E ﬀ ect of Manganese on the Structure-Properties Relationship of Cold Rolled AHSS Treated by a Quenching and Partitioning Process

: The present work focuses on the investigation of both microstructure and resulting mechanical properties of di ﬀ erent lean medium Mn Quenching and Partitioning (Q&P) steels with 0.2 wt.% C, 1.5 wt.% Si, and 3–4 wt.% Mn. By means of dilatometry, a signiﬁcant inﬂuence of the Mn-content on their transformation behavior was observed. Light optical and scanning electron microscopy (LOM, SEM) was used to characterize the microstructure consisting of tempered martensite ( α ”), retained austenite (RA), partially bainitic ferrite ( α B ), and ﬁnal martensite ( α ’ ﬁnal ) formed during ﬁnal cooling to room temperature (RT). Using the saturation magnetization measurements (SMM), a beneﬁcial impact of the increasing Mn-content on the volume fraction of RA could be found. This remarkably determined the mechanical properties of the investigated steels, since the larger amount of RA with its lower chemical stabilization against the strain-induced martensite transformation (SIMT) highly inﬂuenced their overall stress-strain behavior. With increasing Mn-content the ultimate tensile strength (UTS) rose without considerable deterioration in total elongation (TE), leading to an enhanced combination of strength and ductility with UTS × TE exceeding 22,500 MPa%. However, for the steel grades containing an elevated Mn-content, a narrower process window was observed due to the tendency to form α ’ ﬁnal .


Introduction
Increasing requirements of the automotive industry related to lightweight construction and increased passenger safety drive the development of advanced high strength steels (AHSS) [1,2].Currently, research focuses on the development of the third generation AHSS, including the concepts of medium Mn and Quenching and Partitioning (Q&P) steels.These steel grades offer a promising combination of strength and ductility achieved by a microstructure having a substantial amount of retained austenite (RA) which transforms into strain-induced martensite (α') due to the transformation induced plasticity (TRIP) effect [3][4][5].
Medium Mn steels with a typical chemical composition of 0.05-0.2wt.% C and 3-10 wt.% Mn have a microstructure consisting of an ultrafine-grained ferritic (α) matrix and volume fractions of retained austenite (RA) up to 40 vol.%.Therefore, they are characterized by an excellent combination

Materials and Methods
The chemical compositions of the investigated steel grades with varying Mn-contents are given in Table 1.Three 80 kg ingots were cast under laboratory conditions in a medium frequency furnace, followed by hot rolling to a final thickness of 4 mm.In order to provide cold rollability, the hot rolled sheets (finish rolling temperature = 900 • C) were tempered in a batch-annealing-like furnace for 16 h at 550 • C. Subsequently, the material was cold rolled to a final thickness of 1 mm.Specimens of 10 × 4 × 1 mm 3 were heat-treated on a Bähr 805 A/D dilatometer (TA instruments, New Castle, DE, USA) in order to investigate the influence of both, T Q and Mn-content on the transformation behavior of the cold rolled sheets.Figure 1 shows the applied time-temperature schedules for the Q&P process, adapted to suit an industrially feasible continuous annealing line.After full austenitization for t A = 120 s at T A = 850 • C the samples were quenched to various T Q in the range of 130-330 • C with a 20 • C step and isothermally held for 10 s (t Q ).Subsequently, partitioning was performed at T P = 400 • C for 300 s, followed by cooling to RT.For tensile testing, strips of 450 × 20 × 1 mm 3 were heat-treated referring to the equal time-temperature regime using a multipurpose annealing simulator (MULTIPAS, voestalpine Stahl GmbH, Linz, Austria), providing electrical resistance heating and gas jet cooling.Three thermocouples were welded onto the strip in order to ensure temperature control.
Metals 2019, 9, x FOR PEER REVIEW 3 of 14 Specimens of 10 × 4 × 1 mm 3 were heat-treated on a Bähr 805 A/D dilatometer (TA instruments, New Castle, DE, USA) in order to investigate the influence of both, TQ and Mn-content on the transformation behavior of the cold rolled sheets.Figure 1 shows the applied time-temperature schedules for the Q&P process, adapted to suit an industrially feasible continuous annealing line.After full austenitization for tA = 120 s at TA = 850 °C the samples were quenched to various TQ in the range of 130-330 °C with a 20 °C step and isothermally held for 10 s (tQ).Subsequently, partitioning was performed at TP = 400 °C for 300 s, followed by cooling to RT.For tensile testing, strips of 450 × 20 × 1 mm 3 were heat-treated referring to the equal time-temperature regime using a multipurpose annealing simulator (MULTIPAS, voestalpine Stahl GmbH, Linz, Austria), providing electrical resistance heating and gas jet cooling.Three thermocouples were welded onto the strip in order to ensure temperature control.Tensile tests were performed according to DIN EN ISO 6892-1, using flat tensile specimens with 25 mm gauge length.For each heat-treatment condition, two samples were tested in a longitudinal direction.The microstructure was characterized by means of LOM using LePera etching.Furthermore, electrochemically polished samples were investigated by means of SEM using a Zeiss SUPRA 35 microscope (Carl Zeiss Microscopy GmbH, Jena, Germany).The volume fracture of RA was determined by means of SMM.Furthermore, interrupted tensile tests were performed at different strain levels to investigate the SIMT and thus, the RA-stability.By means of SMM, the RAcontent at gradually increased strains was determined and finally the Ludwigson-Berger relation [19] was used to calculate the kP-value as an indicator for the RA-stability.
Here, Vγ is the volume fraction of RA determined at a specific true strain (ε), Vγ0 is the initial RAcontent before straining and kP is a factor indicating the RA-stability.p is a strain exponent related to the autocatalytic effect of martensite formation, which can be considered as 1 for TRIP-steels, since this effect can be neglected close to RT [20].
The C-content in RA was determined by the application of the following equation proposed by Dyson and Holmes [21].
Here, XC is the C-content in RA, aγ is the austenite lattice parameter in Å , which was measured by X-ray diffraction (XRD) using a PANalytical XPert Pro diffractometer (Malvern Panalytical Ltd, Kassel, Germany) with Co-anode (λ = 0.179 nm, U = 35 kV).XAl and XMn are the contents of Al and Mn in wt.% in RA, respectively, determined by energy dispersive X-ray spectroscopy (EDX, Carl Zeiss Microscopy GmbH, Jena, Germany).Tensile tests were performed according to DIN EN ISO 6892-1, using flat tensile specimens with 25 mm gauge length.For each heat-treatment condition, two samples were tested in a longitudinal direction.The microstructure was characterized by means of LOM using LePera etching.Furthermore, electrochemically polished samples were investigated by means of SEM using a Zeiss SUPRA 35 microscope (Carl Zeiss Microscopy GmbH, Jena, Germany).The volume fracture of RA was determined by means of SMM.Furthermore, interrupted tensile tests were performed at different strain levels to investigate the SIMT and thus, the RA-stability.By means of SMM, the RA-content at gradually increased strains was determined and finally the Ludwigson-Berger relation [19] was used to calculate the k P -value as an indicator for the RA-stability.
Here, V γ is the volume fraction of RA determined at a specific true strain (ε), V γ0 is the initial RA-content before straining and k P is a factor indicating the RA-stability.p is a strain exponent related to the autocatalytic effect of martensite formation, which can be considered as 1 for TRIP-steels, since this effect can be neglected close to RT [20].
The C-content in RA was determined by the application of the following equation proposed by Dyson and Holmes [21].
Here, X C is the C-content in RA, a γ is the austenite lattice parameter in Å, which was measured by X-ray diffraction (XRD) using a PANalytical XPert Pro diffractometer (Malvern Panalytical Ltd, Kassel, Germany) with Co-anode (λ = 0.179 nm, U = 35 kV).X Al and X Mn are the contents of Al and Mn in wt.% in RA, respectively, determined by energy dispersive X-ray spectroscopy (EDX, Carl Zeiss Microscopy GmbH, Jena, Germany).

