EP1699582B1 - Method for the generation of hot strips of light gauge steel - Google Patents

Description

The invention relates to a method for producing hot strips of a deformable, especially good cold deep drawable lightweight structural steel according to the preamble of claim 1.

The hotly contested automotive market is forcing manufacturers to constantly seek solutions to reduce fleet consumption while maintaining maximum comfort. The weight saving plays a decisive role here. Suppliers are trying to meet this desire, especially for the bodywork sector, by reducing wall thicknesses through the use of higher-strength steels without having to accept losses in buckling rigidity and forming by deep drawing and / or stretch drawing and coating.

An approach to this is in the EP 0 889 144 A1 released. This document proposes a cold-formable, in particular readily deep-drawable, austenitic lightweight structural steel which has a tensile strength of up to 1100 MPa. The main elements of this steel are Si, Al and Mn in the range 1 - 6% Si, 1 to 8% Al and 10 to 30% Mn, balance iron including common steel elements.

The high degree of deformation is achieved by TRIP (Transformation Induced Plasticity) and TWIP (Twinning Induced Plasticity) properties of the steel. High Mn steels tend to segregate as found in conventional continuous casting by bend, bulge of the strand, sedimentation, and suction segregation in the sump tip region.

The resulting macrosegregation, which can also lead to intermetallic phases, leads to serious belt defects during hot rolling.

High-alloyed steels basically also tend to form internal cracks, which ultimately represent marrow defect defects. These result z. B. from bending stresses during the manufacturing process.

Steels with high Al contents can not be cast with conventional casting powders, since Al in particular reduces the SiO ₂ in the casting powder and thus leads to a reduced friction between the strand shell and mold.

Karl-Heinz Spitzer, et al. "Direct Strip Casting (DSC) - Option for the Production of New Steel Grades", Steel Research vol. 74, 2003, no. 11/12, 724-730 , XP 00 902 8744 discloses a method for producing hot strips of TRiP / TWiP steel in a horizontal strip caster.

The invention has for its object to provide a method for producing hot strips of a formable, especially good cold thermoformable Leichtbaustah, I consisting of the main elements Si, Al and Mn, which has a high tensile strength and TRIP and / or TWIP properties, which avoids the disadvantages described above.

This object is achieved starting from the preamble in conjunction with the characterizing features of claim 1.

• C 0,04 bis ≤ 1,0
• Al 0,05 bis < 4,0
• Si 0,05 bis ≤ 6,0
• Mn 9,0 bis ≤ 30,0

und optional die Gehalte in Masse-% für

• Cr bis ≤ 6,5
• Cu bis ≤ 4,0
• Ti, Zr, in Summe bis ≤ 0,7 und Nb, V in Summe bis ≤ 0,06 und für H₂ ≤ 5 bis 20 ppm betragen, und Rest Eisen und unvermeidliche Verunreinigungen,

und bei dem eine Schmelze in einer horizontalen Bandgießanlage endabmessungsnah sowie strömungsberuhigt und biegefrei zu einem Vorband im Bereich zwischen 6 und 15 mm vergossen wird,
wobei durch Konditionierung der Oberseite des umlaufenden Förderbandes durch Sandstrahlen oder Bürsten oder Aufprägen einer Noppenstruktur oder durch Aufbringen einer thermisch isolierenden Trennschicht, für alle Flächenelemente der mit Beginn der Erstarrung sich bildenden Strangschale eines sich über die Breite des Förderbandes erstreckenden Streifens gleiche Abkühlbedingungen gegeben sind, und anschließend einer Weiterbehandlung zugeführt wird.According to the teachings of the invention, the steel has contents in% by mass
For

