Research on Low Temperature Toughness Mechanism of F500 Grade Ultra-high Strength and Toughness Ship Plate Steel

F500 ship plate steel with composite structure of acicular ferrite and granular bainite were studied by means of metallography, scanning and transmission . The results show that the impact energy of the steel plate at - 80 °C can reach more than 120J , and the acicular ferrite structure has subgrain boundaries with a large number of high-density dislocations pinned by fine precipitates, which can effectively hinder the crack propagation , which is the mechanism for the good low-temperature toughness of F500 ultra-high-strength ship plate steel.
Key words : ultra-high strength ship plate steel; acicular ferrite; low temperature toughness

With the gradual development of ships in the direction of large-scale and light-weight ships, general-strength ship plate steel can no longer meet the requirements of the hull structure, and the application proportion of high / ultra-high-strength ship plate in the shipbuilding industry continues to increase. Therefore, the development of high-strength steel and As for the ultra-high-strength ship plate steel is imperative, in the past, the purpose of increasing the strength of the steel plate was achieved by increasing the carbon content of the steel. With the advancement of refining technology, high-purity low-carbon steel billets can be produced, and the medium-temperature transformation structure can be prepared by using advanced TMCP technology, so that various means such as structure strengthening, grain refinement, and carbonitride precipitation strengthening can be used. Improving the strength of the steel plate [4-5] provides conditions for the development of ultra-high toughness steel for hull structures.
Studies have shown that F500 high-strength ship plate steel has a composite structure of quasi-polygonal ferrite and acicular ferrite, which contains a certain density of dislocations and substructures, and the interface between the two is a large-angle pinned by precipitation. Grain boundaries can effectively hinder the propagation of cracks and make the structure have good low-temperature toughness. Therefore, based on this idea, this paper prepares the trial -manufactured F500 ultra-high-strength ship plate steel and discusses its low-temperature toughness. mechanism.

Experiment materials and methods of F500 ship plate steel
The experimental steel adopts the composition design scheme of low carbon Nb-Cr-Ni-Ti system, and its chemical composition is shown in Table 1 . F500 ship plate steel is smelted in a vacuum induction furnace. After forging, it is prepared into two slabs. It is rolled in two stages on the experimental rolling mill of the Beijing University of Science and Technology Metallurgical Engineering Research Institute. The deformation in the recrystallization zone and the non-recrystallization stage are both >65 % , the final rolling temperature is 810°C , the final cooling temperature of 1# steel plate is 570 °C , the final cooling temperature of 2# steel plate is 530 °C , the finished product is a 15mm thick plate, and the metallographic sample of the steel plate is taken along the side of the plate The full-thickness sample was polished and etched with 5% nitric acid alcohol for metallographic observation and SEM observation. The transmission electron microscope sample used 5% perchloric acid anhydrous ethanol solution as the electrolyte, at - 20 ° C , 50 V The electrolytic double spray is thinned to perforation, the electron microscope used is H-800 , and the working voltage is 200kV .

Table 1 Chemical composition of F500 plate ( wt , % ) Tab.1 Chemical composition of F500 plate (wt , % )

C	Si	mn	P	S	Nb	Cr	Ni	Ti	Ceq
0.05	0.24	1.48	W 0.012	W 0.005	0.03	0.26	0.28	0.011	W 0.44

Experimental Results and Discussion of F500 Ship Plate Steel
Mechanical properties of F500 grade ship plate steel

The tensile and impact properties of the two experimental steels are given. The results show that the yield strength of the two steel plates can reach more than 500 MPa , and the impact energy at -80匸 can reach more than 120J . Due to the low final cooling temperature of 2# steel, its strength is slightly higher than that of 1 # steel, while its elongation is slightly lower than that of 1 # steel. In addition, it can be seen from the data in Table 2 that the steel plate has no obvious sharp drop zone of impact energy as the temperature decreases. The combined strength of the two steel plates is a typical structure of discontinuous islands distributed on the hexagonal ferrite matrix , and its boundary is irregular, and there are a small amount of Mao islands at the boundary. Acicular ferrite and granular bainite are basically formed in the same temperature range, and both belong to the medium temperature transformation structure. However, since the transformation temperature of granular bainite is slightly lower than that of acicular ferrite, from Figure 2(a) and (b) It can be seen from the comparison that the granular bainite in 2 # steel accounts for a large proportion. At the same time, because the toughness of acicular ferrite is higher than that of granular bainite, the strength of 1 # steel plate is lower than that of 2 # steel plate, while the toughness is better than that of 2 # steel plate.
Table 2 Mechanical properties of experimental steel
Tab.2 Mechanical properties of the steels

 

sample	R el /MPa	R m /MPa	A 5(%)	Impact energy /J
				- 20°C	- 40°C	- 60 °C	- 80 °C
1 #	520	650	24.0	197	179	170	148
2 #	538	680	23.0	188	160	152	141

Table2 Mechanical properties of experimental steel
Tab.2 Mechanical properties of the steels

sample	R el /MPa	R m /MPa	A 5(%)	Impact energy /J
				- 20°C	- 40°C	- 60 °C	- 80 °C
1 #	520	6 50	24.0	197	179	170	148
2 #	538	680	23.0	188	160	152	141

Experimental conclusion of F500 ship plate steel

1. V&C7 particle-reinforced iron-based composites can be prepared by casting composite heat treatment process . The composite material has a tight structure, the interface between the reinforcement phase and the matrix is well bonded, the particles are relatively round and there is no segregation phenomenon and obvious structural defects.
2. Under the abrasive wear test conditions, the composite material exhibits good abrasive wear resistance, and its wear resistance is about 11.7 times that of HT300 under a load of 10 N.
3. V8C7 particle-reinforced iron-based composites showed a trend of first increasing and then decreasing with the increase of V8C7 particle volume fraction. In this experiment, the composite sample with V&C7 particle fraction of 33vol% had the best wear resistance.

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