Test Items

Hardness

Superficial Rockwell hardness HR30T is used to express the hardness of tinplate, which is used to assess the temper grade of tinplate. Material properties of tinplate are categorized on the basis of temper grades.

1. Principles for Superficial Rockwell Hardness Test

The superficial Rockwell hardness test method consists of indenting the test sample with an indenter (a diamond cone or steel ball) under a preliminary load P0 and a total load P (P0+P1). Then, remove the major load P1, and calculate the superficial Rockwell hardness value with the remaining depth of penetration t (mm) that is measured under the preliminary load.

2. Precautions

(1) If you have any doubt about the hardness value, remove the tin coating and retest the hardness.

(2) If the surface finish is matte surface, always use a 600# sand paper to polish it before measuring the hardness.

(3) Generally, the distance between two adjacent centers of indentation or the distance from any center of indentation to the edge of the test sample should be no less than 3mm.

(4) Normally, three points of a test sample are tested. When the difference of the measured values of the three points (maximum value – minimum value) is more than 1.0, two more points need to be tested. Then, remove the maximum and minimum values of the five points, and calculate the average value of the remaining three points.

(5) In principle, the steel ball indenter should be changed every month.

(6) If the hardness value of aged tinplate needs to be measured, have the test sample under aging treatment under the condition of 220℃×30 minutes, and then measure the hardness value.

(7) For tinplates of the same temper grade and the same chemical composition, the measurement of the hardness value is affected by tinplate thickness. Normally, a thickness difference of 0.10mm results in a difference of ±1.7HR30T (with tinplate thickness of 0.25mm as the benchmark).

Example: Design temper grade T3 (control HR30T = 57 ± 5), and hardness range of tinplate that is 0.32 thick is (57 ± 5) + (0.25-0.32) × 1.7 ÷ 0.10 = 50.9 ~ 60.9HR30T

Tensile Test, r Value

1. Tensile testing machine: model number: AG-E, 10KN; manufacturer: Shimadzu (Japan)

2. Tensile testing machine composition

(1) Host framework (2) Control cabinet (3) Length extensometer (4) Extensometer D/A converter and controller (5) Calculator (6) Recorder

3. Sample Preparation

Use the shear to cut the sheet strip into samples of 220×33 mm, and send them to the mill to produce JIS 5# test samples, which are 220 mm long, 33mm wide in the holding part and 25mm wide in the parallel part and 25mm long in the parallel part with the gauge length at 50mm and R 25mm.

4. Tensile Test Performance Parameters

(1) Yield strength: The strength of Yp at the yield point is the yield strength.

When Y0.2 has no yield point, the strength of 0.2% extension should be used as the yield strength.

(2)Tensile strength: The ratio of the maximum tensile force (instantaneous maximum force) (Fm) to the original cross-sectional area (S0), expressed as TS = Fm / S0

(3) Elongation: EL=△L/L0×100%

(4) Yield point elongation: EL –Yp=(L2-L1)/L0×100%

(5) Plastic strain range: r=ln(W0/W)/ln(W×L/W0×L0)

r value indicates the ironability and deep drawablity of the material.

Due to the surface anisotropy of cold-rolled plates, r values obtained in different aspects are different. To properly assess and determine the r value of tinplate, use r to evaluate the deep drawability of strip steel, and use △r to express the earing effect of strip steel.

r＝（rL+rc+2r45）/4

△ r=（rL+rc-2r45）/2

rL, rc, and r45 indicate r values in the lengthwise direction, the widthwise direction, and the 45O direction of the test sample.

Fluting

For tinplates used for manufacturing can bodies, including T-2.5 and T-3, the fluting test needs to be conducted to access the fluting level of the tinplate. The samples for the fluting test must be subject to aging treatment under the conditions of 220±2℃×30min, as three-piece cans are usually painted and dried before being machined into cylinders, which is in effect aging treatment.

Tester: three-roll forming machine manufactured by Baoshu Design Studio (Japan)

Test sample: L85 × C120 mm samples for the side and middle parts.

Machining requirement: After molding, the overlap of the cylinder must be controlled within ± 10 mm.

Grade rating: G1, G2, G3, G4, G5, G6 (with photos and reference samples for rating)

Grade Description:

G1 indicates no fluting

G2 indicates no obvious fluting

G3 indicates identifiable fluting

G4 indicates relatively obvious fluting

G5 indicates obvious and conspicuous fluting

G6 indicates obvious and conspicuous fluting with small fluting marks

Assessment criteria: ≤G3

Cupping

Test principle: Use a punch with a spherical end against the test piece clamped between upper and lower dies until the test piece shows signs of cracks (not through). The depth of the indentation is the Erishen cupping index, expressed in mm. 39 Corporate Standard and internal control standards do not require the performance of the cupping test. The Erishen cupping index is only measured upon users' request.