Transformation Behavior
Figure 2 depicts the dilatometer curves for the investigated steel grades quenched to 3 different T Q , respectively.It is evident from the graphs that with increasing Mn-content, the M S -temperature steadily decreased: for the steel grade containing 3.0 wt.% Mn (Figure 2a) the M S -temperature of 333 • C was determined, for the Fe-C-3.5Mn-Sisteel grade (Figure 2b) the M S -temperature was 315 • C and for the steel grade containing 4.0 wt.% Mn (Figure 2c), an M S -temperature of 305 • C was observed.Thus, in order to adjust a comparable amount of α' prim , the increase of the Mn-content requires a lower T Q .As a consequence, the illustrated T Q were chosen in such a way that regardless of the chemical composition the red curves always correspond to a volume fraction of 50% α' prim , the purple curves represent the samples containing of 75% α' prim , and the blue lines depict the samples where 85% α' prim was adjusted in the microstructure.

Transformation Behavior
Figure 2 depicts the dilatometer curves for the investigated steel grades quenched to 3 different TQ, respectively.It is evident from the graphs that with increasing Mn-content, the MS-temperature steadily decreased: for the steel grade containing 3.0 wt.% Mn (Figure 2a) the MS-temperature of 333 °C was determined, for the Fe-C-3.5Mn-Sisteel grade (Figure 2b) the MS-temperature was 315 °C and for the steel grade containing 4.0 wt.% Mn (Figure 2c), an MS-temperature of 305 °C was observed.Thus, in order to adjust a comparable amount of α'prim, the increase of the Mn-content requires a lower TQ.As a consequence, the illustrated TQ were chosen in such a way that regardless of the chemical composition the red curves always correspond to a volume fraction of 50% α'prim, the purple curves represent the samples containing of 75% α'prim, and the blue lines depict the samples where 85% α'prim was adjusted in the microstructure.It is obvious from these dilatometric curves that during isothermal holding at TP γ partially transformed to αB, accompanied by a linear expansion.Figure 3 gives a detailed perspective on the dilatation during the partitioning step as a function of isothermal holding time.It is evident that with increasing TQ, and thus decreasing volume fraction of α'prim, a larger amount of αB was formed.Irrespective of the chemical composition, for the samples with 75% and 85% α'prim (purple and blue lines), rather comparable amount of αB was formed, whereas for the samples with a matrix consisting of 50% α'prim (red curve) a significant influence of the Mn-content on the formation of αB could be stated: the higher the Mn-content, the lower the volume fraction of αB.It is obvious from these dilatometric curves that during isothermal holding at T P γ partially transformed to α B , accompanied by a linear expansion.Figure 3 gives a detailed perspective on the dilatation during the partitioning step as a function of isothermal holding time.It is evident that with increasing T Q , and thus decreasing volume fraction of α' prim , a larger amount of α B was formed.Irrespective of the chemical composition, for the samples with 75% and 85% α' prim (purple and blue lines), rather comparable amount of α B was formed, whereas for the samples with a matrix consisting of 50% α' prim (red curve) a significant influence of the Mn-content on the formation of α B could be stated: the higher the Mn-content, the lower the volume fraction of α B .
In general, for all steels containing 50% α' prim , the formation of α' final was observed during cooling to RT (Figure 2).However, since with increasing Mn-content a decreasing amount of α B was formed during the partitioning step, the largest volume fraction of α' final was inherently formed for the steel grade containing 4.0 wt.% Mn.
transformed to αB, accompanied by a linear expansion.Figure 3 gives a detailed perspective on the dilatation during the partitioning step as a function of isothermal holding time.It is evident that with increasing TQ, and thus decreasing volume fraction of α'prim, a larger amount of αB was formed.Irrespective of the chemical composition, for the samples with 75% and 85% α'prim (purple and blue lines), rather comparable amount of αB was formed, whereas for the samples with a matrix consisting of 50% α'prim (red curve) a significant influence of the Mn-content on the formation of αB could be stated: the higher the Mn-content, the lower the volume fraction of αB.4e,f).During quenching into the M S -M f region α' prim was formed, being subsequently tempered during isothermal holding at T P .This lath-like carbon-depleted α" partially included carbide precipitations visible in the SEM images.The microstructural investigation confirmed the results stemming from the analysis of the dilatometric curves, particularly the continuous increase in α' prim and decrease in α B fraction with declining T Q .By means of SMM 18.5, 20.7 and 12.5 vol.%, finely distributed RA were measured for the samples quenched to 290, 230, and 170 • C, respectively.Owing to the extremely fine distribution of RA in the tempered-martensitic matrix, its localization is rather difficult using both LOM and SEM.It is obvious from the micrographs that for the sample quenched to 290 • C, a considerable amount of α' final was present in the microstructure due to the insufficient chemical stabilization of austenite.This α' final was characterized in the given microstructure observed by SEM by coarser and surface structured areas.For the steel grades containing 3.0 and 4.0 wt.% Mn, a similar influence of T Q on the microstructural constituents could be confirmed using LOM and SEM.Regarding the influence of the chemical composition, no significant effect of Mn on the microstructural evolution could be observed at low T Q , whereas for the higher T Q , a larger volume fraction of α' final was obtained for the Fe-C-4.0Mn-Sisteel, as was also proved by the dilatometric measurements.

Microstructure
Metals 2019, 9, x FOR PEER REVIEW 5 of 14 In general, for all steels containing 50% α'prim, the formation of α'final was observed during cooling to RT (Figure 2).However, since with increasing Mn-content a decreasing amount of αB was formed during the partitioning step, the largest volume fraction of α'final was inherently formed for the steel grade containing 4.0 wt.% Mn.  4e,f).During quenching into the MS-Mf region α'prim was formed, being subsequently tempered during isothermal holding at TP.This lath-like carbon-depleted α'' partially included carbide precipitations visible in the SEM images.The microstructural investigation confirmed the results stemming from the analysis of the dilatometric curves, particularly the continuous increase in α'prim and decrease in αB fraction with declining TQ.By means of SMM 18.5, 20.7 and 12.5 vol.%, finely distributed RA were measured for the samples quenched to 290, 230, and 170 °C , respectively.Owing to the extremely fine distribution of RA in the tempered-martensitic matrix, its localization is rather difficult using both LOM and SEM.It is obvious from the micrographs that for the sample quenched to 290 °C , a considerable amount of α'final was present in the microstructure due to the insufficient chemical stabilization of austenite.This α'final was characterized in the given microstructure observed by SEM by coarser and surface structured areas.For the steel grades containing 3.0 and 4.0 wt.% Mn, a similar influence of TQ on the microstructural constituents could be confirmed using LOM and SEM.Regarding the influence of the chemical composition, no significant effect of Mn on the microstructural evolution could be observed at low TQ, whereas for the higher TQ, a larger volume fraction of α'final was obtained for the Fe-C-4.0Mn-Sisteel, as was also proved by the dilatometric measurements.Figure 5 summarizes the results obtained by dilatometry, microstructural investigations, and SMM for the three different steel grades, wherein the individual phase fractions of α", α B , RA and α' final are plotted as a function of T Q .Irrespective of the chemical composition, an increasing T Q led to a remarkable decrease in the amount of α", whereas the volume fraction of α B simultaneously increased.Furthermore, the RA-content rose, until the specific T Q where α' final was formed as an aftermath of insufficient chemical RA stabilization.This inherently led to a decreasing volume fraction of RA with a further increase of T Q .It can be regarded from the graphs that with an increasing Mn-content the volume fraction of α B decreased, leading to a larger amount of RA.For the Fe-C-3.0Mn-Sisteel the maximum RA-fraction (RA max ) of 19.9 vol.% was achieved at T Q = 290 • C. For the steel grade containing 3.5 wt.% Mn RA max = 22.3 vol.% was measured at T Q = 270 • C, whereas by further increasing the Mn-content to 4.0 wt.% RA max reached 24.6 vol.% at T Q = 230 • C.These results indicate, that regardless of the chemical composition, the adjustment of approximately 70 vol.%α' prim was necessary to stabilize the maximum volume fraction of RA.Thus, it can be inferred that the increase of the Mn-content requires the set of lower T Q .In addition, the value of RA max is positively influenced by an increasing Mn-content as a consequence of the lower fraction of α B formed during the partitioning step.

Microstructure
Metals 2019, 9, x FOR PEER REVIEW 6 of 14 aftermath of insufficient chemical RA stabilization.This inherently led to a decreasing volume fraction of RA with a further increase of TQ.It can be regarded from the graphs that with an increasing Mn-content the volume fraction of αB decreased, leading to a larger amount of RA.For the Fe-C-3.0Mn-Sisteel the maximum RA-fraction (RAmax) of 19.9 vol.% was achieved at TQ = 290 °C.For the steel grade containing 3.5 wt.% Mn RAmax = 22.3 vol.% was measured at TQ = 270 °C, whereas by further increasing the Mn-content to 4.0 wt.% RAmax reached 24.6 vol.% at TQ = 230 °C.These results indicate, that regardless of the chemical composition, the adjustment of approximately 70 vol.%α'prim was necessary to stabilize the maximum volume fraction of RA.Thus, it can be inferred that the increase of the Mn-content requires the set of lower TQ.In addition, the value of RAmax is positively influenced by an increasing Mn-content as a consequence of the lower fraction of αB formed during the partitioning step.

Tensile Testing
Representative engineering stress-strain curves for the investigated steels are presented in Figure 6.For the samples containing approximately 85 vol.% α'prim (blue curves) a comparable stressstrain behavior with very high yield strength (YS), low strain-hardening, and moderate TE was observed.In general, with increasing TQ a significant decrease in YS was detected for all compositions.When increasing TQ and thus adjusting 75 vol.%α'prim (purple curves), UTS remained rather constant for the three investigated steels.However, in this case for all compositions an enhanced TE was obtained.When TQ was further increased, which means that only 50 vol.%α'prim (red curves) was present in the microstructure, a pronounced increase in UTS along with a remarkable decrease in TE was obtained.This was especially in case of the steels containing elevated Mn-contents.The presence of a considerable amount of α'final in the microstructure was particularly responsible for this behavior. Fe-C-4.0Mn-Si

