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Study on the weldability of ultra fine-grained steel has become the focus of research[3].
Fig. 1 shows the macrostructure of resistance spot welding joint for ultra fine-grained steel.
When columnar grains grow to a certain extent, because of the front liquid metal is far from electrode surface, the heat dissipation difficulty and slowly cooling hinder the rapid growth of columnar grains.
At the same time, the exist of forge-delay force impels the melted metal to shock, resulting in the solidified grains fragmentation at the advanced edge of liquid-solid interface, not only hinder the grains growth, but the grains fragments can act as nuclei for new grains to form, thus, the grain structure in fusion zone will be refined.
The nugget zone almost consists of coarse martensite, and the existing of forge-delay force during the cooling process causes the plastic deformation of nugget, promoting the number of dislocations.

A grain refinement to a mean grain size of 10 to 15 µm is achieved by using this process.
The most important limitations of introduced methods are the small size of the product and the large number of steps needed to reach the desired texture.
Pure copper bars were pre-stretched in a tension test machine and then deformed by torsion to a specific number of turns in a torsion test machine.
By increasing the number of torsion turns in a specific length of the wire, a higher degree of refinement due to the higher equivalent strain could be achieved (Fig. 7e).
It is shown that at a low strain (εeq<2), the original grains are deformed and start to be fragmented into sub-grains or grains.

It is therefore necessary to evaluate the Schmid value, m of all potential slip systems in order determine the actual number of active slip systems.
As a consequence, it is very difficult to determine the number of activated slip systems in grains with this or similar orientations.
It is clear that the slip behaviour depends upon the initial crystallographic orientation, i.e. the number slip systems activated is very heterogeneous through the profile thickness.
Fig. 6: Rotation path of the tensile direction during increasing deformation. a) Grain A. b) Grain B c) Grain C d) Grain D and e) Grain E.
• The number of slip systems activated during deformation is strongly dependent upon the initial orientation and is therefore very heterogeneous through the microstructure

With the increase of the number of ECAP-C passes to 4 in accordance with the specified regimes the most part of boundaries of fragments/grains remained non-distinct and wavy.
The view of electron-diffraction patterns with a big number of spots, arranged in the circumference, testifies to formation of a UFG structure with high-angle grain boundaries.
d UTS UTS, MPа d, % Number of passes Fig. 4.
With the increase of the number of passes to 4 the structure is transformed into a UFG structure and there high-angle grain boundaries appear.
With the further increase of the number of passes to 6 the structure is not changed, but the grain size decreases to 250 nm. 3. 

The grain boundary microstructure ( the type, the frequency of grain boundaries and the connectivity of different type of grain boundaries ) was analyzed by SEM-EBSP-OIM for all the specimens.
The initial grain structure is drawn in red lines.
It is reasonable to imagine that the grain boundary connectivity may control the grain growth through the interaction between initially existed grain boundaries and newly formed interphase boundaries during phase transformation.
As shown in Figure 4, significant abnormal grain growth was observed and the magnitude of abnormal grain growth increased with increasing the number of cycling.
The grain size of abnormally grown grains was almost ten times larger than the initial grain size ( d0 =59-65µm).

The number of serial sections was 96 and the area of each scanned section was about 50 2 µm2.
The number of neighboring pixels contributing to the calculation of average disorientation was one for this case.
The number of nearest neighboring pixels included in the calculation of Eq. (1) is one.
The grains adjacent to the edge of the reconstructed region were excluded from the calculation and the total number of bulk grains was 1018.
This work was supported primarily by the MRSEC program of the National Science Foundation under Award Number DMR-0520425.

Finally, it was found that the grain size and the mechanical connection between grains play an important role in determining the electronic and magnetic properties.
By this result, it is expected that the grain size and the mechanical connection between grains play an important role in the electronic conduction.
Therefore, the lower energy shifts in Fig. 4 indcate that the valence number of Mn decreases.
The increase of grain size leads to the reduction of grain boundary.
Acknowledgement This work was supported by the KOSEF through q-Psi and Grant Number 04-2002-000-00009-0 and by Korea Research Foundation Grants (KRF-2001-015-DP0193 and KRF-2001-015-DS0015).

The growth of Goss grains is facilitated when the normal growth of other grains is suppressed.
In Fig. 1(b), an island grain is observed inside one large grain and a small number of finer grains are present in the boundary of the larger grains (noted with Y).
(a) (b) Fig. 1 SEM images of 900°C 30 min RH annealed sample (a) abnormal growth large grain (b) a large grain with an island grain inside it and some finer grains present at the boundary of large grains.
For grains ~ < 100 µm in size, these grains readily grow during the first 5 min of annealing for both the RH and SH samples.
As the annealing time is prolonged, the growth of Goss grains in the SH samples is limited as these grains appear to have already reached saturation.

Results show that ultrafine grains + nano-carbides are obtained in the steel plates.
Introduction Recently, ultrafine-grained materials with mean grain size of less than 1μm have been energetically studied, because they are expected to bear superior mechanical properties to conventionally coarse grained materials.
The SAD patterns were arc-like, which suggests that a number of the high-angle boundaries (>15°) exist within the selected areas of Fig. 1b [6].
Annealing above 873 K, the microhardness decreased slightly but the grain size grew rapidly.
Annealed at 873 K, the equiaxed ferrite grains + nano-carbides formed in most of the areas as shown in Fig. 3c.

Influence of Steel Grain Size on Residual Stress in Grinding Processing M.
The average grain size of NFG600 and SM490 is 3 µm and 15µm respectively.
Nakayama steel works, Ltd. in Japan developed the fine-grained rolling steels named 'Nakayama Fine Grain (NFG)' as commercial materials [1].
Thus, the semblance tri-axial residual stress generated in fine-grained steel would be different from that in conventional steel, because the ratio of the X-ray penetration depth to the distance between grain walls in fine-grained steel is high compared with conventional steel.
(16) Because Xi is a constant, several expressions corresponding to the number of angles ψ can be obtained.