OUTLINE. Dual phase and TRIP steels. Processing Microstructure Mechanical properties Damage mechanisms Strategies to improve strength and ductililty

Transcription

1 OUTLINE Dual phase and TRIP steels Processing Microstructure Mechanical properties Damage mechanisms Strategies to improve strength and ductililty

2 DUAL PHASE STEELS With relatively straightforward thermomechanical processing and lean alloying, a simple ferrite-martensite microstructure develops Provide a wide range of excellent and industrially obtainable mechanical properties DP steels typically have high ultimate tensile strength (UTS) (due to the martensite) combined with low initial yielding stress (enabled by the ferrite) High early-stage strain hardening, and macroscopically homogeneous plastic flow (enabled through the absence of Luders effects) Properties are controlled by martensite volume fraction (VM), martensite grain size (SM), martensite carbon content (CM), martensite/ferrite morphology, ferrite grain size (SF), ferrite texture, density of transformationinduced geometrically necessary dislocations (GNDs), micro- and mesoscale segregation, and the chemical decoration state of the hetero-interfaces

3 DUAL PHASE STEELS Dual phase steels produced by intercritical annealing Heating rate, soaking time and cooling rate are important parameters

4 DUAL PHASE STEELS Cold rolled ferrite-pearlite steel Warm rolled ferrite-pearlite steel

5 DUAL PHASE STEELS Diagram for 0.12C 1.5Mn steel illustrating austenitization stages depending on temperature and time

6 DUAL PHASE STEELS Kinetics of isothermal austenitization at different temperatures : (a) steel 0.06C 1.5Mn, (b) steel 0.12C 1.5Mn; (c) steel 0.20C 1.5Mn

7 DUAL PHASE STEELS SEM micrograph showing fine grained ferrite and martensite after intercritical annealing at 760 C for 2 min. of the cold-rolled sample, (b) bright field TEM micrograph showing dislocation structures in the ferrite and block type of martensite. M: Martensite, F: Ferrite

8 DUAL PHASE STEELS Hall-Petch slope increases with increase in volume fraction of martensite Yield strength increases with grain refinement Motivation for producing ultra fine grained DP steels

9 DUAL PHASE STEELS Damage events captured during the in-situ experiments. (a) High magnification crop showing micro-cracks near a martensite-ferrite phase boundary. (b) Each damage event in the low-magnification image is represented with black dots. (c) Damage incidents are overlaid with the local strain map.

10 DUAL PHASE STEELS

11 DUAL PHASE STEELS

12 DUAL PHASE STEELS Grain refinement, bi-modal grain size distribution, precipitation of carbides, texture optimisation can contribute to high strength and ductility

13 TRIP STEELS These steels rely on the transformation of austenite into martensite during deformation for achieving their mechanical properties and hence are known as transformation-induced plasticity (TRIP) steels There are two types of such steels. Those having a fully austenitic microstructure are called TRIP steels. These steels tend to be rich in nickel and other expensive austenite stabilising elements. By contrast, austenite is only a minor phase in the overall microstructures of TRIPassisted steels. Allotriomorphic ferrite comprises about vol.% of the microstructures of these materials, the remainder being a mixture of bainite and carbon-enriched retained austenite. TRIP-assisted steels are generally lean in solute content with only about 0.2 wt% carbon, 1.5 wt% manganese and some 1-2 wt% silicon. Silicon retards cementite precipitation from the untransformed austenite during bainite formation. The carbon that is partitioned into the untransformed austenite following the formation of bainite, stabilises it and allows it to be retained at ambient temperature

14 TRIP STEELS In steels, austenite can transform to ferrite either by a reconstructive or a displacive mechanism. Martensitic transformation is displacive and can occur at temperatures where diffusion is inconceivable within the time scale of the process. Transformation starts only after cooling to a particular temperature called martensite-start temperature or M S. The fraction transformed increases with the undercooling. below M S. A martensite finish temperature or M f is usually defined as the temperature where 95% of the austenite has decomposed. Martensite can also be induced to form by stress or strain at temperatures above M S. The work done by an external stress compensates for the shortfall in the driving force for transformation to occur above M S

15 TRIP STEELS The higher the temperature above M S, the greater is the magnitude of the stress required. Strength of the austenite is lower at high temperatures. When the stress required for transformation exceeds the strength of the austenite, plastic strain precedes transformation. In steels containing austenite in the temperature range M S -M d, the formation of martensite during plastic deformation helps to maintain strain hardening. This delays the onset of necking, resulting a large uniform elongation. This is the principle of TRIP steels.

