Simultaneously Enhanced Strength , Toughness and Ductility of Cast 40 Cr Steels Strengthened by Trace Biphase TiC x-TiB 2 Nanoparticles

Simultaneously improving the strength, toughness, and ductility of cast steels has always been a difficult problem for researchers. Biphase TiCx-TiB2 nanoparticle-reinforced cast steels are prepared by adding in situ nanosized biphase TiCx-TiB2/Al master alloy during the casting process. The experimental results show that a series of significant changes take place in the microstructure of the steel: the ferrite-pearlite structure of the as-cast steels and the bainite structure of the steels after heat treatment are refined, the grain size is reduced, and the content of nanoparticles is increased. Promotion of nucleation and inhibition of dendrite growth by biphase TiCx-TiB2 nanoparticles leads to a refinement of the microstructure. The fine microstructure with evenly dispersed nanoparticles offers better properties [yield strength (1246 MPa), tensile strength (1469 MPa), fracture strain (9.4%), impact toughness (20.3 J/cm2) and hardness (41 HRC)] for the steel with 0.018 wt.% biphase TiCx-TiB2 nanoparticles, which are increased by 15.4%, 31.2%, 4.4%, 11.5%, and 7.9% compared with the 40Cr steels. The higher content of nanoparticles provides higher strengths and hardness of the steel but are detrimental to ductility. The improved properties may be attributed to fine grain strengthening and the pinning effect of nanosized carbide on dislocations and grain boundaries. Through this work, it is known that the method of adding trace (0.018 wt.%) biphase TiCx-TiB2 nanoparticles during casting process can simultaneously improve the strength, toughness, as well as ductility of the cast steel.


Materials and Methods
The main chemical composition of investigated 40Cr steel is displayed in Table 1.The nanosized TiC x -TiB 2 /Al master alloy was prepared through self-propagating high-temperature synthesis.First, Al (~13 µm), Ti (~25 µm), and B 4 C (~1.5 µm) powders at a ratio corresponding to that of stoichiometric TiC and TiB 2 were sufficiently mixed in a ball mill at a speed of 50 rpm for 18 h.Second, the sufficiently mixed powders were cold pressed into cylindrical compacts (Φ = 28 mm; h = 30 mm).Finally, a self-made vacuum thermal explosion furnace was used to initiate the self-propagating high-temperature synthesis (SHS) reaction in the compacts at a heating speed of about 30 K/min.and then at 900 • C for 10 min.After the reaction was completed, the nanosized TiC x -TiB 2 /Al master alloy was successfully produced.The molten steel melted in 5 kg-capacity medium frequency furnace (GW-0.3t,NHHY, Foshan, China) was poured into a ladle, in which the master alloy had been placed.The molten steel with the ceramic particles dispersed throughout was cast into the sand mold and then cooled to room temperature.The steels were kept at 950 • C for 90 min and oil quenched.Afterwards, the steels were kept at 200 • C for 480 min and air cooled to room temperature, as shown in Figure 1.The chemical composition of the investigated alloy was determined by direct-reading spectrometer (CX-9000, GLMY, WuXi, China).The ceramic particles extracted from the master alloy were characterized by X-ray diffraction (XRD, D/Max 2500PC Rigaku, Japan) and by field emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Tokyo, Japan).Samples after heat treatment were cut into dog-bone shaped tensile specimens (the specimens had a gauge length of 8 mm and cross-sectional area of 2.5 mm × 2 mm) and Charpy impact specimens (10 × 10 × 55 mm, U-notch).The samples used for optical microscopy (OM, Olympus PMG3, Tokyo, Japan) were polished and etched with a 5% nitric acid and 95% alcohol mixture.The samples designated for electron backscatter diffraction (EBSD, Oxford NordlysMax EBSD, Oxford, UK) analysis were electropolished in a mixture of 20% perchloric acid (Beijing Chemical Works, Beijing, China) and 80% acetic acid (Beijing Chemical Works, Beijing, China) at 30 V for 15 s.0.3 µ m was chosen as step size of EBSD scans.The TEM microstructure was acquired by using a JEOL microscope (JEM 2100F, JEOL, Tokyo, Japan).The tensile tests were performed on a servo hydraulic materials testing system (MTS 810, MTS Systems Corporation, Minneapolis, MN, USA).The standard Charpy impact test was conducted using a semiautomatic impact tester (JBW-300B, JNKH, Jinan, China).The fracture surfaces of the tensile and impact specimens were observed by means of scanning electron microscopy (SEM, VEGA 3 XMU, TESCAN, Brno, Czech Republic).

