| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized thin steel sheet superior in processability, and a method for manufacturing the same. <P>SOLUTION: The method comprises: a hot-rolling step of performing hot-rolling of a steel material having a composition containing, in terms of mass%, 0.08-0.15% C, 0.5-1.5% Si, 0.5-1.5% Mn, 0.01-0.1% Al, and ≤0.005% N so as to obtain a hot-rolled sheet; an annealing step of pickling the hot-rolled sheet and then, without performing cold-rolling, holding the same for 5-400 sec at a first temperature range of from the A<SB POS="POST">c1</SB>transformation point to the A<SB POS="POST">c3</SB>transformation point in a continuous hot-dip galvanization line; a cooling step of cooling the sheet from the first temperature range to 700°C at an average cooling rate of ≥5°C/sec and holding the same for 15-400 sec at a second temperature range of from 700°C to a temperature at which the sheet is immersed in a hot-dip galvanizing bath; and a hot-dip galvanization step. Thus, a structure comprising a ferrite phase and a second phase comprising pearlite can be obtained, the ferrite phase and the pearlite accounting for 75-90% by area and 10-25% by area, respectively, with respect to the whole structure. This yields the high-strength hot-dip galvanized steel sheet which has a strength (TS) as high as ≥540 MPa, is superior in stretch flange workability and is superior in processability. The pearlite accounts for ≥70% by area of the whole second phase and has an average grain diameter of ≤5 μm. <P>COPYRIGHT: (C)2012,JPO&INPIT |
