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Sep 9, 2018 - Ductility of Cast 40Cr Steels Strengthened by Trace .... the preparation methods show a large potential application value, and the analysis and ...
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Simultaneously Enhanced Strength, Toughness and Ductility of Cast 40Cr Steels Strengthened by Trace Biphase TiCx-TiB2 Nanoparticles Chuan-Lu Li 1,2 , Feng Qiu 1,2,3, * , Fang Chang 1,2 , Xu-Min Zhao 1,2 , Run Geng 1,2 , Hong-Yu Yang 4 , Qing-Long Zhao 1,2 and Qi-Chuan Jiang 1,2, * 1

2 3 4

*

State Key Laboratory of Automotive Simulation and Control, Jilin University, Renmin Street NO. 5988, Changchun 130025, China; [email protected] (C.-L.L.); [email protected] (F.C.); [email protected] (X.-M.Z.); [email protected] (R.G.); [email protected] (Q.-L.Z.) Key Laboratory of Automobile Materials, Ministry of Education and Department of Materials Science and Engineering, Jilin University, Renmin Street NO. 5988, Changchun 130025, China Qingdao Automotive Research Institute of Jilin University, Renmin Street NO. 5988, Changchun 130025, China School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China; [email protected] Correspondence: [email protected] (F.Q.); [email protected] (Q.-C.J.); Tel.: +86-431-8509-5592 (F.Q.); +86-431-8509-4699 (Q.-C.J.)

Received: 10 August 2018; Accepted: 4 September 2018; Published: 9 September 2018

 

Abstract: 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. Keywords: TiCx -TiB2 nanoparticles; grain refinement; mechanical property; strengthening mechanism

1. Introduction Solving the problem of simultaneously improving the strength and toughness of cast steels is a long-term goal for researchers. In recent years, ceramic particles, such as TiC [1,2], TiB2 [3,4], (NbTi)C [5] and other ceramics [6,7], have been used to enhance metallic materials because of their good thermal stability, high modulus, and good wettability with the metal matrix [6,8–11]. The outstanding role of ceramic particles in enhancing metal properties has aroused researchers’ interest [12–15].

Metals 2018, 8, 707; doi:10.3390/met8090707

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Sung-Mo Hong et al. found that nanosized TiC particles can refine the structure and increase the strength of SA-106B carbon steel [16]. S.W. Hu et al. found that TiC particles 2 µm in size can strengthen the hardness and wear resistance of manganese steel [17]. TiB2 and TiC-reinforced steel matrix composites were produced through powder metallurgy, improving the wear properties of the composites [18–20]. Improved steel properties were expected to be obtained through the combination of TiB2 and TiC particles. Spark plasma sintering was used to produce TiB2 -TiC-reinforced steel matrix composites [21]. Among the various methods, traditional casting is economical and convenient, with its high production efficiency and flexible product shape [22,23]. However, due to the large difference in specific gravity, agglomeration of nanoparticles and surface pollution of nanoparticles, it seems that the traditional casting method is not suitable for producing nanoceramic-reinforced steel. Because of these constraints, there have been few studies on enhancing the strength and toughness of cast steel with dual-phase nanosized ceramic particles (TiCx -TiB2 ). Therefore, it is meaningful to prepare TiCx -TiB2 nanoparticle-reinforced steel through traditional casting method. In this work, we have innovatively added the in situ nanosized biphase TiCx -TiB2 /Al master alloy into the casting process to make trace (TiCx -TiB2 ) nanoparticle-reinforced cast steel. The master alloy, in which nanosized ceramic particles are uniformly dispersed, is prepared by the SHS (self-propagating high temperature synthesis) method. This work helps to solve the problem of floating, agglomerating, and surface contamination of ceramic particles in the steel and makes full use of the benefits of the dual-phase ceramic particles (TiCx -TiB2 ) and conventional casting methods. The experimental results show that the strength and toughness of the dual-phase ceramic particles-reinforced cast steel are simultaneously improved. The influence mechanisms of the ceramic particles on both microstructural evolution and properties improvement of steels have been fully analyzed and discussed. In this paper, the preparation methods show a large potential application value, and the analysis and discussion provide a valuable reference for subsequent researchers. 2. Materials and Methods The main chemical composition of investigated 40Cr steel is displayed in Table 1. The nanosized TiCx -TiB2 /Al master alloy was prepared through self-propagating high-temperature synthesis. First, Al (~13 µm), Ti (~25 µm), and B4 C (~1.5 µm) powders at a ratio corresponding to that of stoichiometric TiC and TiB2 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 TiCx -TiB2 /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. Table 1. Chemical composition of the 40Cr steel. Element

C

Si

Mn

Cr

S

P

Fe

wt.%

0.41

0.25

0.65

0.95

0.004

0.018

Bal.

