EP2366035B1 - Manganese steel strip having an increased phosphorus content and process for producing the same - Google Patents

Description

The invention relates to an austenitic manganese steel strip and to a method for producing austenitic manganese steel strips. Furthermore, the invention relates to a manganese steel sheet with a deformed, in particular stretched or deep-drawn sheet steel section.

Manganese austenites are lightweight structural steels that are particularly strong and elastic at the same time. The weight reduction afforded by the higher strength makes manganese austenite a material of great potential in the automotive industry. Because through lighter bodies fuel consumption can be reduced, with a high elasticity and stability for the production of the body parts and their crash behavior are important.

TRIP-steels (TRANSformation Induced Plasticity), which are increasingly used in the automotive industry, are already known. High-alloyed TRIP steels reach high tensile strengths of up to more than 1000 MPa and can have elongations of up to about 30%. Due to these high mechanical properties thinner plates and thus a reduction in body weight can be achieved in vehicle construction. TRIP steel consists of several phases of iron-carbon alloys, mainly ferrite, bainite and carbon-rich residual austenite. The TRIP effect is based on the deformation-induced transformation of residual austenite into martensite. This remodeling of the crystal structure results in a simultaneous increase in strength and formability in product manufacture or in product use in the event of a crash. The TRIP effect can be specifically influenced by admixing the alloying elements aluminum and silicon.

In the case of TRIP steel, a certain amount of austenite is already being transformed into the high-strength martensitic phase (α-martensite) during deep-drawing of the body part, which can hardly be stretched any longer. It is therefore possible that with TRIP steels for the crash case only a relatively small expansion reserve remains.

The recently developed TWIP steels differ from TRIP steels in that they have a higher elongation at break (50% and more). The abbreviation TWIP stands for "TWinning Induced Plasticity", ie a plasticity induced by twinning. The particular ductility of TWIP steels can be caused by different mechanisms in the crystal structure. For example, the extensibility can be promoted by lattice defects in the crystal structure, at which the crystal structure can fold-induced induced, whereby the folding mechanism runs on a mirror plane and regularly mirrored crystal areas (so-called twins) arise. Different types of twins can be distinguished. Furthermore, it is known that further effects such as the occurrence of slip bands can influence the mechanical properties. Due to their high ductility, TWIP steels are excellently suited for the production of metal sheets in the automotive industry, especially for accident-relevant areas of the body. TWIP steels have an austenitic structure and are characterized by a high manganese content (usually over 25%) and relatively high alloying additions of aluminum and silicon.

For example, hot and cold rolled austenitic manganese steel tapes are from the US 2008/0035248 A1 which have a maximum phosphorus content of 0.05% and whose mean particle sizes are given as ≤ 10 μm for hot-rolled manganese steel or <5 μm for cold-rolled manganese steel.

In the FR 2 857 980 A1 Both hot and cold rolled manganese steel strips with a maximum phosphorus content of 0.08% and corresponding manufacturing processes are described. For hot-rolled manganese steel strip, the mean grain size is below 18 μm, while the mean grain size for cold-rolled manganese steel strip remains below 10 μm after the last production step.

One object of the invention is to provide a steel with improved mechanical properties. In particular, good weldability of the steel and / or good formability should be achievable. Furthermore, the invention aims to provide a method for producing a steel with improved mechanical properties, in particular high ductility in combination with high tensile strength, and in particular a good weldability and a good formability.

The object of the invention is based solved by the features of the independent claims. Advantageous embodiments and further developments are specified in the dependent claims.

It has been found that high mechanical properties and good weldability and good formability can be achieved with an austenitic manganese steel strip according to the invention. The steel according to the invention is characterized, inter alia, by the fact that with a carbon content in% by weight of about 0.4% ≦ C ≦ 1.2%, a manganese content of about 12.0% ≦ Mn ≦ 25.0 % is available. The percentages of chemical constituents in this document always refer to percentages by weight, phosphorus, which increases the yield strength or tensile strength, reduces the elongation at break, promotes brittleness, lowers austenite stability, hampers cementite precipitation, and usually weldability decreases, is alloyed according to the invention in a relatively high proportion of at least 0.03%. It turned out in the case of an extensive omission of the alloying element aluminum (Al ≦ 0.05%) with this alloy concept, high mechanical properties and a surprisingly good weldability with very good formability of the manganese steel strip produced can be achieved.

In a hot-rolled austenitic manganese steel strip having the chemical composition of the present invention, a product of elongation at break in MPa and tensile strength in percent of over 65,000 MPa%, especially over 70,000 MPa% can be obtained. In a cold-rolled austenitic manganese steel strip having the chemical composition according to the invention, this product is above 75,000 MPa% and may be above 80,000 MPa%, in particular also above 85,000 MPa%, preferably above 100,000 MPa%.

