Thick gauge and high strength E36 ship plate

Introduced the development of thick gauge E36 ship plate of Shaoguan Iron and Steel Co., Ltd., adopting the composition design of NbTiNi microalloying, combined with clean steel smelting and reasonable rolling cooling process, and normalizing heat treatment process, the results show that the steel plate after normalizing treatment, The structure is uniform from the surface to the core, and the grain size reaches 9.5 grades. The mechanical properties of the product fully meet the requirements of the ship regulations of nine countries' classification societies, which shows that Shaogang has successfully developed the 60 mm thick E 36- Z3 delivered in a normalized state. 5 ship boards .

With the development of large-scale and specialization of ships, higher requirements are put forward for shipbuilding steel - when building large ships, the thickness of ship plates needs to be increased - and with the continuous development of ships and ocean engineering, low-cost shipbuilding steel is also required Alloy high-strength steels are developing toward thick-gauge ship plates with lamellar tear resistance .
In order to meet the needs of the development of China's shipbuilding industry and improve the market competitiveness of hull structure steel, it is urgent to increase the research and development of high-value-added, high-tech thick E -class high-strength ship plates to form a complete series of ship plate varieties and specifications- In addition to meeting the strength and plasticity indicators, the thickness specification ship plate also requires low - temperature toughness and weldability at -40°C , so it is more difficult to develop-

is the requirements of GB7 12-2000 on the chemical composition of E36 ship plate and the design target of Shaogang E36 ship plate composition. For the ship plate with requirements on thickness direction performance ( hereinafter referred to as Z -direction performance ) , the corresponding sulfur content must also meet Table 2 Requirements. Table 3 shows the performance requirements of ship regulations GB7 12-2000 for thick E36 ship plates .
Table 1 Chemical composition of E36 ship plate

	C	S	P	mn	Si	als	Nb + v + Ti
standard	<0. 18	<0-035	<0-035	0-90 ~ 1-60	<0-50	> 0.015	<0-12
Target	0. 10 ~ 0. 15	<0-005	<0-015	1-20 ~ 1-60	<0-50	>0.02	Appropriate amount

Table 2 Sulfur content W K of thickness direction performance steel plate

	Z performance level		Z 25	Z 35
	Sulfur content, not more than		0 . 007	0.005
Table 3 Mechanical properties of E36 ship plate
steel	Yield strength Tensile strength Degree such as Pa degree /KlPa	back extension Length / %	—40C impact value /J ( longitudinal )	wide cold bend test
E 36	>355 490 ~ 630	>21	>41	intact

2 -ingredient process design
2 1 Component Design Ideas
According to the factory approval requirements of the classification society, the 60 mm thick E36 ship plate not only needs to meet the requirements of strong plasticity , -4 . °C low-temperature impact toughness and good Z -direction performance, it is also necessary to ensure -40C aging impact toughness , Low temperature aging toughness is also a difficult point in the development of thick gauge ship plates . Therefore , It is necessary to use reasonable chemical composition combined with appropriate process design to ensure moderate strength . Good low-temperature impact toughness and low-temperature aging toughness and Z -direction performance • Therefore, the composition design of 60 mmE3 grade 6 board with lamellar tear resistance meets the following requirements .
1 ) Good weldability is essential for hull steel , In order to ensure good weldability and excellent low-temperature impact toughness of steel for hull , need to reduce carbon content , Considering that the subsequent normalizing treatment will greatly reduce the strength , Therefore, the design goal is low carbon .
2) The role of Nk Ti microalloying elements in steel is very significant . In rolling , Through a strain-induced mechanism , Precipitated fine carbides and nitrides are pinned to grain boundaries and dislocations , Inhibit the recrystallization of austenite and the growth of grains, play the role of grain refinement , and play the role of precipitation strengthening in the process of rolling and cooling . At the same time , considering that E36 is delivered in a normalizing state, the normalizing heat treatment will reduce the strength to a certain extent , so it is necessary to add a certain amount of alloying elements to make up for the loss of strength through fine grain strengthening and precipitation strengthening .

