Effect of Rare Earth Ce on Deep Stamping Properties of High

materials

Article Eﬀect of Rare Earth Ce on Deep Stamping Properties of High-Strength Interstitial-Free Steel Containing Phosphorus

Hao Wang 1,2,3, Yanping Bao 1,*, Chengyi Duan 2,3, Lu Lu 2,3, Yan Liu 2,3,* and Qi Zhang 2,3 1 State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China; [email protected] 2 Technical Center of Inner Mongolia Baotou Steel Union Co., Ltd., Baotou 014010, China; [email protected] (C.D.); [email protected] (L.L.); [email protected] (Q.Z.) 3 Inner Mongolia Enterprise Key Laboratory of Rare Earth Steel Products Research & Development, Baotou 014010, China * Correspondence: [email protected] (Y.B.); [email protected] (Y.L.)

Received: 23 February 2020; Accepted: 21 March 2020; Published: 24 March 2020

Abstract: The influence of rare earth Ce on the deep stamping property of high-strength interstitial-free (IF) steel containing phosphorus was analyzed. After adding 120 kg ferrocerium alloy (Ce content is 10%) in the steel, the inclusion statistics and the two-dimensional morphology of the samples in the direction of 1/4 thickness of slab and each rolling process were observed and compared by scanning electron microscope (SEM). After the samples in each rolling process were treated by acid leaching, the three-dimensional morphology and components of the second phase precipitates were observed by SEM and energy dispersive spectrometer (EDS). The microstructure of the sample was observed by optical microscope, and the grain size was compared. Meanwhile, the content and strength of the favorable texture were analyzed by X-ray diffraction (XRD). Finally, the mechanical properties of the product were analyzed. The results showed that: (1) The combination of rare earth Ce with activity O and S in steel had lower Gibbs free energy, and it was easy to generate CeAlO3, Ce2O2S, and Ce2O3. The inclusions size was obviously reduced, but the number of inclusions was increased after adding rare earth. The morphology of inclusions changed from chain and strip to spherical. The size of rare earth inclusions was mostly about 2–5 µm, distributed and dispersed, and their elastic modulus was close to that of steel matrix, which was conducive to improving the structure continuity of steel. (2) The rare earth compound had a high melting point. As a heterogeneous nucleation point, the nucleation rate was increased and the solidification structure was refined. The grade of grain size of products was increased by 1.5 grades, which is helpful to improve the strength and plasticity of metal. (3) Rare earth Ce can inhibit the segregation of P element at the grain boundary and the precipitation of Fe(Nb+Ti)P phase. It can effectively increase the solid solution amount of P element in steel, improve the solid solution strengthening effect of P element in high-strength IF steel, and obtain a large proportion of {111} favorable texture, which is conducive to improving the stamping formability index r90 value.

Keywords: high-strength IF steel containing P; Fe(Nb + Ti)P phase; rare earth Ce; deep stamping properties

1. Introduction Aluminum deoxidized interstitial-free (IF) steel has become the main material for automobile outer panel production in the world because of its good deep stamping performance and economy [1,2]. With the development of the automobile to the direction of weight reduction and lighter weight,

Materials 2020, 13, 1473; doi:10.3390/ma13061473 www.mdpi.com/journal/materials Materials 2020, 13, x FOR PEER REVIEW 2 of 12 rapidly [3,4]. The solid solution strengthened IF steel is one of the main directions of developing high‐ strength IF steel at domestic and foreign plants [5,6]. Due to the good strengthening effect and low price of P element, the strength of IF steel can be improved by solution strengthening by adding appropriate P element and adjusting the proportion of Si and Mn element. The higher the plastic strain ratio r90 is, the better the stamping performance is. The research shows that the shape and size ofMaterials inclusions,2020, 13 the, 1473 grain size, and the proportion of favorable texture all affect the stamping2 of 12 performance of the strip [7,8]. Rare earth has become an important microalloy element in high value‐ added steel materials because of its large atomic size, unique outer electronic structure, variable the automobile plate with high-strength and ultra-deep stamping performance has been developed valence state, and strong activity. The role of rare earth in steel can be summarized in the following rapidly [3,4]. The solid solution strengthened IF steel is one of the main directions of developing aspects: Purification, modification, and microalloying [9,10]. The effect of rare earth Ce on the deep high-strength IF steel at domestic and foreign plants [5,6]. Due to the good strengthening effect and stamping property of high‐strength IF steel containing phosphorus by industrial production and the low price of P element, the strength of IF steel can be improved by solution strengthening by adding application research of more than 50% yield is relatively less. In this paper, the influence of rare earth appropriate P element and adjusting the proportion of Si and Mn element. The higher the plastic strain Ce on inclusions, grain size, and texture in the whole process of aluminum deoxidized IF steel casting ratio r90 is, the better the stamping performance is. The research shows that the shape and size of and rolling was studied by an industrial test system, and the r90 value of a finished strip was inclusions, the grain size, and the proportion of favorable texture all affect the stamping performance characterized, which provides technical guidance for industrial application to improve the deep of the strip [7,8]. Rare earth has become an important microalloy element in high value-added steel stamping property control of IF steel through new ways. materials because of its large atomic size, unique outer electronic structure, variable valence state, and strong activity. The role of rare earth in steel can be summarized in the following aspects: Purification, 2. Test Method modification, and microalloying [9,10]. The effect of rare earth Ce on the deep stamping property of 2.1.high-strength Process IF steel containing phosphorus by industrial production and the application research of more than 50% yield is relatively less. In this paper, the influence of rare earth Ce on inclusions, grain size,The and experimental texture in the materials whole process were produced of aluminum in the deoxidized Rare Earth IF steel Steel casting Plates and Plant rolling of Baotou was studied Iron and Steel Group Corporation, located in Baotou, China. In the whole process of production, except by an industrial test system, and the r90 value of a finished strip was characterized, which provides addingtechnical rare guidance earth, other for industrial processes application were identical. to improve The thedefinition deep stamping of the whole property series control of samples of IF steel withoutthrough adding new ways. rare earth is 1# and the whole series of samples with rare earth is 2#. The production process is described as follow. 2. Test(1) MethodSmelting process. In the hot continuous rolling production line, the production contrast test of IF steel strengthened by phosphorus in the adjacent furnaces of the same pouring time was carried out,2.1. and Process the production process flow and rare earth addition time are shown in Figure 1. DuringThe experimental vacuum circulation materials degassing were produced refining in (RH) the Rare treatment, Earth Steelthe limit Plates vacuum Plant of degree Baotou was Iron ≤0.106and SteelKpa, Groupand the Corporation, total vacuum located treatment in Baotou, time was China. 30 min. In the Due whole to the process strong oxidizability of production, of except rare earth,adding in order rare earth, to avoid other the processes oxygen oxidation were identical. brought The on definition by other of alloys the whole during series the ofalloying samples process, without andadding to ensure rare earth the effective is 1# and boron the whole content series in of the samples steel, after with rarealuminum earth is deoxidization, 2#. The production the addition process is sequencedescribed of as the follow. alloy in the RH vacuum treatment process was ferromanganese, ferrotitanium, ferroniobium,(1) Smelting and process.ferroboron. In the The hot 120 continuous kg rare earth rolling ferrocerium production alloy line, the(Ce production content was contrast 10%) testwas of addedIF steel at strengthened3 min after alloying. by phosphorus After RH in treatment, the adjacent the furnaces sedation of time the same was more pouring than time 15 min was to carried ensure out, theand inclusion the production would fully process float. flow The and recovery rare earth rate additionof rare earth time Ce are was shown 51%. in Figure1.

FigureFigure 1. 1.CeCe-Fe‐Fe alloy alloy added added during during steelmaking steelmaking processes. processes.

