E36 High Strength Ship Plate Steel

This paper introduces the main technical idea and process route of developing E 36 high-strength ship plate steel in wide and heavy plate factory of HENAN BBN Steel Co. , Ltd. Through reasonable composition design, molten steel purity control and controlled rolling and controlled cooling process, E36 ship plate with pure steel quality and fine structure is obtained, which has excellent comprehensive mechanical properties and meets the certification requirements of classification societies .
Key words E36 ship plate steel pure steel controlled rolling controlled cooling

With the rapid development of the domestic shipbuilding industry, my country has become one of the most important shipbuilding countries in the world. Therefore, my country's demand for ship plate steel continues to increase, and at the same time the quality level requirements are also getting higher and higher. Xiangtan Iron and Steel Company (referred to as Xiangtan Iron and Steel) is a new medium and heavy plate production base in my country, and ship plate steel is one of its main varieties. In order to further improve the market competitiveness of ship plate steel, expand the market share of ship plate steel, and increase the benefit of varieties, HENAN BBN Steel Co., Ltd. has carried out the development of E36 high-strength ship plate steel.
1 Ship plate steel production process
The ship plate steel process flow of HENAN BBN Steel Co., Ltd. is: molten iron pretreatment 120 1 top-bottom combined blown converter f LF VD refining * continuous casting * heating * phosphorus removal * rolling f ACC cooling ~ normalizing ~ finishing ~ storage 2 ship plate steel composition design
E36 high-strength ship plate steel not only has high requirements for strength, but also has high requirements for low temperature ( -4 『 C ) impact toughness and welding performance. Therefore, the main basis for designing E36 steel is to adopt microalloying, improve the purity of steel, and optimize the controlled rolling and controlled cooling process .
According to the new design concept of low-alloy high-strength steel , modern welding structures require low-alloy high-strength steel to have low carbon content , which can adapt to on-site welding conditions . The minimum notch toughness and maximum hardness of welds and weld heat-affected zones shall comply with the new requirements. In order to ensure good weldability and low temperature impact toughness at the same time , It is necessary to reduce the carbon content and carbon equivalent in steel, and the carbon equivalent of E36 is required to be less than 0.38 where : Ceq (%)=C + 1 / 6Mn + (Cr + Mo + v) / 5 + (Ni + Cu) / 15 .
In the case of ensuring good low-temperature toughness, the strength of the steel plate is improved by microalloying and optimizing the controlled rolling and controlled cooling process. The core of the composition process design of E36 ship plate is how to refine the grain. Appropriate addition of Ni element in the steel can ensure the good low temperature toughness of the steel, and at the same time increase the content of Mn , and the strength of the steel plate can be appropriately improved through its solid solution strengthening. In addition, by adding an appropriate amount of microalloying element Nb , the fine Nb ( C, N ) precipitated during the hot rolling process can affect the grain boundary
The pinning effect increases the termination temperature of recrystallization, thus providing a higher and wider temperature range for the controlled rolling of steel. However, a certain deformation of the steel below the austenite recrystallization temperature will produce "pancake-shaped grains" that are not recrystallized. In the subsequent transformation process from austenite to ferrite, it is easier to obtain fine ferrite grains, which improves the strength and toughness of steel. Therefore, the chemical composition design of the E36 ship plate steel in this test is shown in Table 1 .
3 Control of purity and inclusions of molten steel
Chemical composition, mechanical properties and processability of steel-to-steel for ship hulls
There are high requirements for performance, especially for high-grade ship plates, which have very high requirements for steel structure uniformity, strength, and low-temperature impact toughness. Therefore, it is necessary to improve the purity of molten steel, reduce inclusions in steel, and control inclusions at the same time. form to improve the plasticity, toughness and elongation of steel. 3 1 Purity control of molten steel
In order to reduce the total oxygen content in Al-killed steel , the point of view of system control is adopted. First, determine a reasonable end-point carbon content, adopt enhanced stirring, improve the end-point control level, and reduce the end-point oxygen content. Secondly, focus on solving the problem of inclusions in the deoxidation process , control the amount of slag lowered by the converter and improve the top slag of the ladle , Aluminum wire is fed after the furnace , Rapidly reduce the oxygen content in steel and ladle slag , Then through soft blowing and long-term sedation, the inclusions are promoted to float up; LF and VD refining processes further reduce the oxidation of slag , Increase the alkalinity of slag , And feed the calcium wire to denature the oxide to form liquid inclusions , and cooperate with reasonable argon gas stirring to further promote the floating and absorption of the inclusions . To improve the purity of molten steel and reduce the total oxygen content in the ladle ; The continuous casting process is mainly to prevent secondary oxidation and further purification , To this end, a large-capacity tundish (25 1 ) , argon blowing protection pouring throughout, and a high-alkalinity tundish covering agent are used to further reduce inclusions and improve the purity of molten steel.
For deep desulfurization , First of all, the amount of slag in converter tapping should be reduced . Create good conditions for the rapid slagging of LF . During the tapping process, the refining process is carried out in advance through the modification of ladle slag and the strengthening of molten steel deoxidation. A reasonable slag system is selected for LF refining. The MnO+FeO content in the slag is less than 2%, which meets the necessary conditions for slag deoxidation and desulfurization. At the same time, the slag basicity is controlled at about 40 , and the MI index is between 0.31 and 0.34 , which is Desulfurization creates favorable thermodynamic and kinetic conditions. In the process of slagging, follow the principle of "fast, white and stable", and cooperate with reasonable argon blowing and stirring process. Under VD vacuum, a larger amount of argon blowing is used to further improve the desulfurization kinetics of molten steel. A large-capacity tundish is used to promote the floating of inclusions. The mass fraction of sulfur in the tundish molten steel is less than 0 003% , Meet the quality requirements of high-grade ship plate steel , Improve low temperature impact toughness of ship plate steel.

