What is Q345 steel?

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 stands for the yield strength of this material. The latter 345 is the yield value of this material, which is about 345 MPa. The yield value will decrease with the increase of material thickness. 


Comprehensive performance of Q345

Q345 has good comprehensive mechanical properties, low temperature performance, good plasticity and weldability. It is used as the structure, mechanical parts, building structure, general metal structural parts, heat, etc. bearing dynamic load for medium and low pressure vessels, oil tanks, vehicles, cranes, mining machinery, power stations, bridges, etc Used in rolling or normalizing state, it can be used for various structures in cold area below – 40 ℃.

Grade classification of Q345

Q345 can be divided into Q345A, Q345B, Q345C, Q345D and Q345E by grade. What they represent is that the impact temperature is different. Q345A, no impact; Q345B, impact at 20 ° normal temperature; Q345C, 0 degree impact; Q345D, is – 20 degree impact; Q345E, is – 40 degree impact. At different impact temperature, the impact value is different.

Chemical composition of Q345

Chemical Property
Name Grade Chemical Composion%
C≤ Mn Si≤ P≤ S≤ V Nb≤ Ti Al≤
Q345 A 0.20 1.00-1.60 0.55 0.045 0.045 0.02-0.15 0.015-0.060 0.2
B 0.20 1.00-1.60 0.55 0.040 0.040 0.02-0.15 0.015-0.060 0.2
C 0.20 1.00-1.60 0.55 0.035 0.035 0.02-0.15 0.015-0.060 0.2 0.015
D 0.18 1.00-1.60 0.55 0.030 0.030 0.02-0.15 0.015-0.060 0.2 0.015
E 0.18 1.00-1.60 0.55 0.025 0.025 0.02-0.15 0.015-0.060 0.2 0.015

Notes: The P and S contents of profiles and bars can be increased by 0.005%, and the upper limit of Grade A steel can be 0.0045%.

Q345 Mechanical Properties

Mechanical Properties: Yield Strength (≥ N/mm2)
Steel Grade Quality Grade Ø ≤16 16 < Ø ≤ 40 40 < Ø ≤ 63 63 < Ø ≤ 80 80 < Ø ≤ 100 100 < Ø ≤ 150 150 < Ø ≤ 200 200 < Ø ≤ 250 250 < Ø ≤ 400
Q345 Q345A ≥345 ≥335 ≥325 ≥315 ≥305 ≥285 ≥275 ≥265
Q345D ≥265

Notes: Ø = Thickness or wire diameter in mm;

1 MPa = 1 N/mm2

Name Grade Tension Test 180℃ bending test d=Diameter of bending center a=Thickness of sample (Diameter)
Yield point, Mpa Tensile Strength Mpa Elongation % Impact Power, AkV, J
Thickness (Diameter Side Length) mm
≤16 >16-35 >35-50 >50-100 +20℃ 0℃ -20℃ -40℃
Thickness of steel (Diameter) mm
≤16 >60-100
Q345 A 345 325 295 275 470-630 21
B 345 325 295 275 470-630 21 34
C 345 325 295 275 470-630 22 34
D 345 325 295 275 470-630 22 34
E 345 325 295 275 470-630 22 27
Ø mm, Elongation (≥%)
Grade Quality Level Ø ≤40 40 < Ø ≤ 63 63 < Ø ≤ 100 100 < Ø ≤ 150 150 < Ø ≤ 250 250 < Ø ≤ 400
Q345 Q345A ≥20 ≥19 ≥19 ≥18 ≥17
Q345C ≥21 ≥20 ≥20 ≥19 ≥18
Q345D ≥17

Charpy Impact

Charpy (V-notch) impact test temperature and impact absorption energy (KV2/J), thickness (t).

Quality Temperature, ℃ 12-150 (mm) 150<t≤250 (mm) 250<t≤400 (mm)
Q345B 20 ≥34J ≥27J
Q345C 0
Q345D -20 27J
Q345E -40

The Q345 is a steel with less than 0.2 percent of its composition made up of carbon, less than 0.55 percent of its composition made of silicon and several impurities. This particular grade of steel is a general purpose manufacturing steel in construction equipment field.