1. (1) Mikro- und Nanozwillingsbildung mit hoher Dichte:
   • Es wurde bei den erfindungsgemäßen Stählen eine Bevorzugung von Mikrozwillingsbildung (d.h. der Bildung von kleinen Zwillingen mit geringer Dicke) beim Umformprozess im Kristallgefüge festgestellt. Die nach Umformbeanspruchung (z.B. Tiefziehen) festgestellte hohe Dichte der Mikrozwillinge und ihre geringe Dicke im Vergleich zur Dichte und Dicke der Mikrozwillinge bei konventionellen hochmanganlegierten Stählen bewirken eine Erhöhung der Bruchdehnung. Dies ist zumindest teilweise auf die Tatsache zurückzuführen, dass mit der Dichte der Zwillinge die Anzahl an Versetzungshindernissen deutlich ansteigt. Die mittlere Dicke der Mikrozwillinge lag bei Proben des erfindungsgemäßen Manganstahlbandes, nachdem sie einem Umformprozess unterzogen worden waren, vorzugsweise unterhalb von 30 nm, insbesondere unterhalb von 20 nm und insbesondere unterhalb von 10 nm. Zwillinge mit einer Dicke von unter 10 nm werden auch als Nanozwillinge bezeichnet. Nach der Umformbeanspruchung lag gegenüber der üblichen Dichte an Zwillingen insbesondere eine signifikant erhöhte Dichte an Nanozwillingen vor. Es wird vermutet, dass mit Erhöhung des Phosphorgehaltes und einer erniedrigten Stapelfehlerenergie die Dichte der Mikrozwillinge und insbesondere der Nanozwillinge zunimmt. Diese wirken direkt auf der Duktilität des Materials und bieten eine ungewohnte sehr hohe Dehnung im Kombination mit hoher Zugfestigkeit.
2. (2) Mischkristallhärtung:
   • Eine Mischkristallhärtung wird durch hohe Anteile an interstitiell gelösten Legierungselementen wie P und C bewirkt. Dadurch können hohe Festigkeiten (insbesondere größer als 1100 MPa) bei gleichzeitig hohen Kaltverfestigungswerten und Bruchdehnungen (gegebenenfalls größer als 90%) eingestellt werden.
3. (3) Dynamische Reckalterung:
   • Das Auftreten der dynamischen Reckalterung ist auf die hohen Gehalte an interstitiell gelösten Legierungselementen im Stahl zurückzuführen und ist anhand der Spannungs-Dehnungskurven zu erkennen. Dieser Effekt kann einen zusätzlichen Beitrag zur Verbesserung der Festigkeit und der Bruchdehnung des Materials leisten.

It is believed that the good mechanical properties of the manganese steel of the invention are based on a combination of at least the following three mechanisms:

1. (1) High-density micro and nano-twin formation:
   • It has been found in the steels of the invention a preference for micro twinning (ie the formation of small twins with a small thickness) during the forming process in the crystal structure. The high density of the micro twins and their small thickness compared to the density and thickness of the micro twins in conventional high manganese alloy steels determined after forming stress (eg thermoforming) increase the elongation at break. This is at least partly due to the fact that with the density of the twins the number of dislocation obstacles increases significantly. The average thickness of the micro-twins in samples of the manganese steel tape according to the invention, after being subjected to a forming process, was preferably below 30 nm, in particular below 20 nm and in particular below 10 nm. Twins with a thickness of less than 10 nm are also called nano-twins designated. After Umformbeanspruchung was compared to the usual density of twins in particular one significantly increased density of nano-twins before. It is believed that as the phosphorus content is increased and the stacking fault energy decreased, the density of the micro-twins, and especially the nano-twins, increases. These act directly on the ductility of the material and offer an unusual very high elongation in combination with high tensile strength.
2. (2) Solid solution hardening:
   • Solid solution hardening is caused by high levels of interstitially dissolved alloying elements such as P and C. As a result, high strengths (in particular greater than 1100 MPa) can be set with simultaneously high strain hardening values and breaking elongations (possibly greater than 90%).
3. (3) Dynamic strain aging:
   • The occurrence of dynamic strain aging is due to the high content of interstitially dissolved alloying elements in the steel and can be recognized by the stress-strain curves. This effect can make an additional contribution to improving the strength and elongation at break of the material.

In addition, with a corresponding heat treatment, the bake-hardening effect can also be used to increase the yield strength.

For the produced steels, the bake hardening values (BH values) were determined according to the European standard EN 10325. The high levels of interstitially dissolved alloying elements ensure an increased bake-hardening potential and can further improve the mechanical properties of the final product. There was an increase in strength after heat treatment of about 30 to 80 MPa depending on the degree of deformation.

It has been shown that a low manganese content has a positive influence on the phase transformations and the forming mechanisms (in particular the formation of nano and micro twins and stronger solid solution hardening) in the final component. In this respect, the manganese content of an austenitic manganese steel tape according to the invention may preferably be in the range of 14% ≦ Mn ≦ 18.0%, in particular 14% ≦ Mn ≦ 16.5%.

It has also been shown that a very uniform and high solids solubility of the elements C and / or P and / or N in the large grains can be achieved by a large grain size. The good solubility of these elements can also be a cause for the preference of small size micro-twin formation or nano-twin formation and their high density in the crystal structure. Furthermore, it is believed that by the preferably achieved high solids solubility of P and C, the usually negative effects of these elements (deterioration of weldability, embrittlement of the steel) did not occur in a surprising manner in the steel according to the invention. In particular, high concentrations of C and P could be achieved without significantly deteriorating the weldability of the steel.

Since aluminum nitride (AlN) hinders (austenitic) grain growth, the grain size can be influenced in a targeted manner by the ratio of N to Al. By deliberately low addition of Al (for example Al ≦ 0.05%, in particular Al ≦ 0.02%), a high grain size can be made possible with an austenitic manganese steel strip. The Al content can be kept very low in the alloying concept pursued here, as much carbon is available for the deoxidation of the liquid steel. In particular, the manganese steel according to the invention can have the lowest possible aluminum content, which is limited only by unavoidable impurities in the production process (ie no aluminum addition). In the case of the steel strip according to the invention, this results in maximum grain size growth during recrystallization (ie during hot rolling or during annealing).

Furthermore, appropriately high phosphorus contents of 0.03% ≤ P, in particular 0.05% ≤ P, 0.06% ≤ P, 0.07% ≤ P, 0.08% ≤ P and also 0.10% ≤ P are used. It may even be provided a phosphorus content 0.20% ≤ P. A high phosphorus content can increase the tensile strength and especially the yield strength at higher particle sizes. Surprisingly, no significant reduction in elongation at break and no significant deterioration in weldability were observed with an increase in phosphorus content.

By adjusting the mean grain size in the metal structure, the tensile strength and the yield strength as well as the elongation at break of the produced steel strip can be changed in a targeted manner. The larger the grain, the lower the tensile strength and the yield strength and the higher the elongation at break. Medium particle sizes of more than 5 μm or more than 10 μm can be set. In particular, it can be provided that in the hot-rolled austenitic manganese steel a large mean grain size of more than 13 .mu.m , in particular set over 18 .mu.m , and that in the cold-rolled austenitic manganese steel a large average grain size of about 15 .mu.m , in particular about 20 μ m is set.