16 TRIP STEELS Typical chemical compositions (wt%) of early TRIP steels [Zackay et al., 1967]

17 TRIP STEELS The elongation obtained can vary with the tensile test temperature due to changes in the stability of austenite to martensitic transformation; this is one of the key design issues

18 TRIP ASSISTED STEELS Typical chemical compositions (wt%) of TRIP-assisted steels

19 TRIP ASSISTED STEELS The microstructures consisted of vol.% allotriomorphic ferrite, vol.% carbide-free bainite, the remainder being high-carbon retained austenite with some martensite and is referred to as TRIP assisted steels A typical multiphase microstructure of a modern TRIP-assisted steel, with allotriomorphic ferrite (F), carbide-free bainite (B) and retained austenite (A)

20 TRIP ASSISTED STEELS Schematic illustration of the two routes to generate the microstructure of TRIPassisted steel, with typical temperature and time indicated. Curves 1 and 2 stand for the transformation from fully austenitic state after hot rolling and intercritical annealing after cold rolling respectively.

21 TRIP ASSISTED STEELS Superior elongation observed in a TRIP-assisted steel compared to a dual phase steel with similar strength level

22 TRIP STEELS The rate at which austenite transforms during deformation appears to be the highly emphasised factor affecting the properties. The austenite stability should be such that it transforms progressively during deformation, so that damage can be accommodated at all stages of deformation. High carbon content, fine austenite grain size increase the austenite stability

23 Advanced High Strength Steels (AHSS) Current focus in AHSS is on 3 rd generation steels

24 Advanced High Strength Steels

25 TWIP steels Background Principles Microstructure Mechanical properties Formability Outlook

26 Background The original austenitic manganese steel, containing about 1.2% C and 12% Mn was invented by Sir Robert Hadfield in 1882 Hadfield`s steel was unique in that it combined high toughness and ductility with high work-hardening capacity and, usually, good resistance to wear Slip lines crossing without any deviation after 35% plastic strain in Hadfield steel

27 Background Initial studies of TWIP steels including those by European, Korean, and Japanese researchers led to attempts to industrially produce steels with ~22 28 % Mn Mn with its specific weight being less than that of iron and additions of Al make these materials lighter than conventional steels A subgroup of carbon-free TWIP steels with slightly lower strength was termed L-IP (Light-Induced Plasticity) steels

28 Principles High-Mn austenitic steels possess the highest product of strength and ductility normally above 50,000 MPa % As the applied stress increases, the volume fraction of twins increases steadily dividing grains continuously into smaller fragments that gradually reduce the effective glide distance of dislocations. This phenomenon is considered as dynamic Hall Petch effect resulting in very high strain hardening observed in TWIP steel. Combination of high strength and ductility of TWIP steels is explained by the high rate of strain hardening associated with the phenomenon of deformation twinning Mechanical twins are formed during deformation due to the low stacking-fault energy. Dynamic Hall Petch effect as strain hardening mechanism in TWIP steels (DMFP - Dislocation Mean Free Path)

29 Principles Stacking-Fault Energy (SFE) is the key factor that controls the mechanical properties of the high-mn alloys and plays essential role in the occurrence of twinning phenomena and the TWIP effect Some reports suggest that SFE required for the TWIP effect should be within the range mj/m 2. The minimum SFE value could be related to suppression of athermal γ to ε martensitic transformation Effect of SFE on deformation mechanism

30 Principles There is uncertainty with regard to the value of SFE required to initiate deformation by twinning mechanism. Low SFE is apparently a necessary condition to initiate twinning but is not sufficient, and activation of dislocation gliding should be hampered. Experimental measurement of SFE with good accuracy (using TEM) is very difficult. Most of the data found in the literature are based on thermodynamic computations and not by direct measurements with TEM. With all other conditions kept constant, lower SFE leads to higher density of straininduced twins, which act as barriers to dislocation glide, and ultimately results in higher strain hardening. Deformation of TWIP steels as in low SFE fcc alloys occurs by a competition between dislocation slip, deformation twinning, and austenite-to-martensitic transformation. In the most promising TWIP steels for automotive applications, the austenite-to martensite transformation is largely suppressed (which is particularly crucial in view of manufacturability as well)

