structures. This month and in part three, we'll examine in-depth the manufacturing process for steel as a whole and LGMF in particular.
A rose by any other nameIt's worth noting at the beginning of our discussion of the manufacturing process itself that the terminology used when referring to base metal thickness is undergoing a change. The "G" in LGMF gives us the familiar term "gauge." I've always found it difficult to recall off-hand the exact base metal thickness of-for example, standard 25-gauge framing components as it is represented by a decimal equivalent of an inch (0.0179). The new terminology solves this problem by using the actual base metal thickness itself as the defining element. Thus, a 25-gauge framing component will now be referred to as "18 mil." A mil represents 1/1,000th (0.001) of an inch (0.0179 ~ 0.18 ~ 18 mil). Manufacturers, design/construction professionals and industry organizations, such as the Steel Framing Alliance and the American Iron and Steel Institute, are adapting to this change. Here is a breakdown:
Gauge Thickness Mils
25 0.018 18
22 0.027 27
20 0.033 33
18 0.043 43
16 0.054 54
14 0.068 68
12 0.097 97
Essentially, there are two methods used for manufacturing steel to produce the rolls of sheet steel used to "cold-roll" LGMF shapes (track, stud, joists, etc.). They are:
• The integrated process
• The electric furnace process
The integrated process (a.k.a. "basic oxygen furnace") method is the older of the two. It is much less energy efficient and uses significantly less recycled material in the form of steel scrap in the manufacturing process. The three basic materials used in the process are:
• Iron ore
Limestone and coal are used to refine the iron ore into molten iron ore within the confines of a blast furnace (limestone removes impurities from the iron). Oxygen and heat (from burning fuel oil or natural gas) are also used in the process. A molten byproduct of the blast furnace, whereby minerals and iron ore combine with lime flux, is called "slag."
THE RIDDLE OF STEELBefore entering the blast furnace, iron ore is processed into pellets or "sinter." This increases the concentration of iron in the ore. Limestone is baked to make lime and coal is converted to coke, which fuels the process. At high temperatures and without the use of oxygen, organics and other contaminants in the coal are removed. At the steel mill, volatile gases removed from the coal are captured for in-plant use and are also used to heat the coking ovens. Water used to cool the hot coke emerging from the ovens is treated and reused or discharged.
Now, with the ingredients ready, the iron ore, lime and coke are placed in the blast furnace where the burning coke provides the heat required to melt the iron into what is known as "pig iron." Periodically in the process, molten iron and slag are removed from taps located at the bottom of the furnace. Slag, the byproduct of the blast furnace, is used to make mineral wool (Thermafiber) insulation, acoustical ceiling tiles, and aggregate for road and railroad beds. The molten pig iron exiting the blast furnace is inundated with carbon and other contaminants. To make it steel, the carbon and contaminants must be removed-this is where the BOF comes into play.
A variation of the BOF, the Open Heart Furnace, is no longer used in North America. Molten pig iron, lime flux and between 25 to 30 percent steel scrap is inserted into the BOF. Pure oxygen injected into the BOF reacts with and removes the carbon in the pig iron. Other impurities in the pig iron and steel scrap bond to the lime flux forming slag. The process is heated by chemical reactions of the oxygen within the BOF and molten iron, which is at a temperature of 2,500 degrees F. In fact, the steel scrap acts as a coolant for the BOF.
The electric furnace process or electric arc furnace is the modern method for manufacturing steel and is most widely used where LGMF is concerned. It is more energy efficient and relies on recycled steel to a much higher degree.
In the '70s, "mini-mills" began to appear in North America. Initially used to fabricate heavier steel products such as rebar, steel plate and "I" beams via "hot-rolling," EAF directly melted steel scrap to make new steel. Nucor Steel Inc. went a step further by producing steel sheets from steel scrap directly using the EAF process-this was revolutionary. Though the process left surface imperfections unacceptable to the automobile and appliance industries, it was ideal for LGMF where these imperfections were irrelevant. At that time, the process was using 91-percent recycled steel. With 21st century technology, the recycled content using the EAF method is typically at or near 100 percent.
Roll out the steelSteel sheets are formed by pouring molten steel into an ingot mold or continuous caster. There, it solidifies into slabs which are large, rectangular shapes. These large slabs are then passed through rollers that reduce the steel slab- based on desired thickness and strength-into thin sheets. These thinner sheets are run through a hot-dip galvanizing process and rolled into coils weighing between 20,000 to 25,000 pounds. The steel industry's own initiatives, advancements and investments in new technology along with increased use of steel scrap is big part of why steel prices have remained so stable for so long and why LGMF is so attractive as compared to lumber-price stability.
In part three, we'll examine the galvanizing and cold-rolling process in our on-going discussion of the many aspects of LGMF.