Microstructure and properties analysis of hot- rolled FH550 grade ultra-high strength steel

The results show that the strength and plasticity of the rolled steel meet the requirements of the classification society. After two-stage rolling, the low-temperature toughness at 1/4 width of the steel plate is better, the ductile-brittle transition temperature is lower, the structure is mainly acicular ferrite, and the impact fracture is ductile fracture; the low-temperature toughness at 1/2 width of the steel plate is lower Poor, the structure is mainly acicular ferrite and pearlite, and the impact fracture is a cleavage fracture. After tempering, the performance of steel changes obviously. With the increase of tempering temperature, the hardness of steel decreases first and then increases. When tempering at 600 ° C , the highest hardness and yield strength are obtained. When tempering above 600 °C , the steel Both hardness and yield strength decreased.
Key words : ship plate steel; rolling; tempering; mechanical properties

steel for hull structure refers to the steel used to manufacture the hull structure produced in accordance with the requirements of the classification society's construction specifications, generally including ship plates, section steel, etc. The steel used for the hull structure should have high strength, good toughness, process adaptability and seawater corrosion resistance. Carbon steel was mostly used in the early ship plates, and the strength was increased by increasing the carbon content. After the 1950s , 550 MPa -grade HY-80 steel with quenched and tempered nickel-chromium-molybdenum alloy elements was used , and it was successfully developed later. HY-100 steel of 660MPa grade . After the 1980s , the United States proposed a development plan for HSLA (high strength low alloys) ship steel. First developed HSLA-80 , and later developed an ultra-low carbon high-strength HSLA-100 . Today's ship plate steel manufacturers mostly adopt the design idea of "low carbon, high manganese, and microalloying" in chemical composition [1-4] . Low-carbon low-alloy steel not only has high strength, but also has low impact transition temperature and excellent welding performance, and these excellent comprehensive mechanical properties can be obtained in the controlled rolling state. The controlled rolling and controlled cooling process has the advantages of saving energy, simplifying the production process, and improving the comprehensive mechanical properties of steel. Therefore, the TMCP ( thermomechanical rolling ) process is used in the production of high-strength steel plates for hull structures at home and abroad.
60% of domestic marine steel plates are grades A and B , and high-strength steels are used from AH32 to FH40 , but the largest proportion is still grade AH . Angang has developed FH550 , EH550 and super high strength ship plate AH690 .
FH550 is an ultra-high-strength ship plate steel, which is a low-carbon bainite steel with acicular ferrite microstructure at room temperature. The rolling test of FH550 ship plate steel lays the foundation for determining a reasonable controlled rolling and controlled cooling system, focusing on the influence of TMCP process and tempering system on the structure and mechanical properties of the steel.
1 Experimental materials and methods
The composition and rolling process parameters of FH550 ship plate steel are shown in Table 1 . Pick
Tab.1 Chemical composition ( wt , % ) and rolling parameters of FH500 ship plate steel

C

Si

mn

S

P

als

Nb

V

Cr

Ni

Ti

0.08

0.26

1.38

0.004

0.013

0.037

0.025

0.03

0.3

0.2

0.015

heating temperature

Rough rolling finish temperature

Finishing temperature

Final rolling thickness

open cold temperature

final cooling temperature

1200°C

M 1120 °C

810 °C

16 mm

766 °C

502 °C

Using the TMCP production process, the thickness of the slab is 80mm , the thickness of the finished product is 16mm , and the thickness of the intermediate billet is 45mm . According to the rolling mill capacity, heating capacity and cooling capacity , the heating temperature is set to 1200 °C . The rolling of the FH550 ship plate steel plate is divided into two stages. The first three passes are high-temperature stage rolling, which is kept above 1120 °C . The purpose is to fully refine the austenite through repeated recrystallization between rolling passes. organize. The last five passes are low-temperature stage rolling, and the rolling temperature is below 880 ° C . The purpose is to increase the nucleation sites of phase transformation through deformation in the non-recrystallized zone. Control different final rolling temperatures, and cool to 502°C after rolling .
mechanical property test, microstructure observation and fracture analysis of the hot-rolled FH550 ship plate steel plate were carried out, and then 6 samples were taken from different positions of the steel plate and heated to 400 , 450 , 500 , 550 , 600 and 650 °C respectively for back After fire treatment, the hardness test, microstructure observation and mechanical property test were carried out on the treated samples respectively.
Experimental Results and Analysis of FH550 Ship Plate Steel
FH550 Ship Plate Steel
The mechanical property data of FH500 ship plate steel measured after hot rolling are shown in Table 2 . It can be seen that the yield strength of the steel after rolling is 580 ~ 590MPa , the elongation is 22% ~ 24% , and the strength and plasticity have reached the requirements of the classification society. The low-temperature toughness of 1/4 is better, the ductile-brittle transition temperature is lower, and the low-temperature toughness of 1/2 is poor.

Table 2 Mechanical properties of FH550 ship plate steel
Tab.2 Mechanical properties of FH550 ship plate steel

Grade	Stretching test			Lateral impact energy				Remark
	Yield strength / MPa	Tensile strength / MPa	Elongation (%)	Experiment temperature / °C	A KV/J
FH550	580	655	twenty four	- 40	193	204	183	1/4
				- 60	193	186	192
				- 80	168	168	152
	590	675	twenty two	- 40	80	72	118	1/2
				- 60	78	90	55
				- 80	12	twenty one	15

Note: 1/4 in the text means the 1/4 width of the steel plate, 1/2 means the 1/2 width of the steel plate
2.4 Microstructure and performance analysis of FH550 ship plate steel after tempering
2.4.1 Tempering hardness and structure
Six samples of FH550 ship plate steel were heated to 400 , 450 , 500 , 550 , 600 and 650 ° C for tempering treatment for 1 h , and the hardness was measured on a HD9-45 optical surface Vickers hardness tester , the obtained data are shown in Table 3 . It can be seen that the hardness of the rolled FH550 ship plate steel is 226HV . When tempering at 400 °C , the hardness value decreases, and when tempering at 400 ~ 500 °C , with the increase of tempering temperature, the average hardness shows a downward trend. When the tempering temperature is 500 °C , the hardness value is the lowest, which is 209HV . After 500 °C , the hardness value increases with the increase of the tempering temperature. When the tempering temperature reaches 600 °C , the hardness value reaches the tempering process. The highest value is 241HV . After 600 °C, the hardness value decreases again with the increase of tempering temperature.

analysis in Table 3 , the hardening peak of FH550 ship plate steel appears when it is tempered at 600° C , because a large number of alloy elements in the steel age and precipitate, which play a hardening role. There are a large number of dislocations in the post-rolling structure, which are deformation dislocations formed during the controlled rolling deformation of the austenite region, and between each pass of deformation, due to the strain induction .

Table 3 Hardness values of FH550 tempered samples
Tab.3 Hardness of tempered FH550 ship plate steel

rolled state		Hardness value (HV)					Average (HV)
		229	231	212	228	228	226
Temper temperature / °C	400	209	223	220	221	207	216
	450	213	210	213	224	207	213
	500	209	205	210	209	212	209
	550	233	221	216	219	231	224
	600	249	244	235	238	240	241
	650	240	231	239	240	231	236

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