1 Introduction

In the last decade, the academic community extensively researched medium-Mn steels. The alloying concept of medium-Mn steels with 4–10 wt-% Mn is proposed as one of the most promising to fulfil the property requirements of the 3rd generation AHSS. Mn partitioning facilitates the stabilization of substantial amounts of austenite to room temperature. This allows the TRIP effect to be utilized, and combinations of tensile strength and total elongation can easily exceed the margin of 30,000 MPa%. This steel class has proved its position as one possible successor of classic AHSS due to the excellent mechanical properties paired with still reasonable alloying costs. Nevertheless, several technical issues have to be resolved in order to enable a successful application of medium-Mn steels in a vehicle, which is the main aim of this contribution. In this context, all presented results were acquired by a large-scale produced material, fulfilling the requirements for a batch-annealed medium-Mn steel with the strength level of 780 MPa, manufactured at the voestalpine steel plant in Linz, Austria.

The present contribution intends to give an overview of the development for a medium-Mn780 grade, focusing on industrial feasibility, formability, and weldability.

2 Experimental

Two industrial heats of approx. 180 t, one with 0.12 wt-% C and 5.8 wt-% Mn and one with an optimized chemistry, were produced via a conventional LD converter route followed by subsequent continuous casting equipped with soft reduction. The slabs were reheated to 1200 °C for ~ 2 h before hot rolling was applied with a finishing and coiling temperature of 880 °C and 550 °C, respectively to 3.2 mm thickness. For scale removal, a pusher-type pickling line (HCl solution) was used. Due to a fully martensitic microstructure and thus a high strength level of the material after hot rolling, a batch annealing step was necessary to ensure cold-rollability. Cold-rolled material in a thickness range of 1.0 to 1.6 mm was produced.

Dilatometric investigations were conducted on a Bähr 805 A/D using cold rolled material (10 × 3.5 × 1 mm3). Heating and cooling rate to intercritical annealing temperature were 100 and 80 Ks‑1, respectively. The microstructure of the annealed materials was observed by means of EBSD on a Jeol JSM-7800F scanning electron microscope with a field emission gun. Simultaneous EDX measurements were performed with a windowless Oxford Extreme detector. The mechanical properties were obtained by tensile testing according to the international standard ISO 6892‑1 on a Zwick-Roell BTC-FR020TN tensile testing machine using tensile samples with a gauge length of 80 mm. The retained austenite content was determined by means of saturation magnetisation measurement. Welding parameters were applied according to SEP1220‑2.

3 Results and Discussion

3.1 Heat Treatment and Mechanical Properties

Building on the experience gained from previous work [1,2,3], a two-step heat treatment comprising full austenitization in a continuous annealing (CA) line followed by intercritical annealing (IA) of prior martensitic microstructure in a batch-annealing (BA) furnace was applied. As shown in Fig. 1a, the initial medium-Mn780 grade 0.12C5.8 Mn possesses a yield strength of 620 MPa, a tensile strength of 810 MPa, a total elongation of over 30%, and a hole expansion ratio of 35%. This means that not only does the material offer a ~ 50% higher elongation than a DP800HD but also outperforms a DP600HD with significantly lower strength. For processability and weldability reasons, the chemical analysis was slightly adjusted. This results in a decrease in elongation (27%), which is, however, still above or equal to that of the previously mentioned HD-grades. At the same time, the hole expansion ratio increases to an average of over 50%. This means that global ductility was slightly reduced but local formability was gained. The adapted chemistry, therefore, offers a more balanced formability behavior. None of the medium-Mn780 grades presented shows a pronounced YPE or serrated flow phenomena, which leads to inhomogeneous thickness distribution across a manufactured component and therefore deteriorates crash performance. The main influencing factor for preventing YPE is the applied two-step heat treatment and the resulting microstructure and texture as described in detail in [1]. Fig. 1b displays the ultrafine-grained microstructure of the optimized medium-Mn780 grade consisting of a highly tempered martensitic matrix (blue) with lath-like austenite films (red). The yellow areas indicate ε‑martensite (hcp), which is believed to originate from γ→ ε transformation during sample preparation. In Fig. 1c, the heterogeneous Mn distribution within the microstructure can be seen. The brighter orange Mn-rich areas are assigned to the austenitic phase.

Fig. 1
figure 1

a tensile properties, b phase fractions (blue: bcc, red: fcc, yellow: hcp), c Mn-distribution

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The industrially produced medium-Mn grades show very homogeneous properties over the coil length (e.g.: YS 586–598 MPa, TS 830–844 MPa, TE 26.8–27.4%, HER 51–53%). Initially, this was one of the largest concerns related to application of a batch annealing process for this steel concept. However, it was found that although the steels are very sensitive to the intercritical annealing temperature (experimental data and simulation as described in [4]), the annealing time plays in this respect a minor role. Fig. 2 shows the detrimental effect of temperature on the retained austenite fraction versus the inferior effect of annealing time greater than 8 h. In-depth investigations on the effect of annealing time revealed a decrease of the austenite stability due to a slight coarsening of the grains, but the effect remains negligible. Therefore, the time-temperature cycles during batch annealing were optimized so that there are only small temperature differences within the coil. The holding time, however, varies considerably in some cases.

Fig. 2
figure 2

Retained austenite fraction as a function a temperature, b annealing time

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3.2 Weldability

As already reported in [3] and displayed in Fig. 3a, the initial chemical composition of the medium-Mn780 grade exhibited very poor spot weldability with respect to the low cross tensile strength (CTS). Furthermore, the failure appearance proved to be an adverse interfacial failure. By optimizing the chemical composition towards weldability, the CTS could be almost doubled and the failure mode changed to a predominantly partial interfacial failure.

Fig. 3
figure 3

a CTS for initial and optimized medium-Mn780 (1.2 mm) Imin-Imax, b interfacial and c partial interfacial failure

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However, these CTS were still not fully satisfactory, especially at Imin. Therefore, a double pulse welding process was used to further raise the CTS. Fig. 4a clearly shows the advantage of a double pulse welding process on the CTS. With the right choice of cooling time between pulses, this can raise the CTS to > 5 kN. In-depth investigations have shown that a Mn-enrichment, introduced by the welding process, can be significantly reduced (Fig. 4b). Mn segregation at grain boundaries is known to have a strong embrittlement effect [5]. A comprehensive compilation of the studies can be found in [6]. It should also be mentioned that the grade does not show any susceptibility to liquid metal embrittlement as it is an issue for many new emerging steel grades for the automotive industry.

Fig. 4
figure 4

a CTS (Fmax) for single pulse (sp) at Imin and double pulse welding as a function of cooling time, b weld spot appearance and Mn distribution (EDX) after different welding conditions

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4 Conclusion

The presented results clearly show that the adapted chemistry for the medium-Mn780 grade offers balanced mechanical properties by increasing the hole expansion ratio on the expanse of total elongation. Nevertheless, the grade is still suitable for highly demanding deep-drawing applications as well offering an excellent crash performance. Furthermore, welding properties were substantially improved in order to meet the requirements for applications in the automotive industry.