2020 年 61 巻 11 号 p. 2228-2235
In the present study, a GCr15/45 carbon steel composite billet is manufactured by the new electroslag remelting cladding (ESRC) method and a systematic analysis of the interface characteristics including the bonding state, element transition, microstructure evolution and tensile strength is carried out. It illustrates that an appropriate smelting power is beneficial to obtain a metallurgical bonding interface. Based on the temperature variation characteristics of the composite system, the bonding state of the bimetals (interface) changes gradually from entrapped slag defect to metallurgical bonding at the early stage of ESRC process, and the widths of elements transition and heat-affected zone (HAZ) become proportional to the composite height. It has an obvious influence on the grain size and precipitated phase at bimetallic interface. Tensile test results on both as-cast and annealed samples prove that the bimetallic interface is not the weakest zone as the fracture occurred at the roll core (45 carbon steel) side. In addition, an appropriate isothermal spheroidization annealing treatment is beneficial to refine the austenite grains and optimize the microstructure of the composite billet.
A composite billet is manufactured by the new electroslag remelting cladding (ESRC) technology and a systematic analysis of the bimetallic interface characteristics is carried out in the present study. As the temperature of the composite system increases with the increase of composite height, a better surface quality of the cladding layer is obtained and the bonding stage of the bimetallic interface changes gradually from entrapped slag defect to metallurgical bonding at the early stage of ESRC process. Then, the cladding process of the bimetals reaches a basically steady state, as a result, a uniform bonding interface with no inclusions, shrinkages and cracks between the bimetals and a certain width of heat affected zone are obtained.
Rolls are the significant part of rolling mills and always operated under high impacts of alternating loads; therefore, the high strength and toughness are necessary for the rolls to bear heavy loads and impact forces as well as the high degree of hardness and abrasion resistance to improve their durability in the service process.1) For the aims of meeting the demands of high performance rolls, the bimetallic composite rolls consisting of two different metals as the roll core and composite layer respectively have been developed. At present, the centrifugal foundry (CF)2–4) and continuous pouring process for cladding (CPC)5–7) are the main preparation methods of composite roll. Compared with the CF, CPC process has a higher solidification rate of the outer layer and can use a strong and tough steel base material as the roll core,8) so the CPC rolls always show high performance and have been widely used in the rolling mills. Similar to the CPC process, the electroslag surfacing with liquid metal (ESSLM)9,10) is developed and it can also use a forged or cast steel as the core and use the liquid metal to form the outer layer. It should be noted that an accurate control of the pouring rate and temperature of the liquid metal is required for both the CPC and ESSLM methods. It is difficult and increases the process complexity. In order to have a further improvement in the metal’s purity and solidification quality of the outer layer, various electroslag remelting (ESR) technologies are used to produce composite roll. Different from the rotational ESR,11) an innovative electroslag remelting cladding (ESRC)12) has been developed by authors based on the advantages of current supplying mold (CSM)13) which can better control the temperature distributions in slag bath and bimetallic interface between the outer layer and roll core by adjusting the conductive circuits.
Bimetallic interface is an important part of the composite billet, analyses of its metallurgical characteristics are of great significance for the deep understanding of the preparation process and the optimization design of its comprehensive performance.14) For the new ESRC method, temperature distributions for the composite system have been analyzed by steady-state13) and transient15) simulations respectively and composite billets of GCr15/45 carbon steel and high speed steel/ductile cast iron16) have been produced. However, only few researches on bimetallic interface have been carried out and that are not enough to reveal its metallurgical characteristics in detail. In this study, an investigation on the bimetallic interface characteristics of GCr15/45 carbon steel composite billet manufactured by the ESRC method was carried out through numerical simulations and experiments.
The schematic diagram of the ESRC method is displayed in Fig. 1 and it includes three main stages of slag melting, slag pouring, and bimetallic cladding. In the ESRC process, the consumable electrode can be either tubular or multi-rod shaped and 20 rod shaped electrodes with the uniform circumferential distribution are used in the present study.
