Container Steel Plate Testing

Container steel plate inspection mainly conducts scientific testing and analysis on the chemical composition analysis, tensile property testing, impact toughness testing, hardness testing, metallographic structure examination and macroscopic defect detection, 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: Through the means such as the spectrometry analysis, the contents of the primary and secondary elements such as the carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulphur (S) in the steel, as well as the key alloying elements such as the chromium (Cr), nickel (Ni) and molybdenum (Mo), are precisely determined. This is the basis for evaluating the weldability, corrosion resistance and mechanical properties of the steel, ensuring that the raw materials meet the requirements of the standard designations.

Tensile property testing: The standard specimens are subjected to the tensile testing on a universal material testing machine, determining their yield strength, tensile strength, elongation after fracture and reduction of area. These data directly reflect the ability of the steel plate to resist the static load deformations and fractures, and are the core indicators for ensuring that the container does not undergo the permanent plastic deformations or damages during the stacking, hoisting and transportation.

Impact toughness testing: The Charpy V-notch impact testing is adopted, and the impact absorbed energy of the steel plate is determined at the specific low temperatures (such as -20 ℃ or -40 ℃). This testing aims to evaluate the ability of the steel plate to resist the brittle fracture under the harsh environments (such as the cold sea areas), and is crucial for the structural safety and reliability of the container.

Hardness testing: The surface hardness of the steel plate is determined with the Brinell (HBW), Rockwell (HRB/HRC) or Vickers (HV) hardness testers. The hardness value indirectly reflects the strength, wear resistance and processing performance of the material, and is an important reference for evaluating the material uniformity and heat treatment effect, helping to judge its ability to resist the local indentations and wear.

Metallographic structure examination: The microstructures of the steel plate, such as the morphologies, sizes and distributions of the ferrite, pearlite and bainite, are observed through an optical microscope or scanning electron microscope, and the types and grades of the non-metallic inclusions are checked. This analysis is used to reveal the intrinsic relationship between the material performance and microstructure, evaluating the quality of the heat treatment process.

Macroscopic defect detection: The surface and near-surface of the steel plate are inspected through the visual inspection or with the aid of the low-power magnifying glasses, identifying whether there are the macroscopic defects such as the cracks, folds, scars, delaminations and bubbles. This is the preliminary and key step for ensuring the material integrity, preventing the materials with the obvious defects from entering the subsequent processes.

Surface quality and dimensional tolerance inspection: The tools such as the callipers, micrometers, tape measures and templates are used to measure the thickness, width, length and flatness of the steel plate; at the same time, it is checked whether there are the defects exceeding the standard allowances on the surface, such as the corrosion, pitting and scratches, ensuring that they meet the processing requirements of the subsequent stamping and welding.

Testing Scope

Incoming inspection of the raw materials: The comprehensive incoming re-inspection of the hot-rolled steel plates or strips for the containers supplied by the steel plants is carried out. This stage of testing is the first gate of the quality control, aiming to verify whether the incoming materials fully meet the requirements of the procurement technical agreements and the corresponding national standards (such as GB/T 11253) or international standards (such as JIS G 3131 SPHC).

Inspection before the stamping forming: Before the steel plate enters the blanking and stamping processes, the sampling testing of the specific batches is carried out, emphatically focusing on the chemical composition, mechanical properties and surface quality. It is ensured that the performance of the material to be formed is stable, avoiding the stamping cracks, abnormal springback or insufficient component strength caused by the material problems.

Special testing of the welding areas: For the steel plates used for the key welding parts (such as the corner posts, top rails and bottom rails), the carbon equivalent (CEV) and cold cracking sensitivity index (Pcm) need to be additionally focused on, to evaluate their weldability. At the same time, the matching between the welding consumables and the base material is verified, preventing the defects such as the welding cracks.

Substrate inspection before the coating: Before the application of the anti-corrosion coatings (such as the zinc-rich primers), the surface cleanliness (such as the derusting grade Sa2.5), roughness and residual chloride content of the steel plate need to be detected. It is ensured that the surface condition of the substrate meets the requirements of the coating adhesion and durability. This is the key to guaranteeing the long-term weather resistance of the container.

Inspection during the service and maintenance: When the periodic inspection or damage repair of the in-service containers is carried out, the performance evaluation of the steel plates in the local corrosion and crack propagation areas needs to be carried out. It may involve the hardness testing, thickness reduction measurement and corrosion product analysis, providing the basis for the structural safety evaluation and maintenance scheme formulation.

Verification testing of the new materials: To meet the needs of the lightweight or higher corrosion resistance, when the high-strength steels (such as Q550), weathering steels or composite materials are introduced, the comprehensive testing schemes stricter than the routine materials need to be implemented, to verify whether their comprehensive performances meet the special working condition requirements of the container.

