To investigate the relation between reduction ratio and mechanical properties of killed mild steel plates, the following tests were carried out.
(1) Six 10-t ingots from a coarse austenitic-grained heat melted with a basic open hearth furnace were hot-rolled into several plates of different thickness, which corresponded the reduction ratios (=ingot thickn./plate thickn.) of 4, 6, 7, 8, 12 and 20, respectively.
(2) For the comparison with test 1, one 10-t ingot from fine austenitic-grained heat melted with a basic open hearth furnace was hot-rolled in to a reduction ratio of 7.5.
(3) One twelve-sided 15t ingot from an acid open hearth furnace was forged into a slab previously, then rolled in to a 100mm thick plate. The total reduction ratio combined with forging and rolling reductions was 10.1.
The results obtained were as follows:
(1) The mechanical properties of rolling-direction specimens were improved with increase of reduction ratio, rapidly up to a reduction ratio of 12 and slowly thereafter. This tendency was remarkable in elongation, reduction of area and notch-toughness but a little in yield and tensile strengths.
(2) The mechanical properties of transversal-direction specimens were improved rapidly with increase of reduction ratio up to 8, but better effects were not expected with further proceeding of rolling. This tendency was conspicuous in elongation and reduction of area.
(3) In mechanical properties in the thickness direction, tensile and yield strengths wereimproved remarkably with plates of reduction ratios between 7 and 8. Elongation, reduction of area and notch-toughness were increased with plates of reduction ratio up to 12, but decreased thereafter.
(4) Hence, in considering the properties in the thickness direction, it was desirable to apply the rolling reduction ratio over 8 for commercial steel plate. At the reduction ratio of 12, best properties were obtained for all directions in the plate.
(5) After normalizing, the properties except tensile strength were improved, but the behavior of reduction ratio was scarcely varied from the as-rolled condition.
(6) Application of forging in slabbing process was effective as well as rough rolling, and a property impro.vement corresponding to total reduction ratio was achieved.
(7) The same behavior was obtained in the case of a fine-grained steel in the relation between reduction ratio and mechanical properties.