EP3433386B1 - Method for temperature-treating a manganese steel intermediate product. - Google Patents

Description

The present invention relates to a method of heat treating a manganese steel intermediate. It is also about a specific alloy of a manganese steel intermediate, which is heat-treated in a special process in order to achieve a significantly reduced fatigue expansion. This application claims priority from European patent application number EP 16 162 073.7 which was submitted on March 23, 2016.

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

It is known that heating, holding and cooling can have an impact on the final structure of a steel product as part of a heat treatment. Furthermore, as already indicated, the alloy composition of the steel product also plays a major role. The thermodynamic and material-technical 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 percent by weight (% by weight) is often in the range between 3 and 12. A medium manganese steel has a high combination of tensile strength and elongation due to its structure. Typical applications in the automotive industry are complex safety-relevant deep-drawn components.

In Fig. 1 A classic, highly schematic diagram is shown, in which the elongation at break A ₈₀ (called total elongation) is plotted in percent above the tensile strength (called tensile strength) in MPa. The tensile strength is abbreviated here with R _m . The diagram of the Fig. 1 provides an overview of the strength classes of steel materials currently used 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. Put simply, it can be stated that the elongation at break A ₈₀ decreases with increasing tensile strength R _m and vice versa. An optimal compromise between the elongation at break A ₈₀ and the tensile strength R _m must therefore be found for each application.

In the automotive sector, you work with a whole range of different steel alloys, each of which has been specially optimized for its respective area of application on the vehicle. Alloys with good energy absorption are used for interior and exterior panels, structural parts and bumpers. Steel panels for the outer skin of a vehicle are relatively "soft" and have, for example, a tensile strength R _m of approximately 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. TRIP (transformation induced plasticity) steels (reference number 1 in.) Are very well suited for this Fig. 1 ).

Steel alloys with a high tensile strength R _m of mostly more than 1000 MPa are used for steel barriers (e.g. for side impact protection), which are intended to prevent the penetration of vehicle parts in the event of an accident. For example, the new generation of high-strength AHSS (Advanced High-Strength Steels) steels is suitable here (reference number 2 in Fig. 1 ). This category includes 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 in Fig. 1 With the reference number 3, the medium-manganese steels already mentioned are summarized schematically. The range designated by the reference number 3 comprises medium-manganese steels with an Mn content between 3 and 12% by weight and with a carbon content ≤ 1% by weight.

Today's medium manganese steels have a pronounced yield strength due to their ultra-fine grain (typically ≤ 1µm), which is clearly shown in the tensile test. An example tension curve 4 (also called stress-strain curve) is the Fig. 2 refer to. In Fig. 2 the stress σ (in MPa) is plotted against the strain ε (in%). The tension curve 4 shows an intermediate maximum 5, which is referred to as the upper yield point (R _eH ), followed by a plateau 6. In the region of the lower yield point (R _eL ), the plateau 6 _merges into an increasing curve area. The "length" of the plateau 6 is referred to as the elongation (A _L ), as in Fig. 2 shown. A steel product with such a pronounced yield strength can form undesirable strains on the surface of the components for the automotive industry. For this reason, the pronounced yield strength must typically be reduced by a re-rolling process. 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 skin training is sometimes quite high. In addition, this process leads to a reduction in the usable elongation.

It is therefore the task of developing a process for producing manganese steel intermediate products in which the elongation is less pronounced. The manganese steel intermediate products should preferably have no (measurable) fatigue strain.

Studies on numerous alloy compositions of medium-manganese steels have shown that there is a connection between the original austenite grain size of these steels and the elongation at break. I.e. the original austenite grain size has an influence on the mechanical properties of these steels. In general, it can be postulated that the elongation is inversely proportional to the original austenite grain size.

It is therefore part of the object of the invention to find an alloy composition and a method for temperature treatment in order to achieve an increase in the original austenite grain size and to manifest the enlarged austenite grains in the structure of the medium-manganese steels. Unlike in the prior art (see e.g. WO2014095082 A1 ), when it comes to providing ultra-fine microstructures (with an ultra-fine grain with an average grain size of approx. 1 µm), the invention aims in a different direction. In addition, the patent application mentioned as an example WO2014095082 A1 a double annealing process is used, which works with different temperatures and processes. Steel products made by the process of WO2014095082 A1 have a clearly defined yield strength.

According to the invention, a particularly suitable manganese steel alloy and an optimized method for the temperature treatment of a manganese steel intermediate product are provided.