Testing Methods

Spectrometry analysis method (OES): The arc/spark emission spectrometer is used to excite the steel plate sample, and the element contents are determined by analysing the characteristic spectral line intensities. This method has the fast analysis speed and high precision, and is the main means for the rapid analysis before the furnace and the chemical composition arbitration of the finished products, conforming to the standards such as GB/T 4336.

Static tensile testing method: In accordance with GB/T 228.1 (equivalent to ISO 6892-1), the rectangular or circular cross-section specimens are prepared, and the axial tensile forces are applied at the specified strain rates on a material testing machine until the fracture. By recording the load-displacement curves, the key mechanical parameters such as the yield strength and tensile strength are calculated.

Charpy pendulum impact testing method: In accordance with GB/T 229 (equivalent to ISO 148-1), the standard specimens with the V-notches are cooled to the specified temperatures in a low-temperature bath, and then quickly moved to the supports of the impact testing machine and struck once by the pendulum. The impact absorbed energy value is read, used to evaluate the low-temperature toughness of the material.

Ultrasonic flaw detection (UT): The high-frequency sound waves propagate inside the steel plate, and the reflections will occur when encountering the defects such as the delaminations, inclusions and internal cracks. By analysing the positions, amplitudes and shapes of the reflected waves, the positions, sizes and natures of the internal defects can be non-destructively detected and evaluated. It is commonly used for the detection of the thick plates or key load-bearing components.

Magnetic particle flaw detection (MT): An effective method for detecting the surface and near-surface defects of the ferromagnetic steel plates. After the magnetization of the steel plate, the magnetic suspension is sprayed on its surface, and the leakage magnetic fields at the defects will attract the magnetic particles to form the magnetic indications. This method has the high sensitivity to the linear defects such as the surface cracks and folds, with the relatively simple operation.

Brinell hardness indentation method: In accordance with GB/T 231.1, a cemented carbide ball indenter of a certain diameter is used to press into the specimen surface under the specified test force, maintained for the specified time and then the test force is removed, and the indentation diameter is measured. The Brinell hardness value is obtained through the calculation or table lookup. This method has the large indentation areas and good result representativeness, and is less affected by the local structure non-uniformity.

Testing Standards

Domestic Standards:

1. GB/T 11253, Cold-rolled carbon structural steel sheets and strips

2. GB/T 4336, Carbon and low-alloy steel — Determination of multi-element contents — Spark discharge atomic emission spectrometric method

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

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

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

6. GB/T 10561-2023, Steel — Determination of content of nonmetallic inclusions — Micrographic method using standards diagrams

7. GB/T 232-2010, Metallic materials — Bend test method

International Standards:

1. JIS G 3131, Hot-rolled low carbon steel sheets and strips

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

3. 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 spectrometer: Used for the fast and precise analysis of the chemical composition. The modern spectrometers are usually equipped with the argon-purged spark stands, CCD or CID detectors and professional software, which can complete the quantitative analysis of dozens of elements such as the C, Si, Mn, P and S within tens of seconds, and automatically correct the matrix effects. It is the core equipment of the laboratories of the steel enterprises and large container manufacturers.

Microcomputer-controlled electronic universal testing machine: The key equipment for carrying out the mechanical property tests such as the tensile and bending. This equipment precisely controls the loading through the servo motors or hydraulic systems, measures the force and displacement with the high-precision sensors, and the computer system automatically collects the data and generates the reports. The testing process fully conforms to the standard requirements, with the reliable data and good repeatability.

Fully automatic impact testing machine: Used for the material impact toughness testing. The equipment integrates the functions of the automatic sample feeding, positioning, impacting and specimen recovery, and is equipped with the low-temperature devices (such as the compressor refrigeration or liquid nitrogen refrigeration) to precisely control the testing temperatures. The digital display system directly reads the impact energy, and the data can be managed through the software, improving the testing efficiency and accuracy.

Metallographic microscope system: Composed of the inverted or upright optical microscopes, high-resolution cameras, image analysis software and automatic stages. It is used to observe and analyse the microstructures, grain sizes and non-metallic inclusions of the steel plate. The software can carry out the quantitative ratings (such as the inclusion ratings in GB/T 10561), providing the micro basis for the process improvement.

Ultrasonic flaw detector: The portable or large-scale automated UT equipment is the main force for detecting the internal defects of the steel plate. The equipment generates the electrical pulses through the pulse generators to excite the probes to produce the ultrasonic waves, and receives the echo signals to display the A-scan waveforms on the screen. The advanced equipment has the B-scan and C-scan imaging functions, which can more intuitively display the defect shapes and distributions.

Coating thickness gauge and adhesion tester: Used to evaluate the surface coating quality of the steel plate. The magnetic or eddy current thickness gauges can non-destructively measure the dry film thickness; the cross-cut testers or pull-off adhesion testers are used to quantitatively evaluate the bonding strength between the coating and the steel plate substrate, ensuring that the anti-corrosion coatings achieve the designed protection effects.