Melting points are high due to strong metallic bonding involving both s and d electrons. The trend shows a maximum in the middle of the series: Cr has the highest m.p. in the 3d series (1907°C), and W has the highest of all metals (3422°C, 5d series). Mn is the anomaly: despite being in the middle, it has an unusually low m.p. (1246°C) due to its complex crystal structure (alpha-Mn has 58 atoms per unit cell with weak metallic bonding). Zn has the lowest m.p. (420°C) because $d^{10}$ electrons contribute little to metallic bonding.