Results and Discussion
Figure 2a presents the XRD pattern of the nanosized biphase TiCx-TiB2/Al master alloy, which shows that the master alloy mainly contains α-Al, TiC, TiB2 and a small amount of Al3Ti.It can be determined that the SHS reaction is relatively complete, with only a small amount of Al3Ti remaining.Figure 2b is a FESEM image of the mixed particles of TiC and TiB2 obtained by extracting the nanosized TiCx-TiB2/Al master alloy with hydrochloric acid.It can be seen that in addition to the spherical TiC particles, there are also hexagonal prisms or lamellar TiB2 particles.The size statistics of the TiCx and TiB2 mixed particles in Figure 2b shows that the average grain size of the mixed particles is 190 nm.The nanoparticles are separated by aluminum, with no significant agglomeration with one another (Figure 2c,d).Then, the nanosized TiCx-TiB2/Al master alloy was melted and gradually dispersed in the molten steel.The aluminum in the master alloy can also be simultaneously used as an oxygen scavenger to react with oxygen at a high temperature to form Al2O3.Then, the Al2O3 is removed during the smelting process.This method not only successfully introduces nanoparticles into the molten steel but also contributes to the uniform dispersion of nanoparticles and to the removal of oxygen from the steel.The chemical composition of the investigated alloy was determined by direct-reading spectrometer (CX-9000, GLMY, WuXi, China).The ceramic particles extracted from the master alloy were characterized by X-ray diffraction (XRD, D/Max 2500PC Rigaku, Japan) and by field emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Tokyo, Japan).Samples after heat treatment were cut into dog-bone shaped tensile specimens (the specimens had a gauge length of 8 mm and cross-sectional area of 2.5 mm × 2 mm) and Charpy impact specimens (10 × 10 × 55 mm, U-notch).The samples used for optical microscopy (OM, Olympus PMG3, Tokyo, Japan) were polished and etched with a 5% nitric acid and 95% alcohol mixture.The samples designated for electron backscatter diffraction (EBSD, Oxford NordlysMax EBSD, Oxford, UK) analysis were electropolished in a mixture of 20% perchloric acid (Beijing Chemical Works, Beijing, China) and 80% acetic acid (Beijing Chemical Works, Beijing, China) at 30 V for 15 s.0.3 µm was chosen as step size of EBSD scans.The TEM microstructure was acquired by using a JEOL microscope (JEM 2100F, JEOL, Tokyo, Japan).The tensile tests were performed on a servo hydraulic materials testing system (MTS 810, MTS Systems Corporation, Minneapolis, MN, USA).The standard Charpy impact test was conducted using a semiautomatic impact tester (JBW-300B, JNKH, Jinan, China).The fracture surfaces of the tensile and impact specimens were observed by means of scanning electron microscopy (SEM, VEGA 3 XMU, TESCAN, Brno, Czech Republic).

