Influence of Heat Treatment Process on Microstructure and Properties of NV-F690 Ship Plate Steel

A low-carbon copper-containing NV_F690 steel was studied on the phase transformation law of continuous cooling , and a new process ( DQ - .T ) was designed , The microstructure and properties of the test steel under the conditions of traditional quenching and tempering , DQ-T and DQ1T were studied. The results show that the microstructure of the test steel after water quenching ( WQ ) is lath martensite ( LM ) , and after tempering at 650 T for 1 h , it reaches the best matching of strength and toughness; Tensile body ( GB ) , its toughness is reduced. The DQ-T steel plate is resistant to temper softening, and its strength and toughness match reaches the best after tempering at 680-705 T for 1 h . The microstructure of the steel plate treated by the DQ-/T process is quasi-polygonal ferrite ( QPF ) as the soft phase and LM and LB grains containing dispersed Cu -rich precipitates as the hard phase , and the toughness is further improved. The best match of strength and toughness of DQ-.T steel is: yield strength ( R p0.2 ) =793 MPa , tensile strength ( R ” ) =908 MPa , elongation ( A ) =19% , Charpy impact energy absorption KV 2 ( -80°C ) = 83J .
Key words: continuous cooling phase transformation; quenching and tempering; direct quenching and tempering ( DQ-T ); direct quenching and two-phase zone quenching and tempering ( DQ-LT ); large-scale toughness ships and the development of polar deep-sea oil and gas fields require large Thick and high-strength steel plate, which provides a broad application prospect for F -quality ultra-high-strength ship plate and offshore platform steel plate. At present, high-strength ship plates and steels for offshore platforms with a yield strength of 500-590 MPa have been developed domestically [ 1-4 ] . The research on the yield strength 690 MPa steel plate is in progress.
Det Norske Veritas ( DNV ) specifies the NV-F690 specification: room temperature yield strength Rpo.2 & 690 MPa , tensile strength R m & 770 MPa , elongation AM14% , transverse Charpy impact absorption energy KV 2 at -60 C (-60C ) & 46J . Similar steel types include copper-containing precipitation-strengthened HSLA-100 steel, whose quenched and tempered ( QT ) state achieves the best matching of strength and low-temperature toughness, and can be used in shipbuilding, offshore platforms, heavy construction machinery and bridge construction, etc., representing a new generation The development direction of hull structural steel. However, due to the high content of alloying elements and high cost, its commercialization in large quantities is limited. Therefore, the development of composition-reduced high-performance ship plate steel is of great significance.
For different industrial production conditions, it is necessary to discuss the change law of the microstructure and properties of NV-F690 ultra-high strength ship plate steel under different process conditions, so as to realize its flexible heat treatment.

this paper, a new type of low-carbon CuNiCrMnMo steel is designed, the austenite continuous cooling phase transformation kinetics is studied, a new process of controlled rolling direct quenching ( DQ ) + two-phase zone quenching and tempering ( DQ-LT ) is designed, and the DQ steel is revealed. According to the changes of microstructure and properties under the conditions of traditional quenching and tempering, direct quenching + tempering ( DQ- T ) and the new process DQ-LT , high-performance NV-F690 steel and corresponding key process parameters were obtained.
1 Experimental materials and methods The experimental steel was smelted in a vacuum induction furnace and cast into an 80 kg ingot. The chemical composition is shown in Table 1 . According to formula 13 : A r3 = 910 - 310% C - 80% Mn - 20% Cu - 15% Cr - 80% Mo - 55% Ni, and the phase transition temperature of the test steel is 647 T. Ingot Warp 1180°C austenitized and kept in a nitrogen atmosphere for 10 h for homogenization, cooled to room temperature, peeled and machined to 140 mm X 115 mm
X 240 mm test billet for hot rolling.．

