Patent ID: 9228244
Filing Date: 2016-01-05
CPC Classification: B32B,C21D,C22C,C23C

Claim Text:
1. A method of production of high strength hot dipped galvanized steel sheet having an average grain size of a low temperature transformed phase of 0.1 to 1 μm, a ratio of average nano hardnesses of a ferrite phase and the low temperature transformed phase of 1.5 to 3.0, and a nano hardness of the low temperature transformed phase at 80% or more of the measurement points of 1 to 5 times the average nano hardness of the ferrite phase, the method comprising: heating a slab to 1000 to 1350° C., the slab having steel ingredients, by mass %, C: 0.05 to 0.1%, Si: 0.1 to 1.0%, Mn: 2.0% to 2.5%, Al: 0.02 to 0.1%, Ti: 0.01 to 0.05%, Cr: 0.1 to 1.0%, Sn: 0.0010 to 0.1%, and a balance of Fe and unavoidable impurities, as the unavoidable impurities, P: 0.03% or less, S: 0.01% or less, Nb: 0.001% or less, V: 0.001% or less, W: 0.001% or less, Mo: 0.001% or less, Zr: 0.001% or less, B: 0.0001% or less, then hot rolling at a final rolling temperature Ar3 or more, coiling at 600° C. or less, pickling, cold rolling at a rolling rate of 30 to 70% and a draft per pass of 30% or less, annealing at a temperature in the range of 720° C. to the lower of 850° C. or an Ac3 temperature, wherein a heating rate from at least 600° C. to an annealing temperature is 0.5-6° C./sec, holding at the annealing temperature for 10 sec or more, then cooling in at least the temperature range of the annealing temperature to 650° C. by a cooling rate of 5° C./sec or more, further cooling in at least the temperature range of 600° C. to 500° C. by a cooling rate of 3° C./sec or less, and then performing hot dip galvanization or hot dip galvannealization.