EP2366035A1

EP2366035A1 - Manganese steel strip having an increased phosphorus content and process for producing the same

Info

Publication number: EP2366035A1
Application number: EP09760726A
Authority: EP
Inventors: Reinhold Schneider; Ludovic Samek; Enno Arenholz; Klemens Mraczek
Original assignee: Voestalpine Stahl GmbH
Current assignee: Voestalpine Stahl GmbH
Priority date: 2008-11-12
Filing date: 2009-11-12
Publication date: 2011-09-21
Anticipated expiration: 2029-11-12
Also published as: US20110308673A1; US9677146B2; WO2010054813A1; EP2366035B1; DE102008056844A1; CN102216474B; KR101387040B1; CN102216474A; KR20110083735A; ES2642891T3

Abstract

A hot-rolled austenitic manganese steel strip having a chemical composition in percent by weight of 0.4% ≤ C ≤ 1.2%, 12.0% ≤ Mn ≤ 25.0%, P ≥ 0.01% and Al ≤ 0.05% has a product of elongation at break in % and tensile strength in MPa of above 65 000 MPa%, in particular above 70 000 MPa%. A cold-rolled austenitic manganese steel strip having the same chemical composition achieves a product of elongation at break in % and tensile strength in MPa of above 75 000 MPa%, in particular above 80 000 MPa%.

Description

description

MANGANE STEEL STRIP WITH INCREASED PHOSPHORIC CONTENT AND METHOD FOR THE PRODUCTION THEREOF

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 are already known (TRANSformation Induced

Plasticity), which are increasingly used in the automotive industry. 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 reshaping of the crystal structure results in a simultaneous increase in the strength and formability during product production or during product use in the event of a crash. The TRIP effect can be specifically influenced by admixing the alloying elements aluminum and silicon. With TRIP steel, a certain amount of austenite is transformed into the high-strength marttensitic phase (α-martensite) during deep-drawing of the body part, which is hardly stretchable. 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 be folded in a deformation-induced manner, wherein the folding mechanism proceeds at a mirror plane and regularly produces mirrored crystal regions (so-called twins). 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 vehicle 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.

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 specify 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 wt .-% 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 the austenite stability, hampers cementitious precipitation and usually reduces weldability, according to the invention in a relatively high proportion of at least 0.01%. to be alloyed. It turned out that with a largely omission of the alloying element aluminum (Al <0.05%) with this alloying concept, high mechanical properties and a surprisingly good weldability with very good formability of the produced manganese steel strip 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 more than 60,000 MPa%, in particular over 70,000 MPa% can be achieved. 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%. 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) High-density micro and nano-twin formation:

In the steels of the present invention, preference has been given to micro-twin formation (i.e., the formation of small twins of small thickness) in the forming process in the crystal structure. The high density of micro-twins found after forming (e.g., thermoforming) and their small thickness compared to the density and thickness of the micro-twins in conventional high manganese steel results in an increase in 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 lay with samples of the manganese steel strip according to the invention after they had been subjected to a forming process, preferably below 30 nm, in particular below 20 nm and in particular below 10 nm. Gemini with a thickness of less than 10 nm also become referred to as nano-twins. After the deformation stress, in comparison to the usual density of twins, in particular a significantly increased density of nano-twins was present. 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) 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) with simultaneously high strain hardening values and Elongation at break (possibly greater than 90%) can be set.

(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 strip 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 the microbial formation of small size or the formation of nano-twins and their high density in the crystal structure. Furthermore, it is assumed that the usually high solubility of P and C, which is usually achieved, does not cause the usually negative effects of these elements (worsening of the weldability, embrittlement of the steel) 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 particle size can be made possible with an austenitic manganese steel strip. The proportion of Al in the alloying concept pursued here can be kept very low, since much carbon is available for the deoxidation of the liquid steel. In particular, the manganese steel according to the invention can have the smallest possible proportion of aluminum, which is limited only by unavoidable impurities in the production process (that is, no aluminum addition). In the case of the steel strip according to the invention, this enables maximum grain size growth during recrystallization (that is to say during hot rolling or annealing).

