Process characteristics of hot rolling, cold rolling and ESP steel coils

From iron ore to finished steel, it needs to go through a series of complex processes, and finally produce different categories. Hot-rolled steel coils and cold-rolled steel coils are the two most common categories. Hot-rolled coils are both the raw materials of cold-rolled coils and can be sold and used separately like cold-rolled coils.

There are significant differences in process characteristics, advantages and disadvantages, and applications between these two categories. In addition, the new ESP process products are also easy to cause confusion among users. This article attempts to analyze and compare these three products.

Hot rolled steel coil

Process characteristics

Hot rolled steel coils are made by rolling steel billets at high temperatures. In this process, the steel is heated above the recrystallization temperature, usually between 1000°C and 1250°C, giving it good plasticity and low deformation resistance. After hot rolling, the steel surface will have a layer of oxide scale, which is usually removed before use (pickling or shot blasting).

Advantages

Good mechanical properties: Hot-rolled steel coils are rolled at high temperatures and have good comprehensive mechanical properties, such as strength, plasticity and toughness.

High production efficiency: The hot rolling process has fewer steps, the rolling thinning is large, and the rolling efficiency is high.

Low cost: Since the hot rolling process has a shorter production process and reduces energy consumption, the production cost of the same specification is lower than that of cold rolling.

Shortcoming

Low dimensional accuracy: The dimensional accuracy and surface finish of hot-rolled steel coils are not as good as those of cold-rolled steel coils.

Residual stress: During the hot rolling process, the rapid cooling of the steel will produce certain residual stress, which may affect its subsequent processing performance.

Poor surface quality: Because there is iron oxide scale on the surface of the steel plate during rolling, it is easy to cause the iron oxide scale to be pressed in, and even after pickling and leveling, the residual traces cannot be completely eliminated.

Application

Hot-rolled steel coils are widely used in construction, bridges, ships, machinery manufacturing and other fields. For example, patterned plates, beam steels, boom steels, etc. are mostly manufactured using hot rolling technology.

Cold rolled steel coil

Process characteristics

Cold rolled coils are made by further rolling hot rolled coils at room temperature.

The dimensions can be precisely controlled during the room temperature cold rolling process, followed by specialized heat treatment to obtain the desired properties.

Advantages

High dimensional accuracy: The thickness, width and other dimensional accuracy of cold-rolled steel coils are high.

Excellent mechanical properties: Through cold rolling and subsequent heat treatment, higher strength and hardness, as well as good plasticity can be obtained.

Good surface quality: The surface of cold-rolled steel coils is free of iron oxide and has high smoothness, which is suitable for direct use in the manufacture of various parts.

Shortcoming

High production cost: The cold rolling process has many steps, the process is complicated, and the energy consumption is high, so the production cost is higher than that of hot-rolled steel coils.

Low production efficiency: The cold rolling process is complicated, the material flow is cumbersome, and the production efficiency is lower than that of hot rolling.

Application

Cold-rolled steel coils are mainly used to manufacture parts for automobiles, home appliances, electronic products, etc. that have high requirements for dimensional accuracy and surface quality.

ESP（Endless Strip Production）

ESP production line is considered to be the third technological revolution of the steel industry after oxygen converter steelmaking and continuous casting, and represents the highest level of hot-rolled strip technology in the world today.

#1 Many people used to think that ESP was a cold-rolled product, but in fact ESP is a thin-gauge hot-rolled pickled product.

ESP adopts a new process of endless strip rolling, realizing full continuous production from molten steel to hot-rolled coils on the coiler, and can produce ultra-thin hot-rolled steel coils such as 0.8mm.

Process characteristics

Endless rolling: The most notable feature of ESP technology is that it realizes full continuous production from molten steel to hot rolled coils without cutting in the middle.

Compact process: The continuous casting and rolling processes are directly connected, eliminating the intermediate links such as slab warehouse and heating furnace.

High-speed production: The continuous casting machine is designed with a high pulling speed of up to 7.0m/min, which greatly improves production efficiency.

High-precision control: The use of advanced control technology can accurately control the thickness, width and straightness of the product.

Advantages

Energy saving and emission reduction: Due to the elimination of intermediate heating and other links, the ESP production line greatly reduces energy and water consumption.

High yield rate: The yield rate of molten steel to hot-rolled coils can reach 97%-98%, which is much higher than the traditional process.

Reduce investment costs: The equipment layout is compact, reducing equipment investment and plant investment.

Improve product quality: It can produce ultra-thin hot-rolled steel coils, with a typical thin specification of 0.8mm (or even thinner), and the product performance is excellent.

Shortcoming

High initial investment: Although it can reduce costs in the long run, ESP technology is not yet popular enough, and the initial investment is relatively high.

Technical threshold: For an automated full-line production line, no problem can occur in any link, otherwise the entire line will be shut down, so the skills of operators and maintenance personnel are required to be high.

Low flexibility: Due to the high degree of automation and continuity of the production line, the change of product types is not as flexible as traditional production lines.

Application

Replacing cold with heat, a comprehensive replacement for cold-rolled products is provided in scenarios where surface and performance requirements are not high but the specifications and thickness are thinner.

Answers to some frequently asked questions

Question 1: What are the core advantages of ESP?

Low production cost + low carbon emissions. Because of the compact process, the production cost of ESP has dropped significantly. Also, because of the compact process, there is little energy waste in the intermediate links, and less waste means lower carbon emissions.

From this perspective, the explosion of ESP technology is not now. When Europe and the United States vigorously promote carbon tariffs, hot-dip galvanized steel coils based on ESP will have a relatively large export advantage.

Question 2: What are the differences between hot rolling, cold rolling and ESP of the same specifications?

Take 2.0mm thick C material as an example:

• Production cost: ESP＜hot rolling＜cold rolling

  Surface quality: cold rolling＞ESP＞hot rolling

  Production accuracy: cold rolling＞ESP＞hot rolling

Question 3: What are the differences in surface quality among the three products?

In terms of surface quality, the roughness of hot-rolled coils after pickling and leveling is relatively high. After washing away the iron oxide scale, the surface is slightly uneven, and sometimes there are iron oxide scale peaks.

Because cold rolling is rolled at room temperature after washing away the iron oxide scale, the rolling deformation after pickling is large, the original surface unevenness will be rolled flat, and the roughness is mainly given by the cold rolling roller, and the surface is smoother and more uniform.

ESP is essentially a hot-rolled pickled sheet. Hot-rolled thin specifications are usually pickled and leveled before leaving the factory, and the surface quality is closer to cold-rolled products.

Question 4: Will ESP significantly reduce the survival space of hot rolling and cold rolling?

It will not be greatly compressed, but it will form advantages in economic specifications and grades.

This question actually depends on the ratio of special steel and commercial general materials. Traditional cold and hot rolling still has advantages in making special steel (complex steel grades with multiple specifications), especially there is more room for users to fine-tune performance.

ESP has an extreme cost advantage when making a single specification of ordinary grades, especially as a coating substrate.

To put it bluntly, ESP technology makes the original low-priced general materials cheaper, and the advantage is reflected in the economy, but if you say it is better than cold rolling quality, it is a bit exaggerated.