Effect of As-Cast Structure and Macrosegregation on Mechanical Properties in Direct-Quenched Low-Alloy Ultrahigh-Strength Steel

Howe (1916) describes the basic principles of the formation of segregation in steel castings. Macroscopic variations in the composition of a casting by the gradual separation of the different chemical constituents during solidification are called macrosegregation and correspondingly the chemical heterogeneity at the scale of the microstructure is known as microsegregation. Both macro- and microsegregation originates from the partitioning of alloying elements between the solid and liquid phases coupled with the movement of the coexisting liquid and solid phases in the mushy zone.[1] Flemings[2] summarized the state of knowledge about macrosegregation in the twentieth century in his review article. Generally, macrosegregation is the result of interdendritic flow, driven by i.e., geometry, gravity, shrinkage, bulging, and solid deformation and in specific circumstances the settling of solid in the early steps of solidification.[2] Ghosh[3] explained the fundamentals of microsegregation, which is confined to the micron-scale interdendritic spaces of the solidifying melt and can be alleviated by subsequent homogenization heat treatments unlike macrosegregation, which is therefore more harmful to the finished steel properties. Later refinements include treatment of dendrite tip undercooling and further research of microsegregation to more complex alloys. In the case of low-alloy steel solidification, accumulation of solute-enriched liquid leads to positive macrosegregation, for example on the centerline of the casting, and the settling of solute-lean solid regions leads to negative segregation.[3] A columnar-to-equiaxed transition (CET) zone is formed once the equiaxed grains in the central part of the solidifying casting are present to prevent the columnar front movement. For example, Choudhary and Ghosh[4] explained that the formation of an equiaxed zone demands the existence of solidification nuclei in the liquid and circumstances under which the nuclei growth and number density is sufficient to arrest the propagation of the columnar dendrites.[4] High superheat increases the amount of columnar dendritic structure and when the superheat is high enough, the columnar dendrites can reach to the centerline of a continuously steel cast slab, while a low superheat can lead to a fully equiaxed dendritic structure as showed by Choudhary and Ganguly.[5] With intermediary superheat in casting, the positive macrosegregation may appear at the CET boundary as Pikkarainen et al. proposed in their article.[6] Zhang et al. have investigated the effect of filling velocity on positive macrosegregation and found that severity of macrosegregation was reduced when the filling rate was increased and secondary arm spacing became slightly smaller in the intermediate and final solidified zones.[7] Dong, Gerin and Zhang[8‐10] have also modeled the macrosegregation phenomena in their recent articles.[8‐10]

Ultrahigh-strength steels (UHSS) can be produced via thermomechanically controlled processing and direct quenching (TMCP-DQ), but typically reaustenitization followed by quenching and tempering is still used. In the TMCP-DQ process route, austenite conditioning is important as it affects both strength and toughness, as Tamura et al.[11] wrote in their comprehensive book about thermomechanical processing of high-strength low-alloy steels. Prior austenite grain (PAG) size is firstly refined by repetitive hot rolling reductions with recrystallization and then controllably deformed in the non-recrystallization temperature range to achieve the desired pancaked prior austenite microstructure. Austenite pancaking is seen as an efficient method to improve the mechanical properties without significantly compromising the uniform elongation as e.g., Kaijalainen et al.[12] proposed.

This paper is concerned with the mechanical properties of a low-alloy TMCP-DQ martensitic ultrahigh-strength steel and how casting temperature and as-cast structure define the microstructure and mechanical properties after rolling and quenching. Particular emphasis was placed on isolating the effects of positive and negative macrosegregations on final hardness and on the formation and intensity of macrosegregation at the CET boundary. Studies concerning the quantification of macrosegregation morphologies and their effects on mechanical properties in industrial scale continuously cast steel flat products have been received relatively little attention. In particular, methodical research addressing these topics in continuously cast UHSS slabs is relatively limited. The present study has been undertaken with the aim of understanding how casting parameters can be optimized with respect to the mechanical properties of such steels.

Experimental heats of a low-alloyed steel were continuously cast on a vertical—curved caster into 270 mm thick and 1975 mm wide slabs with two different superheats. The superheat for Cast 1 (Materials A, B, C) was 16 °C above the liquidus and for Cast 2 (Materials D, E) 34 °C above liquidus. The composition of the studied steel in wt pct was: 0.25C-1.1Mn-0.5Si-0.4Cr-0.4Ni-0.2Mo-0.04Al-0.015Ti-0.0015B.