Results and Discussion
Figure 2a presents the XRD pattern of the nanosized biphase TiC x -TiB 2 /Al master alloy, which shows that the master alloy mainly contains α-Al, TiC, TiB 2 and a small amount of Al 3 Ti.It can be determined that the SHS reaction is relatively complete, with only a small amount of Al 3 Ti remaining.Figure 2b is a FESEM image of the mixed particles of TiC and TiB 2 obtained by extracting the nanosized TiC x -TiB 2 /Al master alloy with hydrochloric acid.It can be seen that in addition to the spherical TiC particles, there are also hexagonal prisms or lamellar TiB 2 particles.The size statistics of the TiC x and TiB 2 mixed particles in Figure 2b shows that the average grain size of the mixed particles is 190 nm.The nanoparticles are separated by aluminum, with no significant agglomeration with one another (Figure 2c,d).Then, the nanosized TiC x -TiB 2 /Al master alloy was melted and gradually dispersed in the molten steel.The aluminum in the master alloy can also be simultaneously used as an oxygen scavenger to react with oxygen at a high temperature to form Al 2 O 3 .Then, the Al 2 O 3 is removed during the smelting process.This method not only successfully introduces nanoparticles into the molten steel but also contributes to the uniform dispersion of nanoparticles and to the removal of oxygen from the steel.The typical ferrite-pearlite hybrid structure of the medium-carbon steel can be seen in Figure 3a.The bright area is pro-eutectoid ferrite, and the dark area is pearlite [24][25][26].Because the carbon content of the steel is less than 0.77 wt.%, ferrite will be precipitated along the austenite grain boundary before the pearlite transformation.Therefore, the long strip ferrite is distributed on the boundary of the coarser pearlite group.After adding the 0.018 wt.% nanoceramic particles, it can clearly be seen that instead of the strip-shaped pro-eutectoid ferrite, the small pieces of ferrite dispersed inside the pearlite group increase, and the size of the pearlite group is also reduced (Figure 3b).When the content of nanoparticles in the steel is increased to 0.054 wt.%, smaller ferrite in the structure is distributed uniformly, and the pearlite structure is further refined Figure 3c.It is known from the structure observed under the optical micrographs that the microstructure of the steel after heat treatment mainly contains lower bainite and a small amount of residual ferrite.The unevenly distributed particles with a large number of large white blocks can be seen in Figure 3d.In contrast, the lath ferrite and carbide in the microstructure of the steel, to which the 0.018 wt.% nanoparticles are added, are significantly finer and more uniform.More lath ferrites and carbides were observed in Figure 3f, but the lath ferrite is thicker than that in Figure 3e.The typical ferrite-pearlite hybrid structure of the medium-carbon steel can be seen in Figure 3a.The bright area is pro-eutectoid ferrite, and the dark area is pearlite [24][25][26].Because the carbon content of the steel is less than 0.77 wt.%, ferrite will be precipitated along the austenite grain boundary before the pearlite transformation.Therefore, the long strip ferrite is distributed on the boundary of the coarser pearlite group.After adding the 0.018 wt.% nanoceramic particles, it can clearly be seen that instead of the strip-shaped pro-eutectoid ferrite, the small pieces of ferrite dispersed inside the pearlite group increase, and the size of the pearlite group is also reduced (Figure 3b).When the content of nanoparticles in the steel is increased to 0.054 wt.%, smaller ferrite in the structure is distributed uniformly, and the pearlite structure is further refined Figure 3c.It is known from the structure observed under the optical micrographs that the microstructure of the steel after heat treatment mainly contains lower bainite and a small amount of residual ferrite.The unevenly distributed particles with a large number of large white blocks can be seen in Figure 3d.In contrast, the lath ferrite and carbide in the microstructure of the steel, to which the 0.018 wt.% nanoparticles are added, are significantly finer and more uniform.More lath ferrites and carbides were observed in Figure 3f, but the lath ferrite is thicker than that in Figure 3e.The EBSD method was used to clarify the quantitative degree of grain refinement, as shown in Figure 4.The average grain size is refined from 9.12 μm in the 40Cr steel to 3.59 μm in the steel with 0.018 wt.% TiCx-TiB2 nanoparticles.The grain size of the steels with trace nanosized TiCx-TiB2 particles was refined by 60.6%.More importantly, the nanoparticles not only changed the cast microstructure of the steel but also refined the grain size of the steel after heat treatment.The TEM images in Figure 5 show the differences in microstructures between the steels without nanosized TiCx-TiB2 particles and the 0.018 wt.% TiCx-TiB2 nanoparticle-reinforced steel.It is determined that a large number of carbide particles, of which the size is less than 100 nm, are evenly distributed in the microstructure of the 0.018 wt.% TiCx-TiB2 nanoparticle-reinforced steel in Figure 5b.On the contrary, a smaller amount of carbide particles is found in the microstructure of the steels without nanosized TiCx-TiB2 particles (Figure 5a).The EBSD method was used to clarify the quantitative degree of grain refinement, as shown in Figure 4.The average grain size is refined from 9.12 µm in the 40Cr steel to 3.59 µm in the steel with 0.018 wt.% TiC x -TiB 2 nanoparticles.The grain size of the steels with trace nanosized TiC x -TiB 2 particles was refined by 60.6%.More importantly, the nanoparticles not only changed the cast microstructure of the steel but also refined the grain size of the steel after heat treatment.The EBSD method was used to clarify the quantitative degree of grain refinement, as shown in Figure 4.The average grain size is refined from 9.12 μm in the 40Cr steel to 3.59 μm in the steel with 0.018 wt.