Process Design Ideas
In order to make the grain size in the thickness direction of the thick gauge E36 steel plate uniform , the reduction and rolling speed of each pass should be reasonably designed during the controlled rolling process . When rolling in the austenite recrystallization zone , the deformation should be fully penetrated to the central part in the thickness direction , In order to fully carry out the recrystallization of the Austrian body , Achieve uniform grain size refinement ; when rolling in the austenite non-recrystallized region, ensure a certain deformation rate (> 60 % ), and at the same time make the deformation of the steel plate thickness direction sufficient and uniform, and fully refine the grain size through the deformation-induced phase transformation mechanism Grain .
E36 ship plate is a low-alloy steel. Combined with previous tests, its normalizing process is determined, and the heating temperature is designed according to Ac 3 + (50 ~ 100 C ) . Normalizing is controlled according to the three temperature zones of heating and holding . Combined with the actual use of the roller hearth heat treatment furnace and the control of the production rhythm of steel plate heat treatment, constant speed heating is adopted , and after normalizing, it is air-cooled to room temperature .

Process route and key technology control of E36 ship plate
3 . 1 process route
Hot metal desulfurization treatment * 120 t converter smelting * LR refining * RH vacuum degassing * slab continuous casting (250 mm thick slab )* cutting and storage * continuous casting slab heating * descaling * 3 450 mm rolling mill rolling * ACC cooling * Straightening * finishing * surface shot blasting * normalizing * air cooling * sampling * inspection and storage .
3 . 2 Key technical control of trial production
3.2.1 Smelting and control of clean steel
In order to obtain better low-temperature toughness , it is necessary to control the P content in the steel. The converter adopts the double slag method to reduce phosphorus and strengthen the technology of slag blocking and tapping to prevent phosphorus return; to ensure the refining time of LF white slag, absorb inclusions in steel, and reduce The content of elements such as &O in steel is treated with Si—Ca line , Improve the shape of inclusions; RH vacuum degassing treatment, to ensure that the vacuum degassing time is not less than 15 minutes to reduce the content of harmful elements such as H and N in the steel as much as possible .
3.2.2 Internal quality control of continuous casting slab
Casting with constant speed and low superheat , Ensure that the temperature fluctuation of molten steel in the tundish is small and controlled at about 20 C above the liquidus temperature ; Using the crystallizer liquid level automatic control system to stabilize the liquid level fluctuations , Prevent slag rolling and slag dropping in the pouring process ; the continuous casting process adopts the whole process of protective pouring , and uses special low-carbon steel mold slag ; uses dynamic secondary cooling water distribution and dynamic light reduction technology to improve the component segregation of the billet , Ensure that the slab segregation is controlled below Class B 10 , and the central shrinkage porosity is below 10. 3. 2. 3 Continuous casting slab heating system
Due to the addition of a certain amount of microalloying elements such as NhTi in the steel , In order to fully make the microalloying elements into solid solution and prevent the grain growth caused by too high temperature, it is necessary to ensure that the heating time is longer than 2.5 h and the heating temperature does not exceed 1 250 C.

 Controlled rolling and cooling
Two-stage controlled rolling ( ie, austenite recrystallization zone + non-recrystallization zone controlled rolling ) is adopted . In order to ensure fine and uniform grains , the rolling start temperature in the rough rolling stage is controlled at about 1 050 C , and the single pass reduction rate in the rough rolling stage is not less than 12 % . In the finishing rolling stage, the thickness of the intermediate slab is controlled to be more than twice the thickness of the finished product , the starting rolling temperature is controlled below 880 °C , the final rolling temperature is controlled below 820 °C , and the final cooling temperature is controlled below 700 ° C . Fine-grain strengthening can effectively improve low-temperature toughness , The ferrite mechanism is induced by large deformation at low temperature in the unrecrystallized region , Combined with appropriate cooling measures to effectively control the grain size .

E36 ship plate normalizing treatment
According to Andrews temperature empirical formula (1) Ac 3 (°C ) =910 — 203 C 1 / — 15 . 2n i + 44 .7 Si + 104 y + 31 . 5m o + 13.1 W to estimate the Ac3 temperature is 851 C, the heating temperature is set at about 930 C according to Ac 3+50 ~ 100C control , the heating speed is set at 20 ~ 2.2 m n/im, after normalizing, air cool to room temperature

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