Nov 19th – 21st 2014, Pilsen, Czech Republic, EU

INFLUENCE OF THERMO-MECHANICAL TREATMENTS ON STRUCTURE AND

MECHANICAL PROPERTIES OF HIGH-MN STEEL

Leszek Adam DOBRZAŃSKI, Wojciech BOREK, Janusz MAZURKIEWICZ

Division of Materials Processing Technology, Management and Computer Techniques in Materials Science,

Institute of Engineering Materials and Biomaterials, Silesian University of Technology, Konarskiego 18A,

44-100 Gliwice, Poland, wojciech.borek@polsl.pl,

Abstract

The aim of this paper is to determine the high-manganese austenite propensity to twinning induced by the

cold working and its effect on structure and mechanical properties, and especially the strain energy per unit

volume of ten new-developed high-manganese Fe – Mn – (Al, Si) investigated steel, containing about 24,5 %

of manganese, 1% of silicon, 3 % of aluminium and microadditions Nb and Ti. with various structures after

their heat- and thermo-mechanical treatments. The new-developed high-manganese Fe – Mn – (Al, Si) steel

provide an extensive potential for automotive industries through exhibiting the twinning induced plasticity

(TWIP) mechanisms. TWIP steel not only show excellent strength, but also have excellent formability due to

twinning, thereby leading to excellent combination of strength, ductility, and formability over conventional

dual phase steels or transformation induced plasticity TRIP steels. Results obtained for high-manganese

austenitic steel with the properly formed structure and properties in the thermo-mechanical processes

indicate the possibility and purposefulness of their employment for constructional elements of vehicles,

especially of the passenger cars to take advantage of the significant growth of their strain energy per unit

volume which guarantee reserve of plasticity in the zones of controlled energy absorption during possible

collision resulting from activation of twinning induced by the cold working as the fracture counteraction factor,

which may result in significant growth of the passive safety of these vehicles' passengers.

Keywords:

High manganese steel, TWIP steel, strengthening mechanisms, mechanical properties, twinning

1. INTRODUCTION

Constructional solutions and steels used in the frontal part of a vehicle are the most significant due to the

possibility of accident occurrence. The goal of structural elements such as frontal frame side members,

bumpers and the others is to take over the energy of an impact. Proper selection of chemical composition

and manufacturing technologies, guarantee obtaining the structure allowing for connections a favorable

strength and plastic properties of steel. In the last three decades new groups of high-manganese steels have

been developed [1-7]. One, I think, most important from group of high manganese steels are TWIP-tape

steels which contains carbon from 0.02 to 0.65% and manganese from 20 to 35% and with diverse

concentrations of Si and Al, introduced primarily to lower the density of about 7.3g/cm3 for this group of

steels. Such steels possess very good plastic properties as a result of an intense curve of twinning during

plastic deformation (TWIP effect, Twinning Induced Plasticity). An advantageous array of mechanical

properties achieved by such steels, i.e. Rm~800-1000MPa, Rp0.2=250-550MPa, un=35-90% is strongly

dependent on chemical composition, and especially a concentration of Mn. The role of silicone and

aluminium consists of solid solution hardening of steel, while carbon is an element stabilising austenite.

Apart from high strength and plastic properties, they possess an especially high ability to absorb energy in

the conditions of plastic deformation at high speeds and formation of parts with a complex shape [1-14].

Thermo-mechanical treatment applied to refine microstructure of austenitic steels used in the automotive

industry has to be done in controlled conditions. Appling too large deformation, or too long isothermal holding

Nov 19th – 21st 2014, Pilsen, Czech Republic, EU

times after the last deformation results in excessive grain refinement of the structure up to about 2 mm, what

has an impact on increasing strength properties in particular increasing the offset yield point by about 150-200

MPa and tensile strength increase to 1000 MPa [1-7, 15-23]. Whereas too large value for the average grain

diameter of about 70 mm can improve plastic properties, especially elongations which can achieve value about

80-90% at relatively low strength properties. Therefore the main objective of the application of thermo-plastic

deformation is the selection of the processing parameters to achieve optimal value for the average grain

diameter at which the product of tensile strength and elongation reaches optimal value. This will allow

increasing strain energy per unit volume of structural components of vehicles during traffic collision [1-3, 15-23].