Three sets of samples intended for laboratory hot rolling and direct quenching were cut from Cast 1 as seen in Figure 1(a) and two sets of samples were cut from Cast 2 (Figure 1(b)). The hypothesis was that Cast 2 would have one Sample (D) without macrosegregation flaws and other (Sample E) with positive segregation at CET and centerline and negative segregation between them. Before hot rolling 10 × 55 × 80 mm³ slices were cut from the ends of the 180 × 55 × 80 mm³ minislabs and etched using Oberhoffer’s reagent (500 mL H₂O, 30 g FeCl₃, 0.5 g SnCl₃, 1 g CuCl₂, 500 mL ethyl alcohol and 500 mL H₂O, 10 to 20 seconds soaking time) to show the as-cast structure (Figure 3). The remaining 170 × 55 × 80 mm³ minislabs were laboratory hot rolled (Carl Wezel Muhlacker) to the eventual plate thickness of 12 mm and direct-quenched in a water tank to room temperature at a cooling rate 40 to 60°C/s. Care was taken to mark the upper surfaces of the slabs and plates to allow corresponding positions to be identified. The hot rolling pass schedule was varied in order to separate the effects of hot rolling from those of as-cast structure and segregation patterns on the impact toughness. The rolling schedules had the finishing rolling temperatures (FRT) of 975 °C and 830 °C where the latter was achieved with waiting period after pass 4 between 1075 °C and 920 °C (Table I).

For analysis of the as-cast structure of the whole slab thickness, slabs were cut from the center of the transverse direction of the slabs to sample sizes of 20 mm (casting direction = CD), 270 mm (rolled plate normal direction = ND) and 100 mm (transverse direction = TD) (Figure 2). From these pieces, nine samples of 50 x 40 x 20 mm³ were cut for macrosegregation analyses as presented in Figure 2.

The slab samples were etched with HCl to show the macro-etched as-cast structure of the slabs (Figure 4). Etchant was 1:1 mixture of water and 100 pct HCl and soaking time was several minutes. An analysis matrix of 30 analysis places was applied as presented in Figure 2 using an optical emission spectrometer (OES, OBLF QSG 750, OBLF, Witten, Germany). Each analyzed point was 6 mm in diameter and by staggering the points as shown in Figure 2 it was possible to obtain mean values for the composition every 1 mm through the slab thickness.

The strength and ductility properties of the rolled samples were defined at room temperature using tensile testing (Zwick/Z100) at the uniform strain rate of 0.008 s⁻¹ in accordance with the European standard EN 10002 using specimens with 6 mm diameter with their axis both in RD and TD. The strain was determined using an extensometer. Charpy-V notch impact testing was performed at 20 °C intervals with three samples per temperature from − 60 °C to 0 °C and two samples per temperature from + 20 °C to 40 °C using samples of 10 × 10 × 55 mm³ taken both longitudinal (T–L) and transverse (L–T) relative to the rolling direction. Hardness was measured through the thickness of the hot-rolled and quenched plates with 0.5 mm intervals utilizing a Duramin-A300 (Struers) under a 50 kg/mm² load (HV5).

Microstructural inspections were executed utilizing optical microscopy (Nikon Eclipse MA100), laser scanning confocal microscope (LSCM) (VK-X200, Keyence Ltd) and field emission scanning electron microscope (FESEM, Zeiss Zigma). The PAG structure was quantified at the quarter-thickness of the laboratory rolled plates by the mean linear intercept method (MLI) with LSCM after picric acid etching (mixture of 4 pct picric acid and ethanol) with 10 to 30 seconds soaking time. Based on these measurements, the mean PAG size, aspect ratio, total reduction below the recrystallization temperature (R_tot.) and relative standard error were determined as described in Higginson and Sellars.[13] Additionally, electron backscatter diffraction (EBSD) measurements were taken at sample centerlines with an accelerating voltage of 15 kV, working distance of 13 mm and step size 0.2 μm from an area of 21500 μm² using (EDAX, TSL OIM Data Analysis software). The EBSD data enabled the determination of the mean effective grain size (d) and the effective grain size at 90 pct in the cumulative size distribution (d90 pct). EBSD grain sizes were expressed as equivalent circle diameters (ECD) and grain boundaries with a misorientation angle greater than 15 deg were used to determine the effective grain size.

The effect of as-cast structure and macrosegregation on the toughness and tensile properties in direct-quenched martensitic steel has been investigated by using two different superheats and hot rolling finishing temperatures. The main results can be summarized as follows: