Introduction

Medium manganese steels (3–10 mass.% Mn) belonging to the third generation of advanced high strength steels (AHSS) can potentially be applied in the automotive industry because they show good strength–ductility balance at an acceptable production cost [1,2,3]. The third generation of AHSS includes steels produced via intercritical annealing (IA) [4, 5], quenching and partitioning (Q&P) process [5, 7] and steels containing bainitic ferrite [8,9,10]. The beneficial combination of strength and ductility of these steels is related to the multiphase microstructure containing a fine-grained matrix (ferrite, martensite or bainite) and some fraction of retained austenite (RA), typically in the range of 15–40 vol.% [4,5,6,7,8,9,10]. The stabilization of the desired fraction of RA is a key issue, which is taken into account during the design and processing routes of medium-Mn steels. Retained austenite is able to undergo strain-induced martensitic transformation (SIMT) during deformation providing the high strength and ductility of steels. The effectiveness of SIMT is closely related to the amount and stability of RA [11, 12].

The successive phase transformations in the solid state play a key role as a fundamental instrument in achieving desired microstructures and mechanical properties of multiphase steels [13]. The Quenching and Partitioning (Q&P) concept was designed to improve the ductility of novel martensitic steels via the Transformation Induced Plasticity (TRIP) effect caused by the transformation of retained austenite into martensite during plastic deformation [14, 15]. The Q&P process consists of a few stages. First, the steel is heated to the austenitization or intercritical temperature, then it is quenched to a temperature between Ms and Mf and the austenite undergoes a partial transformation into martensite. After quenching, the steel is heated to a partitioning temperature, which is slightly higher than Ms. At this stage, carbon partitioning from supersaturated martensite into austenite takes place and the undesired cementite precipitation may also occur. The austenite is enriched in carbon, thus its stability increases and therefore some fraction of austenite can be retained in the final microstructure [6, 7]. The quenching temperature is crucial to determining the austenite–martensite ratio. Typically, the quenching stage of the Q&P process is carried out in a temperature range from 200 to 300 °C [14, 15]. Avoiding the occurrence of ferritic, pearlitic or bainitic transformation during cooling after austenitization to the quenching temperature is crucial to obtaining the microstructure composed of martensite and austenite. To ensure a homogeneous microstructure of products with larger cross sections such as plates, steel should show high hardenability. Moreover, the high hardenability of steel is beneficial from the industrial point of view because simple air-cooling can be applied instead of using water or other medium as a coolant.

The Q&P process has been typically carried out as a heat treatment applied to cold rolled sheets [16, 16]. The innovative energy saving approach integrates the hot rolling and Q&P process in one production cycle. Therefore, the designing of processing routes for Q&P steels requires detailed knowledge on the influence of plastic deformation in the austenite region on the kinetics of phase transformation during cooling. In this work, the influence of plastic deformation on the kinetics of phase transformation during continuous cooling of steels containing 4 and 5 mass.% Mn was studied.

Material and experimental techniques

The chemical composition of the investigated alloys is presented in Table 1. The content of individual chemical elements was designed to obtain the best mechanical properties via Quenching and Partitioning (Q&P) process. Moreover, the chemical composition of alloys took into account their potential use for sheets and plates in automotive applications. The reduced carbon content to 0.17 mass.% is beneficial in terms of weldability and impact toughness of steel [17, 18]. The Mn content 4–5 mass.% was added to provide high hardenability of steel and to stabilize retained austenite. Si and Al were added to prevent carbide precipitation during a partitioning step [6, 19]. Mo was added to improve hardenability and solid solution strengthening [20]. Nb was added to refine the microstructure and thus to improve the strength properties [21].

Table 1 Chemical composition of the investigated alloys
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The investigated alloys were first cast in a vacuum furnace under argon atmosphere. Next, ingots were austenitized at 1200 °C for 2 h to homogenize the chemical composition and subsequently hot-rolled in a temperature range from 1200 to 900 °C to a thickness of 22 mm. The hot-rolling process was carried out using the LPS/B semi-industrial (Łukasiewicz Research Network—Upper Silesian Institute of Technology, Gliwice, Poland). The obtained flat bars were used for further investigations.

Thermodynamic calculations using JMatPro software with the implemented database v11.2 General Steels Module were carried out in order to determine the theoretical phase transformation kinetics under continuous cooling and characteristic parameters of model alloys such as critical temperatures of phase transformations and critical cooling rate.

The transformation kinetics of investigated alloys was tested using high resolution BÄHR dilatometer DIL 805 A/D. The experiments were conducted in a vacuum and helium was used for cooling. The temperature during measurements was controlled by a S-type thermocouple welded to the surface of each sample. The samples for dilatometric measurements were machined from the hot-rolled plates, parallel to the rolling direction. Two types of samples were used in the measurements. Cylindrical samples of 5 mm diameter and 10 mm length deformed with 50% deformation degree at 900 °C and samples of 4 mm diameter and 10 mm length without plastic deformation applied before cooling. The dilatometric data were analyzed according to ASTM A1033-04 standard [22]. The Ac1 and Ac3 temperatures were determined by slow heating of a sample at a rate of 20 °C s-1. The Ms temperature was determined during quenching at a rate of 60 °C s-1 [23]. The samples were austenitized at 1100 °C prior cooling. The continuous-cooling-transformation (CCT) diagrams and deformation continuous-cooling-transformation (DCCT) diagrams of alloys were prepared based on dilatometric measurements to determine a critical cooling rate. The detailed information about the parameters applied during dilatometric measurements is presented in Table 2.

