I. Overview of Q345 Steel
Q345 steel is a low-alloy steel conforming to Chinese national standards, with a carbon content of less than 0.2%. It contains small amounts of alloying elements to enhance strength and is classified into five grades. It offers excellent mechanical properties, low-temperature performance, and workability. It is widely used in construction, bridges, vehicles, and ships. The yield strength of Q345 steel exceeds 345 MPa. According to standard requirements, the lower yield limit decreases correspondingly as the material thickness increases.
Q345 hot-rolled coils are commonly covered by the GB/T 1591 standard, which specifies the technical requirements for low-alloy structural steels. Additionally, the new standard replacement grade for Q345 steel is Q355, which offers improved performance in certain aspects.
II. Classification of Q345 Steel Grades
Q345 steel is classified into five grades—Q345A, Q345B, Q345C, Q345D, and Q345E—based on different impact test temperature requirements. The primary distinction among these grades lies in the impact test temperature:
Grade Q345A: No impact test is required.
Grade Q345B: Impact test at 20°C (room temperature).
Grade Q345C: Impact test at 0°C.
Grade Q345D: Impact test at -20°C.
Grade Q345E: Impact test at -40°C.
Taking Q345D as an example, compared to Q345A, B, and C, its low-temperature impact test is conducted at a lower temperature, resulting in superior low-temperature performance. Additionally, the content of harmful elements—phosphorus (P) and sulfur (S)—in Q345D steel is lower than in Q345A, B, and C, which results in a relatively higher market price for Q345D steel.
Low-temperature impact performance ensures the toughness of materials at low temperatures. It is particularly critical in extremely cold regions, where strict adherence to design requirements is essential; otherwise, the entire structure may face safety hazards.
III. Chemical Composition of Q345 Steel
The chemical composition of Q345 steel primarily includes elements such as carbon, manganese, silicon, phosphorus, sulfur, and vanadium. The chemical composition varies slightly among different grades of Q345 steel.
Taking Q345A as an example, its chemical composition is: C ≤ 0.20, Mn ≤ 1.7, Si ≤ 0.55, P ≤ 0.045, S ≤ 0.045, V 0.02–0.15.
IV. Comparison of Q345 Steel and 16Mn Steel
Q345 steel serves as a replacement for steel grades such as 12MnV, 14MnNb, 18Nb, 16MnRE, 16Mn, and other steel grades. Although historically, Q345 originated from the 16Mn grade defined in the Ministry of Metallurgy standard YB13-69 (which can be traced back even further to the German standard ST52), its chemical composition differs from that of 16Mn steel.
There are also significant differences between Q345 and 16Mn steel in terms of yield strength, thickness grouping dimensions, and low-temperature classification. Overall, the comprehensive mechanical properties and low-temperature performance of Q345 steel are superior to those of 16Mn steel; in most cases, it is not recommended to use 16Mn as a substitute for Q345.
V. Mechanical Properties of Q345 Steel
The mechanical properties of Q345 steel include tensile strength, yield strength, and elongation. Taking the GB standard Q345B hot-rolled steel coil as an example, its mechanical properties are: tensile strength 470–630 MPa, yield strength ≥345 MPa, and elongation ≥20%.
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VI. Welding Characteristics of Q345 Steel
When welding Q345 steel, the following points should be noted:
Calculation of Carbon Equivalent (Ceq): Ceq = 0.49%. If Ceq exceeds 0.45%, weldability is generally poor, and strict welding procedure specifications must be established.
Common Issues During Welding: Hardening tendency in the heat-affected zone, susceptibility to cold cracking, etc.
Selection of Welding Parameters: Includes welding consumables, groove configuration, welding method, welding current, preheating temperature, etc.
Weld Inspection: According to the “Code for Construction and Acceptance of Steel Structures,” welds may be inspected using ultrasonic testing.
The typical welding process is as follows: Groove preparation → Tacking → Preheating → Inner pass welding → Back-side root cleaning (carbon arc gouging) → Outer pass welding → Inner pass welding → Self-inspection/special inspection → Post-weld heat treatment → Non-destructive testing (weld quality must meet Grade 1 standards)
VII. Applications in Extremely Cold Climates
In extremely cold regions, Q345D steel is widely used, particularly in transmission towers and building structures.
In China’s Tibet region, the daily temperature range can reach 40°C under extreme conditions, with winter temperatures dropping below -30°C. Q345D steel is used to manufacture transmission towers and building steel structures in this region (such as scenic areas)