Similar to aluminum, silicon also impedes the precipitation of carbides such as cementite ((Fe, Mn) ₃ C), which occurs during hot rolling and annealing. Since the cementite precipitation lowers the elongation at break, it could be expected that the elongation at break can be increased by a silicon addition.

However, the manganese steel according to the invention preferably has a very low silicon content (Si ≦ 1.0%, in particular Si ≦ 0.2%, particularly preferably Si ≦ 0.05%), which is optionally limited only by unavoidable impurities in the production process ( ie in this case no addition of silicon, the Si content may then be below Si ≤ 0.03%). The reason for this is that silicon has an influence on deformation mechanisms. Silicon impairs twinning, ie a low silicon concentration facilitates the formation of twins and possibly the formation of small micro-twins or nano-twins. Since the deformation mechanism of the micro-twin formation, and in particular the nano-twin formation strongly promote a high elongation at break, this effect causes an increase in the elongation at break with a reduction of the silicon content. In this case, other deformation mechanisms may be preferred by little silicon. Therefore, the silicon content of the manganese steel of the present invention can be set low, preferably as low as possible. The silicon content can be kept very low, as much carbon is available for the deoxidation of the liquid steel, and because the strength of the steel (silicon causes an increase in strength) by other measures such as high concentrations of C and / or P. is guaranteed.

Niobium (Nb), vanadium (V) and titanium (Ti) are elements that form precipitates (carbides, nitrides, carbonitrides) and may optionally be added to improve strength through precipitation hardening. However, these elements have a grain-fine effect, which is why their concentration should be kept low, if a high grain size is to be ensured.

It is known that nickel (Ni) can stabilize the austenite phase (so-called γ-stabilizer). Nickel may optionally be added in larger amounts (e.g., over 1% to 5% or even 10%).

Unlike nickel, the solid solution strengthener chromium (Cr) stabilizes the α-ferrites. Additions of chromium up to 10% by weight prefer the formation of ε-martensite and / or α'-martensite, resulting in higher tensile strength and lower ductility. The proportion of chromium should therefore be limited. For example, Cr ≦ 5%, in particular Cr ≦ 0.2%, can preferably be set. Molybdenum (Mo) and tungsten (W) also show a grain-refining angle. Tungsten has a high affinity for carbon and forms the hard and very stable carbides W ₂ C and WC steel. The proportion of tungsten should be limited. In this case, W ≦ 1%, in particular W ≦ 0.02%, is set. Tungsten is an even better solid solubility enhancer than chromium and also forms carbides (but to a lesser extent than chromium). In addition, Mo ≦ 1%, in particular, Mo ≦ 0.02% is set. The grain size of a hot-rolled steel strip is also greatly influenced by the final rolling temperature during hot rolling. The steel strip according to the invention can be rolled with a final rolling temperature of between 750 ° C and 1050 ° C, preferably between 800 ° C and 900 ° C. For a given chemical composition can be adjusted by the choice of the final rolling temperature, the average grain size.

It could be shown that in the hot-rolled steel according to the invention a high elongation at break of 60% or 65% and more can be achieved. The tensile strength of the hot-rolled steel may preferably be above 1050 MPa.

Cold rolling can increase the mechanical properties of the hot rolled austenitic manganese steel strip. The grain size of a cold-rolled steel strip is strongly influenced by the annealing temperature. For example, the annealing performed after the cold rolling may be carried out at an annealing temperature between 750 ° C and 1050 ° C, and in particular, the annealing temperature may be greater than 900 ° C. Tensile strengths of more than 1100 MPa, in particular more than 1200 MPa, can be achieved with an elongation at break of more than 75%, in particular over 80%.

A manganese steel sheet according to the invention with the said chemical compositions has a reshaped, in particular stretched or deep-drawn sheet steel section whose microstructure micro-twins having an average thickness of less than 30 nm, in particular less than 20 nm and nano-twins having an average thickness of less than 10 nm. As mentioned, these micro- and nano-twins form during the forming process, whereby the high mechanical properties of the starting material are presumably due - at least in part - to this deformation mechanism.

In a method for producing a hot-rolled austenitic manganese steel strip, the semi-finished product is heated to a temperature above 1100 ° C after casting a semi-finished steel. The heated semi-finished product is rolled at a final rolling temperature between 750 ° C and 1050 ° C, preferably between 800 ° C and 900 ° C. Subsequently, the rolled steel strip is cooled at a rate of 20 ° C./s or higher. Preferably, rapid cooling of the hot rolled steel strip is performed at a rate of 50 ° C / s or higher, more preferably 200 ° C / s or higher. Rapid cooling helps to provide high solids solubility of C, N and P elements in the granules. To put it bluntly, the rapid cooling leads to a "freezing" of the dissolved elements without or with only little excretion formation. In other words, the excretion formation can be largely prevented by a rapid cooling. In particular, the occurrence of grain boundary carbides as well as embrittlement (grain boundary segregation) of the steel structure caused by high phosphorus contents can be prevented by a rapid cooling. The higher the cooling rate, the better and more uniformly can carbon and phosphorus be kept in solution. Cooling rates of over 100 ° C / s to 400 ° C / s were used. Cooling rates of more than 400 ° C / s to even more than 600 ° C / s are also possible. If necessary, before the rapid cooling an intermediate phase of several seconds, in particular 1 to 4 seconds, persist, in which the steel strip slowly cools in air to improve the recrystallization of the phosphorus-alloyed steel strip.

To produce a cold-rolled austenitic manganese steel strip, the hot rolled steel strip is cold rolled and then annealed for recrystallization.

In the case of cold rolling, high reduction in thickness in the range of more than 45%, in particular more than 60%, particularly preferably more than 80%, is preferably carried out by using high rolling forces.

The annealing temperature may be between 750 ° C and 1150 ° C and in particular greater than 900 ° C. By annealing, the grain size can be changed again, after annealing, a grain size of about 15 μ m, especially about 20 microns may be provided to achieve a high elongation at break and possibly an improvement in the solid solubility of carbon, phosphorus and optionally nitrogen , A high tensile strength can be ensured in particular by a relatively high proportion of phosphorus (and carbon).