Cutaway view of the new ESRC method of withdrawing process.
During the ESRC process, a series of measures were considered to control the interface temperature and promote the metallurgical bonding of the bimetals, such as machining the roll core surface to remove rust before the experiment, adding the deoxidizer to deoxidize during the cladding process and giving a solidification feeding at the end of the experiment. The essence of the cladding process is that the electrodes were melted by the high-temperature slag bath and then formed the composite layer around the preheated roll core. In the present study, the GCr15 steel of 1.02C–0.26Si–0.34Mn–1.49Cr (mass%) and 45 carbon steel of 0.45C–0.21Si–0.68Mn–0.07Cr (mass%) are used as the composite layer and roll core respectively. A pair of metal level sensors are used to match the pumping speed of composite billet with the melting rate of electrodes to obtain a stable ESRC process. Figure 2 shows the macromorphology of GCr15/45 carbon steel composite billets and their cross sections under different process conditions. The process parameters of the three cases marked as I, II and III are shown in Table 1.
Composite billets produced with different process conditions.
According to the previous simulation results of the ESRC process,13) the highest temperature in the slag bath is located in the area between electrode and current supplying mold and it is mainly influenced by the active power of the slag bath and each process parameter. Changes of roll core diameter and distance between roll core and electrode result in the variations of slag bath shape and resistance as well as the distributions of current density and Joule heat, which have obvious influences on the temperature distribution in slag bath and the bonding state of the bimetals. As the active power of the slag bath has a direct relation with the apparent power outputs of transformer (easy to obtain), the latter values are given in Table 1. The experimental results in Fig. 2 illustrate that the melting power is the most significant factor to the bonding quality of the bimetals and surface quality of the composite layer. The larger melting power, the higher temperature of the slag bath, as a result, the heat transfers from slag bath to roll core and molten metal pool increase obviously which are beneficial to promote the bonding state of bimetallic interface from unbound state (entrapped slag or cracks defects) to metallurgical bonding and improve the solidification quality and surface quality of the composite layer (from shrinks with surface wrinkle to even and dense with smooth surface). But an excessive melting power will lead to a high enough interface temperature and an excessive melting of roll core surface which also results in an obvious composition mixing of the bimetals, that are harmful to obtain the uniform microstructure and performance of the bimetallic interface and composite layer. Here, with the appropriate process parameters, the metallurgical bonding interface with no cracks, porosity and shrinks can be obtained as shown in Fig. 2(c).
In order to give a systematic investigation on the interface characteristics of GCr15/45 carbon steel composite billet, that of the case-III is used in this work. The anatomical diagrams, macroscopic feature, microstructures of the composite billet and its bimetallic interface are exhibited in Fig. 3. As shown in Fig. 3(a) and (b), the composite height of the cladding layer, diameter of the roll core and thickness of the composite layer are 320 mm, 240 mm and 50 mm respectively. Figure 3(d) and (e) illustrate the corresponding microstructures of the samples taken from the cross-section (Fig. 3(b)) and longitudinal section (Fig. 3(c)) of the composite billet.
Morphologies (a), (b), (c) and microstructures (d), (e) of the composite billet and bimetallic interface.
Figure 3(a) reveals that the surface quality of composite billet is improved gradually with the increasing composite height. An unsmooth surface with thick slag shell is formed in the lower part of the billet, whereas a relatively smooth surface with thin slag shell occurs in the upper part. Therefore, it can be inferred that the process characteristics of ESRC are very similar to that of the ESR17) which is gradually heating up at the early stage and then tends to be stable.