Tensile Testing
Representative engineering stress-strain curves for the investigated steels are presented in Figure 6.For the samples containing approximately 85 vol.% α' prim (blue curves) a comparable stress-strain behavior with very high yield strength (YS), low strain-hardening, and moderate TE was observed.In general, with increasing T Q a significant decrease in YS was detected for all compositions.When increasing T Q and thus adjusting 75 vol.%α' prim (purple curves), UTS remained rather constant for the three investigated steels.However, in this case for all compositions an enhanced TE was obtained.When T Q was further increased, which means that only 50 vol.%α' prim (red curves) was present in the microstructure, a pronounced increase in UTS along with a remarkable decrease in TE was obtained.This was especially in case of the steels containing elevated Mn-contents.The presence of a considerable amount of α' final in the microstructure was particularly responsible for this behavior.
Figures 7 and 8 depict the mechanical properties as a function of T Q determined by tensile testing.It is clear that independent of the chemical composition first, with increasing T Q both, YS and UTS decreased until reaching a T Q of 230 • C (Fe-C-3.0Mn-Si), 210 • C (Fe-C-3.5Mn-Si), and 190 • C (Fe-C-4.0Mn-Si), respectively.This was associated with a decreasing amount of α' prim and thus, increasing volume fraction of RA.Consistent with this, both UE and TE significantly increased, until reaching a sharp maximum, especially in case of the steels containing 3.5 and 4.0 wt.% Mn.Further rise of T Q led to an increase in UTS, which was more pronounced for the steels with elevated Mn-contents.This was accompanied by a drastic reduction in both UE and TE.In contrast, for the Fe-C-3.0Mn-Sisteel in the T Q -range of 210-290 • C, a rather constant UTS and TE evolution could be obtained.This indicates that with increasing Mn-content, the sensitivity against T Q fluctuations increased, leading to a narrower process window.Additionally, in case of the 3.5 and 4.0 wt.% Mn steels, at high T Q , a rise in YS was noticeable, caused by the presence of considerable amounts of α' final formed upon final cooling.Figure 6.For the samples containing approximately 85 vol.% α'prim (blue curves) a comparable stressstrain behavior with very high yield strength (YS), low strain-hardening, and moderate TE was observed.In general, with increasing TQ a significant decrease in YS was detected for all compositions.When increasing TQ and thus adjusting 75 vol.%α'prim (purple curves), UTS remained rather constant for the three investigated steels.However, in this case for all compositions an enhanced TE was obtained.When TQ was further increased, which means that only 50 vol.%α'prim (red curves) was present in the microstructure, a pronounced increase in UTS along with a remarkable decrease in TE was obtained.This was especially in case of the steels containing elevated Mn-contents.The presence of a considerable amount of α'final in the microstructure was particularly responsible for this behavior.
Fe-C-4.0Mn-Si Metals 2019, 9, x FOR PEER REVIEW 7 of 14 Figures 7 and 8 depict the mechanical properties as a function of TQ determined by tensile testing.It is clear that independent of the chemical composition first, with increasing TQ both, YS and UTS decreased until reaching a TQ of 230 °C (Fe-C-3.0Mn-Si),210 °C (Fe-C-3.5Mn-Si),and 190 °C (Fe-C-4.0Mn-Si),respectively.This was associated with a decreasing amount of α'prim and thus, increasing volume fraction of RA.Consistent with this, both UE and TE significantly increased, until reaching a sharp maximum, especially in case of the steels containing 3.5 and 4.0 wt.% Mn.Further rise of TQ led to an increase in UTS, which was more pronounced for the steels with elevated Mncontents.This was accompanied by a drastic reduction in both UE and TE.In contrast, for the Fe-C-3.0Mn-Sisteel in the TQ-range of 210-290 °C , a rather constant UTS and TE evolution could be obtained.This indicates that with increasing Mn-content, the sensitivity against TQ fluctuations increased, leading to a narrower process window.Additionally, in case of the 3.5 and 4.0 wt.% Mn steels, at high TQ, a rise in YS was noticeable, caused by the presence of considerable amounts of α'final formed upon final cooling.
Since the maximum TE was approximately 16.5% for all investigated steels, no evident influence of the Mn-content on the ductility of the investigated steels could be stated.Yet, with increasing Mncontent, a lower TQ was necessary in order to adjust the maximum TE (TQ = 230 °C, TQ = 210 °C, and TQ = 190 °C for the steels containing 3.0, 3.5, and 4.0 wt.% Mn, respectively).However, a slight influence of Mn on the strength was observed, since with increasing Mn-content UTS rose from 1304 MPa to 1343 MPa and 1360 MPa for the samples achieving the highest TE of 16.5%.Thereby, the best combination of strength and ductility was obtained for the Fe-C-4.0Mn-Sisteel with a product of UTS × TE exceeding 22,500 MPa%.Figures 7 and 8 depict the mechanical properties as a function of TQ determined by tensile testing.It is clear that independent of the chemical composition first, with increasing TQ both, YS and UTS decreased until reaching a TQ of 230 °C (Fe-C-3.0Mn-Si),210 °C (Fe-C-3.5Mn-Si),and 190 °C (Fe-C-4.0Mn-Si),respectively.This was associated with a decreasing amount of α'prim and thus, increasing volume fraction of RA.Consistent with this, both UE and TE significantly increased, until reaching a sharp maximum, especially in case of the steels containing 3.5 and 4.0 wt.% Mn.Further rise of TQ led to an increase in UTS, which was more pronounced for the steels with elevated Mncontents.This was accompanied by a drastic reduction in both UE and TE.In contrast, for the Fe-C-3.0Mn-Sisteel in the TQ-range of 210-290 °C , a rather constant UTS and TE evolution could be obtained.This indicates that with increasing Mn-content, the sensitivity against TQ fluctuations increased, leading to a narrower process window.Additionally, in case of the 3.5 and 4.0 wt.% Mn steels, at high TQ, a rise in YS was noticeable, caused by the presence of considerable amounts of α'final formed upon final cooling.
Since the maximum TE was approximately 16.5% for all investigated steels, no evident influence of the Mn-content on the ductility of the investigated steels could be stated.Yet, with increasing Mncontent, a lower TQ was necessary in order to adjust the maximum TE (TQ = 230 °C, TQ = 210 °C, and TQ = 190 °C for the steels containing 3.0, 3.5, and 4.0 wt.% Mn, respectively).However, a slight influence of Mn on the strength was observed, since with increasing Mn-content UTS rose from 1304 MPa to 1343 MPa and 1360 MPa for the samples achieving the highest TE of 16.5%.Thereby, the best combination of strength and ductility was obtained for the Fe-C-4.0Mn-Sisteel with a product of UTS × TE exceeding 22,500 MPa%.Since the maximum TE was approximately 16.5% for all investigated steels, no evident influence of the Mn-content on the ductility of the investigated steels could be stated.Yet, with increasing Mn-content, a lower T Q was necessary in order to adjust the maximum TE (T Q = 230 • C, T Q = 210 • C, and T Q = 190 • C for the steels containing 3.0, 3.5, and 4.0 wt.% Mn, respectively).However, a slight influence of Mn on the strength was observed, since with increasing Mn-content UTS rose from 1304 MPa to 1343 MPa and 1360 MPa for the samples achieving the highest TE of 16.5%.Thereby, the best combination of strength and ductility was obtained for the Fe-C-4.0Mn-Sisteel with a product of UTS × TE exceeding 22,500 MPa%.