After annealing, the rolled steel strip is cooled at a rate of 20 ° C / s or higher. Preferably, rapid cooling of the cold-rolled steel strip is conducted at a rate of 50 ° C / sec or higher, more preferably 200 ° C / sec or higher.

As already described in the case of the hot strip process, a rapid cooling also contributes to effecting a high and uniform solid solubility of carbon, phosphorus and nitrogen in the grains and thereby to achieve a high tensile strength even with large grains. Cooling rates of over 100 ° C / s to 400 ° C / s were used. Cooling rates of more than 400 ° C / s to even more than 600 ° C / s are also possible. If necessary, before the rapid cooling an intermediate phase of several seconds, in particular 1 to 6 seconds, persist, in which the steel strip slowly cools in air to improve the recrystallization of the phosphorus-alloyed steel strip.

Fig. 1

ein Schaubild, in welchem für kaltgewalzte Stähle die mittlere Korngröße gegenüber der Glühtemperatur dargestellt ist;

Fig. 2

ein Schaubild, in welchem für mehrere Proben kaltgewalzter Stähle die Kaltverfestigung (n_10/20-Wert) gegenüber der senkrechten Anisotropie (r_0/15-, r_45/15-, und r_90/15-Wert) dargestellt ist;

Fig. 3A-C

schematische Darstellungen von Zwillingen und Mikrozwillingen bzw. Nanozwillingen in der Gefügestruktur von Stählen;

Fig. 4

eine Aufnahme eines Transmissionselektronenmikroskops von einem erfindungsgemäßen Stahlgefüge; und

Fig. 5

einen Mikroschliff der Schweißlinse eines geschweißten erfindungsgemäßen Stahlgefüges.

The invention will now be explained in more detail by way of example with reference to the drawings, by way of example; in the drawings show:

Fig. 1

a graph in which the mean grain size is shown for cold-rolled steels compared with the annealing temperature;

Fig. 2

a graph in which, for several samples of cold-rolled steels, the work hardening (n _10/20 value) versus the vertical anisotropy (r _0/15 , r _45/15 , and r _90/15 value) is shown;

Fig. 3A-C

schematic representations of twins and micro-twins or nano-twins in the microstructure of steels;

Fig. 4

a photograph of a transmission electron microscope of a steel structure according to the invention; and

Fig. 5

a microsection of the weld nugget of a welded steel structure according to the invention.

First, various possibilities for the production of manganese steel according to the invention will be explained by way of example.

In a first process route, pig iron is produced in the blast furnace or with a smelting reduction process such as Corex or Finex. The Tecnored process is also possible. The pig iron is then converted into steel, for example, in an oxygen inflation process (eg in an LD (Linz-Donawitz) / BOF (Bottom Oxygen Furnace) process). Before casting the steel, a vacuum degassing (eg according to the Ruhrstahl-Heraeus process (RH)) can be carried out and a ladle furnace (ladle furnace) for heating and alloying the molten metal.

A second production route, which may be particularly suitable for manganese steels, uses an electric arc furnace (EAF) for steelmaking and an AOD converter for decarburizing the liquid steel. Again, prior to casting the steel, a ladle furnace can be used to heat and alloy the molten metal.

The steel thus produced can be further processed by means of various casting techniques such as block casting, casting rolls, thin strip casting or continuous casting. The steel body produced during casting is called semifinished and may e.g. be realized in the form of slabs, billets or blocks.

The slab is further processed in hot strip mills to hot strip. For this purpose, rolling mills for narrow strip (width less than 100 mm), middle strip (width between 100 mm and 600 mm) and broadband (width greater 600 mm) can be used. Furthermore, the processing of blocks and billets to profiles, pipes or wires is possible.

In the following, a hot strip process (WB) will be described after steel strips according to the invention can be produced.

In the production of steel strips according to the invention, a rolling temperature between about 1100 ° C and 1300 ° C, optionally also higher, can be used. The rolling end temperature may for example be between 750 ° C and 1050 ° C and in particular between 800 ° C and 900 ° C. Different rolling end temperatures result in different average particle sizes of the hot-rolled steel strip according to the dynamic recrystallization at the prevailing temperature. The lower the final rolling temperature, the smaller the average particle size obtained for a given chemical composition. With a reduction of the mean grain size, the tensile strength decreases and the breaking strength of the hot-rolled steel strip, the elongation at break decreases. If the roller end temperature is too low, however, there is the risk that the high grain refining in manganese steels causes a loss of plastic deformability as a result of the increased strength. Furthermore, due to the phase stability, low cementation temperatures increasingly lead to the formation of cementite ((Fe, Mn) ₃ C), which may affect the mechanical properties. The cementite precipitates reached a particle size at rolling end temperatures below 740 ° C., which significantly impaired the mechanical properties.

The mean grain size of the hot strip steel strip is further influenced by the content of aluminum and nitrogen. It is known that manganese increases the solubility of nitrogen in liquid iron. Nitrogen dissolved in liquid iron forms aluminum nitride precipitates with aluminum, which hinder the migration of grain boundaries and thus grain growth. Aluminum nitride may further cause hot working cracking. It has been found that by targeted control of the aluminum and nitrogen content in steel low Endwalztemperaturen well below 950 ° C and especially below 900 ° C down to 750 ° C are possible without causing cracking occurs. However, the formation of large cementite particles, which begins with a lowering of the final rolling temperature below about 740 ° C to 800 ° C, to avoid. Particularly preferred final rolling temperatures in the hot rolling process can therefore be in the range of 800 ° C to 900 ° C.

For example, the avoidance of cracking has been achieved at said final rolling temperatures in the range of 800 ° C to 900 ° C with chemical compositions in which an extremely small amount of aluminum up to 0.008% or 0.010% in combination with a low content of nitrogen to eg 0.030% or 0.036% were used. The respective concentrations of the elements are interdependent. If less nitrogen is used, more aluminum is allowed and vice versa. In this respect, higher nitrogen contents than stated above are possible with a low aluminum content.

