Low Cost Shipbuilding Steel EH36 With Enhanced Weldability

New type low cost shipbuilding steel EH36 with different thickness (20, 26，36 mm) were industrially produced by using advanced steel-making and thermo-mechanically controlled processing technology， the plates with excellent low temperature toughness having a microstructure of polygonal ferrite and acicular ferrite satisfy themechanicalpropertieso EH36．Thermalsimulationwasconductedtoevaluatetheplateweldability， withthe increase of heat input from 30 to 160 kj/cm， austenite grain size (AGS) of coarse grain heataffected zone (CGHAZ)increased， Vickershardnessdecreased， andmicrostructurebecomedgrainboundaryferrite，intra-gran- ularacicularferriteandintra-granularferriteinsteadofgranularbainite， andtheresultsshowtheplatehasexcel- lenttoughness．TherefinedgrainstructureofCGHAZ which lead to excel ent toughness results from such fac- tors： prohibition of austenite grain growth by TiN particles，prevention of Widmanstatten ferrite formation by boron segregation at austenite grain boundary and promote of intra-granular ferrite formation by TiN and BN. The resultsabovewerealsoprovedbytandemsubmergedarcwelding．

Key words : shipbuilding steel EH36 ; thermal simulation ; intra-granular ferrite; low temperature toughness;

Ship plate steel AH36/EH36 is one of the most common steel types currently used in hull construction , with an annual demand of 2 million to 2.5 million tons [ 1 ] . When used in hull construction, single-wire or double-wire submerged arc welding is usually used. In order to ensure the mechanical properties of welded joints, especially the impact performance of the heat-affected zone, it is generally necessary to control the welding heat input to not exceed 50 kj/cm .
20 % to 35 % of the total shipbuilding cycle , so improving welding efficiency is an effective way to improve shipbuilding efficiency. At present, welding methods with a welding heat input of 80 ~ 150 kj/cm are gradually being introduced into the ship welding process, such as single-wire or double-wire gas-electric vertical welding, and large-diameter multi-wire submerged arc welding, all of which can make the thickness specification not greater than 40 mm steel plate to achieve one-time penetration, reducing the number of welding passes, thereby improving efficiency and reducing cost. However, the ship plates on the existing market cannot withstand the above-mentioned large heat input welding. When the welding heat input is greater than 50 kj/cm , the impact toughness of the welding heat-affected zone will deteriorate significantly, which cannot meet the standard requirements of the classification society. It directly restricts the development of high-efficiency shipbuilding. Therefore, the shipbuilding industry urgently needs to be able to adapt to large heat input (150 kj/cm ) of welded ship plate. At present, the EH40 grade ship plate developed by Shagang using "SHTT " technology can realize single-pass welding of 40 mm thick steel plate, the welding line energy is not less than 350 kj/cm , and can ensure the mechanical properties of the welding heat affected zone.

the author has developed a low-cost high-weldability ship plate steel EH36 , which has been smelted and rolled through on-site mass production . According to the requirements of the classification society, the welding performance of the steel plate was evaluated by thermal simulation and high heat input welding method. In this paper, the microstructure and mechanical properties of rolled steel plates and the microstructure and properties of welded joints are described in detail.

Ship plate steel EH36 test method

180 t converter on site , its chemical composition (mass fraction, %) is C 0. 06, Si 0. 15, Mn 1. 50, Nb + V + Ti < 0. 1, A10.03, b 0. 001 ,p < a 0i,s < a 006 . The thickness of the continuous casting slab is 220 mm , the heating temperature is 1 200 °C , and the heat preservation is kept for 2 hours , and the controlled rolling is carried out on a 5 m wide plate rolling mill equipped with Mulpic accelerated cooling equipment , and then the finished steel plate with a thickness specification of 20 ~ 36 mm is obtained , the rolling process parameters are shown in Table 1 .
After the samples were ground, polished and etched with 4 % nitric acid alcohol, Zeiss

Table 1 Hot rolling process parameters of trial steel plate
Table 1 Hot rolling parameters for industrial production of plates

Steel plate thickness / mm	thick	rolling	Refined	rolling		controlled cooling
	Rolling temperature /C	Press down / mm	Rolling temperature / C	Press down / mm	Opening and cooling temperature / C	Final cooling temperature /C	Cooling rate /(C • s	-1)
20	1090	220—60	880	60—20	805	450	10
26	1080	220—80	860	80-26	855	420	11
36	1110	220—110	840	110 —36	810	400	13

Optical microscope and JEOL JSM 7001F field emission scanning electron microscope ( SEM ) were used to observe the structure of the steel plate. Electron Backscatter Diffraction ( EBSD ) analysis samples were ground and oxide polished, and carried out on a JSM 7001F scanning electron microscope with EBSD accessories , with an accelerating voltage of 20 kV and a step size of 0.2 pm. The BCC phase was calibrated using HKL Channel 5 software was used for data analysis. Tensile properties were tested using M12 size specimens on a 250 kN tensile testing machine (model 5585H , Instron ) ; impact tests were performed on a 450 J (IMP 450J , Instron ) pendulum testing machine, and the sample size was 10 mmX 10 mmX55mm . _ _
The welding performance of the steel plate was evaluated by the thermal simulation of the welding coarse grain zone (HAZ ) test and the double-wire double-sided submerged arc welding test. The thermal simulation test is carried out on a Gleeble 3800 thermal simulation testing machine, and the specific parameters are as follows: 1) heating to 1 400 C at a speed of 180 C / s and staying for 3 s ; 2 ) using a 2D heat source model, and setting the simulated plate thickness to 30 mm ; 3 ) 8/5 are set to 13 , 40, 150 and 300 s respectively, and the corresponding welding heat input is 30 , 60, 110 and 160 kJ/cm respectively . The double-wire double-sided submerged arc welding test of the 26 mm thick trial-produced steel plate was carried out on a welding test machine equipped with Lincoln AC/DC1000 power supply. There was no preheating before welding and no heat treatment after welding. The specific welding parameters are shown in Table 2 . The welding material is a self-developed welding wire with high heat input resistance, and its main components (mass fraction, %) are : C 0. 10, Si 0. 35, Mn 1. 60, P < a 02, S 01 ; the flux mainly The composition (mass fraction, %) is: SO2 13 49 , Al2O31774 , TiO2115 , MnO080 , FeO 055 , CaO 3 89,CaF 2 22 13, MgO 31 74, K 2 O 1 36, Na 2 O 2 20 .

	Table 2 welding test process parameters Table 2 Parameters for submerged arc welding
Welding method Groove (X type )	' Welding current /A welding voltage / V welding wire spacing/ welding speed / heat input / Location Front wire and back wire mm (cm • min _1 ) (kJ • cm -1 )
Bluntness of twin wire submerged arc welding : mm	First welding surface 750 600 34 37. 5 67 Rear welding surface 860 600 34 375 20 43 72

• Analysis of test results of ship plate steel EH36

2.1 Microstructure and properties of EH36 steel plate

The yield strength of ship plate steel EH36 is not less than 380 MPa, the tensile strength is not less than 510 MPa, the elongation is not less than 25% , the reduction of area is not less than 75% , and the impact energy at -40°C is not less than 300 J.
The EH36 steel plate has good strength and plasticity, which meets the mechanical performance requirements of the classification society for EH36 .

Table 3 Mechanical properties of the trial-produced steel plates
Table 3 Mechanical properties of the hot rolled plates

Thickness / mm	Yield strength / MPa	Tensile strength / MPa	elongation / %	Reduction of area / %	Yield ratio		0°C		Impact energy /J — 20°C —					-40°C
20	390	530	26	84	0 .	73	385 ,	384 ,	390	389 ,	383 ,	396	305 ,	367 ,	316
26	380	535	25	79	0	72	383 ,	394 ,	385	354 ,	389 ,	372	366 ,	359 ,	369
36 , 1/4 thickness at _	415	515	27	83	0	77	393 ,	388 ,	378	388 ,	385 ,	377	360 ,	327 ,	348
36 , 1/2 thickness at _	390	510	28	82	0	73	319 ,	325 ,	339	303，	321，	327	310，	311，	307

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