**6. Impact of plastic deformation parameters on microstructure and properties evolution**

Some major metal processing parameters such as temperature, strain rate, and strains that impact steel microstructures and their flow curves are illuminated in detail in the following.

### **6.1 The role of work-hardening rate**

The WH rate enables strengthening and hardening to the materials below the recrystallization temperature. Rapid WH rates are realized in low strain regions due to increases in dislocation density while at later strain increase, the effect starts to decrease due to recrystallization of new strain-free grains [31, 36, 40, 50, 56]. In WH, dislocations are preferably pinned, which will impede crack propagation on the microscale. With increasing temperature, the probability of rearrangement of atoms is higher which assists lower strength but increases the ductility of materials. Samantaray et al. [36] have reported for 316 L stainless steel that the WH rate starts rapidly with increasing temperature and strain rate at a specific value of strain (see **Figure 8**). The WH rate gradually decreased at higher temperature with increasing strain while it falls more rapidly under lower temperature conditions.

Lin et al. [44] have derived the following model for the influence of dynamic recovery during WH (see Eq. (2)) and dynamic recrystallization Eq. (3) under different deformation conditions within TMP.

$$\sigma = \left[ \sigma\_{DRV}^2 + \left( \sigma\_0^2 - \sigma\_{DRV}^2 \right) \exp(-\mathcal{Q} \,\varepsilon) \right]^{0.8} \tag{2}$$

**11**

*Plastic Deformation Behavior in Steels during Metal Forming Processes: A Review*

( ) 1

*The impact of temperature and strain rate on work hardening rate of stainless steel [57].*

is peak strain; εc is critical strain; is coefficient of dynamic recovery.

 σ

σ σ ε

*DRV P DRX d c*

<sup>−</sup> =σ − −σ − − <sup>≥</sup>

Where σ is flow stress; σDRV is steady-state stress due to dynamic recovery; σDRV is steady-state stress due to dynamic recrystallization; σ0 is yield stress; ε is strain; ε<sup>P</sup>

Generally, the critical strain acknowledgeable for the start of DRX can be calculated either by deformed microstructure or flow stress curves [58], In which flow stress curve analysis are simple and easier while microstructural are complicated. This flow curve analysis method was proposed in 1981 by Mecking et al. [59] and later it developed by Ryan et al. [60] and McQueen et al. [61] emphasize the point where DRX occurs on the flow curves. This method allows to find out the critical strain point where the flow curve changes due to the formation of new strain-free

**6.2 On properties derived from flow curves and relation to microstructures**

The flow stress–strain curve reflects the changes in the material through plastic deformation during dynamic loading [25, 32, 62–66]. The flow stress can be influenced by several factors like chemical composition, crystal structure (e.g., steel matrix - bcc, fcc, Mg-base - hcp, and others) [50, 67], different phases and compounds [17, 30, 50, 68–70], grain boundaries [25, 50, 71, 72] as well as imperfections [34, 50, 55, 73, 74]. Other factors such as friction (σf), thermal (σt) and athermal (σa) terms also affect flow stresses, as indicative by relations

> σ εε

Where T is temperature, ἐ is strain rate and ε is strain. σa represents the internal stress which occurs due to long range barriers to dislocation motion in the materials, while σf term reflects the stress needed to overcome the lattice friction depending

 σ=+ + <sup>f</sup>( , ,, *T T* ) *t a* ( ) (4)

*exp K*

*d n c*

ε ε (3)

*P*

ε

ε ε

*DOI: http://dx.doi.org/10.5772/intechopen.97607*

σ

grains via DRX.

**Figure 8.**

in Eq. (4) [75].

on strain rate and temperature.

