EP3222734A1 - Method for temperature treating a manganese steel interim product and steel interim product put through corresponding temperature treatment - Google Patents

Abstract

Process for heat treating an intermediate manganese steel product, the alloy of which comprises: o a manganese content that is in the following manganese range 3% by weight ‰¤ Mn ‰¤ 12% by weight, o a proportion of one or more alloying elements from the group: silicon (Si), aluminum (Al), nickel (Ni), chromium (Cr), molybdenum (Mo), phosphorus (P), sulfur (S), nitrogen (N) , Copper (Cu), Boron (B), Cobalt (Co), Tungsten (W), o an optional carbon content (C) of less than 1% by weight, o an optional proportion of one or more micro-alloying elements, the total proportion of micro-alloying elements being less than 0.45% by weight, and o the balance being an iron content (Fe) and unavoidable impurities, the temperature treatment of the intermediate steel product comprising a first temperature treatment process and a subsequent second temperature treatment process, it being - the first heat treatment process is a high temperature process in which the intermediate steel product is subjected to a first annealing temperature during a first holding period, which is above a critical temperature limit defined as follows: T KG = (856 - S K * manganese content) degrees Celsius, where S K is a slope value, - the second temperature treatment process is an annealing process in which the intermediate steel product is subjected to a second annealing temperature which is lower than the first annealing temperature.

Description

The present invention relates to a method of 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.

Both the composition, respectively alloy, and the heat treatment in the manufacturing process have a significant influence on the properties of steel products.

It is known that in the course of a heat treatment, the warm-up, holding and cooling can have an influence on the final structure of a steel product. Furthermore, as already indicated, of course, the alloy composition of the steel product also plays a major role. The thermodynamic and materials-related relationships in alloyed steels are very complex and depend on many parameters.

It has been shown that a combination of different phases in the structure of a steel product can influence the mechanical properties and the deformability.

Depending on the specific requirement profile, different steels are used.

An important component of today's new steel alloys is manganese (Mn). These are so-called 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 structure, 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.

In Fig. 1 is a classical, highly schematic diagram shown in which the elongation at break A ₈₀ (in English total elongation called) in percent over the tensile strength (in English tensile strength called) 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 A _{80 of} this alloy. In simple terms, it can be stated that the breaking elongation A ₈₀ decreases with increasing tensile strength R _m and vice versa. It is therefore necessary to find an optimum compromise between the breaking elongation A ₈₀ and the tensile strength R _m for each application.

In the automotive sector, they work with a whole range of different steel alloys, each of which has been optimized specifically for their particular area of application on the vehicle. For interior and exterior panels, structural parts and bumpers, alloys are used which have good energy absorption. 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 A ₈₀ > 30%. The steel alloys of safety-relevant components, for example, have a tensile strength R _m in the range between 600 and 1000 MPa. For example, the TRIP (transformation-induced plasticity) steels (reference numeral 1 in FIG Fig. 1 ).

For steel barriers (eg for side impact protection), which are intended to prevent the ingress of vehicle parts in an accident, steel alloys are used which have a high tensile strength R _m of usually more than 1000 MPa exhibit. Here, for example, the new generation of higher-strength AHSS (Advanced High-Strength Steels) steels is suitable (reference 2 in Fig. 1 ). In this category are the TBF (Trip Bainitic Ferrite) steels and the Q & P (Quenching & Partitioning) steels. 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.

In the area that is in Fig. 1 designated by the reference numeral 3, the already mentioned medium-manganese steels are schematically summarized. The area designated by reference numeral 3 comprises medium-manganese steels having an Mn content of between 3 and 12% by weight and a carbon content of ≦ 1% by weight.

Due to their ultra-fine grain (typically ≤ 1 μm), today's medium manganese steels have a pronounced yield strength which is clearly evident in the tensile test. An exemplary tensile curve 4 (also called stress-strain curve) is the Fig. 2 refer to. In Fig. 2 the stress σ (in MPa) is plotted over the strain ε (in%). The tensile curve 4 shows an intermediate maximum 5, which is referred to as upper yield strength (R _eH ), followed by a plateau 6. In the region of the lower yield strength (R _eL ), the plateau 6 changes into a rising curve region. The "length" of the plateau 6 is referred to as Lüders stretch (A _L ), as in Fig. 2 shown. A steel product with such a pronounced yield strength can form undesirable luer bands on the surface of components for the automotive industry. Therefore, the pronounced yield strength typically needs to be reduced by a re-rolling process. The aftertreatment in a corresponding Nachwalzwerk (usually with a skin pass mill) is also referred to as tempering.

