In the field of metallic materials, cold-rolled steel and hot-rolled steel are two common forms of steel. Due to the differences in their production processes, they have significant distinctions in performance, appearance and application scenarios. Accurately identifying these two types of steel is of vital importance for material selection, optimization of processing techniques and quality control of products. Today, we will delve into the identification methods between cold-rolled steel and hot-rolled steel from the perspective of inspection.

 

 

 

I. Introduction to Cold-Rolled Steel and Hot-Rolled Steel

 

The cold rolling and hot rolling of steel are essentially based on the process differences of rolling temperature:

 

Hot-rolled steel: It is a type of steel made by heating steel billets above the recrystallization temperature (usually over 1000℃) and then rolling them. At high temperatures, steel has good plasticity and low deformation resistance, allowing for large-scale deformation rolling and the production of thicker steel plates, sections, etc. However, its surface is relatively rough, with lower dimensional accuracy. In terms of mechanical properties, it has better toughness and ductility, but relatively lower strength.

 

Cold-rolled steel: It is made by rolling hot-rolled steel coils at room temperature. Due to the lack of heating, steel has a high resistance to deformation. During the rolling process, work hardening occurs. Therefore, cold-rolled steel has higher strength and hardness, a smooth and flat surface, high dimensional accuracy, and can produce thinner steel plates (such as as thin as 0.1mm). However, the toughness and ductility of cold-rolled steel are inferior to those of hot-rolled steel, and internal stress may occur during the rolling process.

 

The application scenarios of the two also have their own focuses: hot-rolled steel is mostly used in fields such as building structures and mechanical parts blanks where precision requirements are not high; Cold-rolled steel is widely used in scenarios with high requirements for surface quality and dimensional accuracy, such as automotive panels, home appliance casings, and precision instruments.

 

 

 

Ii. Principles of Identification and Detection

 

The principle of distinguishing cold-rolled steel from hot-rolled steel mainly relies on the differences in physical, chemical and mechanical properties formed under the variations in their production processes:

 

- Surface condition difference: During the hot rolling process, the surface of steel is prone to form iron oxide scale (iron oxide), and the high temperature during rolling can cause surface roughness. Cold rolling is a process of rolling at room temperature, with no iron oxide scale on the surface. Under the effect of rolling pressure, the surface becomes smoother and may even retain traces of rolling oil.

 

- Microstructure differences: Hot-rolled steel undergoes recrystallization at high temperatures, and its microstructure is mostly uniform ferrite or pearlite with relatively coarse grains. Cold-rolled steel, due to work hardening, presents a fibrous or banded microstructure, elongated grains, and may have increased dislocation density and other characteristics.

 

- Mechanical property differences: Due to work hardening, cold-rolled steel has significantly higher tensile strength and hardness than hot-rolled steel, while its elongation (plasticity) is lower than that of hot-rolled steel. The mechanical properties of hot-rolled steel are more balanced and its toughness is better.

 

- Dimensional accuracy difference: The thickness tolerance of cold-rolled steel is small (usually within ±0.01mm), and the flatness is good; The thickness tolerance of hot-rolled steel is relatively large (possibly up to ±0.1mm or more), and it is prone to warping.

 

 

 

Iii. Specific Identification and Testing Methods

 

(1) Appearance and macroscopic inspection method

 

1. Surface observation method

 

Hot-rolled steel: The surface is mostly blackish-brown or bluish-gray, with iron oxide scale (commonly known as "iron scale") present. It feels rough when touched by hand, and even obvious roller marks or scratches left during the rolling process can be seen.

 

Cold-rolled steel: The surface is bright, silver-white (without coating), free of iron oxide scale, and has a smooth and fine touch. Some cold-rolled steel plates may have slight rolling streaks (fine streaks along the rolling direction) on the surface.

 

This method is simple to operate and can be used as a basis for preliminary identification. However, it should be noted that if hot-rolled steel undergoes acid washing to remove the iron oxide scale, its surface may approach that of cold-rolled steel. It is necessary to combine other methods for judgment.

 

2. Dimensional accuracy measurement

 

When measuring the thickness of steel with a micrometer or a laser thickness gauge, the thickness deviation of cold-rolled steel is extremely small, and the thickness uniformity of the same batch of steel is good. The thickness deviation of hot-rolled steel is relatively large, and the measured values at different positions may vary significantly. In addition, cold-rolled steel plates have higher flatness and are less likely to bend when placed. Hot-rolled steel plates may have slight warping.

 

(2) Microstructure analysis method

 

Observing the microstructure of steel through a metallographic microscope is an accurate method to distinguish cold-rolled steel from hot-rolled steel:

 

1. Sampling: Take a small sample from the steel to be inspected and treat it by grinding, polishing or etching (such as with 4% nitric acid alcohol solution).

 

2. Observation

 

Hot-rolled steel: In the microstructure, the grains are equiaaxially distributed and have no obvious directionality (for example, ferrite grains are polygonal and pearlite is distributed in layers).

 

Cold-rolled steel: Due to the lack of recrystallization, the grains are elongated by the rolling force, presenting a fibrous or banded structure along the rolling direction, with blurred grain boundaries, and dislocation accumulation characteristics caused by work hardening may be observed.

 

This method can directly reflect the essential differences between the two and is a commonly used authoritative identification means in laboratories.

 

(3) Mechanical property testing method

 

Identification can be made by tensile tests or hardness tests, taking advantage of the differences in mechanical properties between the two:

 

Hardness testing: It is measured using a Rockwell hardness tester (HRB or HRC) or a Vickers hardness tester. The hardness of cold-rolled steel (such as HRB value) is usually 20-50 units higher than that of hot-rolled steel of the same material (for example, the HRB of hot-rolled low-carbon steel is about 60-80, and that of cold-rolled steel can reach 90-120).

 

2. Tensile test: Tested on a universal testing machine, the tensile strength (σb) of cold-rolled steel is 30% to 50% higher than that of hot-rolled steel, while the elongation (δ) is more than 50% lower (for example, the elongation of low-carbon steel in the hot-rolled state is about 30%, while in the cold-rolled state it may only be 10% to 15%).

 

Mechanical property testing can not only identify the type of steel but also evaluate its performance in use, making it suitable for scenarios where there are clear requirements for material performance.

 

(4) Other auxiliary detection methods

 

1. Magnetic detection method

 

Although both cold-rolled steel and hot-rolled steel are ferromagnetic materials, the internal stress of cold-rolled steel increases due to work hardening, and its magnetic property may be slightly higher than that of hot-rolled steel (measured with a precision magnetometer), which can be used as an auxiliary reference.

 

2. Pickling test method

 

Take a small sample and put it into dilute hydrochloric acid (5%-10%). The iron oxide scale on the surface of the hot-rolled steel will react rapidly, generating a large number of bubbles that fall off, and the solution will quickly become turbid. The surface of cold-rolled steel has no iron oxide scale, the reaction is relatively slow, and the turbidity of the solution is low. This method is applicable to the identification of hot-rolled steel that has not undergone pickling.

 

 

 

Summary

 

The identification of cold-rolled steel and hot-rolled steel requires multi-dimensional testing, including appearance, size, microstructure and mechanical properties. Appearance and size inspection can quickly make a preliminary judgment, while microstructure and mechanical property inspection can provide precise basis. In practical applications, appropriate methods can be selected based on the testing conditions and requirements to ensure that the material selection meets the design requirements and guarantee the quality and performance of the products. With the development of detection technologies, such as scanning electron microscopy (SEM) for observing microscopic morphology and energy dispersive spectroscopy (EDS) for assisting component verification, the accuracy and efficiency of identification will be further enhanced.