*Plastic Deformation Behavior in Steels during Metal Forming Processes: A Review DOI: http://dx.doi.org/10.5772/intechopen.97607*

**Figure 8.** *The impact of temperature and strain rate on work hardening rate of stainless steel [57].*

$$\sigma = \sigma\_{DRV} - \left(\sigma\_P - \sigma\_{DRX}\right) \left\{ \mathbf{l} - \exp\left[ -K\_d \left(\frac{\mathbf{c} - \mathbf{c}\_c}{\mathbf{c}\_p}\right)^{n\_d} \right] \right\} \mathbf{c} \ge \mathbf{c}\_c \tag{3}$$

Where σ is flow stress; σDRV is steady-state stress due to dynamic recovery; σDRV is steady-state stress due to dynamic recrystallization; σ0 is yield stress; ε is strain; ε<sup>P</sup> is peak strain; εc is critical strain; is coefficient of dynamic recovery.

Generally, the critical strain acknowledgeable for the start of DRX can be calculated either by deformed microstructure or flow stress curves [58], In which flow stress curve analysis are simple and easier while microstructural are complicated. This flow curve analysis method was proposed in 1981 by Mecking et al. [59] and later it developed by Ryan et al. [60] and McQueen et al. [61] emphasize the point where DRX occurs on the flow curves. This method allows to find out the critical strain point where the flow curve changes due to the formation of new strain-free grains via DRX.

#### **6.2 On properties derived from flow curves and relation to microstructures**

The flow stress–strain curve reflects the changes in the material through plastic deformation during dynamic loading [25, 32, 62–66]. The flow stress can be influenced by several factors like chemical composition, crystal structure (e.g., steel matrix - bcc, fcc, Mg-base - hcp, and others) [50, 67], different phases and compounds [17, 30, 50, 68–70], grain boundaries [25, 50, 71, 72] as well as imperfections [34, 50, 55, 73, 74]. Other factors such as friction (σf), thermal (σt) and athermal (σa) terms also affect flow stresses, as indicative by relations in Eq. (4) [75].

$$
\sigma = \sigma\_f \left( \pounds, T \right) + \sigma\_\iota \left( \pounds, \pounds, T \right) + \ \sigma\_\iota \tag{4}
$$

Where T is temperature, ἐ is strain rate and ε is strain. σa represents the internal stress which occurs due to long range barriers to dislocation motion in the materials, while σf term reflects the stress needed to overcome the lattice friction depending on strain rate and temperature.

*Material Flow Analysis*

tion sub boundaries.

*800°C with strain rate 0.001 s−1 [52].*

**Figure 7.**

detail in the following.

temperature conditions.

different deformation conditions within TMP.

σ σ

**and properties evolution**

**6.1 The role of work-hardening rate**

bulging can be seen in **Figure 7b**. It is obvious that the dDRX nuclei contain a much lower dislocation density than the deformed region and these nuclei are distinct from highly disturbed substructures with twin boundaries and low angle disloca-

*(a) Schematic correlation between cDRX and dDRX [51], (b) dDRX nuclei in austenitic stainless steel at* 

One additional terminology has recently been denoted as post-DRX which

Some major metal processing parameters such as temperature, strain rate, and strains that impact steel microstructures and their flow curves are illuminated in

The WH rate enables strengthening and hardening to the materials below the recrystallization temperature. Rapid WH rates are realized in low strain regions due to increases in dislocation density while at later strain increase, the effect starts to decrease due to recrystallization of new strain-free grains [31, 36, 40, 50, 56]. In WH, dislocations are preferably pinned, which will impede crack propagation on the microscale. With increasing temperature, the probability of rearrangement of atoms is higher which assists lower strength but increases the ductility of materials. Samantaray et al. [36] have reported for 316 L stainless steel that the WH rate starts rapidly with increasing temperature and strain rate at a specific value of strain (see **Figure 8**). The WH rate gradually decreased at higher temperature with increasing strain while it falls more rapidly under lower

Lin et al. [44] have derived the following model for the influence of dynamic recovery during WH (see Eq. (2)) and dynamic recrystallization Eq. (3) under

> σ σ

2 22 0.5 <sup>0</sup> [ ( )exp( )]

 Ω = + − −ε *DRV DRV* (2)

**6. Impact of plastic deformation parameters on microstructure** 

occurs during the annealing process in deformed materials [55].