Furthermore, expediently 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 the 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 as well as the yield strength and the higher the

Elongation at break. It is possible to set average particle sizes of more than 5 μm or more than 10 μm. In particular, it can be provided that in the hot-rolled austenitic manganese steel a large average grain size of more than 13 microns, especially about 18 microns is set, and that in the cold-rolled austenitic manganese steel a large average grain size of more than 15 microns, especially about 20 microns 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. As cementite precipitation lowers the elongation at break, it can be expected that the elongation at break can be increased by adding silicon.

However, the manganese steel according to the invention preferably has a very low silicon content (Si <1.0%, in particular Si <0.2%, more preferably Si <0.05%), which may be limited only by unavoidable impurities in the production process is (ie in this case no silicon addition, 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 the formation of twins, ie a low silicon concentration facilitates the formation of twins and possibly especially the formation of small micro-twins or nano-twins. Since the deformation mechanism of micro-twin formation and in particular of nano-twin formation strongly promotes a high elongation at break, this effect causes an increase in the elongation at break with a reduction in the silicon content. In this case, other deformation mechanism 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, since 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 should be guaranteed.

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 solubility enhancer (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. Preferably, e.g. Cr <5%, in particular Cr <0.2%.

Molybdenum (Mo) and tungsten (W) also show a grain refining effect. 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. Preferably, W <2%, in particular W <0.02%, can be set become. Tungsten is an even better solid solubility enhancer than chromium and also forms carbides (but to a lesser extent than chromium). Preferably, Mo <2%, in particular Mo <0.02% can be 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 with a final rolling temperature between 75O ⁰ C to 1050 ⁰ C, preferably be- see 800 ⁰ C and 900 ⁰ C, are rolled. For a given chemical composition, the choice of the final rolling temperature allows the average particle size to be set.

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. The annealing performed after the cold rolling can be carried out, for example, at an annealing temperature between 75O ⁰ C and 1050 ⁰ C, and in particular the annealing temperature can be greater than 900 ⁰ 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%.

An inventive manganese steel sheet having the said chemical compositions has a formed, 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 above, these micro- and nano-twins form during the reforming process, whereby the high mechanical properties of the probably derived - at least in part - from this deformation mechanism.

In a method for manufacturing a hot rolled austenitic manganese steel bands the semifinished product is heated to a temperature above HOO ⁰ C after casting, a semifinished product made of steel. The heated semi-finished product is rolled with a final rolling temperature between 750 ⁰ C and 1050 ⁰ C, preferably between 800 ⁰ C and 900 ⁰ 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 the E, 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, precipitate formation can be largely prevented by 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 smoother carbon and phosphorus can 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

Slowly cool steel strip 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 of 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 radial forces.

The annealing temperature can be between 750 ⁰ C and 1150 ⁰ C and in particular greater than 900 ⁰ C. By annealing, the grain size can be changed again, after annealing, a grain size of about 15 microns, 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. 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 is a graph in which for cold-rolled steels, the average grain size compared to the annealing temperature is shown;

2 shows a graph in which, for several samples of cold-rolled steels, _strain hardening (n _10/20 value) is compared with perpendicular anisotropy (r _{o /} i ₅ -, r _45/15 -, and r _{90 /} i ₅ - Value) is shown;

FIGS. 3A-C show schematic representations of twins and micro-twins or nano-twins in the microstructure of steels;

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

5 shows 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 a blast furnace or by 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 degasification (eg according to the Ruhrstahl-Heraeus method (RH)) can be carried out and a ladle furnace (Ladle Furnace) can be used for heating and alloying the molten metal. A second production route, which may be particularly suitable for manganese steel, uses an electric arc furnace (EAF: Electric Are Furnace) 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 into profiles, pipes or wires is possible.

