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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • 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
• • 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 present invention relates to a process for temperature treating a manganese steel intermediate. It is also a specific alloy of a manganese steel intermediate, which is subjected to a temperature-controlled process in order to obtain a significantly reduced Lüders elongation.
• This application claims the benefit of European Patent Application Number EP 16162073.7, filed on Mar. 23, 2016. Both the composition, or alloy, as well as the heat treatment in the manufacturing process have a significant impact on the properties of steel products.
• Mn manganese
• medium-manganese steels which are also referred to as medium-manganese steels.
• the manganese content in weight percent (wt.%) Is often in the range between 3 and 12. Due to its microstructure, a medium-manganese steel has a high combination of tensile strength and elongation. Typical applications in the automotive industry are complex safety-relevant deep-drawn components.
• FIG. 1 a classic, highly schematic diagram is shown, in which the breaking elongation A 8 o (in English total elongation called) in percent over the tensile strength (in English called tensile strength) in MPa is plotted.
• the tensile strength is abbreviated here to R m .
• the diagram of FIG. 1 gives an overview of the strength classes of currently used steel materials for the automotive industry. In general, the following statement applies: the higher the tensile strength R m of a steel alloy, the lower the elongation at break Aso of this alloy. In simple terms, it can be stated that the breaking elongation A 8 o decreases with increasing tensile strength R m and vice versa.
• Steel panels for the outer skin of a vehicle are relatively "soft" and have for example a tensile strength R m of about 300 MPa and a good elongation at break Aso> 30%
• the steel alloys of safety-relevant components for example, have a tensile strength R m in the range between 600 and 1000 MPa
• the TRIP (transfomation induced plasticity) steels are very well suited (reference 1 in Fig. 1).
• steel barriers eg for side impact protection
• steel alloys are used, which have a high tensile strength R m of usually more than 1000 MPa.
• the new generation of higher-strength AHSS (Advanced High-Strength Steels) steels is suitable (reference 2 in Fig. 1).
• the TBF (Trip Bainitic Ferrite) steels and the Q & P (Quenching & Partitioning) steels are suitable (reference 2 in Fig. 1).
• These high-strength AHSS steels have, for example, a manganese content in the range between 1.2 and 3% by weight and a carbon content C which is between 0.05 and 0.25% by weight.
• the area designated by the reference numeral 3 in Fig. 1 the already mentioned medium-manganese steels are schematically summarized.
• the area designated by the reference numeral 3 comprises medium-manganese steels having an Mn content of between 3 and 12% by weight and with a carbon content of less than 1% by weight.
• FIG. 2 An exemplary tensile curve 4 (also called stress-strain curve) is shown in FIG. 2 to remove.
• the stress ⁇ in MPa
• the traction curve 4 shows an intermediate maximum 5, which is referred to as the upper yield strength (ReH), followed by a plateau 6.
• the plateau 6 changes into a rising curve region.
• the "length" of the plateau 6 is referred to as the Lüders elongation (A L ), as shown in Fig. 2.
• a steel product having such a pronounced yield strength may form undesirable Lüder straps on the surface of the components for the automotive industry
• the pronounced yield strength must typically be reduced by a re-rolling process, and the aftertreatment in a corresponding re-rolling mill (usually with a skin pass mill) is also referred to as skin-pass.
• the energetic and technical effort for the dressage is sometimes quite high.
• this process leads to a reduction of the usable elongation. It is therefore the task of developing a method for producing manganese steel intermediates in which the Lüders stretching is less pronounced.
• the manganese steel intermediates should have no (measurable) Lüders stretching.
• a subtask of the invention is therefore to find an alloy composition and a method for temperature treatment to achieve an increase in the original austenite grain size and to manifest the increased austenite grains in the structure of the medium-manganese steels.
• the invention aims in a different direction.
• WO2014095082 AI double annealing is used, which works with other temperatures and processes. Steel products produced by the process of WO2014095082 A1 have a distinct yield strength.
• the manganese steel alloy of the invention comprises:
• Mn manganese content which in the following manganese range is 3% by weight ⁇ Mn ⁇ 12% by weight
• micro-alloying elements e.g.
