MnS Precipitation Behavior of High-Sulfur Microalloyed Steel Under Sub-rapid Solidification Process
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A typical high-sulfur microalloyed steel was investigated by a sub-rapid solidification process for grain refinement of the as-cast microstructure. The size and distribution characteristics of the MnS precipitates were analyzed. The variations in the dendrite morphology and secondary dendrite arm spacing (SDAS) under different cooling rates have been studied, which strongly influence the precipitation behavior of MnS. The 3D-morphology of MnS precipitates was revealed by a novel saturated picric acid deep-etching method. Most MnS precipitates with a length smaller than 5 μm were columnar or equiaxed in the corresponding dendrite zones under sub-rapid solidification conditions at cooling rates of 261 to 2484 K/s. Furthermore, an area scan analysis of the precipitates showed the number of small MnS per square millimeter with lengths lower than 3 μm decrease from 200,537 to 110,067. The percentage of large MnS with a length over 5 μm increased from 2.6 to 6.2 pct as the solidification condition changed from sub-rapid to air cooling. In addition, the size of MnS precipitate was found to depend linearly on the SDAS.
High-strength medium carbon sulfur-containing microalloyed steels have been widely used in hot forging parts of automobiles, such as crankshafts and connecting rods, due to the advantage of energy-savings with elimination of traditional quenching and tempering processes.[1,2] During the continuous casting process of this type of steels, the precipitation behavior of MnS is crucial, as MnS precipitates are good lubricants for improving the cutting performance of microalloyed steels. MnS precipitates in as-cast steel slabs can be typically classified according to the morphology: globular MnS (Type I); fine rod-like MnS (Type II); and angular MnS (Type III). It is well known that the mechanical properties of high-sulfur steels are closely related to the MnS precipitates’ shape and distribution. In traditional continuous casting of sulfurized steels, the size of MnS precipitates is generally larger than 10 μm. In order to obtain better cutting performance, the blooms require prolonged heat treatments to decompose the MnS precipitates into finer rod-like shapes (Type II) having a mean length lower than 5 μm. This extended heat treating processes would consume significant amounts of additional energy. Therefore, less energy-intensive new production methods to ensure finely dispersed MnS inclusions in high sulfur-containing microalloyed steels are necessary. Lower sulfur segregation and finer as-cast microstructure with increasing of solidification cooling rates could reduce the precipitation and growth of sulfide.[7,8]
As the only industrialized sub-rapid solidification process, strip casting is an important technological revolution for the steel industry, which can produce thin strips directly from the liquid metal. Strip casting has the potential to greatly reduce operating and investment costs through the elimination of multiple rolling steps.[9,10] Strip casting has been known to provide solutions for steels with difficult casting issues including macro-segregation, precipitation of large inclusions, larger structures. Due to the rapid cooling experienced during strip casting, the morphology of as-cast microstructure could be significantly refined.[11,12] Electrical steels, TRIP steels, dual phase (DP) steels, and other special steels with complex and non-uniform morphologies have been identified as potential products applicable for strip casting. Some past publications have reported the formation of fine manganese sulfides during rapid solidification of low-sulfur steels, such as stainless steels and high-strength low-alloy steels.[8,16]
However, there has been limited research related to strip casting of high-sulfur microalloyed steels. In particular, studies on the relationship between the secondary arm dendrite spacing (SADS), cooling rate, and mean length of MnS precipitates have yet to be studied to the knowledge of the present authors. The aim of this work is to present a novel method for controlling MnS precipitation via sub-rapid solidification process and reveal the relationship between the distribution characteristics and size of MnS precipitates, cooling rate, and SDAS of the as-cast structure.
The Main Chemical Composition of the High-Sulfur Microalloyed Steel (in Mass Percent)
Experimental Apparatus and Procedure
The droplet solidification testing system consists of two parts: droplet ejection and data acquisition. The liquid droplet is obtained by heating the metal specimen using induction coils. The ejection of molten droplets to the copper mold is conducted through a pulse of high-purity Ar (99.999 vol pct). The atmosphere control system allows the control of the oxygen partial pressure. This system allows the metal melting and dropping on the substrate surface under controlled temperature and atmosphere.
