Grain-oriented Electrical Steel Plate Testing

Grain-oriented electrical steel plate inspection mainly conducts scientific testing and analysis on the core loss value determination, magnetic flux density determination, stacking factor measurement, coating performance testing, surface quality and dimensional precision inspection, and hardness and mechanical property testing, 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

Core loss value determination: The core loss is the core indicator for measuring the energy loss of the grain-oriented electrical steel plate in the alternating magnetic field, usually divided into the loss values under the different testing conditions such as P1.5/50 and P1.7/50. This testing aims to evaluate the sum of the hysteresis loss and eddy current loss of the material, directly related to the operating efficiency and temperature rise of the electromagnetic equipment such as the transformers. It is the key basis for judging the product designation grade.

Magnetic flux density determination: The magnetic flux density (such as B8 and B25) refers to the magnetic flux density reached by the material under the specific external magnetic field strengths. This testing is used to evaluate the magnetic permeability of the material. The high magnetic flux density means that a stronger magnetic field can be obtained under the same excitation conditions, thereby helping to realize the miniaturized and lightweight designs of the electromagnetic equipment.

Stacking factor measurement: The stacking factor refers to the ratio of the theoretical thickness to the actual thickness of the plates stacked under a certain pressure. This factor is measured to evaluate the thickness, flatness and interlayer insulation performance of the surface insulating coatings of the steel plate, directly affecting the effective cross-sectional area and filling efficiency of the iron core, and has the important influence on the compactness and performance of the transformer core.

Coating performance testing: Mainly detects the adhesion, insulation resistance, corrosion resistance, interlamination resistance and heat resistance of the inorganic or semi-organic insulating coatings on the steel plate surface. The coating performance directly affects the interlamination insulation, rust prevention ability and the stability during the annealing and use of the iron core, and is the necessary testing for ensuring the long-term reliable operation of the product.

Surface quality and dimensional precision inspection: Including the inspection of whether there are the defects such as the warpage, waviness, corrosion, holes and inclusions on the steel plate surface, as well as the dimensional tolerance testing of the thickness, width, length and same-plate difference. The surface defects and dimensional deviations will affect the punching, stacking processes and the magnetic performance uniformity of the final iron core, and are the routine quality control items in the production process.

Hardness and mechanical property testing: The hardness of the steel plate is measured with a Rockwell or Vickers hardness tester, evaluating its punching and shearing processing performance. Sometimes, the yield strength and tensile strength also need to be tested, to ensure that the material has the sufficient mechanical strength and formability during the subsequent shearing, punching, stacking and transportation processes.

Testing Scope

Conventional grain-oriented electrical steel (CGO): Mainly tests its routine core loss, magnetic flux density, stacking factor and coating performance, suitable for the iron core manufacturing of the small and medium power transformers and reactors without the extremely stringent efficiency requirements. The testing standards are usually based on GB/T 2521 and IEC 60404-8-7, etc.

High magnetic flux density grain-oriented electrical steel (Hi-B): In addition to the routine items, the magnetic flux density under the high magnetic fields (such as B8) and the core loss value under the low magnetic fields (such as P1.7/50) need to be emphatically focused on. This type of material is mainly applied to the high-performance large power transformers, and the testing requirements are more stringent, to verify its superior performances obtained through the technologies such as the magnetic domain refinement.

Laser-scored or plasma-treated oriented electrical steel: The testing emphasis is on the core loss reduction effect after the treatment (especially the loss under the high frequencies or high magnetic flux densities), the uniformity of the treated lines and the influences on the coating and substrate. This type of product refines the magnetic domains through the physical methods to further reduce the loss, and is the current key field of the high-end testing.

Products of the different thickness specifications: The testing scope covers the common thickness specifications such as 0.23 mm, 0.27 mm and 0.30 mm. The thickness directly affects the eddy current loss, so the core loss testing conditions and judgement standards of the products of the different thicknesses are different, and need to be strictly implemented according to the corresponding product standards.

Finished iron cores and punched laminations: The testing scope can extend to the iron cores or punched laminations processed from the oriented steel plates, and the testing items may include the overall no-load loss and noise of the iron cores, as well as the burr heights and dimensional consistency of the punched laminations, to evaluate the final performance of the material in the actual applications.

Comparative verification of the imported and domestic materials: During the material selection or localization substitution process, the testing scope covers the comprehensive performance comparison testing of the grain-oriented electrical steel plates of the different origins and manufacturers, to ensure that their performances meet the design requirements of the specific equipment.

Testing Methods

Epstein frame method: This is the internationally common standard method (such as GB/T 3655 and IEC 60404-2), used to measure the AC magnetic properties of the strip and sheet materials. The strip specimens are overlapped into a square closed magnetic circuit, the sinusoidal current is applied to the primary coil, and the core loss and magnetic flux density are calculated by measuring the induced voltage and power loss of the secondary coil. This method has the good reproducibility and is the main basis for the product grading and trade arbitration.

