Q345 is a steel material. It is a low-alloy steel (C<0.2%), widely used in construction, 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 about 345 MPa. And the yield value will decrease with the increase of material thickness.
Q345 has good comprehensive mechanical properties, acceptable low temperature performance, good plasticity and weldability, and is used as structures, mechanical parts, building structures, general Metal structural parts, hot-rolled or normalized, can be used in various structures in cold regions below -40°C.
Classification
Q345 can be divided into Q345A, Q345B, Q345C, Q345D, Q345E according to grade. What they represent is mainly the temperature of the shock.
Q345A level, no impact;
Q345B level, 20 degrees normal temperature impact;
Q345C level, is 0 degree impact;
Q345D level, is -20 degree impact;
Q345E level, is -40 degree impact.
At different shock temperatures, the shock 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；
vs. 16Mn
Q345 steel is a substitute for the old brands of 12MnV, 14MnNb, 18Nb, 16MnRE, 16Mn and other steel types, not just a substitute for 16Mn steel. In terms of chemical composition, 16Mn and Q345 are also different. More importantly, there is a large difference in the thickness group size of the two steels according to the difference in yield strength, and this will inevitably cause changes in the allowable stress of materials with certain thicknesses. Therefore, it is inappropriate to simply apply the allowable stress of 16Mn steel to Q345 steel, but the allowable stress should be re-determined according to the new steel thickness group size.
The proportion of main constituent elements of Q345 steel is basically the same as that of 16Mn steel, the difference is that trace alloy elements of V, Ti and Nb are added. A small amount of V, Ti, and Nb alloying elements can refine the grains, greatly improve the toughness of the steel, and greatly improve the comprehensive mechanical properties of the steel. It is also 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 performance is not available in 16Mn steel. 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
Q345C seamless pipe mechanical properties:
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 compared with Q345A, B, C steel. The test temperature of low temperature impact energy is low. Good performance. The amount of harmful substances P and S is lower than that of Q345A, B and C. The market price is higher than Q345A, B, C.
Definition of Q345D:
① Composed of Q + number + quality grade symbol + deoxidation method symbol. Its steel number is preceded by "Q", which represents the yield point of the steel, and the number behind it represents the value of the yield point in MPa. For example, Q235 represents a carbon structural steel with a yield point (σs) of 235 MPa.
②If necessary, the symbol indicating the quality grade and deoxidation method can be marked behind the steel number. The quality grade symbols are A, B, C, D respectively. Deoxidation method symbol: F means boiling steel; b means semi-killed steel; Z means killed steel; TZ means special killed steel, and the killed steel can not be marked with symbols, that is, both Z and TZ can be omitted. For example, Q235-AF means Grade A boiling steel.
③ Carbon steel for special purposes, such as bridge steel, marine steel, etc., basically uses the expression method of carbon structural steel, but the letter indicating the purpose is added at the end of the steel number.
Material introduction
| element | C≤ | Mn | Si≤ | P≤ | S≤ | Al≥ | V | Nb | Ti |
| content | 0.2 | 1.0-1.6 | 0.55 | 0.035 | 0.035 | 0.015 | 0.02-0.15 | 0.015-0.06 | 0.02-0.2 |
The mechanical properties of Q345C are as follows (%):
| Mechanical properties index | Elongation(%) | Test temperature 0℃ | Tensile strength MPa | Yield point MPa≥ |
| value | δ5≥22 | J≥34 | σb（470-650） | σs（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%, greater than 0.45%, it can be seen that the welding performance of Q345 steel is not very good, and strict technological measures need to be formulated during welding.
2.2 Problems prone to occur in Q345 steel during welding
2.2.1 Hardening tendency in the heat-affected zone
During the welding and cooling process of Q345 steel, the quenched structure-martensite is easily formed in the heat-affected zone, which increases the hardness and reduces the plasticity of the near-seam area. The result is cracks after welding.
2.2.2 Cold crack sensitivity
The welding cracks of Q345 steel are mainly cold cracks.
Post time: Mar-20-2023