The energetic and technical effort for the dressage is sometimes quite high. In addition, this process leads to a reduction of the usable elongation.

It is therefore the object to develop a process for the production of manganese steel intermediates in which the Lüdersdehnung is less pronounced. Preferably, the manganese steel intermediates should have no (measurable) Lüders stretching.

Studies on numerous alloy compositions of medium-manganese steels have shown that there is a correlation between the original austenite grain size of these steels and the Lüders strain. That the original austenite grain size has an influence on the mechanical properties of these steels. In general, it can be postulated that the Lüders strain behaves inversely proportional to the original austenite grain size.

The object of the invention is therefore to find an alloy composition and a process for temperature treatment in order to increase the original austenite grain size and to manifest the increased austenite grains in the structure of the medium-manganese steels. Unlike in the prior art (see, eg WO2014095082 A1 ), where it comes to the provision of ultrafine structures (with an ultrafine grain having a mean grain size of about 1 micron), the invention aims in a different direction. In addition, in the exemplified patent application WO2014095082 A1 a double annealing process that works with different temperatures and processes. Steel products made by the method of WO2014095082 A1 were produced, have a distinct yield strength.

In accordance with the invention, there is provided a particularly suitable manganese steel alloy and an optimized process for temperature treating a manganese steel intermediate.

• einen Mangananteil (Mn), der im folgenden Manganbereich 3 Gew.% ≤ Mn ≤ 12 Gew.% liegt,
• einen Anteil von einem oder mehreren Legierungselementen der Gruppe: Silizium (Si), Aluminium (Al), Nickel (Ni), Chrom (Cr), Molybdän (Mo), Phosphor (P), Schwefel (S), Stickstoff (N), Kupfer (Cu), Bor (B), Kobalt (Co), Wolfram (W),
• einen optionalen Kohlenstoffanteil (C) von weniger als 1 Gew.%,
• einen optionalen Anteil von einem oder mehreren Mikrolegierungselementen z. B.: Titan (Ti), Niob (Nb) und Vanadin (V), wobei der gesamte Anteil der Mikrolegierungselemente weniger als 0,45 Gew.% beträgt, und
• als Rest einen Eisenanteil (Fe) und unvermeidbare Verunreinigungen.

The manganese steel alloy of the invention comprises:

• a manganese content (Mn) which in the following manganese range is 3% by weight ≦ Mn ≦ 12% by weight,
• a proportion of one or more alloying elements of the group: silicon (Si), aluminum (Al), nickel (Ni), chromium (Cr), molybdenum (Mo), phosphorus (P), sulfur (S), nitrogen (N), Copper (Cu), boron (B), cobalt (Co), tungsten (W),
• an optional carbon content (C) of less than 1% by weight,
• an optional fraction of one or more micro-alloying elements, e.g. For example: titanium (Ti), niobium (Nb) and vanadium (V), the total proportion of micro-alloying elements being less than 0.45% by weight, and
• the remainder being an iron content (Fe) and unavoidable impurities.

The manganese steel intermediates which have been 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 undergoes a first annealing temperature above a critical temperature limit (denoted as T _KG ) during a first holding period, this critical temperature limit (T _KG ) being defined as follows is: T _KG ≥ (856 - S _K * manganese fraction) degrees Celsius, and where S _{K is} a slope value.

The mentioned formula, which serves as a definition of the critical temperature limit (T _KG ), states that the critical temperature limit (T _KG ) in said manganese range decreases with increasing manganese content.

The said slope value is preferably defined in all embodiments as follows: S _K = 7.83 ± 10% and particularly preferably at S _K = 7.83.

The second temperature treatment process is an annealing process in which the steel intermediate is exposed to a second annealing temperature T2, which in any case is lower than the first annealing temperature T1.

Preferably, the first annealing temperature T1 in all embodiments depicts a dependence on said manganese range of the alloy, defined as follows: T1 ≥ T _KG .

Particularly preferred embodiments of the invention, at a critical temperature T _K ≥ (866 - S _K * manganese content) degrees Celsius, where: $${\mathrm{S}}_{\mathrm{K}}=7.83\pm 10\%\mathrm{,}$$

Preferably, 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.