Method A is adopted for the cupping test for tinplates, and the determination of the terminal point for tinplates is different from that for cold-rolled plates. In a cupping test for cold-rolled plates, one should use a punch against the test piece until a through crack appears, while in a cupping test for tinplates, one should use a punch to push upon the test piece until it shows signs of non-through cracks.

Cupping tester: Model number: SAS-120D; manufacturer: JT

Test sample: L90 ± 2 mm × plate width; two testing points on the edge and in the center are measured, with the distance in between at more than 90 mm and the distance from the edge at more than 45 mm.

Surface lubricant: Vaseline or graphite grease.

Earing Value (Earing Rate and Earing)

The earing value is used to assess the anisotropy of strip steel and is in certain correspondence with the earing parameter △ r. Normally, for tinplates used for manufacturing DI cans, the earing rate is required to be no more than 3%.

1. Test sample: 57.3mm; measure three points D, C, and W, and calculate the average value; with both sides of the test piece covered with lubricant: Vaseline.

2. Equipment: The Erichsen tester; the earing test shares the same equipment as the cupping test. While a punch with a spherical end is used for the cupping test, a cup-shaped punch is used for the earing test (33 mm). The test piece of 57.3mm is punched into a deep-drawn cup of 33 mm, the deep drawing rate of which is 1.7 times.

3. Test principle: Use a cup-shaped punch of 33 mm to push out the round test piece of 57.3mm clamped between upper and lower dies at a certain speed to form a deep drawn cup with the diameter of the round base at 33 mm. The cup normally has four valley heights and four hill heights. However, for samples used for testing the earing value, there are six valley heights and six hill heights. Use a measuring instrument to measure and calculate the average hill height, average valley height and average height of the deep drawn cup to determine the earing value.

Surface Roughness

Different surface finishes of tinplates result in different surface roughness values. Different surface roughness values are generated from the temper rolling process of the CAPL line when different temper rolls are used. Use the surface roughness tester to measure the surface roughness.

1. Surface roughness tester: Model number: SARFCON 120A; manufacturer: ToKYo SEIMITSU Co. (Japan)

2. Test sample: L70 × C50 mm; measure three points, namely, D, C, and W, and calculate their average value. For equal coatings, measure either side. For differential coatings, measure the light-coated side. During measurement, drive the probe for 4mm at the speed of 0.3mm/s in the direction of the plate width.

3. Measuring principle: Move the probe of the roughness tester on the surface of the test piece. Depending on different surface shapes, the point of the probe needle will move up and down, and the movement will be converted into electrical signals through the sensor and magnified; cross-section curve and roughness curve of the test piece. The straight-angle curve is transmitted to the recorder; after calculation of the signals sent to the recorder, the average roughness of the center line (umRa) will be shown on the display of the instrument.

Magnetic Particle Testing (To Detect Surface or Internal Defects)

Test principle: Use a test sample of L500 mm × plate width, and make its rolling direction perpendicular to the direction of the magnetic line of force. Connect the magnetizing current of 15A, and spread the magnetic solution on the sample for 30 minutes. Then, observe the strip-like magnetic power concentration area that is in line with the rolling direction, which indicates surface or internal defects. Record the sizes, number and positions of defects. If it's required to identify the root cause of any internal defect, use a stencil pen to mark places whether defects have occurred. Then, prepare the sample and observe the cross section with a metallographic microscope.

Metallographic Examination

1. Crystallite Size

The crystallite size determination is used to identify the temper grade and annealing process of a tinplate.

The crystallite size test for tinplates is a process where a cross section in the rolling direction is used for observation. After cutting, grinding, polishing, crystallite boundary corrosion of the tested cross section, observe the crystallite size under the microscope with a magnification of 200 times and compare it with the standard photograph to determine the exact crystallite size.

2. Inclusions

DI cans made of tinplates are only 0.1mm in terms of wall thickness, so steel base has to be of high purity to prevent cracks on can walls or flange cracks. Therefore, magnetic particle inspection must be conducted on tinplates for DI cans so that internal defects can be identified and the cross-section test sample can be prepared. Then, use a microscope with a magnification of 100 times to observe inclusions and compare them with the standard photograph to determine the level of inclusions.