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It has been reported by previous work that ultrafine-grained (UFG) steels with grain sizes smaller than a few micrometers have super mechanical properties such as high strength, enhanced superplasticity [1-3].
The size and number of carbide become smaller and lesser with decreasing the rolling reduction.
There is no formation of ultrafine ferrite grain.
They stated that the yield drop phenomenon was observed due to the ferrite grain size was smaller than 1 µm and the yielding behavior was slightly different if the grain size was larger than 1 µm.
This means that significant strengthening by grain refinement was achieved in the present ultrafine grained steel (Fig. 4c).

The hydrogen gas evolution from the specimen was evaluated from the hydrogen ion current, where the mass number M/e=2 was selected.
Table 2 Manufacturing condition and the grain size of the plate specimens.
In addition, under the same alloy group, the specimens with relatively finer grains (B and D) showed higher fracture strain than that with coarse grains (A and C).
By comparing the hydrogen evolution results and the fracture morphology between Fig.2 and Fig.3, it is presumed that a number of hydrogen atoms are trapped at the GBs before fracturing.
However, the arrangement of Ag particles on GBs was not observed in all the grain boundaries.

It is suggested that the AlN precipitates restrict the grain grow in one direction, promoting the appearance of an elongated grain in the rolling direction, which is fundamental to obtain a steel with deep drawing quality (DDQ)[3-7] .
It will be characterized by the aspect ratio, defined as the ratio between the grain length and grain width.
At these conditions, the recrystallized ferrite grain shows a "pancake" type of morphology, characterized for elongated grains oriented in the longitudinal direction.
It is observed that a 40°C/h and 100°C/h, a pancake type of structure is obtained, at 200°C/h, the material exhibits a mix microstructure, comprised by a combination of elongated and equiaxial grains, accompanied by a decrease in grain size.
In consequence, the nucleation of new ferrite grains is favored, and as result, the number density of small recrystallized grains increases with a tendency to be equiaxial [3,10,11] .

This definition again implicitly assumes uniformity in the overall grain size distribution.
Sequeira and Dunlop [1] and Rodrigo et al. [6] defined the skin as the region outside the mean hardness number across the specimen's thickness.
After testing the coordinates of the indentation points and hardness numbers were profiled into a 3D hardness surface map using a commercial software package (Surfer).
Figure 2a shows the fine grain structure that prevails along the surface.
The grains structure, however, is not fully uniform, as shown by the mixture of fine and large grains, Fig. 2b, and the presence of occasional microporosity, Fig. 2c.

In addition to coarsening of M23C6 carbides and an equiaxed fine grains in FGHAZ, intermetallic Fe2(Mo,W)Laves phase precipitated on grain boundaries during creep is probably the significant factor caused the type failure.
There have been a number of studies that have investigated the fracture characteristics and cause of type crack in welded joints of P91 and P122[3,4].
The microstructure near the fracture location in the specimen fractured at 100MPa after 1546.5h, as shown in Fig.4, this area exhibits a finer and an equiaxed grains of average size 10μm, whereas initial martensite lath and relative coarse grain with prior austenite grains of size about 125μm microstructure are observed in base metal area of weldment.
A large number of creep cavites on grain boundaries could be determined in the FGHAZ, while hardly any cavities could be observed in the other position of weldment.
●Except for coarser M23C6 carbides and an equiaxed fine grains, intermetallic Fe2(Mo,W) Laves phase precipitated on grain boundaries in FGHAZ during creep is probably the significant factors caused the type failure.

The number of measurements collected ranged from 300 to 500.
For both regimes, evidence of grain growth and elongation of grains along the deformation axis was revealed.
The distance between grains A and B was increased and the grain C became a neighbor of the grain D at 0.87Ti.m., and the same behavior was found for the grains E, F, G, and H at higher temperature.
The grain rotation had a random character.
For both deformation regimes, grain growth and elongation of grains along the deformation axis were observed.

Analysis method of the samples.Two types of castings after heat treatment are processed into the desired size and shape for experiment, then proceeding the analysis of metallographic structure, grain size and the type, number and the distribution form of nonmetallic inclusions.
The microstructure and the austenite grain boundaries of the experiment steel without refined treatment is shown in Fig. 2.
And the microstructure and the austenite grain boundaries of the experiment steel with refined treatment is shown in Fig. 3.
Tab. 4 Content of O, H , N , P, S in steel sample number O(ppm) N(ppm) H(ppm) P(wt%) S(wt%) Refined treatment r1# 2 34 <1 0.024 0.002 r2# 2 35 <1 0.023 0.003 unrefined treatment ur1# 65 100 <1 0.034 0.014 ur2# 63 110 <1 0.032 0.023 Observing the metallographic photographs amplified to 100 times of the experimental steel with and/or without refining we could find that the number of inclusions is in the refined steel has decreased, and the inclusions become more rounded.
The change of these two aspects reduces the number of the potential crack sources and reduces the block function on dislocation motion when the experimental steel happens to plastic deformation.

The former has the grain size of 80 mesh, and Table 1 shows its composition; The grain size of the latter one is less than 50 nm,and it is prepared by Hefei Kaier Nano-meter Energy& TechnologyCo.
When the addition of nano TiN is 1%, the nano TiN play a role of nucleating agentto promote formation of large numbers of close grains.
When the addition amount of TiN is 1%, wear weightlessness is 0.1177 mg,reducing by 5.40% than sample of number 0, this is related to that M7C3 separate out as hupereutectic overeutectic carbide precipitation phase.
M7C3 has superior hardness of 672.89 HV and appear in the form of homogeneous grains.
(2)When the addition amount of TiN is 1%, the grain is refined obviously, the wear weightlessness is 0.1177 mg,which means the best wear resistance.

Green color grain (#2) favorites tension twinning.
From the misorientations of #5 grain with #1 grain (～86°<110>) and #2 grain (～60°<100>)[12] shows in Fig.4d, this is another type of variant of tension twins.
Compare with the matrix, these grains have high quality of Kikuchi band, displayed as grain growth approaches twin boundary.
Under uniaxial tensile condition, the number of tension twin variants has relationship with original orientation explained as follow.
But it is worth to note that the observed variants can be formed at different time; also, limited by 2D testing technique, observed number of variants is less than the result from theoretical calculation.

A number of articles have been written about the mechanisms and kinetics of spheroidization [1-2].
Even numbers (2, 4, 6, and 8) can be used to represent to compromises among the preferences below.
An expert in the end determined the preference numbers in the choice for each pairwise comparison.
The numbers of dimple fracture are much more, and its width-depth are bigger in the optimized material than other annealing materials.
Mullins, Grain boundary grooving by volume diffusion, Trans.