Figure 3(c) expresses that an entrapped slag defect exists in the lower part of composite billet, whereas a good metallurgical bonding occurs in the upper part. Although its existence and the adverse influences on the properties of composite billet can be eliminated through some machining, it increases the machining costs and reduces the productivity. Figure 3(b) confirms that no entrapped slag, shrinkage cavity, cracks, and other metallurgical defects are existed at the bimetallic interface. For the cladding process, it is worth noting that different process parameters will lead to the different bonding states of the bimetallic interface which are closely related to the bonding temperature and time of the bimetals.18)
3.2 Entrapped slag formation at bimetallic interfaceIn order to reveal the formation mechanism of entrapped slag defect at bimetallic interface in the early stage of ESRC process, a transient simulation was carried out by ANSYS software based on the “birth and death element” method. Only a 2D half of the actual model was used due to its axisymmetric nature, and the computation region includes the roll core, slag bath, thin steel sheet, and alumina powder as shown in Fig. 4. The alumina powder was used to prevent the lower part of roll core from being heated by high temperature slag bath which would be used as the roll neck. Distances from the proposed research points at bimetallic interface to the bottom of the slag bath (slag/steel interface in Fig. 4(b)) were marked as h1 (10 mm), h2 (20 mm), h3 (40 mm), h4 (60 mm), and h5 (80 mm).
Geometric model (a) and finite element mesh (b) in ESRC system.
For the ESRC experiment, about 37 s was used to pour the liquid slag into the gap between the mold and roll core, then the consumable electrodes were lowered and inserted into the slag bath to connect the conductive circuit for starting the ESRC process in which 10 s was used. The pouring temperature of liquid slag and the initial temperature of roll core, steel sheet and alumina powder were set to 1893 K and 293 K, respectively. The average growth rate of the slag cell was calculated based on the slag quantity and the pouring time. The cells size of the computation region was 0.0025 m with the shape of quadrangle. The liquidus temperature of the slag was 1655 K (calculated by FactSage software) and it was used to judge whether the molten slag was solidified or not. Figure 5 displayed the temperature distributions at different times during the slag pouring process.
Temperature distributions at 0.2 s (a), 10 s (b), 37 s (c) and 47 s (d) during the slag pouring process.
Figure 5 reveals that the liquid slag poured into the gap is cooled rapidly by the water-cooled mold and roll core. Hence, slag solidification mainly occurs in the lower part of the slag bath after the slag pouring process (t = 37 s) (Fig. 5(c)). Before the conductive circuit is connected at t = 47 s, most of the slag bath has been solidified due to the continuous decrease of the slag temperature (Fig. 5(d)). Figure 6 exhibits the temperature of different research points (marked in Fig. 4) varying with time at bimetallic interface. For a research point, its temperature changes significantly after getting in touch with the liquid slag and subsequently drops to 1200 K within 5∼10 s, thus indicating the occurrence of slag solidification. At t = 47 s, the temperatures of the research points at h = 10 mm, 20 mm, 40 mm, 60 mm, and 80 mm were 857 K, 948 K, 1073 K, 1136 K, and 1153 K, respectively.
Temperature variations of the different research points during the slag pouring process.
After the conductive circuit is connected at t = 47 s, a quick increase of the slag temperature occurs due to the generation of Joule heat in the slag bath. However, because of the lower electrical and thermal conductivity of the solidified slag19,20) and the characteristics of high temperatures mainly distributed at the upper part of the slag bath (simulation results),13) it is very difficult to remelt the solidified slag at the lower part and the interface areas in a short time. Consequently, consumable electrodes are melted and solidified as composite layer and there is not enough time for the solidified slag to remelt and rise, thereby an entrapped slag defect is formed at bimetallic interface of the composite billet (Fig. 3(c)). Therefore, preheating the roll core effectively before the melting of the consumable electrodes (cladding process) is very necessary to reduce the entrapped slag defect and improve the interface quality.