• Ti titanium
• Nb niobium
• V vanadium
• Microalloying elements is less than 0.45% by weight
• the manganese steel intermediates which were produced from a melt of this manganese steel alloy, are subjected to a first temperature treatment process and a subsequent second temperature treatment process within the scope of a temperature treatment according to the invention.
• the first temperature treatment process is a high temperature process in which the steel intermediate is exposed during a first holding period to a first annealing temperature which is above a critical temperature limit (referred to as T KG ), this critical temperature limit (T KG ). is defined as follows: T KG ⁇ (856 - SK * manganese fraction) degrees Celsius, and where SK is a slope value.
• the above formula which serves as a definition of the critical temperature limit (T KG ), states that the critical temperature limit (T KG ) in said manganese region decreases with increasing manganese content.
• the second temperature treatment process is an annealing process in which the steel intermediate product is exposed to a second annealing temperature T2 which is in any case lower than the first annealing temperature T1.
• the first annealing temperature T1 is one in all embodiments Dependence on the stated manganese range of the alloy, which is defined as follows: Tl> T KG -
• the first holding period is at least 10 seconds in all embodiments. Particularly preferably, the first holding period in all embodiments is between 10 seconds and 7000 minutes.
• the second annealing temperature T2 is in all embodiments in the range between the temperatures Ai and A3. [0027] There are obtained advantageous results if the second heat treatment process, including heating the steel ⁇ betweenavess, holding the second annealing temperature and cooling the steel intermediate takes less than 6000 minutes. Preferably, this total time is even less than 5000 minutes.
• the invention enables for the first time the provision of steel intermediates having a Lüders stretching A L which is less than 3% and preferably less than 1%.
• the steel intermediates of the invention preferably have in all embodiments a mean primary austenite grain size greater than 3 pm.
• the alloy of the steel intermediates of the invention preferably has an average manganese content according to the invention, which means that the manganese content is in the range of 3% by weight ⁇ Mn ⁇ 12% by weight. Preferably, the manganese content in all embodiments is in the range 3.5% by weight ⁇ Mn ⁇ 8.5% by weight.
• the carbon content of the steel products of the invention is generally rather low. In addition, the carbon content is optional in all embodiments.
• That the carbon content is in the invention in the range C ⁇ 1 wt.%.
• the first temperature treatment process is carried out in a continuous strip plant (annealing plant).
• annealing plant This process is also known as continuous annealing.
• another possibility is a discontinuous heat treatment (bell annealing) of the steel intermediate.
• the first temperature treatment of the invention can also be performed by a special temperature control during hot rolling.
• this special temperature control care is taken to ensure that the rolling end temperature of the hot strip during hot rolling is in the range above the critical temperature limit T KG .
• the second temperature treatment process is carried out in a discontinuous plant, wherein the steel intermediate is subjected to the annealing process in this plant in a protective gas atmosphere.
• This process is preferably carried out in a bell annealing plant.
• the second temperature treatment process can be carried out in all embodiments but also in a continuous strip plant (annealing plant) or in a hot-dip galvanizing plant.
• the steel intermediate of all embodiments may optionally be subjected to a skin pass coating process, which is primarily directed to conditioning the surface of the steel intermediate. A more intensive skin-pass is not required because the steel intermediates of the invention have a low Lüders stretch.
• the temperament can be reduced or avoided altogether.
• steel intermediates can be prepared which have a Lüders elongation less than 3% and which is preferably less than 1%.
• steel intermediates can be produced which have a tensile strength R m (also called minimum strength) which is greater than 490 MPa. It is an advantage of the invention that steel intermediates can be produced, which have a (minimum) breaking strain (Aso) due to the reduced Lüders stretching, which is greater than 10%.
• the steel intermediates have an increased technically usable elongation due to the reduced Lüders strain.
• the invention can be used to z. B. cold rolled steel products in the form of cold rolled flat products (eg coils).
• the invention can also be used to z. As thin sheets or wire and wire products produce.
• the invention can also be used to provide hot strip steel products.
• FIG. 1 shows a highly schematic diagram in which the (minimum)
• FIG. 2 shows a schematic stress-strain diagram of FIG
• Fig. 3 shows a schematic diagram showing the two
• FIG. 4 shows, in the form of a schematic diagram, the critical one
• Temperature limit T KG shows a schematic diagram showing on the one hand the Lüdersdehnung A L in percent and on the other hand also the average original austenite grain size (DUAK M) as a function of the first annealing temperature Tl, in this diagram the corresponding curves of two different samples are shown;
• FIG. 2 shows a schematic diagram showing the stress ⁇ in MPa as a function of elongation ⁇ in% (analogous to Fig. 2), in which case four identical alloys were subjected to four different temperature treatment processes.