• 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 lies in the following manganese range 3% by weight ≤ Mn ≤ 12% by weight,
• a proportion of one or more alloying elements of the group:
  Silicon (Si), aluminum (Al), nickel (Ni), chrome (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 portion of one or more microalloying elements e.g. For example: titanium (Ti), niobium (Nb) and vanadium (V), the total proportion of the microalloying elements being less than 0.45% by weight, and
• the remainder iron (Fe) and unavoidable impurities.

The manganese-steel intermediate products 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 as part of a temperature treatment according to the invention.

The first temperature treatment process is a high temperature process in which the steel intermediate product is exposed to a first annealing temperature for a first holding period which is above a critical temperature limit (referred to as T _KG ), which defines the critical temperature limit (T _KG ) as follows is: T _KG ≥ (856 - SK ^∗ manganese content) degrees Celsius, and where S _{K is} a slope value.

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

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

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.

In all embodiments, the first annealing temperature T1 preferably shows a dependency on the stated manganese range of the alloy, which is defined as follows: T1 T T _KG .

Embodiments of the invention are particularly preferred at a critical temperature TK ((866 - S _K ^∗ manganese content) degrees Celsius, where: S _K = 7.83 ± 10%.

In all embodiments, the first holding period is preferably at least 10 seconds. The first holding time is particularly preferably between 10 seconds and 7000 minutes in all embodiments.

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

Advantageous 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. This total time is preferably 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% 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.

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

The invention makes it possible for the first time to provide intermediate steel products which have an elongation A _L which is less than 3% and preferably less than 1%.

At the same time, the process according to the invention can be used to produce intermediate steel products which have an average primary austenite grain size which is larger than 3 μm.

According to the invention, the alloy of the steel intermediate products of the invention preferably has an average manganese content, which means that the manganese content is in the range 3% by weight M Mn 12 12% by weight. In all embodiments, the manganese content is preferably in the range from 3.5% by weight M Mn 8 8.5% by weight.

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

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

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

If heat treatment of a hot strip is involved, the first temperature treatment of the invention can also be carried out by special temperature control during hot rolling. With this special temperature control, care is taken to ensure that the end temperature of the hot strip during hot rolling is 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 discontinuously operating plant, the steel intermediate being exposed to the annealing process in this plant in a protective gas atmosphere. This process is preferably carried out in a bell annealer. In all embodiments, however, the second temperature treatment process can also be carried out in a continuous strip system (annealing system) or in a hot-dip galvanizing system.

The steel intermediate of all embodiments can optionally be subjected to a skin pass process, this skin pass process primarily aimed at conditioning the surface of the steel intermediate product. A more intensive skin pass is not necessary since the steel intermediate products of the invention have a low fatigue elongation.

With the invention, the degree of skin passage can thus be reduced or avoided entirely.

It is an advantage of the invention that steel intermediates can be produced which have an elongation at break which is less than 3% and which is preferably less than 1%.

It is an advantage of the invention that intermediate steel products 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 intermediate products can be produced which, owing to the reduced elastic expansion, have a (minimum) elongation at break (A ₈₀ ) which is greater than 10%.

It is an advantage of the invention that the steel intermediate products have an increased technically usable elongation due to the reduced elastic expansion.

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

The invention can also be used to provide hot-rolled 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.

Exemplary embodiments of the invention are described in more detail below with reference to the drawings.

FIG. 1

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

FIG. 2nd

shows a schematic stress-strain diagram of a steel product which has a clearly defined yield strength (elastic strain A _L );

FIG. 3rd

shows a schematic diagram showing the two temperature treatment processes;

FIG. 4th

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

FIG. 6

shows a schematic diagram that shows the stress σ in MPa as a function of the elongation 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 matter of steel products or steel intermediate products which are distinguished by a special structure and properties.

In the following, we sometimes speak of intermediate steel products when it is important to emphasize that it is not the finished steel product but rather a preliminary or intermediate product in a multi-stage manufacturing process. The starting point for such manufacturing processes is usually a melt. In the following, the alloy composition of the melt is given, because on this side of the manufacturing process you can influence the alloy composition relatively precisely (e.g. by adding components such as alloy elements and optional micro-alloy elements). The alloy composition of the steel intermediate normally deviates only insignificantly from the alloy composition of the melt.

Quantities or proportions are mostly given in percent by weight (short% by weight), unless otherwise stated. If information is given on the composition of the alloy or steel product, then the composition includes iron (Fe) and so-called unavoidable in addition to the explicitly listed materials Contamination that always occurs in the weld pool and that is also evident in the resulting steel intermediate. All percentages by weight must therefore always be supplemented to 100 percent by weight and all percentages by volume must always be supplemented to 100 percent of the total volume.

