Low Carbon Steel Plate Testing

Low-carbon steel plate inspection mainly conducts scientific testing and analysis on the chemical composition analysis, tensile property testing, bending property testing, hardness testing, impact toughness testing and metallographic structure examination, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Chemical composition analysis: This is the basis of the low-carbon steel plate inspection, mainly determining the contents of the elements such as the carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulphur (S). Among them, the carbon content is the key, and the low-carbon steel usually requires the carbon content lower than 0.25%. This analysis directly determines the strength, toughness, weldability and cold working performance of the steel, and is the primary basis for judging whether it meets the designation requirements (such as Q195, Q215 and Q235).

Tensile property testing: Used to evaluate the mechanical behaviour of the low-carbon steel plate under the tensile forces. The yield strength, tensile strength, elongation after fracture and reduction of area are mainly determined. These indicators reflect the load-bearing capacity, uniform plastic deformation ability and fracture resistance of the plate, and are the core data of the structural design and safety assessment, usually carried out in accordance with the standard GB/T 228.1.

Bending property testing: Evaluates the plastic deformation ability and cracking resistance of the low-carbon steel plate during the bending processing. The specimen is bent to the specified angle (such as 180°) with the bending mandrels of the specified diameters, and it is checked whether the cracks, peeling or fractures occur on the outer surface of the bent specimen. This testing is crucial for evaluating the cold bending forming adaptability of the steel plate in the fields such as the construction and manufacturing.

Hardness testing: The ability of the steel plate surface to resist the local plastic deformation (such as the indentation) is measured through the methods such as the Brinell hardness (HBW), Rockwell hardness (HRB) or Vickers hardness (HV). The hardness value has a certain correlation with the strength and wear resistance of the material, and the testing method is simple and fast, commonly used for the on-site quality control and material consistency verification.

Impact toughness testing: Usually carried out on a Charpy pendulum impact testing machine, used to determine the ability of the steel plate to resist the impact fracture under the dynamic loads, expressed by the impact absorbed energy (KV2 or KU2, in joules J). This indicator is particularly important for the structural parts used in the low-temperature or dynamic load environments (such as the bridges and ships), and can effectively evaluate the brittle transition trend of the material.

Metallographic structure examination: The microstructures of the steel plate, such as the morphologies, grain sizes and distributions of the ferrite and pearlite, are observed with an optical microscope or electron microscope, and the grades of the non-metallic inclusions (such as the sulphides and oxides) are checked. This examination can intuitively reflect whether the metallurgical quality and heat treatment process of the material are normal, and is an important means for analysing the intrinsic causes of the performance advantages and disadvantages.

Testing Scope

Hot-rolled low-carbon steel plates/strips: Referring to the plates produced through the high-temperature rolling process, including the medium and thick plates, hot-rolled thin plates and hot-rolled steel strips. The testing needs to cover the mechanical properties, dimensions and shapes (thickness, width and flatness) and surface qualities (defects such as the iron oxide scales, cracks and scars) under all the size specifications, ensuring that they are suitable for the fields such as the building structures, vehicle manufacturing and pipeline vessels.

Cold-rolled low-carbon steel plates/strips: Made from the hot-rolled coils through the processes such as the pickling, cold rolling and annealing, with the characteristics of the clean surfaces, high dimensional precision and relatively high strength. The testing emphasis is on the stricter control of the surface defects (such as the roll marks, scratches and colour differences), excellent forming properties (such as the tensile and cupping testing) and the adhesion of the coatings (such as the subsequent galvanizing and colour coating).

Coated low-carbon steel plates: Mainly including the hot-dip galvanized plates, electro-galvanized plates and galvalume plates. The testing includes not only the mechanical and chemical compositions of the base plates, but also focuses more on the coating quality, such as the coating thickness (zinc coating weight per unit area), adhesion (no peeling after the bending and cupping testing), uniformity and corrosion resistance (such as the salt spray testing).

Special low-carbon steel plates: The plates developed for the specific purposes, such as the deep drawing steels (08Al series), steels for the boilers and pressure vessels, and steels for the automobile frames. On the basis of the general items, the special requirements are added to their testing scope. For example, the deep drawing steels need to carry out the stricter aging testing and the plastic strain ratio (r value) and strain hardening exponent (n value) testing.

Plates for the import and export trade: In the cross-border trade, the testing of the low-carbon steel plates is the core link of the commodity inspection and identification. The testing scope needs to strictly follow the trade contracts, letter of credit requirements and the mandatory standards of the destination countries (such as the EU CE certification and US ASTM standards), carrying out the comprehensive conformity verification, to ensure the smooth customs clearance and avoid the trade disputes.

Maintenance and evaluation of the in-service equipment: The detection of the low-carbon steel components in use (such as the old building steel structures, storage tanks and pipelines) is carried out, and the scope focuses on the remaining performance evaluation, including the thickness corrosion reduction measurement, non-destructive flaw detection of the surface cracks, material hardness and strength estimation, and metallographic structure aging (such as the spheroidization and graphitization) analysis, providing the basis for the safe operation and repair.

Testing Methods

Spectrometry analysis method: Used for the fast and accurate chemical composition determination. It mainly includes the spark direct-reading optical emission spectrometry (OES) and X-ray fluorescence spectrometry (XRF). The OES method has the high precision and is the main means of the laboratory; the XRF method is more portable and suitable for the on-site fast screening. The sample needs to be prepared with a flat and clean excitation surface, and the quantitative analysis is carried out by comparing the spectral intensities with the standard samples.

