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Overview of Q235 steel


The material of Q235 steel is ordinary carbon structural steel, and the yield value of Q235 steel is about 235 MPa, and the yield value will gradually decrease with the increase of steel thickness.
Q235 steel is often rolled into wire rod or round steel, square steel, flat steel, angle steel, I-beam, channel steel, window frame steel, etc., and medium and thick steel plates. It is widely used in construction and engineering structures. Today, we will have an overview of Q235 steel.

What is Q235 steel?

Q235 steel is a carbon structural steel. Equivalent to A3 and C3 steel, it is the brand of Russian TOCT. The Q in the steel number represents the yield strength, and the latter 235 means that the yield value of this material is about 235MPa. Normally, the steel is used directly without heat treatment.

Q235 steel consists of Q+number+quality grade symbol+deoxidation method symbol. Its steel number is prefixed with "Q", which represents the yield point of the steel, and the following number represents the yield point value in MPa. For example, Q235 represents a carbon structural steel with a yield stress (σs) of 235 MPa. If necessary, symbols indicating quality grade and deoxidation method can be marked after the steel number. The quality grade symbols are A, B, C, and D, respectively. Symbol of deoxidation method: F means boiling steel; b means semi-killed steel: Z means killed steel; TZ means special killed steel, killed steel may not be marked with symbols, that is, Z and TZ may not be marked. For example, Q235-AF means A-grade boiling steel.

Because Q235 steel has moderate carbon content, good comprehensive properties, and good coordination of strength, plasticity and welding properties, Q235 steel is the most widely used. It is widely used in construction and engineering structures. It is used to make steel bars or build factory building frames, high-voltage transmission towers, bridges, vehicles, boilers, containers, ships, etc. It is also widely used as mechanical parts that do not require high performance. C, D grade steel can also be used as some professional steel.

What is the difference of Q235 A, B, C, D?

Carbon structural steel Q235 is divided into four grades of A, B, C, and D according to metallurgical quality. The chemical composition of each grade of steel is: Q235A, B, C, D four grades (GB/T 700-2006), followed by A Grades are of poorer quality, and Grades D are of the highest quality. The sulfur content of A, B, C, and D decreases in turn; A and B have the same phosphorus content, C has the second highest phosphorus content, and D has the least phosphorus content.

Q235A grade contains C 0.14-0.22% Mn 0.30-0.65% Si ≤0.30% S ≤0.050 P ≤0.045

Q235B grade contains C 0.12-0.20% Mn 0.30-0.70% Si ≤0.30% S ≤0.045 P ≤0.045

Q235C grade contains C ≤0.18% Mn 0.35-0.80% Si ≤0.30% S ≤0.040 P ≤0.040

Q235D grade contains C ≤0.17% Mn 0.35-0.80% Si ≤0.30% S ≤0.035 P ≤0.035

The different grades of Q235A, Q235B, Q235C, and Q235D represent the difference in the impact temperature. A means no impact, B is above 20 degrees, C is above 0 degrees, and D is above 20 degrees. The difference between A and E refers to the difference in impact temperature in their performance. They are: Q235A grade, which is not required for impact toughness test; Q235B grade, for normal temperature (20 ℃) impact toughness test; Q235C grade, for 0 ℃ impact toughness test; Q235D grade, for -20 ℃ impact toughness test.

What classification does Q235 steel belong to?

Generally speaking, there are many classification methods for materials, such as the common classification methods according to material properties such as strength, hardness, density, thermal conductivity, electrical conductivity, weldability, plasticity, and hardenability, and there are also classification methods according to process. Methods, such as material forming process, alloying process, heat treatment process, surface treatment process, etc., in addition to classification methods according to material acceptance conditions, transportation methods, application scenarios, etc. It can be noticed that the classification of materials is very extensive, which is a concrete manifestation of the rich and colorful application of most materials. Material classification is also an important method of applying materials in our life and work. As a widely used steel material, Q235 steel belongs to which more important classifications?

1. Q235 is a medium and low strength steel.

Its yield strength is 235Mpa, which is relatively low in steel materials. Steels with higher strength than it, such as Q345, are medium-strength steels, and then higher strengths, such as Q420D, are high-strength steels, and there are special ultra-high-strength steels. Strength steel such as GC-4 steel has a tensile strength of 1765~1961Mpa.

2. Q235 belongs to carbon structural steel.

Its composition is shown in the table below in "GBT_700-2006_ Carbon Structural Steel". Carbon steel is an alloy with iron and carbon as the main components, but the carbon content is less than 2%, and generally also contains a small amount of silicon, manganese, sulfur, phosphorus and other elements. Carbon steel with a content of more than 0.35% but less than 0.5% is called ordinary carbon steel. The sulfur content of high-quality carbon steel and high-grade high-quality steel is lower, and the sulfur and phosphorus content of the latter is less than 0.3%.

3. Q235 is a steel with good plasticity and toughness.

The plasticity of Q235 steel can be seen from the elongation after fracture shown in the table below, and the impact toughness can be compared by the impact test. Better toughness of steel is a very important advantage of steel, because in actual engineering, if the toughness is low, then it may happen Brittle fracture. Brittle fracture occurs very quickly and causes great damage.

4. Q235 has low flexural strength and is a flexible steel.

As shown in the table below, the design value of the flexural strength of Q235 steel is about 200MPa, which is significantly lower than that of Q345 steel. Moreover, when doing the cold bending test, the diameter of the bending indenter, d=a, is also significantly smaller than the diameter of the bending indenter of the Q275 steel shown in the table, d=2a, so its bending performance is better.

5. Q235 is an easy-to-weld steel.

Because the general welding process is carried out at room temperature, the cooling rate of the weld metal droplets is very fast, and the raw materials and welding materials are fused with each other, resulting in a very violent solidification process, so we can regard the weld as defective solidification thing. The advantage of Q235 steel is that it contains less harmful elements such as S, P and other elements, and it is not easy to produce segregation that can seriously affect the quality of the weld. This leads to the occurrence of hot cracks in the weld.

6. Q235 is easy to form steel.

Whether it is easy to form is mainly the process required to form a unit of material, or more specifically, the cost required. The technological process of Q235 channel steel production mainly includes (1) billet preparation (2) heating (3) rolling (4) finishing.

7. Q235 is easy to rust and belongs to corrosion-prone steel.

Modern research believes that steel will be corroded in weak alkaline, neutral and acidic environments, which is determined by the nature of iron. The so-called natural environment (including the atmosphere, water, and soil) is mostly neutral and weakly acidic, and steel is easily corroded, but the concrete environment is strongly alkaline, so the steel bars in reinforced concrete are not easy to corrode. At the same time, corrosion is also related to the ease with which electrochemical reactions occur. However, