F690 extra-thick ship plate steel

Using automatic submerged arc welding technology to carry out double-sided multi-layer welding test on extra thick (thickness 80 mm ) F690 steel plate, test the tensile properties and hardness of welded joints at room temperature, check the Charpy impact performance at -60 t , and analyze the Microstructure of different subregions of welded joints . The results show that when the heat input (E ) is 35 kj/cm , F690 steel does not appear to soften the heat-affected zone of welding, and the strength and toughness of the welded joint match well . The microstructure of the heat-affected coarse-grained zone ( CGHAZ ) is mainly lath-like bainite (LB ), and the microstructure of the heat-affected fine-grained zone ( FGHAZ ) is mainly fine granular bainite (GB ) .
Key words : F690 extra-thick steel plate; welded joint; microstructure

When developing resources in the cold polar regions and deep sea, the low temperature toughness of hull and platform steel must be guaranteed . Therefore, the demand for steel plates for ships and offshore platforms is developing in the direction of high strength , high toughness and large thickness . The NV-F690 grade ship plate steel with extra thickness (thickness over 80 mm ) , high strength and toughness , easy welding , ultra-wide and ultra- long plate width is an important part of large-scale hull structures and offshore platforms in the future .
High-strength and extra-thick (thickness 80 mm ) F690 ship plate, its alloy is low C steel with composite addition of Cu , Ni , Nb , Cr , Mo , Ti , the steel plate is produced by controlled rolling and controlled cooling ( TMCP ) and advanced heat treatment technology [ 1-4 ] .
F690 extra-thick steel plate is usually a continuous multi-pass welding operation . The microstructure and properties of heat-affected coarse-grain zone ( CGHAZ ) are affected by the heat input ( E ) and the peak temperature of multiple thermal cycles ［ 5 ］ . The thermal simulation test shows that [ 2 ] , the critical coarse-grained heat-affected zone of low- C Cu- containing F690 steel

 ( Intercritically reheated coarse-grained heat-affected zone , IRCGHAZ ) is a brittle zone . When E is lower than 35 kj/cm , its supercritically reheated coarse-grained heat-affected zone ( SRCGHAZ ) is a ductile zone . However, the results of thermal simulation tests still need to be verified by welding tests of steel plates that are closer to the actual production .
In this paper, the industrially produced F690 extra-thick steel plate is taken as the research object, and the automatic submerged arc welding ( Submerged ARC welding , SAW ) technology is used to carry out double-sided multi-layer multi-pass welding experiments, and the microstructure and properties of the welded joints of the F690 steel plate are studied. Provide a basis for the application of ship plate steel .

1. Test materials and methods

The test steel F690 was continuously cast into a 320 mm thick slab through industrial production processes such as pre-desulfurization treatment of molten iron , 80 t converter steelmaking , ladle refining ( LF ) , and vacuum degassing by RH method . Ca treatment is used in steelmaking to control the shape of inclusions, and light reduction is used in continuous casting to reduce the degree of central segregation of the slab . The chemical composition (mass fraction, % ) of continuous casting slab molten steel is: 0.07C , 0.21Si , 1.03Mn , 1.68Ni , 1.08Cu , 0.036Al , 0.008P , 0.002S , 0.042Nb , 0.014Ti And the right amount of Cr and Mo , the balance of Fe . Carbon equivalent C eq =0. 61 , cold crack sensitivity coefficient P cm =0. 27 [ 3 ] .

