GB/T 24180-2020Cold-reduced electrolytic chromium/chromium oxide coated steel sheet and strip (English PDF)

6.2 Permissible Deviations for Dimensions

6.2.1 Thickness

6.2.1.1 The thickness of steel sheet and strip shall satisfy the following requirements:

a) The deviation of thickness measured at the centerline position in the width direction from the nominal thickness shall not exceed ± 5% of the nominal thickness.

b) The deviation of thickness measured at a position not less than 6 mm from the side in the width direction from the nominal thickness is -8% ~ +5% of the nominal thickness.

c) When the quantity of delivery is greater than 10,000 pieces of steel sheet (or steel coils of equal length), the permissible deviation between the average thickness of all steel sheets and the nominal thickness shall not exceed ± 2% of the nominal thickness.

6.2.1.2 The measurement of thickness is carried out with a micrometer with an accuracy of 0.001 mm. The measurement result shall be rounded off to 0.001 mm. The thickness measurement position is any point not less than 6 mm from the side in the width direction.

6.2.2 Thin edge

6.2.2.1 Thin edge is the thickness change of steel sheet and strip along the width direction. It is characterized by thickness reduction occurring near the edge of steel sheet and strip. The deviation between the thickness measured at 6 mm from both sides of the width direction of the steel sheet and strip and the actual thickness measured along the centerline of the width direction of the steel sheet and strip shall not exceed 6.0% of the actual thickness measured at the centerline position.

6.2.2.2 In terms of the measurement of thin edge, use a micrometer to measure 3 points on the same straight line perpendicular to the rolling direction. The 3 measured points are located along the straight line and respectively at the center of the width and 6 mm from the two rolling width cutting edges.

6.2.3 Width

A.1 Principle

Firstly, measure the thickness of the rectangular test piece. Make a 180° bend around the cylindrical mandrel, then, loosen it to measure the rebound angle. It provides a simple and convenient method for the evaluation of the yield strength of secondary cold-reduced sheet.

A.2 Sample

From each piece of sample steel sheet of secondary cold-reduced electrolytic chromium coated sheet, along the rolling direction at the edge and in the middle, take two 200 mm x 25 mm samples. The distance between the edge sample and the edge of the steel sheet is not less than 25 mm.

A.3 Test

A.3.1 Test instrument

Rebound tester.

A.3.2 Test Procedures

A.3.2.1 Measure the thickness of the sample, accurate to 0.001 mm.

A.3.2.2 Insert the sample into the rebound tester. Use moderate pressure to tighten the clamping screw; fix the sample in the test position.

A.3.2.3 Stably swing the builder arm, so that the sample is bent 180° around the axis.

A.3.2.4 Return the builder arm to the starting position. Along the sample, directly observe, read and record the rebound angle. Then, remove the sample.

A.3.2.5 In accordance with the measured value of the sample thickness and the rebound angle, on the nomogram matched with the rebound tester, measure the rebound index (SBI).

A.3.2.6 In order to ensure the accuracy of the test results, a standard sample or another reference rebound tester shall be used to calibrate the rebound tester being used.

A.4 Test Result Report

B.4.1.2

B.4.1.2.8 NaNO2 solution (2 g/L): weigh-take 0.2 g of analytically pure sodium nitrite; dissolve it in 100 mL of deionized water.

B.4.1.2.9 Urea solution (200 g/L): weigh-take 20 g of analytically pure urea; use deionized water to dissolve it and reach a constant volume of 100 mL.

B.4.1.2.10 Diphenylcarbazide solution (2.5 g/L): weigh-take 0.25 g of diphenylcarbazide; dissolve it in 100 mL of analytically pure acetone. This reagent should be prepared for immediate use.

B.4.1.2.11 Hexavalent chromium standard solution (1,000 mg/L): a commercially available national standard solution.

B.4.1.2.12 Hexavalent chromium standard solution (10 mg/L): use a transfer pipette to accurately dispense 10.0 mL of hexavalent chromium standard solution (1,000 mg/L); reach a constant volume in a 1,000 mL volumetric flask. […]

B.4.1.3 Operating procedures

B.4.1.3.1 Add 25 mL of NaOH solution (40 g/L) to a conical flask. Add 1 drop of phenolphthalein (1%) indicator; use H2SO4 solution (1+3) to titrate it, until the red color of the indicator just fades. Then, record the volume of the consumed sulfuric acid VH2SO4 (mL).