Prior to the experiment, the copper mold substrate is cleaned and polished with an abrasive paper of grit number 3500 to ensure comparable surface roughness for each experiment. The oxygen partial pressure is lower than 10−5 atm through a stream of high-purity Ar (99.999 vol pct). The metal specimen is placed within a quartz tube that has a small hole on the bottom. The specimen lies in the middle of the induction coil installed inside the bell jar, where a controlled atmosphere can be ensured. The sample is heated and melted using the induction furnace, and the temperature was measured with a pyrometer placed above the tube. A PID controller receives the temperature signals from the pyrometer and controls the target temperature by adjusting the power of the induction furnace. When the desired temperature is reached, the liquid droplet is ejected through the small hole at the bottom of the tube with the help of a pulse of high-purity Ar (99.999 pct). The droplet subsequently impinges onto the water-cooled copper substrate and solidifies. A charge coupled device (CCD) camera is placed adjacent to the bell jar to record the entire melting and solidification process of the sample.
The target temperature of the liquid metal before ejection is 1550 °C. In order to obtain samples with different cooling rates, one melt sample was retained in the quartz tube and subject to air cooling, and the other melt sample was ejected onto the surface of the water-cooled copper substrate for sub-rapid cooling.
The solidified samples were cut into halves along the longitudinal direction and prepared through standard metallographic procedures for cross-sectional analysis. In addition, a novel saturated picric acid deep-etching method was developed to reveal the 3D-morphology of MnS precipitates. The samples were subject to morphological examinations using optical microscopy (OM, Jiangnan MR5000, China), scanning electron microscopy (SEM, TESCAN MIRA 3 LMU, Czech) equipped with X-ray energy-dispersive spectrometer (EDS, Oxford X-Max20, England), and electron probe micro-analysis (EPMA, JEOL JXA-8530F, City, Japan) equipped with wavelength dispersive X-ray spectrum system (WDS, XM-86030). The hardness of the samples was identified using a microhardness tester (HMV-2T, Japan). The number and size of MnS precipitates were analyzed by an inclusion automatic detection scanning electron microscopy (ASPEX). The secondary dendrite arm spacing and mean size of MnS precipitates in the as-cast microstructures were analyzed by GetData software from the OM and SEM images.
Results and Discussion
Identification of the SDAS
The SDAS and Corresponding Calculated Cooling Rates
Cooling Rates (K/s)
Distribution of MnS
Size of MnS
The Mean Length of MnS Precipitates
Mean Length of MnS (μm)
Moreover, in order to confirm the mechanical properties of MnS, microhardness tests have also been conducted. The results indicate that the average Vickers microhardness value of MnS (~ 167 HV) is much smaller than the metal matrix (~ 712 HV), which further shows the reason behind the good lubrication ability of MnS to improve the cutting performance of microalloyed steels.
Control Mechanism of MnS Precipitates
It should be noted that the control of MnS precipitation in high-sulfur microalloyed steels using a sub-rapid solidification method was not noticed before because the existing works usually focused on optimizing the traditional continuous casting process and its subsequent heat treating process. The size of MnS precipitates in the present work is significantly decreased compared to the typical industrial process. The control of MnS precipitates with a length smaller than 5 μm is difficult through the conventional continuous casting process.
The 3D-morphology of MnS precipitate was revealed by a simple saturated picric acid deep-etching method. Most MnS precipitates with a length smaller than 5 μm were columnar or equiaxed in the corresponding dendrite zones under sub-rapid solidification conditions at cooling rates of 261-2484 K/s
As the solidification condition changed from sub-rapid to air cooling, the number of small MnS per square millimeter with a length lower than 3 μm decreased from 200,537 to 110,067, and the percentage of the large MnS with a length over than 5 μm increased from 2.6 to 6.2 pct.
The length of MnS reduced from 4.51 to 1.98 μm as the SDAS decreased from 26.11 to 11.56 μm due to the different cooling rates from the top to the bottom of the molten droplet. A formula which can be used to predict the size of MnS precipitates was established.
A novel experimental droplet solidification apparatus was developed to simulate the process of strip casting. The sub-rapid solidification process is probably a viable method for obtaining small rod-like form MnS precipitates and thus may has good prospects for industrial application.
This work is supported by the National Natural Science Foundation of China (U1760202), Hunan Provincial Key Research and Development Program (2018WK2051), Opening Foundation of the State Key Laboratory of Advanced Metallurgy (KF19-04), Hunan Provincial Innovation Foundation for Postgraduate (CX2018B089), and Fundamental Research Funds for the Central Universities of Central South University (2018zzts018).