Effect of Heat Treatment on AH36 Ship Plate Steel

The effect of normalizing on the microstructure and mechanical properties of AH36 ship plate steel was studied.
The results show that with the increase of normalizing temperature, the yield strength and tensile strength of AH36 ship plate steel decrease, the elongation after fracture and impact toughness first increase and then decrease, and the change of mechanical properties is closely related to the dislocation density and grain size . The optimal normalizing temperature of AH36 ship plate steel is 900 °C . After normalizing at 900 °C, the grains of the steel plate are fine and uniform, the abnormal segregation zone is completely replaced by pearlite, the fracture delamination phenomenon disappears, and the ductility and toughness are significantly improved.
Key words : normalizing ; ship plate steel ; delamination ; abnormal segregation zone

In recent years, the large-scale and high-speed ships and the development of deep-sea oil and gas fields have not only provided a huge market for high-strength ship plate steel, but also put forward higher requirements for its strength and toughness. Delamination defects of ship plate steel have seriously affected the qualified rate of steel plates, and have been downgraded to iron and steel enterprises, resulting in large economic losses, which have attracted widespread attention from scholars at home and abroad. Delamination defects cause anisotropy of the mechanical properties of the steel plate, especially the deterioration of the mechanical properties in the thickness direction, resulting in a decrease in the Z -direction properties of the steel plate and prone to lamellar tearing. Relevant studies have shown that compositional segregation, inclusions, porosity, and structural defects all present a sheet-like structure parallel to the surface of the steel plate. Decreased tensile strength and fracture toughness in the thickness direction are intrinsic factors in the formation of delamination defects.
At present, the research on reducing the delamination defects of ship plate steel mainly focuses on improving the steelmaking and continuous casting process, and a wide consensus has been obtained. However, the more effective method to improve the delamination defects of ship plate steel after hot rolling is the heat treatment of the finished plate. This can not only effectively reduce the downgrading rate of ship plate steel, but also significantly improve economic benefits.
on the D36 ship plate steel in the hot-rolled and normalized states. It is believed that the proper normalizing heat treatment process can significantly improve the toughness of the ship plate steel and obtain better comprehensive mechanical properties.
VC after normalizing heat treatment of 34Mn2V steel and its effect on mechanical properties were studied by TEM . It is believed that normalizing can significantly improve the impact toughness of the steel and improve the plasticity without compromising the strength.
AH36 ship plate steel is the most demanded in domestic and foreign shipbuilding industry , and the influence of normalizing process on the microstructure and mechanical properties of AH36 ship plate steel is analyzed.

test is industrially produced 25 mm thick AH36 ship plate steel . The production process is a two - stage controlled rolling + water cooling process. The thickness of the continuous casting slab is 270 mm . Rolling in the recrystallization zone, rolled to 60 mm after 18 passes , with a total reduction rate of 77.8 % ; rolling at 940 ~ 850 C in the non-recrystallization zone, rolled to the final thickness through 6 passes, and the total reduction rate 58.3% ; water cooling, initial cooling temperature 820 ~ 840 C , final cooling temperature 610 ~ 630 C , cooling speed 7 ~ 8 C/s . The main chemical composition and mechanical properties of the steel used in the test are shown in Table 1 and Table 2. The mechanical properties test found that the fracture delamination appeared in the center of the tensile fracture of the hot-rolled steel plate.

Table 1 Main chemical composition of test steel (mass fraction, % )
Table 1 Main chemical composition of the test plate ( mass fraction , % )

C	Si	mn	P	S	Nb	V	Ti	als	Fe
0. 160	0. 340	1. 460	0.010	0.006	0.032	0.003	0. 013	0. 036	Bal.

Table 2 Mechanical properties of test steel
Table 2 Mechanical properties of the test plate

^ s / MPa ^ b / MPa	8 /%	KV 2 ( 0C ) /J
		ABC
390 570	20	147. 3 158.1 160. 7

Test conclusion
AH36 ship plate steel gradually increase with the normalizing temperature, and the elongation after fracture and impact toughness show a trend of first increasing and then decreasing. The change of mechanical properties is related to the dislocation density and grain size. closely related. The optimal normalizing temperature of the test steel plate is 900 C. After normalizing at 900 C , the grains of the steel plate are fine and uniform, the abnormal segregation zone is completely replaced by pearlite, the fracture delamination phenomenon disappears, and the plasticity and toughness are significantly improved.

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