Preferably, the second annealing temperature T2 is in all embodiments in the range between the temperatures A ₁ and A ₃ .

Beneficial results are obtained if the second temperature treatment process, including heating the steel intermediate, maintaining the second annealing temperature, and cooling the steel intermediate, takes less than 6000 minutes. Preferably, this total time is even less than 5000 minutes.

• Silizium (Si) ≤ 3 Gew.%, und vorzugsweise ≤ 2 Gew.%,
• Aluminium (Al) ≤ 8 Gew.%, und vorzugsweise ≤ 6 Gew.%,
• Nickel (Ni) ≤ 2 Gew.%, und vorzugsweise ≤ 1 Gew.%,
• Chrom (Cr) ≤ 2 Gew.%, und vorzugsweise ≤ 0,5 Gew.%,
• Molybdän (Mo) ≤ 0,5 Gew.%, und vorzugsweise ≤ 0,25 Gew.%,
• Phosphor (P) ≤ 0,05 Gew.%, und vorzugsweise ≤ 0,025 Gew.%,
• Schwefel (S) ≤ 0,03 Gew.%, und vorzugsweise ≤ 0,01 Gew.%,
• Stickstoff (N) ≤ 0,05 Gew.%, und vorzugsweise ≤ 0,025 Gew.%,
• Kupfer (Cu) ≤ 1 Gew.%, und vorzugsweise ≤ 0,5 Gew.%,
• Bor (B) ≤ 0,005 Gew.%, und vorzugsweise ≤ 0,0035 Gew.%.
• Wolfram (W) ≤ 1 Gew.%, und vorzugsweise ≤ 0,5 Gew.%.
• Kobalt (Co) ≤ 2 Gew.%, und vorzugsweise ≤ 1 Gew.%.

The invention can be applied particularly advantageously to alloys in which the proportion of the one or more alloying elements lies in the following range:

• Silicon (Si) ≦ 3% by weight, and preferably ≦ 2% by weight,
• Aluminum (Al) ≦ 8% by weight, and preferably ≦ 6% by weight,
• Nickel (Ni) ≦ 2% by weight, and preferably ≦ 1% by weight,
• Chromium (Cr) ≦ 2% by weight, and preferably ≦ 0.5% by weight,
• Molybdenum (Mo) ≤ 0.5% by weight, and preferably ≤ 0.25% by weight,
• Phosphorus (P) ≦ 0.05% by weight, and preferably ≦ 0.025% by weight,
• Sulfur (S) ≤0.03% by weight, and preferably ≤0.01% by weight,
• Nitrogen (N) ≦ 0.05% by weight, and preferably ≦ 0.025% by weight,
• Copper (Cu) ≦ 1% by weight, and preferably ≦ 0.5% by weight,
• Boron (B) ≤ 0.005 wt%, and preferably ≤ 0.0035 wt%.
• Tungsten (W) ≤ 1% by weight, and preferably ≤ 0.5% by weight.
• Cobalt (Co) ≦ 2% by weight, and preferably ≦ 1% by weight.

Advantageous results are shown in all embodiments in which elements of the following group are used as micro-alloying elements: titanium (Ti), niobium (Nb), vanadium (V).

The invention makes it possible for the first time to provide steel intermediates having a Lüders elongation A _L which is less than 3% and preferably less than 1%.

At the same time, the steel intermediates of the invention preferably have in all embodiments a mean primary austenite grain size greater than 3 μm.

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 of 3.5 wt% ≤ Mn ≤ 8.5 wt%.

1. a. 0,01 ≤ C ≤ 0,8 Gew.%, oder
2. b. 0,05 ≤ C ≤ 0,3 Gew.%.

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 is, the carbon content is in the invention in the range C ≤ 1 wt.%. Embodiments in which the carbon content is in one of the following ranges are particularly preferred

1. a. 0.01 ≤ C ≤ 0.8 wt%, or
2. b. 0.05 ≤ C ≤ 0.3 wt%.

In a preferred method of the invention, the first temperature treatment process is carried out in a continuous strip plant (annealing plant). This process is also known as continuous annealing. Or another possibility is a discontinuous heat treatment (bell annealing) of the steel intermediate.

If it concerns the temperature treatment of a hot strip, the first temperature treatment of the invention can also be carried out by a special temperature control during hot rolling. At this special Temperature control is taken to ensure that the rolling end temperature of the hot strip during hot rolling in the range above the critical temperature limit T _KG .

