ISO/DIS 4948-1

ISO/TC 17

Secretariat: JISC

Date: 2025-10-13

Steels — Classification —

Part 1:
Classification of steels based on chemical composition

DIS stage

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Contents

Foreword

Scope

Normative references

Terms and definitions

General

Classification by chemical composition

Non-alloy steel

Low-alloy steel

High-alloy steel

Special cases for the classification by chemical composition

Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

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This document was prepared by Technical Committee ISO/TC 17, Steel, WG 30, Revision of ISO 4948-1.

This second edition cancels and replaces the first edition (ISO 4948-1:1982), which has been technically revised.

The main changes are as follows:

• Main classes of steels based on chemical compositions are: a) non-alloy steels, b) low alloy steels and c) high alloy steels;
• Micro-alloy steels are for the first time included in this standard since during the last decades the class of micro-alloy steel grades has gained importance and it is assumed that it is now the class of steels with the greatest amounts produced worldwide.

A list of all parts in the ISO 4948 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html.

Steels — Classification —

Part 1:
Classification of steels based on chemical composition

This document specifies the definition and classification of steels. It defines the term "steel" (see clause 3) and it classifies the steel grades as main criteria by the chemical composition into non-alloy, low-alloy and high-alloy steels.

Micro-alloy steels are a subclass of low alloy steels and stainless steels are a subclass of high-alloy steels.

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements 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.

ISO 4885:2018, Ferrous materials — Heat treatments — Vocabulary

ISO 6306:2020, Chemical analysis of steel — Order of listing elements in steel standards

ISO 6929:2013, Steel products — Vocabulary

ISO 15510:2014, Stainless steels — Chemical composition

For the purposes of this document, the terms and definitions given in ISO 4885:2018, ISO 6929:2013, ISO 15510:2014 as well as the following apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp

steel

material which contains by mass more iron than any other single element, having a carbon content generally less than 2 % and containing other elements - a limited number of chromium steels may contain more than 2% of carbon, but 2 % is the usual dividing line between steel and cast iron

Note 1 to entry: The presence of large quantities of carbide-forming elements can modify the upper limit of the carbon content.

Note 2 to entry: This definition is similar to the definition 3.195 in ISO 4885:2018

Note 3 to entry: Other elements and substances may be existing or added into steel for different reasons: a) to improve the steel making process such as de-oxidation, castability and rolling; b) to remove detrimental elements resulting from raw materials such as phosphorus and sulphur; c) to improve the properties of steel products such as strength, toughness, hardenability, formability, weldability, corrosion and wear resistance

Note 4 to entry: If producing steel based on recycled raw materials, some residual elements are practically impossible to remove such as e.g.: Cu, Ni, Co.

Note 5 to entry: If the amount of alloying elements exceeds the iron content, the material is defined as e.g.: Ni-based alloy or Co-based alloy.

non-alloy steel

non-alloy steels are grades where no alloying elements are intentionally added. The use of additions aimed to deoxidation or to guarantee the castability or to guarantee other steel making processes and performances is allowed. Residual elements from raw material and/or process equipment can exist in non-alloy steels

Note 1 to entry: The limit values between non-alloy and low alloy steel are given in Table 1.

low-alloy steel

low-alloy steels are grades where alloying elements are intentionally added apart from additions for deoxidation, castability and/or for other steel making processes and performances, low alloy steels are grades with a content of any alloying element less than 5,0 % or with a total content of alloying elements of less than 8,0 %

Note 1 to entry: The limit values between non-alloy and low alloy steels are given in Table 1.

micro-alloy steel

micro-alloy steels contain small amounts of alloying elements (0,0008 to 0,25 %) including niobium, vanadium, titanium, molybdenum, zirconium, boron and lanthanides. They are used to improve the properties by refining the grain microstructure, to facilitate precipitation or improve hardenability in quench hardening

Note 1 to entry: Micro-alloy steels are part of the low alloy steels. The limit values are given in Table 2.

high-alloy steel

high-alloy steels are grades with a content of any alloying element equal to or greater than 5,0 % or with a total content of alloying elements of equal to or greater 8,0 %. For the upper limit the amount of one alloying element shall not exceed the iron content, otherwise the material is defined according to the main element, i.e. as a nickel or cobalt base alloy

Note 1 to entry: High-alloy steels include, for example, stainless steels, other corrosion and heat resistant steels and partly also tool and bearing steels.

stainless steel

high-alloy steels with at least 10,5 % (mass fraction) Cr and maximum 1,2 % (mass fraction) C

SOURCE: ISO 15510:2014, 3.1

Steels are divided into non-alloy, low-alloy and high-alloy steels.

