An R-T-B based permanent magnet includes main phase grains composed of R.sub.2T.sub.14B type compound. R is a rare earth element. T is iron group element(s) essentially including Fe or Fe and Co. B is boron. An average grain size of the main phase grains is 0.8 μm or more and 2.8 μm or less. The R-T-B based permanent magnet contains at least C and Zr in addition to R, T, and B. B is contained at 0.75 mass % or more and 0.88 mass % or less. Zr is contained at 0.65 mass % or more and 5.00 mass % or less. A formula (1) of 5.0≦[B]+[C]−[Zr]≦5.6 is satisfied, where [B] is a B content represented by atom %, [C] is a C content represented by atom %, and [Zr] is a Zr content represented by atom %.

A high-strength steel sheet comprises: a chemical composition containing C, Si, Mn, P, S, Al, N, Mo, Cr, Ca, and Sb with a balance consisting of Fe and inevitable impurities, wherein [% Si], [% Mn], [% P], [% Mo], and [% Cr] satisfy a predetermined relationship; a steel microstructure that contains ferrite, hard phase, and retained austenite and in which a carbon concentration in the retained austenite is 0.55% or more and 1.10% or less, an amount of diffusible hydrogen in the steel sheet is 0.80 mass ppm or less, a surface layer softening thickness is 5 μm or more and 150 μm or less, and a corresponding grain boundary frequency in a surface layer of the steel sheet after a high-temperature tensile test is 0.45 or less; and a tensile strength of 980 MPa or more.

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.

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.

Provided is a resistance spot welding method suitable for manufacturing a weld joint exhibiting excellent delayed fracture resistance. The resistance spot welding method includes sandwiching two or more overlapped steel sheets between a pair of welding electrodes, applying current to the steel sheets while pressing the steel sheets, forming a nugget on overlapping surfaces of the steel sheets to join the steel sheets, and after the joining, directly or indirectly irradiating the nugget with sound waves having a frequency of 10 Hz or more and 100000 Hz or less so that a sound pressure level on a surface of the steel sheet is 30 dB or more.

Provided is a dehydrogenation method capable of efficiently reducing hydrogen content in steel for thick or complexly-shaped steel materials and steel products in general. In a dehydrogenation method for a steel material, in a series of steel material production process including: a process of supplying a steel raw material; a process of subjecting the steel raw material to hot working; a process of inspecting a steel material obtained from the steel raw material; and a process of shipping the steel material, at least one of the steel raw material and the steel material at any stage from the supply process to the shipment process is subjected to, at least once, a sound wave irradiation treatment so that a sound pressure level at a surface of the at least one of the steel raw material and the steel material will be 30 dB or more.

Provided is a dehydrogenation method capable of efficiently reducing hydrogen content in steel for thick or complexly-shaped steel materials and steel products in general. In a dehydrogenation method for a steel material, in a series of steel material production process including: a process of supplying a steel raw material; a process of subjecting the steel raw material to hot working; a process of inspecting a steel material obtained from the steel raw material; and a process of shipping the steel material, at least one of the steel raw material and the steel material at any stage from the supply process to the shipment process is subjected to, at least once, a sound wave irradiation treatment so that a sound pressure level at a surface of the at least one of the steel raw material and the steel material will be 30 dB or more.

Provided are a method of predicting hydrogen content in steel of a steel strip etc. Provided is, in a continuous galvanizing line that performs manufacturing processes including an annealing process, a coating process, and a reheating process of a steel strip, a method of predicting hydrogen content in steel of a steel strip downstream of the reheating process, including acquiring at least one parameter selected from operation parameters of the continuous galvanizing line and transformation rate information measured in at least one of the annealing process and the reheating process as input data, and predicting hydrogen content in steel of a steel strip downstream of the reheating process using a prediction model of hydrogen content in steel that has been trained by machine learning and that outputs information on hydrogen content in steel of a steel strip downstream of the reheating process as output data.

Provided are a steel sheet dehydrogenation apparatus, a steel sheet production system, and a steel sheet production method capable of producing a steel sheet excellent in hydrogen embrittlement resistance without changing the mechanical properties of the steel sheet. A dehydrogenation apparatus comprises: a housing configured to house a steel sheet coil obtained by coiling a steel strip; and a sound wave irradiator configured to irradiate the steel sheet coil housed in the housing with sound waves to obtain a product coil.

Disclosed is a method for manufacturing a corrosion-resistant hot-stamping part and a device thereof. The method includes the following steps: blanking a bare steel plate into a required blank shape; heating the blank to above AC3 in an oxygen-free heating furnace to austenite the blank; putting the austenitized blank into a mold to mold a part; and conducting a surface treatment of the part to form a corrosion-resistant coating layer on a surface of the part. The hot-stamping part manufactured using the described method has good surface quality and great corrosion-resistant performance