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

In the production of steel strips according to the invention, a rolling temperature between about 1100 ⁰ C and 1300 ⁰ C, optionally also higher, can be used. The rolling end temperature may, for example, between 750 ⁰ C and 1050 ⁰ C and in particular between 800 ⁰ C and 900 ⁰ 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 in the average grain size, the tensile strength and the breaking strength of the hot-rolled steel strip increase, the elongation at break decreases. At a However, if the roller end temperature is too low, there is a risk that the high grain refining in the case of manganese steels causes a loss of plastic deformability as a result of the increased strength. Furthermore, due to the phase stability, lower cementation temperatures lead to the formation of cementite ((Fe, Mn) ₃ C), which may affect the mechanical properties. The cementite reached at rolling temperatures below 740 ⁰ C, a particle size by which the mechanical properties were significantly adversely affected.

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 the steel low Endwalztemperaturen significantly below 950 ⁰ C and especially below 900 ⁰ C down to 750 ⁰ 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 ⁰ C to 800 ⁰ C, to avoid. Particularly preferred final rolling in the hot rolling process can therefore be in the range of 800 ⁰ C to 900 ⁰ C.

For example, the avoidance of cracking at said final rolling temperatures in the range of 800 ⁰ C to 900 ⁰ C has been achieved 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 minium permissible 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 in a continuously working

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. Cold rolling further reduces the thickness of the hot strip and sets the mechanical and technological properties of the strip. 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-rolled 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. Rolling mills with 2 to 20 rollers can be used. To apply the high cold rolling forces, for example, designed for high rolling pressures rolling stands with 12 or 20 rolls, in particular Sendzimir type (Cluster roller) can be used. A Sendzimir rolling mill with 12 rolls, for example, consists of a symmetrical arrangement of 3 back rolls, 2 intermediate rolls and 1 pressure roll defining the roll gap. 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 pressure roll defining the roll gap. 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 at temperatures between 750 ⁰ C to 1250 ⁰ C, in particular 750 ⁰ C are made to 1150 ⁰ C and continue for approximately 5 seconds to 5 minutes, in particular 2 to 5 minutes at the annealing temperature. The annealing time is sufficient to heat the band substantially full volume to the respective annealing temperature. It can also be carried out several rolling steps and intermediately Zwischenglühschritte at a suitable temperature, for example about 950 ⁰ C.

After annealing, the hot steel strip is rapidly cooled, preferably quenched by exposure to water or in the gas stream (Gasj et). It turned out provides that a particularly rapid cooling is helpful to cause a high solids solubility of the elements C, N and P in the grains. In particular, the embrittlement (grain boundary segregations) critical with a high phosphorus content could be largely or completely prevented by increasing the cooling rate. Cooling speeds of more than about 50 ^° C. or more than 100 ^° C. per second are advantageous. Furthermore, preferably also cooling rates of over 200 °, 300 ⁰ C or 400 ⁰ C can be seen the ERI second, which also attempts at cooling than 500 ⁰ C and 600 ⁰ C per second have been successfully carried out.

After cold rolling, annealing and cooling, cold rolling can be performed to achieve a suitable flatness of the cold 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) may be added depending on the field of application and customer requirements.

The chemical composition of the steel may vary over a wide range in other alloying elements. For example, optional upper limit values are 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 use 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, extremely high phosphorus Phor concentrations of, for example, over 0.10% <P or even 0.12% <P be provided.

The invention will be explained in more detail below by way of example with reference to exemplary embodiments.

Table 1 shows the chemical composition of four steel strips X80Mnl6-0.01P, X80Mnl6-0.03P, X80Mnl6-0.08P and X80Mnl6-0.1OP with a phosphorus concentration between 0.011 and 0.102% by weight.