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

The heat treatment of the intermediate steel 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 side-by-side temperature-time diagrams.

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

The course of this critical temperature limit T _KG depends (among other things) on the manganese content Mn of the alloy of the manganese steel intermediate, as has been determined on the basis of numerous studies. In Fig. 4 the critical temperature TK (represented by straight line 7) and the course of the corresponding critical temperature limit T _KG (represented by straight line 8) are shown.

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

In Fig. 4 the measurement results of four samples are shown using small circle symbols. Further details on these four samples to be understood as examples and on further samples of the invention can be found in Tables 1 and 2. Table 1 alloy C. Mn Al Nb P.1 P.2 Type 1 0.096 5.08 Cont. Glow Hood Type 2 0.097 5.13 0.09 Cont. Glow Hood Type 3 0.100 6.38 Cont. Glow Hood Type 4 0.106 3.53 Cont. Glow Hood Type 5 0.110 3.56 0.095 Cont. Glow Hood Type 6 0.148 7.73 2.09 Cont. Glow Cont. Glow Type 7 0.098 9.95 Cont. Glow Hood

The alloy composition of the respective type can be found in Table 1, only the essential alloy components being mentioned here. There are a number of exemplary embodiments that have been tested for each type. The corresponding examples are numbered 1 to 26 in the left column in Table 2.

In Fig. 4 the following four samples are shown by the circle symbols mentioned: type 4, 18; Type1, 1; Type 3, 14 and Type 7, 24 (the designation Type 4, 18 stands for example for the alloy composition of Type 4, Example No. 18).

If you look at the circular symbols of the Fig. 4 , or the measurement results interpolated by a straight line, results in a constantly decreasing straight line 7, as in Fig. 4 shown. This straight line 7 can be described by the following equation (1), where T _{K is given} in degrees Celsius: $${\mathrm{T}}_{\mathrm{K}}=\left(866-{\mathrm{S}}_{\mathrm{K}}\ast \mathrm{Manganese\; content}\right)$$

The absolute value 866 in degrees Celsius defines the point of intersection with the vertical axis and the value S _K defines the slope. S _K is therefore also referred to as the slope value.

The investigations have shown that the slope value S _{K is} preferably = 7.83 ± 10% in all embodiments.

It was also possible to show that the critical temperature TK 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 straight line 8.

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

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

The following condition can be postulated: In the case of 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 in} order to ensure that a manganese steel intermediate is obtained in which the fatigue expansion occurs A _{L is} less than 3%.

It could be shown that the second temperature treatment process S.2 also has an influence on the elongation. In order to maintain the grain size of the austenite grains in the structure, 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 .

Using the schematic example of the 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 . In this case, the second temperature treatment S.2 is also referred to as intercritical annealing.

In all embodiments, the first holding period Δ1 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 times Δ1 and Δ2 are only shown as examples. The time interval between the first temperature treatment process S.1 and the second temperature treatment process S.2 can be selected as required. The second temperature treatment process S.2 is typically carried out shortly after the first temperature treatment process S.1.

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

Embodiments are preferred in which the second temperature treatment process S.2, including heating E2 of the steel intermediate, maintaining H2 of the second annealing temperature T2 and cooling Ab2 of the steel intermediate, takes less than 6000 minutes and preferably less than 5000 minutes.

It could also be shown that the significant reduction in the elongation 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 system (for example in a continuous system) or in a discontinuously operating system (for example in a bell annealer).

The invention can be applied both to cold strip intermediate products and to hot strip intermediate products. In both cases there is a significant reduction in the elongation A _L.

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 elongation under stress A _L.

Fig. 5 shows both the reduction in the elongation A _L in percent and the dependence of the mean original austenite grain size (D _{UAK M} ) in µm 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 microalloy:

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

Balance 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 alloy samples with microalloy:

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

Balance iron Fe and unavoidable impurities.

You can do that Fig. 5 derive from the fact that the critical temperature _limit T _KG1 ∼820 ° C is for the examined alloy composition of type 1 (represented by curve 9), if one wants to achieve an elongation at break for this type composition 1 which is less than 3%. Curve 10 shows the associated course of the mean original austenite grain boundary D _{UAK M 1} , as a function of temperature T1. For the Type 1 example, this results in a grain size of> 3 µm.

