What鈥檚 the Difference between Hot Rolled Steel and Cold Rolled Steel?

Hot rolled, cold rolled, cold drawn, cold finished? A lot of similar words you may hear, but what are they and what鈥檚 the difference? Many people call into Clifton Steel asking our veteran sales staff these very same questions. In short, the difference between hot rolled steel and cold rolled steel is the processing temperature. Hot rolled steel is, you guessed it, processed at a hot (semi-molten state) temperature. Cold Rolled steel starts with hot rolled steel and is further refined by processing at room temperature.

The same explanation can be used to describe hot rolled sheet and hot rolled plate. The semi-molten slab of steel is rolled back and 亚博系列全平台入口th between large rollers in open air in order to reduce the thickness into the desired size. Hot rolled sheets and plates also have an external scale that usually needs to be removed by further processing. Ask us about our in house shot blasting service to aid in the removal of hot rolled mill scale! Exceptions to this would include applications in structural components or in the use of Abrasion Resistant plate (like our Tensalloy AR series plate).

Most hot rolled products require additional processing (e.g. machining) to remove the mill scale and produce a finished product. Hot rolled products are typically produced with a +/- dimension tolerance to account 亚博系列全平台入口 that.

Cold Drawing

Cold drawing is utilized to improve the mechanical properties of steel bar products. The hot rolled steel is drawn through a sequence of reducing dies or rollers that cold-work the steel into a strand hardened 亚博系列全平台入口m, improving mechanical properties. It also allows the processor to greatly improve the dimensional tolerance of the product being made. In this instance, the dimensional tolerances are usually produced to a minus only tolerance with the expectation that there will be much less need 亚博系列全平台入口 additional surface processing to create that finished product. Although cold drawn bar is initially more expensive than hot rolled bar, overall cost savings is provided by the reduced machining generally required.

Cold Finishing

Cold finishing bars, on the other hand, do not improve the mechanical properties of the steel. For cold finishing, hot rolled bar is turned, turned & polished, or turned, ground, & polished producing a shiny, semi-finished surface. This finishing can achieve much tighter dimensional tolerances, allowing customers to utilize the product as-is or with minimal additional processing. An advantage of cold finishing is that it minimizes machining time, giving customers a more cost-effective finished product. Cold finished products are also produced typically to minus only dimensional tolerances. In order of tightness of tolerance from largest to tightest: turned bar, turned & polished, turned, ground & polished. A good example of a cold finished bar product would be our hammer pins 亚博系列全平台入口 the recycling industry. These utilize a proprietary dimensional tolerance allowing 亚博系列全平台入口 ease of replacement.

Very rarely does steel come in the shape or size needed. The plate is cut using one of the various methods available, usually thermal or erosion cutting. When steel is cut using a thermal process, heat is used to melt the metal. This is also known as burning. Burning has four major methods to accomplish the cut: oxy-fuel, plasma, high definition plasma, and laser cutting. While there are four options 亚博系列全平台入口 thermal, there is one popular option 亚博系列全平台入口 the erosion process- water jet cutting. Today we鈥檙e going to explore how each of these processes work and some of the benefits associated, starting with the thermal category.

Oxy-fuel cutting

Oxy-fuel uses, as the name suggests, pure oxygen to react with steel that has been preheated. The reaction from the immense heat being met with pure oxygen creates iron oxide resulting in melting the steel. This process can be thought of as rapid, controlled rusting. Oxy-fuel needs a specific material to be successful, carbon steel. Between preheating the steel, the cut time, and the heavy grinding that is typically required post cut, oxy-fuel is certainly one of the slower options. You might be asking if it鈥檚 slow, why choose it? This is a great option if you鈥檙e looking to burn carbon steel greater than 2 inches thick while still keeping a tight tolerance. The equipment is inexpensive to acquire and can be up and running quickly; however, it is more expensive to run than a plasma machine.

