High Tension Steel Plate with Small Acoustic Anisotropy and with Excellent Weldability and Method of Production of Same

Abstract

The present invention provides a high tension steel plate of a tensile strength of 570 MPa or more with a small acoustic anisotropy and with excellent weldability tensile strength and a method of production of steel plate predicated on the high productivity accelerated cooling stop process, which steel sheet is a high tension steel plate with a value of 0.045%≦Nb+2Ti≦0.105%, A=(Nb+2Ti)×(C+N×12/14) satisfying 0.0025 to 0.0055 and with a steel composition containing bainite in an amount of 30 vol % or more and pearlite+martensite-austenite constituent (MA) in an amount of less than 5 vol %. The steel is heated to 1200° C. or more, rough rolled at 1020° C. or more, then rolled at a cumulative reduction rate from over 920° C. to less than 1020° C. of 15% or less and a cumulative reduction rate from 860 to 920° C. of 20 to 50%, then starting accelerated cooling giving a cooling rate of 2 to 30° C./sec at 800° C. or more, stopping the accelerated cooling at 600 to 700° C., then cooling by 0.4° C./sec or less.

1 . A high tension steel plate of a tensile strength of 570 MPa or more with a small acoustic anisotropy and with excellent weldability, characterized by having a composition of steel containing, by mass %,
 C: 0.03% to 0.07%,   Si: 0.1 to 0.6%,   Mn: 0.8 to 2.0%,   Al: 0.003% to 0.1%,   Nb: 0.025 to 0.1%,   Ti: 0.005 to 0.1%,   [Nb]+2×[Ti]: 0.045 to 0.105%   N: over 0.0025% to 0.008%,   
       containing Nb, Ti, C, and N in a range where the value A expressed by the A of the following equation (1) is 0.0022 to 0.0055, and containing a balance of Fe and unavoidable impurities, having a steel structure with a volume fraction of bainite of 30% or more and a sum of volume fractions of pearlite and martensite-austenite constituent (MA) of less than 5%:
     A =([Nb]+2×[Ti])×([C]+[N]x×12/14)  (1) 
 where [Nb], [Ti], [C], and [N] mean the mass$ of Nb, Ti, C, and N 
 
     
     
         2 . A high tension steel plate of a tensile strength of 570 MPa or more with a small acoustic anisotropy and with excellent weldability as set forth in  claim 1 , characterized in that said steel sheet further contains, by mass %, one or more of
 Mo: 0.05% to 0.3%,   Cu: 0.1% to 0.8%,   Ni: 0.1% to 1%,   Cr: 0.1% to 0.8%,   V: 0.01% to 0.03%,   W: 0.1% to 3%,   B: 0.0005% to 0.005%,   Mg: 0.0005% to 0.01%,   Ca: 0.0005% to 0.01%.   
     
     
         3 . A high tension steel plate of a tensile strength of 570 MPa or more with a small acoustic anisotropy and with excellent weldability characterized by heating a slab having a composition of ingredients as set forth in  claim 1  to 1200° C. to 1300° C., rough rolling it at a temperature range of 1020° C. or more, then hot rolling it by a cumulative reduction rate of 15% or less in the temperature range of less than 1020° C. to over 920° C. and by a cumulative reduction rate of 20% or more in the temperature range of 920° C. to 860° C., then starting accelerated cooling by a cooling rate of 2° C./sec to 30° C./sec from a temperature of 800° C. or more, stopping said accelerated cooling at a temperature of the center of plate thickness of 700° C. to 600° C., then cooling by a cooling rate of 0.4° C./sec or less.