**10**

In addition, processing temperatures and strain rates are equally important for the plastic deformation behavior. Therefore, the dynamics of TMP can be understood through the investigation of microstructural changes combined with interpretations of trends of flow stress–strain curves which depend on DRV and DRX, and SRX [76, 77]. I.

It is noticed in most cases that flow stress decreases with increase in temperature and depends upon the applied strain rate [27, 32, 36, 78]. In terms of temperature, strain, and strain rates, the flow curves can be expressed by Eq. (5) [75].

$$
\sigma = \frac{2}{\sqrt{3 \left( 1 - m \right)}} K e^{\
u} \text{ è, } \exp \left( -\beta T \right) \tag{5}
$$

Where m stands for strain rate sensitivity, n represents the strain hardening exponent, and K, β represents material constant.

In the following, some flow curve trends of different steels and underlying phenomena are discussed.

Researchers reported that the series of flow curves are subjected to different temperatures and strain rates for different grades of steels [26, 27, 44, 45, 64]. Lin et al. [44] have reported interesting results for hot deformation of 42CrMo grade high strength steel in which they found that flow stress increased with decreasing temperature (**Figure 9a**) while it increased with strain rates (**Figure 9b**). At the slower strain rate in different ranges of temperatures, the flow stress will decrease with increase in temperature due to increase in the amount of cross slip screw dislocations and climb of edge dislocations, as well as vacancy diffusion. This results in an increase of grain boundary mobility and energy accumulation at boundaries for the nucleation and growth in DRX grains and dislocation annihilation which is responsible for the decrease in flow stress [27, 64].

Kumar et al. [27] have found for hot deformed condition in high strength steel that flow stress increases continuously at lower deformation temperature (750–850°C) due to continuation of work hardening phenomena High temperature showed the higher steady-state condition where DRX was dominant. While both DRV and DRX were dominant at all strain rates with decreasing temperature, the dDRX phenomena was more prominent at slow strain rate (0.001 s-1) at 900°C due to nucleation of unstrained grains that occurs normally in low SFE high strength steels. A flow curve without pronounced peak stress, but which exhibits

**13**

*Plastic Deformation Behavior in Steels during Metal Forming Processes: A Review*

a steady-state, is generally associated with dynamic recovery being the dominant

Zhang et al. [31] have reported that several original grains were broken and recrystallized new grains showed up along the grain boundaries at the deformed condition at 900°C with a high strain rate 10 s−1, which indicates that the deformed morphology is inhomogeneous. In contrast, at the same temperature but with a lower strain rate (1 s−1), DRX was observed along the grain boundaries. This is due to local temperature rise within the samples during deformation. It is interesting to note that at the initial stage of strain, the flow stress increases steeply due to work hardening phenomena in materials having higher carbon content and less austenite

stabilizing alloying elements. It reaches a peak value before going into the

temperatures with different strain rates in austenitic stainless steel. Also, some differences could be seen in the work hardening phenomena; the slopes of the flow stress curves changed. In the initial work hardening region, the increase of dislocation density during deformation is controlled by the competition between storage and annihilation of dislocations, i.e. opposing contributions of work hardening and the dynamic recovery due to the change of dislocation density with deformation.

**7. SPD impacts on the structure and mechanical properties of steels**

indirectly associated with material stability and durability.

SFE between 15 to 50 mJ/m2 [31, 85, 87].

steelmakers Kobo steel, Nippon, and Sumitomo steel organizations.

