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GB/T 24174-2022 English PDF (GBT24174-2022)

GB/T 24174-2022 English PDF (GBT24174-2022)

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GB/T 24174-2022: Steel -- Determination of Bake-Hardening-Index (BH)

This Standard specifies the principle, test equipment, specimens, test methods, calculation of test results and test reports for the determination of the Bake-Hardening-Index (BH) of steel. This Standard applies to the determination of the Bake-Hardening-Index of steels with bake hardening phenomena.
GB/T 24174-2022
NATIONAL STANDARD OF THE
PEOPLE REPUBLIC OF CHINA
ICS 77.040.10
CCS H 22
Replacing GB/T 24174-2009
Steel - Determination of Bake-Hardening-Index (BH)
ISSUED ON: APRIL 15, 2022
IMPLEMENTED ON: NOVEMBER 01, 2022
Issued by: State Administration for Market Regulation;
Standardization Administration of the People's Republic of China.
Table of Contents
Foreword ... 3
1 Scope ... 5
2 Normative references ... 5
3 Terms and definitions ... 5
4 Symbols and descriptions ... 5
5 Principle ... 6
6 Test equipment ... 7
7 Specimens ... 7
8 Test methods ... 7
9 Calculation of test results ... 9
10 Test report ... 13
Steel - Determination of Bake-Hardening-Index (BH)
1 Scope
This Standard specifies the principle, test equipment, specimens, test methods, calculation of test results and test reports for the determination of the Bake-Hardening- Index (BH) of steel.
This Standard applies to the determination of the Bake-Hardening-Index of steels with bake hardening phenomena.
NOTE: This Standard is mainly used in the automotive industry.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. GB/T 228.1-2021, Metallic materials - Tensile testing - Part 1: Method of test at room temperature
GB/T 8170, Rules of rounding off for numerical values and expression and judgement of limiting values
3 Terms and definitions
For the purposes of this document, the terms and definitions defined in GB/T 228.1- 2021 as well as the followings apply.
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.
4 Symbols and descriptions
The symbols given in Table 1 apply to this document.
6 Test equipment
6.1 The accuracy requirements of the tensile testing machine and extensometer are in accordance with the provisions of GB/T 228.1-2021.
6.2 The temperature control accuracy of the heating device is ??2??C. The resolution of the temperature measuring device shall not exceed 1??C.
7 Specimens
The specimen shall use the P5, P6 and P17 lead specimens in Annex B of GB/T 228.1- 2021. When the product standard or contract does not specify the specimen type number, the P17 specimen shall be used.
8 Test methods
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??? ~ 35???. 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

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