• steel intermediates are sometimes referred to when it comes to stress that it is not a question of the finished steel product but of a preliminary or intermediate product in a multi-stage production process.
• the starting point for such production processes is usually a melt.
• the alloy composition of the melt is given, since on this side of the manufacturing process it is possible to influence the alloy composition relatively precisely (eg by zag-targetting constituents, such as alloying elements and optional micro-alloying elements).
• the alloy composition of the steel intermediate usually deviates only insignificantly from the alloy composition of the melt. Quantities or proportions are here largely in weight percent (short wt.%) Made, unless otherwise mentioned.
• the composition comprises iron (Fe) and so-called unavoidable raw materials Contaminants that always occur in the molten bath and that also show up in the resulting steel intermediate. All% by weight must always be supplemented to 100% by weight and all% by volume must always be completed to 100% of the total volume.
• the temperature treatment of the steel intermediate product comprises a first temperature treatment process S. l and a subsequent second temperature treatment process S.2. These two temperature treatment processes S. l and S.2 are shown in Fig. 3 in two temperature-time diagrams shown side by side.
• the first temperature treatment process S.sub.1 is a high-temperature process in which the steel intermediate product is exposed to a first annealing temperature Tl during a first holding period .DELTA.l (this step is also referred to as holding Hl).
• the annealing temperature Tl is during holding H l above a critical temperature limit T KG -
• the course of this critical temperature limit T KG is (inter alia) dependent on the manganese content Mn of the alloy of the manganese steel intermediate, as could be determined by numerous investigations.
• the critical temperature T K represented by the straight line 7
• the course of the corresponding critical temperature limit T KG shown by the straight line 8) are shown.
• the manganese range MnB is plotted in percent by weight.
• the invention provides excellent results, especially with a manganese content in the following manganese range MnB: 3% by weight ⁇ Mn ⁇ 12% by weight.
• the measurement results of four samples are shown by way of example using small circle symbols. Further details of these four samples to be understood by way of example and to further samples of the invention are shown in Tables 1 and 2.
• the alloy composition of the respective type is shown in Table 1, wherein only the essential alloying ingredients are mentioned here. For each type, there are a number of embodiments that have been tested. The corresponding examples are numbered in the left column in Table 2 with the numbers 1 to 26.
• Type 4, 18 stands for Type 4 alloy composition, Example No. 18, for example.
• T K (866 - S K * manganese content) (1)
• the absolute value 866 in degrees Celsius defines the intersection with the vertical axis and the value S K defines the slope. S K is therefore also called the slope value.
• the investigations have shown that the slope value S K is preferably 7.83 ⁇ 10% in all embodiments.
• This straight line 8 can be circumscribed by the following equation (2), where TKG is given in degrees Celsius:
• the straight line 8 is parallel to the straight line 7.
• the following condition can be postulated:
• the first annealing temperature Tl must always be above the lower critical temperature limit T KG , to ensure that a manganese steel intermediate is obtained in which the Lüders strain A L is less than 3%.
• the second temperature treatment process S.2 has an influence on the Lüders stretch.
• the second annealing temperature T2 In order to maintain the grain size of the austenite grains in the microstructure, the second annealing temperature T2 must in any case be lower than the first annealing temperature T1. Since the first annealing temperature Tl is always above the lower critical temperature limit TKG, it can be concluded that the second annealing temperature T2 should preferably be below the lower critical temperature limit TKG. On the basis of the schematic example of FIG. 3 it can be seen that the first annealing temperature Tl is above the temperature limit T KG and that the second annealing temperature T2 is in the range between Ai and A3.
• the second temperature treatment S.2 is referred to in this case as intercritical annealing.
• the first holding period ⁇ 1 in all embodiments is preferably at least 10 seconds and preferably between 10 seconds and 6000 minutes.
• the second holding period ⁇ 2 is at least 10 seconds in all embodiments.