B.4.1.3.2 Take an appropriate amount of NaOH solution (300 g/L); on an electric heating plate or electric furnace, heat it up to 90 °C. Then, put the sample into it. Maintain it immersed and heated for 5 min ~ 10 min. Dissolve it to remove the chromium oxide coating on the surface of the sample.

B.4.1.3.3 Take out the sample which has the chromium oxide coating peeled off. Put it into the electrolysis device as shown in Figure B.1. Take the sample as an anode. Add 25 mL of NaOH solution (40 g/L) as the electrolytic solution. At the current density of 0.4 mA/cm2 ~ 2.5 mA/cm2, carry out electrolysis at room temperature. When small bubbles emerge from the sample, stop the electrolysis.

B.4.1.3.4 Transfer all the electrolyte to a 100 mL volumetric flask. Rinse the sample, the electrode and electrolytic cell. Incorporate all the rinsing fluid into the volumetric flask. Add the H2SO4 solution (1+3) VH2SO4 (mL) obtained through the test in B.4.1.3.1. Use deionized water to reach a constant volume to the scale, then, shake it well.

NOTE: since alkali will corrode glass container, and the dispensed test solution consumes about 0.5 mL of H2SO4 solution (1+3), it is quite difficult to adjust the acidity. Meanwhile, neutral solution makes it easy for dispensing and rinsing.

B.4.1.3.5 From the above-mentioned constant-volume electrolyte, dispense 20.0 mL […]

B.4.3 X-ray fluorescence method

B.4.3.1 Instrument

X-ray fluorescence instrument that complies with the requirements of GB/T 16597.

B.4.3.2 Instrument conditions

The target material of the X-ray tube shall be made of materials suitable for the determination of metal chromium. The setting of tube voltage and tube current shall consider the lowest excitation voltage of X-ray fluorescence and the counting loss of the determination. The selection of slits, spectroscopic crystals and detectors shall consider the transition energy of the X-ray fluorescence being determined and the measurement range of chromium element.

B.4.3.3 Operating procedures

B.4.3.3.1 Take an appropriate amount of NaOH solution (300 g/L); on an electric heating plate or electric furnace, heat it up to 90 °C. Then, put the sample into it. Maintain it immersed and heated for 5 min ~ 10 min. Dissolve it to remove the chromium oxide coating on the surface of the sample.

B.4.3.3.2 Correctly place the above-mentioned sample in the sample box of the X-ray fluorescence instrument. Use the set instrument conditions to irradiate the test piece with X-ray fluorescence, so as to determine the X-ray fluorescence intensity of chromium.

B.4.3.3.3 Immerse the above-mentioned sample into H2SO4 solution (1+3) for 3 min ~ 5 min, so as to completely remove the metal chromium on the surface of the sample.

C.1 General Rules

This Appendix applies to the detection of the amount of chromium oxide coating on the surface of chromium coated sheet.

C.2 Outline

This Appendix includes the following three test methods:

--- Diphenylcarbazide spectrophotometric method: immerse the chromium-plated sample in a hot sodium hydroxide solution to dissolve the chromium oxide coating on the lower surface. After oxidizing the chromium in the solution to hexavalent chromium, use diphenylcarbazide for chromogenic reaction. Use a spectrophotometer to determine the absorbance, so as to obtain the amount of chromium oxide coating of the sample.

---X-ray fluorescence method: firstly, determine the X-ray fluorescence intensity of chromium on the surface of the sample, then, immerse the chromium-plated sample in a hot hydroxide solution to remove the chromium oxide coating on the surface. Then, determine the X-ray fluorescence intensity of the sample.

Through the difference between the two intensities, calculate the amount of chromium oxide coating of the sample.

--- Electrolytic stripping method: in phosphate buffer solution, through the constant electrolysis current, oxide the sample to generate a potential jump. Measure the electrolysis time. Through the working curve, calculate the amount of chromium oxide.

C.3 Sample

The sample to be tested is square or circular. Specifically speaking, the stripping area of the diphenylcarbazide spectrophotometric method and the electrolytic stripping method is not less than 700 mm²; for the sample of the X-ray fluorescence method, the area irradiated by X-ray fluorescence is not less than 700 mm².

C.4 Test Methods

C.4.1 Diphenylcarbazide spectrophotometric method

C.4.1.1 Instruments

C.4.1.2

C.4.1.2.9 Diphenylcarbazide solution (2.5 g/L): weigh-take 0.25 g of diphenylcarbazide; dissolve it in 100 mL of analytically pure acetone. This reagent should be prepared for immediate use.