In a preferred method of the invention, the second temperature treatment process is carried out in a discontinuous plant, the steel intermediate being 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 belt plant (annealing) 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.

Thus, with the invention, the degree of tempering can be reduced or completely avoided.

It is an advantage of the invention that steel intermediates can be made having a Lüders elongation less than 3% and preferably less than 1%.

It is an advantage of the invention that 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, due to the reduced Lüders strain, have a (minimum) breaking elongation (A ₈₀ ) which is greater than 10%.

It is an advantage of the invention that the steel intermediates have increased technically usable elongation due to the reduced Lüders strain.

The invention can be used to e.g. Cold rolled steel products in the form of cold rolled flat products (e.g., coils). The invention can also be used to e.g. To produce thin sheets or wire and wire products.

The invention can also be used to provide hot strip steel products.

Further advantageous embodiments of the invention form the subject of the dependent claims.

DRAWINGS

FIG. 1

zeigt ein stark schematisiertes Diagramm, bei dem die (Mindest-) Bruchdehnung (A₈₀) in Prozent über die Zugfestigkeit (R_m) in MPa für verschiedene Stähle für die Automobilindustrie aufgetragen sind;

FIG. 2

zeigt ein schematisiertes Spannungs-Dehnungs-Diagramm eines Stahlprodukts, das eine deutlich ausgeprägte Streckgrenze (Lüdersdehnung A_L) aufweist;

FIG. 3

zeigt ein schematisiertes Diagramm, das die beiden Temperaturbehandlungsprozesse zeigt;

FIG. 4

zeigt in Form eines schematisierten Diagramms die kritische Temperatur T_K und den Verlauf der entsprechenden kritischen Temperaturgrenze T_KG;

FIG. 5

zeigt ein schematisiertes Diagramm, das einerseits die Lüdersdehnung A_L in Prozent und andererseits auch die mittlere ursprüngliche Austenit-Korngrösse (D_{UAK M}) als Funktion der ersten Glühtemperatur T1 darstellt, wobei in diesem Diagramm die entsprechenden Kurven von zwei unterschiedlichen Proben gezeigt sind;

FIG. 6

zeigt ein schematisiertes Diagramm, das die Spannung σ in MPa als Funktion der Dehnung ε in % zeigt (analog zu Fig. 2), wobei hier vier identische Legierungen vier verschiedenen Temperaturbehandlungsprozessen unterzogen wurden.

Embodiments of the invention will be described in more detail below with reference to the drawings.

FIG. 1

shows a highly schematic diagram in which the (minimum) breaking elongation (A ₈₀ ) in percent over the tensile strength (R _m ) in MPa for various steels for the automotive industry are plotted;

FIG. 2

shows a schematic stress-strain diagram of a steel product, which has a distinct yield strength (Lüders stretching A _L );

FIG. 3

shows a schematic diagram showing the two temperature treatment processes;

FIG. 4

shows in the form of a schematic diagram the critical temperature T _K and the course of the corresponding critical temperature limit T _KG ;

FIG. 5

shows a schematic diagram, on the one hand represents the Lüdersdehnung A _L in percent and on the other hand, the average original austenite grain size (D _{UAK M} ) as a function of the first annealing temperature T1, in this diagram corresponding curves of two different samples are shown;

FIG. 6

shows a schematic diagram showing the stress σ in MPa as a function of the strain ε in% (analogous to Fig. 2 ), where four identical alloys were subjected to four different temperature treatment processes.

Detailed description

According to the invention, it is a question of steel products, or steel intermediates, which are distinguished by a special microstructure constellation and properties.

In the following, the term "intermediate steel products" is sometimes used when it comes to emphasizing that it is not about the finished steel product but about a preliminary or intermediate product in a multi-stage production process. The starting point for such production processes is usually a melt. The following is an indication of the alloy composition of the melt, since on this side of the manufacturing process it is possible to influence the alloy composition relatively precisely (for example by tartrating 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 stated. If information is provided on the composition of the alloy, or the steel product, then in addition to the explicitly listed materials or materials, the composition comprises iron (Fe) and so-called unavoidable impurities, which always occur in the molten bath and also in the resulting steel intermediate demonstrate. 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.