• the two holding periods .DELTA.1 and .DELTA.2 are shown only by way of example.
• the interval between the first temperature treatment process S.1 and the second temperature treatment process S.2 can be selected as needed.
• the second temperature treatment process S.2 is performed shortly after the first temperature treatment process S.l.
• Preferred embodiments are those in which the first temperature treatment process S.I. including the heating of the steel intermediate product El, the holding HI of the first annealing temperature Tl and the cooling Abi of the steel intermediate takes less than 7000 minutes.
• the second temperature treatment process S.2 including the heating E2 of the steel intermediate, the holding H2 of the second annealing temperature T2 and the cooling Ab2 of the steel intermediate takes less than 6000 minutes and preferably less than 5000 minutes.
• the significant reduction of Lüders stretching A L is independent of whether the first temperature treatment process Sl and / or the second temperature treatment process S.2 in a continuous belt plant (for example in a continuous system) or in a discontinuous Plant (for example, in a bell annealing) is / will be performed.
• the invention can be applied to both cold strip intermediates and hot strip intermediates. In both cases a clear reduction of the Lüders strain A L is shown .
• Increasing the first annealing temperature Tl to a value above the critical temperature limit T KG clearly leads to an increase in the average original austenite grain size and to a significant reduction in the Lüders stretching A L.
• the critical temperature limit TKGI ⁇ 820 ° C if one wants to achieve a Lüders stretching for this alloy composition of Typl, the smaller one than 3%.
• the curve 10 shows the associated course of the mean original austenite grain boundary DU AK MI , as a function of the temperature Tl. For the example Typl a grain size for this results with> 3pm.
• the critical temperature limit T KG is 2 ⁇ 970 ° C, if one wants to achieve a Lüders elongation for this type 2 alloy composition, which is less than 3%.
• the curve 12 shows the associated course of the mean original austenite grain boundary DUAK M, as a function of the temperature Tl.
• Tl temperature
• the micro-alloying element niobium (Nb) has a recognizable influence, which manifests itself as a shift from T KG 2 (compared to T KG I) to a higher critical temperature for A L ⁇ 3%.
• TKGI lower critical temperature limit
• the lower critical temperature limit T KG 2 can be determined as follows:
• the microalloying leads to an increase in the critical temperature limit T KG .
• the corresponding effective lower critical temperature limit T * K G2 is shown as a dashed vertical line.
• the resulting average original austenitic grain size in this case is> 8 pm.
• FIG. 6 shows a schematic diagram showing the stress ⁇ in M Pa as a function of the strain ⁇ in%.
• the illustration of FIG. 6 is with the illustration of FIG. 2, wherein FIG. 6 shows only a small section.
• Type 3 alloys of Table 1 were compared here.
• the type 3 alloys also meet the requirements of the invention. All four samples were each subjected to a first temperature treatment process S.sub.1 and a subsequent second temperature treatment process S.sub.2. All process parameters were identical, except that in the first temperature treatment process S.sub.1, the first annealing temperature T.sub.1 was varied as follows (see column 2 of the following table 3):
• the alloys of type 3 in these experiments had the following main composition:
• the solid curve 13.1 of FIG. 6 (Type 3, 14 of Table 2) shows a clearly visible pronounced yield strength and has a Lüders stretching of A L ⁇ 2.6%.
• the curve 13.2 represents another exemplary sample (Type 3, 15 of Table 2) of type 3, wherein here yield strength is still slightly pronounced.
• the curve 13.4 represents a further exemplary sample of the type 3, wherein here too no pronounced yield strength is more visible. These are Type 3, 17 of Table 2. [0091] Considering now the manganese steel intermediates of the invention in connection with Figure 1, the corresponding measurements (eg for the alloy compositions of Typl, Type 2 and Type 3) in the range of approx. 700 to 1000 MPa and with an elongation at break Aso in the range of approx. 20 to 40%.

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• 2016
  • 2016-03-23 EP EP16162073.7A patent/EP3222734A1/de not_active Withdrawn
• 2017
  • 2017-03-10 KR KR1020187030461A patent/KR102246704B1/ko active IP Right Grant
  • 2017-03-10 CN CN201780019271.3A patent/CN108884507B/zh active Active
  • 2017-03-10 EP EP17709124.6A patent/EP3433386B1/de active Active
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