After hot rolling, rapid cooling of the hot strip is performed at as high a cooling rate as possible (e.g., above 50 ° C / s or higher). The cooling can be done by applying the hot strip with water.

The hot strip is then removed in a continuous pickling plant e.g. cleaned with sulfuric acid (descaled). The hot strip may have a thickness of 1.5 to 2.0 mm, for example. However, it is also possible to realize hot-rolled strip products with strip thicknesses which are smaller or larger than those specified above. An annealing step is usually not carried out in the hot strip products produced here. In a particular embodiment, however, such an annealing step is carried out and causes a grain coarsening as well as an increase in the elongation at break.

The hot strip produced in the manner described above can be further processed by cold rolling and annealing to the cold strip product. By cold rolling, the hot strip is further reduced in thickness and the mechanical and technological properties of the band are set. For example, low strip thicknesses in the range of about 0.7 mm to 1.75 mm of the cold strip can be produced. Cold-strip products with such small thicknesses are of particular interest in the automotive sector for crash-absorbing components. However, it is also possible to realize cold-rolled strip products with strip thicknesses which are smaller or larger than those specified above.

The cold rolling is preferably carried out using high rolling forces. Roll stands with 2 to 20 rolls can be used. In order to apply the high cold rolling forces, it is possible, for example, to use roll stands with 12 or 20 rolls, in particular of the Sendzimir type (cluster roll), designed for high rolling pressures. A Sendzimir rolling mill with 12 rolls consists for example of a symmetrical arrangement of each of 3 back rolls, 2 intermediate rolls and 1 roller defining the nip pressure roller. A Sendzimir rolling mill with 20 rolls, for example, consists of a symmetrical arrangement of 4 back rolls, 3 outer intermediate rolls, 2 inner intermediate rolls and 1 roller defining the nip pressure roller. It showed a surprisingly good rolling and low cracking compared to other manganese steels.

The percent reduction in thickness (cold rolling degree) achieved during cold rolling may be above 40%, e.g. between 40% and 60%. Cold rolling was also carried out with cold rolling degrees above 60%, especially above 80%. It was cold rolled with and without train.

After cold rolling or in an intermediate step during cold rolling, the steel strip is annealed for recrystallization. The annealing may e.g. be carried out after the continuous annealing or annealing process. By annealing, the solidification of the microstructure occurring during cold rolling is reduced again. It comes here about nucleation and grain growth to a rebuilding of the structure.

The annealing can be carried out at temperatures between 750 ° C and 1250 ° C, in particular 750 ° C to 1150 ° C and about 5 seconds to 5 minutes, in particular 2 to 5 minutes to annealing temperature persist. The annealing time is sufficient to heat the band substantially full volume to the respective annealing temperature. There may also be multiple rolling steps and intervening intermediate annealing steps at a suitable temperature, e.g. about 950 ° C, to be performed.

After annealing, the hot steel strip is rapidly cooled, preferably quenched by exposure to water or in the gas stream (Gasjet). It has been found that a particularly rapid cooling is helpful to a high solids solubility of the elements C, N and P in the grains too cause. In particular, the embrittlement (grain boundary segregation) critical with a high phosphorus content could be largely or completely prevented by increasing the cooling rate. Cooling rates of over about 50 ° C or over 100 ° C per second are advantageous. Furthermore, cooling rates of more than 200 ° C., 300 ° C. or 400 ° C. per second may preferably also be provided, whereby experiments with cooling rates above 500 ° C. and above 600 ° C. per second have also been successfully carried out.

After cold-rolling, annealing and cooling, cold-rolling can be carried out to set a suitable flatness of the cold-rolled strip. In cold rolling, thickness reductions of e.g. 0.5%, 1.5%, 5%, 25% and more than 40%, or appropriate intermediate values.

Further process steps, such as galvanizing (for example hot-dip galvanizing or electrolytic galvanizing), can follow depending on the area of application and customer requirements.

The chemical composition of the steel may vary over a wide range in other alloying elements. As upper limit values are provided: 0.5% ≥ V, 0.5% ≥ Nb, 0.5% ≥ Ti, 10% ≥ Cr, 10% ≥ Ni, 1% ≥ W, 1% ≥ Mo, 3% ≥ Cu, 0.02% ≥ B, the rest as mentioned iron and production-related impurities. Specific embodiments of the invention and other examples utilize the following ranges: 0.85% ≥ C ≥ 0.70%, 16.2% ≥ Mn ≥ 15.5%, 0.015% ≥ Al ≥ 0.0005%,
0.028% ≥ Si ≥ 0.001%, 0.039% ≥ Cr ≥ 0.020%,
0.08% ≥ Ni ≥ 0.02%, 0.025% ≥ Nb ≥ 0.020%,
0.002% ≥ Ti ≥ 0.0015%, 0.0056% ≥ V ≥ 0.002%,
0.04% ≥ N ≥ 0.015%, 0.2% ≥ P ≥ 0.01%. In particular, as the following examples show, it is also possible to provide extremely high phosphorus concentrations of, for example, above 0.10% ≦ P or even 0.12% ≦ P.

In the following, the invention will be explained in more detail by way of example with reference to exemplary embodiments and further examples.

Table 1 shows the chemical composition of four steel strips X80Mn16-0.01P, X80Mn16-0.03P, X80Mn16-0.08P and X80Mn16-0.1.0P with a phosphorus concentration between 0.011 and 0.102% by weight. Table 1 - chemical composition - element X80Mn16-0.01P X80Mn16-0.03P X80Mn16-0.08P X80Mn16-0.10P C 0.79 0.79 0.75 0.81 Mn 16.0 15.8 16.0 16.1 P 0.011 0.032 0.083 0,102 Si 0.001 0.001 0.001 0.001 al 0.009 0,010 0.005 0.005 N 0.033 0,336 0.034 0,035 Cr 0.031 0.027 0.026 0.032 Ni 0,029 0,025 0.024 0, 031 Nb 0,022 0,022 0,022 0,025 Ti 0,002 0,002 0,002 0, 002 V 0,006 0,003 0,004 0.005 S 0.0035 0.0025 0.001 0.001 Cu 0,017 0.016 0.016 0,018 Not a word 0,017 0,017 0,015 0,017 sn 0.005 0.005 0,004 0,006 Zr 0.001 0.001 0.001 0.001 ace 0.005 0.005 0.005 0.005 B 0.0001 0.0001 0.0001 0.0001 Co 0,006 0.009 0,006 0,006 sb 0.001 0.001 0.001 0.001 Ca 0.0001 0.0001 0.0001 0.0001

