Study on Thermal Stability of F40 Ship Plate Steel

The hull is linear and complex, with various types of single curves or hyperbolas , Various forming operations such as cold and hot bending and straightening are required, and the steel is required to be adaptable to the shipbuilding process . The F40 ship plate produced by controlled rolling and controlled cooling ( TMCP ) technology is an unbalanced structure steel composed of acicular ferrite composite structure; considering the adaptability to the service environment and the impact of subsequent processing, keep its Thermal stability of non-equilibrium tissues is essential . Based on this idea, this study focused on analyzing the performance changes and microstructure evolution of F40 ship plate steel in the laboratory at different temperatures ranging from 400-750 C for 1 h , and discussed the thermal properties of F40 ship plate steel. The stability mechanism provides theoretical support for the subsequent hot working process of the steel plate .

F40 ship plate steel test
The test steel was smelted in a 25 kg vacuum induction furnace and vacuum cast. The steel ingot was hot forged into a 100 mm thick slab and then heated to 1200 C in a heating furnace . After homogenization for 1.5 h , it passed through the recrystallization temperature zone and non-recrystallization The deformation of 50% and 60% in each temperature zone is rolled into a 16 mm plate, and controlled cooling is carried out after rolling, and the cooling rate is 10 C / s and the red temperature is 550 C.
The performance of the test steel meets the requirements of the classification society for F40 ship plate . The main chemical composition ( mass fraction, %) of the test steel is 0.072C , 0.30 Si , 1.45 Mn , 0.008 P , 0.003 S , 0.030 Al , 0.024 Nb , 0.25 N] take a 20 mm X15 mm ( length X width ) rectangle on the plate For the sample , take the sample out immediately after incubating at 400 , 450 , 500 , 550 , 600 , 650 , 700 , 750 C for 1 h , and cool it in the air .
Vickers Hardness Test and Microstructure Observation of F40 Ship Plate Steel

The F40 steel plate samples reheated at different temperatures were cut in the middle by wire cutting , and after grinding and polishing on both sides, the hardness was tested on a HVS100 0-790 surface Rockwell / Vickers hardness tester with a load of 294 N. The metallographic and scanning samples were prepared by electropolishing and etching with 5% nitric acid alcohol , The scanning electron microscope used is a JSM- 5800 transmission electron microscope sample using 5% perchloric acid anhydrous ethanol solution as the electrolyte, and the electrolytic double spray is thinned to perforation at -20C and 50V . The electron microscope used is H -800 , The working voltage is 175 kV .

Hardness test results of F40 ship plate steel
After the steel plate sample is heated at different temperatures, its hardness decreases obviously, but the reheating process is not purely a softening process, it is accompanied by fluctuations in hardness . When the reheating temperature is 600°C, the hardness of the sample is the highest, which is 225 HV 30; when the reheating temperature is 500°C , the hardness of the sample is the lowest , which is 209.2 HV 30 . After the sample was heated at 400 C for 1 h , the hardness began to drop sharply. With the increase of reheating temperature, it dropped from 230 HV 3 to 217.6 HV 30. At 500 C , the hardness trough began to appear, and then the hardness began to increase. 600 C peaks . As the reheating temperature continues to increase, the hardness begins to decrease again. It decreases slowly in the process of 650 C to 700 C , and the hardness changes little. Has dropped to 210.8HV30.

 

Experimental conclusion of F40 ship plate steel :

1. F40 ship plate steel is not a monotonous softening process, and its hardness fluctuates with the increase of reheating temperature .
2. Due to the pinning effect of carbonitrides during reheating , The disappearance of deformation dislocations is slower, which is the reason for the better thermal stability of F40 ship plate steel structure .
3. F40 ship plate is determined by the mobility of dislocations, and the final microstructure evolution is caused by the nucleation and growth of polygonal ferrite .

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