2. MATERIALS AND EXPERIMENTAL PROCEDURE

Examinations were carried out on new-developed high-manganese steel TWIP-type steel -

X8MnSiAlNbTi25-1-3 containing 24,5 % Mn, 1 % Si, 3 % Al, and with microadditions Nb and Ti. The

chemical compositions of steel were shown in Table 1. For the investigated melts Nb and Ti microadditions

were added in order to refine the structure and achieve precipitation hardening. Steel is characterized by

high metallurgical purity, associated with low concentrations of S and P contaminants and gases. Melt were

modified with rare earth elements.

Table 1.

Chemical composition of new-developed high-manganese TWIP-type steel, mass fraction

Steel designation Chemical composition, mass fraction

C Mn Si Al Nb Ti P max S max Ce La Nd

X8MnSiAlNbTi25-1-3 0,08 24,60 0,91 3,10 0,040 0,024 0,002 0,003 0,005 0,001 0,002

Based on the results obtained during thermo-mechanical treatment carried out in continuous axisymetrical

compression test and multi-stage compression tests using the Gleeble 3800 thermo-mechanical simulator at

the Institute of Engineering Materials and Biomaterials, Silesian University of Technology [15-23] allowed to

work out a schedule of three different variants of hot-rolling of high-manganese austenitic steel (Fig. 1),

where the processes eliminating the consequences of strain hardening were, respectively, dynamic

recovery, dynamic, metadynamic and static recrystallisation.

Fig. 1. Schematic and parameters of the multi-stage hot-rolling carried out on single-cage reversing hot-

rolling mill line for semi-industrial simulation (LPS) module B at the Institute of Ferrous Metallurgy in Gliwice.

TA - austenitizing temperature, t - time of the isothermal holding of specimens at a temperature of last

deformation - 850°C

Nov 19th – 21st 2014, Pilsen, Czech Republic, EU

In the next step after four-stage hot-deformations specimens for static tensile tests were cut down in the

direction of hot-rolling of high-manganese austenitic sheets. Static tests were performed on tensile testing

machine ZWICK Z100 in order to investigate mechanical properties, especially strain energy per unit volume

of high-manganese austenitic steel, with various structures after their thermo-mechanical treatment.

Metallographic examinations of samples were carried out using the LEICA MEF4A light microscope. In order

to reveal austenite structure, the samples etched in a mixture of nitrous and hydrochloric acid in various

proportions. The structure of the investigated steel was also characterised using the SUPRA 25 scanning

electron microscope and the high resolution S/TEM TITAN 80-300 transmission electron microscope working

at accelerating voltage of 300 kV. TEM observations were carried out on thin foils. The specimens were

ground down to foils with a maximum thickness of 80 µm before 3 mm diameter discs were punched from

the specimens. The disks were further thinned by ion milling method with the Precision Ion Polishing System

(PIPS™), using the ion milling device (model 691) supplied by Gatan until one or more holes appeared. The

ion milling was done with argon ions, accelerated by voltage of 15 kV.

3. RESULTS AND DISCUSSION

After four-stage hot-rolling according to scheme shown in figure 1, static tensile tests were performed in

order to investigate mechanical properties, especially strain energy per unit volume of high-manganese

steel, with various structures after their thermo-mechanical treatment. After the solution heat treatment of

X8MnSiAlNbTi25-1-3 steel were noticed that from the end temperature of hot-rolling with a strain 0.23, the

structure of steel is represented by dynamically recovered austenite grains elongated in the direction of

rolling containing numerous twins (Fig. 2a). In addition, twins are visible occurring especially in large-sized

grains. Fine, recrystallized grains are arranged dynamically at the boundaries of dynamically recovered

austenite grains elongated in the direction of rolling. The dynamically recovered grains are refined twice less

than the specimens deformed in a Gleeble 3800 simulator [15-23] as a result of too rapid give up the heat

from very thin sheets (about 3-4mm) of high-manganese steel to the environment and thus it creates a lot of

problems to control of temperature of individual rolling possess. Isothermal holding for 30s after hot-rolling

with final pass reduction 20% (equal true strain 0.23), causes that the fraction of statically and

metadynamically recrystallized grains occurs in about 30% and is considerably lower than for specimens

after hot-working in the Gleeble 3800 simulator. They are arranged mainly at the boundaries of the elongated

grains of the statically recovered austenite, and are often situated also on the boundaries of twins. Creation

of refined structure has no influence on phase composition of the investigated steel and should increase

mechanical properties during cold plastic deformation.

a) b) c)