% TiCx-TiB2 nanoparticles.The grain size of the steels with trace nanosized TiCx-TiB2 particles was refined by 60.6%.More importantly, the nanoparticles not only changed the cast microstructure of the steel but also refined the grain size of the steel after heat treatment.The TEM images in Figure 5 show the differences in microstructures between the steels without nanosized TiCx-TiB2 particles and the 0.018 wt.% TiCx-TiB2 nanoparticle-reinforced steel.It is determined that a large number of carbide particles, of which the size is less than 100 nm, are evenly distributed in the microstructure of the 0.018 wt.% TiCx-TiB2 nanoparticle-reinforced steel in Figure 5b.On the contrary, a smaller amount of carbide particles is found in the microstructure of the steels without nanosized TiCx-TiB2 particles (Figure 5a).The TEM images in Figure 5 show the differences in microstructures between the steels without nanosized TiC x -TiB 2 particles and the 0.018 wt.% TiC x -TiB 2 nanoparticle-reinforced steel.It is determined that a large number of carbide particles, of which the size is less than 100 nm, are evenly distributed in the microstructure of the 0.018 wt.% TiC x -TiB 2 nanoparticle-reinforced steel in Figure 5b.On the contrary, a smaller amount of carbide particles is found in the microstructure of the steels without nanosized TiC x -TiB 2 particles (Figure 5a).TiCx and TiB2 have been proven to be effective austenitic heterogeneous nucleation sites [2, 10,12].Nanoparticles are successfully introduced and dispersed into the liquid steel herein.As shown in Figure 6, because structural undulations and energy fluctuations of the heterogeneous nucleation are small, nucleation occurs preferentially on the nanoparticles.Usually, larger-sized nanoparticles act as the nucleation site, while smaller-sized nanoparticles adsorb onto the solid-liquid interface and hinder the transition of the interface, thus inhibiting the growth of the dendrite [27].The primary dendrites gradually grow, and finally, the material becomes completely austenitized.The size of the prior austenite grains seriously affects the later ferrite-pearlite structure and the bainite structure [28].The increased boundaries due to austenite grain refinement and dispersed nanoparticles provide more precipitation sites for pro-eutectoid ferrite, which avoid the formation of long ferrite and limit the growth of pearlite.The most important process when the bainite transformation occurs is the diffusion of carbon.The rapid diffusion channels and thermal mismatches produced by the grain boundaries and nanoparticles promote the diffusion of carbon and the nucleation of carbides, so there are a large number of dispersed carbides throughout the structure.Figure 7 shows the tensile engineering stress-strain curves and fracture surfaces of the 40Cr steels with different TiCx-TiB2 nanoparticle contents.It is not difficult to determine from Figure 7a that the yield strength and tensile strength of the steel with nanoparticles are greatly improved.In addition to a large increase in the yield strength and tensile strength compared to the 40Cr steel, the fracture strain of the steel containing 0.018 wt.% TiCx-TiB2 nanoparticles is also improved.The yield and tensile strengths of the steel with 0.054 wt.% nanoparticles added are further enhanced, but the TiC x and TiB 2 have been proven to be effective austenitic heterogeneous nucleation sites [2, 10,12].Nanoparticles are successfully introduced and dispersed into the liquid steel herein.As shown in Figure 6, because structural undulations and energy fluctuations of the heterogeneous nucleation are small, nucleation occurs preferentially on the nanoparticles.Usually, larger-sized nanoparticles act as the nucleation site, while smaller-sized nanoparticles adsorb onto the solid-liquid interface and hinder the transition of the interface, thus inhibiting the growth of the dendrite [27].The primary dendrites gradually grow, and finally, the material becomes completely austenitized.The size of the prior austenite grains seriously affects the later ferrite-pearlite structure and the bainite structure [28].The increased grain boundaries due to austenite grain refinement and dispersed nanoparticles provide more precipitation sites for pro-eutectoid ferrite, which avoid the formation of long ferrite and limit the growth of pearlite.The most important process when the bainite transformation occurs is the diffusion of carbon.The rapid diffusion channels and thermal mismatches produced by the grain boundaries and nanoparticles promote the diffusion of carbon and the nucleation of carbides, so there are a large number of dispersed carbides throughout the structure.TiCx and TiB2 have been proven to be effective austenitic heterogeneous nucleation sites [2, 10,12].Nanoparticles are successfully introduced and dispersed into the liquid steel herein.As shown in Figure 6, because structural undulations and energy fluctuations of the heterogeneous nucleation are small, nucleation occurs preferentially on the nanoparticles.Usually, larger-sized nanoparticles act as the nucleation site, while smaller-sized nanoparticles adsorb onto the solid-liquid interface and hinder the transition of the interface, thus inhibiting the growth of the dendrite [27].The primary dendrites gradually grow, and finally, the material becomes completely austenitized.The size of the prior austenite grains seriously affects the later ferrite-pearlite structure and the bainite structure [28].The increased grain boundaries due to austenite grain refinement and dispersed nanoparticles provide more precipitation sites for pro-eutectoid ferrite, which avoid the formation of long ferrite and limit the growth of pearlite.The most important process when the bainite transformation occurs is the diffusion of carbon.The rapid diffusion channels and thermal mismatches produced by the grain boundaries and nanoparticles promote the diffusion of carbon and the nucleation of carbides, so there are a large number of dispersed carbides throughout the structure.Figure 7 shows the tensile engineering stress-strain curves and fracture surfaces of the 40Cr steels with different TiCx-TiB2 nanoparticle contents.It is not difficult to determine from Figure 7a that the yield strength and tensile strength of the steel with nanoparticles are greatly improved.In addition to a large increase in the yield strength and tensile strength compared to the 40Cr steel, the fracture strain of the steel containing 0.018 wt.