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

The chemical composition of the investigated alloy was determined by direct-reading spectrometer The chemical composition of the investigated alloy was determined by direct-reading (CX-9000, GLMY, WuXi, China). The ceramic particles extracted from the master alloy were spectrometer (CX-9000, GLMY, WuXi, China). The ceramic particles extracted from the master alloy characterized by X-ray diffraction (XRD, D/Max 2500PC Rigaku, Japan) and by field emission scanning were characterized by X-ray diffraction (XRD, D/Max 2500PC Rigaku, Japan) and by field emission electron microscopy (FESEM, JSM-6700F, JEOL, Tokyo, Japan). Samples after heat treatment were cut scanning electron microscopy (FESEM, JSM-6700F, JEOL, Tokyo, Japan). Samples after heat treatment into dog-bone shaped tensile specimens (the specimens had a gauge length of 8 mm and cross-sectional were cut into dog-bone shaped tensile specimens (the specimens had a gauge length of 8 mm and area of 2.5 mm × 2 mm) and Charpy impact specimens (10 × 10 × 55 mm, U-notch). The samples cross-sectional area of 2.5 mm × 2 mm) and Charpy impact specimens (10 × 10 × 55 mm, U-notch). used for optical microscopy (OM, Olympus PMG3, Tokyo, Japan) were polished and etched with The samples used for optical microscopy (OM, Olympus PMG3, Tokyo, Japan) were polished and a 5% nitric acid and 95% alcohol mixture. The samples designated for electron backscatter diffraction etched with a 5% nitric acid and 95% alcohol mixture. The samples designated for electron backscatter (EBSD, Oxford NordlysMax EBSD, Oxford, UK) analysis were electropolished in a mixture of 20% diffraction (EBSD, Oxford NordlysMax EBSD, Oxford, UK) analysis were electropolished in a perchloric acid (Beijing Chemical Works, Beijing, China) and 80% acetic acid (Beijing Chemical Works, mixture of 20% perchloric acid (Beijing Chemical Works, Beijing, China) and 80% acetic acid (Beijing Beijing, China) at 30 V for 15 s. 0.3 µm was chosen as step size of EBSD scans. The TEM microstructure Chemical Works, Beijing, China) at 30 V for 15 s. 0.3 µ m was chosen as step size of EBSD scans. The was acquired by using a JEOL microscope (JEM 2100F, JEOL, Tokyo, Japan). The tensile tests were TEM microstructure was acquired by using a JEOL microscope (JEM 2100F, JEOL, Tokyo, Japan). The performed on a servo hydraulic materials testing system (MTS 810, MTS Systems Corporation, tensile tests were performed on a servo hydraulic materials testing system (MTS 810, MTS Systems Minneapolis, MN, USA). The standard Charpy impact test was conducted using a semiautomatic Corporation, Minneapolis, MN, USA). The standard Charpy impact test was conducted using a impact tester (JBW-300B, JNKH, Jinan, China). The fracture surfaces of the tensile and impact specimens semiautomatic impact tester (JBW-300B, JNKH, Jinan, China). The fracture surfaces of the tensile and were observed by means of scanning electron microscopy (SEM, VEGA 3 XMU, TESCAN, Brno, impact specimens were observed by means of scanning electron microscopy (SEM, VEGA 3 XMU, Czech Republic). TESCAN, Brno, Czech Republic). 3. Results and Discussion 3. Results and Discussion Figure 2a presents the XRD pattern of the nanosized biphase TiCx -TiB2 /Al master alloy, which Figure the XRD pattern of theα-Al, nanosized biphase TiCx-TiB2/Al master alloy, which shows that 2a thepresents master alloy mainly contains TiC, TiB 2 and a small amount of Al3 Ti. It can be shows that the alloy mainly contains α-Al, TiC, with TiB2 only and aa small amount of of Al Al3Ti Ti.remaining. It can be determined thatmaster the SHS reaction is relatively complete, small amount 3 determined that the SHS reaction is relatively complete, with only a small amount of Al 3Ti remaining. Figure 2b is a FESEM image of the mixed particles of TiC and TiB2 obtained by extracting the nanosized Figure 2b/Al is amaster FESEM image the mixed particles of be TiC and TiB obtainedtoby TiCx -TiB alloy withofhydrochloric acid. It can seen that in2 addition theextracting spherical the TiC 2 nanosized TiCx-TiB 2/Al master alloy with hydrochloric acid. It can be seen that in addition to the particles, there are also hexagonal prisms or lamellar TiB2 particles. The size statistics of the TiCx and spherical TiCparticles particles, hexagonal lamellar 2 particles. The size statistics TiB2 mixed in there Figureare 2balso shows that theprisms averageorgrain size TiB of the mixed particles is 190 nm. of the TiC x and TiB2 mixed particles in Figure 2b shows that the average grain size of the mixed The nanoparticles are separated by aluminum, with no significant agglomeration with one another particles is 190 Then, nm. The are-TiB separated by aluminum, with no significant agglomeration (Figure 2c,d). thenanoparticles nanosized TiC x 2 /Al master alloy was melted and gradually dispersed with one another (Figure 2c,d). Then, the nanosized TiCalso x-TiB2/Al master alloy was melted and in the molten steel. The aluminum in the master alloy can be simultaneously used as an oxygen gradually dispersed in the molten steel. The aluminum in the master alloy can also be simultaneously scavenger to react with oxygen at a high temperature to form Al2 O 3 . Then, the Al2 O3 is removed used as an oxygen scavenger to react with oxygen at a high temperature to form Al 2O3. Then, during the smelting process. This method not only successfully introduces nanoparticles into the the Al 2O3 is removed during the smelting process. This method not only successfully introduces molten steel but also contributes to the uniform dispersion of nanoparticles and to the removal of nanoparticles intosteel. the molten steel but also contributes to the uniform dispersion of nanoparticles oxygen from the and to the removal of oxygen from the steel.

