HIGH-STRENGTH PLATED STEEL PLATE FOR WELDED STRUCTURAL MEMBER, AND METHOD FOR PRODUCING THE SAME
A high-strength plated steel plate for a welded structural member having a steel composition containing from 0.050 to 0.150% of C, from 0.001 to 1.00% of Si, from 1.00 to 2.50% of Mn, from 0.005 to 0.050% of P, from 0.001 to 0.020% of S, and from 0.005 to 0.100% of Al, having a steel base material having a metal structure containing a ferrite phase and a second phase containing mainly martensite having an average crystal grain diameter of 8 mm or less, and having a chemical composition and a thickness t (mm) of the steel base material regulated to obtain a value, C/0.2+Si/5.0+Mn/1.3+Cr/1.0+Mo/1.2+0.4t−0.7(Cr+Mo) 1/2 , of 2.9 or less. The high-strength plated steel plate is excellent in corrosion resistance of the welded portion, resistance to liquid metal embrittlement cracking, and bending workability.
1 - 2 . (canceled)
3 . A method for producing a high-strength plated steel plate for a welded structural member, comprising subjecting a steel slab to steps of hot rolling, pickling, cold rolling, annealing, and hot-dip plating, sequentially, so as to produce a plated steel plate containing a steel base material having on a surface thereof a hot-dip Zn—Al—Mg based alloy plated layer,
the steel slab having a chemical composition containing from 0.050 to 0.150% of C, from 0.001 to 1.00% of Si, from 1.00 to 2.50% of Mn, from 0.005 to 0.050% of P, from 0.001 to 0.020% of S, from 0.005 to 0.100% of Al, from 0.01 to 0.10% of Ti, from 0.0005 to 0.0100% of B, from 0 to 0.10% of Nb, from 0 to 0.10% of V, from 0 to 1.00% of Cr, from 0 to 1.00% of Mo, and from 0.001 to 0.005% of N, all in terms of percentage by mass, with the balance of Fe and unavoidable impurities,
in the cold rolling, assuming that a thickness after the cold rolling is t (mm), the cold rolling reduction ratio being from 45 to 70%, and the H value defined by the following expression (1) being 2.9 or less, and
the annealing and the hot-dip plating being performed in a continuous plating line, and the annealing being performed under such conditions that the material is heated to a temperature of from 740 to 860° C., and then an average cooling rate at least from 740° C. to 650° C. in a cooling process until immersing in a plating bath is 5° C. per second or more, so as to regulate a metal structure of the steel base material after the hot-dip plating to contain a ferrite phase and a second phase having an area ratio of 15% or more and less than 45%, the second phase being constituted by martensite or by martensite and bainite, and the second phase having an average crystal grain diameter of 8 μm or less:
H value=C/0.2+Si/5.0+Mn/1.3+Cr/1.0+Mo/1.2+0.4 t −0.7(Cr+Mo) 1/2 (1)
wherein in the expression (1), the element symbols each represent the content of the corresponding element in the steel in terms of percentage by mass.
4 . The method for producing a high-strength plated steel plate for a welded structural member according to claim 3 , wherein the hot-dip plating bath has a composition containing from 3.0 to 22.0% of Al, from 0.05 to 10.0% of Mg, from 0 to 0.10% of Ti, from 0 to 0.05% of B, from 0 to 2.0% of Si, and from 0 to 2.0% of Fe, all in terms of percentage by mass, with the balance of Zn and unavoidable impurities.