Q345 is a steel material. It is low alloy steel (C<0.2%) and is widely used in buildings, bridges, vehicles, ships, pressure vessels, etc. Q represents the yield strength of this material, and the following 345 refers to the yield value of this material, which is around 345MPa. And as the thickness of the material increases, its yield value decreases.
Q345 has good comprehensive mechanical properties, acceptable low temperature performance, good plasticity and weldability. It is used as structures, mechanical parts, building structures, general Metal structural parts, used in hot-rolled or normalized conditions, can be used for various structures in cold areas below -40°C.
Level classification
Q345 can be divided into Q345A, Q345B, Q345C, Q345D and Q345E according to the grade. What they represent is mainly that the temperature of the impact is different.
Q345A level does not do impact;
Q345B grade, is a normal temperature impact of 20 degrees;
Q345C level, is 0 degree impact;
Q345D level, -20 degree impact;
Q345E grade, -40 degree impact.
At different impact temperatures, the impact values are also different.
chemical composition
Q345A: C≤0.20, Mn ≤1.7, Si≤0.55, P≤0.045, S≤0.045, V 0.02~0.15;
Q345B: C≤0.20, Mn≤1.7, Si≤0.55, P≤0.040, S≤0.040, V 0.02~0.15;
Q345C: C≤0.20, Mn≤1.7, Si≤0.55, P≤0.035, S≤0.035, V 0.02~0.15, Al≥0.015;
Q345D: C≤0.20, Mn≤1.7, Si≤0.55, P≤0.030, S≤0.030, V 0.02~0.15, Al≥0.015;
Q345E: C≤0.20, Mn≤1.7, Si≤0.55, P≤0.025, S≤0.025, V 0.02~0.15, Al≥0.015.
Compare 16Mn
Q345 steel is a substitute for many old steel types such as 12MnV, 14MnNb, 18Nb, 16MnRE, 16Mn, etc., rather than just replacing 16Mn steel. In terms of chemical composition, 16Mn and Q345 are also different.
What's more important is that there are large differences in the thickness grouping sizes of the two steel materials according to their different yield strengths, which will inevitably cause changes in the allowable stress of materials with certain thicknesses. Therefore, it is not appropriate to simply apply the allowable stress of 16Mn steel to Q345 steel. Instead, the allowable stress should be re-determined based on the new steel thickness group size.
The proportion of the main constituent elements of Q345 steel is basically the same as that of 16Mn steel. The difference is that trace alloying elements of V, Ti, and Nb are added. A small amount of V, Ti, and Nb alloy elements can refine the grains, greatly improve the toughness of the steel, and greatly improve the comprehensive mechanical properties of the steel.
It is precisely because of this that the thickness of the steel plate can be made larger. Therefore, the comprehensive mechanical properties of Q345 steel should be better than 16Mn steel, especially its low-temperature properties, which 16Mn steel does not have. The allowable stress of Q345 steel is slightly higher than that of 16Mn steel.
Performance comparison
Q345D seamless pipe mechanical properties:
Tensile strength: 490-675 Yield strength: ≥345 Elongation: ≥22
Q345B seamless pipe mechanical properties:
Tensile strength: 490-675 Yield strength: ≥345 Elongation: ≥21
Q345A seamless pipe mechanical properties:
Tensile strength: 490-675 Yield strength: ≥345 Elongation: ≥21
Mechanical properties of Q345C seamless pipe:
Tensile strength: 490-675 Yield strength: ≥345 Elongation: ≥22
Q345E seamless pipe mechanical properties:
Tensile strength: 490-675 Yield strength: ≥345 Elongation: ≥22
Product Series
Q345D steel is compared with Q345A, B, and C steel. The test temperature for low temperature impact energy is low. Good performance. The amount of hazardous substances P and S contained is lower than that of Q345A, B and C.
The market price is higher than Q345A, B, and C.
Q345d definition: ① It consists of Q + number + quality grade symbol + deoxidation method symbol. Its steel number is prefixed with "Q", which represents the yield point of the steel. The following number represents the yield point value in MPa. For example, Q235 represents a carbon structural steel with a yield point (σs) of 235MPa.
② If necessary, symbols indicating the quality grade and deoxidation method can be marked after the steel number. The quality grade symbols are A, B, C, and D respectively.
Deoxidation method symbols: F represents boiling steel; b represents semi-killed steel; Z represents killed steel; TZ represents special killed steel. Killed steel may not be marked with symbols, that is, both Z and TZ may be unmarked. For example, Q235-AF represents Grade A boiling steel.
③Special-purpose carbon steel, such as bridge steel, marine steel, etc., basically uses the representation method of carbon structural steel, but a letter indicating the purpose is appended to the end of the steel number.
Q345 (low alloy high strength steel) excerpted relevant information from the Internet
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Material introduction
1. The chemical composition of Q345 is as follows (%):
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2. The mechanical properties of Q345C are as follows (%):
Mechanical performance indicators
Elongation(%)
Test temperature 0℃
Tensile strengthMPa
Yield pointMPa
numerical value
≥22
≥34
470-650
324-259
When the wall thickness is between 16-35mm, σs≥325Mpa; when the wall thickness is between 35-50mm, σs≥295Mpa
2. Welding characteristics of Q345 steel
2.1 Calculation of carbon equivalent (Ceq)
Ceq=C+Mn/6+Ni/15+Cu/15+Cr/5+Mo/5+V/5
Calculate Ceq=0.49%, which is greater than 0.45%. It can be seen that the welding performance of Q345 steel is not very good, and strict process measures need to be formulated during welding.