The hot strip process (WB) was carried out in each case as described above. The used final rolling temperatures

(between 750 ⁰ C and 1030 ⁰ C) and the resulting mechanical properties of the hot rolled products produced X80Mnl6-0.01P, X80Mnl6-0.03P, X80Mnl6 -0.08P and X80Mnl6 -0.1OP are given in Table 2. The mechanical values obtained in the tensile tests were according to the 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.

As already mentioned, the hot strip (WB) can optionally be further processed into a cold strip (KB). In the embodiments illustrated here, 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 X80Mnl6-0.01P, X80Mnl6 - 0.03P, X80Mnl6 -0.08P and X80Mnl6-0.1OP 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.

As can be seen from Table 3, the cold strip were products with the KB numbers 1 to 7 and 9 with a finish rolling - rolling temperature of 900 ⁰ C in the hot strip process. Otherwise, the same hot strip process was used as that underlying 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 ⁰ 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 ⁰ C).

Table 3 shows that tensile strengths Rm above 1100 MPa and even above 1200 MPa are achieved and that even with large average particle sizes (over 15 μm in the case of X80Mn16- 0.03P (KB No. 6) and X80Mnl6-0.1OP ( KB No. 10) as well as over 20 μm or possibly even 25 μm in the case of the other ben) tensile strengths Rm above 1000 MPa can 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 products, the weldability tests showed a very good weldability, i. as break-off, break-outs were achieved in all samples.

Table 4 shows the results of a study of

Weldability of the steels of the chemical compositions X80Mnl6-0.01P, X80Mnl6-0.03P, X80Mnl6- 0.08P and X80Mnl6-0.1OP indicated: Table 4 - Investigation of the weldability -

According to Table 4, for all steel strips, a welding range deltal of at least 1.1 kA is found, which exceeds the 1.0 kA required for good weldability.

In FIG. 1, the mean grain size of the cold rolled aluminum nitride-poor steel strips with the chemical compositions X80Mnl6-0.01P, X80Mnl6-0.03P, X80Mn16-0.08P and X80Mn16-0.1OP, given in Table 3, is dependent on the annealing temperature Cold strip process shown. Those illustrated cold-rolled products was a final rolling temperature of 900 ⁰ C in the hot strip process is based. The graph shows that the steel strips X80Mnl6-0.01P and X80Mnl6-0.03P reach annealing temperatures of about 920 ⁰ C average grain sizes over 15 microns. The phosphorus-rich steel strips of the chemical compositions X80Mnl6-0.08P and X80Mnl6-0.1OP 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 ni _{O / 2} o value) of the above-mentioned steel strips, which is also referred to as solidification _exponent , compared to the vertical anisotropy (r _{o /} is-, r _45/15 -, and r _{90 /} is-value) is shown. The n-value was determined in accordance with standard ISO 10275, issue 2006-07, which is hereby incorporated by reference in the Of content of this document is included. The vertical anisotropy is defined 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 have a greater scattering than the mean grain size shown in FIG. 1, several samples of said steel strips were examined. The higher the r _{o /} i ₅ , r ₄ 5 _/ i5, and r _{90 /} i5 values, the better the thermoformability of the material. A high n value favors in particular the iron drawability. The graph shows that nio _/ 2o values above 0.5 at r _{o /} i ₅ , r _{45 /} i ₅ , and r _{90 /} i ₅ values range from 0.6 to 1, 5 are available. The phosphorus-rich steel strips of the chemical compositions X80Mnl6-0.08P and X80Mnl6-0.1OP achieve slightly higher n values than the steel strips of the chemical compositions

X80Mnl6-0.01P and X80Mnl6 - 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 zugbeanspruchten samples of the invention

Steels are very many and fine micro- and nano-twins are present, the average thickness, for example, was less than 30 nm and, for example, in the range between 5 and 25 nm, in particular 10 and 20 nm, was. For example, a value of 17 nm for the average thickness of the micro- and nano-twins was determined on the cold-rolled product X80Mnl6-0.03P. The presence of these small micro-twins, in particular the nano-twins, can explain the high elongation at break values, since it leads, rather than the usual twinning, to an increasing inhibition of the dislocation movement and an increase of dislocation sources. Figures 3A-C are schematic representations of microstructures observed in electron beam microscopic studies on reshaped samples of steels of the invention. FIG. 3A shows a unidirectionally activated system with conventional twinning, wherein 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 greater 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 system

Micro- or nano-twins 2. It can be seen that micro- or nano-twins 2 extending in both directions occur.