In addition to the special combination of alloying elements, a specially optimized process for temperature treatment is used. A corresponding diagram is in Fig. 3 and will be explained in more detail below.

The temperature treatment of the steel intermediate product comprises a first temperature treatment process S.1 and a subsequent second temperature treatment process S.2. These two temperature treatment processes S.1 and S.2 are in Fig. 3 shown in two temperature-time diagrams shown side by side.

The first temperature treatment process S.1 is a high-temperature process in which the steel intermediate is subjected to a first annealing temperature T1 during a first holding period Δ1 (this step is also referred to as holding H1). The annealing temperature T1 is during holding H1 above a critical temperature limit T _KG .

The course of this critical temperature limit T _KG is dependent (inter alia) on the manganese content Mn of the alloy of the manganese steel intermediate, as determined by numerous investigations. In Fig. 4 are the critical temperature T _K (shown by the straight line 7) and the course of the corresponding critical temperature limit T _KG (shown by the straight line 8) shown.

On the horizontal axis, the manganese range MnB is plotted in percent by weight. As already mentioned, the invention provides excellent results, especially with a manganese content in the following manganese range MnB: 3% by weight ≦ Mn ≦ 12% by weight. This manganese range MnB is in Fig. 4 by two vertical boundary lines at Mn = 3% by weight and Mn = 12% by weight.

In Fig. 4 For example, the measurement results of four samples are shown 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. Table 1 alloy C Mn al Nb S.1 S.2 Type 1 0.096 5.08 Cont. Annealing Hood type2 0.097 5.13 0.09 Cont. Annealing Hood typ3 0,100 6.38 Cont. Annealing Hood Typ4 0.106 3.53 Cont. Annealing Hood Typ5 0,110 3.56 0,095 Cont. Annealing Hood Type6 0.148 7.73 2.09 Cont. Annealing Cont. Annealing type 7 0.098 9.95 Cont. Annealing Hood

The alloy composition of the respective type is shown in Table 1, wherein only the essential alloying constituents 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.

In Fig. 4 the following four samples are shown by the circle symbols mentioned: Type 4, 18; Type 1, 1; Type 3, 14 and Type 7, 24 (the designation Type 4, 18 represents, for example, the alloy composition of Type 4, Example No. 18).

If you use the circle symbols of Fig. 4 , respectively, the measurement results interpolated by a straight line, so there is a constant sloping straight line 7, as in Fig. 4 shown. This straight line 7 can be circumscribed by the following equation (1), where T _{K is given} in degrees Celsius: $${\mathrm{T}}_{\mathrm{K}}=\left(866-{\mathrm{S}}_{\mathrm{K}}*\mathrm{manganese\; content}\right)$$

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.

In addition, it has been shown that the critical temperature T _K for alloy compositions according to the invention is always above a lower critical temperature limit T _KG . This lower critical temperature limit T _KG is in Fig. 4 shown as a straight line 8.

This straight line 8 can be circumscribed by the following equation (2), where T _{KG is given} in degrees Celsius: $${\mathrm{T}}_{\mathrm{KG}}=\left(856-{\mathrm{S}}_{\mathrm{K}}*\mathrm{manganese\; content}\right)$$

The straight line 8 is parallel to the straight line 7.

The following condition can be postulated: For steel alloys of the manganese steel intermediate, as already defined, the first annealing temperature T1 must always be above the lower critical temperature limit T _KG to ensure that a manganese steel intermediate is obtained in which the Lüders stretching A _{L is} less than 3%.

It could be shown that also the second temperature treatment process S.2 has an influence on the Lüders strain. 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 T1 is always above the lower critical temperature limit T _KG , it can be concluded that the second annealing temperature T2 should preferably be below the lower critical temperature limit T _KG .

Based on the schematic example of Fig. 3 It can be seen that the first annealing temperature T1 is above the temperature limit T _KG and that the second annealing temperature T2 is in the range between A ₁ and A ₃ . The second temperature treatment S.2 is referred to in this case as intercritical annealing.

The first holding period Δ1 in all embodiments is preferably at least 10 seconds and preferably between 10 seconds and 6000 minutes.

The second holding period Δ2 is at least 10 seconds in all embodiments. In Fig. 3 the two holding periods Δ1 and Δ2 are shown by way of example only. The interval between the first temperature treatment process S.1 and the second temperature treatment process S.2 can be selected as needed. Typically, the second temperature treatment process S.2 is performed shortly after the first temperature treatment process S.1.