• C 0.04 to ≤ 1.0
• Al 0.05 to <4.0
• Si is 0.05 to ≤ 6.0
• Mn 9.0 to ≤ 30.0

and optionally the contents in% by mass for

• Cr to ≤ 6.5
• Cu to ≤ 4.0
• Ti, Zr, in total up to ≤ 0.7 and Nb, V in total up to ≤ 0.06 and for H ₂ ≤ 5 to 20 ppm, and balance iron and unavoidable impurities,

and in which a melt in a horizontal strip casting plant close to the final dimensions and flow-smoothed and bend-free is poured into a preliminary strip in the range between 6 and 15 mm,
wherein by conditioning the top of the circulating conveyor belt by sandblasting or brushing or imprinting a knobbly structure or by applying a thermally insulating release layer, for all surface elements of forming at the beginning of solidification strand shell of a extending over the width of the conveyor belt strip same cooling conditions are given, and subsequently fed to a further treatment.

The steel of the invention is gefügemig pronounced either as a stabilized γ-crystal or as a partially stabilized γ-mixed crystal with a defined stacking fault energy, the z. T. multiple TRIP effect shows.

fcc

= face centred cubic

bcc

= body centred cubic

hcp

= hexogonal closed packed

The latter effect is the stress- or strain-induced transformation of a surface-centered γ-mixed crystal into a martensitic ε-structure with hexagonal close-packed spherical packing, which then partially transforms into a body-centered α-martensite and retained austenite. $${\mathrm{\gamma }}_{\mathrm{fcc}}\to \mathrm{\epsilon }\frac{\mathrm{ms}}{\mathrm{hcp}}\to \mathrm{\alpha }\frac{\mathrm{ms}}{\mathrm{bcc}}$$

fcc

= face centric cubic

bcc

= body centric cubic

hcp

= hexagonally closed packed

Numerous experiments have led to the realization that in the complex interaction between Al, Si and Mn, the carbon content is of paramount importance. On the one hand it increases the stacking fault energy and on the other hand it widens the metastable austenite area. This inhibits the deformation-induced martensite formation and the associated solidification and also increases the ductility.

Further improvements can be achieved by targeted additions of copper and / or chromium. With the addition of copper, the ε-martensite is stabilized and the galvanic nature is improved. In addition, copper increases the corrosion resistance of the steel. Chromium also stabilizes ε-martensite and improves corrosion resistance.

The advantage of the proposed lightweight steel number is the fact that a wide range of strength and ductility requirements can be met by targeted alloy composition and choice of process parameters such as degree of deformation and heat treatment, with tensile strengths up to 1400 MPa are possible. The carbon addition plays a key role.

So far, experts have argued that it is best to zero the carbon content to avoid the formation of κ carbides. The invention overcomes this prejudice by proposing a balanced ratio of the addition of aluminum and manganese, which also allows for a targeted addition of carbon.

For the phenomenon "delayed fracture", which can occur in steels with predominantly TRIP properties, the hydrogen content in the steel plays an important role. The phenomenon manifests itself in that z. B. on deep-drawn cups after some time in the edge area cracks occur. The cracking process can last for several days.

For this reason, it is proposed to limit the hydrogen content to <20 ppm, preferably to <5 ppm. This can be achieved by a careful treatment during the melting, z. B. by a special rinsing and vacuum treatment.

Depending on the requirements, it may be necessary to equip the lightweight steel mainly with TRIP or with TWIP properties. This can be achieved most easily by controlling the Mn content. If the lower range of about 9 - 18% is selected, then an end product with predominantly TRIP properties is to be expected, whereas if the upper range is preferred with about 22-30%, the TWIP properties predominate. As already mentioned, this control is also possible by targeted addition of other elements, in particular carbon. In this connection, it should be mentioned that, from the viewpoint of sufficient corrosion resistance, a higher Cr content is advantageous for the lower Mn range specified and a lower Cr content is advantageous for the upper Mn range.

In terms of process technology, it is proposed to achieve flow calming by using a follower electromagnetic brake, which ensures that in the ideal case the speed of the melt feed is equal to the speed of the circulating conveyor belt.

The considered disadvantageous bending during solidification is avoided in that the underside of the casting tape receiving the melt is supported on a plurality of juxtaposed rollers. The support is reinforced in such a way that in the region of the casting belt, a negative pressure is generated, so that the casting belt is pressed firmly on the rollers.

