Referring to FIGS. 7 and 8 together, the protrusion step portion 21 may be formed in the second boundary portion P2a of the second plate P2, and the protrusion step portion 21 may be formed in each of the second boundary portion P2a (the second boundary portion P2a adjacent to the main outer boundary portion MO) formed at the edge of the second plate P2 and the second boundary portion P2a (the second boundary portion P2a adjacent to the main inner boundary portion MI) formed around the slit S of the second plate P2. The protrusion step portion 21 may include the upper surface upwardly protruding and the lower surface which is concave and indented from below, and the concave lower surface of the protrusion step portion 21 may function as the jig groove 22. The protruding upper surface of the protrusion step portion 21 may function as a wall structure for defining the cooling fluid passage F therebetween, and the cooling fluid passage F may be formed between the protrusion step portion 21 formed at the edge of the second plate P2 and the protrusion step portion 21 formed around the slit S of the second plate P2. The protrusion step portion 21 may protect the cooling fluid passage F. For example, the protrusion step portion 21 may prevent or reduce the blockage of the cooling fluid passage F due to a leakage of the sealing portion 50 (the sealing member included in the sealing portion 50) formed around the cooling fluid passage F into the cooling fluid passage F.