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

Fig. 5 shows a micrograph of a weld nugget ^inventions' to the invention the steel structure by a weld. X80Mnl6-0.1OP 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 ¹ -martensite may be present in the microstructure of the end products. Thus, there does not have to be 100% austenite phase in the final product, although preferably a 100% austenite phase is present. should be. Measurements on the cold-rolled product X80Mnl6-0.03P gave eg about 3% ε-martensite and 1% α ¹ -martensite. Since α'-martensite increases the tensile strength, it is conceivable that the high tensile strength values, which are maintained especially even at high particle sizes, may also be favorably influenced by the (but relatively low) α 'martensitic fraction in the end 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 exploited, whereby in recent investigations tensile strength values of 1100 MPa with an extremely high elongation at break of 95% could be measured. 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 attributed in particular to the use of high phosphorus concentrations and the associated increase in strength-in particular with relatively large average particle sizes.

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). When converting Forming process, at least portions of the steel sheets of a mechanical stress (usually tensile stress) are exposed, so that in these areas, the above-described deformation mechanisms are effective. In particular, it comes in the formed areas to the described formation of many, thin micro- and nano-twins, which favorably influence the forming behavior and on the (reshaped) steel sheet are detectable.

Claims

claims

1. Hot-rolled austenitic manganese steel strip having a chemical composition in weights of 0.4% <C <1.2%

12, 0% <Mn <25, 0% P> 0, 01% Si <2%

Al <0.05%, whereby a product of elongation at break in% and tensile strength in MPa of over 65,000, in particular over 70,000 MPa% is achieved.

2. Hot rolled austenitic manganese steel strip according to claim 1, having the property that a sample of manganese steel strip, after it has been subjected to a forming process, micro twigs having an average thickness of less than 30 nm, in particular less than 20 nm, in particular smaller in their microstructure as 10 nm.

3. Hot rolled austenitic manganese steel strip according to claim 1 or 2, with 14.0% <Mn <18.0%, in particular 14.0% <Mn <16.5%.

4. Hot rolled austenitic manganese steel strip according to one of the preceding claims, with P> 0.03%, in particular P> 0.05%, in particular P> 0.06%, in particular P> 0.08%, especially P> 0.10 %.

5. Hot rolled austenitic manganese steel strip according to one of the preceding claims, with 0.6% <C <0.9%.

6. Hot rolled austenitic manganese steel strip according to one of the preceding claims, with Al <0.05%, in particular Al <0, 02%.

7. Hot rolled austenitic manganese steel strip according to one of the preceding claims, with Si <1.0%, in particular Si <0.2%, especially Si <0.05%.

8. Hot rolled austenitic manganese steel strip according to one of the preceding claims, with an average particle size of about 13 microns, in particular about 18 microns, more preferably about 20 microns.

9. Hot rolled austenitic manganese steel strip according to one of the preceding claims, having a tensile strength above 1050 MPa.

10. Hot rolled austenitic manganese steel strip according to one of the preceding claims, with an elongation at break over

65%.

11. Cold-rolled austenitic manganese steel strip having a chemical composition in weights of 0.4% <C <1, 2%

12, 0% <Mn <25, 0%

P> 0, 01%

Si <2%

Al <0.05%, with a product of elongation at break in% and tensile strength in MPa of more than 75,000, in particular more than 80,000

MPa% is achieved.