3.3 Microstructural evolution at bimetallic interfaceFigure 7 illustrates the microstructural evolution in the sample taken from the end of the entrapped slag defect as shown in Fig. 3(c). The grain size in the roll core (45 carbon steel) side of Fig. 7(a) is significantly smaller than that of Fig. 7(b) and (c). On the one hand, the temperatures of the interface and roll core surface in Fig. 7(a) are lower than that of Fig. 7(b) and (c), on the other hand, the appearance of the entrapped slag defect prevents the heat transfer from composite layer to roll core. However, after the disappearance of the entrapped slag defect, direct contact between composite layer and roll core occurred and the heat transfer from high-temperature outer material to roll core was strengthened sharply; hence, the high-temperature austenitization at the roll core surface caused a noticeable increase in grain size. With the ongoing cladding process, the continuous increase in the composite system temperature further facilitated the austenitization of the roll core surface. In this study, the grain coarsening zone is named as the heat-affected zone (Fig. 3(d) and (e)). After the disappearance of the entrapped slag defect, the width of the heat-affected zone became proportional to the composite height (Fig. 3(e)) and finally reached 3.57 mm (Fig. 3(d)) where the ESRC process has attained a steady state.
Microstructure of the bimetallic interface with the entrapped slag defect.
The scanning electron microscopy (SEM) was used to examine the bonding state of the bimetals at the end of the entrapped slag defect. Figure 8 illustrated that numerous tiny holes were formed between the solidified slag and the metal matrix, when the entrapped slag defect disappeared, a good metallurgical bonding with no cracks between the bimetals was obtained. The microstructure of the matrix that was in contact with the solidified slag was mainly composed of fine pearlites. The elements migration and diffusion behavior were investigated through the line scan analysis (marked as line 1 in Fig. 3(e)).
Bonding state and microstructure of the bimetallic interface with the entrapped slag defect.
Figure 9 displays the microstructure of the composite sample taken from the cross-section of the composite billet (Fig. 3(b)). It illustrates that the microstructure of the heat-affected zone on the roll core side was mainly composed of lamellar pearlites and few reticular proeutectoid ferrites (Fig. 9(c), (d) and 3(d), (e)). On the 45 carbon steel side, when it was far away from the heat-affected zone, the grain size decreased significantly due to temperature changes and the proeutectoid ferrite content increased greatly due to the migration of elements (especially C) between the bimetals.
Microstructure of the composite layer side (a), (b) and roll core side (c), (d) at the interface.
At bimetallic interface, the microstructure of the composite layer side (GCr15 steel) was more uniform and finer than that of the roll core side (45 carbon steel) and both sides were mainly composed of lamellar pearlites. Unlike 45 carbon steel, few carbides were formed at the grain boundary of GCr15 steel due to its higher carbon content. On the GCr15 side, the liquid metal had a rapid cooling rate due to the water-cooled mold, thus finer pearlites with a smaller lamellar spacing were obtained.
3.4 Migration of elements at bimetallic interfaceThe migration behavior of elements at bimetallic interface manifests direct effects on the microstructure and mechanical properties of composite billet. The Cr concentration distribution at the interface was analyzed by energy dispersive spectroscopy (EDS), and the corresponding results are displayed in Fig. 10 (yellow lines represent the positions of the line scan analyses, black lines signify the measured Cr concentration fluctuations, and red lines denote the detected concentration curves after the smoothing treatment). In addition, the widths of the Cr concentration transition zone at different analysis positions (line 1, line 2, line 3 and line 4 in Fig. 3(d) and (e)) of the bimetallic interface are also marked in Fig. 10.
Concentration of Cr at different analysis positions of bimetallic interface.
The widths of the Cr concentration transition zone for lines 1, 2, 3, and 4 were evaluated as 137 µm, 610 µm, 840 µm, and 1140 µm, respectively. Obviously, the width of the Cr concentration transition zone became proportional to the composite height after the disappearance of the entrapped slag defect at bimetallic interface due to the better thermodynamic and kinetic conditions for the migration and diffusion of elements. Further, the smooth and continuous change in Cr concentration at the interface proves a perfect metallurgical bonding between GCr15 and 45 carbon steel during the ESRC process.