Q345 Mechanical Properties - What is Q345 steel?

  • Yield Strength

A yield strength or yield point is the material property defined as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Yield strength of Q345B is 345 Mega Pascals. Like most steels of its grade, it suffers elongation before it pulls apart, typically at 20-21 percent of its initial length.

  • Tensile strength

The tensile strength of a material is the maximum amount of tensile stress that it can take before failure, for example breaking. Steel tensile strength is expressed in units of force divided by cross sectional area; in the Metric system, this unit is known as the Pascal. One Pascal is a Newton  per square meter. The tensile strength of Q345 steel is rated at 470 to 660 Mega Pascals.

  • Hardness

Brinell scale hardness characterizes the indentation hardness of materials through the scale of penetration of an indenter, loaded on a material test-piece. It is one of several definitions of hardness in materials science.

Q345 Equivalent

Q345 steel: Q345A, Q345B, Q345C, Q345D, Q345E equivalent to US ASTM, European Germany DIN, British BS, France NF, Japan JIS, and ISO Standard (For Reference).

Notes: Q345 steel has been replaced by Q355 in the latest version of GB standard “GB/T 1591 – 2018”, the previous version is “GB/T 1591 – 2008”.

Q345 Equivalent
China USA Germany Japanese ISO
Standard Grade Standard Grade Standard Grade (steel number) Standard Grade Standard Grade
GB/T 1591 – 2008 (Year) Q345A ASTM A529/A529M; ASTM A572/A572M; ASTM A588/A588M; ASTM A678/A678M; ASTM A808/A808M Grade 50; Grade 50; Grade B; Grade D; A808M JIS G3135 SPFC590 ISO 630-2 S355C, S355D (S355)
GB/T 1591 – 2008 Q345B Ditto Ditto DIN EN 10025-2; DIN 17100 S355, S355JR (1.0045), E355 (1.0060); St52-3 JIS G3135 SPFC 590 ISO 630-2 S355C, S355D (S355)
GB/T1591 – 2008 Q345C Ditto Ditto DIN EN 10025-2; DIN 17100 S355J0 (1.0553), E335 (1.0060); St52-3 JIS G 3135 SPFC590 ISO 630-2 S355D (S355)
GB/T 1591 – 2008 Q345D ASTM A656/A656M; ASTM A529/A529M Type 7; Grade 50 DIN EN 10025-2; DIN EN 10025: 1993; DIN 17100 S355J0 (1.0553), E335 (1.0060); S355J2G3 (1.0570); St52-3 JIS G 3135 SPFC590 ISO 630-2 S355D (S355)
GB/T 1591 – 2008 Q345E ASTM A656/A656M; ASTM A529/A529M Type 7; Grade 50 DIN EN 10025-2; DIN EN 10025: 1993 S355J2 (1.0553), S355K2 (1.0596), E335 (1.0060); S355J2G4 (1.0577), S355K2G3 (1.0595), S355K2G4 (1.0596) JIS G3135 SPFC 590
Contrast 16Mn
Q345 steel is the replacement of old brand 12mnv, 14MnNb, 18nb, 16mnre, 16Mn and many other steel grades, rather than just one material of 16Mn steel. In chemical composition, 16Mn and Q345 are also different. More importantly, the thickness group sizes of the two steels are quite different according to the different yield strength, which will cause the change of the allowable stress of some thickness materials. Therefore, it is not suitable to simply apply the allowable stress of 16Mn steel to Q345 steel, and the allowable stress should be re determined according to the new steel thickness grouping size. The proportion of main elements in Q345 steel is basically the same as that in 16Mn steel, the difference is that V, Ti, Nb microalloy elements are added. A small amount of V, Ti, Nb alloy elements can refine the grains, greatly improve the toughness of the steel, and greatly improve the comprehensive mechanical properties of the steel. Because of this, the thickness of steel plate can be made larger. Therefore, the comprehensive mechanical properties of Q345 steel should be better than that of 16Mn steel, especially its low temperature properties. The allowable stress of Q345 steel is slightly higher than that of 16Mn steel.

