EP2649214B1

EP2649214B1 - Process for manufacturing high manganese content steel with high mechanical resistance and formability, and steel so obtainable

Info

Publication number: EP2649214B1
Application number: EP11820814.9A
Authority: EP
Inventors: Alessandro Ferraiuolo
Original assignee: Centro Sviluppo Materiali SpA
Current assignee: Centro Sviluppo Materiali SpA
Priority date: 2010-12-07
Filing date: 2011-12-07
Publication date: 2016-12-07
Anticipated expiration: 2031-12-07
Also published as: EP2649214A2; WO2012077150A3; ITRM20100641A1; CN103339279B; WO2012077150A2; IT1403129B1; CN103339279A; KR20140025324A

Description

The present invention refers to the field of products made of high manganese content austenitic steel and with high mechanical resistance and high formability (steels named as TWIP, Twinning Induced Plasticity).
High manganese content TWIP type steels constitute within high resistance steel field a unique family being characterized in that the same display peculiar mechanical properties. TWIP steels have austenitic structure with face-centered cubic lattice (FCC) along with a low stacking fault energy (SFE) promoting the activation of twinning deformation mechanisms (mechanically induced twinning).
In the metals the two fundamental and competitive mechanisms by means of which the plastic deformation occurs are dislocation slip and geminate formation (gemination).
In order to obtain a steel which is deformed preferentially by means of gemination it is necessary to set up the metallurgical design of the steel in such way that the content of solute is suitable to simultaneously increase shear stress for dislocation slip and reduce the stacking fault energy (SFE).
The atoms of solute interact with dislocations through two mechanisms:

1) Interaction among solute and dislocation produced reticular distortions. This interaction is related in some degree to dimensional difference of solute and solvent atoms. Substitutive atoms reduce stress conditions in proximity of the dislocation line: it results therefrom that a greater tension is necessary in order the dislocation to be moved.
2) In face-centered cubic lattice (FCC) the ordinary dislocations under determined conditions (low stacking fault energy) become unstable and tend to separate in two partial dislocations (Shockley dislocations). The lattice region between the two partial dislocations is characterized by a structure defect named stacking defect, that is the normal sequence of highly stacked crystalline sheets is altered passing from typical sequence of a face-centered cubic lattice (FCC) to that of compact hexagonal lattice (HCP). The concentration of solute in the zone of stacking defect, at equilibrium, tends to be greater than the average value. This heterogeneous solute atom distribution exerts a further resistance to dislocation slip (Suzuki effect).

In a typical curve true stress-true strain of a TWIP steel three steps can be distinguished:

1° step: at the beginning of plastic deformation phase wherein the deformation occurs predominantly by means of dislocation slip.
2° step: as the deformation and relative shear stress increase as a result of material hardening, the gemination demanding a greater activation energy, compared to dislocation slip, but constant one (independent of the deformation), progressively starts. Therefore a threshold deformation value, above which further material deformation occurs predominantly by germination, exists. This threshold value can be controlled by means of an opportune steel metallurgical design. For still increasing deformations the deformation proceeds by gemination and this results in the characteristic high ductility TWIP effect. The non-dependence of the gemination tension on deformation results in the fact that the deformation proceeds homogenously in the material and without necking (high uniform elongation).
3° step: when the deformation reaches high values also the gemination is hindered and from this point on the deformation occurs non-homogenously and the failure occurs by localized necking.