You can do that Fig. 5 derive from the fact that the critical temperature _limit T _{KG2 is} ∼970 ° C for the examined alloy composition of type 2 (represented by curve 11), if one wants to achieve a fatigue strain which is less than 3% for this type 2 alloy composition. Curve 12 shows the associated course of the mean original austenite grain boundary D _{UAK M} , depending on the temperature T1. For the Type 2 example, this results in a grain size of> 8 µm. The _{microalloying element} niobium (Nb) has a noticeable influence, which is shown as a shift from T _KG2 (compared to T _KG1 ) to a higher critical temperature for A _L <3%.

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

Using the aforementioned equation (2), the lower critical temperature _limit T _KG1 can be determined as follows for the alloy compositions of type 1: $${\mathrm{<figure-callout\; id="1"\; label="T\; KG"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T\; </figure-callout>}}_{\mathrm{KG}}=\left(856-7.83\ast 5\right)=\sim 817°\mathrm{C.}$$

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 _have an average grain size which is> 3 μm from an annealing temperature T1> T _KG1 . The lower critical temperature _limit T _KG1 is in Fig. 4 identified by a small black triangle.

The lower critical temperature _limit T _KG2 can be determined as follows for the type 2 alloy compositions _using equation (2): $${\mathrm{T}}_{\mathrm{KG}\mathrm{2nd}}=\left(856-7.83\ast 5\right)=\sim 817°\mathrm{C.}={\mathrm{<figure-callout\; id="1"\; label="T\; KG"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T\; </figure-callout>}}_{\mathrm{KG}}$$

In alloy compositions containing an Nb content, the microalloy leads to an increase in the critical temperature limit T _KG . In Fig. 5 can be seen from the example of type 2 that the critical temperature _limit T _KG2 around is approx. 150 ° C higher than with Type 1 alloy compositions. 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. In this case, the resulting mean original austenitic grain size is ≥ 8 µm.

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

Specifically, four identical samples (type 3 alloys in Table 1) were compared here. 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 Type 3 810 640 13.1 Type 3 850 640 13.2 Type 3 900 640 13.3 Type 3 950 640 13.4

• Mn = 6,38 Gew.%,
• C= 0,1 Gew.%,

Rest Eisen Fe und unvermeidbare Verunreinigungen.The Type 3 alloys had the following main composition in these tests:

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

Balance iron Fe and unavoidable impurities.

The solid curve 13.1 Fig. 6 (Type 3, 14 of Table 2) shows a clearly visible pronounced yield point and has an elongation of A _L ∼2.6%. The temperature T1 here was 810 ° C., which for a type 3 alloy and a slope value S _K = 7.83 is a bit above the lower critical temperature limit T _KG .

Curve 13.2 represents another exemplary sample (type 3, 15 of table 2) of type 3, the yield strength here still being slightly pronounced.

Another identical sample (see dash-dotted 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 point is visible. This is type 3, 16 of table 2.

Curve 13.4 represents another exemplary sample of type 3, with no pronounced yield point being visible here either. This is type 3, 17 of table 2.

Considering the manganese steel intermediates of the invention in connection with the figure Fig. 1 considered, the corresponding measurement values (eg for the alloy compositions of type 1, type 2 and type 3) are in the range from approx. 700 to 1000 MPa and with an elongation at break A ₈₀ in the range from approx. 20 to 40%. Reference numerals TRIP steels 1 Q&P and TBF steels 2nd Medium manganese steels 3rd Tensile curve 4th Intermediate maximum 5 plateau 6 Just 7 Just 8th Curve 9 Curve 10th Curve 11 Curve 12 Curves 13.1, 13.2, 13.3, 13.4 Austenitization start temperature A ₁ Start temperature of the full austenitization A ₃ Elongation at break A ₈₀ Stretching A _L first cooling From1 second cooling Starting at 2 Mean original austenite grain limit D _{UAK M} first holding period Δ1 second holding period Δ2 first warming up E1 second heating E2 strain ε first stop H1 second hold H2 Manganese area MnB Residual austenite content RA upper yield point R _eH lower yield strength R _eL tensile strenght R _m 0.2% proof stress R _p0.2 first temperature treatment process P.1 second temperature treatment process P.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 (14)

1. Method for temperature-treating a manganese steel intermediate product whose alloy comprises:

   ∘ a manganese content (Mn), which lies in the following manganese range (MnB):
   3 wt.% ≤ Mn ≤12 wt.%,