(2)During Casting vacuum process. circulation In the continuous degassing casting reﬁning (CC) (RH) process, treatment, strict theprotective limit vacuum casting degreemeasures was against0.106 secondary Kpa, and oxidation the total vacuumwere adopted treatment to prevent time was nitrogen 30 min. and Due oxygen to the increase. strong oxidizabilityThe chemical of ≤ compositionrare earth, inof orderthe tundish to avoid sample the oxygen was analyzed oxidation by broughtdirect reading on by otherspectrum alloys analyzer during (ARL the alloying‐4460, Thermoprocess, Fisher and toScientific, ensure theSwitzerland), eﬀective boron and the content content in the of rare steel, earth after in aluminum the steel deoxidization,was detected by the chemicaladdition analysis. sequence The of theyield alloy of Ce in thewas RH 51%. vacuum Selected treatment steel grades process and was the ferromanganese, final chemical composition ferrotitanium, offerroniobium, the samples is andshown ferroboron. in Table 1. The 120 kg rare earth ferrocerium alloy (Ce content was 10%) was added at 3 min after alloying. After RH treatment, the sedation time was more than 15 min to ensure

the inclusion would fully ﬂoat. The recovery rate of rare earth Ce was 51%. (2) Casting process. In the continuous casting (CC) process, strict protective casting measures against secondary oxidation were adopted to prevent nitrogen and oxygen increase. The chemical composition of the tundish sample was analyzed by direct reading spectrum analyzer (ARL-4460, Thermo Fisher Scientiﬁc, Switzerland), and the content of rare earth in the steel was detected by Materials 2020, 13, 1473 3 of 12

Materialschemical 2020 analysis., 13, x FOR The PEER yield REVIEW of Ce was 51%. Selected steel grades and the ﬁnal chemical composition3 of 12 of the samples is shown in Table1. (3) Rolling processes. The specific technological process of each rolling process is shown in Figure 2. Table 1. Chemical composition of the samples (wt.%). Heat Number C Si Mn P S Als 1 Nb Ti B Ce Table 1. Chemical composition of the samples (wt.%). 2nd 0.0020 0.15 0.68 0.078 0.005 0.029 0.027 0.022 0.0012 Heat 3nd 0.0018 0.14 0.70 0.074 0.005 0.032 0.025 0.025 0.0010 0.0022 C Si Mn 1 P S Als 1 Nb Ti B Ce number Als, acid soluble aluminum. 2nd 0.0020 0.15 0.68 0.078 0.005 0.029 0.027 0.022 0.0012 (3) Rolling processes. The speciﬁc technological process of each rolling process is shown in 3nd 0.0018 0.14 0.70 0.074 0.005 0.032 0.025 0.025 0.0010 0.0022 Figure2. 1.Als, acid soluble aluminum.

Discharging temperature 1200 1220℃ Roughing temperature Beginning temperature of finish rolling 1100℃ 1000 1050℃ End temperature of finish rolling Heating-up and Soaking section temperature 910℃ 800 Coiling temperature 825℃ Slow cooling section temperature 600 650℃ Cold

Temperature / ℃ Air cooling Rapid cooling section temperature 400 rolling Overaging section temperature 400℃ process 350℃ Final cooling section 200 Continuous Room temperature Hot rolling process Room temperature annealing process 120℃ temperature 0 Air cooling Time / min Figure 2. TheThe specific specific technological process of each rolling process. 2.2. Method 2.2. Method The size of the slab was 230 1550 mm2, and the samples of 1# and 2# were cut at the end of the The size of the slab was 230 × 1550 mm2, and the samples of 1# and 2# were cut at the end of the second strand of the second slab of each heating, respectively. In order to compare and count the types second strand of the second slab of each heating, respectively. In order to compare and count the and sizes of inclusions in the whole casting and rolling process and ensure the representativeness of types and sizes of inclusions in the whole casting and rolling process and ensure the the test data and the accuracy of the analysis, the sampling position was selected at 1/4 of the slab representativeness of the test data and the accuracy of the analysis, the sampling position was width and 1/4 of the slab thickness. The selected sample size was 10 10 10 mm. selected at 1/4 of the slab width and 1/4 of the slab thickness. The selected× × sample size was 10 × 10 × The sampling positions of strips in hot rolling (HR), cold rolling (CR), and continuous annealing 10 mm. (CA) correspond to the slab positions, which were 1/4 of the width direction of the plate. The sample The sampling positions of strips in hot rolling (HR), cold rolling (CR), and continuous annealing sizes were 4.3 10 10 mm3, 1.15 10 10 mm3, and 0.7 10 10 mm3. (CA) correspond× to× the slab positions,× which× were 1/4 of the× width× direction of the plate. The sample The samples of CC and each rolling process were cut, inlaid, ground, and polished, and then the sizes were 4.3 × 10 × 10 mm3, 1.15 × 10 × 10 mm3, and 0.7 × 10 × 10 mm3. inclusions were detected by ASPEX-Scanning electron microscope (ASPEX-SEM, ASPEX eXplorer, The samples of CC and each rolling process were cut, inlaid, ground, and polished, and then the ASPEX, Pembroke Park, FL, USA). In this study, the edge of the metallographic samples was avoided, inclusions were detected by ASPEX‐Scanning electron microscope (ASPEX‐SEM, ASPEX eXplorer, and the field area of view was not less than 50 mm2. SEM (ZEISS LEO EVO 50HV, Carl Zeiss, ASPEX, Pembroke Park, FL, USA). In this study, the edge of the metallographic samples was avoided, Oberkochen, Germany) and energy dispersive spectrometer (EDS, EDAX, Mahwah, NJ, USA) were and the field area of view was not less than 50 mm2. SEM (ZEISS LEO EVO 50HV, Carl Zeiss, used to observe the morphology and composition of the typical inclusions found in the above samples. Oberkochen, Germany) and energy dispersive spectrometer (EDS, EDAX, Mahwah, NJ, USA) were The samples of each rolling process were etched by 4% nitric acid alcohol, the original morphology used to observe the morphology and composition of the typical inclusions found in the above was extracted, and the three-dimensional morphology and composition of typical inclusions and samples. second phase precipitates were observed by SEM-EDS. The samples of each rolling process were etched by 4% nitric acid alcohol, the original The samples of the HR and CA processes were cut, inlaid, ground, and polished. After processing, morphology was extracted, and the three‐dimensional morphology and composition of typical the surface of the samples was etched with 4% nitric acid alcohol. Then, the microstructure of the inclusions and second phase precipitates were observed by SEM‐EDS. samples was observed by optical microscope (Axio observer D1m, Carl Zeiss, Oberkochen, Germany) The samples of the HR and CA processes were cut, inlaid, ground, and polished. After and the grain size were tested according to the standard GB/T 6394-2017. processing, the surface of the samples was etched with 4% nitric acid alcohol. Then, the The sample size of each rolling process was cut into 20 25 mm2, and then the sample was microstructure of the samples was observed by optical microscope× (Axio observer D1m, Carl Zeiss, polished. The content and strength of favorable texture were analyzed by X-ray Diffraction (XRD, Oberkochen, Germany) and the grain size were tested according to the standard GB/T 6394‐2017. X’ Pert Pro MPD, Malvern Panalytical B.V., Almelo, The Netherlands). The sample size of each rolling process was cut into 20 × 25 mm2, and then the sample was polished. The content and strength of favorable texture were analyzed by X‐ray Diffraction (XRD, Xʹ Pert Pro MPD, Malvern Panalytical B.V., Almelo, Netherlands). The strip was sampled according to the standard GB/T 2975, rolling 20 coils in sample 1# and sample 2#, respectively. Each coil weighed 7 tons, and sampling was carried out at 1/4 of the width

Materials 2020, 13, 1473 4 of 12

Materials 2020, 13, x FOR PEER REVIEW 4 of 12 The strip was sampled according to the standard GB/T 2975, rolling 20 coils in sample 1# and sampleat 3 m from 2#, respectively. the tail of the Each strip. coil The weighed mechanical 7 tons, properties and sampling of products was carried were tested out at according 1/4 of the widthto the atstandard 3 m from GB/T the 5027. tail of Through the strip. the The comparison mechanical of properties the above of test products results, were the testedinfluence according of rare toearth the standardaddition GBon /theT 5027. deep Through stamping the property comparison of high of the‐strength above testIF steel results, containing the inﬂuence phosphorus of rare earth was additionanalyzed. on the deep stamping property of high-strength IF steel containing phosphorus was analyzed.