Retained Austenite Stability
The volume fraction of RA in dependency of true strain is displayed in Figure 9.At a very low T Q , where approximately 85 vol.% α' prim was adjusted in the microstructure (blue curves), a rather moderate decline in RA-content was observed.This indicates a very stable RA that underwent only minor transformation during straining.The increase of T Q led to a decline in α' prim fraction to 75 vol.%(purple curves) and therefore to a higher initial volume fraction of RA.This contributed to the pronounced TRIP-effect under these conditions.However, for the samples containing only 50 vol.%α' prim (red curves), a considerable amount of α' final was present in the initial microstructure due to a very low chemical RA-stability.Therefore, in this case, a fast SIMT was observed, especially for the steel grades with elevated Mn-contents.

Retained Austenite Stability
The volume fraction of RA in dependency of true strain is displayed in Figure 9.At a very low TQ, where approximately 85 vol.% α'prim was adjusted in the microstructure (blue curves), a rather moderate decline in RA-content was observed.This indicates a very stable RA that underwent only minor transformation during straining.The increase of TQ led to a decline in α'prim fraction to 75 vol.%(purple curves) and therefore to a higher initial volume fraction of RA.This contributed to the pronounced TRIP-effect under these conditions.However, for the samples containing only 50 vol.%α'prim (red curves), a considerable amount of α'final was present in the initial microstructure due to a very low chemical RA-stability.Therefore, in this case, a fast SIMT was observed, especially for the steel grades with elevated Mn-contents.In order to enable a better comparison of the RA-stability of the individual steel grades, the volume fraction of γ transformed to α' is plotted as a function of true strain (Figure 10), following a linear relationship according to the Ludwigson-Berger equation (Equation ( 1)).With rising TQ increasing kP-values were observed, indicating a declining RA-stability.For the samples containing 85 vol.% α'prim (blue curves) rather comparable kP-values between 27 and 48 were determined for the investigated steels, whereas for the samples containing lower α'prim fractions, a substantial influence of the chemical composition was observed.With increasing Mn-content, a significant decrease in RAstability was found, represented by markedly increasing kP-values.

true strain [%]
0.00 0.02 0.04 0.06 0.08 0.10 Fe-C-3.0Mn-Si k P, 50% = 248 0.00 0.02 0.04 0.06 0.08 0.10  In order to enable a better comparison of the RA-stability of the individual steel grades, the volume fraction of γ transformed to α' is plotted as a function of true strain (Figure 10), following a linear relationship according to the Ludwigson-Berger equation (Equation ( 1)).With rising T Q increasing k P -values were observed, indicating a declining RA-stability.For the samples containing 85 vol.% α' prim (blue curves) rather comparable k P -values between 27 and 48 were determined for the investigated steels, whereas for the samples containing lower α' prim fractions, a substantial influence of the chemical composition was observed.With increasing Mn-content, a significant decrease in RA-stability was found, represented by markedly increasing k P -values.
linear relationship according to the Ludwigson-Berger equation (Equation ( 1)).With rising TQ increasing kP-values were observed, indicating a declining RA-stability.For the samples containing 85 vol.% α'prim (blue curves) rather comparable kP-values between 27 and 48 were determined for the investigated steels, whereas for the samples containing lower α'prim fractions, a substantial influence of the chemical composition was observed.With increasing Mn-content, a significant decrease in RAstability was found, represented by markedly increasing kP-values.In Figure 11a the kP-values are plotted in dependence of TQ, summarizing the results displayed in Figure 10.Regardless of the chemical composition, with increasing TQ continuously rising kP-values were observed.Since the Mn-content has significant influence on the MS-temperature, the true

true strain [%]
Fe-C-4.0Mn-SiIn Figure 11a the k P -values are plotted in dependence of T Q , summarizing the results displayed in Figure 10.Regardless of the chemical composition, with increasing T Q continuously rising k P -values were observed.Since the Mn-content has significant influence on the M S -temperature, the of the investigated steel grades consisted of different volume fractions of its individual constituents at comparable T Q .For this reason, to ensure a better comparability of the steels, Figure 11b presents the k P -values as a function of α' prim .In general, with decreasing T Q and hence increasing volume fractions of α' prim , declining k P -values were observed.This can be attributed to the decreasing fractions of γ remain being present in the microstructure at the onset of the partitioning step.Thus, the C-content in the supersaturated α' prim had to distribute to a lower volume fraction of γ remain , leading to its enhanced stabilization.Furthermore, it is apparent from the graph that for the samples containing volume fractions of α' prim exceeding 70 vol.%, the chemical composition rarely influenced the RA-stability.However, for the samples containing lower α' prim fractions, the increase of the Mn-content led to a pronounced rise in k P -values related to a substantial decline in RA-stability.The C-content in RA is displayed over the volume fraction of α' prim in Figure 11c.For all steels, with increasing α' prim fraction the C-content in RA rose.For the samples containing at least 70 vol.%α' prim the increase of the Mn-content marginally influenced the C-content in RA.In contrast, at lower α' prim fractions, the influence of the chemical composition was more remarkable, since a sharp decrease in C γ was observed with increasing Mn-content.
Metals 2019, 9, x FOR PEER REVIEW 9 of 14 microstructure of the investigated steel grades consisted of different volume fractions of its individual constituents at comparable TQ.For this reason, to ensure a better comparability of the steels, Figure 11b presents the kP-values as a function of α'prim.In general, with decreasing TQ and hence increasing volume fractions of α'prim, declining kP-values were observed.This can be attributed to the decreasing fractions of γremain being present in the microstructure at the onset of the partitioning step.Thus, the C-content in the supersaturated α'prim had to distribute to a lower volume fraction of γremain, leading to its enhanced stabilization.Furthermore, it is apparent from the graph that for the samples containing volume fractions of α'prim exceeding 70 vol.%, the chemical composition rarely influenced the RA-stability.However, for the samples containing lower α'prim fractions, the increase of the Mn-content led to a pronounced rise in kP-values related to a substantial decline in RA-stability.The C-content in RA is displayed over the volume fraction of α'prim in Figure 11c.For all steels, with increasing α'prim fraction the C-content in RA rose.For the samples containing at least 70 vol.%α'prim the increase of the Mn-content marginally influenced the C-content in RA.In contrast, at lower α'prim fractions, the influence of the chemical composition was more remarkable, since a sharp decrease in Cγ was observed with increasing Mn-content.