Table 2 Heat treatment parameters applied for 4Mn and 5Mn alloys in order to obtain CCT and DCCT diagrams
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A light microscope (Zeiss, Z1m Axio Observer) was used to characterize the microstructure of the investigated alloys. Specimens were cut to one third of their length and embedded in a thermosetting resin. The samples were ground using SiC paper with a grid up to 2000. Then, the samples were polished with 3 and 1 µm diamond pastes and finally with the colloidal silica. The microstructure was revealed by etching the samples for 5 s with a 5% nital solution at room temperature.

Results and discussion

Determining a critical cooling rate to obtain a fully martensitic microstructure is crucial for Quenching and Partitioning steels because after heating to the austenite region and hot deformation (thermomechanically treated steels), quenching to a temperature between Ms and Mf is carried out to achieve a desired martensite–austenite ratio [24, 25]. Determining the range of cooling rates for which the ferrite, pearlite or bainite are not present in the microstructure is important from the technological point of view because it allows adjustment a proper cooling medium.

The theoretical CCT (continuous cooling transformation) diagrams of investigated alloys obtained using JMatPro are presented in Fig. 1. The austenitization at 1100 °C was applied and the range of cooling rates was from 110 to 0.01 °C s-1. The characteristic temperatures and critical cooling rates of investigated alloys calculated using JMatPro are listed in Table 3.

Fig. 1
figure 1

CCT diagrams calculated using JMatPro obtained at different cooling rates from 110 to 0.01 °C s-1 for the: 4Mn alloy (a) and 5Mn alloy (b)

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Table 3 Characteristic temperatures and critical cooling rates of investigated alloys calculated using JMatPro
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It was observed that the Mn addition had a strong effect on reducing Ac3 and Ms temperatures of steel because Mn is an austenite stabilizing element [26,27,28]. Kaar et al. [28] reported that addition of 1 mass.% of Mn decreases the Ms temperature of steel by − 32 °C. It was noted that the formation of pearlite and bainite in 5 mass.% Mn alloy was shifted to lower cooling rates compared for the alloy containing 4 mass.% Mn. Figure 2 shows the microstructural evolution of structural constituents as a function of temperature determined by JMatPro. It was noted that the 5Mn alloy shows higher hardenability than the alloy containing 4 mass.% of Mn. For the 5Mn alloy, the fraction of ferrite was close to zero even at a very slow cooling rate of 0.1 °C s-1. Detrimental pearlite disappears in 5Mn steel at a cooling rate of 1 °C s-1 (Fig. 2f), whereas in the 4Mn steel it is the case after increasing the cooling rate to 5 °C s-1 (Fig. 2c).

Fig. 2
figure 2

Microstructural evolution of structural constituents as a function of temperature determined based on JMatPro software at different cooling rates in: 4Mn alloy (a–c) and 5Mn alloy (d–f)

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The results obtained using thermodynamic calculations were compared with the results of dilatometric measurements. Figure 3 presents the dilatometric curves of investigated alloys obtained during heating at 20 °C s-1 and cooling at 60 °C s-1. Comparing to the data obtained from the theoretical calculations (Table 3) the results showed moderate agreement; however, a similar tendency was observed, i.e., the Ac3 and Ms temperatures were lower for the steel containing 5 mass.% of Mn. The discrepancy in theoretical and experimental values was related to the inaccuracy of current databases and/or non-equilibrium thermal routes applied during experimental investigations. Modeling approach assumes equilibrium heating (infinitely long). However, the heating rate used for dilatometric measurements was 20 °C s-1. The non-equilibrium conditions resulted in higher critical temperatures of investigated alloys [29]. A wide temperature range between Ms and Mf was noted for both investigated alloys: 190 °C for 4Mn steel and 176 °C for 5Mn steel, respectively. It provides a large technological window for the quenching step. The characteristic temperatures during heating and cooling of investigated alloys obtained using dilatometry without hot deformation are listed in Table 4.