Classification is based on the cast analysis specified in the product standard or specification and is determined by the minimum value specified for each element.

If there is a maximum value only specified as nominal content, a value of 70% of the max. content shall be taken (70% rule). For manganese and boron see also footnote a to Table 1.

NOTE For this document all the % values for the chemical composition are % by mass.

Non alloy steels are steel grades in which none of the limit values of elements in Table 1 is reached by the contents as defined in clause 4.

NOTE Non alloy steel grades are grades where no alloying elements are intentionally added. The use of additions aimed to deoxidation or to guarantee the castability or to guarantee other steel making processes and performances is allowed. Residual elements from raw material and/or process equipment can exist in non-alloy steels in the allowed limits of Table 1.

Table 1 — Boundary between non-alloy and low-alloy steels based on the cast analysis

EXAMPLE 1 ISO 683-1, C45

1. classification according to Table 1 last column:
2. max. Si = 0,40                                                                          < 0,60
3. max. Mn = 0,80                                                                       < 1,80
4. max. Cr = 0,40                                                                         < 0,43
5. max. Ni = 0,40                                                                         < 0,43
6. max. Mo = 0,10                                                                       < 0,12
7. max. Cu = 0,55                                                                        < 0,71 
8. none of the elements is above the limits in Table 1 => non-alloy steel grade
9. classification according to Table 1 second last column, 70% rule
10. max. Si = 0,40                  => 0,40 x 0,7 = 0,28                    < 0,60
11. max. Mn =0,80                 => 0,80 x 0,7 = 0,56                     < 1,65
12. max. Cr = 0,40                 => 0,40 x 0,7 = 0,28                      < 0,30
13. max. Ni = 0,40                 => 0,40 x 0,7 = 0,28                      < 0,30
14. max. Mo = 0,10               => 0,10 x 0,7 = 0,07                      < 0,08
15. max. Cu = 0,55                => 0,55 x 0,7 = 0,385                    < 0,50
16. none of the elements is above the limits in Table 1 => non-alloy steel grade

EXAMPLE 2 ISO 630-2, S355D

1. classification according to Table 1 last column:
2. max. Si = 0,55                                                                          < 0,60               
3. max. Mn =1,60                                                                        < 1,80              
4. max. Cu = 0,55                                                                        < 0,71               
5. none of the elements is above the limits in Table 1 => non-alloy steel grade
6. classification according to Table 1 second last column, 70% rule
7. max. Si = 0,55                   => 0,55 x 0,7 = 0,385                  < 0,40          
8. max. Mn =1,60                 => 1,60 x 0,7 = 1,12                    < 1,65          
9. max. Cu = 0,55                 => 0,55 x 0,7 = 0,385                  < 0,50           
10. none of the elements is above the limits in Table 1 => non-alloy steel grade

The alloy limits are specified as follows: For the lower alloy limit at least one element is equal to or greater than the limits given in Table 1. The upper alloy limit for any element is less than 5,0 % or totally less than 8,0 %.

Low-alloy steels are subdivided into micro-alloy steels and other low alloy steels (heat treatable steels).

Micro-alloy steels contain small amounts of alloying elements (each min. 0,0008 % to 0,25 %) including niobium, vanadium, titanium, zirconium, boron and lanthanides. They are used to refine the grain microstructure or to facilitate precipitation or increase hardenability in quench hardening. Micro-alloy steel grades are grades with at least one microalloying element (niobium, vanadium, titanium, zirconium, boron and lanthanides) with limit values for the elements given in Table 2.

These kinds of steels are often used for construction purposes and for cold forming of multiphase steels.