The migration and diffusion of elements at bimetallic interface are mainly affected by the temperature, crystal structure, crystal defects, solid solution type, and the chemical composition.21) As the diffusion activation energy of interstitial C atom is smaller than that of substitutional Cr atom, the diffusion distance of C atom is much larger than that of Cr atom under the same conditions. According to the research results of elements (Cr and C) diffusion behavior22) and the widths of Cr element in Fig. 10, the diffusion distance of C atoms must exceed the width of the heat-affected zone. Hence, the C concentration in the matrix of the 45 carbon steel side was inversely proportional to the distance from the bimetallic interface. For a hypoeutectoid steel, the percentage of proeutectoid ferrites always decreases with the increasing C content; therefore, a continuous decrease of the proeutectoid ferrite content was noticed in the heat-affected zone and other radial regions of the roll core in this study.
3.5 Isothermal spheroidization annealing at the bimetallic interfaceBased on the previous report by authors,23) the coarse grains and harmful ferritic Widmanstatten structure were observed in the heat-affected zone of the as-cast GCr15/45 carbon steel composite samples. In order to eliminate the above undesirable microstructure, an isothermal spheroidization annealing treatment was carried out. The samples were first austenitized at 1063 K for 5 h and then cooled down to 983 K and kept for 2.5 h at this temperature. The microstructure around the bimetallic interface after the annealing treatment are exhibited in Fig. 11.
Microstructure of the bimetallic interface after the annealing treatment. (a), (d), (g)-GCr15 steel side; (b), (e), (h)-bimetallic interface; (c), (f), (j)–45 carbon steel side.
For the hypoeutectoid steel, the amount of proeutectoid ferrite precipitated from the austenite grain is closely associated with the cooling process. Longer stay time at high temperature facilitates more proeutectoid ferrites to be precipitated. As the isothermal temperature was only 983 K and a lower cooling rate in this work, a large amount of flaky-shaped proeutectoid ferrites precipitated at bimetallic interface as shown in Fig. 11(b) and (e), it also results in the elimination of coarse grains and the ferritic Widmanstatten structure. Moreover, the proeutectoid ferrite content in the transition zone of the bimetallic interface was significantly lower than that in the roll core side due to a higher C content in the transition zone (Fig. 11(e)). In addition, the microstructure of GCr15 steel changed from lamellar pearlites and reticulated carbides to spherical pearlites (uniform and fine spheroidal carbides were distributed on the ferrite matrix) as shown in Fig. 11(a), (d), and (g).
3.6 Tensile testing of the interface specimensThe tensile tests for the samples at different state were carried out with a tensile speed of 1 mm·min−1. The tensile strength of the annealed sample (590 MPa) is lower than that (661 MPa) of the as-cast sample but an obvious plastic deformation and yielding phenomenon occurred for the former one. Moreover, the fractions of section shrinkage and elongation of the annealed sample were 29.6% and 24.0%, respectively, whereas the values were 13.5% and 12.8%, respectively for the as-cast sample.
Figure 12 displays the morphologies and microstructures of tensile fracture for both as-cast and annealed samples. Both samples ruptured in the roll core side rather than at the bimetallic interface; hence, it implies that the bimetallic interface wasn’t the weakest zone during tensile testing. Further, the pearlite and ferrite deformation along the tensile direction greatly improved the plasticity of the annealed sample (Fig. 12(c)), and this phenomenon can effectively resist the impact load and reduce the occurrence of roll breakage accident during its service process.
Morphologies (a) and microstructures (b), (c) of the tensile fracture for the different samples.
The fracture morphologies of both as-cast and annealed samples were observed by field-emission scanning electron microscopy (FESEM) (Fig. 13). It was evident that both kinds of samples mainly presented ductile fracture with numerous dimples, in addition, a partial cleavage fracture morphology (river patterns) was also noticed in the as-cast sample (Fig. 13(d)). As a whole, the dimple numbers and uniformity in the annealed sample were much more and better than that of the as-cast sample, while, the dimple sizes were smaller in the annealed sample. As the size, depth, number and uniformity of the dimples always reflected the plasticity of the material, the annealed sample had a better plasticity than the as-cast sample and the latter one presented the mixed features of both ductile and brittle fracture with the nonuniform plasticity and toughness.
