metals Article Precipitates in Compact Strip Production (CSP) Process Non-Oriented Electrical Steel Jia-long Qiao 1,* , Fei-hu Guo 1, Jin-wen Hu 1,2, Li Xiang 1, Sheng-tao Qiu 1 and Hai-jun Wang 2 1 National Engineering Research Center of Continuous Casting Technology, China Iron & Steel Research Institute Group, Beijing 100081, China; [email protected] (F.-h.G.); [email protected] (J.-w.H.); [email protected] (L.X.); [email protected] (S.-t.Q.) 2 School of Metallurgy and Resources, Anhui University of Technology, Maanshan 243002, China; [email protected] * Correspondence: [email protected]; Tel.: +86-188-0102-8675 Received: 16 July 2020; Accepted: 18 September 2020; Published: 29 September 2020 Abstract: Nitrogen and Sulfur in non-oriented electrical steel would form precipitates, which would severely affect its magnetic properties. Precipitates in compact strip production (CSP) process non-oriented electrical steel were investigated using a transmission electron microscope (TEM) and scanning electron microscopy (SEM). The precipitation mechanism and influence on grain growth were analyzed experimentally and theoretically. The results showed that the main particles in steel were AlN, TiN, MnS, Cu2S, and fine oxide inclusions. The spherical or quasi-spherical of MnS and Cu2S were more liable to precipitate along grain boundaries. During the soaking process, the amount of MnS precipitated on the grain boundary was much larger than that of Cu2S. AlN and TiN in cubic shape precipitated inside grains or grain boundaries. Precipitates preferentially nucleated at grain boundaries, and TiN, MnS mainly precipitated during soaking. In the subsequent processes after soaking, AlN and Cu2S would precipitate unceasingly with the decrease in the average size. The distribution density, the volume fraction, and the average size of the precipitates in the annealed sheets were 9.08 1013/cm3, 0.06%, and 54.3 nm, respectively. Precipitates with the grain size of × 30–500 nm hindered the grain growth, the grains with 100–300 nm played a major role in inhibiting the grain growth, and the grains with the grain size of 70–100 nm took the second place. Keywords: non-oriented electrical steel; CSP process; magnetic properties; precipitates; particle growth; driving force and pinning force 1. Introduction Non-oriented electrical steel is a kind of soft magnetic alloys, which desire high magnetic induction and low core loss [1]. On account of the small columnar crystals, which belong to the cubic texture ({100}<001>) of continuous cast slab that would inherit to the finished product, the magnetic induction intensity of compact strip production (CSP) process non-oriented electrical steels would be higher than the traditional products. However, the magnetic properties would be deteriorated by inclusions, which are difficult to float during the casting process, and the small size of precipitates through inhibiting grain growth [2–7]. As a result of the solubility fall of certain elements, the major residual elements or impurities of Cu, Ti, S, N, etc. would form nitride and sulfide precipitates [8]. AlN, TiN, MnS, Cu2S, etc. would obviously hinder the growth of crystal grains and strengthen γ-fiber texture components, leading to the decrease of magnetic properties of non-oriented electrical steel [1,5,9,10]. Many research works have been reported on the study of nitride and sulfide precipitates0 precipitation mechanism in non-oriented electrical steel [5–7,9–14]. The particle nucleation rate and Metals 2020, 10, 1301; doi:10.3390/met10101301 www.mdpi.com/journal/metals Metals 2020, 10, 1301 2 of 15 Metals 2020, 10, x FOR PEER REVIEW 2 of 15 growth depend on the nucleation driving force, the diffusivity diffusivity of controlling element M in the the matrix, matrix, and the interfacial energy associated with the matrix [[8,15].8,15]. In In the the CSP CSP process’ process’ non-oriented non-oriented electrical electrical steel, the size of precipitates would grow more in the soaking process, but the precipitation contents are less [[5].5]. Meanwhile, models on the grain growth inhibiti inhibitionon by precipitates are presented with the hypotheses thatthat thethe grain grain growth growth will will stop stop by by the th pinninge pinning force force of particles of particles [12–14 [12–14,16,17].,16,17]. Studies Studies show showthat the that relationship the relationship between between grain sizes grain would sizes be stronglywould be inhibited strongly by increasinginhibited by the increasing interfacial areathe interfacialbetween grains area between and particles grains [ 18and–20 particles]. The e ff[18–20].ect of di Thfferente effect size of different of precipitates size of on precipitates the ferrite on grain the ferritegrowth grain is also growth different is also in non-oriented different in no electricaln-oriented steel electrical [7,10,14 ,steel21–26 [7,10,14,21–26].]