Using a 1 750 mm ( RAL-NEU ) two-roll reversing mill, the test steel was rolled into steel plates with thickness ( t ) of 20 mm and 32 mm , respectively. The heating temperature of the billet is 1180 C , and the heat preservation is 2 h ; the starting temperature of rough rolling is 1120 C ; the starting temperature of finishing rolling is 890 C. The rough rolling total reduction ratio ( 115-74 mm ) of the steel plate with a thickness of 20 mm is 36% , and the finishing rolling total reduction ratio ( FR ) is 73% . The finishing rolling reduction rules are as follows: 74-74 mm 61—48—38—30—24—20 ( mm ) ; after rolling, cool to 400 C at 15 C/s ( DQ ) , and air cool. The rough rolling reduction ratio ( 140-74 mm ) with a thickness of 32 mm is 47 % , and the FR is 57 % . Cool at 13 C/s to 400 C , air cooled. During the experiment, a UX 70P portable thermometer was used to measure the surface temperature of the rolled piece.
Use Gleeble 3800 thermal simulator to measure the continuous cooling phase transition ( Continuous Cooling Transformation , CCT ) curve. The sample is a 120 mm long round bar (the diameter of the gauge section is 6 mm and the length is 10 mm ). Experiment at 1.3 X 10 - 4 Pa vacuum chamber. The testing process is as follows: the sample is heated to 905 C at 5 C/s ( Q- y ), keep warm for 5 min , and then cool down to room temperature at different cooling rates ( v ). Weld a K -type thermocouple on the surface of the center of the sample length to control the temperature; install a phase change dilatometer at the center of the sample length to measure the diameter change caused by the phase change; according to the lever law, convert the expansion caused by the Y — a phase change into phase Variable fraction, draw the phase transition fraction - temperature kinetic curve corresponding to different v .
series of heat treatment experiments were carried out on DQ tempered steel plates using a box-type air furnace . ( 1 ) Water quenching and tempering ( WQ-T ) and oil quenching and tempering ( OQ-T ) : heat the steel plate to 905 °C , hold it for 60 minutes , water quenching and oil quenching respectively, and then temper at 600-700 ° C Fire 1 h . ( 2 ) Direct quenching and tempering ( DQ-T ) : Temper the steel plate in DQ state at 600~705 C for 1h . ( 3 ) Direct Quenching Two-phase Quenching and Tempering ( DQ-LT ) : Heating the steel plate in DQ state to Y + a ( 720 ~ 780 C ), heat preservation for 1 h , water quenching, and then tempering at 550 - 650 C for 1 h . All tempered steel plates are air cooled after coming out of the furnace.
Take a transverse ( TD ) round bar tensile sample with a gauge section diameter of ! 8 mm , and use a 100 kN tensile testing machine ( Instron 5585 ) to test the tensile properties at room temperature. The Charpy impact specimens are cut according to GB /T 2975-1998 " Sampling Position and Sample Preparation for Mechanical Properties Test of Steel and Steel Products", and processed into a standard V -shaped notch along the thickness ( ND ) direction ; using an impact testing machine ( IMP450J Dynatup , Instron ), according to GB/T 229-2007 " Metallic Materials Charpy Pendulum Impact Test Method", measure the Charpy impact absorbed energy ( KV2 ) at -40 , -60 and -80 C respectively .
The polished surface was corroded by 4% nital solution, and the microstructure was analyzed by optical microscope and scanning electron microscope ( SEM , Quanta 3D FEG ) . The hardness was tested using an Instron Vickers hardness tester with a load of 10 kg , and the average value was obtained from 5 points. For steel plates, inspect the structure and hardness of the rolling direction ( RD ) XND ; for CCT samples, inspect the structure and hardness of the vertical axial section at the center of its length. The thin film sample was thinned mechanically first, and double-sprayed with 5% perchloric acid and 95% acetic acid solution to less than 30 "m. The microstructure of the thin film sample was observed with a JEM2100F transmission electron microscope ( TEM ) with an accelerating voltage of 200 kV .
Table 2 Mechanical properties of test steel under various heat treatment conditions
Table 2 M echanical properties of the experimental steel plates under various heat treatment conditions

Process	Thickness / mm	Temperature / C		Tensile properties				Charpy absorbed energy ( KV 2 ) / J
		Reheating ( 1 h )	Tempering ( 1 h )	R p0.2 / MPa	Rm / MPa	A / %	YR	- 40 C	- 60 C	- 80 C
	32	-	600	903	937	16	0. 96	-	43 ( 40 )	-
	32	-	625	892	890	16	0. 98	-	79 ( 46 )	-
WQ-T	32	-	650	748	791	23	0. 95	172 ( 148 )	162 ( 122 )	93 ( 81 )
	32	-	680	681	732	23	0. 93	177 ( 156 )	185 ( 99 )	76 ( 62 )
	32	-	700	752	785	23	0. 96	194 ( 172 )	155 ( 139 )	26 ( 23 )
	32	-	600	843	918	17	0. 92	-	26 ( 23 )	-
	32	-	625	818	884	18	0. 93	-	28 ( 15 )	-
OQ-T	32	-	650	730	790	19	0. 92	-	49 ( 45 )	-
	32	-	675	680	753	20	0. 90	-	78 ( 73 )	-
	20	-	-	625	854	16	0. 73	96 ( 90 )	64 ( 60 )	56 ( 52 )
	20	-	600	801	932	18	0. 86	61 ( 53 )	52 ( 51 )	25 ( 19 )
DQ-T	20	-	650	778	891	17	0. 87	75 ( 68 )	76 ( 61 )	30 ( 18 )
	20	-	680	754	854	18	0. 88	84 ( 81 )	61 ( 57 )	62 ( 58 )
	20	-	705	762	849	19	0. 90	89 ( 62 )	56 ( 54 )	53 ( 53 )
	20	720	-	640	813	20	0. 79	93 ( 93 )	96 ( 87 )	53 ( 45 )
DQ-L	20	750	-	626	840	20	0. 75	100 ( 88 )	83 ( 82 )	65 ( 44 )
	20	780	-	634	900	17	0. 71	103 ( 103 )	134 ( 96 )	110 ( 75 )
	20	720	550	827	885	18	0. 93	107 ( 102 )	79 ( 61 )	61 ( 52 )
	20	750	550	805	864	20	0. 93	79 ( 68 )	68 ( 64 )	51 ( 51 )
DQ-LT	20	780	550	793	908	19	0. 87	96 ( 95 )	78 ( 72 )	83 ( 63 )
	20	780	600	740	807	20	0. 92	125 ( 114 )	118 ( 87 )	85 ( 76 )
	20	780	650	702	750	22	0. 94	170 ( 165 )	138 ( 124 )	109 ( 59 )

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