Fig. 2. Austenitic structures of high manganese X8MnSiAlNbTi25-1-3 TWIP-type steel; a) obtained after

four-stage hot-rolling with a true strain equal 4x0.23 and natural air-cooling after final deformation at

temperature 850°C, according to Variant No. II of thermo-mechanical treatment (Fig 1); b) and c) shows

structures with mechanical and micro twins and slip bands obtained after hot-rolling with a true strain 4x0.23

b) according to Variant No. II, c) according to Variant No. III of thermo-mechanical treatment (Fig 1) and after

static tensile tests

Nov 19th – 21st 2014, Pilsen, Czech Republic, EU

a)

b)

c)

d)

Fig. 3. Twinned austenite area in the X8MnSiAlNbTi25-1-3 steel in the heated state with a true strain equal

4x0.23, with water cooling after isothermal holding 30s at temperature 850°C, according to variant No. III of

thermo-mechanical treatment (Fig. 1), and a following strain test until elongation of 30%: a) bright field, b)

dark field from the ( 200 ) plain Fe, c) diffraction pattern, d) solution of the diffraction pattern in fig. c

This group of steels can be applied on constructional elements of the car body which should transfer loads

during front or side impact collisions. On figures 2b, 2c and 3 are presented austenitic structures of high

manganese steel with mechanical and micro twins and slip bands obtained after hot-rolling with a true strain

0.23 according to thermo-mechanical treatment variant No. II or III and after static tensile tests. The

substantial contribution of mechanical twinning into the plastic deformation of steel in this variant causes a

substantial increase in plastic properties and continuous increase in the temporary strain hardening

coefficient. An increase in the parallel elongation of steel is closely linked to the intensive progress of

mechanical twinning in the places where local narrowing in the specimen occurs. The twin boundaries

formed are strong barriers for the dislocation movement; hence further plastic deformation is situated in

areas with a smaller local flow stress.

Fig. 4. Influence of various parameters of thermo-mechanical treatment on mechanical properties

of high manganese X8MnSiAlNbTi25-1-3 TWIP-type steel: yield strength Rp0.2, tensile strength Rm

and total elongation um

Nov 19th – 21st 2014, Pilsen, Czech Republic, EU

On the figure 4 are presented results mechanical properties of new developed high-manganese TWIP-type

steel, with various structures after various variants of thermo-mechanical treatments. Mechanical twinning

induced by the cold working of the high-manganese austenitic TWIP steels has a significant effect on

forming their structure and mechanical properties (Figs 2b, 2c and 3). Mechanism of twinning induced by the

cold working of the high-manganese austenitic steel results in growth of the strain energy per unit volume

after the successive cold deformation. On figure 5 is presented representative tensile curve of the TWIP-type

steel with designated strain energy per unit volume after the cold deformation equal 263 MJ/m3. To increase

strain energy per unit volume the temperature of plastic deformation should be decrease below ambient

temperature. Energy increase can be also achieved by increasing the strain rate cold plastic deformation, for

instance for TWIP type steel acceleration of deformation to 500 s-1 causes that energy per unit volume

increases by approximately 200 MJ/m3 in comparison with energy determined in static condition. The high-

manganese austenitic steels with the properly formed structure and properties and especially with the big

strain energy per unit volume yield the possibility to be used for the constructional elements of cards

affecting advantageously the passive safety of the vehicles' passengers.

Fig. 5. Representative tensile curve of investigated X8MnSiAlNbTi25-1-3 TWIP-type steel with designated

strain energy per unit volume after the cold deformation in static conditions

4. CONCLUSIONS

High-manganese Fe – Mn – (Al, Si) steels provide an extensive potential for automotive industries

through exhibiting the twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP)

mechanisms.

It was concluded on the basis of the own investigations [15-23] that mechanical properties of high-

manganese austenitic steels in the condition after supersaturation state and on the basis of data from the

literature that the examined steel subjected to thermo-mechanical processing are characterised by a yield

point higher by approx. 200-250MPa, which is a result of precipitation strengthening of such steels by the

dispersive particles of Nb and Ti carbonitrides and is also connected with thermo-mechanical treatment

employed.

Twinning mechanism induced with cold plastic deformation, forming the structure of the investigated steel

after prior thermo-mechanical processing with varied share of dynamic, metadynamic and static

mechanisms removing the consequences of strain hardening, has a substantial effect on increasing the

strain energy per unit volume and strength and plastic properties of steel in the conditions of cold plastic

deformation.

Nov 19th – 21st 2014, Pilsen, Czech Republic, EU

ACKNOWLEDGEMENTS

Project was founded by the National Science Centre based on the decision number

DEC-2012/05/B/ST8/00149.

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