GB/T 24174-2022Steel - Determination of the bake hardening index (English PDF)

3.1 Bake-Hardening-Index; BH

The increase in yield strength of the specimens pre-stretched with or without the specified amount of strain after baking at 170°C for 20min relative to the yield strength of the same or another specimen in the original state.

8.1 Test classification

According to the number of specimens used in the test, the test methods are divided into two categories.

a) Use a specimen to determine. According to the difference of determination methods, it is divided into BH2, BH2H, BH2L and BH2P. When the product standard or contract does not stipulate, BH2 shall be used.

b) Use two specimens to determine. Use BH0 to express.

8.2 Test conditions

8.2.1 Test temperature

Unless otherwise specified, the test shall be carried out at room temperature within the range of 10°C ~ 35°C. For tests with strict temperature requirements, the test temperature shall be 23°C ± 5°C.

8.2.2 Tensile test rate

8.2.2.1 To better maintain the consistency of test results, a uniform tensile test rate shall be used.

8.2.2.2 When measuring Rt2.0, Rp2.0, Rp0.2, Rp0.2,t, ReL and ReL,t, it is recommended to set the test rate according to the beam displacement rate of 5%Lc per minute. When measuring ReH,t, it is recommended to set the test rate according to the beam displacement rate of 2.5% Lc per minute. The test rate shall not be switched from the start of the stretch to the measurement of the above indicators.

8.2.3 Baking treatment

8.2.3.1 After the temperature of the heating device reaches 170°C, put the specimen to be baked. After the temperature of the heating device reaches 170°C again, keep the temperature for (20.0±0.5) min. Take out the specimen immediately. Place in air to cool to room temperature.

8.2.3.2 During baking, the specimens shall not be stacked. They shall be placed separately on the shelf.

8.2.3.3 The same tensile testing machine shall be used for the two tensile tests before and after baking.

8.3 Determination methods

8.3.1 Determination of BH2

Use a tensile test specimen. According to the provisions of GB/T 228.1-2021, the specimen is first subjected to pre-strain stretching with a total strain of 2.0%. Obtain Rt2.0. After the pre-tensile specimen is subjected to the prescribed baking treatment, the tensile test is performed on the sample again. Obtain ReL,t or Rp0.2,t (when there is no apparent yield).

8.3.2 Determination of BH2H

Use a tensile test specimen. In accordance with the provisions of GB/T 228.1-2021, the specimen is first subjected to pre-strain stretching with a total strain of 2.0%. Obtain Rt2.0. After the pre-tensile specimen is subjected to the prescribed baking treatment, the tensile test is performed on the specimen again. Obtain ReH,t or Rp0.2,t (when there is no apparent yield).

8.3.3 Determination of BH2L

Use a tensile test specimen. According to the provisions of GB/T 228.1-2021, the specimen is first subjected to pre-strain stretching with a total strain of 2.0%. Obtain Rp0.2. After the pre-tensile specimen is subjected to the prescribed baking treatment, the tensile test is performed on the specimen again. Obtain ReL,t or Rp0.2,t (when there is no apparent yield).

8.3.4 Determination of BH2P

Use a tensile test specimen. According to the provisions of GB/T 228.1-2021, the specimen is first subjected to pre-strain stretching with a total strain of 2.2%. Obtain Rp2.0. After the pre-tensile specimen is subjected to the prescribed baking treatment, the tensile test is performed on the specimen again. Obtain ReL,t or Rp0.2,t (when there is no apparent yield).

NOTE: For high strength steel, the total strain corresponding to Rp2.0 may be greater than 2.2%. At […]