The hot strip process (WB) was carried out in each case as described above. The used final rolling temperatures (between 750 ° C and 1030 ° C) as well as the obtained mechanical properties of the produced hot rolled products X80Mn16-0.01P, X80Mn16-0.03P, X80Mn16-0.08P and X80Mn16-0.10P are given in Table 2. The mechanical values obtained in the tensile tests were determined according to European standard "EUROPEAN STANDARD EN 10002-1, July 2001", which is hereby incorporated by reference into the disclosure of this document. All values given in Table 2 are also disclosed as lower limits on the size to which they relate. Table 2 - Mechanical properties (hot strip) - WB-No. chemical composition Final rolling temperature (° C) Rm (MPa) Elongation at break (%) Bruchdehn. x Rm (% MPa) average particle size (μm) 1 X80Mn16-0.01P 750 1081 60.9 65787 5 2 X80Mn16-0.01P 890 1103 67.5 74496 15.7 3 X80Mn16-0.01P 1030 1065 62.6 66701 18 4 X80Mn16-0.01P 1015 987 70.6 69676 26.3 5 X80Mn16-0.03P 870 1200 71.1 85320 13.9 6 X80Mn16-0.08P 920 1098 59.4 65221 17.6 7 X80Mn16-0.08P 950 928 81.9 70550 30.5 8th X80Mn16-0.08P 975 983 77.6 80614 31.6 9 X80Mn16-0.10P 950 946 85.9 76602 34.3 10 X80Mn16-0.10P 975 981 81.3 79783 30.1

As already mentioned, the hot strip (WB) can optionally be further processed into a cold strip (KB). In the embodiments illustrated here and further examples, the cold strip processing was carried out with the processing parameters given in Table 3. The mechanical properties of the cold-rolled products of the chemical compositions X80Mn16-0.01P, X80Mn16-0.03P, X80Mn16-0.08P and X80Mn16-0.10P prepared in this way are given in Table 3. All values given in Table 3 are also disclosed as lower limits on the size to which they relate. Table 3 - Mechanical properties (cold strip) - Survey no. composition Final rolling temperature (° C) in the hot strip process Annealing temperature (° C) Cold rolling degree (%) Tensile strength Rm (MPa) Break elongation A ₅₀ (%) Bruchdehn. x Rm (% MPa) average particle size (μm) 1 X80MR16-0.01P 900 750 48.0 1240 62.0 76545 5.4 2 X80Mn16-0.01P 900 850 48.0 1162 88.5 102883 7.7 3 X80Mn16-0.01P 900 1050 48.0 1065 94.0 100238 29.4 4 X80Mn16-0.03P 900 750 53.8 1261 61.0 76530 5.0 5 X80Mn16-0.03P 900 750 47.5 1217 77.4 94256 4.9 6 X80Mn16-0.03P 900 950 47.5 1100 94.0 103147 18.9 7 X80Mn16-0.08P 900 950 65.4 1046 81.8 84527 28.3 8th X80Mn16-0.08P 950 950 65.4 1146 91.8 105203 29.4 9 X80Mn16-0.10P 900 950 65.4 1021 78.1 79781 27.8 10 X80Mn16-0.10P 850 950 65.4 1121 88.3 98984 15.2

As can be seen from Table 3, the cold-rolled products with KB numbers 1 to 7 and 9 were rolled at a final rolling temperature of 900 ° C in the hot strip process. Otherwise, the same hot strip process was used, which underlies the hot strip products in Table 2.

The cold-rolled products with the KB numbers 1 to 3 are therefore based on approximately the hot-rolled product with the WB number 2 (the final rolling temperatures differ only by 10 ° C) and the cold-rolled products with the KB numbers 4 to 6 is approximately based on the hot-rolled product with the WB number 5 (the final rolling temperatures differ only by 30 ° C).

Table 3 shows that tensile strengths Rm over 1100 MPa and even above 1200 MPa are attained, and that even with large average particle sizes (15 μ m in the case of X80Mn16-0.03P (KB no. 6) and X80Mn16-0.10P (KB No. 10) as well as over 20 μm or possibly even 25 μm in the case of the other samples), tensile strengths Rm above 1000 MPa can still be achieved. The tensile strength Rm is defined as the stress occurring at maximum tensile force on the workpiece.

The elongation at break A ₅₀ given in Table 3 is the percentage permanent change in length after breakage of the tensile test specimen (according to EN 10002-1), based on the initial measuring length, based on an initial measuring length of 50 mm. For the steel strips, it was found that high elongation at break values of more than 75% and in particular with large mean grain sizes of more than 80% and even over 90% can be achieved.

Another important parameter for the mechanical properties of steel strips is the product of tensile strength and elongation at break. Especially with large average particle sizes, high product values are achieved. The reason for this is that large grains lead to higher elongation at break values and the tensile strength, which usually decreases markedly with increasing grain size, is maintained as far as possible according to the invention by the relatively high carbon and / or phosphorus content.

Even at the higher P-contents of 0.08% and 0.1% (X80MN16-0.08P and X80MN16-0.10P, respectively) in the hot-rolled and cold-rolled strips, very good weldability was observed in the weld tests, i.e., in the weld tests. as break-off, break-outs were achieved in all samples.

Table 4 gives the results of a study of the weldability of the steels of the chemical compositions X80Mn16-0.01P, X80Mn16-0.03P, X80Mn16-0.08P and X80Mn16-0.10P: Table 4 - Investigation of weldability - composition Imine (n / a) Imax (n / a) deltaI (n / a) X80Mn16-0.01P 5.2 6.3 1.1 X80Ma16-0.03P 4.7 5.8 1.1 X80Mn16-0.08P 5.2 6.4 1.2 X80Mn16-0.08P 5.3 6.6 1.3 X80Mn16-0.10P 5.2 6.4 1.2 X80Mn16-0.10P 5.1 6.6 1.5

According to Table 4, a weld area deltaI of at least 1.1 kA is found for all steel strips, which exceeds the 1.0 kA necessary for good weldability.