Severe plastic deformation where metal grains are heavily deformed is realized by using several setups of plastic deformations like high-pressure torsion, equal channel angle pressing, multi-axial forging, twist extrusion, accumulated roll bending, and constrained groove pressing [22, 34, 80]. Severe deformation produces not only a strong direct impact on the mechanical properties i.e. high strength, lowtemperature toughness, superior plasticity, good ductility, and good wear resistance of high manganese grades steel but also on other important properties such as thermal stability, diffusion, radiation tolerance, and corrosion properties, which are

The high manganese steels (Mn) are advanced high strength austenitic steels that contain Mn between 3 to 31% wt. These steels are known as Hadfield steel, damping steel, complex steel, transformation induced plasticity steel (TRIP), and twinning induced plasticity steel (TWIP) [81, 82]. In all of these, Hadfield steel was firstly discovered in 1882 by Sir Robert Hadfield [83] while TWIP steel is one of the latest fully austenitic steel which is developed in the early 1990s by Japanese

It is well known that ultrafine and nanocrystalline structure depends on three mechanisms; martensitic transformation, dislocation motion, and twinning and twin evolution where stacking fault energy (SFE) of material plays an important role. **Figure 10** reflects the relation between strain-induced mechanism vs. temperature and SFE for Fe-20Mn-4Cr-0.5C steel. It shows that retained austenite can be converted into ε-martensite and strain-induced by a twinning mechanism at a lower temperature. Thus, the calculation of martensitic start temperature *and* SFE value is necessary to achieve the right combination of mechanical and other properties in low SFE high Mn steels. It is well known that the SFE of materials depends on chemical composition and on temperature [84–86]. The high Mn steels have a low

Allain et al. [86] reported results for Fe-22Mn-0.06C steel where the temperature influences the SFE values and strain-induced mechanism, which can be seen in **Table 3**. The strain hardening and mechanical behavior of steels strongly depend

Souza et al. [45] have documented results of hot deformation testing at elevated

*DOI: http://dx.doi.org/10.5772/intechopen.97607*

restoration mechanism [79].

softening stage.

**Figure 9.** *The true stress–strain curves at different temperatures and strain rates for 42CrMo steel [44].* *Plastic Deformation Behavior in Steels during Metal Forming Processes: A Review DOI: http://dx.doi.org/10.5772/intechopen.97607*

a steady-state, is generally associated with dynamic recovery being the dominant restoration mechanism [79].

Zhang et al. [31] have reported that several original grains were broken and recrystallized new grains showed up along the grain boundaries at the deformed condition at 900°C with a high strain rate 10 s−1, which indicates that the deformed morphology is inhomogeneous. In contrast, at the same temperature but with a lower strain rate (1 s−1), DRX was observed along the grain boundaries. This is due to local temperature rise within the samples during deformation. It is interesting to note that at the initial stage of strain, the flow stress increases steeply due to work hardening phenomena in materials having higher carbon content and less austenite stabilizing alloying elements. It reaches a peak value before going into the softening stage.

Souza et al. [45] have documented results of hot deformation testing at elevated temperatures with different strain rates in austenitic stainless steel. Also, some differences could be seen in the work hardening phenomena; the slopes of the flow stress curves changed. In the initial work hardening region, the increase of dislocation density during deformation is controlled by the competition between storage and annihilation of dislocations, i.e. opposing contributions of work hardening and the dynamic recovery due to the change of dislocation density with deformation.

### **7. SPD impacts on the structure and mechanical properties of steels**

Severe plastic deformation where metal grains are heavily deformed is realized by using several setups of plastic deformations like high-pressure torsion, equal channel angle pressing, multi-axial forging, twist extrusion, accumulated roll bending, and constrained groove pressing [22, 34, 80]. Severe deformation produces not only a strong direct impact on the mechanical properties i.e. high strength, lowtemperature toughness, superior plasticity, good ductility, and good wear resistance of high manganese grades steel but also on other important properties such as thermal stability, diffusion, radiation tolerance, and corrosion properties, which are indirectly associated with material stability and durability.

The high manganese steels (Mn) are advanced high strength austenitic steels that contain Mn between 3 to 31% wt. These steels are known as Hadfield steel, damping steel, complex steel, transformation induced plasticity steel (TRIP), and twinning induced plasticity steel (TWIP) [81, 82]. In all of these, Hadfield steel was firstly discovered in 1882 by Sir Robert Hadfield [83] while TWIP steel is one of the latest fully austenitic steel which is developed in the early 1990s by Japanese steelmakers Kobo steel, Nippon, and Sumitomo steel organizations.

