GB/T 20887.6-2017Continuously hot-rolled high strength steel sheet and strip for automobiles - Part 6: Complex phase steel (English PDF)

1 Scope

China's national standard for hot-rolled complex phase steel sheet for automotive use. It is Part 6 of GB/T 20887 and specifies the classification and designation, the order content, the dimensions, shape and weight, the technical requirements, the test methods, the inspection rules, and the packaging, marking and quality certificate of hot-rolled high strength complex phase steel sheet and strip. Complex phase steel in hot-rolled form goes to the parts that carry load and take a beating: chassis arms, subframes, torsion beams, bumper reinforcements and wheel discs. What distinguishes it from the dual phase steel of the same tensile strength is its yield strength, which is far higher, and its edge behaviour, which is far better. Both matter in a hot-rolled part for the same reason: these parts are made by roll forming, bending and stretch flanging from thicker material, and they carry punched holes. A steel with a low yield ratio deforms early and springs back unpredictably, and a steel with poor hole expansion splits at the edge of a punched hole when that edge is flanged - the characteristic and expensive failure of high strength chassis parts. Complex phase steel avoids both because its microstructure is fine ferrite and bainite with only small amounts of martensite and retained austenite, strengthened by precipitation and grain refinement, so it is uniformly hard rather than being a soft matrix with hard islands. It also gives high energy absorption in a crash, since it resists from the first millimetre of deformation. The standard sets the strength grades and the mechanical requirements, including hole expansion, together with the composition, the tolerances and the surface classes. Issued on 7 September 2017 and in force since 1 June 2018.

This Part of GB/T 20887 specifies the classification and designation representing method, ordering content, dimensions, shape, weight, technical requirements, test methods, inspection rules, package, mark, and quality certificate of hot rolled high strength complex phase steel sheet and strip. This Part applies to the steel strip with a thickness of no more than 8 mm and steel plate cut transversely from steel strip and longitudinally-cut steel strip (hereafter known as "steel plate and strip" for short) for the production of automobile structural parts and reinforcements.

2 Normative references

The following documents are indispensable for the application of this document. For the dated references, only the versions with the dates indicated are applicable to this document. For the undated references, the latest version (including all the amendments) are applicable to this document.

GB/T 222 Permissible Tolerances for Chemical Composition of Steel Products

GB/T 223.5 Steel and Iron - Determination of Acid-Soluble Silicon and Total Silicon Content - Reduced Molybdosilicate Spectrophotometric Method

GB/T 223.9 Iron Steel and Alloy - Determination of Aluminum Content - Chrome Azurol S Photometric Method

GB/T 223.11 Iron, Steel and Alloy - Determination of Chromium Content - Visual Titration or Potentiometric Titration Method

GB/T 223.17 Methods for Chemical Analysis of Iron Steel and Alloy - The Diantipyrylmethane Photometric Method for the Determination of Titanium Content

GB/T 223.26 Iron, Steel and Alloy - Determination of Molybdenum Content - The Thiocyanate Spectrophotometric Method

GB/T 223.40 Iron, Steel and Alloy - Determination of Niobium Content by the Sulphochlorophenol S Spectrophotometric Method

GB/T 223.58 Methods for chemical analysis of iron, steel and alloy - The sodium arsenite-sodium nitrite titrimetric method for the determination of manganese content

GB/T 223.59 Iron, steel and alloy - Determination of phosphorus content - Bismuth phosphomolybdate blue spectrophotometric method and antimony phosphomolybdate blue spectrophotometric method

GB/T 223.60 Methods for chemical analysis of iron, steel and alloy - The perchloric acid dehydration gravimetric method for the determination of silicon content

GB/T 223.68 Methods for chemical analysis of iron, steel and alloy - The potassium iodate titration method after combustion in the pipe furnace for the determination of sulfur content

GB/T 223.69 Iron, steel and alloy - Determination of carbon contents - Gas- volumetric method after combustion in the pipe furnace

GB/T 223.75 Iron, steel and alloy - Determination of boron content - Methanol distillation-curcumin photometric method

3 Terms and definitions

The following terms and definitions are applicable to this document.

3.1 Complex phase steel; CP The microstructure of the steel is a small amount of martensite, retained austenite, or pearlite distributed on the ferrite or bainite matrix. Fine-grain strengthening or precipitation strengthening by microalloying elements. Compared with dual phase steels with the same tensile strength, it has higher yield strength and certain flanging hole expanding property.

6 Dimensions, shape, weight

The dimensions, shape, weight, and permissible tolerances of steel sheet and strip shall comply with the provisions of GB/T 709.

9 Inspection rules

9.1 The inspection and acceptance of steel sheet and strip shall be performed by the supplier's inspection department.

9.2 The steel sheet and strip shall be accepted in batches. Each batch shall consist of steel sheets and strips of the same designation, the same furnace number, the same thickness, and the same rolling mechanism with a weight of no more than 60 t. For the steel strip and continuously rolled sheet with roll weight greater than 30 t, each two rolls shall be regarded as one batch.

9.3 The sampling quantity and sampling methods for steel sheet and strip shall conform to the provisions of Table 3.

9.4 The re-inspection and judgement rules of steel sheet and strip shall conform to the provisions of GB/T 17505.

9.5 The test results of chemical composition and mechanical properties shall be rounded off according to the rounding off value comparison method. The rounding off rules shall be in accordance with the provisions of GB/T 8170.