3. Results and Discussion

3.1. Effect Effect of Rare Earth Ce on Inclusions According to the statistics of the number, size, and proportion of all kinds of inclusions in the samples of casting and rolling process by ASPEX-SEM,ASPEX‐SEM, the main types of inclusions in the samplessamples 1# are AlAl22O3,, Al Al-O-Ti,‐O‐Ti, TiN, MnS, silicate, etc. The The inclusions inclusions in in the the samples 2# are mainly Al Al-O-Ce,‐O‐Ce, Ce-S-O,Ce‐S‐O, Ce-O,Ce‐O, Al2O3,, Al Al-O-Ti,‐O‐Ti, TiN, TiN, MnS, etc. Figure3 3 shows shows the the number number of of inclusionsinclusions inin didifferentfferent size ranges under corresponding processes and schemes.schemes. TheThe amountamount of of inclusion inclusion with with CC, CC, HR, HR, CR, CR, and and CA CA process process greater greater than than 10 µ10m μ underm under the statisticalthe statistical field field of view of view of sample of sample 1# group 1# group were 12,were 29, 12, 25, 29, 26, 25, respectively. 26, respectively. The sample The sample 2# group 2# were group 5, 6,were 9, and 5, 6, 7 9, respectively. and 7 respectively. The proportion The proportion of inclusions of inclusions less than less 5 µm than in each 5 μm process in each of process sample of 1# sample group were1# group 18.7%, were 27.4%, 18.7%, 37.7%, 27.4%, and 37.7%, 18.6%, and respectively. 18.6%, respectively. The sample The 2#sample group 2# were group 76.5%, were 79.2%,76.5%, 79.2%, 78.9%, and78.9%, 75%, and respectively. 75%, respectively. It can be It seen can thatbe seen the inclusionthat the inclusion scale decreased scale decreased and the number and the of number inclusions of increasedinclusions obviously increased inobviously the whole in castingthe whole and casting rolling and process rolling after process adding after rare adding earth. rare earth.

150 140 >10μm 130 6 5~10μm 120 <5μm 110 21 100 90 9 80 11 7 70 29 25 13 60 50 103 5 26 Number of inclusions of Number 7 40 32 23 75 30 60 12 20 39 22 29 10 14 23 11 0 6 CC1# CC2# HR1# HR2# CR1# CR2# CA1# CA2# Sampling process and corresponding scheme Figure 3. The statistics of inclusion size and quantity in each process. Figure 3. The statistics of inclusion size and quantity in each process. Because of the high oxidation property of Ce, it will react with oxygen, sulfur, and other elements Because of the high oxidation property of Ce, it will react with oxygen, sulfur, and other elements in the molten steel. The thermodynamic calculation of the rare earth compound formed by adding in the molten steel. The thermodynamic calculation of the rare earth compound formed by adding cerium was carried out. cerium was carried out. Table2 shows the chemical reactions related to the addition of rare earth to the molten steel and Table 2 shows the chemical reactions related to the addition of rare earth to the molten steel and the standard Gibbs free energy [11,12]. According to the reaction and the activity coefficient of each the standard Gibbs free energy [11,12]. According to the reaction and the activity coefficient of each element in the molten steel, the Gibbs free energy of production of each rare earth inclusion at 1873 K element in the molten steel, the Gibbs free energy of production of each rare earth inclusion at 1873 was calculated as shown in Figure4. It can be concluded that when the content of wt.% Ce in the rare K was calculated as shown in Figure 4. It can be concluded that when the content of wt.% Ce in the earth was less than 0.01, only CeAlO3, Ce2O3, Ce2O2S, and CeO2 were generated in the molten steel. rare earth was less than 0.01, only CeAlO3, Ce2O3, Ce2O2S, and CeO2 were generated in the molten When the content of wt.% Ce was greater than 0.01, CeS, Ce3S4, and other inclusions were successively steel. When the content of wt.% Ce was greater than 0.01, CeS, Ce3S4, and other inclusions were precipitated out. As the alloying and solidification process of molten steel is unsteady, there may be successively precipitated out. As the alloying and solidification process of molten steel is unsteady, different contents of inclusions, such as Al-O-Ce and S-O-Ce, in the steel [13]. there may be different contents of inclusions, such as Al‐O‐Ce and S‐O‐Ce, in the steel [13].

Materials 2020, 13, x FOR PEER REVIEW 5 of 12 Materials 2020, 13, x FOR PEER REVIEW 5 of 12 Materials 2020, 13, 1473 5 of 12 Table 2. Thermodynamic calculation of the rare earth inclusions formation. Table 2. Thermodynamic calculation of the rare earth inclusions formation. Table 2.ReactionThermodynamic Equations calculation in of the∆G rareθ = A earth + B inclusions× T, J·mol formation.‐1 Reaction Equations in ∆Gθ = A + B × T, J·mol‐1 Molten Steel 1 Molten Steel A ∆Gθ=A+B BT, J mol− Reaction Equations in Molten Steel A ×B · 2Al + 3O = Al2O3 (s) −1,208,271 390.91 2Al + 3O = Al2O3 (s) −1,208,271 AB390.91 2Ce + 3O = Ce2O3 (s) −1,431,090 360.06 2Ce2Al ++ 3O3O == CeAl22OO33 (s)(s) −1,431,0901,208,271 360.06 390.91 Ce + 2O = CeO2 (s) −854,274.7− 249.11 Ce2Ce + +2O3O == CeOCe22O (s)3 (s) −854,274.71,431,090 249.11 360.06 − Ce ++ 2OS == CeSCeO (s)(s) −422,783854,274.7 120.58 249.11 Ce + S = CeS (s)2 −422,783− 120.58 3CeCe + 4S+ S == CeCeS3S4 (s) (s) −1,493,010422,783 438.9 120.58 3Ce + 4S = Ce3S4 (s) −1,493,010− 438.9 3Ce + 4S = Ce S (s) 1,493,010 438.9 2Ce + 2O + S = Ce32O4 2S (s) −1,353,592.4− 331.6 2Ce2Ce ++ 2O2O ++ SS == CeCe2OO2SS (s) (s) −1,353,592.41,353,592.4 331.6 331.6 Ce + Al + 3O = CeAlO2 23 (s) −1,366,460− 364 CeCe + +AlAl + +3O3O == CeAlOCeAlO3 (s)(s) −1,366,4601,366,460 364 364 3 −

