University of Groningen
Precipitate evolution in grain oriented electrical steel and high strength low alloy steel
Zhang, Xukai
DOI:
10.33612/diss.133163231
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Publication date: 2020
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Citation for published version (APA):
Zhang, X. (2020). Precipitate evolution in grain oriented electrical steel and high strength low alloy steel. University of Groningen. https://doi.org/10.33612/diss.133163231
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Chapter 7 Summary and Outlook
7.1 Summary
7.1.1 GO electrical steel
GO electrical steels have a microstructure consisting of centimeter-size grains that have the so-called Goss orientation, which results in low core losses. Therefore, GO electrical steels are used as the core of power transformers. This extraordinary microstructure develops during the so-called high temperature coiling (HTC) annealing process where secondary recrystallization is activated due to removal of the inhibition effect of nanoscale precipitates. Developing in-depth understanding of the evolution of multiple types of precipitates during various stages of HTC annealing is crucial for optimizing the magnetic properties of GO electrical steels by tailoring the precipitates.
In Chapter 3, precipitate evolution in GO electrical steel during simulated HTC annealing was investigated with state-of-the-art STEM techniques. STEM EDS elemental mapping was employed to investigate precipitate composition. The core-shell structure of precipitates was revealed, especially for the type II AlN based precipitates, showing that AlN needs a seed, like copper sulphide, to nucleate. Low kV HAADF-STEM image stitching over larger areas was used to obtain precipitate size distribution in the range of 10 - 100 nm. The type II AlN-based precipitates were observed to be present in distinct areas with high and low number density and the precipitate size and number density evolution in both areas was quantitatively assessed. The type III FCC Cu2-xS precipitates with a size of
several nanometers were observed to dissolve between 830 ºC and 900 ºC. However, they
can precipitate out again as FCC Cu2-xS precipitates during slow furnace cooling when
annealing to above 900 ºC. Finally, the role of precipitates with different compositions and sizes play during the secondary recrystallization process was illuminated: the large type I (Al,Si)N-(Cu,Mn)S precipitates are too large to prevent grain boundary migration; the dissolution of type II AlN-based precipitates is responsible for activating secondary recrystallization process; whereas type III Cu2-xS precipitates only indirectly affect this
process by acting as nuclei on which AlN-based precipitates can grow.
7.1.2 HSLA steel
Cold-rolled and annealed high strength low alloy (HSLA) steels exhibit a combination of high strength, good ductility, weldability and therefore they are widely applied in the automotive industry to reduce vehicle weight. The excellent properties of the steel are achieved by nanoscale precipitates, which result in precipitate strengthening and grain refinement. Therefore, investigating precipitation behavior is of great significance. Recrystallization interacts with precipitation behavior and also affects the final mechanical proprieties, and hence, recrystallization behavior during annealing was also studied.
Chapter 7 Summary and Outlook
In this thesis, two sets of annealed cold-rolled Ti-V HSLA samples were produced. One set was isothermally annealed (see Chapter 4) and the other set was obtained during continuous annealing (see Chapter 6). During isothermal annealing, the cold-rolled samples were annealed to ferrite temperatures (650 °C and 700 °C) for different times up to several hours, whereas during continuous annealing, the cold-rolled samples were annealed to ferrite-austenite two phase region temperatures (780 °C and 800 °C) for different times up to several hundred seconds.
In Chapters 4 and 5, precipitation and recrystallization behavior of isothermally annealed cold-rolled samples were separately investigated and quantification methods were developed. In Chapter 6, the simultaneous precipitation and recrystallization behavior at the early stage of continuous annealing of cold-rolled samples was studied using the characterization techniques developed in Chapters 4 and 5.
In chapter 4, a unique combination of techniques including state-of-the-art TEM and STEM, a matrix dissolution based method and SANS was used to quantify precipitates. In this way precipitate size, composition and volume fraction was always quantitatively obtained by two techniques allowing their close comparison. In the case of matrix dissolution, we combined for the first time filtering with centrifuging, increasing the accuracy of this method and obtaining separate volume fractions of precipitates with different size ranges. We demonstrated that the matrix dissolution results on the precipitate volume fractions are quite accurate and comparable to (even somewhat better than) those obtained from SANS measurements. Moreover, the composition variation in ternary (Ti,V)C precipitates with size was considered and implemented in the SANS data analysis.
