Heat Treating and Annealing Steel Strip
A plethora of heat treating processes can change the crystalline structure of metal. This can improve its formability, machinability and a slew of other properties.
A strand anneal is one such process. This resets the properties of stainless steel so it can be rolled down to extremely thin gauges.
Heat Treatment
Heat treatment is a series of processes that alter the physical properties of metal, most notably steel. It involves heating the material to a certain temperature, then cooling it in a specified manner. This can be done to improve the machinability of the material or to relieve internal stress. The process is typically carried out in a closed furnace that can be used for both continuous or batch operations.
Annealing is a common heat treatment that can soften steel, making it easier to work with. It works by giving the atoms in the metal enough energy to move within the structure, and this causes them to rearrange themselves into a more uniform crystal lattice. This reduces brittleness, removes internal stresses that may have been created during the manufacturing process, and increases the ductility of the metal. It can also improve a metal’s ability to be machined, and it can extend the lifespan of shop tools.
There are a few different ways to anneal metal, but most of them involve bringing the material up to its critical temperature. This can be done by hand with a torch or in a kiln, and it is important to Annealing Steel Strip know the exact temperature that you are working at so that you don’t overheat the metal. One good way to check that you have reached the critical temperature is to use a magnet. The metal will lose its magnetic properties once it reaches this point.
Annealing
Annealing is a heat treatment process that alters the microstructure of metal strip to change its physical properties. This is generally done to increase the ductility of the material to reduce stress, and to facilitate further processing. This process is typically used in steels, but also applies to other metals such as aluminium and copper. This heat treatment process is usually carried out in a protective gas atmosphere to prevent surface oxidation. It involves heating the steel to a temperature above its recrystallisation point, holding it there for a specified period of time, then cooling at a predetermined rate.
This allows the atoms in the metal to migrate within the structure and reorganise the crystal lattice, restoring its original physical properties. The resulting reduction in brittleness and work hardening also improves machinability.
Depending on the specific composition of the steel, different annealing temperatures are required. For instance, if the steel is to be cold rolled it should be annealed at around 1600 F. During this annealing, the tensile strength of the steel is raised, and it can be rolled into sheets or tracks for use in automotive applications. This type of annealing is known as temper annealing or controlled incomplete annealing. The precise temperature used to carry out this annealing is determined by the composition of the steel, as well as its width range and stack position in the strand anneal line.
Pickling
A clean, dry surface is needed for annealing. Pickling is a chemical process that removes oxide scale, dirt and other impurities from the strip surface. It also descales the surface of carbon and low alloy steel strip, leaving it ready to be rolled, forged or cold worked. Depending on the steel grade, strip size and required properties, it may be further treated in subsequent sections to enhance its performance.
Alternatively, the steel can be cooled from ambient in a controlled environment. This is called intermediate annealing, sub-critical annealing or in-process annealing. This annealing is done below the critical temperature to improve the ductility of the steel, and is commonly used for cold working.
Modern pickling lines can operate at high speeds, with maximum speeds in the pickling section up to 460 m/min for C steel. They use a series of tanks with different acid concentrations, in which the solution is recirculated to ensure that the strip is exposed to the fresh solution only. The tanks can be sized upto 1.2 m in depth and 36 m long, with the last tank having the highest acid concentration.
At the exit end of a push-pull pickling line, there is a steering roll to guide the steel strip as it coils, a strip inspection station, dual side trimmers and an oiler. The oiler adds oils containing S additives to the surface of the steel strip to improve its corrosion resistance. This is especially important for galvanized products.
Overaging
The annealing of strip steel is often followed by a precipitation hardening treatment. This step adds carbon and nitrogen to the base metal, increasing the material’s strength and hardenability. The precipitation hardening process can be controlled to achieve desired mechanical properties, such as hardenability and toughness. The process is commonly carried out in a continuous annealing line. The annealing step removes work hardening stress created by cold-drawing or deep drawing operations. This stress can crack or tear the metal, so annealing reduces it to facilitate additional forming and drawing operations. The resulting strip is often specified as fully annealed, dead soft, or half hard.
In the present study, DP590 cold-rolled dual phase (DP) steel was subjected to intercritical annealing in a continuous annealing line and online overaging. The microstructure and mechanical behavior were analyzed by optical microscopy, scanning electron microscopy, and tensile test.
The results showed that the overaging process can significantly improve the quality of DP steel. The tensile Hardened & Tempered Steel Strip Supplier and yield strengths of the specimens subjected to overaging at different temperatures were examined. Optimal overaging conditions were determined, and the results indicated that a higher overaging temperature is favorable for improving tensile and yield strengths. The aging of the DP steel also promoted the formation of high-C martensite. This second phase strengthens the ductility of the DP steel and reduces the elongation.