As it is known, this TWIP steel type, in particular in strip form, is particularly appreciated in the automotive field. In fact the TWIP steel strips allow the manufacturing of complexly shaped automotive components in a relatively simple way and use thereof under conditions requiring high mechanical performances, in particular for parts involved in energy absorption and structure reinforcing.
According to the state of the art such as US20080035248 there are some proposals aiming to obtain TWIP steels exhibiting high mechanical properties and suitable for providing the above reported performances in the automotive field.
In US 2010 258212 a process for the production of high-tensile TWIP type steel involving the control of the starting steel composition and in particular of titanium content (4.0-5.0%) as an essential alloy element is proposed.
However, it does not exist a process, according to known art, completely satisfactory from the point of view of the best compromise between formability and mechanical resistance, as well as the stability of austenitic phase and the surface quality of the obtained product.
Therefore there is a need in the specific field to provide a production process of a TWIP steel, in order ah optimal compromise between mechanical resistance and formability and high superficial quality to be reached.
This requirement is satisfied by the process according to the present invention wherein conditions in order the stability of the austenitic phase using the control of the chemical composition and heat treatment steps to be obtained are developed.
It is therefore the subject of the present invention a process for the production high Mn content steel of TWIP (Twinning Induced Plasticity) type, with high mechanical resistance and formability, that has the following chemical composition, in percentage by weight :

C 0.2-1.5; Mn 10-25; optionally Ni<2; Si 0.05-2.00; Al 0.01-2.0; 0.01=<N<0.1; P+Sn+Sb+As<0.2 ; S+Se+Te<0.5; and Nb+Co 0.1-0.4 and Re+W<l, the balance being Fe apart from unavoidable impurities, and that, after having been subjected to cold rolling, is subjected to recrystallization annealing carried out continuously at a temperature within the range of 900°C-1100°C for a time comprised between 60 seconds and 120 seconds or carried out at batches at a temperature within the range of 700°C-800°C for a time comprised between 30 minutes and 400 minutes, annealing atmosphere being such that carbon activity a_c is between 0.1 and 1.0, nitrogen N2 content is between 90% and 100% for continuous annealing and between 0% and 100% for batch annealing, hydrogen content is between 0% and 10% for continuous annealing and between 0% and 100% for batch annealing, dew point is lower than 0[deg.]C and preferably between - 10°C and -50°C.

The preferred ranges of composition for single alloy elements or combinations of alloy elements, independently from each other, are:

C 0.4-0.8; Mn 16-19; optionally Ni<1.0; Si 0.2-0.4; Al 0.1-1.5; N 0.01-0.05; Nb+Co 0,1-0.4 and Re+W 0.3-0.7.

According to an embodiment thereof the proposed process comprises also the further operating step to produce a metallic coating on the hot obtained strip using a zinc based alloy containing magnesium and aluminium.
It is also a subject of the present invention a free coating austenitic TWIP steel, obtainable as above indicated, with the following mechanical features:

Rp0.2 between 250 and 350 MPa
Rm between 850 and 1100 MPa
A80 between 60 and 100 %.

It is also a subject of the invention the above said austenitic steel coated with a zinc based magnesium and aluminium containing alloy obtainable using the further above reported operating step .
The steel according to the invention, optionally coated with zinc alloy, can be used for the manufacturing of complexly shaped components to be employed for energy absorption, structure reinforcing and in general terms for automotive applications.
Austenitic TWIP steel according to the invention, optionally coated with zinc alloy, can be used in form of strip, sheet, bar, billet, pipe.
Finally it is also another subject of the invention Mn high content steel used as starting material for the above described process.
The metallurgical role exerted by the various alloy elements of TWIP steels according to the invention can be distinguished in at least four different effects:

1) Stabilization of austenitic phase with face-centered cubic lattice (FCC). Relatively to this effect Mn and C as alloy elements exert the main role.
2)Control of the stacking fault energy (SFE). The energy optimal range of the stacking fault energy in order the best properties of the steel in terms of ductility to be obtained is 20-40mJ/m². Relatively to this effect the alloy elements Mn and C, together with Al and Si, exert the main role.
3) Optimization of TWIP behaviour by means of the control of plastic deformation threshold, starting therefrom the deformation occurs almost exclusively by gemination. This effect can be controlled in fine way with typically limited percents of large sized atoms like Nb, Co, Re and W.
4) Some elements, among which Al, when added at appropriate amounts, in addition to the effect on SFE, tend to act against the HCP martensite formation during the deformation of the alloy.