Performance comparison:

1. Mechanical properties of Q345D seamless pipe Tensile strength: 490-675 Yield strength: ≥ 345 Elongation: ≥ 22

2. Mechanical properties of Q345B seamless pipe Tensile strength: 490-675 Yield strength: ≥ 345 Elongation: ≥ 21

3. Mechanical properties of Q345A seamless pipe Tensile strength: 490-675 Yield strength: ≥ 345 Elongation: ≥ 21

4. Mechanical properties of Q345C seamless pipe Tensile strength: 490-675 Yield strength: ≥ 345 Elongation: ≥ 22

5. Mechanical properties of Q345E seamless pipe Tensile strength: 490-675 Yield strength: ≥ 345 Elongation: ≥ 22

Q345 product line

Q345D steel is compared with Q345A, B and C steel. The test temperature of low temperature impact energy is low. Good performance. The quantity of P and s containing harmful materials is lower than that of Q345A, B and C. Market price is higher than Q345A, B, C, Q345D definition
  • ① it is composed of Q + number + quality level symbol + deoxidation method symbol. Its steel grade is crowned with “Q”, which represents the yield point of the steel. The following numbers represent the yield point value. For example, Q235 represents the carbon structural steel with a yield point (σ s) of 235 MPa.
  • ② if necessary, the symbols indicating the quality grade and deoxidization method can be marked after the steel grade. The quality level symbols are a, B, C and D respectively. Symbols of deoxidization method: F for rimmed steel; B for semi killed steel: Z for killed steel; TZ for special killed steel, which can be unmarked, i.e. Z and TZ can be unmarked. For example, q235-af indicates A-grade rimmed steel.
  • ③ carbon steel for special purpose, such as bridge steel and ship steel, basically adopts the expression method of carbon structural steel, but at the end of steel grade, the letter indicating the purpose is added.

Welding characteristics of Q345 steel

1.1 calculation of carbon equivalent (CEQ)
  • Ceq=C+Mn/6+Ni/15+Cu/15+Cr/5+Mo/5+V/5
  • When CEQ = 0.49%, more than 0.45%, it can be seen that the welding performance of Q345 steel is not very good, so it is necessary to formulate strict process measures during welding.
1.2 problems of Q345 steel during welding
1.2.1 hardening tendency of heat affected zone In the process of welding and cooling of Q345 steel, martensite is easy to form in the heat affected zone, which makes the hardness of near seam zone increase and plasticity decrease. As a result, cracks occur after welding. 1.2.2 cold crack sensitivity The welding cracks of Q345 steel are mainly cold cracks.
Welding construction process
Groove preparation → spot welding → preheating → inner opening welding → back gouging (carbon arc air gouging) → outer opening welding → inner opening welding → self inspection / special inspection → post weld heat treatment → non-destructive inspection (weld quality level I is qualified)
Selection of welding process parameters
Based on the weldability analysis of Q345 steel, the following measures are formulated: 1. Selection of welding materials As the cold crack tendency of Q345 steel is relatively large, the welding material of low hydrogen type should be selected, and the principle of equal strength of welding joint and base metal should be considered, the E5015 (J507) electrode should be selected. See the following table (%) for chemical composition:
See the following table for mechanical properties:
Index of mechanical properties
Mumerical value
164 114 76

(tensile strength should be greater than yield

 2. Groove form: (according to drawings and equipment supply)

3. Welding method: adopt manual arc welding (d).

4. Welding current: in order to avoid the coarse structure of the weld and the decrease of impact toughness, small specification welding must be adopted. The specific measures are: select small diameter welding rod, narrow weld bead, thin welding layer, multi-layer and multi pass welding process. The width of the weld bead shall not be more than 3 times of the electrode, and the thickness of the welding layer shall not be more than 5mm. Ф 3.2 electrode is used from the first layer to the third layer with welding current of 100-130a; Ф 4.0 electrode is used from the fourth layer to the sixth layer with welding current of 120-180a.