Document 1 gives the TMCP production process of the 80 mm thick steel plate in this paper . The hot-rolled steel plate was heated to 895吆, kept at the temperature for 170 minutes , water-quenched to room temperature by a roller quenching machine, and the quenched steel plate was heated to 640吆and tempered for 240 minutes , and water-cooled to room temperature after being released from the furnace .
welding test was carried out by using the American Lincoln submerged arc automatic welding machine ( LINCOLN Power Wave AC/DC 1000 ) . The specification of the welded butt joint test plate is 380 mm x 200 mm x 80 mm (length x width x thickness), where the length direction is the rolling direction of the steel plate . The butt joint groove is a K -type groove . Use arc starting plate and arc extinguishing plate, preheat 100 °C before welding . Use welding wire and flux produced by ESAB , Sweden . Among them, the diameter of the welding wire is ! 4 . 0 mm Tubrod 15 . _ 27S flux cored wire, the flux is OK Flux 10 . 62 . The chemical composition of welding consumables is listed in Table 1 . Before the test, clean the groove, degrease and reveal the metallic luster . The flux is dried at 300 C , and the temperature stay time is not less than 60 min . Strictly control the welding process parameters and interlayer temperature during the welding process to prevent the weld seam from overheating . Clean up the interlayer welding slag between passes to avoid welding defects such as inclusions and pores in the weld . The automatic welding process parameters are listed in Table 2 . The docking polarity is DC reverse polarity ( DCRP ), and the heat input E =35 kj/cm . After the last welding pass, the test plate was kept at 200 C for 120 min for dehydrogenation treatment .

Table 1 Chemical composition of welding wire ( mass fraction , % )
Table 1 Chemical composition of the weld wire ( wt% )

C Si	Mn PS Ni Cr Mo Cu	Ti Al V
0. 23 0. 19	1.740.0110.007 2.350.016 0.27 0.01 0.0220.0490.013
Table 2 Test steel welding process parameters Table 2 Welding parameters
Welding current	Welding voltage Welding speed Interpass temperature	Corresponding heat input
I/A	U/V V/ (cm ・min-1) T/C	E/ (kJ ・ cm-1)
700	32 38.4 150 〜200	35

rolling-direction x -thickness section microstructure was analyzed using an optical microscope ( OM ) . The polished surface was etched by 4% nitric acid alcohol solution . The thin film sample was firstly thinned mechanically, and thinned to perforation by double spraying with 5% perchloric acid and 95% ethanol solution at 50 V. The microstructure of the thin film sample was observed with a jEM-2100F transmission electron microscope ( TEM ) . 200kV . _ Process the tensile specimen at room temperature according to the provisions of literature [ 6 ], and the thickness of the tensile specimen with welded seam is 40 mm . The test was carried out on a 1200 kN tensile testing machine ( Instron 8850 ) according to GB / T 2651-2002 " Welded Joint Tensile Test Method" and GB /T228-2002 " Metallic Materials Tensile Test Method at Room Temperature " . The specimens for the Charpy impact test ( KV 2 ) were taken from the center of the weld , the fusion line , 2 mm ( FL+ 2 ) and 5 mm (FL +5 ) from the fusion line . The impact test is carried out according to GB/ T 2650-2002 " Welded Joint Impact Test Method" and GB /T 19748-2005 " Steel Charpy V - notch Pendulum Impact Test Instrumented Test Method " using 450 J drop weight testing machine ( IMP450j Dynatup , Instron ) , the test temperature is -60C . Using the Instron Vickers hardness tester, test the hardness distribution of the thickness section of the welded joint along the width direction of the steel plate, and the load weight is 10 kg .

• Test Results and Analysis

2. 1 Microstructure and properties of the parent steel plate
The F690 base metal steel plate has excellent comprehensive properties. The transverse non-proportional elongation strength ( R po.2 ) at the thickness center (t/2 ) is 722 MPa , the tensile strength ( R m ) is 784 MPa, and the elongation after fracture ( A ) 18% , -60 C lateral KV 2 166 J , -80 C lateral KV 2 is 81 J ; R po.2 at t /4 is 766 MPa , R m is 804 MPa , A is 21% , -60 C lateral KV 2 230 J , -80 C lateral KV 2 for 205J .

2 Room temperature tensile and low temperature impact properties of welded joints
Table 3 Tensile properties of test steel welded joints
Table 3 Tensile properties of the welded joint

sample	R p0. 2 /MPa	R m /MPa	Fracture location
1	749	798	base material
2	764	809	base material

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