Introduction to the principles of steel selection for steel structures

In this paper, we sort out the classification of structural steel by chemical composition, strength grade, and supply condition, list the steel grades commonly used in steel structures, focus on the performance characteristics of low-alloy high-strength structural steel, structural steel for buildings, structural steel for bridges, weathering structural steel, etc., and point out that structural steel should be selected according to the use of steel, and take into account the importance of the structure, static or dynamic load nature, service temperature, and corrosion environment. It is pointed out that the steel for steel structures should be selected according to the purpose of use, considering the structural importance, static or dynamic loading properties, service temperature, and corrosive environment.

0. Introduction

Steel structure projects are widely used in construction, such as large stadiums, large cultural facilities, transportation hubs, steel bridges, modern urban high-rise buildings, etc., characterized by large scale, high technical difficulty, and novel structure, all relying on a steel structure. China has built several world-renowned projects, marking the comprehensive strength of China’s steel structure construction at the world’s leading level. Controlling the quality of steel structure projects is the primary consideration of countless builders, and the selection and acceptance of steel for steel structures is an important step before production. The existing varieties on the market are steel for building structures, thickness direction steel, weathering steel, fire-resistant steel, hot-rolled or cold-formed profiles with excellent cross-sectional characteristics, etc. The specialized steel varieties produced by steel mills gradually meet the development requirements of the steel structure industry.
Steel structure engineering is divided into engineering design, processing and manufacturing, construction and installation and supervision and acceptance, and other important links, after designing drawings and processing and manufacturing before selecting steel raw materials.

1. Steel acceptance points for steel structure

When the steel for steel structure is delivered, the quality certificate or inspection report issued by the steel company should be checked, focusing on whether the chemical composition and mechanical properties of the steel meet the provisions of national standards and whether the steel material, type specification, and supply state should meet the design requirements of the project.
Special cases of steel such as steel mixed group batch; imports of steel; plate thickness equal to or greater than 40mm, and the design has Z to the performance requirements of the thick steel plate; building structural safety level of one, large span steel structure in the main stress members used in the steel; drawings design requirements for re-inspection of steel; there are doubts about the quality of steel. These cases should be sampled for re-inspection.
Steel mechanical properties, chemical composition, and other items should be rechecked; these important indicators directly affect the material bearing capacity, weldability, and the safety of welded joints important factor, is the steel structure welding construction enterprises as the development of welding process evaluation program, component welding production, on-site installation process parameters.
Welding electrodes, welding wire and flux, and other welding materials should be in line with the provisions of the engineering acceptance standards; welding filler materials are optional principles of welded joint strength, plasticity, and impact toughness is not less than the lower limit of the standard provisions of the steel parent material.
The new materials selected for the steel structure project should be agreed upon by the engineering design unit and issued a certificate. The new materials should be identified as new products by the government industrial department, with complete welded joint performance, welding process, thermal processing, and heat treatment process parameters for reference.

2. Principles for the selection of steel for steel structures

Domestic steel classification: First, according to the chemical composition, steel grades can be divided into carbon structural steel, low-alloy high-strength structural steel, high-quality carbon structural steel, structural steel for bridges, structural steel for buildings, weathering structural steel, welded structural castings, general engineering, and structural low-alloy steel castings. Second, according to the yield strength, steel can be divided into Ⅰ, Ⅱ, Ⅲ, Ⅳ four grades, commonly used steel categories, and nominal yield strength, see Table 1. Third, according to the supply state, steel can be divided into hot rolled steel, normalized steel, controlled rolling steel, controlled rolling and cooling (TMCP) steel, TMCP + tempered steel, quenched + tempered steel, quenched + self-tempered steel, etc. Any combination of the three classifications can meet different types of steel structures.

Table.1 Commonly used steel grades.

Type	Nominal yield strength	Corresponding national standards
I	≤295MPa	GB/T700、GB/T1591
Ⅱ	>295MPa-370MPa	GB/T699、GB/T714
Ⅲ	>370MPa-420MPa	GB/T19879、GB/T4171
V	≥420MPa	GB/T7659、GB/T14408

2.1 Commonly used steel grades for domestic steel structures

Commonly used steel grades can be found in GB/T50661 “Steel Structure Welding Code Standard” Examples of commonly used grades are shown in Table 2, with the standard constantly updated; some of the original standard grades have become obsolete or expression changes. For example, GB/T1591-2018 standard, implemented on February 1, 2019, there is no longer a Q295 grade in the standard; Q355 steel grade instead of a Q345 steel grade; the purpose is to align with international standards, mechanical properties in the test upper yield strength instead of the previous lower yield strength, the indicator correspondingly increased by 10MPa – 15MPa; the standard mainly involves yield strength of 460MPa and below, and for more than 460MPa steel grades need to be used after welding process evaluation.

Table.2 Examples of commonly used steel grades

National standard	Example of steel grade
GB/T700	Q195、Q215、Q235、Q275
GB/T1591	Q355、Q390、Q420.Q460 Q550、Q620、Q690
GB/T714	Q345q、Q370q Q420q、Q460q、Q500q 、Q550q
GB/T19879	Q235GJ、Q345GJ、Q390GJ、Q420GJ、Q460GJ、Q500GJ Q550GJ、Q620GJ、Q690GJ
GB/T41 71	Q235NH、Q295NHQ355NH、Q415NH、Q460NH、Q500NH