Ce2O3 200000 CeCeOO 200000 2 23 CeOCeS2 CeCeS3S4

Ce Ce3S2O4 2S

） CeCeAlO2O2S3 -1 ） 0 CeAlO3 -1 0 mol  J mol  J （

（ -200000 G/

 -200000 G/ 

-400000 -4000000.00 0.01 0.02 0.03 0.04 0.00 0.01 0.02 0.03 0.04 w[Ce]/wt.% w[Ce]/wt.% FigureFigure 4.4. Precipitation ofof rarerare earthearth inclusionsinclusions inin steel.steel. Figure 4. Precipitation of rare earth inclusions in steel. FigureFigure5 5 is is a a comparative comparative analysis analysis of of the the two-dimensional two‐dimensional morphology morphology of of CC CC and and each each rolling rolling Figure 5 is a comparative analysis of the two‐dimensional morphology of CC and each rolling process.process. ItIt cancan bebe seenseen thatthat thethe typicaltypical inclusions inclusions ofof sample sample 1# 1# were were mainly mainly Al Al22OO33 withwith sharpsharp angleangle process. It can be seen that the typical inclusions of sample 1# were mainly Al2O3 with sharp angle andand longlong stripstrip MnSMnS withwith clustercluster shape,shape, andand thethe sizesize cancan reachreach moremore than than 10 10 μµm.m. TheThe inclusionsinclusions inin thethe and long strip MnS with cluster shape, and the size can reach more than 10 μm. The inclusions in the samplessamples 2#2# werewere mainly mainly CeAlO CeAlO33,, CeCe22O33, and Ce 2OO22SS and and the the shape shape was was round round with with a a smooth smooth surface. samples 2# were mainly CeAlO3, Ce2O3, and Ce2O2S and the shape was round with a smooth surface. TheThe sizesize waswas 2–52–5 μµm,m, and and the the distribution distribution was was independent independent dispersion. dispersion. The size was 2–5 μm, and the distribution was independent dispersion.

FigureFigure 5.5. Comparison of typical inclusion morphology between the samples 1# and 2# in each process. Figure 5. Comparison of typical inclusion morphology between the samples 1# and 2# in each process. 1#:1#: ((aa)) CC,CC, ((bb)) HR,HR, ((cc)) CR,CR, ((dd)) CA.CA. 2#:2#: (e)) CC, ((f)) HR, ((g)) CR, ((h)) CA. 1#: (a) CC, (b) HR, (c) CR, (d) CA. 2#: (e) CC, (f) HR, (g) CR, (h) CA.

AlAl22O3 inclusions are brittle inclusions with obvious edgesedges andand corners,corners, andand becomebecome chain-likechain‐like Al2O3 inclusions are brittle inclusions with obvious edges and corners, and become chain‐like distributionsdistributions afterafter rolling.rolling. Large size inclusions are easyeasy toto becomebecome thethe sourcesource pointpoint ofof steelsteel fracturefracture distributions after rolling. Large size inclusions are easy to become the source point of steel fracture

Materials 2020, 13, 1473 6 of 12 under stress, which is easy to scratch the matrix and produce the crack source [11]. Meanwhile, the long strip and large-size MnS inclusions in the aluminum deoxidized IF steel have a large amount of deformation in the deep stamping process, which is very easy to cause the impact of material transverse, radial toughness and plasticity, reduce the continuity of the structure, and cause the product stamping cracking and other problems [14]. When rare earth Ce was added to the steel, the combination of Ce with activity O and S in the steel had lower Gibbs free energy, and it was easy to generate CeAlO3, Ce2O2S, and Ce2O3. On the one hand, it reduced the concentration and supersaturation of aluminum and oxygen elements, and reduced the ability of single particle Al2O3 to aggregate into large-scale cluster inclusions [15]. On the other hand, Ce first combined with S in steel and precipitated earlier than MnS in the solidification process, resulting in small-size spherical inclusions, which can significantly reduce the size and quantity of MnS inclusions at various positions of the slab [16,17]. Therefore, Ce plays a role in the modification of inclusions. The morphology of inclusions changes from chain and strip to spherical. The size of the round inclusions was mostly about 2–5 µm. The distribution of the inclusions was dispersive, which is conducive to improving the structure continuity of the steel. The small spherical rare earth inclusions were not easy to deform in the process of impact deformation, which can slow down the stress concentration at the crack tip in the process of crack growth, thus playing an effect of cushioning and impeding crack growth [18]. In the process of hot working, the damage caused by large-scale inclusions and their shapes was reduced, and the anisotropy of materials was reduced. Besides, after the addition of rare earth, it preferentially segregated at the grain boundary, purified the grain boundary, and improved the grain boundary strength. The transition from intergranular fracture to transgranular fracture can effectively improve the impact toughness and deep stamping performance of the strip [19].

3.2. Effect of Rare Earth Ce on Microstructure Refinement of Steel Strip Figure6 is the comparison of microstructure and grain size detection of a strip in each rolling process. It can be seen that the microstructure was ferrite, the grain of sample 1# obviously had a certain orientation, grain growth was not enough, the structure was not fully recrystallized, some grains were isometric, and a small part of grains were long strip. The grain of sample 2# were more uniform than sample 1#. After annealing, the structure realized complete recrystallization, and the grains grew more evenly, all of which were transformed into equiaxed grains. After the test, the grain sizes of sample 1# were 7 and 9.5, and the sample 2# were 8.5 and 11. The results show that the structure of the strip was more uniform and compact, and the grain size was finer. Due to the addition of rare earth Ce, a series of high melting point compounds such as CeO2, Ce2O3, Ce2O2S, and CeAlO3 with small size were formed in the steel. During the solidification process, homogeneous nucleation needed to generate a crystal nucleus larger than the critical size from the liquid phase, which required a high degree of supercooling. However, the activation energy of heterogeneous nucleation based on the second phase particles in the melt was greatly reduced. Based on the empirical electron theory and lattice mismatch theory [12,20], the lattice mismatch of CeO2, Ce2O3, Ce2O2S, and CeAlO3 was very small, which had a smaller interface free energy required for transformation. Its nucleation supercooling was much smaller than that of A12O3, SiO2, and MnO, so it can increase the nucleation density and refine the grains. In addition, based on the solidification theory, the difference between heterogeneous nucleation work and homogeneous nucleation work was 1/4[2-2cosθ-sin2θcosθ], the wetting tendency of crystal on inclusion surface was expressed by θ, and the wetting angle θ indicated the effectiveness of heterogeneous nucleation. The high surface activity of rare earth elements resulted in the decrease of interfacial tension, the increase of viscosity, the decrease of the wetting angle, and the production of smaller heterogeneous nucleation work [21]. The fine and dispersed rare earth compounds with high melting point were distributed in the steel. These particles can be used as heterogeneous nucleation points, effectively improving the nucleation rate, obtaining the inoculant effect for the heterogeneous Materials 2020, 13, x FOR PEER REVIEW 6 of 12

under stress, which is easy to scratch the matrix and produce the crack source [11]. Meanwhile, the long strip and large‐size MnS inclusions in the aluminum deoxidized IF steel have a large amount of deformation in the deep stamping process, which is very easy to cause the impact of material transverse, radial toughness and plasticity, reduce the continuity of the structure, and cause the product stamping cracking and other problems [14]. When rare earth Ce was added to the steel, the combination of Ce with activity O and S in the steel had lower Gibbs free energy, and it was easy to generate CeAlO3, Ce2O2S, and Ce2O3. On the one hand, it reduced the concentration and supersaturation of aluminum and oxygen elements, and reduced the ability of single particle Al2O3 to aggregate into large‐scale cluster inclusions [15]. On the other hand, Ce first combined with S in steel and precipitated earlier than MnS in the solidification process, resulting in small‐size spherical inclusions, which can significantly reduce the size and quantity of MnS inclusions at various positions of the slab [16,17]. Therefore, Ce plays a role in the modification of inclusions. The morphology of inclusions changes from chain and strip to spherical. The size of the round inclusions was mostly about 2–5 μm. The distribution of the inclusions was dispersive, which is conducive to improving the structure continuity of the steel. The small spherical rare earth inclusions were not easy to deform in the process of impact deformation, which can slow down the stress concentration at the crack tip in the process of crack growth, thus playing an effect of cushioning and impeding crack growth [18]. In the process of hot working, the damage caused by large‐scale inclusions and their shapes was reduced, and the anisotropy of materials was reduced. Besides, after the addition of rare earth, it preferentially segregated at the grain boundary, purified the grain boundary, and improved the grain boundary strength. The transition from intergranular fracture to transgranular fracture can effectively improve the impact toughness and deep stamping performance of the strip [19].

