% in the tensile test.
• the hot strip can be optionally annealed after coiling at an annealing temperature of 200-1000° C. over an annealing period of 1 to 200 h. This serves to increase ductility at room temperature.
• a bell annealing process with a peak temperature above 650° C. is suitable for annealing the hot strip.
• Lower annealing temperatures or holding times show no effect, whereas higher annealing temperatures or holding times can lead to ductility loss due to grain coarsening as a result of coarsening of the Ti boride particles and the Fe 3 Al matrix.
• the hot strip according to the invention can also be optionally subjected to a pickling treatment with common media, wherein the pickling time must be chosen so that even the stable Al-oxides that appear on the hot strip are eliminated.
• a flat steel product alloyed according to the invention therefore has high yield points and tensile strengths. At the same time, its density is greatly reduced compared to conventional steels of the same strength class.
• the typical density of steels according to the invention is in the range of 6.2-6.7 g/cm 3 and is on average typically 6.4 g/cm 3 . This results in a high strength/density ratio compared to other heat-resistant materials.
• the BDTT value (brittle-ductile transition) can be lowered to surprisingly low temperatures of about 75-100° C.
• Typical hot yield points of flat steel products according to the invention are at 650° C. with approx. 130-170 MPa in the range of conventional ferritic Cr steels, such as the steel standardised under material number 1.4512 (hot yield point approx. 70 MPa) and the steel designed for high heat resistance, standardised under material number 1.4509 (hot yield point approx. 150 MPa).
• the tensile strength of the flat steel product according to the invention is still regularly at at least 100 MPa.
• flat steel products produced according to the invention are particularly suitable for the production of, in particular, heat-resistant components for plant construction (e.g. heavy plate), gas turbines, offshore installations and, in particular, heat-resistant components for the automotive industry, in particular exhaust systems or turbocharger housings (hot strip).
• heat-resistant components for plant construction e.g. heavy plate
• gas turbines gas turbines
• offshore installations e.g., offshore installations
• heat-resistant components for the automotive industry in particular exhaust systems or turbocharger housings (hot strip).
• exhaust systems or turbocharger housings e.g. biogas plants, brake discs, vehicle underbodies.
• the heated blooms were each hot rolled into a hot strip with a thickness of 3 mm in a conventional manner at a hot rolling end temperature HRET from a hot rolling start temperature HRST.
• the obtained hot strips were cooled from the respective hot rolling end temperature HRET to the respective coiling temperature CT and wound into a coil at that temperature.
• samples A1-F3 The following was determined for samples A1-F3: the mechanical properties yield strength Rp0.2, tensile strength Rm and elongation A50 at room temperature (see Table 3), and also for some selected samples, the mechanical properties yield strength Rp0.2, tensile strength Rm and elongation at break A at 650° C. (see Table 4), as well as the texture properties “Grain size of the matrix”, “Matrix” and “TIB 2 content in the microstructure” (see Table 5) and the brittle transition temperature BDTT (see Table 6).
• the mechanical properties were determined in the tensile test according to DIN EN 10002, whereas the brittle-ductile transition temperature was determined in the four-point bending test.
• alloys A-F could easily be rolled using industrial conditions on a laboratory scale.
• the tests have thus confirmed that the tensile strengths Rm of flat steel products according to the invention at room temperature typically have 550-700 MPa and yield points Rp0.2 of 400-650 MPa at an elongation A50 of typically 2-5%.
• the tensile strength could be increased in particular if the pre-roll and finishing roll occurred in different rolling directions.
• the Vickers hardness HV5 typically varies between 335 and 370 in the flat steel products according to the invention.
• the hot yield point 60.2 (measured transversely to the rolling direction according to DIN EN 10002) at 650° C. is typically 120 ⁇ 170 MPa.
• the sheets do not have a pronounced brittle-ductile transition temperature of 75-100° C. They are fully ductile even at 100° C. This is an improvement of at least 150° C. compared to the cast material, and is due to the microstructure being refined by rolling. Ductility can be increased by hot strip annealing of the type described above.

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• Crystallography & Structural Chemistry (AREA)
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• 2016
  • 2016-01-20 CN CN201680079664.9A patent/CN108603257B/zh active Active
  • 2016-01-20 EP EP16701442.2A patent/EP3405593B1/fr active Active
  • 2016-01-20 US US16/071,566 patent/US20190032161A1/en not_active Abandoned
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