   ∘ a content 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),

   ∘ an optional carbon content (C) of less than 1 wt.%,

   ∘ an optional content of one or more microalloying elements, wherein the total content of the micro-alloying elements is less than 0.45 wt.%; and

   ∘ as a remainder an iron content (Fe) and unavoidable impurities, wherein the temperature-treating 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

   - the first temperature treatment process (S.1) is a high-temperature process in which the steel intermediate product is subjected during a first holding period (Δ1) to a first annealing temperature (T1) which lies above a critical temperature limit (T_KG), which is defined as follows: T_KG = (856 - S_K ^∗ manganese content) degrees Celsius, wherein S_K is a slope value, and wherein said slope value S_K = 7.83 ±10%, preferably S_K = 7.83,

   - the second temperature treatment process (S.2) is an annealing process in which the steel intermediate product is subjected to a second annealing temperature (T2) which is lower than the first annealing temperature (T1).
2. Method according to claim 1, characterized in that the first annealing temperature (T1) in said manganese range (MnB) has a dependence defined as follows: T₁ ≈ (866 - S_K ^∗ manganese content) degrees Celsius.
3. 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.
4. Method according to one of the claims 1 to 4, characterized in that the second annealing temperature (T2) is in the range between the temperatures A₁ and A₃, wherein A₁ is the start temperature of austenitization and A₃ is the start temperature of full austenitization.
5. Method according to claim 1, characterized in that the second annealing temperature (T2) is in the range of 630°C to 675°C.
6. Method according to one of the claims 1 to 5, characterized in that within the scope of the second temperature treatment process (S.2), the second annealing temperature (T2) is held during a second holding period (Δ2) of at least 10 seconds.
7. Method according to one of the claims 1 to 5, characterized in that the second temperature treatment process (S.2), including a heating process (E2) of the steel intermediate product, the holding (H2) of the second annealing temperature (T2) and a cooling process (A2) of the steel intermediate product, takes less than 6000 minutes and preferably less than 5000 minutes.
8. Method according to one of the claims 1 to 7, characterized in that the content of the one or more alloying elements is in the following range:

   - silicon (Si) ≤ 3 wt.%, and preferably ≤ 2 wt.%,

   - aluminum (Al) ≤ 8 wt.%, and preferably ≤ 6 wt.%,

   - nickel (Ni) ≤ 2 wt.%, and preferably ≤ 1 wt.%,

   - chromium (Cr) ≤ 2 wt.%, and preferably ≤ 0.5 wt.%,

   - molybdenum (Mo) ≤ 0.5 wt.%, and preferably ≤0.25 wt.%,

   - phosphorus (P) ≤ 0.05 wt.%, and preferably ≤ 0.025 wt.%,

   - sulfur (S) ≤ 0.03 wt.%, and preferably ≤ 0.01 wt.%,

   - nitrogen (N) ≤ 0.05 wt.%, and preferably ≤ 0.025 wt.%,

   - copper (Cu) ≤ 1 wt.%, and preferably ≤ 0.5 wt.%,

   - boron (B) ≤ 0.005 wt.%, and preferably ≤ 0.0035 wt.%,

   - tungsten (W) ≤ 1 wt.%, and preferably ≤ 0.5 wt.%,

   - cobalt (Co) ≤ 2 wt.%, and preferably ≤ 1 wt.%.
9. Method according to one of the claims 1 to 7, characterized in that the micro-alloying elements are elements of the group: titanium (Ti), niobium (Nb), vanadium (V).
10. Method according to one of the claims 1 to 9, characterized in that the first temperature treatment process (S.1) is a process which is carried out in a continuous strip plant or in a discontinuously operating plant.
11. Method according to one of the claims 1 to 10, characterized in that the second temperature treatment process (S.2) is a process which is carried out in a continuous strip plant or in a discontinuously operating plant, wherein the steel intermediate product in this plant is exposed in the annealing process to a protective gas atmosphere.
12. Method according to claim 11, characterized in that a hood-type annealing device is used as a discontinuously operating plant.
13. Method according to one of the claims 1 to 12, characterized in that the steel intermediate product is subjected to a skin-pass rolling process in a step which is downstream of the second temperature treatment process (S.2), wherein this skin-pass rolling process is primarily directed to condition the surface of the steel intermediate product.
14. Method according to one of the claims 1 to 12, characterized in that the first temperature treatment process (S.1) is carried out during a hot rolling process, wherein said hot rolling process is carried out with a rolling end temperature which lies in the range above the critical temperature limit (T_KG).

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