Bethlehem�s High-Strength, Hot-Rolled Sheet Steels

Introduction:
High-strength, hot-rolled sheet steels are defined generally as having a yield strength greater than 35 ksi (240 MPa). These high strength levels are achieved through combination of alloying, rolling and controlled cooling practices.

Our line of high-strength sheet steels was developed in the 1970�s to meet the growing need for weight savings while maintaining the desirable properties of mild, carbon sheet steels. All of our high-strength grades are strand cast, which assures a highly uniform and consistent chemistry throughout the heat as well as a high degree of internal cleanliness.

Availability:
High-strength, hot-rolled sheet steels are available from Bethlehem in a wide range of widths and thicknesses as well as in coils and cut lengths. Bethlehem uses a code designation for high-strength steels which allows easy identification of the various grades and strength levels. The code consists of the letter B prefix to denote Bethlehem Steel, a number of designate the minimum yield strength in ksi, and a two-letter suffix to further describe additional processing. These suffixes are listed below:

S = plain carbon, structural quality
P = plain carbon, rephosphorized
X = low alloy
K = killed
F = killed with inclusion control

Yield strengths available range from 37 to 100 ksi (255 to 690 MPa) or more, thicknesses, from 0.061 inch (1.55 mm) to 0.250 inch (6.35 mm) and widths from 24 inches (610mm) to 72 inches (1829 mm). There are certain size restrictions within the ranges given depending on the grade and strength combination.

Where corrosion resistance is a consideration, some of the grades, strength levels and sizes can be supplied with a hot-dip coating operation may influence the mechanical properties of the base steel.

Performance Attributes

Formability:
Hot-rolled sheet steels are normally supplies to satisfy the requirements for making a given part of to meet customer-defined specifications which usually cite yield strength, tensile strength and total elongation. Typically, as yield strength (or yield-to-tensile ratio) increases, relative ductility decreases.

Total elongation is also a general measure of formability, with higher elongations generally providing better formability. Elongations normally become lower as the strength increases. Total elongation correlates well with bend capacity, hole expansion, elongation of a blank edge and other forming operations.

Fatigue:
High -strength sheet steels have better fatigue resistance than mild steel at low stresses over a large number of cycles, the opposite is true because lower strength steel accommodates the stress conditions through plastic deformation. To avoid exaggerated fatigue effects at notches, the design should place notches at low stress locations.

Welding:
Although slight adjustments are required, high-strength steels can be readily welded with conventional equipment. To obtain joints with strengths equivalent to that of the base steel, high-strength are welding filler metals are necessary.

Higher electrode force, longer welding times and lower currents are required for resistance spot welding. The tensile strength of high-strength resistance welds is proportional to the tensile strength of the sheet at a constant thickness and button diameter.