Two types of precipitates were distinguished after cold rolling: type I large cuboidal Ti(C,N) precipitates and type II medium-size ellipsoidal (Ti,V)(C,N) precipitates. They form during casting and hot rolling, respectively. Apart from type I and II precipitates, type III spherical (Ti,V)C precipitates were observed in the isothermally annealed samples (see Chapter 4), whereas type III spherical (Ti,V)C precipitates and type IV rectangular (Ti,V)C precipitates were discerned in continuously annealed samples (see Chapter 6). This indicated that type III spherical (Ti,V)C precipitates are formed in ferrite, whereas type IV rectangular (Ti,V)C precipitates form in austenite. For type III spherical (Ti,V)C precipitates, the Ti/(Ti+V) atomic ratio decreases with increasing precipitate radius and annealing temperature, indicating that the core is TiC rich. Their lattice parameters also decrease with the increase of precipitate radius. For type IV rectangular (Ti,V)C precipitates, their sizes are quite uniform and their compositions keep stable when their sizes change.
In Chapter 5, recrystallization kinetics, geometrically necessary dislocation (GND) density and texture evolution were investigated in detail based on EBSD data. The texture in the cold rolled sample is mainly {001}<110> of α fibers and {111}<110> of γ fibers. The recrystallization fraction increases with the increase of annealing time and temperature,
135 whereas the dislocation density has the opposite evolution trend. The average density of GNDs in γ fibers is slightly higher than that in α fibers. α fibers and γ fibers start to recrystallize at the same time, but the recrystallization kinetics of γ fibers is faster than that of α fibers.
In Chapter 6, the effects of different soaking temperatures and times on the microstructure and mechanical properties were revealed after precipitation and recrystallization behavior investigation at the early stage of continuous annealing. All microstructural experimental inputs (and their change during annealing) allowed quantitative modeling of the strengthening mechanisms, which together are in good agreement with the obtained yield strengths during annealing of the steel.
7.2 Outlook
7.2.1 GO electrical steel
(1) Secondary recrystallization already occurred in the sample annealed to 1175 °C but not in the sample annealed to 1100 °C. The precipitates in these two samples were investigated, but precipitate evolution inside and outside Goss grains during secondary recrystallization could not be captured and is still unclear. Therefore, the D-line sample can be annealed to a temperature where secondary recrystallization just starts to occur with distinct larger Goss grains still with limited sizes of 50-100 m (compared to ~20 m for other grains). The precipitates inside the Goss grains, their grain boundaries and in grains that are adjacent to the Goss grains with different crystal orientations can be investigated combining high resolution bright field SEM image stitching with EBSD measurements. Through such a study, important new insights into precipitate evolution during the secondary recrystallization process are expected to be obtained.
(2) Both high and low number density areas of type II AlN-based precipitates were observed. The high number density areas are inferred to originate from the former austenite regions during austenite–ferrite transformation in hot-band annealing. Therefore, hot rolled and hot-band annealed samples can be investigated for further confirmation.
(3) In this thesis, only one set of annealed samples was produced. The information on precipitates is not enough to build the relationship between precipitation and the annealing condition. Therefore, the D-line sample can be annealed with different processing routes to study precipitation behavior (such as annealing with different heating rates), through which sufficient data can be obtained for precipitate modelling and guiding industrial manufacturing.
Chapter 7 Summary and Outlook
(4) In this thesis, only one composition of GO steel was investigated. Therefore, Al-free or Cu-free GO electrical steels (where other compositions should be the same with the current one) can be produced and investigated as a reference sample to reveal the effects of Al and Cu, and the role nitrides and sulphides play in the secondary recrystallization process.
7.2.2 HSLA steel
(1) In Chapter 6, the effects of different soaking temperatures and times during continuous annealing on the microstructure and mechanical properties were investigated. However, the effect of different heating rates was not studied. Therefore, the cold-rolled HSLA steel can be annealed with different heating rates during continuous annealing.
(2) Recrystallization affects precipitation behavior. Therefore, hot rolled HSLA steel (with same composition) can be annealed using the same annealing route to compare the precipitation behavior with the cold rolled HSLA steel studied in the present thesis. (3) Different cold reductions lead to different recrystallization behavior, and hence
influence the precipitation behavior and the final mechanical properties. Therefore, the HSLA steel can be cold rolled with different amounts of reduction and annealed using the same annealing route to investigate the effect the degree of deformation has on the precipitation behavior.
(4) Current precipitation models are valid for binary precipitates or ternary carbonitrides. New precipitation model for ternary precipitates (like (Ti,V)C) can be developed to simulate the evolution of precipitate size, composition and volume fraction. The influence of dislocations and grain boundaries on precipitate nucleation and growth must be considered.
(5) The steel studied in this thesis contains both Ti and V. The individual effect of Ti and V is unclear since reference samples are lacking. Therefore, steels with different Ti and V concentrations (such as Ti-free or V-free) can be designed and investigated to reveal the individual effect of Ti and V on precipitation behavior and recrystallization behavior.