% TiCx-TiB2 nanoparticles is also improved.The yield and tensile strengths of the steel with 0.054 wt.% nanoparticles added are further enhanced, but the Figure 7 shows the tensile engineering stress-strain curves and fracture surfaces of the 40Cr steels with different TiC x -TiB 2 nanoparticle contents.It is not difficult to determine from Figure 7a that the yield strength and tensile strength of the steel with nanoparticles are greatly improved.In addition to a large increase in the yield strength and tensile strength compared to the 40Cr steel, the fracture strain of the steel containing 0.018 wt.% TiC x -TiB 2 nanoparticles is also improved.The yield and tensile strengths of the steel with 0.054 wt.% nanoparticles added are further enhanced, but the fracture strain is obviously decreased.From Table 2 one can observe that the yield strength, tensile strength, and fracture strain of the 40Cr steel are 1080 MPa, 1120 MPa and 9%, respectively.Compared with the 40Cr steel, the yield strength (1246 MPa), tensile strength (1469 MPa), and fracture strain (9.4%) of the steel containing 0.018 wt.% TiC x -TiB 2 nanoparticles are increased by 15.4%, 31.2% and 4.4%, respectively.The yield strength (1415 MPa) and tensile strength (1554 MPa) of the steel with 0.054 wt.% TiC x -TiB 2 nanoparticles are increased by 31% and 38.8% compared with the 40Cr steel, respectively.The fracture surfaces (Figure 7b-d) also reflect the tensile properties of the material to some extent.There are some dimples with uneven sizes on the tensile fracture of the 40Cr steel, which shows a ductile fracture.The dimples on the fracture surface of the steel with 0.018 wt.% nanoparticles are more obvious, and the size more uniform, but it is also a ductile fracture.However, there are fewer dimples on the fracture surface of the steel with 0.054 wt.% nanoparticles, and most of the areas have a rock or sugar-like morphology, which shows a brittle intergranular fracture.Owing to the increase of carbides inside and the refinement of grain size, hardness of steels with nanoparticles has increased, as Table 2  fracture strain is obviously decreased.From Table 2 one can observe that the yield strength, tensile strength, and fracture strain of the 40Cr steel are 1080 MPa, 1120 MPa and 9%, respectively.Compared with the 40Cr steel, the yield strength (1246 MPa), tensile strength (1469 MPa), and fracture strain (9.4%) of the steel containing 0.018 wt.% TiCx-TiB2 nanoparticles are increased by 15.4%, 31.2% and 4.4%, respectively.The yield strength (1415 MPa) and tensile strength (1554 MPa) of the steel with 0.054 wt.% TiCx-TiB2 nanoparticles are increased by 31% and 38.8% compared with the 40Cr steel, respectively.The fracture surfaces (Figure 7b-d) also reflect the tensile properties of the material to some extent.There are some dimples with uneven sizes on the tensile fracture of the 40Cr steel, which shows a ductile fracture.The dimples on the fracture surface of the steel with 0.018 wt.% nanoparticles are more obvious, and the size is more uniform, but it is also a ductile fracture.However, there are fewer dimples on the fracture surface of the steel with 0.054 wt.% nanoparticles, and most of the areas have a rock or sugar-like morphology, which shows a brittle intergranular fracture.Owing to the increase of carbides inside and the refinement of grain size, hardness of steels with nanoparticles has increased, as Table 2 lists.Figure 8 reflects the work-hardening ability of the steels with different TiCx-TiB2 nanoparticles contents.Combined with the analysis of Figure 7, it is determined that the yield strength of the 40Cr steel is low, the yielding occurs very quickly, and the work-hardening rate rapidly decreases after yielding.Compared with the 40Cr steel, the steel with 0.018 wt.% TiCx-TiB2 nanoparticles has a higher yield strength, and a higher work-hardening rate with the same plastic deformation after yielding.The steel with 0.054 wt.% TiCx-TiB2 nanoparticles has the highest yield strength, and the highest work-hardening rate, but its work-hardening rate is less than the steel with 0.018 wt.% nanoparticles  Figure 8 reflects the work-hardening ability of the steels with different TiC x -TiB 2 nanoparticles contents.Combined with the analysis of Figure 7, it is determined that the yield strength of the 40Cr steel is low, the yielding occurs very quickly, and the work-hardening rate rapidly decreases after yielding.Compared with the 40Cr steel, the steel with 0.018 wt.% TiC x -TiB 2 nanoparticles has a higher yield strength, and a higher work-hardening rate with the same plastic deformation after yielding.The steel with 0.054 wt.% TiC x -TiB 2 nanoparticles has the highest yield strength, and the highest work-hardening rate, but its work-hardening rate is less than the steel with 0.018 wt.% nanoparticles when the true strain at a value exceeding 4.5% as shown in Figure 8, and there is no buffering stage of the work-hardening rate similar to that of the steel with 0.018 wt.% TiC x -TiB 2 nanoparticles.when the true strain at a value exceeding 4.5% as shown in Figure 8, and there is no buffering stage of the work-hardening rate similar to that of the steel with 0.018 wt.% TiCx-TiB2 nanoparticles.