Fracture morphologies of the as-cast (a), (c), (d), (e) and annealed (b) samples after tensile tests.
During the ESRC process, influenced by the high-temperature austenitization occurred in the roll core side, coarse austenite grains were generated especially in the heat-affected zone near the bimetallic interface. In this condition, with a certain carbon content and an appropriate cooling rate, some of the proeutectoid ferrite grew into austenite grains from the grain boundary along a certain austenite crystal surface and precipitated as acicular ones, and the composite of this parallel acicular ferrites and lamellar pearlites is named as the Widmanstatten structure.21) For the GCr15/45 carbon steel composite billet, few Widmanstatten structures were always generated in the heat-affected zone of the roll core.23) The Widmanstatten structure is always considered as an overheating defect, its existence generally results in a serious separation of the matrix and reduces the material’s toughness and plasticity. Since to the large angle grain boundary has a certain blocking effect on the crack growth, and the fact of the larger grain size, the less grain boundary, so few obstacles are encountered during the crack growth, and finally, a higher brittleness performance is obtained. After the isothermal spheroidization annealing treatment, the coarse grains and ferrite Widmanstatten structures in the roll core side were eliminated, in addition, an increasing content of proeutectoid ferrite and a decreasing content of pearlite were obtained, so the plasticity and toughness had been effectively promoted while a certain reduction of the tensile strength occurred.
The elemental composition of particles in the dimples was examined by EDS (Fig. 14). It is discernible from Fig. 14(b) and (c) that the detected dimples were mainly composed of spherical or elongated (Mn,Fe)S inclusions, and larger (Mn,Fe)S inclusions corresponded to the larger dimples (Fig. 14(a)). Under the action of stress, the micropores were always generated at the separation interface between the particles (precipitation phases or inclusions) and the metallic matrix due to their elastoplastic difference or the cracks occurred due to the fracture of inclusions. Under the action of external forces, these micropores continued to grow and expand, and the matrix cross-section between adjacent micropores continued to shrink until they were joined to form dimples. With the increasing plastic deformation intensity, the fracture occurred.
Dimple morphology (a) and composition analyses for the particles (b), (c) in the dimples.
In the present work, a GCr15/45 carbon steel composite billet has been manufactured by the new ESRC method and the metallurgical characteristics of its bimetallic interface has been also analyzed in detail.
The liquid slag poured into the gap between the roll core and the mold was cooled rapidly by these two parts, thus leading to the solidification of the slag. Due to the low electrical and thermal conductivity of the solidified slag and the characteristics of high temperature distributed in the upper part of the slag bath, a long time was required to improve the temperature of the entire composite system and eliminate the entrapped slag defect at the bimetallic interface even after the connection of conductive circuit. After the disappearance of the entrapped slag defect, a good metallurgical bonding was obtained between the bimetals.
After the disappearance of entrapped slag defect at bimetallic interface, direct contact between the bimetals occurred and the widths of elements migration and the heat-affected zone became proportional to the composite height. The widths of Cr migration at bimetallic interface and the heat-affected zone in the roll core side were 1140 µm and 3.57 mm, respectively. With the increasing temperature and elements migration (especially C), the grain size and the proeutectoid ferrite content in the heat-affected zone increased greatly.
The appropriate isothermal spheroidization annealing treatment can effectively eliminate the coarse grains and the harmful Widmanstatten structure in the heat-affected zone of the as-cast sample and improve its plasticity and toughness. In addition, the fact of tensile fractures occurred at the roll core side rather than the bimetallic interface proves that a good metallurgical bonding has been obtained for the composite billet produced by the ESRC method.
The authors are grateful to the China Postdoctoral Science Foundation (2019M652720) for support of this research.