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Figure Figure 2. 2. (a) (a) XRD XRD pattern pattern of nano-sized TiCxx-TiB22/Al /Almaster masteralloy, alloy, (b) (b)morphologies morphologies and and sizes analysis of the nano-sized nano-sized TiC TiCxx-TiB -TiB22 particles in the nano-sized nano-sized TiC TiCxx-TiB -TiB22/Al /Al master master alloy. alloy. TEM images (c) and electron diffraction patterns (d) of the nano-sized nano-sized TiC TiCxx-TiB -TiB22/Al /Almaster masteralloy. alloy.

ferrite-pearlite hybrid hybrid structure structure of of the the medium-carbon medium-carbon steel steel can can be be seen seen in in Figure Figure 3a. 3a. The typical ferrite-pearlite The bright brightarea areaisispro-eutectoid pro-eutectoid ferrite, is pearlite [24–26]. Because the content carbon ferrite, andand the the darkdark areaarea is pearlite [24–26]. Because the carbon content of the steelthan is less than 0.77ferrite wt.%,will ferrite will be precipitated along the austenite grain boundary of the steel is less 0.77 wt.%, be precipitated along the austenite grain boundary before before the pearlite transformation. Therefore, the long strip ferrite is distributed on theof boundary of the pearlite transformation. Therefore, the long strip ferrite is distributed on the boundary the coarser the coarser pearlite adding thenanoceramic 0.018 wt.% particles, nanoceramic particles, it seen can clearly be seen pearlite group. Aftergroup. addingAfter the 0.018 wt.% it can clearly be that instead of that instead of the strip-shapedferrite, pro-eutectoid ferrite, of ferrite dispersed inside the the strip-shaped pro-eutectoid the small piecesthe of small ferritepieces dispersed inside the pearlite group pearlite increase, andpearlite the sizegroup of theispearlite group (Figure is also reduced (Figure 3b). When the content increase,group and the size of the also reduced 3b). When the content of nanoparticles of nanoparticles in the steel is increased to 0.054 wt.%, smaller ferrite in the structure is distributed in the steel is increased to 0.054 wt.%, smaller ferrite in the structure is distributed uniformly, and the uniformly, and the pearlite structure is further refinedfrom Figure It is known from thethe structure pearlite structure is further refined Figure 3c. It is known the 3c. structure observed under optical observed under micrographs that theafter microstructure of the steel after heat lower treatment mainly micrographs thatthe theoptical microstructure of the steel heat treatment mainly contains bainite and contains lower bainite andferrite. a smallThe amount of residual ferrite. The unevenly distributed with a small amount of residual unevenly distributed particles with a large number particles of large white ablocks largecan number of in large white can bethe seen Figure 3d.carbide In contrast, lath ferrite and carbide be seen Figure 3d.blocks In contrast, lathinferrite and in thethe microstructure of the steel, in microstructure of nanoparticles the steel, to which the 0.018 nanoparticles aremore added, are significantly to the which the 0.018 wt.% are added, are wt.% significantly finer and uniform. More lath finer and more uniform. lath ferrites and3f,carbides observed in Figure butinthe lath 3e. ferrite ferrites and carbides wereMore observed in Figure but the were lath ferrite is thicker than3f, that Figure is thicker than that in Figure 3e.