Figure 8.The corresponding work-hardening rate-true strain curves that were obtained from the tensile curve.
Figure 9 shows the impact properties and impact fracture morphologies of the steels with different contents of TiCx-TiB2 nanoparticles at room temperature.The impact toughness of steel with 0.018 wt.% TiCx-TiB2 nanoparticles is 11.5% higher than that of the 40Cr steel, but the impact toughness of the steel with 0.054 wt.% nanoparticles is reduced.Some of the reasons for this can be seen from the impact fracture surfaces in Figure 9.In the impact fracture surfaces of the 40Cr steel, the quasi-cleavage fracture is dominant, and there is a small intergranular fracture section.Except for a small amount of cleavage fracture, there are many dimples on the impact of the steel with 0.018 wt.% TiCx-TiB2 nanoparticles, indicating that the impact toughness is better than that of the 40Cr steel.The fracture of the steel with 0.054 wt.% TiCx-TiB2 nanoparticles is dominated by brittle intergranular fracture and some cracks, which are unfavorable for the toughness of the material.Figure 9 shows the properties and impact fracture morphologies of the steels with different contents of TiC x -TiB 2 nanoparticles at room temperature.The impact toughness of steel with 0.018 wt.% TiC x -TiB 2 nanoparticles is 11.5% higher than that of the 40Cr steel, but the impact toughness of the steel with 0.054 wt.% nanoparticles is reduced.Some of the reasons for this can be seen from the impact fracture surfaces in Figure 9.In the impact fracture surfaces of the 40Cr steel, the quasi-cleavage transgranular fracture is dominant, and there is a small intergranular fracture section.Except for a small amount of cleavage fracture, there are many dimples on the impact fracture of the steel with 0.018 wt.% TiC x -TiB 2 nanoparticles, indicating that the impact toughness is better than that of the 40Cr steel.The fracture of the steel with 0.054 wt.% TiC x -TiB 2 nanoparticles is dominated by brittle intergranular fracture and some cracks, which are unfavorable for the toughness of the material.when the true strain at a value exceeding 4.5% as shown in Figure 8, and there is no buffering stage of the work-hardening rate similar to that of the steel with 0.018 wt.% TiCx-TiB2 nanoparticles. Figure 9 shows the impact properties and impact fracture morphologies of the steels with different contents of TiCx-TiB2 nanoparticles at room temperature.The impact toughness of steel with 0.018 wt.% TiCx-TiB2 nanoparticles is 11.5% higher than that of the 40Cr steel, but the impact toughness of the steel with 0.054 wt.% nanoparticles is reduced.Some of the reasons for this can be seen from the impact fracture surfaces in Figure 9.In the impact fracture surfaces of the 40Cr steel, the quasi-cleavage transgranular fracture is dominant, and there is a small intergranular fracture section.Except for a small amount of cleavage fracture, there are many dimples on the impact fracture of the steel with 0.018 wt.% TiCx-TiB2 nanoparticles, indicating that the impact toughness is better than that of the 40Cr steel.The fracture of the steel with 0.054 wt.% TiCx-TiB2 nanoparticles is dominated by brittle intergranular fracture and some cracks, which are unfavorable for the toughness of the material.According to the Hall-Petch formula, a smaller average grain size can result in a larger yield strength [29].The difference in grain orientation on both sides of the grain boundary increases the slip resistance of the dislocation near the grain boundary and causes dislocations to accumulate at the grain boundary.According to the Orowan mechanism, a large number of dispersed carbides can hinder the movement of dislocations and cause an entanglement of the dislocations [30][31][32][33].The dislocation loop will be left behind when dislocations pass through the carbides and nanoparticles, which produces a reaction force to the dislocation source.All of the above factors can improve the tensile strength of the steels.In a certain volume, the smaller grains size means a greater number of grains.The deformation is more uniform because it can be dispersed in more grains with the same amount of plastic deformation.On the other hand, when there is stress concentration in the material, voids tend to form in the weaker part.the strain increasing, voids grow up, and adjacent voids gather together to form dimples.However, materials with uniform internal structure such as steels with 0.018 wt.% nanoparticles have more uniform distribution of vacancies when strain occurs, so the dimples on the fracture surface are more uniform and the size of dimples is larger before fracture.On the contrary, materials with less uniform internal structure, such as the matrix, tend to concentrate a large number of voids to form crack sources when strain occurs.Therefore, the ductility of steels with 0.018 wt.% nanoparticles is better than that of the matrix.Materials can withstand a large amount of deformation before breaking when fewer cracks caused by a stress concentration occur.It is difficult for many nanoparticles to disperse uniformly in the steel.In the case of a large deformation, the second phase of agglomeration often becomes the source of cracks.This is why the strength and plasticity of the steel with 0.018 wt.% TiC x -TiB 2 nanoparticles are simultaneously improved, while the strength of the steel with 0.054 wt.% TiC x -TiB 2 nanoparticles is greatly increased but the fracture strain drops.The smaller grain size means more grain boundary area and more zigzag grain boundary, which is not conducive to the crack propagation.Therefore, the impact performance of the steel with 0.018 wt.% TiC x -TiB 2 nanoparticles is improved.For the steels with 0.054 wt.% TiC x -TiB 2 nanoparticles, the enhancement caused by grain refinement does not offset the damage of the poorly dispersed particles to impact toughness.

