United States Department of Transportation
i
Formability of new high performance A710 grade 50 structural steel.
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2014-01-01
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Abstract:This project compared the formability of modified ASTM A710 Grade B50 ksi yield strength steel, jointly developed by
Northwestern University and the Illinois Department of Transportation, with ASTM A606 Type 4 weathering steel used in Illinois
and many other states for sign and signal structures, light poles, and other highway structures. ASTM E290 Type 1 free-bend
180° guided-bend test was used to evaluate formability, with side clearances as per ASTM E190. Bend and tensile specimens
were cut in longitudinal and transverse directions from plates and sheets of numerous thicknesses (from 0.38" down to 0.10"
thick). Mandrels of three diameters (0.25", 0.313", and 0.375") were used, each having a rounded nose with radius of bend
equal to ½ of the mandrel diameter. All specimens of A710 and A606 steels of all thicknesses passed the guided-bend tests. No
cracks, side tears, or fractures were observed. Tensile tests showed excellent ductility in both longitudinal and transverse rolling
directions, making the modified ASTM A710 steel very suitable for use in sign and signal structures, light poles, and highway
structures, along with structural tubing and other applications requiring weathering steel sheets and plates.
Comparable A606 Type 4 steel had a yield strength range of 65 to 73 ksi and an ultimate tensile strength range of 79 to
89 ksi. The elongation to failure for the A606 steel sheets was in the range of 41% to 49%, averaging 43% ± 2.5% based on a 1"
gage length. The microstructure of A606 steel is primarily ferrite with a limited amount of pearlite. The grain size of A606 in
thinner plates was much smaller; grains were more rounded in thicker plates.
Two heats of modified A710 Grade B steel were tested. Variations in mechanical properties were a function of steel
composition, hot-rolling procedures, and thickness of the plates or sheets. In the first heat, yield strength varied in a range of 48
to 119 ksi, and tensile strength ranged from 64 to 119 ksi. The variation in the strength in the first heat was most likely caused
by excessive section reduction during hot-rolling. The microstructure of the A710 steel was fine-grained ferrite and was
significantly smaller than the A606 steel grains of similar thickness, which accounted for the higher strengths of the A710 steel
in the first heat. Some bands of pearlite and fine-grained ferrite were observed in the modified A710 sheets, but did not affect
the formability of the steel.
Copper, nickel and manganese contents were decreased and better rolling procedures were used in the second heat, which
had a uniform 49 to 56 ksi yield strength range. The second heat also had a uniform yield to tensile ratio of 0.74, high ductility
and formability, making this alloy a very attractive weathering steel for many transportation and structural applications. An
optimized 50 ksi minimum yield strength composition with an ASTM G101 index of 6.0 or more was developed for A710 sheet
steel, and is proposed for general use in transportation and other construction applications.
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