Influence of Heat-Treatment Parameter
Regarding the influence of TQ on the transformation behavior, the results presented in this contribution (Figures 2 and 3) could evidently confirm the findings already known for Q&P steels from the literature [22][23][24][25].With an increasing TQ, the volume fraction of α'prim gradually decreased as a result of the reduced driving force for the γ → α' transformation, as described by Koistinen and Marburger [26].Coinciding with the decreasing amount of α'prim, a larger volume fraction of γremain

Influence of Heat-Treatment Parameter
Regarding the influence of T Q on the transformation behavior, the results presented in this contribution (Figures 2 and 3) could evidently confirm the findings already known for Q&P steels from the literature [22][23][24][25].With an increasing T Q , the volume fraction of α' prim gradually decreased as a result of the reduced driving force for the γ → α' transformation, as described by Koistinen and Marburger [26].Coinciding with the decreasing amount of α' prim , a larger volume fraction of γ remain was present in the microstructure at the onset of isothermal holding at T P .Hence, with a rising T Q , a larger amount of γ remain transformed to α B .Furthermore, by increasing T Q , a rising volume fraction of RA (Figure 5) was stabilized until exceeding a critical T Q , leading to the formation of α' final owing to the insufficient stabilization of γ remain .Thus, in accordance with Speer et al. [27], a triangular shape of the RA-fraction as a function of T Q could be observed (Figure 5).
The considerable influence of T Q on the microstructural constituents obviously affected the mechanical properties of the investigated steels.With increasing T Q , a decline in both UTS and YS accompanied by a rise in TE could be observed (Figures 6-8), which is consistent with the findings of De Moor et al. [12].This behavior is linked to the decreasing volume fractions of α' prim and in turn to rising RA-contents.Therefore, it is inferred that at very low T Q , RA was hyper-stable, i.e., it underwent almost no transformation during deformation (Figures 9-11).With increasing T Q the chemical RA-stability decreased (Figure 11c), resulting in a pronounced TRIP-effect, which contributed to the enhanced combination of strength and ductility.Independent of the chemical composition, the best combination of UTS and TE was observed 40 • C below a T Q amount where the maximum RA-content could be stabilized at RT (T Q = 230 • C, 210 • C, and 190 • C for the steels containing 3.0, 3.5, and 4.0 wt.% Mn, respectively).At a higher T Q than the optimal one, a significant increase in UTS associated with a sharp decline in TE was observed, resulting in a remarkable deterioration of UTS × TE.This was linked to the presence of α' prim in the initial microstructure related to low chemical RA-stability.Therefore, in this case, RA could not contribute to the enhanced TRIP-effect, which is in correlation with results reported in literature [28][29][30].