3.2. Effect of Rare Earth Ce on Microstructure Refinement of Steel Strip Figure 6 is the comparison of microstructure and grain size detection of a strip in each rolling process. It can be seen that the microstructure was ferrite, the grain of sample 1# obviously had a certain orientation, grain growth was not enough, the structure was not fully recrystallized, some

Materialsgrains2020 were, 13, 1473isometric, and a small part of grains were long strip. The grain of sample 2# were7 of more 12 uniform than sample 1#. After annealing, the structure realized complete recrystallization, and the grains grew more evenly, all of which were transformed into equiaxed grains. After the test, the grain nucleationsizes of ofsampleδ-Fe and1# wereγ-Fe, 7 further and 9.5, expanding and the thesample equiaxed 2# were crystal 8.5 area,and limiting11. The theresults development show that of the columnarstructure crystal, of the andstrip thus was realizing more uniform the reﬁnement and compact, of solidiﬁcation and the grain structure size was [22 finer.–24].

Materials 2020, 13, x FOR PEER REVIEW 7 of 12

FigureFigure 6. Comparison6. Comparison of microstructureof microstructure of stripof strip during during rolling rolling process. process. 1#: 1#: (a) ( HR,a) HR, (b) ( CA.b) CA. 2#: 2#: (c) ( HR,c) HR, (d)( CA.d) CA.

InDue the HR to the process, addition due toof therare large earth rare Ce, earth a series atoms, of high the solid melting solution point in compounds the crystal wassuch limited, as CeO2, andCe the2O3, distortion Ce2O2S, and energy CeAlO caused3 with bysmall dissolution size were informed the crystal in the wassteel. far During greater the than solidification that dissolved process, inhomogeneous the grain boundary nucleation area, needed so the to rare generate earth elementsa crystal nucleus will preferentially larger than segregate the critical in size the from grain the boundaryliquid phase, [25,26 ].which However, required the meltinga high degree points ofof CeOsupercooling.2, Ce2O3, Ce However,2O2S, and the CeAlO activation3 particles energy are of relativelyheterogeneous high, which nucleation can preferentially based on the precipitate second phase on theparticles grain in boundary, the melt was subgrain greatly boundary, reduced. andBased dislocationon the empirical line to hinder electron the theory grain boundary and lattice migration, mismatch stabilize theory the [12,20], subgrain, the lattice slow downmismatch the process of CeO2, ofCe dynamic2O3, Ce recrystallization,2O2S, and CeAlO static3 was recrystallization, very small, which and had grain a smaller growth, interface and obtain free the energy rolled required structure for oftransformation. grain refinement Its [20 nucleation,27]. supercooling was much smaller than that of A12O3, SiO2, and MnO, so it canAs aincrease result of the the nucleation heredity density of the structure, and refine based the grains. on the fine grains, the smaller parent phase austeniteIn was addition, produced based in on the the CA solidification process, which theory, provides the difference favorable conditions between heterogeneous for the next refinement nucleation ofwork ferrite and structure homogeneous (Figure6 ).nucleation The finer work and uniform was 1/4[2 grains‐2cosθ‐ indicatedsin2θcos thatθ], the more wetting grains tendency were involved of crystal inon deformation, inclusion surface so that thewas strip expressed had better by θ plastic, and deformation.the wetting angle Meanwhile, θ indicated the finer the theeffectiveness grain was, of theheterogeneous more the number nucleation. of grains The per high unit surface volume activity was, the of largerrare earth the totalelements area resulted of grain in boundary the decrease was, of theinterfacial more dislocation tension, barriersthe increase were of generated, viscosity, thethe more decrease work of was the wetting consumed angle, before and fracture, the production and the of highersmaller the heterogeneous plastic deformation nucleation resistance work of [21]. metal The was fine [ 18and]. Therefore,dispersed rare the additionearth compounds of rare earth with can high improvemelting the point grain were size distributed rating of the in finished the steel. strip These by 1.5particles grades, can which be used is conducive as heterogeneous to improving nucleation the strengthpoints, and effectively plasticity improving of the strip. the nucleation rate, obtaining the inoculant effect for the heterogeneous nucleation of δ‐Fe and γ‐Fe, further expanding the equiaxed crystal area, limiting the development 3.3. Effect of Rare Earth Ce on Texture of Strip of columnar crystal, and thus realizing the refinement of solidification structure [22–24]. TheIn excellent the HR process, deep stamping due to the properties large rare of earth IF steel atoms, mainly the solid come solution from its in {111} the crystal texture. was Table limited,3 showsand the statisticsdistortion of energy the favorable caused texture by dissolution content ofin thethe stripcrystal detected was far in greater each rolling than that process. dissolved It can in the grain boundary area, so the rare earth elements will preferentially segregate in the grain boundary [25,26]. However, the melting points of CeO2, Ce2O3, Ce2O2S, and CeAlO3 particles are relatively high, which can preferentially precipitate on the grain boundary, subgrain boundary, and dislocation line to hinder the grain boundary migration, stabilize the subgrain, slow down the process of dynamic recrystallization, static recrystallization, and grain growth, and obtain the rolled structure of grain refinement [20,27]. As a result of the heredity of the structure, based on the fine grains, the smaller parent phase austenite was produced in the CA process, which provides favorable conditions for the next refinement of ferrite structure (Figure 6). The finer and uniform grains indicated that more grains were involved in deformation, so that the strip had better plastic deformation. Meanwhile, the finer the grain was, the more the number of grains per unit volume was, the larger the total area of grain boundary was, the more dislocation barriers were generated, the more work was consumed before fracture, and the higher the plastic deformation resistance of metal was [18]. Therefore, the addition of rare earth can improve the grain size rating of the finished strip by 1.5 grades, which is conducive to improving the strength and plasticity of the strip.

Materials 2020, 13, x FOR PEER REVIEW 8 of 12

Materials 2020, 13, 1473 8 of 12 3.3. Effect of Rare Earth Ce on Texture of Strip The excellent deep stamping properties of IF steel mainly come from its {111} texture. Table 3 beshows seen the that statistics the sample of the 2# percentagefavorable texture contents content of the of favorable the strip texturedetected {110} in each<001 rolling> in the process. HR process It can andbe seen {111} that (< 110 the >sample + < 112 2#> percentage) in the CR contents and CA processesof the favorable were 5.78%, texture 22.7%, {110} and <001> 24.16% in the respectively, HR process whichand {111} were (< greater 110 > + than < 112 sample >) in the 1# CR (3.79%, and CA 18.8%, processes and 21.62%) were 5.78%, in the corresponding22.7%, and 24.16% processes. respectively, which were greater than sample 1# (3.79%, 18.8%, and 21.62%) in the corresponding processes. Table 3. The statistics of favorable texture content in each rolling process. Figure 7 shows the content statistics of orientation distribution function (ODF) cross section (Ф2 HR CR CA = 45°)Processes of strip steel detected in each rolling process. It can be seen that the sample2# polar density max values of texture in{110} HR,<001 CR,> and /% CA processes{111} (<110 were,> + < 112respectively,>) /% {111}4.1, 13.3, (<110 and> + 7.9, <112 which> /%) were greater than1# the sample1# (2.9, 3.79 12.4, and 6.5) in corresponding 18.8 processes. The tensile 21.62 strength of body centred cube2# lattice (BCC) metal 5.78 in <111> direction was 22.7 the highest, so the deformation 24.16 resistance in <111> direction was the largest, and the <111> direction of {111} texture was perpendicular to the plate surface,Figure so 7this shows texture the made content the statisticssteel plate of difficult orientation to deform distribution in the thick function direction (ODF) during cross stamping, section (Φmaking2 = 45◦ the) of r strip value steel larger. detected The inmore each the rolling {111} process. texture content It can be was, seen the that higher the sample2# the strength polar was density and maxthe higher values the of texture plastic instrain HR, ratio CR, and r value CA processeswas [28,29]. were, Therefore, respectively, the sample 4.1, 13.3, 2# andhad 7.9,better which annealing were greaterstructure than and the deep sample1# stamping (2.9, properties. 12.4, and 6.5) in corresponding processes. The tensile strength of body centred cube lattice (BCC) metal in <111> direction was the highest, so the deformation resistance in <111> direction wasTable the 3. largest, The statistics and the of< favorable111> direction texture of content {111} texturein each rolling was perpendicular process. to the plate surface, so this texture made the steel plate diﬃcult to deform in the thick direction during stamping, HR CR CA makingProcesses the r value larger. The more the {111} texture content was, the higher the strength was and {110} <001> /% {111} (<110> + <112>) /% {111} (<110> + <112> /%) the higher the plastic strain ratio r value was [28,29]. Therefore, the sample 2# had better annealing 1# 3.79 18.8 21.62 structure and deep stamping properties. 2# 5.78 22.7 24.16