Conclusions
Trace biphase TiC x -TiB 2 nanoparticle-reinforced steels are innovatively produced by adding nanosized TiC x -TiB 2 /Al master alloy during the casting process.The pearlitic-ferrite structure of the as-cast steels and the bainite structure of the steels after heat treatment have been refined and the dispersed nanosized carbides are increased.The average grain size is refined from 9.12 µm for the 40Cr steel to 3.59 µm for the steel with 0.018 wt.% TiC x -TiB 2 nanoparticles.Microstructural refinement benefits from the promotion of nucleation and inhibition of dendrite growth by TiC x -TiB 2 nanoparticles.Compared with the 40Cr steel (σ 0.2 = 1080 MPa, σ UTS = 1120 MPa, ε f = 9%, ak = 18.2 J/cm 2 , HRC = 38), the yield strength (1246 MPa), tensile strength (1469 MPa), fracture strain (9.4%), impact toughness (20.3 J/cm 2 ), and hardness of the steel containing 0.018 wt.% TiC x -TiB 2 nanoparticles are increased by 15.4%, 31.2%,4.4%, 11.5%, and 7.9%, respectively.The yield strength (1415 MPa) and tensile strength (1554 MPa) of the steel with 0.054 wt.% TiC x -TiB 2 nanoparticles are increased by 31% and 38.8% compared with the 40Cr steel, respectively.The improved tensile properties may be attributed to fine grain strengthening and the pinning effect of nanosized carbides on dislocations and grain boundaries.The results indicate that the trace TiC x -TiB 2 nanoparticles are successfully introduced into the cast steel through the method of adding nanosized TiC x -TiB 2 /Al master alloy during the casting process and that trace (0.018 wt.%) TiC x -TiB 2 nanoparticles can simultaneously improve the strength, toughness, as well as ductility of the cast steel.

