FH36 low temperature resistant high strength ship plate steel

This paper introduces the practice of Steel's trial production of FH36 low temperature resistant high strength ship plate steel. By designing reasonable chemical composition and adopting LF+RH double refining process to smelt, TM-CP process rolling, successfully developed FH36 high-strength ship plate steel, all performance indicators meet the standard requirements , and the impact energy at -60 wu reaches 200J.

FH36 high-strength ship plate steel has extremely strict requirements on steel performance and quality due to its harsh use environment . In addition to high requirements on strength, there are high or very strict requirements on toughness, weldability, corrosion resistance, lamellar tear resistance and many other aspects. Therefore, it is necessary to design a reasonable chemical composition, formulate a suitable process system, strictly control the composition of molten steel, and control the microstructure of the steel in order to obtain good performance indicators and meet the use requirements . Through a series of exploratory industrial trials, Laiwu Steel determined the production process of FH36 low-temperature resistant high-strength ship plate , and successfully developed a qualified product that meets the requirements of the classification society.
of FH36 high-strength ship plate Trial production process route: hot metal desulfurization pretreatment- 120t converter smelting - LF refining - RH refining - slab continuous casting - slab heating - high pressure water descaling-rough rolling-finish rolling-controlled cooling-straightening-flaw detection-finished product.

FH36 High Strength Ship Plate
FH36 high - strength ship plate steel requires high strength, and must have good low-temperature impact toughness and good welding performance . The ship plate adopts the composition design of low-carbon addition of Nb and Ti microalloying elements to determine the control range of steel composition. The selection of microalloys should not only consider the strengthening effect of microalloying elements, but also not have adverse effects on the structure of steel. At the same time, the influence of microalloying elements on the brittle transition temperature of steel is also an important factor to consider. The influence of steel purity on the toughness of FH36 high-strength ship plate, especially the low-temperature toughness, is the key to control technology in smelting. Therefore, it is required to reduce the content of P and S in steel as much as possible .

1. Converter smelting

The converter is mixed with desulfurized molten iron ( SW0.008 %). In order to reduce the desulfurization of the converter , the desulfurized slag should be removed . The focus of converter control is to minimize P and S content, reasonably control the end point C content to avoid peroxidation of molten steel. During the tapping process of the converter, the operation of slag blocking is strictly controlled, and the thickness of the slag in the ladle is controlled to be >80 mm. In the later stage of tapping, an appropriate amount of pre-melted synthetic slag is added for slag washing.

1. LF refining

LF furnace can further desulfurize, deoxidize, remove inclusions, adjust the composition and temperature of molten steel, and obtain a good refining effect. The entire refining process is blown with argon and stirred, and the molten steel must not be exposed to prevent secondary oxidation. Molten steel enters the station for bottom argon blowing for 1-3 minutes, then heats and refines, and adjusts according to the composition of the initial sample to make each chemical element meet the specified requirements. The refining process produces white slag, and the white slag retention time is >10 min .

1. RH refining

RH furnace refining is to carry out vacuum degassing to reduce the gas content in steel, reduce the defects caused by gas inside the steel plate, and improve the pass rate of steel plate flaw detection. Since FH36 high- strength ship plate steel has high requirements on low temperature toughness (requires -60 t impact test), it requires high purity of steel, and the influence of S is the main one. In addition to reducing the S content, it is also required to control the sulfide form during the smelting process. Therefore , calcium treatment is performed after RH refining to reduce the content of strips of MnS in the steel, and to denature the clustered Al2O3 into calcium aluminate, thereby It is easy to remove by floating or evenly distributed in the steel in the form of spherical inclusions. At the same time, after feeding the line, it should be ensured that the stirring time of soft blowing argon gas is less than 10 min, so as to ensure that the deoxygenated product fully floats up.

1. Continuous casting

The continuous casting adopts full protection pouring, the ladle to the tundish is protected by long nozzle and argon sealing, the tundish is protected by covering agent and covered, the tundish to the crystallizer is immersed in the nozzle and argon sealed, the mold is protected by mold slag, and the protection The slag is selected from special mold slag for ship plate. During the pouring process, control the superheat of molten steel in the tundish within an appropriate range and keep it stable. Generally, the superheat is controlled at about 20 °C ; maintain a constant casting speed, generally controlled by 0.85 m/min ; Improve the internal quality of the slab.

1. rolled steel
2. Slab heating

Considering that the austenite grains are prevented from further coarsening under the condition of ensuring sufficient solid solution of the microalloying elements, the heating temperature of the selected slab should not exceed the austenite coarsening temperature of the steel, and the heating temperature is controlled at
1 150- 1 250 O CO The heating time of slabs with different thicknesses is specifically determined according to different heating conditions of the heating furnace and the furnace temperature of the slab. High-pressure water descaling is used to ensure the descaling effect.

1. Controlled rolling

The key to high-temperature recrystallization zone controlled rolling in rough rolling is to control the rolling start temperature, pass reduction rate and total deformation rate. In the recrystallization temperature zone, a larger pass reduction should be used as much as possible to create more nucleation probability for austenite recrystallization and refine austenite recrystallization grains. The rough rolling start temperature is generally around 1 150 C , and the cumulative reduction rate is controlled at 60%~65% . The key to the controlled rolling in the non-recrystallized area of the finishing rolling is to control the starting rolling temperature, the thickness of the intermediate billet and the finishing rolling temperature. The starting rolling temperature is controlled at 880-950 C, the final rolling temperature is controlled at about 820 C , and the thickness of the intermediate billet is controlled according to different finished product specifications .

1. controlled cooling

Controlled cooling is mainly to control the starting cooling temperature, cooling speed and final cooling temperature after rolling, so as to control the phase transformation type, refine the grain, prevent or delay the premature precipitation of carbides in the cooling process, and make austenite in the iron Dispersed and precipitated in the matrix, improving the strength and toughness of the steel. Generally, the final cooling temperature is controlled at about 580 C , and the cooling rate is controlled at about 10 C/s .
actual effect

1. Chemical composition of molten steel

The high-strength ship plate steel is added with appropriate amount of Nb , Ti , Cu , Cr and Ni elements, and other chemical compositions and standard requirements are shown in Table 1. O
Table 1 Chemical composition of molten steel (mass fraction) %

furnace number	C	Si	mn	P	S	als	Ceq
1384	0.06	0.24	1.40	0.007	0.002	0.036	0.31
1387	0.06	0.19	1.39	0.010	0.005	0.036	0.30
Standard W0.16		W0.50 0.90- 1.60		W0.025	W0.025	Xi 0.015 W0.38

The carbon equivalent calculation formula in the table is Ceq=C+Mn/6+ ( Cr+Mo+ V ) /5+ ( Ni+Cu)/15 . From Table 1 , the chemical composition stability of the trial-produced FH36 high- strength ship plate steel is better, and the fluctuation of each element content is small, which lays the foundation for ensuring the performance of high-strength ship plate steel. At the same time, the carbon equivalent fully meets the standard requirements, ensuring that the steel plate has high strength and good welding performance.

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