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

The EBSD EBSD method method was was used used to to clarify clarify the the quantitative quantitative degree degree of of grain grain refinement, refinement, as as shown shown in in The The EBSD method was used to clarify the quantitative degree of grain refinement, as shown in Figure 4. The The average average grain grain size size is is refined refined from from 9.12 9.12 μm µm in in the the 40Cr 40Cr steel steel to to 3.59 3.59 μm µm in in the the steel steel with with Figure 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 TiC x-TiB nanoparticles. The grain the steels withnanosized trace nanosized x-TiB2 0.018 wt.% The grain sizesize of theofsteels with trace TiCx -TiB2TiC particles x -TiB 2 2nanoparticles. 0.018 wt.% TiC x-TiB2 nanoparticles. The grain size of the steels with trace nanosized TiC x-TiB2 particles was by 60.6%. More importantly, the not nanoparticles notthe only the cast was refined byrefined 60.6%. More importantly, the nanoparticles only changed castchanged microstructure of particles was refined by 60.6%. More importantly, the nanoparticles not only changed the cast microstructure ofof the steel but also refined the grain size of the steel after heat treatment. themicrostructure steel but also refined the grain size of the steel after heat treatment. the steel but also refined the grain size of the steel after heat treatment.

Figure 4. 4. EBSD particlescontents contentsofof0 0wt.% wt.%(a)(a) and 0.018 wt.% after Figure EBSDimages imagesofofsteels steelswith withTiC TiCxx-TiB -TiB22 particles and 0.018 wt.% (b)(b) after Figure 4. EBSD images of steels with TiC x-TiB 2 particles heat treatment. (c)(c)shows the grain size of (a,b).contents of 0 wt.% (a) and 0.018 wt.% (b) after heat treatment. shows theaverage average grain size of (a,b). heat treatment. (c) shows the average grain size of (a,b).

The TEM in microstructures microstructuresbetween between the steels without The TEMimages imagesininFigure Figure55show showthe the differences differences in the steels without nanosized TiC -TiB particles and the 0.018 wt.% TiC -TiB nanoparticle-reinforced steel. It is The TEM images in Figure 5 show the differences in microstructures between the steels without x x 2 2 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 nanosized TiC x -TiB 2 particles and the 0.018 wt.% TiC x -TiB 2 nanoparticle-reinforced steel. It determined that a large number of carbide particles, of which the size is less than 100 nm, are evenly is distributed in the of the wt.% TiC TiC -TiB nanoparticle-reinforced steel inare Figure determined that a microstructure large number of particles, ofxx-TiB which the size is less than steel 100 nm, evenly distributed in the microstructure ofcarbide the 0.018 0.018 22nanoparticle-reinforced in Figure 5b. 5b. OnOn thethe contrary, a asmaller of carbide particles found themicrostructure microstructure of the steels contrary, smalleramount amount of0.018 carbide particles isis2found ininthe ofin the steels distributed in the microstructure of the wt.% TiCx-TiB nanoparticle-reinforced steel Figure 5b. without nanosized TiC x -TiB 2 particles (Figure 5a). without nanosized TiC -TiB particles (Figure 5a). On the contrary, a smaller amount of carbide particles is found in the microstructure of the steels x 2 without nanosized TiCx-TiB2 particles (Figure 5a).

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Figure 5. TEM images of steels without nano-sized TiCx-TiB2 particles (a) and steels with 0.018 wt.% TiCx-TiB nanoparticles Figure 5.2 TEM images of(b). steels without nano-sized TiCxx-TiB -TiB2 particles particles (a) (a) and and steels steels with with 0.018 0.018 wt.% wt.% 2

TiCxx-TiB -TiB22 nanoparticles nanoparticles (b). (b).