12. Cold-rolled austenitic manganese steel strip according to claim 11, having the property that a sample of the manganese steel strip, after it has been subjected to a forming process, microspheres with an average thickness of less than 30 nm, in particular less than 20 nm, in particular less than 10 nm.

13. Cold rolled austenitic manganese steel strip according to claim 11 or 12, with 14.0% <Mn <18.0%, especially 14, 0% <Mn <16, 5%.

14. Cold-rolled austenitic manganese steel strip according to one of claims 11 to 13, with P> 0.03%, in particular P> 0.05%, in particular P> 0.06%, in particular P> 0.08%, in particular P> 0 , 10%.

15. cold rolled austenitic manganese steel strip according to any one of claims 11 to 14, with 0.6% <C <0.9%.

16. Cold-rolled austenitic manganese steel strip according to one of claims 11 to 15, with Si <1.0%, in particular Si <0.2%, especially Si <0.05%.

17. Cold-rolled austenitic manganese steel strip according to one of claims 11 to 16, with an average particle size of more than 15 μm, in particular more than 20 μm.

18. cold rolled austenitic manganese steel strip according to any one of claims 11 to 17, having a tensile strength above 1100 MPa.

19. Cold-rolled austenitic manganese steel strip according to one of claims 11 to 18, with an elongation at break of more than 75%, in particular more than 80%.

20. Manganese steel sheet, with a chemical composition in weights of 0.4% <C <1.2% 12, 0% <Mn <25, 0% P> 0, 01% Si <2%

Al <0, 05%, and with a deformed, in particular stretched or deep-drawn sheet steel section, the microstructure micro twins with egg ner average thickness less than 30 ran, in particular less than 20 nm.

21. Manganese steel sheet according to claim 20, the microstructure of which has micro-twigs with an average thickness of less than 10 nm.

22 manganese steel sheet according to claim 20 or 21, with a mean grain size over 13 microns, in particular about 18 microns, especially more than 20 microns.

A manganese steel sheet according to any one of claims 20 or 22, having an un-formed portion which obtains a product of elongation at break in% and tensile strength in MPa of more than 75,000, in particular more than 80,000, and still especially more than 85,000 MPa%.

24. A process for producing a hot-rolled austenitic manganese steel strip having a chemical composition in terms of weight of

0, 4% <C <1, 2%

12, 0% <Mn <25, 0% P> 0, 01%

Si <2%

Al <0.05%, with the steps:

Casting a semifinished product from steel;

Heating the semifinished product to a temperature above 1100 ⁰ C; Rolling the semifinished product with a final rolling temperature between

75O ⁰ C and 1050 ⁰ C; and

Cooling the rolled steel strip at a rate of 20 ° C / s or higher.

25. The method of claim 24, wherein the final rolling temperature between 750 ⁰ C and 95O ⁰ C, in particular between 800 ⁰ C and 900 ⁰ C.

26. The method according to claim 24 or 25, wherein the steel strip at a rate of 50 ° C / s or higher, in particular 200 ° C / s or higher, is cooled.

27. The method according to any one of claims 24 to 26, wherein the average grain size after hot rolling over 15 microns, especially about 20 microns.

28. A method of producing a cold-rolled austenitic manganese steel strip comprising the steps of:

Providing a hot-rolled steel strip produced by one of the processes of claims 24 to 27; Cold rolling of the steel strip; and annealing the cold-rolled steel strip to recrystallize the same.

29. The method of claim 28, wherein the annealing temperature between 750 ⁰ C and 1150 ⁰ C and in particular greater than 900 ⁰ C.

A method according to claim 28 or 29, wherein the annealed steel strip is cooled at a rate of 50 ° C / s or higher, more preferably 200 ° C / s or higher.

31. The method according to any one of claims 28 to 30, wherein the average grain size after annealing over 15 microns, in particular more than 20 microns.

32. The method according to any one of claims 28 to 31, wherein the

Thickness reduction during cold rolling is over 45%, in particular more than 60%.