High strength and toughness NV-F690 tempering process

At present , the development of ultra-high-strength ship plate steel with excellent performance and lower cost is an urgent need for the sustainable development of the hull manufacturing and offshore platform construction industries.

Use advanced steelmaking and slab continuous casting industrial processes to produce low C and Cu - containing continuous casting slabs , and produce 80 mm thick steel plates in a 5000 mm wide and thick plate hot rolling production line [ 1 ] . Through direct quenching ( QT ) heat treatment and making full use of the precipitation strengthening effect of Cu , the NV-F690 ship plate is obtained , the yield strength of which is greater than 690 MPa , and the Charpy impact energy absorbed at -80 t is greater than 100 J. Compared with the HSLA400 steel developed in the United States ( the mass fraction of Ni is 3.5 % , and the mass fraction of Cu is 1.8 % ) [ 2-3 ] , the content of alloy elements Ni and Cu in the developed NV-F690 steel is low ( Ni mass fraction The fraction is 1.68% , and the mass fraction of Cu is 1.08% ), so it has a great cost advantage .

Studies have shown that steel sheets with higher strength and high toughness can be produced using the hot-rolled Direct Quenching + Tempering ( DQ-T ) process . The research group also used low C Cu - containing continuous casting slabs, and developed a low- C

Cost NV-F690 extra thick ( 100 mm ) steel plate , its yield strength is greater than 790 MPa, and the Charpy impact absorption energy at -60 t is greater than 140 J 0 o However, studies on the fine structure of the microstructure of the developed steel under DQ-T conditions have not received attention . In this paper , the effect of tempering process on the fine structure of NV-F690 extra-thick ( 100 mm ) steel plate is studied by scanning electron microscope ( SEM ) , electron backscatter diffraction ( EBSD ) and transmission electron microscope ( TEM ) , and the steel plate is discussed in depth. Relationship between low temperature impact fracture behavior and fine structure .

1. Test materials and methods

The test materials are 100 mm thick DQ steel plate and DQ-T steel plate . The chemical composition ( mass fraction , % ) of the steel plate is : 0.07C , 0.23Si , 1.06Mn , 0.008P , 0.001S , 0.036Al , 1 . 25Cu , 1. 74Ni , 0.043Nb, 0.019Ti, appropriate amount of Cr, appropriate amount of Mo , balance Fe . The addition amount of alloying elements Ni and Cu in the steel plate is much lower than that of HSLA400 steel . Among them , the preparation of DQ steel plate is carried out on the domestic NEU-RAL type 750 mm two-roll reversing rolling mill . The billet is heated to 1180 t , and the rolling temperature is 1120 t for 2.5 hours. After rolling, the rolled piece immediately enters the rapid cooling device to cool down to room temperature . A 100 mm (thickness, ND ) X150 mm ( width, TD ) X70 mm ( length, RD ) sample was cut from the DQ steel plate for a series of tempering tests to obtain the DQ-T steel plate . The tempering temperature is 650 , 675 and 690 °C , and the tempering time is 6 h .
Take a metallographic sample and observe the microstructure of the ND X RD section. The sampling position is at the core ( ND /2 ) and 1/4 thickness ( ND /4 ) of the steel plate . The polished surface was etched with 4% nital solution, and the microstructure was analyzed by SEM . The influence of tempering on the fine structure of the steel plate was studied using the EBSD instrument that comes with the SEM . The samples for EBSD analysis were first polished with 180 , 600 , 1000 and 1500 grit sandpaper , and then electropolished with perchloric acid alcohol solution . The test acceleration voltage is 20 kV and the working distance is 20 mm . EBSD data analysis was performed using HKL Channel 5 software . The film sample was first thinned mechanically , and double-sprayed with 5% perchloric acid + 95% acetic acid solution to less than 30 ^ m. The microstructure of the film sample was observed with a JEM2100F transmission electron microscope ( TEM ) , and the accelerating voltage was 200 kV .

research conclusion

• core ( ND/2 ) structure of DQ steel plate is lath bainite + granular bainite , the lath spacing is wide , there are relatively coarse MA components in the granular bainite, and the low temperature toughness is poor ; 1/4 thickness ( ND /4 ) The structure is lath bainite , the lath spacing is narrow , and the low temperature toughness is good .
• DQ steel plate was tempered at 650 ~ 690 T for 6 h , part of the lath bainite structure of the obtained DQ-T steel plate was merged . A large number of s-Cu dispersed precipitates precipitated in the matrix structure of the steel plate , the coarse granular MA components decomposed at the ND/2 of the steel plate , the fraction of high-angle grain boundaries increased , and the effective grain size decreased .

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