TiCx and TiB2 have been proven to be effective austenitic heterogeneous nucleation sites [2,10,12]. arebeen successfully introduced and austenitic dispersed into thenucleation liquid nucleation steel herein. As TiCx Nanoparticles and proven to beto effective austenitic heterogeneous sites [2,10,12]. and TiB TiB havebeen proven be effective heterogeneous sites 2 2have shown inNanoparticles Figure because structural undulations and ofherein. thesteel heterogeneous Nanoparticles are6,successfully introduced and dispersed into thefluctuations liquid steel Asherein. shownAs in [2,10,12]. are successfully introduced and energy dispersed into the liquid nucleation are small, nucleation occursundulations preferentially the nanoparticles. Usually, larger-sized Figure 6, structural undulations and energy fluctuations of the heterogeneous nucleation are shown inbecause Figure 6, because structural andonenergy fluctuations of the heterogeneous nanoparticles as thenucleation nucleation site, while adsorbUsually, onto thelarger-sized solid-liquid small, nucleation occurs preferentially onpreferentially thesmaller-sized nanoparticles. larger-sized nanoparticles act nucleation areact small, occurs on nanoparticles theUsually, nanoparticles. interface and act hinder the transition of the interface, thus inhibiting thethe growth of thethe dendrite [27]. as the nucleation site, while smaller-sized nanoparticles adsorb onto solid-liquid interface and nanoparticles as the nucleation site, while smaller-sized nanoparticles adsorb onto solid-liquid The primary dendrites grow, and finally,thus the completely austenitized. hinder the transition ofgradually the interface, thus inhibiting thematerial growth becomes ofthe thegrowth dendrite The primary interface and hinder the transition of the interface, inhibiting of [27]. the dendrite [27]. size ofgradually thedendrites prior austenite grains seriously affectsbecomes the ferrite-pearlite structure and the dendrites grow, and finally, theand material completely austenitized. The sizebainite of the The primary gradually grow, finally, thelater material becomes completely austenitized. structure The austenite increased grain boundaries duethe tolater austenite grainand refinement dispersed priorsize austenite grains seriously affects the lateraffects ferrite-pearlite structure the bainiteand structure [28]. The of[28]. the prior grains seriously ferrite-pearlite structure the bainite nanoparticles provide more precipitation sites for pro-eutectoid ferrite, which avoid the formation of The increased grain boundaries due to austenite grain refinement and dispersed nanoparticles provide structure [28]. The increased grain boundaries due to austenite grain refinement and dispersed long ferrite and limit thepro-eutectoid growth of ferrite, pearlite. Theavoid mostthe important process when the limit bainite more precipitation sitesmore for which formation of long ferrite and the nanoparticles provide precipitation sites for pro-eutectoid ferrite, which avoid the formation of transformation is diffusion of carbon. The rapid diffusion channels thermal growth of pearlite. The most important when the bainite transformation occurs isand the long ferrite andoccurs limit thethe growth ofprocess pearlite. The most important process when thediffusion bainite mismatches produced by boundaries and nanoparticles promote by the diffusion ofthermal carbon of carbon. The rapid diffusion channels and mismatches the grain boundaries transformation occurs is the thegrain diffusion of thermal carbon. The rapid produced diffusion channels and and nanoparticles the nucleation of carbides, so there are a large number of dispersed carbides throughout the promote thegrain diffusion of carbon and the nucleation of carbides, so there are a large mismatches produced by the boundaries and nanoparticles promote the diffusion of carbon structure. number of dispersed carbides throughout the structure. and the nucleation of carbides, so there are a large number of dispersed carbides throughout the structure.

Figure 6. The effect of TiC TiCxx-TiB22 nanoparticles Nanoparticles in nanoparticles on on the refinement of austenite. (a) Nanoparticles molten steel; (b) Nucleation and growth of austenite; (c) Nanoparticles in austenite. austenite. 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 77 shows stress-strain curves andand fracture surfaces of theof40Cr showsthe thetensile tensileengineering engineering stress-strain curves fracture surfaces the steels 40Cr with Figure different TiCx -TiB contents. It is notItdifficult toand determine from Figure that the steels with different TiC x-TiB 2 nanoparticle contents. is curves not difficult to determine from Figure 7a 2 nanoparticle 7 shows the tensile engineering stress-strain fracture surfaces of7a the 40Cr yield strength and tensile strength of the steel with nanoparticles are greatly improved. In addition that the yield strength and tensile strength of the steel with nanoparticles are greatly improved. In steels with different TiCx-TiB2 nanoparticle contents. It is not difficult to determine from Figure 7a to a large increase in the yield strength and tensile strength compared to the 40Cr steel, the fracture addition to a large increase the yield strength and tensile compared to the 40Cr steel, the that the yield strength and in tensile strength of the steel withstrength nanoparticles are greatly improved. In strain ofstrain thea large steel containing 0.018 wt.% TiC is compared alsoisimproved. The yield and fracture of the steel containing 0.018 wt.% TiC xtensile -TiB2 nanoparticles also to improved. The yield x -TiB 2 nanoparticles addition to increase in the yield strength and strength the 40Cr steel, the and tensile strengths of thecontaining steel with0.018 0.054wt.% wt.%TiC nanoparticles added areisfurther enhanced, but the fracture strain of the steel x-TiB2 nanoparticles also improved. The yield and tensile strengths of the steel with 0.054 wt.% nanoparticles added are further enhanced, but the