2.2 Problems that easily occur during welding of Q345 steel
2.2.1 Hardening tendency of heat affected zone
During the welding and cooling process of Q345 steel, the quenching structure - martensite is easily formed in the heat-affected zone, which increases the hardness of the near-seam area and decreases the plasticity. The result is post-weld cracking.
2.2.2 Cold crack sensitivity
The welding cracks of Q345 steel are mainly cold cracks.
Welding construction process
Groove preparation → spot welding → preheating → inner welding → back root cleaning (carbon arc gouging) → outer welding → inner welding → self-inspection/special inspection → post-weld heat treatment → non-destructive inspection (welding Seam quality is first-class qualified)
Selection of welding process parameters
Through the analysis of the weldability of Q345 steel, the following measures have been formulated:
1. Selection of welding materials
Since Q345 steel has a greater tendency of cold cracking, low-hydrogen welding materials should be selected. At the same time, taking into account the principle that the welding joint should be as strong as the base metal, E5015 (J507) type welding rods should be selected.
2. Bevel form: (supplied according to drawings and equipment)
3. Welding method: manual arc welding (D).
4. Welding current: In order to avoid the coarse structure of the weld seam, which will cause a decrease in impact toughness, small specification welding must be used. The specific measures are: select small-diameter welding rods, narrow welding beads, thin welding layers, and multi-layer and multi-pass welding processes (the welding sequence is shown in Figure 1). The width of the weld bead shall not be greater than 3 times that of the welding rod, and the thickness of the welding layer shall not be greater than 5 mm. The first to third layers use Ф3.2 welding rods with a welding current of 100-130A; the fourth to sixth layers use Ф4.0 welding rods with a welding current of 120-180A.
5. Preheating temperature: Since the Ceq of Q345 steel is >0.45%, it should be preheated before welding. The preheating temperature T0=100-150℃, and the interlayer temperature Ti≤400℃.
6. Post-weld heat treatment parameters: In order to reduce welding residual stress, reduce the hydrogen content in the weld, and improve the metal structure and performance of the weld, the weld should be heat treated after welding. The heat treatment temperature is: 600-640℃, the constant temperature time is 2 hours (when the plate thickness is 40mm), and the temperature rise and fall rate is 125℃/h.
On-site welding sequence
1. Preheat before welding
Before welding the flange plate, first preheat the flange plate and start welding after holding the temperature for 30 minutes. The preheating, interlayer temperature and heat treatment of welding are automatically controlled by the heat treatment temperature control cabinet. Far-infrared crawler heating furnace is used. The microcomputer automatically sets and records the curve, and the thermocouple measures the temperature. During preheating, the measuring point of the thermocouple should be 15mm-20mm away from the edge of the groove.
2. Welding
2.1 In order to prevent welding deformation, each column joint is welded symmetrically by two people, and the welding direction is from the middle to both sides. When welding the inner opening (the inner opening is the groove close to the web), small specifications must be used for the first to third layers, because its welding is the main cause of welding deformation. After welding the first to third layers, the backside is cleaned. After using carbon arc gouging to remove the root, the weld must be mechanically polished to clean the surface of the weld for carburization and expose the metallic luster to prevent serious surface carbonization and cracks. The welding of the outer port should be completed in one go, and finally the remaining part of the inner port should be welded.
2.2 When welding the second layer, the welding direction should be opposite to the direction of the first layer, and so on. The welded joints of each layer should be staggered by 15-20mm.
2.3 The welding current, welding speed and number of welding layers of the two welders should be consistent during welding.
2.4 During welding, welding should start from the arc starting plate and end at the arc closing plate. After welding is complete, cut it off and polish it clean.
3. Post-weld heat treatment: Heat treatment should be carried out within 12 hours after welding is completed. If heat treatment cannot be carried out in time, heat preservation and slow cooling measures should be taken. During heat treatment, two thermocouples should be used to measure the temperature, and the thermocouples are spot welded on the inside and outside of the welding joint.
4. Welding inspection
According to the requirements of the "Steel Structure Engineering Construction and Acceptance Code", the welding joints are inspected using ultrasonic flaw detection method, and the inspection rate is 100%.
On-site technical management
1. Prepare detailed welding construction operation instructions.
2. Full-process control of the welding process is the core of ensuring quality.
When welding each column joint, there should be a dedicated person to monitor the welding process. If the welder does not follow the work instructions, the welding should be terminated immediately. During the welding process, heat treatment personnel should monitor the interlayer temperature throughout the process. If it exceeds the standard, the welder should be notified immediately to stop.
3. Improving the quality awareness of construction workers is the key to implementing welding technology
Before construction, all employees will be briefed and a construction technology card will be issued. The briefing explains in detail the characteristics of the welding process and the necessity and control points of strictly controlling the on-site welding process.
