DEVELOPMENT OF NON-GRAIN ORIENTED SILICON STEEL GRADE M310-50KWITH HIGH PERMEABILITY AND LOW ENERGY LOSS

ABSTRACT The development of motor engine with improved efficiency is driving the adoption of non oriented steel grade with quite low power loss and high permeability. In this frame NGO grades M310-50K and M290-50K are increasingly requested by the market. In the present work is presented the development of the semi-processed grade M310-50K, based on a 2.0 wt% Si non-oriented electrical steel, at Marcegaglia Carbon Steel through the optimization of the microstructural characteristics of final product by means of the thermo-mechanical processing (TMP) conditions and the following cold rolling, annealing cycle and skin-pass processes. Microstructure and ODF analysis were carried out on hot rolled sample, cold rolled and annealed and after final decarburization and secondary recrystallization annealing. The results confirmed that to develop the favorable texture and achieve the target magnetic properties a through process approach has to be adopted.

MATERIALS AND METHODS The chemical compositions used for the industrial production of NGO electrical steels grade M310-50K is reported in the following Table 1. In Fig. 1 is reported the phase diagram of the steel tested. As can be noted there is no α-γ phase transformation during the hot rolling temperature range.

TextureThe ideal texture of a soft iron core for motor engine is the one that maximizes the density of <100> crystal directions in the plane of the sheet. Therefore for rotating machine in addition to the well-known Goss component {110}001, the orientations of the cube fibre for which the {001} planes are parallel to the rolling plane are also desired (Fig. 6).

γ-fibre texture components, which evolve during both the hot and cold rolling operations, are unfavorable for magnetic properties since they have hard magnetization directions. In addition to these specific textures, a coarse grained microstructure is required to minimize the magnetic losses. So generally to improve the final magnetic properties it is important to maximise the Cube + Goss texture with respect to γ-fiber, since Goss texture together with cube texture are efficient in improving the magnetic properties of electrical steels.

In Fig.7, 8, 9 are shown the texture of hot rolled, cold rolled and annealed strip, and fully finished strip (after laboratory decarburization and secondary recrystallisation). The values reported in the plots are proportional to the volume fractions of grains with the considered plane orthogonal to the “normal direction” of the sheet, measured respect the volume fraction of same plane in a random sample (I0). The hot rolled sample texture was characterised both close the strip surface and in the centre thickness. As can be noted (Fig.7) a weak Goss orientation develops close to the sheet surface. The origin of the Goss component in the hot rolling stage, is due to high friction and the resulting shear deformation. Fig.8 illustrates the texture of the cold rolled and annealed sample revealing a strong g-fibre, as expected after recrystallization.

The fully finished strip (Fig.9) was obtained heating the semi-finished samples at 840 °C for 2 hours in controlled wet hydrogen atmosphere. This sample reveals a weak volume fraction of Goss oriented grains, inherited through the cold rolling and primary recrystallisation process, and a higher volume fraction of grains with the cube fibre i.e. with the [001] planes parallel to the rolling plane.

For measurement of hysteresis curves and losses of soft magnetic sheets of steel the standardized 30 cm Epstein frame is measuring equipment was adopted according to IEC 60404-2 norm. In the Table 2 are reported the results of the measurements in terms of power loss and polarisation carried out with the Epstein frame. As can be noted the losses Ps at B=1.5T and 50Hz are quite good and fits well the requirements of the steel grade M310-50K.

Tailoring the texture of the final product requires an optimized processing strategy, which involves a thorough understanding of texture evolution along the manifold processing steps. The current contribution presents the results of the M310-50K product development carried out by means of optimization of the texture evolution through the selection of the best hot and cold rolling processing parameters. Microstructure and texture evolution was characterized by means of optical microscope analysis and ODF analysis (Mo target x-ray tube) on hot rolled strip, after cold rolling and batch annealing till to the laboratory decarburization and secondary recrystallization treatment.

Figure 2 illustrates the whole manufacturing route of the product. The slabs were reheated to 1200 °C and kept in the furnace for a soaking time of 90 minutes, and then hot rolled to 2.0 mm. The finishing rolling process started at temperature 1050 °C and finished at 860 °C. After hot rolling, the strip was coiled at 680 °C.

(A. Ferraiuolo, D. Rambelli, L. Solazzi)

C Si Mn Al P S N

<0.005 2.0 0.4 0.5 0.015 <0.01 <0.005

Ps (50Hz, B=1,5T) J ( Hs=2500A/m) J ( Hs=5000A/m) J ( Hs=1000A/m)

≤3.1 ≥1.55 ≥1.64 ≥1.76

Table 1: Steel chemical composition (wt%).

Table 2: Power loss measured by Epstein frame.

Fig.1: Phase diagram

Fig.2: Manufacturing route of a semi-processed at Marcegaglia Carbon Steel

Fig.3: Hot rolled strip microstructure: a) surface - b) centre thickness. Fig.5: Finished sample microstructure: a) surface - b) centre thickness.

Fig.6: Schematic plot of the main textures

Fig.4: Cold rolled and annealed microstructure: a) surface - b) centre thickness.

The hot rolled strip was pickled and cold rolled down to 0.5 mm and then annealed in a bell furnace. The annealing conditions are of paramount importance for the enhancement of the target texture. The annealing atmosphere consists of 100% H2 to obtain a smooth surface without oxidization, and the dew point was kept under -50 °C. The final process is the skin-pass with a typical elongation ranging between 4-8%.

RESULTS MicrostructureFigure 3 illustrates the microstructure of hot rolled strip characterized by almost complete recrystallization in the surface zone and a partially recrystallized structure in the centre thickness. The microstructure of cold rolled and annealed strip (Fig.2) is characterized by a fully recrystallized structure with an average grain size of 20 mm. The fully finished strip after secondary recrystallization is characterized by a quite homogeneous grain size distribution with an average grain size of 120 mm.

CONCLUSIONSIn the present paper the results of the industrial development of the non oriented grade M310-50K at Marcegaglia Carbon Steel (Ravenna plant) has been presented. The magnetic properties of electrical steels such as magnetization curves, permeability and specific losses are, to a large extent, correlated with the microstructure and crystallographic texture. The maximisation of these properties depends on the optimisation between of the thermomechanical process (TMP) adopted and the following cold processing of the strip and the need of cost reductions. Of course this optimisation is strictly related to the specific chemical composition adopted. The results confirmed that the best magnetic performances were achieved with the TMP conditions consisting of slabs reheated at 1200°C, finish hot rolling at 860 °C and coiling at 680 °C. The following process consisted in cold rolling with a reduction of 75%. The fine tuning of batch annealing and skin-pass conditions (4-8%) allowed to achieve power loss below 3.1W/Kg and J50≥1.64T.

HOT ROLLED FULLY ANNEALED

COLD ROLLED AND ANNEALED

Fig.8: Main fiber texture of cold rolled and annealed strip (1/4 strip thickness)

COLD ROLLED AND ANNEALED

Fig.9: ODF of fully finished strip (1/4 strip thickness)

FULLY FINISHED

Fig.7: ODF of hot rolled strip: a) surface - b) centre thickness.

HOT ROLLEDa) b)