Figure 7. The ODF cross section (Φ2 = 45◦) in the rolling process. 1#: (a) HR, (b) CR, (c) CA. 2#: (d) HR, Figure 7. The ODF cross section (Ф2 = 45°) in the rolling process. 1#: (a) HR, (b) CR, (c) CA. 2#: (d) HR, (e) CR, (f) CA. (e) CR, (f) CA. Figure8 shows the typical Fe(Nb +Ti)P precipitates of the sample 1# observed by SEM in the rollingFigure process. 8 shows It can bethe seen typical from Fe(Nb+Ti)P the figure thatprecipitates the types of were the FeTiP,sample Fe(Nb 1# observed+Ti)P, FeNbP, by SEM etc., in with the therolling size rangeprocess. of 1–3It canµm. be No seen inclusion from the of figure this type that was the found types inwere each FeTiP, field ofFe(Nb+Ti)P, view in the FeNbP, sample etc., 2#. Thewith strength the size of range IF steel of 1–3 containing μm. No inclusion phosphorus of this was type mainly was enhanced found in each by the field solution of view strengthening in the sample e2#.ffect The of phosphorus. strength of However, IF steel it containing was easy for phosphorus phosphorus thatwas precipitatedmainly enhanced in the precipitated by the solution phase ofstrengthening FeTiP to cause effect the grainof phosphorus. boundary However, brittleness it of was steel easy due for to grainphosphorus boundary that segregation. precipitated In in the the processprecipitated of annealing phase of and FeTiP recrystallization, to cause the FeTiP grain played boundary a role brittleness of a pinning of steel effect due on the to graingrain boundary,boundary hinderedsegregation. the growthIn the process of {111} of oriented annealing grains, and andrecrystallization, weakened the FeTiPγ texture played strength, a role of which a pinning is harmful effect

Materials 2020, 13, 1473 9 of 12 Materials 2020, 13, x FOR PEER REVIEW 9 of 12

on the grain boundary, hindered the growth of {111} oriented grains, and weakened the γ texture to the r value [30–32]. In addition, the precipitation of Fe(Nb+Ti)P will also consume solid solution P strength, which is harmful to the r value [30–32]. In addition, the precipitation of Fe(Nb+Ti)P will also element,consume reduce solid the solution strength P element, of the strip reduce steel, the consume strength of Ti the element, strip steel, increase consume the Ti interstitial element, increase atom C, N, and damagethe interstitial the formability atom C, N, ofand the damage material the [formability33,34]. of the material [33,34].

FigureFigure 8. The 8. The Fe(Nb Fe(Nb+Ti)P+Ti)P precipitation precipitation ofof sample 1# 1# detected detected in the in therolling rolling process: process: (a) HR, (a ()b) HR, CR, ((bc)) CR, (c) CA.CA.

The resultsThe results show show that that [35 [35,36],,36], because because the the formation formation process of of Fe(Nb+Ti)P Fe(Nb+Ti)P phase phase was was that thatthe the mixedmixed rich grouprich group of FeTi of FeTi and and FeNb FeNb phase phase formed formed firstfirst in steel, the the Fe(Nb+Ti)P Fe(Nb+Ti)P phase phase formed formed as P as P element diffused into the rich group of FeTi and FeNb. With the addition of rare earth, the diffusion element diffused into the rich group of FeTi and FeNb. With the addition of rare earth, the diffusion ability of Nb and Ti elements dissolved in the steel was weakened, which led to a small number of ability of Nb and Ti elements dissolved in the steel was weakened, which led to a small number of FeTi and FeNb phases in the steel containing rare earth, and the precondition for the formation of FeTi andFe(Nb+Ti)P FeNb phases phase was in therestrained steel containing to some extent. rare Moreover, earth, and rare the earth precondition made the diffusion for the formation rate and of Fe(Nbability+Ti)P of phase P element was weaken. restrained That to is, some the ability extent. of P Moreover, element diffusion rare earth into the made phase the of diFeTiffusion and FeNb rate and abilitywas of Pweakened, element weaken.which made That the is, further the ability formation of P of element Fe(Nb+Ti)P diffusion phase into difficult. the phase of FeTi and FeNb was weakened,Other studies which show made that the [26] further the atomic formation radius of of Fe(Nb Ce was+ Ti)Pabout phase 50% larger difficult. than that of Fe, which Otheris a solid studies solution show in thatthe matrix, [26] the resulting atomic in radius a large of lattice Ce was distortion about 50% energy larger and than an increase that of in Fe, the which is a solidsystem solution energy. in Because the matrix, the interaction resulting inbetween a large Ce lattice and distortiondislocation energywas larger and than an increase that of P, in the systemaccording energy. Becauseto the theory the interaction of solute between atom equilibrium Ce and dislocation segregation, was Ce larger and than P had that competitive of P, according segregation, and Ce interacted with dislocation first, occupying the distortion area and vacancy on to the theory of solute atom equilibrium segregation, Ce and P had competitive segregation, and Ce the grain boundary preferentially, thus inhibiting the segregation of P on the grain boundary and interacted with dislocation first, occupying the distortion area and vacancy on the grain boundary hindering the precipitation of Fe(Nb+Ti)P phase. Therefore, Ce can inhibit the precipitation of P preferentially,element and thus Fe(Nb+Ti)P inhibiting phase, the segregation effectively ofincreasing P on the the grain solid boundary solution amount and hindering of P element the precipitation in steel, of Fe(Nbimproving+Ti)P phase. the solid Therefore, solution strengthening Ce can inhibit effect the of precipitation P element in high of P‐strength element IF and steel, Fe(Nb and obtaining+Ti)P phase, effectivelya large increasing proportion theof {111} solid favorable solution texture, amount which of P is element conducive in to steel, improving improving the formability the solid index solution strengtheningr90 value. effect of P element in high-strength IF steel, and obtaining a large proportion of {111} favorable texture, which is conducive to improving the formability index r90 value. 3.4. Comparison of Stamping Properties 3.4. ComparisonBased ofon Stamping the above Properties analysis, according to the comparison results of the mechanical properties Basedof the onsamples the above in Figure analysis, 9, the overall according r90 value to the of comparison the sample 2# results was higher of the than mechanical that of the properties sample of 1#. The high‐strength IF steel containing phosphorus with added rare earth had a better deep drawing the samples in Figure9, the overall r 90 value of the sample 2# was higher than that of the sample 1#. The property. After rare earth addition, the inclusion size was reduced, the inclusion morphology was high-strength IF steel containing phosphorus with added rare earth had a better deep drawing property. optimized, the solidification nucleation rate was increased, the as‐cast and as‐rolled grains were Afterrefined, rare earth and addition, the strength the inclusionand toughness size was of the reduced, strip were the inclusioneffectively morphology improved by was fine optimized, grain the solidificationstrengthening. nucleation Meanwhile, rate the was solution increased, strengthening the as-cast effect and of as-rolledP element grains in high were‐strength refined, IF steel and the strengthwas and improved, toughness a large of the proportion strip were of {111} effectively favorable improved texture bywas fine obtained, grain strengthening.and the r90 value Meanwhile, of the the solutionproduct strengthening was increased. e ffTheect above of P element results have in high-strength certain guiding IF steelsignificance was improved, for improving a large the proportion deep of {111}stamping favorable property texture of high was‐strength obtained, IF steel and and the lightweight r90 value of steel the application. product was increased. The above results have certain guiding significance for improving the deep stamping property of high-strength IF steel and lightweight steel application.

