normalizing on the performance of E -class ship plate

E -class ship plate strength change
After normalizing, the strength decreases in different degrees, the yield strength of 36 mm decreases by 30 MPa, and the tensile strength decreases by 40 MPa ; while the strength of 12 mm decreases greatly, the yield strength decreases by 130 MPa , and the tensile strength decreases by 65 MPa . See Table 2 for details .
E -class ship plate elongation change
grade D 12 mm with a larger decrease in strength increased by 12.5!. The elongation of the 36 mm sample increased by 1! .
2. 3 -40 C and -60 C low - temperature V -shaped impact properties -40 C and -60 C low-temperature impact properties have been improved, 36 mm, after normalizing -40 C and -60 C impact from the original 38 J , 12 J all increased to more than 200 J , and the three values were relatively uniform .

aging impact of the 40C normalized sample is 2 to 3 times higher than that of the rolled sample .

Microstructure and grain size After normalizing, the microstructure of the original rolled sample has been improved . Table 3 shows the banded structure and grain size. The banded structure of the 36 mm plate has been improved .

Table 3 Banded structure and grain size before and after normalizing

Comparison and Analysis of Rolled Microstructure The rolled microstructure is elongated fine ferrite plus pearlite. The reason is that rolling is controlled in two stages of recrystallization zone and non-recrystallization zone. When rolling in the non-recrystallization zone , the austenite grains elongate along the rolling direction, and deformation bands are generated inside the austenite grains . At this time, not only the nucleation density of ferrite is increased due to the increase of the grain boundary area, but also appear on the deformation bands There are a large number of ferrite crystal nuclei, so the ferrite grains are significantly refined after the austenite - ferrite phase transformation. The 12 mm rolled sample is one level finer than the 36 mm grain size, because the thin gauge The accumulative reduction rate of the non-recrystallized area is 67! , 36 mm . Due to the limitation of the billet thickness, the cumulative reduction rate of the non-recrystallized area is only 50!. With the increase of the total reduction in the non-recrystallized area, the The size of the austenite grains in the thickness direction becomes smaller, the grain boundary area increases, and the number of deformation bands in the austenite grains increases, so the ferrite grains are significantly refined after the austenite - ferrite phase transformation Therefore, the as-rolled 12 mm steel plate can meet the certification requirements, while the rolled 36 mm steel plate cannot meet the certification requirements due to insufficient grain refinement .

Microstructure comparison and analysis after normalizing
The structure after normalizing the two specifications, the normalizing process can improve the abnormal structure such as uneven grains, banded structure and other structural defects in the rolling process through recrystallization and homogenization, and obtain fine equiaxed Ferrite + uniformly distributed massive pearlite structure, thereby improving its mechanical properties and process performance .
12 mm , the grain grade is the same, but the ferrite grain shape becomes equiaxed and the grain is more uniform .
36 mm steel plate, the grains are uniform and refined, and the grain refinement increases the number of grain boundaries, and the grain boundaries are obstacles to dislocation movement, which increases the strength, and the grain boundaries can also limit the plastic deformation to a certain range, so that Uniform deformation improves the plasticity of the material; the grain boundary is the resistance to crack propagation, thus improving the toughness of the material . The Hall - Petch formula expresses the relationship between the grain size D and the ductile-brittle transition temperature Tc [ 3 ] :
Tc=A - mD - 1/2 ( 1 )
Where A and m are constants, for structural steel m =12 C /mm1 / 2.
36 mm steel plate after normalizing is 22.5 ^ m , which is 4.2 ^ m finer than the average grain diameter of 26.7 ^ m in as-rolled state . According to the theoretical calculation of formula ( 1 ), the ductile-brittle transition temperature Tc is reduced by 7 C. Judging from the impact toughness comparison in Table 2 , the ductile-brittle transition temperature Tc of the actual normalized steel plate is reduced by about 20 C compared with the as-rolled state . In addition to the contribution of grain refinement and the improvement of billet purity, the grain uniformity It is also an important factor affecting toughness .
Compared with the as-rolled state, although the strength of the 12 mm and 36 mm thick E -grade ship plate after normalizing has decreased, it still meets the ship plate certification requirements, and the key toughness indicators including low temperature impact and strain aging impact are in line with the classification society regulations Certification requirements and a large margin provide a basis for the formulation of the actual certification production process .