Abstract: The invention provides a multistage heating system including a compact multistage furnace of which the installation area in a factory is decreased and a work carrier machine. A multistage furnace is configured by piling up a plurality of furnace units in the vertical direction. Each of the furnace units includes an upper heater and a lower heater, support pipes disposed on the upper heater and extending in the horizontal direction, and a plurality of work support bars mounted over the support pipes. A work carrier machine includes work carrier bars extending in the horizontal direction, a horizontal motion mechanism connecting the ends of the work carrier bars and move the work carrier bars on horizontal rails, and a vertical motion mechanism moving the body of the work carrier machine including the horizontal rails on vertical rails.

Abstract: The invention provides a compact multistage furnace of which the installation area in a factory is decreased. A multistage furnace is configured by piling up a plurality of furnace units in the vertical direction. Each of the furnace units includes an upper heater and a lower heater layered in the vertical direction and holding a heat insulator therebetween, a support pipe disposed on one end of the upper heater and extending in the horizontal direction, a support pipe disposed on other end of the upper heater and extending in the horizontal direction, and a plurality of work support bars mounted over the support pipes. The back surface of a work supported by the work support bars is opposed to the upper heater and the front surface of the work is opposed to the lower heater of the adjacent furnace unit disposed above.

Abstract: The invention provides a method of laser welding zinc plated steel plates which surely realizes excellent welding without a melting defect such as blowholes. By irradiating a line on a superposed portion of a lower plate and an upper plate with a first laser beam having a high energy density and a small irradiation region by moving the first laser beam therealong, steel plate portions in the small irradiation region are melted and zinc on the superposed surfaces around the small irradiation region including in the small irradiation region is evaporated and allowed to escape outside. Then, after the irradiation with the first laser beam, the same line is irradiated with a second laser beam having a lower energy density than the first laser beam and a larger irradiation region than the first laser beam by moving the second laser beam therealong to melt steel plate portions in the larger irradiation region, thereby completing weldbonding.