Influence of Mn-Content
Concerning the main topic of this contribution, the influence of the Mn-content on microstructure and resulting mechanical properties, interesting findings could be observed.First, the effect of an increasing Mn-content on the decrease in M S -temperature reported in literature [31,32] was confirmed by means of dilatometry.Therefore, for the steels containing elevated Mn-content, lower T Q were necessary in order to adjust comparable amounts of α' prim (Figure 2).
In the case of the samples containing at least 80 vol.% α' prim , only very low volume fractions of α B were formed during isothermal holding at T P .The increase of the Mn-content from 3.0 to 3.5 led to a slight decrease in volume fraction of α B , and thus to a minor increase in RA-content (Figure 12a).These marginal differences in microstructure were directly reflected in the mechanical properties of the Fe-C-3.0Mn-Siand Fe-C-3.5Mn-Sisteels, since the slightly increased volume fractions of RA contributed to a more pronounced TRIP-effect.As a consequence, a slight rise in UTS × TE was observed by increasing the Mn-content from 3.0 to 3.5 wt.% (Figure 12b) for the samples containing at least 80 vol.% α' prim .Regarding the RA-stability, almost no differences in k P -values were measured for these samples (Figures 10 and 11b).In counterpart, at comparable volume fractions of α' prim , the C-content in RA shown in Figure 11c) was slightly lower for the steel containing 3.5 wt.% Mn, compared to the Fe-C-3.0Mn-Sisteel grade.This could be attributed to the marginally larger RA-contents, leading to a lower C-enrichment during partitioning.In this context, higher products of RA-fraction and C-content in RA were achieved by an increase of the Mn-content from 3.0 to 3.5 wt.% (Figure 12c).On the contrary, the further increase of the Mn-content from 3.5 to 4.0 wt.% barely influenced the volume fraction of RA (Figure 12a).Therefore, the mechanical properties (Figure 12b) as well as the product of C γ and RA (Figure 12c) remained almost unchanged.
at least 80 vol.% α'prim.Regarding the RA-stability, almost no differences in kP-values were measured for these samples (Figures 10 and 11b).In counterpart, at comparable volume fractions of α'prim, the C-content in RA shown in Figure 11c) was slightly lower for the steel containing 3.5 wt.% Mn, compared to the Fe-C-3.0Mn-Sisteel grade.This could be attributed to the marginally larger RAcontents, leading to a lower C-enrichment during partitioning.In this context, higher products of RAfraction and C-content in RA were achieved by an increase of the Mn-content from 3.0 to 3.5 wt.% (Figure 12c).On the contrary, the further increase of the Mn-content from 3.5 to 4.0 wt.% barely influenced the volume fraction of RA (Figure 12a).Therefore, the mechanical properties (Figure 12b) as well as the product of Cγ and RA (Figure 12c) remained almost unchanged.In general, in case of the samples containing at least 80 vol.% α'prim, the C-diffusion from the supersaturated α'prim to the γremain was the predominant mechanism for RA-stabilization.In contrast, the formation of αB barely contributed to the C-enrichment of γremain due to its relatively low fraction.Furthermore, apart from the C-content in RA, its chemical stability is improved by an increased Mncontent in RA, as well.Since no evident Mn-parititoning during isothermal holding at TP was observed by means of EDX, the higher Mn-content in the bulk composition counterbalanced the lower Cγ and inherently enhanced the chemical RA-stability for the steels containing 3.5 and 4.0 wt.% Mn.Nevertheless, it has to be considered that Mn influences the carbide formation in steels, since Mn is soluble in cementite [33].This could lead to the precipitation of larger amounts of carbides during In general, in case of the samples containing at least 80 vol.% α' prim , the C-diffusion from the supersaturated α' prim to the γ remain was the predominant mechanism for RA-stabilization.In contrast, the formation of α B barely contributed to the C-enrichment of γ remain due to its relatively low fraction.Furthermore, apart from the C-content in RA, its chemical stability is improved by an increased Mn-content in RA, as well.Since no evident Mn-parititoning during isothermal holding at T P was observed by means of EDX, the higher Mn-content in the bulk composition counterbalanced the lower C γ and inherently enhanced the chemical RA-stability for the steels containing 3.5 and 4.0 wt.% Mn.Nevertheless, it has to be considered that Mn influences the carbide formation in steels, since Mn is soluble in cementite [33].This could lead to the precipitation of larger amounts of carbides during heat-treatment with increasing Mn-content, acting as carbon sinks and thus reducing the capacity for partitioning of C into γ.Hence, further research efforts are necessary to investigate the influence of Mn on the carbide precipitation and as a consequence C partitioning from the supersaturated α' prim to γ remain in the case of the present steels.
For the samples quenched to higher T Q , in order to adjust 70-80 vol.% α' prim , the influence of Mn on the phase transformation was more apparent compared to those containing higher fractions of α' prim .This was due to the larger volume fractions of γ remain being present at the onset of isothermal holding at T P , influencing both the partitioning process and the formation of α B .(Figures 2 and 3).According to the T 0 -concept, a thermodynamic limit exists for the γ → α transformation [34].As γ remain is enriched in C during the α B formation, a diffusion-free transformation is thermodynamically impossible, as soon as the carbon content reaches a critical value.Since the difference in Gibb's free energy (∆G m ) between γ and α is the driving force for the γ → α transformation, the bainitic reaction stops if ∆G m reaches 0 due to the C-enrichment in γ.This point is determined by the T 0 -temperature.It is well known that Mn shifts the T 0 -line to lower C-contents [35], allowing the formation of lower amounts of α B in case of an increasing Mn-content owing to the above-mentioned thermodynamic limit.However, this effect is negligible compared to the general retarding effect of Mn on the bainitic transformation kinetics [36,37].Mn enriches at the former austenitic grain boundaries, hindering the ferritic nucleation due to the local decrease in A e3 .In addition, Mn reduces the diffusion rate of C in γ, which has a clear retardation effect on the formation of α B [38].Therefore, in case of the samples containing 70-80 vol.% α' prim the increase of the Mn-content led to lower fractions of α B , and in turn to a significant rise in RA-content (Figure 12a).This increase in volume fraction of RA was accompanied by its decreasing chemical stabilization as proven by interrupted tensile tests (Figures 10 and 11), since the overall C-content had to partition to a larger volume fraction of γ remain .Irrespective of the steel composition, the remarkably lower RA-stability compared to the samples quenched to lower T Q contributed to the pronounced TRIP-effect, leading to enhanced strain hardening and thus improved combinations of UTS and TE (Figure 12b).Regarding the influence of the Mn-content, only minor differences in terms of mechanical properties could be found for these samples.This was linked to rather small deviations of C γ × RA-content (Figure 12c), especially in the case of the samples containing 3.5 and 4.0 wt.% Mn.
However, for the samples containing even lower volume fractions of α' prim (<70 vol.%), a substantial influence of the chemical composition on the phase transformation behavior and resulting structure-properties relation was found.In case of the Fe-C-3.0Mn-Sisteel, the formation of up to 25 vol.%α B (Figure 5) led to a moderate increase in RA-content along with decreasing α' prim fractions.Since for the samples containing elevated Mn-contents the bainitic transformation was delayed, significant larger volume fractions of γ remain transformed to α' final during final cooling to RT, resulting in a sharp decrease in RA-contents with declining amounts of α' prim (Figure 12a).In case of the Fe-C-3.0Mn-Sisteel, this resulted in rather constant and high values of C γ × RA (Figure 12c), whereas with increasing Mn-contents a drastic drop was observed, caused by the considerably declining RA-contents.Obviously, this was reflected in the stress-strain behavior of the investigated steels.For the composition containing 3.0 wt.% Mn, the mechanical properties remained almost unchanged by the increase of T Q , resulting in almost consistent values of UTS × TE (Figure 12b).In contrast, for the Fe-C-3.5Mn-Siand Fe-C-4.0Mn-Sisteels containing less than 70 vol.%α' prim , the presence of α' final related to very low RA-stabilities resulted in a significant increase in UTS, accompanied by a radical loss in TE.These results are coherent with those published by De Moor et al. [15] who reported a high sensitivity against T Q -fluctuations in case of Q&P steels containing elevated Mn-contents.On that account, the formation of increased α B fractions in case of the Fe-C-3.0Mn-Sisteel contributed to the wider process window in terms of constant mechanical properties, compared to the steels containing 3.5 and 4.0 wt.% Mn.