Materials 2020, 13, 1473 10 of 12 Materials 2020, 13, x FOR PEER REVIEW 10 of 12

2.4

2.3 1# 2# 2.2

2.1

2.0

value 1.9 90 r 1.8

1.7

1.6

1.5 2# 4# 6# 8# 10# 12# 14# 16# 18# 20# The samples order

Figure 9. Comparison of r 90 values.values. 4. Conclusions 4. Conclusion (1) Based on the addition of rare earth in steel, the combination of Ce with activity O and S in the steel (1) Based on the addition of rare earth in steel, the combination of Ce with activity O and S in the had lower Gibbs free energy, and it was easy to generate CeAlO3, Ce2O2S, and Ce2O3. Meanwhile, steel had lower Gibbs free energy, and it was easy to generate CeAlO3, Ce2O2S, and Ce2O3. the inclusion size decreased obviously, but the amount of inclusions increased. The morphology Meanwhile, the inclusion size decreased obviously, but the amount of inclusions increased. The of inclusions changed from chain and strip to spherical. The size of rare earth inclusions was morphology of inclusions changed from chain and strip to spherical. The size of rare earth mostly about 2–5 µm, distributed and dispersed, and their elastic modulus was close to that of inclusions was mostly about 2–5 μm, distributed and dispersed, and their elastic modulus was steel matrix, which is conducive to improving the structure continuity of steel. close to that of steel matrix, which is conducive to improving the structure continuity of steel. (2)(2) TheThe rare rare earth earth compound compound had had a a high high melting point. As As a a heterogeneous nucleation nucleation point, point, the the nucleationnucleation rate rate was was increased increased and and the the solidification solidification structure structure was refined. refined. Meanwhile, Meanwhile, in in the the processprocess of of rolling rolling heat heat treatment, treatment, rare rare earth earth can can preferentially preferentially segregate segregate grain grain boundaries boundaries to to slow slow downdown grain grain growth. growth. The The addition addition of of rare rare earth earth can can improve improve the the grain grain size size rating rating of of a a continuous continuous annealingannealing strip by 1.51.5 grades,grades, which which is is conducive conducive to to improving improving the the strength strength and and plasticity plasticity of strip. of (3) strip.Rare earth Ce can inhibit the segregation of P element at the grain boundary and the precipitation (3) Rareof Fe(Nb earth+Ti)P Ce phase.can inhibit It can the effectively segregation increase of theP element solid solution at the amount grain ofboundary P element and in steel, the precipitationimprove the solidof Fe(Nb+Ti)P solution strengthening phase. It can e ffeffectivelyect of P element increase in high-strengththe solid solution IF steel, amount and obtain of P elementa large proportionin steel, improve of {111} the favorable solid solution texture, strengthening which is conducive effect of P toelement improving in high the‐strength stamping IF steel,formability and obtain index a rlarge90 value. proportion of {111} favorable texture, which is conducive to improving the stamping formability index r90 value. Author Contributions: Conceptualization, H.W. and Y.B.; methodology, H.W. and Y.B.; software, L.L.; validation, AuthorL.L. and Contributions: Q.Z.; formal analysis, Conceptualization, H.W.; investigation, H.W. H.W.and Y.B.; and Y.L.; methodology, resources, C.D.;H.W. data and curation, Y.B.; software, H.W. and L.L.; L.L.; writing—original draft preparation, H.W.; writing—review and editing, H.W. and Y.B.; visualization, H.W. and validation,Y.B.; supervision, L.L. and Y.B.; Q.Z.; project formal administration, analysis, H.W.; C.D.; investigation, funding acquisition, H.W. and Y.B. Y.L.; and resources, C.D. All authors C.D.; data have curation, read and H.W.agreed and to the L.L.; published writing—original version of thedraft manuscript. preparation, H.W.; writing—review and editing, H.W. and Y.B.; visualization,Funding: This H.W. research and was Y.B.; supported supervision, financially Y.B.; project by National administration, Natural Science C.D.; Foundationfunding acquisition, of China (No.Y.B. 51574019)and C.D. and Open Project of State Key Laboratory of Advanced Special Steel, Shanghai University (SKLASS 2017-12). All authors have read and agreed to the published version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to Funding: This research was supported financially by National Natural Science Foundation of China (No. publish the results. 51574019) and Open Project of State Key Laboratory of Advanced Special Steel, Shanghai University (SKLASS 2017‐12). References Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the 1. Wang, M.; Bao, Y.-P.; Yang, Q.; Zhao, L.-H.; Lin, L. Coordinated control of carbon and oxygen for study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to ultra-low-carbon interstitial-free steel in a smelting process. Int. J. Miner. Metall. Mater. 2015, 22, 125. publish the results. [CrossRef] Reference2. Guo, J.-L.; Bao, Y.-P.; Wang, M. Cleanliness of Ti-bearing Al-killed ultra-low-carbon steel during different heating processes. Int. J. Miner. Metall. Mater. 2017, 24, 1370. [CrossRef] 1.3. Wang,Di Giovanni, M.; Bao, C.; Y. Biro,‐P.; Yang, E.; Zhou, Q.; N.-Y.Zhao, Impact L.‐H.; Lin, of liquid L. Coordinated metal embrittlement control of cracks carbon on and resistance oxygen spotfor ultra weld‐ lowstatic‐carbon strength. interstitialSci. Technol.‐free steel Welding in a Joiningsmelting2019 process., 24, 218. Inter. [CrossRef J. Miner.] Metallurgy and Mater. 2015, 22, 125. 2. Guo, J.‐L.; Bao, Y.‐P.; Wang, M. Cleanliness of Ti‐bearing Al‐killed ultra‐low‐carbon steel during different heating processes. Inter. J. Miner. Metallurgy and Mater. 2017, 24, 1370. 3. Di Giovanni, C.; Biro, E.; Zhou, N.‐Y. Impact of liquid metal embrittlement cracks on resistance spot weld static strength. Sci. Technol. Welding Joining 2019, 24, 218.