In order to maintain these conditions during the critical phase of solidification, the length of the conveyor belt is selected so that at the end of the conveyor belt before its deflection, the Vorband is largely solidified.

At the end of the conveyor belt is followed by a homogenization zone, which is used for temperature compensation and possible stress relief. This is followed by a further treatment, which may be a direct Aufcoilen the Vorbandes or consists of an upstream rolling process to apply the required deformation of at least 50%, preferably of> 70%.

The direct Aufcoilen the Vorbandes has the advantage that you can choose the casting speed in terms of optimal solidification conditions, regardless of the cycle of the subsequent rolling process.

On the other hand, it may be advantageous, in particular for economic reasons (high productivity), to roll the material according to the invention in-line entirely or partially directly after casting to its final thickness.

In the formation of the strand shell at the beginning of solidification, it may locally come to lifting the strand shell from the circulating belt of the strip casting. Under certain circumstances, this leads to unacceptable unevenness of the underside of the produced pre-strip. To avoid this, it is necessary for all surface elements of the forming strand shell of a strip extending over the width of the conveyor belt to ensure the same cooling conditions as possible. This can be achieved by conditioning the top of the rotating belt, z. B. by a targeted structuring or by applying a thermally insulating release layer.

One of the aforementioned structuring measures is z. B. sandblasting or brushing the top of the rotating belt. An example of the thermally insulating release layer is the coating by plasma spraying with, for example, aluminum oxide or zirconium oxide. Another embodiment of a structuring is the imprinting of a nub structure, for. B. with upward pimples of some 100 microns in height and a few millimeters in diameter and a distance of the pimples of a few millimeters.

• C = 0,06 %
• Mn = 15,5 %
• Al = 2,0 %
• Si = 2,6 %
• H₂ = 4 ppm
• wurde ein Warmband mit einer Dicke von 2,5 mm erzeugt.

Based on an embodiment, the achievable values are demonstrated.
Starting from a steel with the analysis

• C = 0.06%
• Mn = 15.5%
• Al = 2.0%
• Si = 2.6%
• H ₂ = 4 ppm
• a hot strip with a thickness of 2.5 mm was produced.

The rolled tensile specimen gave a tensile strength of 1046 MPa and an elongation (A80) of 35%. Depending on the degree of deformation and heat treatment, the tensile strength can be increased to over 1100 MPa and the elongation (A80) over 40%. A second example shows the possibility of increasing the strength and ductility properties by increasing the carbon content at nearly the same Mn content.

• C = 0,7 %
• Mn = 15 %
• Al = 2,5 %
• Si = 2,5 %
• H₂ = 3 ppm

Das aus diesem Stahl hergestellte Kaltband von 1,0 mm wurde unter Schutzgas bei 1050 °C und einer Haltezeit von 15 Minuten rekristallisierend geglüht. Die Zugfestigkeit ist auf 817 MPa abgesunken, dafür aber die A80-Dehnung auf 60 % gestiegen. Dies bedeutet, dass trotz des niedrigen Mn-Gehaltes durch die höhere Kohlenstoffzugabe der Stahl mehr in den Bereich mit TWIP-Eigenschaften verschoben worden ist.The steel of this embodiment has the following contents

• C = 0.7%
• Mn = 15%
• Al = 2.5%
• Si = 2.5%
• H ₂ = 3 ppm

The cold rolled strip of 1.0 mm made of this steel was recrystallized under protective gas at 1050 ° C. and a holding time of 15 minutes. The tensile strength dropped to 817 MPa, but the A80 elongation increased to 60%. This means that despite the lower Mn content due to the higher carbon addition, the steel has shifted more into the TWIP properties range.