31 Principles Dislocation slip occurs during early stages of deformation with deformation twinning becoming active after a threshold level of strain has been reached. The threshold strain is of the order of 5 %, although it can vary with SFE, grain size, temperature, etc. Strain-induced twins formed due to low stacking-fault energy gradually reduce the effective dislocation slip distance Low SFE is critical for increasing the dislocation density while maintaining relatively long homogeneous slip distances With additional strain, wavy slip is promoted leading to dislocation configurations that are progressively refined as the applied stress increases The nucleation of deformation twins requires the critical dislocation density. The activation of deformation twinning raises strain hardening of the alloy even further and therefore increases its ductility The secondary twin system can be activated triggering intensive twin intersections that further strain harden the alloy

32 Alloy Design Fe Mn C phase stability diagram at room temperature Alloy design should facilitate austenitic structure at all processing temperatures Prevention of martensite formation during cold working operations Optimization (increase) of yield strength, tensile strength and elongation at room temperature Prevention of carbide formation under normal processing conditions, Optimization of austenite stability/sfe so that twinning is activated at sufficiently high strains as well as the resistance to delayed fracture

33 Alloy Design

34 Alloy Design Fe Mn C phase stability diagram after tensile testing at room temperature Wt %Mn + 13wt%C 17 Concentrations of C and Mn required for stabilization of the fully austenitic Microstructure

35 Microstructure (a) Bright-field image of Fe 30Mn deformed up to 20% in tension showing well-developed dislocation cell-like structure and (b) Dark-field image of Fe 22Mn 0.6C after 50% strain and showing extensive mechanical twinning

36 Microstructure Dark-field TEM micrographs of the deformation microstructure of a Fe 22Mn 0.6C steel at 77 K (e-martensite platelets) and at room temperature (mechanical twins). Twins and e-martensite variants have been highlighted simultaneously using SAD techniques (inverted contrast).

37 Mechanical Properties Effect of grain size on tensile response of Fe 22Mn 0.6C Yield and tensile strength increase with decrease in grain size (Hall-Petch) Elongation decreases with grain refinement The number of deformation twins increases with grain size leading to greater TWIP effect in coarse-grained material than in fine grain one. This can be attributed to grain size dependence of the critical stress for onset of deformation twinning

38 Mechanical Properties Room temperature tensile response of 22Mn 0.6C TWIP steel at different strain rates. Note: strain offset is pointed out for clarity Jerky serrated flow during tensile test of FeMnC-type TWIP steels is typically observed, which is explained by the formation of Portevin-Le Chatelier (PLC) bands. The underlying phenomenon is related to the classic dynamic strain aging (DSA) mechanism based on dynamic interaction between mobile dislocations and diffusing solute atoms

39 Mechanical Properties Results of the tensile tests (UE, TS, and 0.2 % YS) carried out at different temperatures on laboratory Fe 22Mn 0.6C cold strips (grain size 3 μm). The activated deformation mechanisms were confirmed by TEM observation

40 Formability Maximum height before failure of a stamped cross-shape part which combines deep drawing, plane strain and expansion deformation paths A 22Mn 0.6C grade referred as FeMn TWIP 1000 exceeds by far the performance of various steel grades

41 Outlook There are many stimulating and necessary challenges that need to be addressed in TWIP steels A more detailed understanding of the fundamental reason why these alloys twin through observations of twin nucleation mechanisms (in situ TEM) and ab initio calculations (SFE and interactions between plasticity defects and interstitial atoms). A deeper analysis of the effect of carbon on the strain-hardening which seems to improve the efficiency of the twins as obstacles to gliding dislocations In the domain of fracture, stress corrosion cracking and delayed fracture by hydrogen have to be analysed Welding of TWIP steels with the other more conventional steels has to be handled. Large gradients in the weldment chemical composition, thermophysical properties (solidification range, thermal and electrical conductivities,...) and in the microstructure between TWIP and carbon steels are indeed expected to make life exciting for welders!!