Embodiments are preferred in which the first temperature treatment process S.1 including the heating of the steel intermediate product E1, the holding H1 of the first annealing temperature T1 and the cooling Ab1 of the steel intermediate takes less than 7000 minutes.

Embodiments are preferred in which the second temperature treatment process S.2 including the heating of the steel intermediate product E2, 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.

Furthermore, it could be shown that the significant reduction of Lüders stretching A _L is independent of whether the first temperature treatment process S.1 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 annealer).

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 T1 to a value above the critical temperature limit T _KG clearly leads to an increase in the mean original austenite grain size and to a significant reduction in the Lüders strain A _L.

Fig. 5 shows both the reduction of Lüders elongation A _L in percent and the dependence of the mean original Austenitkorngröße (D _{UAK M} ) in microns with increasing annealing temperature T1 for two exemplary samples of type 1 and type 2 (see also Table 1), as follows.

• Mn = 5,08 Gew.%,
• C = 0,096 Gew.%,

Rest Eisen Fe und unvermeidbare Verunreinigungen.Chemical composition of type 1 alloy samples without microalloying:

• Mn = 5.08% by weight,
• C = 0.096% by weight,

Residual iron Fe and unavoidable impurities.

• Mn = 5,13 Gew.%,
• C = 0,097 Gew.%,
• Nb = 0,90 Gew.%,

Rest Eisen Fe und unvermeidbare Verunreinigungen.Chemical composition of type 2 micro-alloy alloy samples:

• Mn = 5.13% by weight,
• C = 0.097% by weight,
• Nb = 0.90% by weight,

Residual iron Fe and unavoidable impurities.

You can do that Fig. 5 For example, in the Type 1 alloy composition tested (represented by curve 9), the critical temperature _limit T _{KG1 is ~} 820 ° C if it is desired to achieve a Lüders strain of less than 3% for this Type 1 alloy composition. The curve 10 shows the associated course of the mean original austenite grain boundary D _{UAK M 1} , as a function of the temperature T1. For the example Type1, a grain size for this results with> 3μm.

You can do that Fig. 5 For example, in the type 2 alloy composition tested (represented by curve 11), the critical temperature _limit T _{KG2 is ~} 970 ° C, if it is desired to achieve a Lüders strain of less than 3% for this type 2 alloy composition. The curve 12 shows the associated course of the mean original austenite grain boundary D _{UAK M} , as a function of the temperature T1. For the example Type2, a grain size for this results with> 8μm. The micro-alloying element niobium (Nb) has a recognizable influence, which is shown as a shift from T _KG2 (compared to T _KG1 ) to a higher critical temperature for A _L <3%.

The curves 10 and 12 in Fig. 5 show that the original austenite grain size increases with increasing temperature T1.

From the aforementioned equation (2), for the alloy compositions of type 1, the lower critical temperature _limit T _KG1 can be determined as follows: $${\mathrm{<figure-callout\; id="1"\; label="T\; KG"\; filenames="imgf0001.png,imgf0004.png"\; state="\{\{state\}\}">T\; </figure-callout>}}_{\mathrm{KG}}=\left(856-7.83*5\right)=~817\mathrm{°\; C}$$

In Fig. 5 the corresponding lower critical temperature _limit T _{KG1 is shown} as a dashed vertical line. It can be seen that the alloy compositions of type 1 from an annealing temperature T1> T _{KG1 have} a mean grain size which is> 3 μm. The lower critical temperature _limit T _KG1 is in Fig. 4 characterized by a small black triangle.

From the equation (2), for the alloy compositions of type 2, the lower critical temperature _limit T _KG2 can be determined as follows: $${\mathrm{<figure-callout\; id="2"\; label="T\; KG"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">T\; </figure-callout>}}_{\mathrm{KG}}=\left(856-7.83*5\right)=~817\mathrm{°\; C}={\mathrm{<figure-callout\; id="1"\; label="T\; KG"\; filenames="imgf0001.png,imgf0004.png"\; state="\{\{state\}\}">T\; </figure-callout>}}_{\mathrm{KG}}$$

For alloy compositions containing an Nb content, the microalloying leads to an increase in the critical temperature limit T _KG . In Fig. 5 can be seen on the example _Type2 that the critical temperature _limit T _{KG2 is higher} by approx. 150 ° C than with the alloy compositions of Type1. In Fig. 5 the corresponding effective lower critical temperature _limit T * _{KG2 is shown} as a dashed vertical line. For Type 2 alloy compositions, the annealing temperature must be T1> T * _KG2 = T _KG2 + 150 ° C. The resulting average original austenitic grain size in this case is ≥ 8 μm.