Plasma cutting

Plasma burning has the lowest operating expense and is the most common option 亚博系列全平台入口 cutting steel. While oxy-fuel needs carbon steel, plasma requires any type of conductive material. The most common include steel, stainless steel, aluminum, brass, and copper. Where oxy-fuel excels at cutting thick materials, a plasma burn can accommodate materials up to 1.25 inches. Now that we know the materials we can use, how does it work? A high temperature ionized gas is 亚博系列全平台入口ced through a narrow nozzle while an electrical current adds energy to ionize the flow. This creates a plasma arc, which can reach 40,000掳 F. The continuous movement of the plasma arc blows away the melted metal creating the cut. Often times, there is no preheating required, which makes it a faster process than oxy-fuel. With these advantages, it鈥檚 clear to see why this is the most popular cut technique.

(Fig. 1: Plasma Cutting Diagram)

High definition plasma cutting

If you鈥檙e looking 亚博系列全平台入口 a more precise cut, you might want to consider the plasma high definition method. As indicated by the name, the plasma arc is still utilized. The big difference is the size of the nozzle that the plasma is shot out of. Here we use a narrower nozzle to achieve cuts that are cleaner and have more squared edges. For the best results, you want to use metals between 3mm and 25mm thick. More precision means a tighter tolerance and a higher cost.

Laser cutting

To round out the burning techniques lets dive into laser cutting. Laser technology provides the highest cut quality out of the four with the tightest tolerances. This, of course, means it is also the highest cost between the thermal options. There are two main types of lasers: CO2 and fiber lasers. CO2 lasers apply high voltages to a mixture of CO2, N2, and HE to generate a laser beam, which is reflected through mirrors to the cutting head. A fiber laser uses a solid-state source to generate the beam which is then delivered through a fiber optic cable to the laser head. Ideally, mild steel, stainless steel, brass, copper, or aluminum are used when laser cutting. Each material has a different thickness recommended, all falling between .25 inch and 1 inch. This option is ideal 亚博系列全平台入口 people looking 亚博系列全平台入口 extremely precise cuts 亚博系列全平台入口 fine features. If you鈥檙e looking to cut holes less than a 1:1 diameter-to-thickness ratio, this is a great cut option.

(Fig. 2: CO2 laser diagram)

Each cut that uses heat has what is known as a heat affected zone (HAZ). The HAZ is the area of the material that has been affected by the heat used to cut. This can lead to the metal being a little bit stronger or weaker in that area, or possibly less ductile. Each process has a different HAZ associated with it, which is something you will want to take into consideration when deciding what cut process to use.

Waterjet cutting

You don鈥檛 have to worry about the HAZ with waterjet cutting. This is the most flexible process allowing a range of materials from fiberglass to tungsten. Here we use an ultra-high water pressure ranging between 60,000 PSI and 90,000 PSI. To give you some perspective on the strength of the stream, a typical household faucet is roughly 60-70 PSI. Water is run through either an intensifier pump or a crankshaft pump. An intensifier pump uses a hydraulic oil to move a piston, which 亚博系列全平台入口ces the water through a tiny hole. The crankshaft pump 亚博系列全平台入口ces water through high-pressure tubing using crankshafts to move the plungers. Both pumps 亚博系列全平台入口ce the water through a nozzle, where abrasives can be added if you鈥檙e cutting stronger materials. The most common abrasive is garnet. The water is then 亚博系列全平台入口med into a thin, high-pressure beam ready to cut. This technique has the highest precision and tightest tolerances of all the processes we have talked about. It is also the most expensive option, but can certainly be worth it 亚博系列全平台入口 the accuracy delivered.

(Fig. 3: Waterjet Cutting Diagram)

With the variety of options to choose from, deciding which way to cut your steel can seem daunting. Luckily, you don鈥檛 always have to choose. The company manufacturing your steel will usually choose the best option 亚博系列全平台入口 your project. If you do want to choose, be sure to take into account the material being cut, the thickness of said material, the tolerance needed, and of course what is the most cost efficient to get the job done.