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Metalsstrengths 2018, 8, x FOR REVIEW of 11 tensile of PEER the steel with 0.054 wt.% nanoparticles added are further enhanced, but the7fracture strain is obviously decreased. From Table 2 one can observe that the yield strength, tensile strength, fracture strain is obviously decreased. From Table 2 one can observe that the yield strength, tensile and fracture strain of the 40Cr steel are 1080 MPa, 1120 MPa and 9%, respectively. Compared with strength, and fracture strain of the 40Cr steel are 1080 MPa, 1120 MPa and 9%, respectively. the 40Cr steel, the yield strength (1246 MPa), tensile strength (1469 MPa), and fracture strain (9.4%) Compared with the 40Cr steel, the yield strength (1246 MPa), tensile strength (1469 MPa), and of the steel containing 0.018 wt.% TiCx -TiB2 nanoparticles are increased by 15.4%, 31.2% and 4.4%, fracture strain (9.4%) of the steel containing 0.018 wt.% TiCx-TiB2 nanoparticles are increased by respectively. The yield strength (1415 MPa) and tensile strength (1554 MPa) of the steel with 0.054 wt.% 15.4%, 31.2% and 4.4%, respectively. The yield strength (1415 MPa) and tensile strength (1554 MPa) TiC -TiB nanoparticles are increased 31% and 38.8% compared withand the 38.8% 40Cr steel, respectively. 2 ofx the steel with 0.054 wt.% TiCx-TiB2 by nanoparticles are increased by 31% compared with The surfaces (FigureThe 7b–d) also reflect the(Figure tensile7b–d) properties of thethe material to some extent. thefracture 40Cr steel, respectively. fracture surfaces also reflect tensile properties of There are some dimples with uneven sizes on the tensile fracture of the 40Cr steel, which shows the material to some extent. There are some dimples with uneven sizes on the tensile fracture of the a ductile fracture. dimples onfracture. the fracture surface of with 0.018 of wt.% nanoparticles 40Cr steel, whichThe shows a ductile The dimples onthe the steel fracture surface the steel with 0.018are more obvious, and the size is more uniform, butsize it isisalso a ductile fracture. there are fewer wt.% nanoparticles are more obvious, and the more uniform, but it isHowever, also a ductile fracture. dimples on the fracture surface of the with 0.054 wt.% and most the areas have However, there are fewer dimples onsteel the fracture surface ofnanoparticles, the steel with 0.054 wt.% of nanoparticles, a rock sugar-like morphology, which a brittle intergranular fracture. the increase of and or most of the areas have a rock or shows sugar-like morphology, which shows aOwing brittletointergranular fracture. Owing to the increase of carbides and the refinement of grain size, hardness steels carbides inside and refinement of graininside size, hardness of steels with nanoparticles hasof increased, has increased, as Table 2 lists. aswith Tablenanoparticles 2 lists.

Figure 7. 7.The of steels steelswith withdifferent differentTiC TiC x -TiB 2 nanoparticles Figure Thetensile tensileengineering engineeringstress-strain stress-strain curves curves of x-TiB 2 nanoparticles content inin(a), in (b–d). (b–d). content (a),and andthe thecorresponding corresponding fracture fracture surfaces surfaces in Table 2. Tensile properties, impact toughness and hardness of the steels with different content of Table 2. Tensile properties, impact toughness and hardness of the steels with different content of TiCx -TiB2 nanoparticles. TiCx-TiB2 nanoparticles. Samples Samples 40Cr steels 40Cr steels 0.018 wt.% TiCx + TiB2 0.018 wt.% TiCx + TiB2 0.054 wt.% TiCx + TiB2

0.054 wt.% TiCx + TiB2

σ0.2σ0.2 (MPa)

