Abstract: An aspect of the present invention relates to a cold-rolled steel plate for hot forming, which is excellent in corrosion-resistance and spot-weldability, contains, by weight %, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.5% to less than 3.0%, N: 0.001-0.02%, and a balance of Fe and inevitable impurities, satisfying formula (1) below, and includes an Si amorphous oxidation layer continuously or discontinuously formed at a thickness of 1 nm-100 nm on the surface thereof. Formula (1): 1.4? 0.4*Cr+Si?3.2 (wherein element symbols denote measurements of respective element contents by weight %).

Abstract: An aspect of the present invention relates to a cold-rolled steel plate for hot forming, which is excellent in corrosion-resistance and spot-weldability, contains, by weight %, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.5% to less than 3.0%, N: 0.001-0.02%, and a balance of Fe and inevitable impurities, satisfying formula (1) below, and includes an Si amorphous oxidation layer continuously or discontinuously formed at a thickness of 1 nm-100 nm on the surface thereof. Formula (1): 1.4?0.4*Cr+Si?3.2 (wherein element symbols denote measurements of respective element contents by weight %).

Abstract: The present invention relates to a lightweight bidirectional hollow core slab, and a doughnut-shaped hollow core body which may be advantageously used in the construction of a bidirectional hollow core slab. The doughnut-shaped hollow core body according to the present invention includes an outer case formed in a generally doughnut shape, wherein a hollow portion with a circular section is formed in the center thereof and corners are rounded with curved surfaces. The bidirectional hollow core slab according to the present invention is made by stably locating the doughnut-shaped hollow core bodies in concrete in such a manner that the doughnut-shaped hollow core body is restrained and mounted in steel bar cages or on the upper and lower steel bars.