Materials 2020, 13, 1473 11 of 12

4. Oliveiraa, J.-P.; Ponderb, K.; Brizesb, E.; Abkec, T.; Edwardsd, P.; Ramirezb, A.-J. Combining resistance spot welding and friction element welding for dissimilar joining of aluminum to high strengths steels. J. Mater. Proc. Technol. 2019, 273, 01. [CrossRef] 5. Zhu, R.-Q.; Song, X.-L.; Zheng, A.-Q.; Chen, R.-D.; Jia, J.; Fan, L.-X. Strain hardening and microstructure characteristic of high strength IF bearing phosphorus steel. Iron Steel 2018, 53, 55. 6. Wang, R.; Bao, Y.-P.; Li, Y.-H.; Yan, Z.-J.; Li, D.-Z.; Kang, Y. Inﬂuence of metallurgical processing parameters on defects in cold-rolled steel sheet caused by inclusions. Int. J. Miner. Metall. Mater. 2019, 26, 440. [CrossRef]

7. Wang, R.; Bao, Y.-P.; Yan, Z.-J.; Li, D.-Z.; Kang, Y. Comparison between the surface defects caused by Al2O3 and TiN inclusions in interstitial-free steel auto sheets. Int. J. Miner. Metall. Mater. 2019, 26, 178. [CrossRef] 8. Wang, Y.-P.; Zhao, X.-L.; Liu, J. Inﬂuence of B content on recrystallization and texture of P-containing high strength IF steel. Heat Treat. Metals 2018, 43, 54–59. 9. Yang, J.-C.; Li, H.-W.; Zhou, L.; Zhang, J.; Liu, N. Mechanism of trace cerium in ultra-clean IF steel. Iron Steel 2015, 50, 81. 10. Xiao, J.-G.; Chen, H.-J.; Wang, F.-M. Eﬀects of rare earth in ship hull plate steel on its microstructure and low temperature toughness. China Rare Earths 2010, 31, 52.

11. Li, H.; Yu, Y.-C.; Ren, X.; Zhang, S.-H.; Wang, S.-B. Evolution of Al2O3 inclusions by cerium treatment in low carbon high manganese. Iron Steel Res. Int. 2017, 24, 925. [CrossRef] 12. Gao, S.; Wang, M.; Guo, J.-L.; Wang, H.; Zhi, J.-G.; Bao, Y.-P. Characterization transformation of inclusions using rare earth Ce treatment on Al-killed titanium alloyed interstitial free steel. Steel Res. Inter. 2019, 90, 01. [CrossRef] 13. Yang, J.-C.; Li, H.-W.; Zhang, J.; Zhou, L.; Zhao, G.-S. Thermodynamic analysis and experimental study of cerium inclusions in IF steel containing cerium. Chin. Rare Earths 2018, 39, 01. 14. Tian, Y.-Q.; Wang, A.-D.; Song, J.-Y.; Zhen, X.-P.; Zhao, D.-G.; Chen, L.-S. Formation reason and control measures of surface quality defects of cold rolled steel sheet for galvanization. J. Iron Steel Res. 2017, 29, 312. 15. Wang, C.; Yu, Y.; Liu, K. Forming reason and control of strip fracture in high strength IF steel containing phosphorus during hot-rollling process. China Metallurgy 2016, 26, 17. 16. Chen, S.-F.; Liu, X.; Lei, H. Precipitation behavior of MnS inclusions during solidification of manganese steel. J. Univer. Sci. Technol. Liaoning 2017, 40, 241. 17. Zhou, Y.; Liu, W.-D.; Yan, J.; Liu, D. Valence electron theoretical analysis of the influence of rare earth elements on the deformation ability of MnS inclusions. Chin. J. Rare Metals 2006, 02, 185. 18. Wang, H.; Bao, Y.-P.; Zhao, M.; Wang, M.; Yuan, X.-M.; Gao, S. Effect of Ce on the cleanliness, microstructure and mechanical properties of high strength low alloy steel Q690E in industrial production process. Int. J. Miner. Metall. Mater. 2019, 26, 1372. [CrossRef] 19. Wang, H.; Bao, Y.-P.; Liu, Y.; Wang, M.; Wang, J.-G.; Su, C. Effect of microstructure and grain size on DWTT properties of 22mm thick X80M hot rolled steel strip. Metal. Res. Technol. 2019, 220, 116. 20. Xu, J.; Qi, G.-P.; Zhu, J.-X. Effect of Rare Earth on Solidification Structure and Formability of 430 Ferritic Stainless Steel Shee. J. Chin. Social. Rare Earths 2006, 24, 486. 21. Li, G.; Lu, M.-G.; Lan, P. Research progress of rare earth Ce on improvement of microstructure and homogeneity of as-cast steel. J. Iron Steel Res. 2018, 30, 79. 22. Huang, Y.; Cheng, G.-G.; Xie, Y. Modification Mechanism of Cerium on the Inclusions in Drill Steel. J. Acta Metall. Sin. 2018, 54, 1253. 23. Wang, X.-P.; Zhao, A.-M.; Zhao, Z.-Z.; Huang, Y.; Li, L.; He, Q. Mechanical properties and characteristics of nanometer-sized precipitates in hot-rolled low-carbon ferritic steel. Int. J. Miner. Metall. Mater. 2014, 21, 266. [CrossRef] 24. Hao, L.; Ren, H.-P.; Jin, Z.-L.; Wang, C.-Y.; Li, D.-Y.; Li, Y.-H. Effects of rare earth La and annealing process on microstructure of high strength IF steel. Heat Treat. Metals 2017, 42, 35. 25. Yu, X.; Ren, H.-P.; Jin, Z.-L. Effect of rare earth on Recrystallization Behavior of high strength IF steel during hot rolling. Chin. Rare Earths 2018, 39, 117. 26. Yang, J.-C.; Yang, C.-Q.; Wang, S.-J. Effect of micro rare earth Ce on high temperature mechanical properties of IF steel. Special Steel 2014, 35, 65. 27. Lin, Q.; Guo, F.; Zhu, X.-Y. Behaviors of Lanthanum and Cerium on Grain Boundaries in Carbon Manganese Clean Steel. J. Rare Earths 2007, 25, 485. [CrossRef] Materials 2020, 13, 1473 12 of 12

28. Song, X.-L.; Yuan, Z.-X.; Li, P.-H.; Guo, A.-M.; Wang, D. Effect of recrystallization annealing on Microstructure of cold rolled IF steel containing phosphorus. J. Chin. Electron Microsc. Soc. 2008, 27, 473. 29. Gupta, R.P.; Martin, G.; Lanteri, S.; Maugis, P.; Guttmann, M. Relationship between the electronic structure and the precipitation of FeTiP in interstitial-free ferritic steels. Philosoph. Magaz. 2000, 80, 2393. 30. Xiong, D.-L.; Mao, W.-M. Desolvation and hardening of FeTiP phase in high strength if steel containing phosphorus. Chin. J. Engineer. 2000, 22, 350. 31. Ghosh, P.; Ray, R.-K.; Bhattacharjee, D. Determination of the crystal structure of FeTiP-type precipitates in a few interstitial-free high-strength steels. Scripta Materialia 2007, 57, 241. [CrossRef] 32. Zhang, J.-C.; Cao, Y.; Jiang, G.-W.; Di, H.-S. Effect of Annealing Temperature on the Precipitation Behavior and Texture Evolution in a Warm-Rolled P-Containing Interstitial-Free High Strength Steel. Acta Metall. Sin. 2014, 27, 395. [CrossRef] 33. Guo, W.-M.; Sun, D.P.; Zhang, H.-Y.; Ding, N.; Shi, J.-B.; Zhao, B.-L. Precipitates and deep-drawability of containing phosphorus high strength IF steel. Heat Treat. Metals. 2015, 40, 99–103. 34. Guo, W.-M.; Wang, Z.-C.; Li, Y.-D.; Xu, N.; Shi, J.-B. Effect of Phosphorus Content on Properties of Warm-Rolled Interstitial-Free Steel Sheets. Acta Metall. Sin. 2014, 27, 395. [CrossRef] 35. Ren, D.; Jin, Z.-L.; Ren, H.-P.; Li, Y.-Y.; Li, Y.-M.; Zhang, J.-S. Effect of Rare Earth Lanthanum on P Element and Precipitation P-containing Precipitate in High Strength IF Steel. Chin. J. Rare Met. 2019, 01, 01. 36. Jia, J.; Zhu, W.-W.; Song, X.-L.; Yuan, Z.-X. Precipitation Behavior of FeTiP in Ti-added Interstitial Free High Strength Steels. J. Iron Steel Res. 2016, 23, 692. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).