5. Preheating temperature: since CEQ of Q345 steel is more than 0.45%, preheating shall be carried out before welding, with preheating temperature t0 = 100-150 ℃, 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 shall 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 speed is 125 ℃ / h.

Field welding sequence:

1. Preheating before welding Before flange plate welding, the flange plate shall be preheated first, and welding shall be started after 30 minutes of constant temperature. The preheating, interpass temperature and heat treatment of welding are automatically controlled by the heat treatment temperature control cabinet. The far-infrared crawler type heating furnace is used. The microcomputer automatically sets the curve and records the curve, and the thermocouple measures the temperature. During preheating, the measuring point of thermocouple shall be 15mm-20mm away from the edge of 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), the first layer to the third layer must be operated with small specifications, because its welding is the main reason affecting the welding deformation. After one to three layers of welding, back gouging shall be carried out. After using carbon arc gouging to clear the root, the weld must be mechanically polished, and the surface of the weld must be cleaned for carburization to expose the metallic luster, so as to prevent the surface from serious carbonization and crack. The external welding shall be completed at one time, and the remaining part of the internal welding shall be welded at last.

2.2 when welding the second layer, the welding direction shall be opposite to that of the first layer, and so on. The welded joints of each layer shall be staggered by 15-20mm.

2.3 the welding current, welding speed and number of welding layers of two welders shall be consistent during welding.

2.4 the welding shall be started from the arc striking plate and ended on the arc striking plate. Cut off and polish clean after welding.

3. Post weld heat treatment: heat treatment shall be carried out within 12 hours after the completion of welding. If the heat treatment cannot be carried out in time, measures of heat preservation and slow cooling shall be taken. During heat treatment, two thermocouples shall be used for temperature measurement, and the thermocouples shall be spot welded on the inside and outside of the welded junction.

4. Welding inspection

According to the requirements of code for construction and acceptance of steel structure engineering, the welded junction is inspected by ultrasonic flaw detection method, and the inspection proportion is 100%.

On site technical management

  • Prepare detailed welding operation instructions.
  • The whole process control welding process is the core to ensure the quality. During the welding of each column joint, special personnel shall be assigned to monitor the welding process. If the welder does not follow the operation instruction, the welding shall be terminated immediately. During the welding process, the heat treatment personnel shall monitor the interpass temperature in the whole process, and inform the welder to suspend immediately if it exceeds the standard.
  • Improving the quality awareness of construction personnel is the key to the implementation of welding process before construction, the whole staff shall be disclosed and the construction process card shall be issued. In the disclosure, the characteristics of welding process and the necessity and key points of strict control of on-site welding process are explained in detail.
Conclusion According to the welding process measures, 102 welding joints were welded on site, and the one-time qualified rate reached 100% after NDT. Through the verification of actual construction, this welding process measure can not only guide the welding of Q345 steel on site, but also ensure the welding quality.

Q345 steel grade upgraded to Q355

According to GB / T 1591-2018 low alloy high strength structural steel, from February 1, 2019, Q345 steel brand will be cancelled and changed to Q355, corresponding to S355 steel brand of EU standard.

What is S355?

S355 is a non-alloy European standard (EN 10025-2) structural steel, most commonly used after S235 where more strength is needed. It got great weldability and machinability, let us see more mechanical details of this steel.

Identification of Different S355

S – stands for structural steel
355 – stands for minimum yield strength (though it will vary based on thickness, refer below section for more details)
Normally S355 will have following variation based on impact strength at different temperatures, as steel become more brittle when temperature drops. If your structure need to withstand -20°C, it’s better and safer to choose S355J2.
S355JR – Can withstand an impact energy of 27J at +20°C
S355J0 – Can withstand an impact energy of 27J at 0°C
S355J2 – Can withstand an impact energy of 27J at -20°C
S355K2 – Can withstand an impact energy of 27J at -30°C (40J at -20°C)
And sometime followed by a letter H, like S355J2H, here H stands for hollow section.