Single sheet testing method: A single sheet tester (such as the SST) is used to measure a single square specimen. This method avoids the influence of the overlap gaps in the Epstein frame method, and can better reflect the intrinsic magnetic properties of the material itself, especially suitable for the high magnetic flux density oriented steels and high-precision research and development testing. The standard is IEC 60404-3.

Stacking factor measurement method: In accordance with the standards such as GB/T 19289, a certain number of steel plate specimens are stacked, the specified pressure is applied, the total thickness is measured, and the stacking factor is calculated. Usually, the pressure is required to be about 1.0 MPa, and this method directly simulates the actual compression state of the iron core.

Coating performance testing method: The adhesion is commonly evaluated with the bending method or tape peeling method; the insulation resistance is measured with the volt-ampere method under the specific pressure; the corrosion resistance is evaluated through the salt spray testing or damp heat testing; the interlamination resistance is measured under the specific pressure and temperature. These methods mainly follow the standards such as GB/T 2522 and ASTM A976.

Dimension and surface testing method: The thickness is measured with the high-precision micrometers or laser thickness gauges; the plate shape (flatness) is measured with the surface plate and feeler gauge method or optical plate shape instruments; the surface defects mainly rely on the visual inspection, and a surface roughness instrument or microscope is used for the quantitative analysis of the specific defects when necessary.

Hardness testing method: The Rockwell hardness (HRB scale) or Vickers hardness (HV) is usually adopted for the measurement. During the testing, attention needs to be paid to selecting the flat areas, avoiding the edges and the areas with the excessively thick coatings, and taking the multi-point average values to ensure the representativeness of the results. The specific operations refer to GB/T 230.1 or GB/T 4340.1.

Testing Standards

Domestic Standards:

1. GB/T 2521, Cold-rolled grain-oriented and non-oriented electrical steel strip (sheet) — Magnetic, mechanical and physical properties

2. GB/T 3655, Methods of measurement of the magnetic properties of electrical steel strip and sheet by means of an Epstein frame

3. GB/T 19289, Methods of measurement of density, resistivity and stacking factor of electrical steel strip and sheet

4. GB/T 2522, Test methods for surface insulation resistance and coating adhesion of electrical steel strip and sheet

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

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

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

8. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

International Standards:

1. IEC 60404-2, Magnetic materials — Part 2: Methods of measurement of the magnetic properties of electrical steel strip and sheet by means of an Epstein frame

2. IEC 60404-3, Magnetic materials — Part 3: Methods of measurement of the magnetic properties of electrical steel strip and sheet by means of a single sheet tester

3. IEC 60404-8-7, Magnetic materials — Part 8-7: Specifications for individual materials — Grain-oriented electrical steel strip and sheet

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

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

Epstein frame testing system: This system is composed of a standard Epstein frame (usually 25 cm or 50 cm), high-precision power analyser (or core loss tester), programmable AC power supply, excitation current transformer and standard transformer, etc. The system needs to be calibrated at the power frequency (50/60 Hz) and the specified magnetic flux densities, ensuring that the accuracy of the core loss and magnetic flux density measurements is better than 1%.

Single sheet magnetic property testing system: The core is the single sheet testing fixture, whose magnetic yoke is made of the high-permeability materials, ensuring the uniform closure of the magnetic circuit. The system is equipped with a B-H analyser, power amplifier and temperature control device, and can carry out the automatic measurements in a wide frequency range (such as 50 Hz-1 kHz) and at the different magnetic flux densities, with the fast data acquisition speed and high automation degree.

Stacking factor measuring instrument: The dedicated equipment includes a precision pressure loading device (usually the hydraulic or mechanical type), high-precision displacement sensors (such as the grating rulers) and pressure sensors. The equipment can automatically apply and maintain the specified standard pressure, precisely measure the overall thickness of the stacked plate group, and directly calculate and display the stacking factor result.

Coating comprehensive performance tester: This type of equipment may integrate multiple functions, such as the insulation resistance testers (equipped with the standard electrodes and constant voltage sources), adhesion bending testing machines and salt spray test chambers. For the interlamination resistance, there are the dedicated interlamination resistance testers, which can simulate the operating conditions under the heating and pressurization for the measurement.

Precision dimension measuring tools: Including the electronic micrometers with the suitable ranges and resolutions up to 0.001 mm, large vernier callipers and laser scanning thickness gauges for the online or offline thickness detection; the plate shape instruments use the laser triangulation or image processing technologies to non-contact measure the waviness and warpage of the steel plate.

Hardness tester and metallographic microscope: The Rockwell hardness tester or Vickers hardness tester is used for the material hardness testing. The metallographic microscope is used to observe the grain sizes, orientation distributions (Goss texture) and coating micro-morphologies of the material, and is an important auxiliary equipment for analysing the intrinsic causes of the material performance. It often needs to be equipped with the image analysis software for the quantitative statistics.