Assume that the yield strength of 345MPa grade, 20mm thick, is used in the steel plate’s northern region. Northern use, to consider low-temperature performance, no emphasis on the use of the field, you can first understand the market available steel:
(1) Low-alloy high-strength structural steel, you can choose GB/T1591-2018 standard Q355D, Q355ND, Q355MD, and other grades. The characteristics of such steel apply to general structures and engineering; need to specify whether the delivery state is hot-rolled, normalized, or thermomechanically rolled; to choose with thickness direction performance, need to choose the grade after the grade plus the performance level symbol representing the thickness direction (Z direction); have the equivalent carbon CEV, welding crack sensitivity index Pcm requirements.
(2) Building structural steel, can choose GB/T19879-2015 standard Q345GJD grade, the characteristics of such steel, suitable for the manufacture of high-rise building structures, large-span structures, and other important building structures; need to specify the delivery state is hot-rolled, controlled-rolled, normalized or thermomechanically controlled rolling; if you want to choose a thickness direction performance, need to choose after the grade plus a representative thickness direction (Z) performance level symbol (Z-direction) performance level symbol of the steel grade, there are thickness performance requirements, P, S content requirements are shown in Table 3, the P, S content requirements are high, some converter steel cannot reach this level, the need to use electric furnace steelmaking. Building structural steel plate and ordinary carbon structural steel, low-alloy high-strength structural steel comparison has its characteristics; the chemical composition is different, the same grade C, P, S content, tensile strength, impact work requirements more stringent, the carbon equivalent or welding crack susceptibility index requirements, and especially put forward the requirements of the flexural strength ratio.

Table.3 With thickness direction performance when the P, S content requirements

Performance level in the thickness direction	Phosphorus content (mass fraction) %	Sulfur content (mass fraction) (%)
Z15	≤0.02	≤0.01
Z25		≤0.007
Z35		≤0.005

(3) Structural steel for bridges, can choose GB/T714-2015 standard Q345qD grade, the characteristics of such steel have, suitable for bridge engineering; need to specify the delivery state is hot-rolled, normalized, hot mechanical rolling or tempered steel, heat treatment mode is very strict, normalized state delivery of steel, when the use of other media than in the air cooling rate faster than the cooling rate, should be high-temperature tempering (not less than 580 ℃) treatment, for the bare use of steel with atmospheric corrosion resistance, cannot be delivered in the normalized state, when cooled with a faster cooling rate than in the air, should be tempered; chemical composition testing of residual elements B, H content restrictions; need to clarify whether the steel plate with weathering (NH) performance and thickness direction (Z direction) performance; chemical composition control has Special requirements, such as D grade steel P ≤ 0.025%, S ≤ 0.020%, E grade steel P ≤ 0.020%, S ≤ 0.010%, very good to improve the stability of low temperature toughness in -20 ℃, -40 ℃ environment.
(4) Weathering structural steel, you can choose GB/T4171-2008 standard Q355DNH, Q355DGNH, and other grades; the characteristics of such steel are suitable for atmospheric corrosion resistance of construction projects and vehicles, containers, towers, and other structural components; need to specify the delivery state is hot-rolled, controlled-rolled or normalized state; steel by adding a small amount of alloying elements such as Cu, P, Cr, Ni, etc., to form a protective layer on the surface of the metal matrix to improve atmospheric corrosion resistance.

2.4 Principles of steel selection in steel structure engineering

2.4.1 Strictly according to the use of selection

To save energy, steel structure selection comply with the principle of “to meet the performance, to ensure safety,” combined with the characteristics of the industry, select the industry-specific use of steel. For important projects such as bridge projects, high-rise building structures, large-span structures, stadium building structures, structures in corrosion-resistant environments, etc., steel mills must choose special steel varieties for the industry.

2.4.2 Material selection taking into account other factors

Considering the importance of steel structures, different types can be divided into three categories: important, general and minor, and important structures with high strength grades of steel. China’s large temperature difference between the north and south, the temperature difference between the winter and summer seasons, the daily morning and evening temperature difference, the need to choose at least D grade and above, making full use of steel -20 ℃ and below the low-temperature toughness performance.
Steel bridge manufacturing of single components in large sizes and complex shapes cannot be heat-treated; need to choose hot-rolled or normalized, hot mechanical rolling, tempering treatment, and another factory state of steel directly produced. Bridge structures mostly use cold-formed and welded fabrication processes, requiring low aging sensitivity of metal materials and excellent welding performance. Bridge steel structures long-term low temperature or exposure to the external environment, requiring steel with good low-temperature toughness and weather resistance. Bridge structure long-term dynamic load, steel needs good strength, plasticity, and impact toughness; bridge structural steel transverse specimen impact absorption energy KV2 value is much greater than other uses of steel, well meet the low temperature, dynamic load requirements; several commonly used steel impact absorption energy comparison of the same grade is shown in Table 4.

Table.4 Impact absorption energy of the same grade steel

Steel products	Grade	Impact absorption energy (lateral, -20 ° C) KV2/J
Low alloy high-strength structural steel	Q355	>27
Steel for building structures	Q345GJ	>45
Structural steel for bridges	Q345q	>120
Weathering resistant structural steel	Q355NH	>34

Steel structure design, corner nodes, cross-shaped, T-shaped joints, flange plate thicker (≥ 40mm), complex node form, weld concentration, due to the node constraint, welding shrinkage stress, in the weld near the fusion line or steel plate thickness center area prone to laminar tearing, to fundamentally solve this phenomenon in engineering construction, shall be selected with thickness direction that is Z performance of steel.

3. Conclusion

To ensure the quality of the project, ensure structural safety and reduce the construction cost, the steel structure project should be a reasonable selection of materials, strictly according to the use of steel. Combined with the characteristics of the industry, the bridge project, high-rise building structure, large-span structure, stadium building structure, corrosion-resistant environment structure, and other important projects, considering the importance of the structure, static or dynamic load nature, service temperature, and corrosive environment and other factors, the use of steel companies to produce industry-specific steel varieties.
Author: Fu Firchun

Source: China Steel Structures Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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