S355 Yield Strength

S355 is named based on it’s minimum yield strength of 355 Mpa (N/mm2), However the yield strength reduces when you go up in thickness above 16 mm for flat products & hollow sections Here is the table showing reduction in yield strength based on thickness Up to 16 mm – 355 Mpa 16 mm < t  ≤ 40 mm –  345 Mpa 40 mm < t  ≤ 63 mm –  335 Mpa 63 mm < t  ≤ 80 mm –  325 Mpa 80 mm < t  ≤ 100 mm –  315 Mpa 100 mm < t  ≤ 150 mm –  295 Mpa

S355 Tensile Strength

S355 ultimate tensile strength ranges also varies based on thickness. Up to 3 mm – 510 Mpa to 680 Mpa 3 mm < t  ≤ 100 mm –  470 Mpa to 630 Mpa 100 mm < t  ≤ 150 mm –  450 Mpa to 600 Mpa

S355 Density

Density of S355 is 7850 kg/m3 like all other mild steel

S355 Hardness (Brinell Hardness – HB)

Hardness for S355 J2 ranges from 146 HB to 187 HB

S355 Young’s Modulus

S355 Youngs’s Modulus is 190 to 210 Gpa, in a conservative design go for 190 Gpa

S355 Equivalents

S355 EN 10025:1990 Equivalent

S355J2 → S355J2G4 (1.0577)
S355K2 → S355K2G4 (1.0596)

S355 DIN 17100 Equivalent

S355J0 → St 52-3 U
S355J2G3 → St 52-3 N

S355 ASTM Equivalent

S355 → A572 Grade 50 (Yield Strength 345 Mpa)

S355 Equivalent BS 4360 (British / Indian)

S355JR → 50 B S355J0 → 50 C S355J2G3 → 50 D

S355 Chemical Composition

C %
Si %
Mn %
0.55 1.6 0.035 0.035 0.012
Q355 is a common low-alloy high-strength steel with a yield strength of 355MPa. Farewell! With our Q345 steel for many years! Q345-a is the original 16MnR, which is a widely used container steel. Its biggest advantage is excellent welding performance. After welding, it can eliminate welding stress without heat treatment. Replacing Q345 with Q355, that is to say, the yield strength of steel is increased from 345mpa to 355MPa, which indicates the progress of steel-making technology in China. After the implementation of the new standard, the change is not only the strength improvement, but also many changes. The brand has been changed and the original Q345A and other steel grades have been cancelled. For example, welding properties, microstructure. The production cost of iron and steel plants will also increase slightly. There is no doubt that these costs will be transferred to users. Q345 means that the minimum yield strength is not less than 345mpa. Actually, it is not exactly 345mpa, which is often greater than this value. So it’s not a big problem to change the name. It’s the same thing. It’s like a high-speed railway that can run 350, others can only drive 250, some dare to drive 300, but the margin is different.

Source: China Steel Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

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What’s the difference between Q235 and Q345 in steel?

Q235 steel is a common carbon structural steel, that is, the yield point of the old A3 steel is 235mpa, the tensile strength is 375-500mpa, and it has many general purposes. Q345 steel is a low-alloy high-strength structural steel, which is a general designation of a class of steel including 16Mn, 12mnv, 14MnNb, 18nb and 16mnre. Its yield point is 345mpa, and its tensile strength is 470-630mpa, which belongs to the general category.

What material is Q345A?

Q345A is a kind of steel with good strength but poor performance. It is a low alloy steel, widely used in bridges, vehicles, ships, buildings, pressure vessels, etc. ① it is composed of Q + number + quality level symbol + deoxidation method symbol. Its steel grade is titled with “Q”, which represents the yield point of the steel. The following number represents the yield point value, and the unit is MPa. For example, Q345 represents yield.

What is Q345B?

Q345B is a low alloy high strength structural steel. Q – the first letter of the Chinese Pinyin of the word “Qu” of the yield strength of steel; 345 – the value of the yield strength, in MPa; B – the quality grade is B, which means that 20 degree impact test is required. Q345B chemical composition: C: ≥ 020si: ≥ 055mn: ≥ 17p: ≥ 004S: ≥ 004q345qd is bridge steel.

what is q345 steel - What is Q345 steel?
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What is Q345 steel?
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 stands for the yield strength of this material.
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