Effect of tempering temperature on microstructure of EH40 high-strength ship plate steel

68 mm thick EH40 high-strength ship plate steel were studied by optical microscope, scanning electron microscope and mechanical property testing equipment .
The results show that after tempering in the temperature range of 500-670 C , the microstructure of EH40 grade steel plate does not change basically, but with the increase of tempering temperature, the carbides in the steel plate are dispersed and precipitated and evenly distributed, and the strength of the steel plate decreases , the toughness is improved; the steel plate after tempering at 600 LX3. 5 h has the best matching of strength and toughness.

With the rapid development of the shipbuilding industry, the requirements for the performance of high-strength ship plate steel are getting higher and higher. High-strength steel for ships generally adopts controlled rolling and controlled cooling process (TMCP) to control grain refinement and carbonitride precipitation, and obtain high strength, high toughness, good weldability and formability in the hot rolled state. Cooperate. However, for the high-strength steel plates used for hull structures with a large thickness and grade EH40 , the microstructure and properties of the steel plates obtained by the controlled rolling and controlled cooling process are usually not uniform, so it needs to be tempered to obtain the best quality. Comprehensive performance. At present, there are not many research reports on this aspect. Therefore, the influence of tempering temperature on the microstructure and mechanical properties of EH40 grade high-strength ship plate steel produced by 68 mm thick TMCP process was studied in detail.

Sample preparation and test method for EH40 ship plate
The test material is 68 mm thick EH40 high-strength ship plate steel, and its chemical composition is shown.
Table 1 Chemical composition (mass fraction) of EH40 high-strength ship plate steel
Tab. 1 Chemical composition of high strength ship

plate steel (mass)					%
condition	c	Si Mn	P	S	other
Measured	0.07	0. 21 1. 53	WO. 010	^0.010	Add Al ,
CCS (Standard)	<0.18	^0. 50 0. 90-1. 60	W ( ). 035	<0.035	Nb , Ti , etc.

Process a sample with a size of 200 mm X 200 mm X 68 mm . Use an air heating furnace for tempering treatment. After the furnace temperature rises to the set temperature of 500, 550, 600, 630, 650, and 670 C , put the sample in, and wait for the furnace temperature to rise to the set temperature again. After 3.5 hours of heat preservation , it is air - cooled out of the furnace (insulation coefficient 2.5 ~ 3.5 C - mm -1 ) o
In accordance with the China Classification Society ( CCS) " Materials and Welding Specifications", samples were taken at the 1/4 position and 1/2 position of the sample plate thickness after heat treatment (equal to the corresponding position of the plate thickness) to process tensile samples and impact samples. , and according to room temperature tensile test method (GB/T 228-2002) and metal Charpy notched impact test method ( GB/T 229-2007) to carry out tensile performance detection and - 40C Charpy low temperature impact performance detection; The sample was ground, polished and corroded. The etchant was 4% nitric acid alcohol. The microstructure was observed with a LEICAQ550IW optical microscope and a JSM - 6460LV scanning electron microscope; Time 12 s .

Influence of Tempering Temperature on Strength of EH40 High Strength Ship Plate Steel
EH40 high-strength ship plate steel has different changes in tensile strength at different positions of plate thickness after tempering at different temperatures. The yield strength and tensile strength of the sample at the 1/4 position of the plate thickness decrease with the increase of tempering temperature. After tempering at 670 C , the yield strength is 405 MPa, and the tensile strength is 485 MPa. Compared with the steel plate, the reduction reaches 70 MPa and 90 MPa respectively . After tempering at 500 ~ 600 C , the yield strength and tensile strength of the sample at the 1/2 position of the plate thickness increased, but the increase was very small. The yield strength at 600 t tempered was the largest. The increase reached 45 MPa, and then as the tempering temperature continued to increase, the strength gradually decreased, but overall, the tempering temperature had little effect on the strength of the sample at the 1/2 position of the plate thickness. At the same time, when the tempering temperature is in the range of 500-600 C , the elongation of the steel plate basically does not change, but when the tempering temperature is greater than 600 C , the elongation of the steel plate is greatly improved.
EH40 High Strength Ship Plate Steel on Low Temperature Impact Properties
of tempering temperature, the impact energy in different directions at the 1/4 position of the plate thickness increased greatly . J and 126 J ; at the 1/2 position of the plate thickness, as the tempering temperature increases, the impact energy generally increases, but when the tempering temperature is 550 C , the impact performance of the steel plate is the lowest point; overall It can be seen that the impact of tempering temperature on the impact properties at the 1/4 and 1/2 positions of the plate thickness is consistent, that is, as the tempering temperature increases, the impact properties of the steel plate have been improved.

Influence of Tempering Temperature on Hardness of EH40 Ship Plate
It can be seen that at the 1/4 position of the EH40 ship plate thickness , the hardness value of the steel plate gradually decreases with the increase of the tempering temperature, but the hardness at the 1/2 position of the plate thickness gradually decreases with the increase of the tempering temperature. It reached the maximum at 600 C , and then gradually decreased with the increase of tempering temperature, but the overall change range was not large. The changing trend of the hardness value is consistent with the influence of tempering temperature on the strength at the 1/4 and 1/2 positions of the plate thickness .
After tempering the EH40 high-strength ship plate steel produced by the TMCP process in the temperature range of 500-670 C , the structure of the steel plate basically does not change, but with the increase of the tempering temperature, the carbides in the steel plate are dispersed and precipitated. Evenly distributed, the strength of the steel plate is reduced and the toughness is improved; the steel plate tempered at 600 C for 3.5 h has the best matching of strength and toughness .

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