ICS 77.140.01
CCS H 40
National Standard of the People’s Republic of China
GB/T 8731-2025
Replaces GB/T 8731-2008
Free-cutting structural steels
易切削结构钢
(English translation)
Issue date: 2025-10-31 Implementation date: 2026-05-01
Issued by the State Administration for Market Regulation of the People's Republic of China
the Standardization Administration of the People's Republic of China
Free-cutting structural steels
1 Scope
This document specifies the designation method, order information, manufacturing processes, technical requirements, test methods, inspection rules, as well as packaging, marking and certification for free-cutting structural steels.
This document applies to free-cutting structural steel bars, wire rods, steel wires, steel plates and steel strips (hereinafter referred to as steel) for machining. The chemical components are also applicable to steel ingots, billets and their products.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of this document. For dated references, only the edition cited applies. For undated references, the latest edition (including any amendments) applies.
GB/T 222 Permissible tolerances for chemical composition of steel products
GB/T 223 (all parts) Methods for chemical analysis of iron, steel and alloy
GB/T 224 Determination of the depth of decarburization of steels
GB/T 226 Test method for macrostructure and defect of steel by etching
GB/T 228.1 Metallic materials - Tensile testing - Part 1: Method of test at room temperature
GB/T 229 Metallic materials - Charpy pendulum impact test method
GB/T 231.1 Metallic materials - Brinell hardness test - Part 1 : Test method
GB/T 247 General rule of package, mark and certification for steel plates (sheets) and strips
GB/T 342-2017 Dimension,shape,mass and tolerance of cold-drawn round,square and hexagonal steel wires
GB/T 702-2017 Hot-rolled steel bars - Dimensions,shape,weight and tolerances
GB/T 905-1994 Dimension, shape, weight and tolerance for cold-drawn round, square and hexagonal steels
GB/T 908-2019 Forged bars for dimensions, shape, weight and tolerances
GB/T 1979 Standard diagrams for macrostructure and defect of structural steels
GB/T 2101 General requirement of acceptance, packaging, marking and certification for section steel
GB/T 2103 General requirements for acceptance, packing, marking and quality certification of steel wire
GB/T 2975 Steel and steel products - Location and preparation of samples and test pieces for mechanical testing
GB/T 3207 Bright steel
GB/T 4162 Method for ultrasonic testing of forged and rolled steel bars
GB/T 4336 Carbon and low-alloy steel - Determination of multi-element contents - Spark discharge atomic emission spectrometric method (routine method)
GB/T 6402 Method for ultrasonic testing of steel forgings
GB/T 8170 Rules of rounding off for numerical values & expression and judgement of limiting values
GB/T 10561 Determination of content of nonmetallic inclusions in steel - Micrographic method using standard diagrams
GB/T 13298 Inspection methods of microstructure for metals
GB/T 13299 Determination of free cementite,pearlite and Widmanstaten structure in steel
GB/T 14981-2009 Dimension shape mass and tolerance for hot-rolled round wire rod
GB/T 15711 Inspection of non-metallic inclusions of steel - Etching test method of tower samples
GB/T 17505 Steel and steel products - General technical delivery requirements
GB/T 20066 Steel and iron - Sampling and preparation of samples for the determination of chemical composition
GB/T 20123 Steel and iron-Determination of total carbon and sulfur content Infrared absorption method after combustion in an induction furnace (routine method)
GB/T 20125 Low-alloy steel - Determination of multi-element contents - Inductively coupled plasma atomic emission spectrometric method
GB/T 20126 Unalloyed steel - Determination of low carbon content - Part 2: Infrared absorption method after combustion in an induction furnace (with preheating)
GB/T 22368 Low-alloy steel - Determination of multi-element contents - Glow discharge optical emission spectrometry（Routine method）
GB/T 28300 Surface quality classes for hot-rolled bars and rods technical delivery conditions
GB/T 34474.1 Determination of banded structure of steel - Part 1 : Micrographic method using standards diagrams
3 Terms and definitions
For the purposes of this document, the terms and definitions given in GB/T 20566 and the following apply.
3.1
free-cutting structural steel
structural steel with good machinability due to the addition of one or more high contents of sulfur, lead, tin, calcium, bismuth and other free-cutting elements
4 Designation methods
The designation of free-cutting structural steel consists of the following three parts:
a) part 1: the symbol "Y" for free-cutting structural steel;
b) part 2: the average carbon content is expressed in two Arabic numerals (in tens of thousands);
c) part 3: material series represented by symbols of free-cutting elements such as sulfur, lead, tin, calcium and bismuth: for sulfur-based free-cutting steel (see Example 1), if the average sulfur content (in tens of thousands) is less than 0.20%, the symbol S shall be omitted, indicating a lower sulfur content (see Example 2); for lead, tin, calcium and bismuth-based free-cutting structural steel, if it is added with sulfur, only the symbols of free-cutting elements Pb, Sn, Ca and Bi shall be retained, and the symbol S shall be omitted (see Examples 3 and 5). In addition, if the average manganese content (in tens of thousands) is not less than 1.0%, Mn shall be marked before the symbol of free-cutting elements, indicating a higher manganese content (see Examples 4 and 5).
Example 1:
Example 2:
Example 3:
Example 4:
Example 5:
5 Order contents
Order contracts made based on this document shall cover the followings:
a) Product name;
b) Number of this standard;
c) Designation;
d) Nominal dimension;
e) Dimension, shape and accuracy grade;
f) Delivery weight and quantity;
g) Processing method used;
h) Delivery state;
i) Other special requirements.
6 Manufacturing processes
6.1 Smelting method
Unless otherwise specified by the purchaser, the smelting method shall be selected by the supplier.
6.2 Processing method
Unless otherwise specified by the purchaser, the processing method shall be selected by the supplier.
6.3 Delivery state
Steel products shall be delivered in hot rolling, hot forging, cold rolling, cold drawing or silvery bright state. The specific delivery state shall be indicated in the contract.
7 Technical requirements
7.1 Designations and chemical components
7.1.1 The designation and chemical components (melting analysis) of steel shall meet those specified in Table 1 to Table 5.
GB/T 8731-2025 Free-cutting structural steels English