 

Control of rolling process of ship plate steel
Controlled rolling and controlled cooling is a forming process . Its final deformation process is carried out in a specific temperature range. In the first rolling stage ( rough rolling ) , the austenite grains are coarsened due to heating , and then gradually refined due to continuous deformation and recrystallization process. In order to achieve complete recrystallization, rough rolling of austenite should be completed above 950C . The second rolling stage, that is, the finish rolling stage in the austenite non-recrystallized zone , Rolling at this stage causes recrystallized grains to form elongated deformed bands. After the transformation is complete, ferrite with very fine grains is formed, so that good strength and toughness can be obtained. In the first stage , the slab is rolled to 25 to 4 times the thickness of the final product , depending on the required strength and toughness , After the intermediate billet is cooled to about 950C , the second stage of rolling starts and is rolled to the final product thickness.
When the recrystallized austenite is controlled and cooled, the grain refinement of ferrite occurs to some extent, but the effect is not significant. If the unrecrystallized austenite is controlled and cooled, not only the deformed austenite grain interface can also generate crystal nuclei in the deformation zone, and ferrite nuclei can also be generated in the austenite grains, realizing the ferrite substantial grain refinement.
After the steel plate is cast in the last pass of finished rolling , it is transported by the roller table and directly enters the ACC device. When the steel plate passes through the ACC device , The upper and lower sides are sprayed with water at the same time for rapid cooling, so that the temperature of the steel plate is rapidly dropped from about 700C-800C ( that is, the austenite zone or duplex zone ) to 600C-650C (ACC controlled cooling). For steel plates with a thickness of more than 25mm , the passing speed of the ACC device is about 0 3 - 1 0m/ ; for steel plates with a thickness of less than 25mm , the passing speed can reach up to 3 0mA .

Mechanical Properties of E36 Ship Plate Steel
After being rolled by the controlled rolling and controlled cooling technology in the medium and heavy plate rolling mill , the strength and elongation of the ship plate meet the requirements of the national standard .
Its mechanical properties : mechanical properties of E36 ship plate steel

Sampling position	Rel/MPa	Rm / MPa	A /%	Sampling position	AKv ( —4CTC ) / J	AKv (—60C )/ J
horizontal head	520	600	25 . 5	horizontal head	245 , 234 , 225	145 , 219 , 265
				head portrait	244 , 259 , 264	188 , 191 , 216
tail lateral	470	570	25 . 5	tail lateral	239 , 201 , 183	159 , 206 , 186
				longitudinal tail	230 , 185 , 222	193 , 168 , 213

1. HENAN BBN Steel Co., Ltd. adopts hot metal desulfurization, converter smelting and LF/VD refining to control [S]< 0.003 %, [P]V0.0120 %, T[0]<20X10 — 6 , n <55X10 — 6 ; The inclusions in the slab are completely denatured and spheroidized , and the average microscopic inclusions are 2 . 83 / dish, the average large-grained inclusions are 156 mg /kg , which has a relatively high purity, which can fully meet the quality requirements of high-grade ship plate steel for casting slabs .
2. Through microalloying and reasonable controlled rolling and controlled cooling process, the structure of the rolled material is refined, so that the mechanical properties of the E36 ship plate steel are good, the difference in the properties of the transverse and longitudinal steels is reduced, and the brittle transition temperature of the steel is lowered.
3. The performance of high-strength ship plate steel produced by this process can fully meet the requirements of shipbuilding.

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