DH36 High Strength Ship Plate Steel

Abstract : This paper introduces the practice of developing and trial-manufacturing DH36 high-strength ship plate steel at Tiangang . Through the design of reasonable chemical composition and strict control of the process operation of steelmaking , steel rolling and other processes , DH36 high-strength ship plate steel has been developed , and has successfully passed the certification of China Classification Society .
Key words : DH36; high-strength ship plate; refining; controlled rolling and controlled cooling

With the increasing development of the global shipbuilding industry, large-scale and light-weight hulls have become the main development direction of the world's shipbuilding industry . The quality requirements for marine steel plates are getting higher and higher . Since the high-strength ship plate steel has high strength and good comprehensive performance , it can reduce the weight of the hull , The advantages of increasing the load capacity , Therefore, its usage is increasing year by year . It has also greatly promoted the development of China's low-alloy high-strength ship plate steel market . After Tianjin Iron and Steel Co., Ltd. ( hereinafter referred to as Tiangang ) eastward project was successfully put into operation , In terms of process equipment, it has the conditions for the development of high-strength ship plate steel . In May 2007 , Tiangang started the development and trial production of DH36 high-strength ship plate steel .
1 Composition design of high-strength ship plate steel
1 and Table 2 for the chemical composition and mechanical properties of DH36 high-strength ship plate steel required by GB712-2000 ; at the same time, the national standard stipulates that microalloying elements such as Nb , V , and Ti can be added alone or in any mixed form . When added alone, its content should not be less than the lower limit specified in the standard . When mixing two or more kinds, the total content shall not exceed 0.12 % .

Table 1 Chemical composition (mass fraction ) of national standard high-strength ship plate steel DH3 6 (GB71 2-2O00 ) %

C	Si	M n PS	Nb V	Ti	Al t
C0.18	<0.50	0.9 ~ 1.6 < 0.035 < 0.035	0 . 02 ~ 0 . 05 0 . 05 ~ 0 . 10	<0.02	>0.020
Table 2 National standard high-strength ship plate steel DH36 mechanical properties ( GB712-2000 )
Specifications/ mm	steel grade	R el /MPa R m /MPa	A /%	A kv /J
			0 °C -20°C	-40C _	-60C _
<50	DH36	> 355 490 ~ 630	> 22 34 / 24 34 / 24	34/24	34/24

Note: Before the slash in the table is the vertical value, and after the slash is the horizontal value.
Referring to the national standard and related information , considering the ratio of strength and plasticity of DH36 ship plate steel, reserve of toughness, weldability, corrosion resistance , delivery status and other requirements , the design principle of low carbon and high manganese is adopted in the composition design At the same time, a certain amount of aluminum content is guaranteed in the steel; two schemes of vanadium and niobium are tried to be added to the micro-alloying process , and corresponding controlled rolling and controlled cooling processes are formulated respectively to develop the high-strength ship plate steel of Tiangang .

2 Process equipment and process selection

1. 1 Process and equipment situation

Steelmaking : a 120 t top-bottom combined blowing converter ;
Refining : one 120 t LF, one 20 t VD unit with single cover and double pits ;
Continuous casting : 2 straight mold arc-shaped slab continuous casting machines ;
Steel rolling : 3500 mm double-stand plate rolling mill ;
2.2 Process flow
Top-bottom compound blowing converter * LFf slab continuous casting * inspection * heating furnace * plate mill rolling * cooling * shearing * inspection and storage .
3 Key control points of DH36 steel production
Harmful elements and inclusions in steel seriously deteriorate the related properties of steel, especially because the content of alloy elements in low-alloy high-strength steel is low, only when impurity elements are removed thoroughly, alloy elements can fully play their role . Therefore, the higher the requirements for strength, toughness and weldability, the more important the use, and the harsher the use conditions , the higher the purity required . At the same time, since the shape and distribution of inclusions also have a great influence on the performance of the steel, if they cannot be removed, they should be spheroidized and distributed evenly in the steel to reduce their damage . Therefore, the key point of operation from steelmaking to continuous casting is the control of inclusions and harmful elements in steel .
In the steel rolling process, in order to effectively improve the strength and toughness of the steel and improve the welding performance and various processing properties of the steel under the condition of low carbon content and alloy elements, the controlled rolling and cooling process is required . The essence of controlled rolling is to obtain fine austenite grains, so that fine ferrite grains and other fine phase transformation products can be obtained after Y*a transformation, which can effectively improve the strength and toughness of steel . The essence of controlled cooling is to prevent the recrystallization or growth of fine austenite and ferrite grains after controlled rolling, so as to obtain fine grains and corresponding structures and improve the strengthening effect[1 ] o
High-strength ship plate steel requires high strength, toughness, plasticity and weldability, so the following aspects should be controlled from smelting to rolling .

1. 1 converter smelting

0.035 %, w ( P ) <0.080 %.