The function of the single alloy elements, as well as the reason for the specific selection of upper and lower range limits of the relative weight percentages, are explained hereinafter.
Carbon contributes to the stabilization of austenite. Compositionally the range thereof is 0.2-1.5%.
When C is lower than 0.2% crack formation is observed during the steel working. When C is higher than 1.5% a lower formability is observed.
Also manganese plays a determining role in the stabilization of the austenitic phase. Compositionally the range thereof is 16-18% according to the present invention. In correspondence of this interval of Mn percentages the maximum stabilization of austenite is observed.
The silicon exerts the function to increase the mechanical resistance and ductility of the steel. Si content is comprised between 0.05 and 2.0%. When the percentage thereof is lower than 0.05, often a thick iron and manganese oxide layer is formed resulting in increased pickling duration, and in decreased corrosion resistance of the annealed steel and surface quality of the cold rolled sheet. When the percentage thereof is higher than 2.0%, stability properties of the steel are decreased.
The aluminium presence is finalized to the increment of the ductility of the steel. The content thereof in the austenitic steel according to the invention is comprised between 0.01 and 2.0%. When the content of Al is lower than 0.01%, the mechanical resistance is increased but a quick degradation of the ductility occurs. When, on the contrary, it is present in percentage higher than 2.0%, the steel exhibits decreased ductility along with lower castability during the continuous casting and corrosion susceptibility during hot rolling with consequent worsening of the surface quality of the resulting product.
Nitrogen promotes the gemination generation, by means of reaction with aluminium and precipitation of fine nitrides (during the solidification) within the austenitic grains, the presence thereof improves both mechanical resistance and suitability to be elongated during the steel working. Nitrogen is present in the steel used according to the invention in percentage lower than 0.1%. In fact when the content of N is higher than 0.1%, excess nitride precipitation takes place resulting in cold machinability and formability degradation.
Nb+Co and Re+W promote the formation of geminations, and improve both the mechanical resistance and the suitability to be elongated during steel working.
Through the experimental activity the present invention in based on, it has been found that it is fundamental to control the activity of carbon in the atmosphere of annealing furnace in order a control of non-decarburizing or re-carburizing to be carried out.
Up to now a general description has been offered for the present invention. With the aid of the following figures and examples it will be now supplied a more detailed description of embodiments thereof, in order to better understand its objects, features and advantages.

Figure 1 shows the microstructure of a non-deformed TWIP steel, according to example 2 of the present invention.
Figure 2 shows the microstructure, of a TWIP steel according to example 2 of the present invention after deformation, wherein the presence of geminates is observed.

Example 1

A steel, containing C 0.6; Mn 18; Ni 0.5; 0.3; Al 1.0; P+Sn+Sb+As 0.1; S+Se+Te 0.01; N 0.05; Nb+Co 0.1, apart from iron and unavoidable impurities, is subjected to the process according to the present invention.
A 1.0 mm thick strip of claimed steel is obtained using a continuous casting plant and it is hot rolled, cold rolled and subjected to recrystallization annealing according to the invention as reported hereinafter.
Continuous recrystallization annealing occurs at 1000°C for 90s in non-decarburizing atmosphere, carbon activity within annealing atmosphere is 0.15, nitrogen content is 100% and dew point -25°C.
The final product exhibits the following mechanical features: Rp0.2 290 MPa, Rm 1000 MPa and A80 90%, and has austenitic microstructure.
This product is used for the manufacturing of auto-motive components requiring high mechanical resistance and high formability, as for example car structural elements.

Example 2

For comparative and illustrative purposes a steel, containing C 0.6; Mn 17; Si 0.3; Al 0.04; P+Sn+Sb+As<0.1; S+Se+Te 0.01; N 0.05; Re+W 0.2, apart from iron and unavoidable impurities, is subjected to the process according to the present invention.
A 2.0 mm thick tube made of this steel obtained using a continuous casting plant is hot rolled, cold rolled and subjected to recrystallization annealing according to the invention as reported hereinafter.
The recrystallization annealing is carried out in batch at 750°C for 180 minutes. The carbon activity within annealing atmosphere is 0.15, the nitrogen percentage within the furnace is 95%, the dew point is - 30°C, and hydrogen is 5%.
The final strip exhibits the following mechanical features Rp0.2 310 MPa, Rm 950 MPa and A80 80% and has austenitic microstructure.
This product is used for the preparation of automotive components requiring high mechanical resistance and high ductility, as for example structure reinforcing bars of cars.