It is well known that ultrafine and nanocrystalline structure depends on three mechanisms; martensitic transformation, dislocation motion, and twinning and twin evolution where stacking fault energy (SFE) of material plays an important role. **Figure 10** reflects the relation between strain-induced mechanism vs. temperature and SFE for Fe-20Mn-4Cr-0.5C steel. It shows that retained austenite can be converted into ε-martensite and strain-induced by a twinning mechanism at a lower temperature. Thus, the calculation of martensitic start temperature *and* SFE value is necessary to achieve the right combination of mechanical and other properties in low SFE high Mn steels. It is well known that the SFE of materials depends on chemical composition and on temperature [84–86]. The high Mn steels have a low SFE between 15 to 50 mJ/m2 [31, 85, 87].

Allain et al. [86] reported results for Fe-22Mn-0.06C steel where the temperature influences the SFE values and strain-induced mechanism, which can be seen in **Table 3**. The strain hardening and mechanical behavior of steels strongly depend

*Material Flow Analysis*

and SRX [76, 77]. I.

phenomena are discussed.

In addition, processing temperatures and strain rates are equally important for the plastic deformation behavior. Therefore, the dynamics of TMP can be understood through the investigation of microstructural changes combined with interpretations of trends of flow stress–strain curves which depend on DRV and DRX,

It is noticed in most cases that flow stress decreases with increase in temperature and depends upon the applied strain rate [27, 32, 36, 78]. In terms of temperature,

> ( ) ( ) <sup>2</sup> , exp

εε

Where m stands for strain rate sensitivity, n represents the strain hardening

In the following, some flow curve trends of different steels and underlying

Researchers reported that the series of flow curves are subjected to different temperatures and strain rates for different grades of steels [26, 27, 44, 45, 64]. Lin et al. [44] have reported interesting results for hot deformation of 42CrMo grade high strength steel in which they found that flow stress increased with decreasing temperature (**Figure 9a**) while it increased with strain rates (**Figure 9b**). At the slower strain rate in different ranges of temperatures, the flow stress will decrease with increase in temperature due to increase in the amount of cross slip screw dislocations and climb of edge dislocations, as well as vacancy diffusion. This results in an increase of grain boundary mobility and energy accumulation at boundaries for the nucleation and growth in DRX grains and dislocation annihilation which is

Kumar et al. [27] have found for hot deformed condition in high strength steel that flow stress increases continuously at lower deformation temperature (750–850°C) due to continuation of work hardening phenomena High temperature showed the higher steady-state condition where DRX was dominant. While both DRV and DRX were dominant at all strain rates with decreasing temperature, the dDRX phenomena was more prominent at slow strain rate (0.001 s-1) at 900°C due to nucleation of unstrained grains that occurs normally in low SFE high strength steels. A flow curve without pronounced peak stress, but which exhibits

*The true stress–strain curves at different temperatures and strain rates for 42CrMo steel [44].*

<sup>−</sup> <sup>−</sup>

*n m K T*

 β

(5)

strain, and strain rates, the flow curves can be expressed by Eq. (5) [75].

*m*

3 1

σ=

exponent, and K, β represents material constant.

responsible for the decrease in flow stress [27, 64].

**12**

**Figure 9.**

#### **Figure 10.**

*Effect of SFE and temperature on deformation mechanism in Fe-20Mn-4Cr-0.5C steel [84].*


#### **Table 3.**

*The deformation mechanism at different temperatures for Fe-22Mn-0.6C steel [86].*

on the SFE, which is responsible for the activation energy of a deformation mechanism [87].

