| 摘要 |
PURPOSE: To improve both toughness of weld heat affected zone and strength of steel plate while restraining surface crack on slab that is often observed in high nitrogen steel, TiN precipitates, together with Al2O3·MnO(Galaxite), are homogeneously formed in low nitrogen steel that is designed to have a complex microstructure of ferrite and pearlite by nitriding, thereby controlling grain size thereof smaller than 20μm. CONSTITUTION: The method for manufacturing high strength steel plate having superior toughness in weld heat-affected zone includes: (i) manufacture of a low nitrogen steel slab comprising C 0.03-0.17%, Si 0.01-0.5%, Mn 0.4-2.0%, Ti 0.005-0.2%, Al 0.03-0.3%, N 0.005% or less, B 0.0003-0.01%, W 0.001-0.2%, P 0.03%or less, S 0.03% or less, O 0.002-0.03%, 8<=Al/O<=22, 250<=MnO<=530, a balance of Fe and other inevitable impurities; (ii) nitriding the low nitrogen slab in the temperature range 1100-1250deg.C for 60-180min until nitrogen content of the steel slab become 0.008-0.03% while satisfying below inequalities: 1.2<=Ti/N<=2.5, 10<=N/B<=40, 2.5<=Al/N<= 7, 6.5<=(Ti+2Al+4B)/N<=14; (iii) hot rolling of the heated steel slab within the temperature range for austenite recrystallization at a reduction ratio higher than 40%; and (iv) cooling the hot rolled slab to terminal temperature±10deg.C for ferrite transformation at a cooling rate higher than 1deg.C/min. Further, the steel slab comprises (i) one or two element selected from Ni 0.1-3.0%, Cu 0.1-1.5%, Nb 0.01-0.1%, Mo 0.05-1.0%, Cr 0.05-1.0%; (ii) one or two selected from Ca 0.0005-0.005%, Rem 0.005-0.05%; and (iii) V 0.01-0.2%, wherein V/N is 0.3-9, 7<=(Ti+2Al+4B+V)/N<=17.
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