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Analysis of Q355 Structural Steel

Q355, as the most mainstream low-alloy high-strength structural steel in current steel structures, carries the history of China's steel standards from imitation to independent innovation. In engineering procurement and design replacement, issues such as whether Q355 can replace 16Mn and whether ABCD grades can be mixed are not uncommon.

In the current production of structural components, both sheet metal structural components and machined parts are made of Q345 material. We can perform processing such as cutting, bending, welding, and turning on them.

This article will provide in-depth analysis from three aspects: brand evolution, underlying principles, and common misconceptions.

1、 Brand explanation and the process of 16Mn ->Q345->Q355

1. Meaning of brand name

According to the national standard GB/T1591-2018 "Low alloy High strength Structural Steel":

Q: The first letter of the pinyin representing the yield strength  

355: represents a lower limit of yield strength of 355MPa.

Code (A/B/C/D): Represents quality level, with the core difference being the temperature of the impact test.

2. Development process: from imitation to benchmarking

Phase 1: origin (1959) - originating from German standards

16Mn was first introduced in YB13-59 (a standard issued by the Ministry of Metallurgical Industry) in 1959. At the beginning of the founding of the People's Republic of China, the industrial foundation was weak, and the formulation of standards mainly adhered to the principle of "borrowing". 16Mn referred to the St52 steel grade in the advanced German standard DIN17100 at that time.

Phase 2: Transformation (1979) - Upgrading from section Standard to National Standard

With the development of China's standardization process, YB13 has been transformed into the national standard GB 1591. In the 1979 version of the national standard, the 16Mn brand was retained and used.

Phase Three: Replacement (1994) - Unification of Q345

In 1994, GB 1591 was revised to implement a new system named after yield strength. Please note that Q345 is not a simple renaming of 16Mn. Q345 is a general term used to merge and replace multiple old standard grades such as 16Mn, 12MnV, 14MnNb, 18Nb, 16MnRE, etc.

Afterwards, the number of grades gradually decreased, and Q345 became the mainstream in the market, which is also the significance of standardization~

Phase Four: Upgrade (2008-2018) - Internationalization of Q355

In order to align with international standards such as the S355 series of European standard EN10025, GB/T1591-2018 upgraded Q345 to Q355. By changing to an upper yield strength and increasing the nominal value by 10MPa, but due to comprehensive benchmarking with international standards in terms of chemical composition and overall performance, it has become more convenient for steel mills to organize production and for enterprises to negotiate business overseas.

2、 Differences and underlying principles of low-temperature codes A/B/C/D/E

1. Code meaning and test method

The difference in quality levels is mainly reflected in the Charpy (V-notch) impact test:

2. Reasons for differences in low-temperature performance (impurities, structure, and process)

The improvement in low-temperature performance from A to D is mainly due to the reduction of impurity elements and optimization of microstructure. Multiple methods are used in the production process to control:

Purity control: High grade steel is refined outside the furnace to strictly control impurities such as phosphorus (P) and sulfur (S), reducing the tendency of the steel to become "cold brittle".

Composition and microstructure optimization: By adding microalloying elements such as V, Ti, Nb, and using controlled rolling and controlled cooling (TMCP) technology, the grain size is refined to form a better microstructure, thereby simultaneously improving strength and toughness.

Heat treatment state: C and D grades usually require normalizing or normalizing rolling state to eliminate internal stress, stabilize microstructure and properties, and ensure that the material still has excellent toughness at low temperatures. The E-grade is -40 ℃, and the standard requires the use of grades such as Q355NE, while N requires a mandatory normalization process.

3、 Frequently Asked Questions (FAQ)

1. Can a lower grade brand replace a higher grade if its impact energy value exceeds twice?

Answer: Absolutely not.

This is the most dangerous misconception. Even though the impact performance of Q355C has doubled in value. But it cannot guarantee compliance even at -20 ℃. The results of the Charpy impact test are nonlinear, and the current temperature test can only represent the results of the current temperature, without any conversion relationship between different temperatures.

2.Is it feasible to replace lower levels with higher levels? For example, Q355C replacing Q355B?

Answer: In principle, it is feasible, but the process needs to be considered.

The performance of Q355C covers Q355B, and there is no problem from a structural safety perspective. However, two points should be noted:

Cost: C-level is more expensive than B-level, resulting in waste.

Process drawing evaluation: If the original specification process document specifies a B-level, strictly speaking, it should be explained and evaluated.

 3.16Mn board still be purchased? Can Q345/Q355 be used as a substitute?

Answer: In the current national standard, 16Mn sheet metal has been eliminated, and only some environmental component standards are still included. If the old standard is specified for futures trading, in principle, steel mills can also produce and supply according to the old standard.

Suggestion: When 16Mn sheet metal cannot be purchased, the Q355 series can be considered. But we cannot simply replace it, the evaluation process that should be followed must be followed.

4.What are the same grades and extreme low-temperature steels with low-temperature impact?

Similar grades: In addition to the Q355 series, there are also container steels such as 16MnDR and 09MnNiDR.

Extreme low temperature: For extremely low operating conditions such as liquefied natural gas (LNG) (-162 ℃), Ni (nickel) steel such as 5Ni and 9Ni steel should be used.

5. Typical extreme low temperature application cases

In typical extreme low-temperature application cases, the structural materials used are extremely sophisticated, such as:

Round tube steel columns, H-beams, and foundation steel plates: 09MnNiDR (yield strength 300MPa, resistance to -70 ℃) is commonly used.

Rectangular steel pipes, round steel and floor purlins: Q345E (resistant to -40 ℃) is commonly used.

Bolt: Made of 42CrMo (resistant to -60 ℃).

These materials collectively ensure structural safety and stability in extreme environments.