Conclusions
This contribution focused on the study of the influence of the Mn-content on the microstructural evolution and mechanical properties of lean medium Mn Q&P steels containing 0.2% C, 1.5% Si, and 3.0-4.0wt.% Mn, with a special emphasis on the RA-stability.The findings of the present paper are as follows:

•
Regardless of the chemical composition, by increasing T Q the volume fraction of α' prim steadily decreased, accompanied by a rising amount of α B and RA.The exceedance of a critical T Q , depending on the Mn-content, resulted in an insufficient chemical stabilization of RA, triggering the formation of α' final during final cooling to RT.

•
A significant influence of the Mn-content on the phase transformation behavior could be observed, particularly with increasing T Q and thus decreasing α' prim fraction.The addition of enhanced Mn-contents led to an appreciable delay in γ → α B transformation during the partitioning step.Thus, on the one hand, larger volume fractions of RA could be stabilized with increasing Mn-content.On the other hand, the increase of the Mn-content adversely affected the RA-stability due to the declining C-content in RA, which was only partially counterbalanced by the enhanced Mn-content in RA.

•
The mechanical properties achieved by the Q&P process were pronouncedly determined by both, volume fraction and stability of RA.With increasing Mn-content, a remarkably stronger sensitivity against T Q -fluctuations in terms of RA-content and its stability was observed.As a result, the increase of the Mn-content resulted in a narrower process window with regard to the robustness of mechanical properties.

•
For all investigated steels, the best combination of UTS and TE was observed for microstructures containing 75-80 vol.% α' prim .For this reason, a T Q 40 • C below the maximum RA-content had to be set in order to obtain the optimum mechanical properties.By increasing the Mn-content, the maximum value of UTS × TE could exceed 22,500 MPa%, since the larger volume fraction of RA by approximately 5% contributed to an enhanced TRIP-effect.

Figure 1 .
Figure 1.Schematic representation of the applied Quenching and Partitioning (Q&P) heat-treatment cycles for the investigated steels.

Figure 1 .
Figure 1.Schematic representation of the applied Quenching and Partitioning (Q&P) heat-treatment cycles for the investigated steels.

Figure 3 .
Figure 3. Dilatation due to formation of αB as a function of isothermal holding time for the (a) Fe-C-

Figure 4
Figure 4 depicts representative LOM (upper row) and SEM (lower row) images exemplarily shown for the Fe-C-3.5Mn-Sisteel grade quenched to T Q = 290 • C (Figure 4a,b), T Q = 230 • C (Figure 4c,d), and T Q = 170 • C (Figure4e,f).During quenching into the M S -M f region α' prim was formed, being subsequently tempered during isothermal holding at T P .This lath-like carbon-depleted α" partially included carbide precipitations visible in the SEM images.The microstructural investigation confirmed the results stemming from the analysis of the dilatometric curves, particularly the continuous increase in α' prim and decrease in α B fraction with declining T Q .By means of SMM 18.5, 20.7 and 12.5 vol.%, finely distributed RA were measured for the samples quenched to 290, 230, and 170 • C, respectively.Owing to the extremely fine distribution of RA in the tempered-martensitic matrix, its localization is rather difficult using both LOM and SEM.It is obvious from the micrographs that for the sample quenched to 290 • C, a considerable amount of α' final was present in the microstructure due to the insufficient chemical stabilization of austenite.This α' final was characterized in the given microstructure observed by SEM by coarser and surface structured areas.For the steel grades containing 3.0 and 4.0 wt.% Mn, a similar influence of T Q on the microstructural constituents could be confirmed using LOM and SEM.Regarding the influence of the chemical composition, no significant effect of Mn on the microstructural evolution could be observed at low T Q , whereas for the higher T Q , a larger volume fraction of α' final was obtained for the Fe-C-4.0Mn-Sisteel, as was also proved by the dilatometric measurements.

Figure 4
Figure 4 depicts representative LOM (upper row) and SEM (lower row) images exemplarily shown for the Fe-C-3.5Mn-Sisteel grade quenched to TQ = 290 °C (Figure 4a,b), TQ = 230 °C (Figure 4c,d), and TQ = 170 °C (Figure4e,f).During quenching into the MS-Mf region α'prim was formed, being subsequently tempered during isothermal holding at TP.This lath-like carbon-depleted α'' partially included carbide precipitations visible in the SEM images.The microstructural investigation confirmed the results stemming from the analysis of the dilatometric curves, particularly the continuous increase in α'prim and decrease in αB fraction with declining TQ.By means of SMM 18.5, 20.7 and 12.5 vol.%, finely distributed RA were measured for the samples quenched to 290, 230, and 170 °C , respectively.Owing to the extremely fine distribution of RA in the tempered-martensitic matrix, its localization is rather difficult using both LOM and SEM.It is obvious from the micrographs that for the sample quenched to 290 °C , a considerable amount of α'final was present in the microstructure due to the insufficient chemical stabilization of austenite.This α'final was characterized in the given microstructure observed by SEM by coarser and surface structured areas.For the steel grades containing 3.0 and 4.0 wt.% Mn, a similar influence of TQ on the microstructural constituents could be confirmed using LOM and SEM.Regarding the influence of the chemical composition, no significant effect of Mn on the microstructural evolution could be observed at low TQ, whereas for the higher TQ, a larger volume fraction of α'final was obtained for the Fe-C-4.0Mn-Sisteel, as was also proved by the dilatometric measurements.

Figure 5
Figure5summarizes the results obtained by dilatometry, microstructural investigations, and SMM for the three different steel grades, wherein the individual phase fractions of α'', αB, RA and α'final are plotted as a function of TQ.Irrespective of the chemical composition, an increasing TQ led to a remarkable decrease in the amount of α'', whereas the volume fraction of αB simultaneously increased.Furthermore, the RA-content rose, until the specific TQ where α'final was formed as an

Figure 11 .
Figure 11.kP-values indicating the RA-stability as a function of (a) TQ and (b) α'prim.(c) C-content in RA as a function of α'prim for the investigated steels.

Figure 11 .
Figure 11.k P -values indicating the RA-stability as a function of (a) T Q and (b) α' prim .(c) C-content in RA as a function of α' prim for the investigated steels.

Table 1 .
Chemical composition of the investigated cold rolled steel grades.