Detailed Explanation Of The Differences Between 16Mn, Q345 And Q355 Steel Grades

Detailed explanation of the differences between 16Mn, Q345 and Q355 steel grades

Family tree from 16Mn to Q355

16Mn is an old brand in the 1988 national standard GB 1591-88. Its main ingredients include 0.12% – 0.20% carbon, plus 1.20% – 1.60% manganese. This grade was widely used in bridges, boilers, and pressure vessels before the 1990s. For example, a thermal pipeline renovation project in Beijing completed in 1992 used nearly 2,000 tons of 16Mn steel plates. However, with the advancement of steel smelting technology, the phosphorus and sulfur control standards of 16Mn are relatively loose, with the upper limit of S content reaching 0.045%, and the low-temperature impact toughness is only 0℃ level, which no longer meets the safety requirements of modern engineering.

In 1994, Q345 officially replaced 16Mn in the GB/T 1591-94 standard and became a new general-purpose low alloy steel grade. Among them, Q represents the yield strength, but 345 represents the minimum yield strength of 345MPa, which is slightly higher than the actual measured value of 16Mn. In 2008, in the steel structure project of the Shanghai World Expo venue, more than 60% of the load-bearing components were made of Q345B steel plates. Its phosphorus and sulfur content dropped to less than 0.030%, and the low-temperature impact test temperature was also increased to – 20°C.

Why does Q355 replace Q345?

In the GB/T 1591-2018 standard released in 2018, Q345 was directly canceled and replaced with Q355. The core upgrade lies in the microalloying process. During the smelting process of Q355, 0.02% – 0.15% vanadium was actively added, and 0.02% – 0.10% titanium or 0.015% – was added. With the addition of 0.060% niobium, it is these trace elements that can refine the grain size, allowing the steel to maintain good toughness while increasing its strength. In the 2020 steel structure bidding for the Xiongan New District High-speed Railway Station, it was clearly stated that all Q355C grade steel plates should be used, and the total usage reached 45,000 tons. It was decided.

The minimum yield strength of Q355 has been increased to 355MPa, which is 10MPa higher than Q345, and the low-temperature impact toughness has been extended to the -40°C level. For example, the longitudinal impact energy absorption requirement of the Q355D grade at -40°C is no less than 47J, but the Q345E grade can only guarantee performance at -20°C. In addition, the carbon equivalent of Q355 is more strictly controlled, the CEV upper limit is reduced from 0.45% of Q345 to 0.42%, and the welding cold crack sensitivity coefficient is reduced by about 15%.

Performance differences in actual use

The advantages of Q355 are even more obvious when thick plates are thicker than 30mm. Take a 4,000-ton coastal cargo ship built by a certain shipyard in 2022 as an example. The thickened part of the hull was originally designed to use Q345D, and then Q355D was used for alternative welding operations. The thickness of the plate reached 35mm, and the preheating temperature during welding dropped from the original 100°C to 80°C. The time for heat treatment after welding was reduced by 20%, but the impact toughness of the weld was increased by 12%.

As far as riveting and welding processing companies are concerned, the improvement in the strength of Q355 will directly produce two benefits. One is that the thickness of the steel plate can be reduced under the same load. For example, a heavy machinery factory changed the crane boom from the 25mm plate of Q345 to the 22mm plate of Q355, which reduced the weight by 12%; the second is that during welding, due to the decrease in carbon equivalent, the sensitivity to heat input becomes weaker. In 2023, a steel structure factory in Shenyang observed through actual comparison that The occurrence rate of Q355 thick plate welding pores is 8% lower than that of Q345.

Procurement and processing price comparison

In the current market, Q355 thick plates are those with a thickness of more than 30mm. Generally speaking, its price per ton of steel is 200 to 400 yuan more expensive than Q345. Take the quotation given by a steel factory in Jiangsu in May 2024 to explain the differences between 16Mn, Q345, and Q355 steel grades. The ex-factory price of Q345B, a 35mm steel plate, is 4150 yuan per ton. As for the Q355B with the same specifications, the price is 4,380 yuan per ton. However, considering that the Q355 can reduce weight by 10% to 15%, in fact, the cost of a single unit may be reduced by 3% to 5%.

In terms of the cost of processing involved in riveting welding, Q345 thick plate (above 30mm) riveting price Q345 thick plate (above 30mm) riveting price Q345 thick plate (above 30mm) riveting price , Q355 is simpler in view of the process used to connect and fuse (the temperature required for preheating is relatively low, and the types of materials that can be used as welding wires are wider). On average, the cost of workers and materials consumed per ton of welding can be saved by fifty to eighty yuan. According to statistics from a bridge component factory in Shandong in 2023, when processing 200 tons of Q355 plates with a thickness of 32 mm, the total man-hours required were reduced by 8% compared to Q345, and the number of welding rods consumed was reduced by 6%.

Key points to note when replacing old with new ones

When old projects use 16Mn steel to carry out renovation work, it is feasible to choose Q345 or Q355 to replace them, but it is necessary to strictly check the grade matching. Once the original 16Mn steel plate is used in a -10°C environment, at least grade Q345C or Q355C must be selected; if the original structure is subject to dynamic loads, it is recommended to directly choose grade Q355D. When a chemical plant in Liaoning was updating its pressure vessels in 2021, Q345B was mistakenly used to replace 16Mn. As a result, the -15°C low temperature impact did not meet the qualified standard, and the final rework caused losses of more than 800,000 yuan.

Controlling welding process adjustments is extremely critical. For Q355, the recommended control range for welding heat input is 15 to 25kJ/cm, which is narrower than the 20 to 30kJ/cm range for Q345. In terms of welding rods, it is recommended to use E5015 type, and for welding wire, ER50-6 can be selected. What needs special attention is that when the plate thickness exceeds 30mm, the preheating temperature of Q355 can be maintained at 60 to 80°C, but Q345 thick plates may need to be preheated at 100 to 120°C, otherwise, cold cracks will easily occur.

Optimal solution for material selection for new projects

When carrying out a new design project, it is very strongly recommended to give priority to Q355, which is a mandatory recommended grade in the current national standard. In 2024, new regulations issued by the Ministry of Housing and Urban-Rural Development require that steel structures with load-bearing functions in civil buildings must use Q355 and higher grades. Therefore, if you purchase Q345 now, you will face problems in supply and certification. For example, a large gymnasium insisted on using Q345 during design. In the end, it was discovered that the production schedules of major domestic steel mills were as long as 60 days, but with Q355, it only took 20 days.

Compared with other situations, in scenarios where there are low temperatures and fatigue requirements, Q355 shows a more significant cost performance. In 2023, a bridge in Harbin will undergo a bridge deck system replacement project. The entire project will use Q355D steel plates. This method of use can ensure a 50-year service life in a -40°C environment. If you insist on using Q345D, then in view of the same fatigue life requirements, you need to increase the cross-sectional area by 12%. As a result, the overall cost will be 7% higher.

Is the steel grade currently used by your company Q345 or Q355? Have you encountered welding or price problems during the replacement process? Welcome to share your own practical experience in the comment area, and give it a like so that more peers can see this useful information.