Fig. 3
figure 3

Dilatometric curves of investigated alloys obtained during heating (a) and cooling (b); RCL—relative change in length

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Table 4 Characteristic temperatures of investigated alloys obtained using dilatometry without hot deformation
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The investigated alloys are intended to automotive Q&P sheets and plates, which can be produced via heat treatment or hot-rolling process with the finishing deformation typically applied at 850–900 °C integrated with the Q&P process [30, 31]. Therefore, in the present study, plastic deformation at 900 °C was applied before cooling (DCCT) and the CCT and DCCT diagrams are compared in Fig. 4. The characteristic temperatures obtained using dilatometry for the investigated alloys for deformed and non-deformed specimens cooled at different rates are shown in Fig. 4. The CCT curves of 4Mn alloy showed that the bainitic transformation occurred only in a specimen cooled at a very low rate of 0.05 °C s-1 (Fig. 4a). In case of steel containing 5 mass.% of Mn, regardless of the applied cooling rate in a range from 0.05 to 60 °C s-1, a fully martensitic microstructure was obtained. Typically, plastic deformation accelerates and increases the kinetics of ferritic, pearlitic or bainitic transformation of the steel [13]. Dislocations generated during plastic deformation act as nucleation sites for the new diffusion-controlled phases [33]. It was also observed that for the deformed alloys, the Ms temperature increased with decreasing cooling rates (Fig. 5). It may be related to static recrystallization, which may be more advanced for slowly cooled material [8]. However, in case of non-deformed alloys the observed trend in Ms temperature was almost constant (Fig. 4). At high cooling rates of 60 °C s-1 and 10 °C s-1, Ms temperatures of both alloys were lower for the deformed samples. However, with decreasing cooling rate the Ms temperature of undeformed samples was slightly lower. The Ms temperatures of both investigated alloys in deformed and non-deformed states did not exceed 384 °C in a whole range of applied cooling rates, which creates good conditions for the stabilization of retained austenite (Fig. 5). The fully martensitic microstructure in investigated alloys can be obtained in a wide range of cooling rates both in the deformed and non-deformed states. It is beneficial because the investigated alloys can be used for producing elements of large cross sections such as plates.

Fig. 4
figure 4

CCT and DCCT diagrams obtained using dilatometry for: 4Mn alloy (a) and 5Mn alloy (b)

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Fig. 5
figure 5

Dilatometric curves of investigated alloys obtained during cooling at different rates for non-deformed 4Mn alloy (a) and 5Mn alloy (b) and deformed 4Mn alloy (c) and 5Mn alloy (d)

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Figures 6 and 7 show the microstructures of investigated alloys obtained at different cooling rates from 60 to 0.05 °C s-1. Some fraction of bainite was only observed in the microstructure of non-deformed 4Mn alloy cooled at a rate of 0.05 °C s-1 (Fig. 6b). For cooling rates higher than 0.05 °C s-1, only martensitic microstructures were observed. In case of the alloy containing 5 mass.% of Mn (Fig. 7), a pure martensitic microstructure was observed regardless of the applied cooling rate. It is beneficial because simply air cooling can be applied during the quenching step, which does not require modification of existing technological lines. Obtained results are in good agreement with the dilatometric measurements (Fig. 4).

Fig. 6
figure 6

Microstructures of 4Mn alloy obtained for specimens non-deformed (CCT) and deformed (DCCT) prior to cooling at different rates

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Fig. 7
figure 7

Microstructures of 5Mn alloy obtained for specimens non-deformed (CCT) and deformed (DCCT) prior to cooling at different rates

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This paper is focused on the kinetics of phase transformations in non-deformed and plastically deformed alloys. The obtained results are the basis for further research concerning the design and optimization of time–temperature variants of Quenching and Partitioning processing routes and the characterization of resulting mechanical properties.

Conclusions

This work presents insights into the effect of plastic deformation on the kinetics of phase transformation during continuous cooling of novel steels containing 4 and 5 mass.% of Mn intended for Q&P processing. According to the results, the following conclusions can be drawn:

  • The addition of manganese had a strong effect on reducing the Ac3 and Ms temperatures due to the austenite stabilizing effect of this element. Both critical temperatures were lower for the steel containing 5 mass.% of Mn.

  • The investigated alloys are well-suited for Quenching and Partitioning processing routes. A wide temperature range between Ms and Mf was noted for both investigated alloys: 190 °C for 4Mn steel and 176 °C for 5Mn steel, respectively. It provides a large technological window for the quenching step.

  • The occurrence of undesired ferritic, pearlitic or bainitic transformations during continuous cooling after austenitization was eliminated for both deformed and non-deformed states due to the increased Mn addition (4–5 mass.%). The formation of a small fraction of bainite was observed only in the CCT curves of 4Mn alloy at a very slow cooling rate of 0.05 °C s-1.

  • The investigated alloys can be used both for sheets and plates of increased thickness produced via conventional Q&P heat treatment or thermomechanical Q&P processing integrated with hot rolling in one production cycle because the homogeneous martensitic microstructure can be obtained in a wide range of cooling rates applied during the quenching step.

  • The Ms temperature of deformed alloys increased with decreasing cooling rates. It may be related to static recovery and recrystallization and a corresponding decrease in dislocation density, which was more advanced for slowly cooled samples. In case of non-deformed alloys the Ms temperatures were almost constant.

  • The results of simulations and experimental tests show moderate agreement; however, a similar tendency was observed. The Ac3 and Ms temperatures were lower for the steel containing 5 mass.% of Mn. It was related to the inaccuracy of current databases and/or non-equilibrium thermal routes applied during experimental investigations.