The mechanical properties of Hadfield Mn austenitic steels can be improved through high rate strain hardening where two phenomena (i.e. dislocation accumulation and twinning) act preferentially during plastic deformation [82]. This is attributed to the strain hardening transformation where austenite phase transforms into ε or α-martersite, and twinning, dynamic strain aging, dispute between dislocations with stacking faults occurs. In this connection, Yan et al. [88] have tried to improve the hardness values by shot pinning method, whereby hardness values could be increased with increasing in operation time. This was attributed to the increment in density of dislocations, dislocation accumulation, and formation of twinning. The influence of higher strain rate (between 103 to 105 /s) attains great impact on mechanical behavior and wear resistance properties of high austenitic Mn steel which may be linked to dynamic strain aging and may delay fracture [81, 89–93].

Over the past few years, many researchers have reported work on TRIP and TWIP steels and achieved better mechanical properties by plastic deformation at high strains (more than 1) [22, 73, 74, 81, 89, 91, 94–96]. Both TRIP and TWIP steels are fully austenitic steels with less carbon content than hadfield steel. The initial microstructure of TRIP steel is consisted of martensite, bainite and ferrite with retained austenite. The fraction of carbon enriched retained austenite in TRIP steels is between 5 to 30% which transforms into martensite by displacive mechanism during SPD process. This behavior has attained great improvement in strength and toughness properties [96, 97].

A critical issue remains hydrogen embrittlement in TRIP steels, promoted by a displacive mechanism where the relevance of different solubility and diffusivity in the parent austenite has been discussed [96].

Sevsek et al. [90] reported the effect of strain rate on medium Mn X6MnAl12–3 steel. The softer austenite region was strained locally and transformed into

**15**

was higher.

**Figure 11.**

*(b) 18%, (c) 26%, and (d) 34% [81].*

**8. Concluding remarks**

*Plastic Deformation Behavior in Steels during Metal Forming Processes: A Review*

martensite which depended on the strain rate sensibility. The deformation induced phase transformation of austenite to martensite is partially suppressed at lower and higher strain rates. The impact of high strains in fully austenitic Fe-22Mn-0.6C steel is predominantly controlled by twinning plasticity mechanism (see **Figure 11**) as suggested by Jacob et al. [81]. The initial microstructure of Fe-22Mn-0.6C steel is a single-phase austenitic steel with few twinned grains (**Figure 11a**). They found that the fraction of twinning is increased with increasing strains (**Figure 11b–d**), where most of the internal energy was used for recrystallization and rest for grain growth [87]. They concluded that twin boundaries act as a hindrance to the dislocation

*Optical morphologies of Fe-22Mn-0.6C steel subjected to high strain deformation: (a) unstained; strained with* 

In the same way, Kang et al. [98] have reported HPT tests for TWIP steel, in which they found that both stress and hardness values increased with an increase in the number of turns. This was related to grain refinement. It was also noticed that the inhomogeneity in morphology and volume of low and high angle grain boundaries increased with the increase in the number of turns, associated with higher stress and lower elongation. The hardness at tip location was found to be lower in all strain ranges due to the lower extent of plastic deformation while at the edge it

This review chapter focuses on plastic deformation behavior which can be controlled via processing parameters. Their optimisation is responsible for a refined microstructure, typically associated with beneficial mechanical properties of metals and steel due to metal forming. In other words, an appropriate combination of processing parameters enables one to fabricate products that will be defect-free on

*DOI: http://dx.doi.org/10.5772/intechopen.97607*

glide providing work hardening effect.

*Plastic Deformation Behavior in Steels during Metal Forming Processes: A Review DOI: http://dx.doi.org/10.5772/intechopen.97607*

**Figure 11.**

*Material Flow Analysis*

mechanism [87].

**Table 3.**

**Figure 10.**

influence of higher strain rate (between 103

and toughness properties [96, 97].

the parent austenite has been discussed [96].

on the SFE, which is responsible for the activation energy of a deformation

**Temperature, K SFE value, mJ/m2 Plasticity mechanism** 77 10 Dislocation gliding

293 19 Dislocation gliding and twinning

*Effect of SFE and temperature on deformation mechanism in Fe-20Mn-4Cr-0.5C steel [84].*

*The deformation mechanism at different temperatures for Fe-22Mn-0.6C steel [86].*