In Fig. 1 the mean grain size of the aluminum nitride-poor cold-rolled steel strips specified in Table 3 with the chemical compositions X80Mn16-0.01P, X80Mn16-0.03P, X80Mn16-0.08P and X80Mn16-0.10P is shown as a function of the annealing temperature during the cold strip process. The cold-rolled strip products shown here were based on a final rolling temperature of 900 ° C in the hot strip process. The graph it can be seen that the steel bands and X80Mn16-0.01P X80Mn16-0.03P at annealing temperatures of about 920 ° C achieve average grain sizes more than 15 μ m. The phosphorus-rich steel strips of the chemical compositions X80Mn16-0.08P and X80Mn16-0.10P achieved even larger average particle sizes at comparable annealing temperatures. The mean particle sizes were determined by light microscopic investigations on micrographs.

Fig. 2 shows a graph in which the work hardening n (here the n _10/20 value) of the steel strips mentioned above, which is also referred to as solidification _exponent , compared with the vertical anisotropy (r _0/15 -, r _45/15 -, and r _90/15 value) is shown. The n value was determined in accordance with standard ISO 10275, issue 2006-07, which is hereby incorporated by reference into the disclosure of this document. The vertical anisotropy is in accordance with standard ISO 10113, edition 2006-09 which is hereby incorporated by reference into the disclosure of this document. Since the mechanical properties of a larger dispersion than those in Fig. 1 having shown average grain size, several samples of said steel strips were examined. The higher the r _0/15 , r _45/15 , and r _{90/15 values} , the better the thermoformability of the material. A high n value favors in particular the iron drawability. It can be seen from the graph that n _10/20 values above _{0.5 can be achieved} at a r _0/15 , r _45/15 and r _90/15 value in the range from _0.6 to 1.5 , The phosphorus-rich steel strips of the chemical compositions X80Mn16-0.08P and X80Mn16-0.10P achieve slightly higher n-values than the steel strips of the chemical compositions X80Mn16-0.01P and X80Mn16-0.03P. Thus, the steel strips according to the invention have a good cold workability, which is particularly important for further processing in drawing and deep drawing processes.

After the tensile stresses on the steel products according to the invention, various deformation mechanisms could be detected. Characteristic was the appearance of different types of twins. It turned out that in the tensile-stressed samples of the steels according to the invention there are very many and fine micro- and nano-twins whose mean thickness was for example less than 30 nm and, for example, in the range between 5 and 25 nm, in particular 10 and 20 nm. For example, on the cold-rolled product X80Mn16-0.03P, a value of 17 nm was determined for the average thickness of the micro and nano-twins. The presence of these small micro-twins, especially the nano-twins, can explain the high elongation at break values because it leads, rather than the usual twinning, to an increasing impediment to dislocation movement and an increase in dislocation sources.

The Figures 3A-C show schematic representations of microstructures observed in electron beam microscopic studies on reshaped samples of the steels of the invention. Fig. 3A shows one direction activated System with conventional twinning, where lines 1 represent the mirror lines of the twins.

Fig. 3B shows a unidirectional system with micro- or nano-twins 2. The micro- or nano-twins 2 are lath-shaped and often arranged side by side in larger numbers. The lath thickness is referred to as the thickness d of the micro- or nano-twins 2 and is typically much smaller than the thickness of common twins.

Fig. 3C shows a bi-directionally activated micro- or nano-twin system 2. It can be seen that bi-directional micro- or nano-twins 2 occur.

Fig. 4 shows an electron micrograph of a steel structure according to the invention after a deformation or tensile stress. A large number of pale-shaped micro- and nano-twins are recognizable in the bright field.

Fig. 5 shows a microsection of the weld nugget of a steel structure according to the invention after a weld. X80Mn16-0.10P samples were used. It can be seen that the basic hardness as well as the maximum hardness in the heat-affected zones and the hardness in the weld nugget agree well and have only slight deviations. These deviations are in the range of the measuring tolerance. It is further recognized that there are no cracks or martensite in the structure.

Furthermore, it emerged from the TEM microstructural investigations that proportions of ε-martensite and possibly also α'-martensite may be present in the microstructure of the end products. Thus, there must be no 100% austenite phase in the final product, although preferably a 100% austenite phase should be present. For example, measurements on the cold-rolled product X80Mn16-0.03P gave about 3% ε-martensite and 1% α'-martensite. Since α'-martensite increases the tensile strength, it is conceivable that the high tensile strength values, in particular even at high temperatures Grain sizes are maintained, may also be favorably influenced by the (but relatively low) α'-martensite in the final product.

The n-value is largely determined by the chemical composition. That is, the strength of the final product that can be achieved by deformation depends on how easily dislocations can travel in the crystal. In the fcc crystal lattice, the solid solubility of C and N is greater than in the bcc crystal lattice. Here, as already mentioned, the increase in tensile strength caused by solid solution of C and P is utilized, whereby in recent investigations tensile strength values of 1100 MPa could be measured with an extremely high breaking elongation of 95%. The hardening achieved by solid solution of said elements makes it possible to increase the n-value considerably. As a result, the highest reported product values of tensile strength and elongation at break are achieved. This is particularly due to the use of high phosphorus concentrations and the associated increase in strength - especially at relatively large mean particle sizes - attributed.

The hot strip or cold strip is cut in further processing into steel sheets, e.g. be used in automotive technology for the production of body parts. Furthermore, the steel according to the invention can also be used in rails, switches, in particular switch hearts, rod material, pipes, hollow profiles or high-strength wires.