Table 2 — Boundary between micro-alloy and other low-alloy steels based on the cast analysis

EXAMPLE 1 ISO 630-3, S420ML

microalloying elements max. Nb=Ti=0,05% and max. V=0,12 are in the range of limit values of Table 2

further classification according to Table 2

max. Cr = 0,30                                                                          < 0,40

max. Ni = 0,80                                                                           < 0,90

max. Mo = 0,20                                                                          < 0,22

limit values for Cr, Ni, Mo were not exceeded

and limit values for the other elements (see Table 2, note 4) according to Table 1

max. Si = 0,50                                                                             < 0,60

max. Mn =1,70                                                                           < 1,80

max. Cu = 0,55                                                                            < 0,71

are not exceeded  => micro-alloy steel grade

EXAMPLE 2 ISO 4954-1, 33B2

microalloying element min. B=0,000 8 is in the range of limit value of Table 2

further classification according to Table 2

max. Cr = 0,30                                                                          < 0,40

limit values for Cr is not exceeded

and limit values for the other elements (see Table 2, note 4) according to Table 1

max. Si = 0,30                                                                             < 0,60

max. Mn =0,90                                                                           < 1,80

max. Cu = 0,25                                                                            < 0,71

are not exceeded  => micro-alloy steel grade

Other low-alloy steels are grades as defined in 5.2.1 and which do not belong to the micro-alloy steel grades.

These kinds of steels are mainly alloyed with Cr, Mo and/or Ni for better heat treatability. In addition, they are often alloyed with elements like Bi, Pb, Se, Te (e.g. for machinability reasons), Si (e.g. for spring steels), Mn, Al (e.g. for nitriding steels) and Co, Cu, W (for tool steels). They are used for transmission parts, high strength fasteners and pressure equipment.

EXAMPLE 1 ISO 683-3, 20Cr4

grade without microalloying elements (i.e.: niobium, vanadium, titanium, zirconium, boron and lanthanides less than the limits given in Table 1), further classification according to Table 1 min. Cr =0,90 > 0,30 is above the limit value in Table 1 => other low-alloy steel grade

EXAMPLE 2 ISO 630-4, S690QE

grade with microalloying elements max. B=0,005, max. Nb=0,06, max. Ti=0,05, max. V=0,12, max. Zr=0,15 are in the range of limit values of Table 2, further classification according to Table 2

max. Cr=1,50                                                                           > 0,40

max. Ni = 4,0                                                                           > 0,90

max. Mo = 0,70                                                                         > 0,22

Cr, Ni, Mo, are above the limit in Table 2 => other low-alloy steel grade

High alloy steels are steels as defined in 3.4. High alloy steels are stainless steels, other corrosion and heat resistant steels and partly also tool and bearing steels.

High-alloy steels are subdivided by mainly two principles: a) use properties: corrosion resistant and heat resistant steels; b) microstructure: austenitic, austenitic-ferritic (duplex), ferritic, martensitic and precipitation hardening steels.

This is the most important subgroup of high-alloy steels. Stainless steels are steels with at least 10,5 % (mass fraction) chromium and maximum 1,2 % (mass faction) carbon [Source: ISO 15510, Definition 3.1].

Other high-alloy steels are steels not complying with the definition of stainless steels.

EXAMPLE ISO 4995, X18CrMnMoNbVN12

min. Cr = 10,0 % >5,0/8,0 %

high-alloy steel, but not a stainless steel because Cr is less than 10,5 % min

In case where a product analysis is performed, e.g. because the standard or specification is based on product analysis, an equivalent cast analysis is calculated using the permitted deviations from cast analysis specified in the corresponding product standard or specification.

In the absence of a product standard or specification or a precisely specified chemical composition classification is based on the actual cast analysis reported by the manufacturer.

Composite or coated products are classified according to the specified chemical composition of the product, which has been coated or clad.

For each alloying element, the specified, calculated or actual cast analysis value is expressed according to ISO 6306:2020 and to the same number of decimal places as the corresponding limit value shown in Table 1. For example, in this document a specified range of 0,3 % to 0,5 % corresponds to a range of 0,30 % to 0,50 %. Similar, a specified content of 2 % is taken to mean a content of 2,00 %.