What Are The Differences Between Steel Q355 And Q345?

What are the differences between steel Q355 and Q345?

The root of the replacement of old and new brands

The two grades Q345 and Q355 are essentially different names for the same steel type under different standards. The "Low Alloy High-Strength Structural Steel" GB/T 1591-2018, which came into effect on February 1, 2019, officially replaced the Q345 grade with Q355. The purpose of this modification is to achieve docking with the European standard. The European standard names Q345 thick plate (above 30mm) riveting price based on the minimum upper yield strength of 355N/mm². However, the old national standard Q345 is taken from the 345N/mm² corresponding to the lower yield point. In short, the chemical composition and production process of steel have not fundamentally changed, only the way the grade is marked has changed.

In the actual procurement situation, the nameplates of steel produced after 2019 no longer appear as Q345, but become Q355. If you go to the steel market to buy thick plates with a thickness of more than 30mm, the "Q345 thick plate" quoted by the merchant is most likely an old material in stock or the previous name is used. Newly produced goods will be marked as Q355. Such changes directly affect the parameter values ​​of design institutes and drawing review units.

Specific differences in design strength parameters

Although the grade has been changed, Q355 and Q345 are almost identical in terms of the distinguishing area above the thickness and the design value of strength. The interval definitions that distinguish the thickness setting range, such as t≤16mm, 16-40mm, and 40-63mm, are exactly the same. There is still no change in basic mechanical properties such as tensile strength and elongation. The only difference is the yield strength value. Within each separated thickness range, Q355 is 10N/mm² higher than Q345.

For example, when t is less than or equal to 16 mm, the yield strength of Q345 is 345 Newtons per square millimeter, while that of Q355 is 355 Newtons per square millimeter. This increase of 10 Newtons per square millimeter will trigger a chain reaction in design calculations. This difference also exists when it comes to thick plates of more than 30 mm, which means that within the steel range of the same cross-section, the load-bearing capacity of Q355 will be slightly improved.

The actual impact of the steel grade correction factor

First of all, the change in yield strength directly changes it. This change is for the steel number correction coefficient εk? This coefficient is used to calculate key design parameters such as stability limit and slenderness ratio. Then, taking the case of t≤16mm as an example, the εk of Q345 has the following value, εk is equal to √(235/345) and this value is equal to 0.8253. The εk of Q355 also has the corresponding value, εk is √(235/355) and this value is equal to 0.8136. The correlation coefficient of Q355 is 0.985 times that of Q345, which means that the stability requirements have become slightly stricter.

For thick plates with a thickness of more than 30mm, this change in coefficient indicates that the stability calculation of pressure components will be more stringent. For example, if there is a column whose slenderness ratio is just at the edge of the specification limit, it can pass the calculation using Q345. However, if it is calculated using Q355, it may be necessary to increase the cross-section or encrypt the stiffeners. Designers must pay attention to this coefficient difference of 0.985 times and cannot directly apply the old data.

Confusion caused by unrevised current steel standards

At present, the GB50017-2017 called "Steel Structure Design Standard" is still compiled based on Q345 and has not been updated simultaneously with the update of Q355 parameters. This resulted in an embarrassing situation: when the design institute drew the drawings, the requirements put forward by various construction drawing review units were not consistent. Among them, some drawing review units agree to write Q355 directly and adopt new coefficients, some insist on using the parameters of Q345 for calculations, and some require both parameters to be written in detail in the design description.

This problem is becoming more and more prominent when it comes to the processing of riveted welding parts with thicknesses over 30 mm. Although the heat input of thick plate welds, the selection of welding materials, preheating temperature and other process parameters are not directly affected by the grade , Q345 thick plate (above 30mm) riveting price , however, inconsistent opinions on the drawing review will cause the drawing to be revised repeatedly, thereby delaying the construction period. Many processing plants have to set aside additional costs caused by uncertainty in drawing reviews when quoting.

Practical suggestions for construction drawing design

When faced with inconsistent standards, the safest approach is to list both Q345 and Q355 parameters in the construction drawing design instructions. For example, it can be stated that "the steel grade used is Q355, and the design strength is calculated according to Q345" or "during the stable calculation, the value of εk is 0.8136", so that both the drawing review unit and the construction unit can clearly understand the relevant basis. Especially in the node design and weld calculation of thickened plates with a thickness of more than 30mm, what are the differences between steel Q355 and Q345? , it must be very clear which yield strength the labeling is based on.

For riveting and welding processing enterprises, after receiving the design drawing, they need to confirm the grade and parameters in the design description as soon as possible. If the drawing only writes Q345, but also marks the thickness grouping of Q355, then it is necessary to communicate with the designer to reach a consensus. Currently, many large projects, such as bridges and large factories, have begun to fully adopt the Q355 grade, and the drawing review is also gradually transitioning. However, mixed use still exists in small and medium-sized projects.

Factors affecting the price of thick plate riveting and welding

The riveting price of Q345 thick plates with a thickness of more than 30mm is greatly affected by the purchase cost of the plates and the complexity of the processing technology. In terms of plates, the market prices of Q355 and Q345 remain roughly the same. The reason is that the chemical composition and production costs have not changed, but the brand has changed its label. However, the old standard Q345 plates in the inventory will gradually decrease. In the future, the new standard Q355 plate may show slight fluctuations due to price adjustments by steel mills.

For the processing end, the thick plate riveting welding situation is Q345 thick plate (above 30mm) riveting price , which requires larger tonnage equipment, as well as more stringent preheating process and post-heating process. For plates above 30mm, the cost of groove preparation, multi-layer and multi-pass welding, flaw detection and other processes is more than 30% higher than that of thin plates. The switching of brand names will cause repeated drawing review, which will increase the cost of drawing modifications and rework, and these costs can be indirectly reflected in the riveting and welding quotation. If you hold a riveting and welding order for plates with a thickness of 30mm or more, it is recommended to indicate "calculated based on Q355 grade" when quoting. If you encounter drawing review requirements and switch parameters, you will need to negotiate separately.

Among the steel procurement matters you have encountered recently, including riveting and welding processing, have you encountered any obstacles in the drawing review process due to problems with the Q345 and Q355 grades? You are welcome to share your own experience in the comment area, like it and forward it so that more peers can witness it.