Example 3

In this comparative example TWIP steels from examples 1 and 2, obtained according to the process of the present invention, and comparative steels TRIP 800, HSLA S700MC and DP 980 are compared in terms of mechanical properties, microstructure and supply conditions.

The comparison results are reported in the following table:

	Rp0.2 (MPa)	Rm (MPa)	A80 (%)	Microstructure	Supply conditions
Example 1	290	1000	90	Austenite	Hot and cold rolled
Example 2	310	950	80	Austenite	Hot and cold rolled
TRIP 800	500	850	30	Ferrite + Bainite/Martensite + residual Austenite	Hot and cold rolled
HSLA (S700MC)	700	850	15	Ferrite + Bainite	Hot rolled
DP 980	650	1000	14	Ferrite + Martensite	Cold rolled

Claims

Process for manufacturing high Mn content austenitic steel of the TWIP (Twinning Induced Plasticity) type, with high mechanical resistance and formability, characterized in that the steel has the following chemical composition, in percentage by weight:
C 0.2-1.5; Mn 10-25; optionally Ni<2; Si 0.05-2.00; Al 0.01-2.0; 0.01=<N<0.1; P+Sn+Sb+As<0.2 ; S+Se+Te<0.5 ; and Nb+Co 0.1-0.4 and Re+W<l, the balance being Fe apart from unavoidable impurities, and in that, after having been subjected to cold rolling, it is subjected to recrystallization annealing carried out
- for continuous annealing, at a temperature within the range of 900°C-1100°C for a time interval between 60 seconds and 120 seconds, or

- for batch annealing, at a temperature within the range of 700°C-800°C for a time interval between 30 minutes and 400 minutes,
the annealing atmosphere comprising Carbon activity a_c between 0.1 and 1.0, nitrogen N₂ content between 90% and 100% for continuous annealing, or between 0% and 100% for batch annealing, hydrogen H₂ content between 0% and 10% for continuous annealing, or between 0% and 100% for batch annealing, being the dew point for continuous and batch annealing lower than 0°C and preferably between - 10°C and - 50°C.
Process according to claim 1, wherein preferred composition ranges for single alloy elements or for combination of alloy elements, independently from each other, are :
C 0.4-0.8; Mn 16-19; optionally Ni<1.0; Si 0.2-0.4; Al 0.1-1.5; N 0.01-0.05; and Nb+Co 0.1-0.4 and Re+W 0.3-0.7.
Process according to claim 1 or 2, comprising the further operation of making a metallic coating obtained under hot conditions by a zinc based alloy containing magnesium and aluminum.
Austenitic steel with twinning induced plasticity (TWIP), characterized in that the steel has the following chemical composition, in percentage by weight:
C 0.2-1.5; Mn 10-25; optionally Ni<2; Si 0.05-2.00; Al 0.01-2.0; 0.01=<N<0.1; P+Sn+Sb+As<0.2 ; S+Se+Te<0.5; and Nb+Co 0.1-0.4 and Re+W<1, the balance being Fe apart from unavoidable impurities with the following mechanical features :
Rp 0.2 between 250 and 350 MPa

Rm between 850 and 1100 MPa

A80 between 60 and 100 %.
Austenitic steel according to claim 4, which has the composition C 0.4-0.8; Mn 16-19; optionally Ni<1.0; Si 0.2-0.4; Al 0.1-1.5; N 0.01-0.05; and Nb+Co 0.1-0.4 and Re+W 0.3-0.7.
Use of austenitic steel according to claim 4 or 5 for manufacturing complex geometry components, for absorption of energy, structural reinforces and automotive applications.