1. Field of the Invention
This invention relates to a method of producing H-beams by universal mills. More particularly this invention relates to a method of producing H-beams which have excellent strength and toughness in the joints between the web and flanges, referred to hereinbelow as the "fillets."
2. Prior Art
The conventional method of producing H-beams by rolling comprises: a breakdown process in which the piece 10 having a cross section as shown in FIG. 1 (a) is rolled by a twohigh mill having breakdown rolls 12 of a pass cross section as shown in FIG. 1 (b); a roughing process in which rolling in one pass or in multiple passes is performed by a roughing universal mill group consisting of at least one universal mill having roughing and intermediate horizontal rolls 14 and roughing and intermediate vertical rolls 16 as shown in FIG. 1 (c), and at least one edger mill having edger rolls 18 of a cross section as shown in FIG. 1 (d); and a finishing process in which rolling in one pass is performed by a finishing universal mill having finishing horizontal rolls 20 and finishing vertical rolls 22 of a cross section as shown in FIG. 1 (e). The H-beam 23 thus produced has flanges 24, web 26, and joints (fillets) 28 therebetween. An example of the mechanical properties of each part of the conventional H-beam thus rolled is given in FIG. 2. FIG. 2 (a) shows the relationship between the finish temperature and the yield strength. FIG. 2 (b) shows the relationship between the finish temperature and the tensile strength. FIG. 2 (c) shows the relationship between the finish temperature and the transition temperature of brittleness-ductility fractured surface. In the figure, the full line A, broken line B and dot-and-dash line C show the mechanical properties of the web 26, flange 24 and fillet 28 respectively. As is seen from the figure, when the finish temperature is the same, the yield strength and tensile strength of the fillet 28 in the tensile test are lower than those of the flange 24 and web 26, and the transition temperature of brittleness-ductility fractured surface in the Charpy test is the highest. The possible cause of such weakness in mechanical properties of the fillet 28 in comparison with other parts is considered to be the insufficient draft of the fillet 28 as compared with other parts, and because the fillet receives the highest temperature during rolling. That is, as the fillet 28 is supported only by the web 26 that is high in temperature and flexible, reductions by vertical rolls 16 to 22 in the roughing and the finishing processes are not effective. Further, the fillet 28 is larger in thickness than the web 26 and flange 24, so that heat radiation to the rolls is small. Therefore, the fillet receives the highest temperature during rolling.
FIG. 3 shows the state of deformation in cross section by rolling of each part of the H-beam. If the flange, fillet and web of the piece 10 have square section a, b and c respectively, these square sections become sections a', b', and c' in the H-beam 23 after rolled. As is apparent from the figure, the change in cross section of the flange from a to a' and that of the web from c to c' are featured each by a large decrease in either the vertical dimension or the horizontal dimension, while in the change in cross section of the fillet from b to b', the vertical and horizontal dimensions of the section b are decreased similarly, to almost the same extent, as the result of the metal flow that takes place from the fillet to the web because reductions by vertical rolls 16 to 22 are not effective as described in the above. Supposing that the deformation of the web and flange is plane strain and that the deformation of the fillet is one-dimensional tensile strain, the amount of true strain of the web and flange is equal to about 1.15 times that of the fillet.
Usually in the manufacture of H-beams, the product processed in the above rolling processes is straightened by a roller or press straightener to improve its straightness. However, when the H-beam produced by the above-mentioned conventional method is being straightened by rollers 30 as shown in FIG. 4 (a), due to its inferior mechanical properties the fillet 28 may occasionally be fractured as shown at the hatched portion 32 of FIG. 4 (b), with increasing amounts of reduction by the rollers 30. Therefore, for H-beams produced by the conventional method, press straightening has to be employed if straightness cannot be improved without heavy reductions, which results in a considerable decrease of the working efficiency.
H-beams before use are often subjected to gas cutting, that is, part of the flange 24 of the H-beam 23 is gas cut as shown by oblique lines 34 in FIG. 5 (a), and part of the web 26 of the H-beam is gas cut as shown by oblique lines 36 in FIG. 5 (b). However, when conventional H-beams are subjected to the above gas cutting, notches 37 resulting from the gas cutting may give rise to a crack 38 along the fillet 28 as shown in FIG. 5 (c) or (d), due to the inferior mechanical properties of the fillet. The crack 38 is caused by the influence of the residual stress existing in the fillet 28. The lower the low-temperature toughness of the fillet 28 is in a cold working environment, the more the crack progresses. To prevent this crack, the following measures have hitherto been taken. A hole is made in advance in the fillet 28 for prevention of crack propagation, troublesome operations such as preheating or post heating of the fillet 28 are performed, or costly killed steel, excellent in toughness, is used in place of semi-killed steel used for ordinary H-beams, as the result of which the cost of production of H-beams is raised.
Further, H-beams sometimes are used for monorails as a special application thereof as shown in FIG. 6 in which the reference numerals 39, 40 and 41 designate respectively a vehicle, a guide wheel and a carrying track on which a H-beam, the monorail, is fixed. In using H-beams for monorails, it has so far been required to make the fillet 28 larger in thickness in order to compensate for its insufficient strength.