The steel sheets are brought by forming processes such as deep drawing in the desired shape and then further processed into the final products (eg body part). In the forming process, at least portions of the steel sheets are subjected to a mechanical stress (usually tensile stress), so that in these areas, the deformation mechanisms described above are effective. In particular, in the reshaped areas to the described Formation of many, thin micro- and nano-twins, which favorably influence the forming behavior and are detectable on the (formed) steel sheet.

Claims (15)

1. A hot-rolled austenitic manganese steel strip having a chemical composition in percent by weight of $$0.4\%\le \mathrm{C}\le 1.2\%$$

   

   $$12.0\%\le \mathrm{Mn}\le 25.0\%$$

   

   $$\mathrm{P}\ge 0.03\%$$

   

   $$\mathrm{Si}\le 2\%$$

   

   Al ≤ 0,05%, V ≤ 0,5%, Nb ≤ 0,5%, Ti ≤ 0,5%, Cr ≤ 10%, Ni ≤ 10%, W ≤ 1%, Mo ≤ 1%, Cu ≤ 3%, B ≤ 0,02%, N ≤ 0,04%, the rest iron and impurities due to production, and having a mean grain size above 13 µm, wherein a product of elongation at break in % and tensile strength in MPa of above 65,000, in particular above 70,000 MPa% is obtained.
2. The hot-rolled austenitic manganese steel strip according to claim 1, having the property that the structure of a sample of the manganese steel strip, which has been subjected to a reshaping process, has microtwins with a mean thickness below 30 nm, in particular below 20 nm, more in particular below 10 nm.
3. The hot-rolled austenitic manganese steel strip according to any one of the preceding claims, comprising P ≥ 0.05%, in particular P ≥ 0.06%, in particular P ≥ 0.08%, more particularly P ≥ 0.10%.
4. The hot-rolled austenitic manganese steel strip according to any one of the preceding claims, comprising Si ≤ 1.0%, in particular Si ≤ 0.2%, more particularly Si ≤ 0.05%.
5. The hot-rolled austenitic manganese steel strip according to any one of the preceding claims, comprising a mean grain size above 18 µm, in particularly above 20 µm.
6. A cold-rolled austenitic manganese steel strip having a chemical composition in percent by weight of $$0.4\%\le \mathrm{C}\le 1.2\%$$

   

   $$12.0\%\le \mathrm{Mn}\le 25.0\%$$

   

   $$\mathrm{P}\ge 0.03\%$$

   

   $$\mathrm{Si}\le 2\%$$

   

   Al ≤ 0,05%, V ≤ 0,5%, Nb ≤ 0,5%, Ti ≤ 0,5%, Cr ≤ 10%, Ni ≤ 10%, W ≤ 1%, Mo ≤ 1%, Cu ≤ 3%, B ≤ 0,02%, N ≤ 0,04%, the rest iron and impurities due to production, and having a mean grain size above 5 µm, wherein a product of elongation at break in % and tensile strength in MPa of above 75,000, in particular above 80,000 MPa% is obtained.
7. The cold-rolled austenitic manganese steel strip according to claim 6, having the property that the structure of a sample of the manganese steel strip, which has been subjected to a reshaping process, has microtwins with a mean thickness below 30 nm, in particular below 20 nm and in particular below 10 nm.
8. The cold-rolled austenitic manganese steel strip according to any one of claims 6 or 7, comprising P ≥ 0.05%, in particular P ≥ 0.06%, in particular P ≥ 0.08%, more particularly P ≥ 0.10%.
9. The cold-rolled austenitic manganese steel strip according to any one of claims 6 to 8, comprising Si ≤ 1.0%, in particular Si ≤ 0.2%, more particularly Si ≤ 0.05%.
10. The cold-rolled austenitic manganese steel strip according to any one of claims 6 to 9, comprising a mean grain size above 15 µm, in particular above 20 µm.
11. A manganese steel strip having a chemical composition in percent by weight of $$0.4\%\le \mathrm{C}\le 1.2\%$$

    

    $$12.0\%\le \mathrm{Mn}\le 25.0\%$$

    

    $$\mathrm{P}\ge 0.03\%$$

    

    $$\mathrm{Si}\le 2\%$$

    

    Al ≤ 0,05%, V ≤ 0,5%, Nb ≤ 0,5%, Ti ≤ 0,5%, Cr ≤ 10%, Ni ≤ 10%, W ≤ 1%, Mo ≤ 1%, Cu ≤ 3%, B ≤ 0,02%, N ≤ 0,04%, the rest iron and impurities due to production, and comprising a stretch-formed or deep-drawn sheet steel portion of which the structure comprises microtwins with a mean thickness below 30 nm, in particular below 20 nm.
12. A process for producing a hot-rolled austenitic manganese steel strip having a chemical composition in percent by weight of $$0.4\%\le \mathrm{C}\le 1.2\%$$

    

    $$12.0\%\le \mathrm{Mn}\le 25.0\%$$

    

    $$\mathrm{P}\ge 0.03\%$$

    

    $$\mathrm{Si}\le 2\%$$

    

    Al ≤ 0,05%, V ≤ 0,5%, Nb ≤ 0,5%, Ti ≤ 0,5%, Cr ≤ 10%, Ni ≤ 10%, W ≤ 1%, Mo ≤ 1%, Cu ≤ 3%, B ≤ 0,02%, N ≤ 0,04%, the rest iron and impurities due to production, comprising the following steps:

    casting a semi-finished product made of steel;

    heating the semi-finished product to a temperature above 1100°C;

    rolling the semi-finished product with a final rolling temperature between 750°C and 1050°C, wherein the mean grain size after the hot-rolling process is above 13 µm; and

    cooling the rolled steel strip at a rate of 20°C/s or quicker.
13. The process according to claim 12, wherein the mean grain size after the hot-rolling process is above 20 µm.
14. A process for producing a cold-rolled austenitic manganese steel strip, comprising the following steps:

    preparing a hot-rolled steel strip produced according to any one of the processes of claims 12 or 13;

    cold-rolling the steel strip; and

    annealing the cold-rolled steel strip for recrystallization thereof.
15. The process according to claim 14, wherein the annealing temperature is between 750°C and 1150°C, and in particular is greater than 900°C.

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