Figure 1 .
Figure 1.Heat treatment process diagram for cast steels.

Figure 1 .
Figure 1.Heat treatment process diagram for cast steels.

Figure 2 .
Figure 2. (a) XRD pattern of nano-sized TiC x -TiB 2 /Al master alloy, (b) morphologies and sizes analysis of the nano-sized TiC x -TiB 2 particles in the nano-sized TiC x -TiB 2 /Al master alloy.TEM images (c) and electron diffraction patterns (d) of the nano-sized TiC x -TiB 2 /Al master alloy.

Figure 3 .
Figure 3. OM images of the as-cast steels (a-c), and steels after heat treatment (d-f).

Figure 3 .
Figure 3. OM images of the as-cast steels (a-c), and steels after heat treatment (d-f).

11 Figure 3 .
Figure 3. OM images of the as-cast steels (a-c), and steels after heat treatment (d-f).

Figure 4 .
Figure 4. EBSD images of steels with TiC x -TiB 2 particles contents of 0 wt.%(a) and 0.018 wt.%(b) after heat treatment.(c) shows the average grain size of (a,b).

Figure 6 .
Figure 6.The effect of TiCx-TiB2 nanoparticles on the refinement of austenite.(a) Nanoparticles in molten steel; (b) Nucleation and growth of austenite; (c) Nanoparticles in austenite.

Figure 6 .
Figure 6.The effect of TiCx-TiB2 nanoparticles on the refinement of austenite.(a) Nanoparticles in molten steel; (b) Nucleation and growth of austenite; (c) Nanoparticles in austenite.

Figure 6 .
Figure 6.The effect of TiC x -TiB 2 nanoparticles on the refinement of austenite.(a) Nanoparticles in molten steel; (b) Nucleation and growth of austenite; (c) Nanoparticles in austenite.

Figure 7 .
Figure 7.The tensile engineering stress-strain curves of steels with different TiCx-TiB2 nanoparticles content in (a), and the corresponding fracture surfaces in (b-d).

Figure 7 .
Figure 7.The tensile engineering stress-strain curves of steels with different TiC x -TiB 2 nanoparticles content in (a), and the corresponding fracture surfaces in (b-d).

Table 2 .
Tensile properties, impact toughness and hardness of the steels with different content of TiC x -TiB 2 nanoparticles.

Figure 9 .
Figure 9. Impact toughness of steels with different contents of TiCx-TiB2 nanoparticles and the corresponding impact fracture surfaces.

Figure 8 .
Figure 8.The corresponding work-hardening rate-true strain curves that were obtained from the tensile curve.

Figure 8 .
Figure 8.The corresponding work-hardening rate-true strain curves that were obtained from the tensile curve.

Figure 9 .
Figure 9. Impact toughness of steels with different contents of TiCx-TiB2 nanoparticles and the corresponding impact fracture surfaces.

Figure 9 .
Figure 9. Impact toughness of steels with different contents of TiC x -TiB 2 nanoparticles and the corresponding impact fracture surfaces.

Table 1 .
Chemical composition of the 40Cr steel. lists.

Table 2 .
Tensile properties, impact toughness and hardness of the steels with different content of TiCx-TiB2 nanoparticles.