Although BS 970 is now obsolescent, it is still widely used but should not be quoted when specifying wrought steels for new designs and products. It has now been superceded by the BS EN 10 000 series of specifications which are English language versions and compatible with the Continental ISO specifications.

One advantage of BS 970: 1991 is that it changed the random numbering system of the earlier version and designates the steels by an alphanumeric code that indicates the actual composition of the material in a logical manner. The designation code of each steel is built up as follows:

(a) The first three symbols are a number code indicating the type of steel:

000 to 199 are carbon and manganese steels. The numbers represent the manganese content 100.

200 to 240 are free-cutting steels. The second and third numbers indicate the sulphur content 100.

250 are silicon manganese valve steels.

300 to 499 are stainless and heat-resisting steels.

500 to 999 are alloy steels.

(b) The fourth symbol is a letter code:

A The steel is supplied to a chemical composition determined by chemical analysis of a batch sample.

H The steel is supplied to a hardenability specification.

M The steel is supplied to a mechanical property specification.

S The material is a stainless steel.

(c) The fifth and sixth symbol comprise a number code indicating the actual carbon content 100

The following are examples of the BS 970: 1991 six-figure code:

070M20 A plain carbon steel with a composition of 0.7% manganese and 0.2% carbon. The M indicates that the steel has a prescribed mechanical property specification.

230M07 A low-carbon, free-cutting steel with a sulphur content of 0.3% and a carbon content of 0.7%.

080A15 A plain carbon steel with a manganese content of 0.8% and a carbon content of 0.15%. The A indicates that the steel must meet a prescribed chemical composition specification.

Unfortunately for alloy steels coded between 500 and 999 the logicality of the firstthree digits breaks down. In addition to the six-figure grading code, a condition code is applied. The code letter indicates the tensile strength range for a given steel after heat treatment. The final factor to be considered in the coding of wrought steels is the limiting ruling section. As explained in Section 6.1.9 this is the maximum diameter of bar of a given composition which, after heat treatment, will attain the specified mechanical properties.

For example, a plain carbon steel of composition 070M55(R) containing 0.7% manganese and 0.55% carbon can attain condition (R) after heat treatment providing it is not greater than 100-mm diameter, however if it is to attain condition (S) then its maximum diameter must not exceed 63 mm. In the first example the limiting ruling section is 100 mm and in the second example the limiting ruling section is 63 mm in diameter. This letter/strength relationship code is listed in Table below. The condition code is applied in brackets after the six-symbol code as shown above.

In addition to the standard chemical symbols used to denote the composition of a metal (e.g. C = carbon, M_n= manganese, P = phosphorus, S = sulphur, etc.), the symbols

Code letter/strength relationship

Condition code letter	Tensile strength (MPa)		Condition code letter	Tensile strength (MPa)
	min.	max.		min.	max.
P	550	700	V	1000	1150
Q	620	770	W	1080	1240
R	700	850	X*	1150	1300
S	770	930	Y	1240	1400
T	850	1000	Z	1540	–
U	930	1080

used in tables of mechanical properties are as follows:

Tensile properties

R_mTensile strength

R_eYield strength

A Percentage elongation after fracture

S_oOriginal cross-sectional area of the gauge length

R_p02 0.2% proof stress (non-proportional elongation)

R_p1.0 1.0% proof stress (non-proportional elongation)

R_t0.50.5% proof stress (total elongation)

R_t1.01.0% proof stress (total elongation)

R_eHUpper yield stress

Impact properties

KCV Charpy V-notch impact value

Hardness

H_BBrinell hardness

H_VVickers hardness

HRC Rockwell hardness (C scale)

Other

LRS the limiting ruling section.

Note that the specifications for the chemical composition and mechanical properties for plain carbon and alloy steels are now being superceded by the BS EN 10000 series of standards. For example:

• BS EN 10277-2 1999 deals with the composition and mechanical properties of plain carbon bright steel products.
• BS EN 10277-3 deals with the composition and mechanical properties of free-cutting steels.
• BS EN 10277-4 deals with the chemical composition and mechanical properties of case-hardening steels.
• BS EN 10277-5 deals with non-alloy and alloy steels suitable for quench hardening and tempering.

The steels are now designated by a steel name and a steel number. For example a plain carbon steel containing 0.13–0.18% carbon and 0.7–0.9% manganese would be designated 080A15 in BS 970 it is now designated: steel name C15 and steel number 1.0401 in BS EN 10277-2.

BS EN 10277 and related standards for steels are highly detailed and comprehensive These standards should be consulted together with BS Guide Lines PD 970:2005 when selecting a suitable steel for a new product design and manufacture.