(MPa) +10+10 1080 1080 −70−70 8 1246+ −6 +8 1246 +5 −6 1415−2 +5

1415−2

σσUTS UTS (MPa)

(MPa) ++20 20 1120 1120−−10 10

20 1469+ −+20 10 1469+ −10 8 1554−+835

1554−35

Fracture Strain Strain Fracture εf (%) εf (%) 1.6 +1.6 9.0+ 9.0 − 2.0 −2.0 1.7 +1.7 9.4+ − 1 9.4 −1 +0.8 5.5.6+0.8 −1.3

5.6−1.3

2 ak ak (J/cm (J/cm2))

Hardness Hardness HRC

18.2 18.2±± 0.85 0.85 20.3 ± 1.66 20.3 ± 1.66 13.8 ± 0.80

38−1.0 38 −1.0

13.8 ± 0.80

HRC +1.5 +1.5 +1.1

41−0.8 41 −0.8 +2.2 +1.1

47+2.2

−2.1 47−2.1

Figure the steels steelswith withdifferent differentTiC TiC Figure8 8reflects reflectsthe thework-hardening work-hardening ability ability of the x-TiB 2 nanoparticles x -TiB 2 nanoparticles contents. Figure 7, 7, ititisisdetermined determinedthat thatthe the yield strength of the 40Cr contents.Combined Combinedwith withthe theanalysis analysis of Figure yield strength of the 40Cr steel low,the theyielding yieldingoccurs occurs very very quickly, quickly, and and the decreases after steel is islow, the work-hardening work-hardeningrate raterapidly rapidly decreases after yielding. Comparedwith withthe the40Cr 40Crsteel, steel, the the steel with 2 nanoparticles has a higher yielding. Compared with 0.018 0.018wt.% wt.%TiC TiCx-TiB -TiB nanoparticles has a higher x 2 yield strength,and andaahigher higherwork-hardening work-hardening rate after yielding. yield strength, rate with withthe thesame sameplastic plasticdeformation deformation after yielding. The steelwith with0.054 0.054wt.% wt.%TiC TiCx x-TiB -TiB22 nanoparticles nanoparticles has and thethe highest The steel hasthe thehighest highestyield yieldstrength, strength, and highest work-hardening rate, but its work-hardening rate is less than the steel with 0.018 wt.% nanoparticles

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work-hardening rate, but its work-hardening rate is less than the steel with 0.018 wt.% nanoparticles when the true strain atatataavalue Figure 8, and there is buffering stage of when the true strain exceeding 4.5%asas asshown shownin Figure and there is buffering stage when the true strain avalue valueexceeding exceeding4.5% 4.5% shown inin Figure 8,8, and there is no nono buffering stage the work-hardening raterate similar to that of the steel with 0.018 wt.% TiC ofof the work-hardening similar to of the the steel with 0.018 wt.% TiC x-TiB 2 nanoparticles. x -TiB 2 2nanoparticles. the work-hardening rate similar to that that of steel with 0.018 wt.% TiC x-TiB nanoparticles.

Figure 8. The corresponding work-hardening rate-true strain curves that were obtained from the Figure 8. The Figure The corresponding corresponding work-hardening work-hardening rate-true rate-true strain strain curves curves that that were were obtained obtained from from the the tensile8.curve. tensile curve. tensile curve.