• C = 0,041 %
• Mn = 25 %
• Al = 3,4 %
• Si = 2,54 %
• H₂ = 4 ppm

Nach einer vergleichbaren Wärmebehandlung wie zuvor beschrieben, betrug die Zugfestigkeit im Mittel 632 MPa und die A80-Dehnung 57 %. Auch dieses Beispiel zeigt deutlich, dass man mit hohen Mn-Gehalten die Dehnung stark steigern kann, dies aber immer zulasten der Festigkeit geht, solange der Kohlenstoffgehalt niedrig ist.Another example shows the results of high Mn content and low carbon content. The contents amounted

• C = 0.041%
• Mn = 25%
• Al = 3.4%
• Si = 2.54%
• H ₂ = 4 ppm

After comparable heat treatment as described above, the average tensile strength was 632 MPa and the A80 elongation was 57%. This example also clearly shows that with high Mn contents one can greatly increase the elongation, but this always comes at the expense of strength, as long as the carbon content is low.

Overall, the three examples show the range of variation in strength and elongation, with the Mn and C content playing a key role. The influence of the analysis is superimposed by treatments of the hot strip in the form of annealing and / or by combined cold forming (eg rolling, drawing, deep drawing) and intermediate annealing or final annealing.

Claims (15)

1. A method for producing hot strips from a deformable lightweight structural steel, in particular one which can be readily cold deep-drawn, consisting of the main elements Si, Al and Mn, which has a high tensile strength and TRIP and/or TWIP properties,
   characterised in that
   the content in % by weight for

   C is 0.04 to ≤ 1.0

   Al is 0.05 to < 4.0

   Si is 0.05 to ≤ 6.0

   Mn is 9.0 to ≤ 30.0

   and optionally the content in % by weight for

   Cr is up to ≤ 6.5

   Cu is up to ≤ 4.0

   Ti, Zr, in total is up to ≤ 0.7, and Nb, V in total is up to ≤ 0.06, and for H₂ is ≤ 5 to 20 ppm, and remainder iron and unavoidable impurities,

   and in which a melt is cast to near final dimensions and with flow-calming and free from bending in a horizontal strip casting plant to produce a roughed strip in the range between 6 and 15 mm,
   wherein identical cooling conditions are provided by conditioning the upper side of the revolving conveyor belt by sandblasting or brushing or embossing a knob structure or by applying a heat-insulating separating layer, for all surface elements of the strand shell, which forms with the beginning of solidification, of a strip extending over the width of the conveyor belt, and then is sent for further treatment.
2. A method according to Claim 1
   characterised in that
   the carbon content is 0.06 to ≤ 0.7%.
3. A method according to Claims 1 and 2
   characterised in that
   the Mn content is 9 - 18%.
4. A method according to Claims 1 and 2
   characterised in that
   the Mn content is 18 - 22%.
5. A method according to Claims 1 - 4
   characterised in that
   the Cr content is 0.3 - 1.0%.
6. A method according to Claims 1 - 2
   characterised in that
   the Mn content is 22 - 30%.
7. A method according to Claim 1 and 6
   characterised in that
   the Cr content is 0.05 - 0.2%.
8. A method according to Claims 1 - 7
   characterised in that
   the Si content is 2.0 - 4.0%.
9. A method according to Claims 1 - 8
   characterised in that
   the Al content is 2.0 - 3.0%.
10. A method according to Claims 1 - 9
    characterised in that
    the speed of the melt inflow is identical to the speed of the revolving conveyor belt.
11. A method according to one of Claims 1 - 10
    characterised in that
    the melt which is charged onto the conveyor belt is very largely thoroughly solidified at the end of the conveyor belt.
12. A method according to Claim 1 and 11
    characterised in that
    after the thorough solidification and before the start of the further treatment the roughed strip passes through a homogenisation zone.
13. A method according to Claim 1 and 12
    characterised in that
    the further treatment is coiling-up of the roughed strip.
14. A method according to Claim 1 and 12
    characterised in that
    the roughed strip is subjected to a rolling process in-line and is then coiled up.
15. A method according to Claim 14
    characterised in that
    the degree of deformation is at least 50%, preferably > 70%.