Universal material testing machine method: This is the standard method for carrying out the static mechanical property tests such as the tensile, bending and compression. In accordance with the standards such as GB/T 228.1, the standard specimens are clamped on the testing machine, the loads are applied at the specified rates, and the load-displacement curves are recorded in real time through the sensors and software, automatically calculating and outputting the various mechanical property parameters.

Pendulum impact testing method: In accordance with the standard GB/T 229, the standard Charpy V-notch or U-notch specimens are placed on the supports of the impact testing machine, and the pendulum released from the specified height breaks the specimens. By measuring the potential energy difference before and after the pendulum breaks the specimens, the impact absorbed energy of the specimens is precisely calculated, thereby evaluating the toughness of the material under the impact loads.

Hardness indentation method: The suitable methods are selected according to the expected hardness and thickness of the material. The Brinell hardness (GB/T 231.1) uses the cemented carbide ball indenter of the larger diameters, reflecting the macroscopic hardness; the Rockwell hardness (GB/T 230.1) uses the diamond cone or steel ball indenters, with the simple operation; the Vickers hardness (GB/T 4340.1) uses the diamond pyramid indenter, suitable for the thin plates and micro-area hardness testing.

Metallographic microscope analysis method: After the sampling, cutting, mounting, grinding, polishing and chemical or electrolytic etching, the metallographic specimens are made. The microstructures are observed under an optical microscope, the grain sizes are rated according to the standards such as GB/T 13298 and GB/T 6394, the non-metallic inclusion grades are rated according to the standard GB/T 10561, and the photos are taken and archived for the analysis.

Non-destructive testing method: Detects the internal and surface defects without damaging the material. The common methods include the ultrasonic testing (UT, for the internal delamination and inclusion flaw detection), magnetic particle testing (MT, for the surface and near-surface crack flaw detection) and penetrant testing (PT, for the surface open defect flaw detection). These methods are widely applied to the online monitoring in the steel plate production process and the periodic inspection of the in-service equipment.

Testing Standards

Domestic Standards:

1. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

2. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

3. GB/T 231.1, Metallic materials — Brinell hardness test — Part 1: Test method

4. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

5. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: Test method

6. GB/T 6394-2017, Determination of estimating the average grain size of metal

7. GB/T 10561, Steel — Determination of content of nonmetallic inclusions — Micrographic method using standards diagrams

8. GB/T 13298, Inspection methods of microstructure for metals

International Standards:

1. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

2. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Direct-reading optical emission spectrometer: The core equipment of the chemical composition analysis. It is usually composed of an excitation light source, spectroscopic system, detector and computer control system. Its working principle is to use the sample as the electrode, excite the characteristic spectra of the elements through the high-voltage spark discharge, and after the grating dispersion, the spectra are received by the photomultiplier tubes or CCD detectors and converted into the electrical signals for the analysis. The contents of the multiple elements can be simultaneously determined within tens of seconds.

Microcomputer-controlled universal material testing machine: The mainstream equipment of the modern mechanical property testing. It adopts the servo motor or hydraulic drives, and is equipped with the high-precision load sensors, extensometers and full digital control systems. It can precisely control the loading rates and modes, automatically complete the tests such as the tensile, bending, compression and shear, and display the curves in real time, calculate the data and generate the standard-compliant testing reports.

Pendulum impact testing machine: Used for the impact toughness testing. It is mainly composed of a frame, pendulum, specimen supports, energy indicating mechanism and safety protection device. The equipment needs to be regularly verified for its energy loss and friction loss, ensuring that the impact speed, pendulum lift angle and initial energy conform to the standard provisions, to ensure the accuracy and comparability of the testing results.

Hardness tester series: Including the Brinell hardness testers, Rockwell hardness testers, Vickers hardness testers and portable Leeb hardness testers. The first three are the bench types with the high precision, used for the precise measurement in the laboratory; the Leeb hardness tester is based on the rebound principle, small in size and easy to carry, commonly used for the fast hardness screening and comparison of the large workpieces or installed parts on site.

Metallographic specimen preparation system and microscope: This system includes the cutting machines, mounting machines, pre-grinding machines, polishing machines and etching devices. The metallographic microscope is equipped with the multiple observation modes such as the bright field, dark field and polarization, and is connected to the high-resolution digital camera systems, which can carry out the structure observation, image acquisition, measurement and quantitative analysis. It is the "eyes" for the microstructure research of the material.

Non-destructive testing equipment: The ultrasonic flaw detector (A-scan or phased array) detects the defects by transmitting and receiving the high-frequency sound waves through the probes; the magnetic particle flaw detector displays the defect leakage magnetic fields by magnetizing the tested workpiece and applying the magnetic particles; the penetrant testing reagent kits make the coloured or fluorescent penetrants enter the surface open defects through the capillary action. These equipment are the key tools for ensuring the internal and surface quality integrity of the steel plate.

Dimension and morphology measuring tools: Including the high-precision micrometers, vernier callipers, plate thickness micrometers, flatness measuring instruments and surface roughness instruments. They are used to precisely measure the dimensions and shapes such as the thickness, width, length, flatness and camber of the steel plate, as well as the surface roughness parameters, ensuring that its geometric precision meets the subsequent processing and assembly requirements.