The mechanical properties of Hadfield Mn austenitic steels can be improved through high rate strain hardening where two phenomena (i.e. dislocation accumulation and twinning) act preferentially during plastic deformation [82]. This is attributed to the strain hardening transformation where austenite phase transforms into ε or α-martersite, and twinning, dynamic strain aging, dispute between dislocations with stacking faults occurs. In this connection, Yan et al. [88] have tried to improve the hardness values by shot pinning method, whereby hardness values could be increased with increasing in operation time. This was attributed to the increment in density of dislocations, dislocation accumulation, and formation of twinning. The

673 80 Dislocation gliding and ε-martensitic transformation.

to 105

ical behavior and wear resistance properties of high austenitic Mn steel which may

Over the past few years, many researchers have reported work on TRIP and TWIP steels and achieved better mechanical properties by plastic deformation at high strains (more than 1) [22, 73, 74, 81, 89, 91, 94–96]. Both TRIP and TWIP steels are fully austenitic steels with less carbon content than hadfield steel. The initial microstructure of TRIP steel is consisted of martensite, bainite and ferrite with retained austenite. The fraction of carbon enriched retained austenite in TRIP steels is between 5 to 30% which transforms into martensite by displacive mechanism during SPD process. This behavior has attained great improvement in strength

A critical issue remains hydrogen embrittlement in TRIP steels, promoted by a displacive mechanism where the relevance of different solubility and diffusivity in

Sevsek et al. [90] reported the effect of strain rate on medium Mn X6MnAl12–3

steel. The softer austenite region was strained locally and transformed into

be linked to dynamic strain aging and may delay fracture [81, 89–93].

/s) attains great impact on mechan-

**14**

*Optical morphologies of Fe-22Mn-0.6C steel subjected to high strain deformation: (a) unstained; strained with (b) 18%, (c) 26%, and (d) 34% [81].*

martensite which depended on the strain rate sensibility. The deformation induced phase transformation of austenite to martensite is partially suppressed at lower and higher strain rates. The impact of high strains in fully austenitic Fe-22Mn-0.6C steel is predominantly controlled by twinning plasticity mechanism (see **Figure 11**) as suggested by Jacob et al. [81]. The initial microstructure of Fe-22Mn-0.6C steel is a single-phase austenitic steel with few twinned grains (**Figure 11a**). They found that the fraction of twinning is increased with increasing strains (**Figure 11b–d**), where most of the internal energy was used for recrystallization and rest for grain growth [87]. They concluded that twin boundaries act as a hindrance to the dislocation glide providing work hardening effect.

In the same way, Kang et al. [98] have reported HPT tests for TWIP steel, in which they found that both stress and hardness values increased with an increase in the number of turns. This was related to grain refinement. It was also noticed that the inhomogeneity in morphology and volume of low and high angle grain boundaries increased with the increase in the number of turns, associated with higher stress and lower elongation. The hardness at tip location was found to be lower in all strain ranges due to the lower extent of plastic deformation while at the edge it was higher.

#### **8. Concluding remarks**

This review chapter focuses on plastic deformation behavior which can be controlled via processing parameters. Their optimisation is responsible for a refined microstructure, typically associated with beneficial mechanical properties of metals and steel due to metal forming. In other words, an appropriate combination of processing parameters enables one to fabricate products that will be defect-free on

#### *Material Flow Analysis*

the microscale, which represents an important demand of customers. It is noticed that the flow stress increases with an increase in strain rate when the temperature is constant while it decreases with an increase in temperature when the strain rate is constant. The dDRX phenomena occur under axial stress hot deformation conditions while cDRX phenomena are linked to torsion deformation conditions during severe plastic deformation at relatively low temperatures. Plastic deformation acts differently in the case of high Mn austenitic TRIP steels where retained austenite is transformed into martensite by displacive mechanism and induced strain forms twinning which improves strength and toughness of steels. In contrast, the high-Mn fully austenitic steel such as TWIP steels generate huge amount of twinning structure by induced high strains and do not show phase transformation like TRIP steels.