C.4.1.2.10 Hexavalent chromium standard solution (1,000 mg/L): a commercially available national standard solution.

C.4.1.2.11 Hexavalent chromium standard solution (10 mg/L): use a transfer pipette to accurately dispense 10.0 mL of hexavalent chromium standard solution (1,000 mg/L); reach a constant volume in a 1,000 mL volumetric flask. […]

C.4.1.3 Operating procedures

C.4.1.3.1 Add 15 mL of NaOH solution (300 g/L) to a conical flask. Add 1 drop of phenolphthalein (1%) indicator; use H2SO4 solution (1+3) to titrate it, until the red color of the indicator just fades. Then, record the volume of the consumed sulfuric acid VH2SO4 (mL).

C.4.1.3.2 In accordance with the mode shown in Figure C.1, properly install the sample. Add 15 mL of hot NaOH solution (300 g/L), then, put the device into a water bath kettle (at 90 °C) to heat it up for 5 min ~ 10 min, until the chromium oxide coating on the surface of the sample completely dissolves.

C.4.1.3.3 Transfer all the above-mentioned dissolved solution to a beaker. Add the H2SO4 solution (1+3) VH2SO4 (mL) obtained through the test in C.4.1.3.1. Use deionized water to reach a constant volume to the scale. Then, add 3 mL of sulfur-phosphorous mixed acid.

C.4.1.3.4 Heat up and boil the test solution. Then, add 2 mL of KMnO4 solution (5 g/L). Continue to boil it for 3 min ~ 4 min to oxidize all chromium to Cr6+. After the test solution cools down, add 10 mL of urea solution (200 g/L); shake it well. Then, while stirring it, dropwise add NaNO2 solution [firstly, dropwise add NaNO2 solution (20 g/L); wait till the color of the test solution changes from dark purple red to light purple red, then, change to dropwise addition of NaNO2 solution (2 g/L)], until the test solution becomes colorless and transparent.

C.4.1.3.5 Transfer the above-mentioned test solution to a 100 mL volumetric flask. Use deionized water to dilute it to about 80 mL. Thoroughly shake the test solution in the volumetric flask; drive off the gas in the test solution. Add 3 mL of diphenylcarbazide solution (2.5 g/L), shake it well, so that the chromogenic reaction of the test solution manifests light red. Then, use deionized water to reach a constant volume to the scale.

C.4.1.3.6 After the above-mentioned test solution is placed still for 2 min, use a spectrophotometer to measure the absorbance at 540 nm. From the calibration curve prepared on the spectrophotometer, measure the chromium content (mg/100 mL) of the test solution. […]

C.4.2

C.4.2.3.2 Take an appropriate amount of NaOH solution (300 g/L); on an electric heating plate or electric furnace, heat it up to 90 °C. Then, put the above-mentioned sample into it. Maintain it immersed and heated for 5 min ~ 10 min. Dissolve it to remove the chromium oxide coating on the surface of the sample.

C.4.2.3.3 Correctly place the above-mentioned sample in the sample box of the X-ray fluorescence instrument. Use the set instrument conditions to irradiate the test piece with X-ray fluorescence, so as to re-determine the X-ray fluorescence intensity of chromium.

C.4.2.3.4 Calculate the difference between the above-mentioned two X-ray fluorescence intensities of chromium. On the X-ray fluorescence calibration curve that has already been prepared, obtain the amount of chromium oxide coating of the sample. […]

C.4.2.4 Drawing of calibration curve

By measuring the X-ray fluorescence intensity of a chromium-plated sheet sample with a gradient distribution of already-known metal chromium content, draw a calibration curve. By periodically measuring the X-ray fluorescence intensity of the sample used for calibration, calibrate the curve.

C.4.3 Electrolytic stripping method

C.4.3.1 Electrolysis device and electrolysis conditions

C.4.3.1.1 The electrolysis device and circuit composition are shown in Figure C.2.

C.4.3.1.2 The electrolysis conditions are as follows:

a) Electrolyte: take 15.3 g of Na2HPO4-12H2O and 2.09 g of NaH2PO4-2H2O; dissolve it in 1,000 mL of distilled water or deionized water. The pH value of this solution shall be 6 ~ 8;

b) Electrolysis temperature: normal temperature;

c) Electrolysis constant current: 50 µA/cm2. […]