Fig. 6 shows a schematic diagram showing the stress σ in MPa as a function of the strain ε in%. The presentation of the Fig. 6 is with the representation of Fig. 2 to compare, where Fig. 6 only a small section shows.

Specifically, four identical samples (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.1 and a subsequent second temperature treatment process S.2. All process parameters were identical, except that in the first temperature treatment process S.1, the first annealing temperature T1 was varied as follows (see column 2 of the following Table 3): Table 3 alloy T1 [° C] T2 [° C] Curve typ3 810 640 13.1 typ3 850 640 13.2 typ3 900 640 13.3 typ3 950 640 13.4

• Mn = 6,38 Gew.%,
• C= 0,1 Gew.%,

Rest Eisen Fe und unvermeidbare Verunreinigungen.The alloys of type 3 had the following main composition in these experiments:

• Mn = 6.38 wt.%,
• C = 0.1% by weight,

Residual iron Fe and unavoidable impurities.

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 ~2.60%. The temperature T1 here was 810 ° C, which is a bit far above the lower critical temperature limit T _KG for a Type 3 alloy and a slope value S _K = 7.83.

The curve 13.2 represents another exemplary sample (Type 3, 15 of Table 2) of the type 3, wherein here yield strength is still slightly pronounced.

Another identical sample (see the dot-dashed curve 13.3 in Fig. 6 ) was temperature treated at a higher temperature T1 = 900 ° C (ie at T1> T _KG ) and no pronounced yield strength is more visible. This is Type 3, 16 of Table 2.

The curve 13.4 represents a further exemplary sample of the type 3, whereby also here no pronounced yield strength is more visible. This is Type 3, 17 of Table 2.

Turning now to the manganese steel intermediates of the invention in connection with the Figure Fig. 1 considered, the corresponding measured values (eg for the alloy compositions of Type 1, Type 2 and Type 3) in the range of approximately 700 to 1000 MPa and with an elongation at break A ₈₀ in the range of approximately 20 to 40%. reference numeral TRIP steels 1 Q & P and TBF steels 2 Medium-manganese steels 3 tensile curve 4 between maximum 5 plateau 6 Just 7 Just 8th Curve 9 Curve 10 Curve 11 Curve 12 curves 13.1, 13.2, 13.3, 13.4 Starting temperature of austenitizing A ₁ Start temperature of full austenitizing A ₃ elongation A ₈₀ Lueders elongation A _L first cooling Ab1 second cooling Starting at 2 middle original austenite grain border D _{UAK M} first holding period Δ1 second holding period Δ2 first heating E1 second heating E2 strain e first stop H1 second stop H2 Manga area MnB austenite RA upper yield strength R _eH lower yield strength R _eL tensile strenght R _m 0.2% proof stress R _p0.2 first temperature treatment process S.1 second temperature treatment process S.2 tension σ slope value S _K first annealing temperature T1 second annealing temperature T2 critical temperature limit T _KG critical temperature limit T _KG1 critical temperature limit T _KG2 effective critical temperature limit T * _KG2

Claims (17)