. Given the above, the precipitation mechanism of precipitates in the CSP process’ non-oriented electrical steel has not been studiedstudied comprehensively, and most of these studies only simulate the qualitative relationship between precipitation and and grain grain growth growth theoretically. theoretically. In In the the present present study, study, by means of the thermodynamicsthermodynamics analysis, kinetics calculation, TEM, and SEM, the precipitation behavior of AlN, TiN, MnS, and Cu22S was studied. The The pinning pinning force force of of precipitates precipitates and driving force of grain growth were analyzed as we well,ll, with the aim to reduce the inhibition effects effects of precipitates on grain growth, thus enhancing the magnetic properties ofof CSPCSP process’process’ non-orientednon-oriented electricalelectrical steel.steel. 2. Materials and Methods The mainmain chemical chemical composition composition of compactof compact strip productionstrip production (CSP) process(CSP) non-orientedprocess non-oriented electrical electricalsteel used steel in theused present in the present study is study given is ingiven Table in1 Table. The 1. continuous The continuous casting casting billet billet with with 70 mm 70 mm in inthickness, thickness, which which was was soaked soaked at at about about 1373 1373 K, K was, was hot-rolled hot-rolled to to 2.3 2.3 mm mm inin thicknessthickness byby aa six-high rolling mill. The hot bands were cold rolled in 79% deformation to 0.5 mm in thickness by a six-high rolling mill. Then, the cold-rolled sheetssheets werewere annealedannealed atat 10931093 KK forfor 22 minmin inin NN22:H:H22 = 1:11:1 atmosphere atmosphere for recrystallizationrecrystallization andand grain grain growth. growth. The The heating heating rate rate and and cooling cooling rate rate of the of annealingthe annealing process process were were50 K/ s50 and K/s 25 and K/s, 25 respectively. K/s, respectively. The schematic The schema of thetic thermos-mechanicalof the thermos-mechanical cycle used cycle in used the present in the presentstudy is study shown is inshown Figure in1 .Figure 1. Table 1. The main chemical composition of non-oriented electrical steel. ElementsElements C C Si Si Mn Mn P PS Als NN CuCu Ti Ti Content,Content, wt% wt% 0.0030 0.0030 0.65 0.65 0.25 0.25 0.075 0.075 0.0040 0.0040 0.30 0.00350.0035 0.0300.030 0.00300.0030 2000 CSP process casting to 70mm 1600 Soaking: Annealing: 1373K+40min Initial rolling at 1323K 1093k+2min Heating rate:50K/s 1200 Hot rolling to 2.3mm Colding rate:25K/s Coiling: 800 993K Temperature, K 400 Cold rolling to 0.5mm 0 Time Figure 1. SchematicSchematic of of the the thermos-mechanical thermos-mechanical cycle used in the present study. The microstructure and and morphology morphology of of precipitates precipitates in in steel steel were were scientifically scientifically studied studied using using a transmissiona transmission electron electron microscope microscope (TEM; (TEM; JEM-2100F, JEM-2100F, JEOL, JEOL, Tokyo, Tokyo, Japan) Japan) and and scanning electron microscopy (SEM; Quanta 650FEG, FEI, Morristown, NJ, USA). Combining with energy dispersive spectrometer (EDS) and selected area electron diffraction (SEAD), the compositions and morphology of precipitates could be characterized. Metals 2020, 10, 1301 3 of 15 spectrometerMetals 2020, 10, x (EDS) FOR PEER and REVIEW selected area electron diffraction (SEAD), the compositions and morphology3 of 15 of precipitates could be characterized. TheThe carboncarbon extraction extraction replica replica test test sample sample for for TEM TEM was was prepared prepared into into a sample a sample with with a size a ofsize 8 mmof 8 (TD)mm (TD)10 mm× 10 (RD) mm by(RD) wire by cutting wire cutting and then and roughly then roughly and finely and ground. finely Theground. samples The weresamples prepared were × byprepared electro-polishing by electro-polishing at 90 mA in at 10% 90 mA AA electrolytein 10% AA for electrolyte 120 s. The for electrolyzed 120 s. The sampleselectrolyzed were samples coated withwere a coated layer of with carbon a layer film of with carbon a thickness film with of a aboutthickness 30 nm of about using 30 a vacuum nm using carbon a vacuum spray carbon instrument. spray Afterinstrument. dividing After the dividing carbon film the intocarbon a size film of into about a size 2 mm of about2 mm, 2 mm it was × 2 mm, placed it was in a placed 10% perchloric in a 10% × acidperchloric alcohol acid solution alcohol for solution electrolytic for electrolytic release, and release, then the and molybdenum then the molybdenum net with 3 net mm with in diameter 3 mm in wasdiameter used was to extract used to the extract carbon the film. carbon The film. samples The samples were also were prepared also prepared into SEM into samples, SEM samples, and then and electropolishingthen electropolishing and observation and observation with 100 with fields 100 in each fields sample in each were sample done at were 3000-10000 done magnificationat 3000-10000 undermagnification SEM.