A non-oriented electrical steel sheet having a high magnetic flux density and a low iron loss at a high frequency is produced by subjecting a slab containing, in mass %, C: not more than 0.0050%, Si: 2.8 to 6.5%, Mn: 0.05 to 2.0%, P: not more than 0.10%, S: not more than 0.0050%, Al: 0.3 to 2.0%, N: not more than 0.0050% and Zn: 0.0005 to 0.0050% to a hot rolling, a hot-band annealing, a cold rolling and a finish annealing, a dew point in the hot-band annealing is set to 0 to 70 C. and an atmosphere of the finish annealing has a nitrogen content of not more than 30 vol % and a dew point of not higher than-20 C., and a ratio of the amount of nitrogen present as AlN in an entire sheet thickness to the amount of nitrogen present as AlN in a layer from one-side surface of steel sheet to a depth of 1/20 of sheet thickness is made to not less than 5.0.

A method of establishing a furnace atmosphere in a directly heated heat treatment furnace is provided. A heat treatment furnace has at least one burner which is operated with a fuel gas and an oxygenous gas that are combusted to give a combustion gas. The combustion gas has a defined composition having a defined partial water vapor pressure. Hydrogen is used in the fuel gas with a proportion of at least 10% by volume. The heat treatment furnace is additionally flooded with a water vapor-free and/or hydrogen-free gas, as a result of which the water vapor-free and/or hydrogen-free gas mixes with the combustion gas so as to bring about a partial water vapor pressure of the mixture in the furnace atmosphere of the heat treatment furnace that is less than the defined partial water vapor pressure of the combustion gas.

Provided is a continuous annealing line capable of producing a steel sheet excellent in hydrogen embrittlement resistance. A continuous annealing line 100 comprises: a payoff reel 10 configured to uncoil a cold-rolled coil C to feed a cold-rolled steel sheet S; an annealing furnace 20 configured to continuously anneal the cold-rolled steel sheet S and including a heating zone 22, a soaking zone 24, and a cooling zone 26 that are arranged from an upstream side in a sheet passing direction; a downstream line 30 configured to continuously pass the cold-rolled steel sheet S discharged from the annealing furnace 20 therethrough; a tension reel 50 configured to coil the cold-rolled steel sheet S; and a sound wave irradiator 60 configured to irradiate the cold-rolled steel sheet S being passed from the cooling zone 26 to the tension reel 50 with sound waves.