1. During the smelting process, the dephosphorization operation should be strengthened , and the lance position should be appropriately raised in the early stage to quickly dissolve slag, so as to form slag with high alkalinity, high oxidation and good fluidity in the early stage as soon as possible, so as to realize the early dephosphorization of molten steel; in the middle stage, the lance position should be appropriately lowered to prevent slag Re-drying; the end point temperature is not higher than 1660 °C, effectively preventing the molten steel from returning to phosphorus; preventing end-point overoxidation ; Require end point w(C) >0.08%, w(P) <0.010 %.
2. During the tapping process, the slag blocking operation is strictly controlled, and the thickness of the slag in the ladle is controlled not to exceed 50 mm ; Ladle top slag modifier is added in the later stage of tapping to promote the adsorption and removal of deoxidized products .
3. Use Fe-AkBa-Si composite deoxidizer for final deoxidation, the usage amount is 2 ~ 3 kg/1 .

3.2 LF

1. When molten steel enters the station, it is first fed with aluminum wires to strengthen deoxidation .
2. Adopt high alkalinity and reducing white slag refining process to realize desulfurization , deoxidation and adsorption removal of molten steel . It is required that the slag basicity is greater than 3.0 , w (FeO ) + w ( MnO) <1.0%; and the white slag refining time should not be less than 20 min .
3. Bottom-blown argon is stirred throughout the refining process, and the stirring intensity of bottom-blown argon is adjusted reasonably according to different refining tasks in different periods .
4. Before the refined molten steel leaves the station, the Ca-Si line is fed to the molten steel for calcium treatment to promote the denaturation and removal of A12O3 inclusions in the molten steel. At the same time, ensure that the argon blowing and stirring time after feeding the Ca-Si wire is not less than 10 min . 3.3 Continuous casting
5. ladle * tundish * mold is adopted to effectively prevent secondary oxidation of molten steel to ensure w(Al s )/w(Al t )>85 % in steel .
6. Since the carbon of the designed high-strength ship plate steel is in the peritectic reaction zone, in the selection of mold powder, special powder for peritectic steel is selected to improve the heat transfer between the billet and the mold and reduce the occurrence of cracks . At the same time, due to the presence of crack-sensitive elements V and Nb, the secondary cooling strength is reasonably controlled in the continuous casting process, so that the slab straightening temperature is not lower than 950 C , avoiding the brittle temperature zone of steel, and effectively preventing the occurrence of slab straightening cracks .
7. Use of alkaline tundish covering agent to promote non-metallic clips

1. Adopt dynamic soft reduction technology to improve the internal quality of the billet .
2. Control the superheat of molten steel in the tundish at 15-25 ° C , and control the casting speed at 0.85-0.95 m /min .

3-4 Steel rolling process

1. Three-stage heating is adopted , and the temperature of the slab out of the furnace is controlled at

1130 ~ 1150C . _

1. The reduction rate of each rough rolling pass ( except the cross rolling pass ) is 10 %~ 12 %; the cumulative deformation rate in the finishing rolling stage is not less than 60 %, and the cumulative rolling reduction rate in the last 3 passes of finishing rolling is not less than 45 % .
2. Add V high-strength ship plate steel finish rolling start rolling temperature 880 C , The finish rolling temperature is 810 ~ 830 C ; the finish rolling start temperature of Nb high strength ship plate steel is 930 C , The finishing temperature is 820 ~ 840C .
3. Large-scale varieties adopt accelerated cooling process , and the cooling rate is 5 ~ 8 C/s .

4 actual effect
For certification, we have completed 4 heats of DH36 high-strength ship plate steel trial and rolling . The chemical composition control of the finished product , the low-magnification inspection of the continuous casting slab and the mechanical property inspection of the steel plate are detailed in Table 4 ~ Table 6 .

Table 4 Tiangang DH3 6 high-strength ship plate steel finished chemical composition (average value)

steel type	w B /%
	C	Si	mn	P	S	Al t	Al 's	V Nb
DH36 ( plus V)	0.135	0 . 28	1.36	0.011	0.005	0.0411	0.038	0.0614
D H36 ( with Nb added )	0.114	0.31	1.36	0.012	0.005	0.0338	0.033	0.0245

DH36 high-strength ship plate steel billet quality is better , The chemical composition of the finished product and the mechanical properties of the steel plate fully meet the requirements of the national standard . Finally , Tiangang's DH36 high-strength ship plate steel passed the certification of China Classification Society . At the same time , after analyzing Table 5 , we found that :

1. V- added steel is low , Compared with the requirements of the national standard, the margin is less ( the minimum yield strength margin is 15 MPa, and the tensile strength minimum margin is only 10 MPa), and the elongation index has a slightly larger difference between the horizontal and vertical directions ( the difference is 25 %). However, the strength and elongation index of the Nb- added steel plate are generally better than those of the V-added steel plate . And the elongation of the steel plate has little difference in the transverse direction and the longitudinal direction .
2. In terms of the impact properties of the two , the impact properties at 0 C and -20 C , the 16 mm and 30 mm Nb -added steels are better than the V -added steels ; - 40 C impact test , the 16 mm specifications of the two steel plates Equivalent to merit ; However, for the 30 mm steel plate, the impact energy value of the added V steel has a certain improvement compared with the added Nb steel .

- 60 C impact performance, 16 mm and 30 mm Nb- added steels are better than V- added steels . In summary , we believe that the development of DH36 high-strength ship plate steel adding Nb as a microalloying element has a better effect .

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