Production of EH36 ship plate steel

High-strength ship plate is a kind of ship plate steel that requires high performance , and its development trend is towards high strength and high toughness  . In recent years , with the advancement of microalloying and controlled rolling technology , mainly Nb , V , Ti and other alloying elements are alloyed to refine the grains . The performance requirements of EH36 ship plate steel are relatively high . The difficulty in the production of this steel type is the cooling system in alloying and continuous casting. The production process requires strict requirements and the production is difficult . The process of EH36 ship plate produced by Tangshan Iron and Steel Plate Co., Ltd. is stable , the continuous casting production is smooth , the quality of casting slab meets the technical requirements , and the performance of the plate is good .
1 process flow
Technological process: 1 780 (1 580 ) m 3 blast furnace hot metal — 120 t top-bottom combined blowing converter — 120 tLF refining furnace — slab continuous casting machine — heating furnace — 3 500 mm rolling mill. Among them, the working pulling speed is 0 . 8 1 m/min , the cross-sectional size of the billet is 250 mm X2 000 mm , and the nominal capacity of the tundish is 38 t .

2 Chemical composition of EH36 ship plate steel
EH36 high-strength ship plate steel is mainly low-carbon , manganese - enriched and micro-alloyed . Since the EH36 ship plate requires a -40 Ω impact test and has high requirements for low temperature toughness , the composition design idea is C -Mn -Si - Al system , adding Nb and Ti microalloying elements . A small amount of Nb ( 0.02% ~ 0.05% ) plays the role of grain refinement and precipitation strengthening in the production of high-strength ship plate steel , forming fine carbides and nitrides, and inhibiting the growth of austenite grains . At the same time, increase the recrystallization temperature , inhibit the recrystallization of austenite, form Nb ( N , C ) in the non-recrystallized area , effectively inhibit the growth of ferrite , and accelerate the formation of ferrite grains to achieve obvious fine grain effect . The composition of EH36 ship plate steel is shown in Table 1 .
Table 1 Main composition % of EH36 ship plate steel
Tab. 1 Main composition of ship plate steel of EH36 class %

3 Process control of EH36 ship plate steel
3.1 Converter
The furnace charge structure is scrap steel + molten iron , which improves the hit rate of the end point and reduces the number of supplementary blowing; the end point composition of molten steel: C W O. 05% , P W O. 015% , and the content of residual elements meets the standard requirements; the tapping temperature is 1 620 ~ 1 640 Yi ; other alloys are added during the tapping process , and the main components such as Si , Mn , and Nb are adjusted . Slag is strictly prohibited during the tapping process , and synthetic slag is added after tapping .
3.2 Refining ( LF )
Without hot metal pretreatment equipment , the sulfur content of the initial molten steel during LF refining is relatively high , and it is relatively difficult to smelt low-sulfur steel . In order to meet the requirement that the sulfur content of the finished product is less than 0.008 % , in addition to conventionally increasing the amount of slag appropriately , increasing the tapping temperature of the converter and the basicity of the slag , according to the actual situation of weathering steel varieties, it is mainly to take measures to reduce the oxidation of molten steel and increase Acid-soluble aluminum content in steel .
Except for steel types with high carbon and silicon content , sulfur activity in general steel
degree coefficient is close to 1 , and a can be replaced by ® , so we have :
L s = 5s ) / 3 = C s _ / a [ O ⑴
the formula , L S - the distribution ratio of sulfur in slag and steel ;
C s —— sulfur capacity .
from formula ( 1 ) that the effect of increasing the sulfur capacity to n times and reducing the oxygen activity to 1 /n is equivalent to the sulfur distribution ratio . Therefore , in the production of weathering steel , the sulfur capacity is mainly increased by deep deoxidation and greatly reducing the oxygen activity .
The following rules exist between the deoxidation and desulfurization of molten steel :
3 [ s = 30 a [ O ( 2 )
In the formula , a [ O —— activity of oxygen in molten steel .
Practice shows that 3 ( Als ) in steel < 70 x 10 _6 , oxygen content in steel > 20 x 10 「 6 , is not conducive to the desulfurization reaction; if 3 ( Als ) in steel reaches 300 x 10 － 6 , oxygen in steel The content can reach ( 2 ~ 4 ) x10 - 6 , and the desulfurization rate is significantly improved . Therefore , the 3 ( Als ) of weathering steel in actual production is required to be greater than 200 x10 - 6 .
Other parameter control : LF furnace adopts aluminum wire to quickly deoxidize and adjust white slag , control the stability of slag , reduce the oxidation of slag , ( FeO ) + ( MnO ) W 1.5% , ( SiO 2 ) W 15% , alkalinity M 3.0 , Sampling and analysis after strong deoxidation . According to the analysis results , the ingredients are adjusted according to the internal control standards . Feed the aluminum wire before tapping the LF furnace to adjust the Al ] s content in the steel to 0.020% ~0.040% , and feed the calcium wire when Ca / Al = 0.06 ~ 0.10 to modify the inclusions and deep Deoxygenation , soft argon blowing after treatment
M 10min . The composition of LF furnace refining slag is shown in Table 2 .
Table 2 Main composition % of LF furnace refining slag

Tab. 2	Main composition of refining slag of LF furnace %
CaO	SiO 2	MgO	Al 2 O 3
45 ~ 55	12 ~ 15	8 ~ 10	15 ~ 20

EH36 ship plate steel includes impact energy , tensile test, etc. The thickness of the finished plate is W 16 mm, and its physical mechanical properties are qualified to meet the needs of customers . The physical properties are shown in Table 3 .
Table 3 Mechanical properties of EH36 ship plate steel products
Tab. 3 M echanical properties

	R eH a /MPa	Rm a /MPa	A/%	-40 seven
standard requirement	$ 355	490 ~ 620	$ 21	$ 34
detection value	368 ~ 450	500 ~ 598	22. 5 ~ 33. 5	104 ~ 260
average value	380	561	26	198

 in conclusion
( 1 ) The LF process reduces the oxidizability of molten steel and controls the Als content in the steel to be greater than 200 X 10 -6 , so that the sulfur content in molten steel is less than 0.008% .
(2 ) The cleanliness of the EH36 ship plate molten steel produced by the LD-LF-CC process is relatively high , the steel has high comprehensive mechanical properties , and the quality of the steel meets the requirements of users .

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