Figure 9 shows the impact properties and impact fracture morphologies of the steels with Figure 9 shows the impact properties and impact fracture morphologies of the steels with different Figurecontents 9 shows thex-TiB impact propertiesatand impact fracture morphologies of the steels with different of TiC 2 nanoparticles room temperature. The impact toughness of steel with contents of TiCx -TiB2 nanoparticles at room temperature. The impact toughness of steel with 0.018 wt.% different contents of TiC x-TiB2 nanoparticles at room toughness of impact steel with 0.018 wt.% TiCx-TiB 2 nanoparticles is 11.5% highertemperature. than that ofThe the impact 40Cr steel, but the TiCx -TiB2 nanoparticles is 11.5% higher than that of the 40Cr steel, but the impact toughness of the 0.018 wt.% of TiC x-TiB 2 nanoparticles is nanoparticles 11.5% higheris than thatSome of the 40Cr steel, for butthis thecanimpact toughness the steel with 0.054 wt.% reduced. of the reasons be steel with 0.054 impact wt.% nanoparticles is reduced. Somethe ofimpact the reasons forsurfaces this can be seen from the seen fromofthe fracture surfaces in Figure 9. In is of thefor 40Cr toughness the steel with 0.054 wt.% nanoparticles reduced.fracture Some of the reasons thissteel, can be impact fracture surfaces in Figure 9.fracture In the impact fractureand surfaces 40Cr intergranular steel, the quasi-cleavage the from quasi-cleavage is dominant, there of is the a small seen the impacttransgranular fracture surfaces in Figure 9. In the impact fracture surfaces of the fracture 40Cr steel, transgranular fracture is dominant, and there is a small intergranular fracture section. Except for section. Except for a small amount of cleavage fracture, there are many dimples on the impact fracture the quasi-cleavage transgranular fracture is dominant, and there is a small intergranular fracture a small of cleavage fracture, there are many dimples on the the impact fracture of theissteel with of theamount steel with wt.% TiCx-TiB 2 nanoparticles, impact toughness better section. Except for 0.018 a small amount of cleavage fracture,indicating there are that many dimples on the impact fracture 0.018 wt.% TiC -TiB nanoparticles, indicating that the impact toughness is better than that of x 2 wt.% that of the0.018 40Cr steel.TiC Thex-TiB fracture of the steel with 0.054 wt.% TiCx-TiB2 nanoparticles isthe ofthan the steel with 2 nanoparticles, indicating that the impact toughness is better 40Cr steel. The fracture of the steel with 0.054 wt.% TiC nanoparticles is dominated by brittle x -TiB dominated fracture and some cracks, which arewt.% unfavorable for the toughness 20.054 than that of by thebrittle 40Crintergranular steel. The fracture of the steel with TiCx-TiB 2 nanoparticles is intergranular fracture and some cracks, which are unfavorable for the toughness of the material. of the material. dominated by brittle intergranular fracture and some cracks, which are unfavorable for the toughness

of the material.

Figure different contents contentsofofTiC TiCx-TiB nanoparticles and Figure9. 9.Impact Impact toughness toughness of of steels steels with different 2 2nanoparticles and thethe x -TiB corresponding correspondingimpact impactfracture fracturesurfaces. surfaces.

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

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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 [30–33]. 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. With 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.% TiCx -TiB2 nanoparticles are simultaneously improved, while the strength of the steel with 0.054 wt.% TiCx -TiB2 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.% TiCx -TiB2 nanoparticles is improved. For the steels with 0.054 wt.% TiCx -TiB2 nanoparticles, the enhancement caused by grain refinement does not offset the damage of the poorly dispersed particles to impact toughness. 4. Conclusions Trace biphase TiCx -TiB2 nanoparticle-reinforced steels are innovatively produced by adding nanosized TiCx -TiB2 /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.% TiCx -TiB2 nanoparticles. Microstructural refinement benefits from the promotion of nucleation and inhibition of dendrite growth by TiCx -TiB2 nanoparticles. Compared with the 40Cr steel (σ0.2 = 1080 MPa, σUTS = 1120 MPa, εf = 9%, ak = 18.2 J/cm2 , HRC = 38), the yield strength (1246 MPa), tensile strength (1469 MPa), fracture strain (9.4%), impact toughness (20.3 J/cm2 ), and hardness of the steel containing 0.018 wt.% TiCx -TiB2 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.% TiCx -TiB2 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 TiCx -TiB2 nanoparticles are successfully introduced into the cast steel through the method of adding nanosized TiCx -TiB2 /Al master alloy during the casting process and that trace (0.018 wt.%) TiCx -TiB2 nanoparticles can simultaneously improve the strength, toughness, as well as ductility of the cast steel. Author Contributions: Conceptualization, C.-L.L., F.Q. and Q.-C.J.; Data curation, C.-L.L. and F.Q.; Formal analysis, C.-L.L., F.Q., F.C., X.-M.Z., R.G., H.-Y.Y. and Q.-L.Z.; Investigation, C.-L.L., F.Q., F.C., X.-M.Z. and R.G.; Supervision, C.-L.L. and F.Q.; Writing—review & editing, C.-L.L., F.Q. and Q.-C.J. Funding: This research received no external funding.

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Acknowledgments: This work is supported by the National Natural Science Foundation of China (NNSFC, No. 51571101 and No. 51771081), the Source Innovation Plan of Qingdao City, China (No. 18-2-2-1-jch), the Science and Technology Development Program of Jilin Province, China (20170101178JC) and the Project 985-High Properties Materials of Jilin University. Conflicts of Interest: The authors declare no conflict of interest.

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