A method of treating a manganese steel intermediate whose alloy comprises: o a manganese content (Mn) which in the following manganese range (MnB) is 3% by weight ≦ Mn ≦ 12% by weight, o a proportion of one or more alloying elements of the group: silicon (Si), aluminum (Al), nickel (Ni), chromium (Cr), molybdenum (Mo), phosphorus (P), sulfur (S), nitrogen (N) , Copper (Cu), boron (B), tungsten (W), cobalt (Co), o an optional carbon content (C) of less than 1% by weight, o an optional fraction of one or more micro-alloying elements, the total proportion of micro-alloying elements being less than 0.45% by weight, and o, as the remainder, an iron component (Fe) and unavoidable impurities, wherein the temperature treatment of the steel intermediate product comprises a first temperature treatment process (S.1) and a subsequent second temperature treatment process (S.2), characterized in that in the first temperature treatment process (S.1) is a high temperature process in which the steel intermediate is exposed during a first holding period (Δ1) to a first annealing temperature (T1) which is above a critical temperature limit (T _KG ) defined as follows T _KG = (856 - S _K * manganese fraction) degrees Celsius, where S _{K is} a slope value, in the second temperature treatment process (S.2) is an annealing process in which the steel intermediate product is exposed to a second annealing temperature (T2) which is lower than the first annealing temperature (T1). A method according to claim 1, characterized in that the first annealing temperature (T1) in said manganese region (MnB) a Dependency, which is defined as follows: T1 ≈ (866 - S _K * manganese content) degrees Celsius. A method according to claim 1 or 2, characterized in that the slope value S _K = 7.83 ± 10% and preferably S _K = 7.83. A method according to claim 1 or 2, characterized in that the first holding period (Δ1) is at least 10 seconds and preferably between 10 seconds and 6000 minutes. Method according to one of claims 1 to 4, characterized in that the second annealing temperature (T2) is in the range between the temperatures A ₁ and A ₃ . Method according to one of claims 1 to 5, characterized in that in the second temperature treatment process (S.2) the second annealing temperature (T2) during a second holding period (.DELTA.2) of at least 10 seconds is maintained. Method according to one of claims 1 to 5, characterized in that the second temperature treatment process (S.2) including a heating process (E2) of the steel intermediate, the holding (H2) of the second annealing temperature (T2) and a cooling process (A2) of the steel intermediate less takes as 6000 minutes and preferably less than 5000 minutes. Method according to one of claims 1 to 7, characterized in that the proportion of one or more alloying elements in the following range: Silicon (Si) ≦ 3% by weight, and preferably ≦ 2% by weight, Aluminum (Al) ≦ 8% by weight, and preferably ≦ 6% by weight, Nickel (Ni) ≦ 2% by weight, and preferably ≦ 1% by weight, Chromium (Cr) ≦ 2% by weight, and preferably ≦ 0.5% by weight, Molybdenum (Mo) ≦ 0.5% by weight, and preferably ≦ 0.25% by weight, Phosphorus (P) ≦ 0.05% by weight, and preferably ≦ 0.025% by weight, Sulfur (S) ≦ 0.03% by weight, and preferably ≦ 0.01% by weight, Nitrogen (N) ≦ 0.05% by weight, and preferably ≦ 0.025% by weight, Copper (Cu) ≦ 1% by weight, and preferably ≦ 0.5% by weight, Boron (B) ≦ 0.005% by weight, and preferably ≦ 0.0035% by weight, Tungsten (W) ≦ 1% by weight, and preferably ≦ 0.5% by weight, Cobalt (Co) ≦ 2% by weight, and preferably ≦ 1% by weight. Method according to one of claims 1 to 7, characterized in that the micro-alloying elements are elements of the group: titanium (Ti), niobium (Nb), vanadium (V). Method according to one of claims 1 to 9, characterized in that it is in the first temperature treatment process (S.1) is a process which is carried out in a continuous belt plant or in a discontinuous plant. Method according to one of claims 1 to 10, characterized in that it is in the second temperature treatment process (S.2) is a process which is carried out in a continuous belt plant or in a discontinuous plant, wherein the steel intermediate in this Appendix the annealing process is exposed in a protective gas atmosphere. A method according to claim 11, characterized in that a Haubenglühvorrichtung serves as a discontinuous plant. A method according to any one of claims 1 to 12, characterized in that the steel intermediate is subjected to a skin pass- through process in a step downstream of the second heat treatment process (S.2), this dressing process being primarily directed to condition the surface of the steel intermediate. Method according to one of claims 1 to 12, characterized in that the first temperature treatment process (S.1) is carried out during a hot rolling process, said hot rolling process is carried out with a rolling end temperature in the range above the critical temperature limit (T _KG ). A steel intermediate product which has been subjected to a temperature treatment according to a process of claims 1 to 14, characterized in that it has a Lüders elongation (A _L ) which is less than 3% and preferably less than 1%. An intermediate steel product which has been temperature treated by a process of claims 1 to 14, characterized in that a Lüders elongation (A _L ) of less than 3% of the steel intermediate is measurable before the steel intermediate is subjected to a downstream skin pass. Intermediate steel product which has been subjected to a temperature treatment by a process according to claims 1 to 14, characterized in that it has a greater usable technical elongation due to a reduced Lüders stretching (A _L ) compared to the reduction of Lüders stretching with temper rolling.

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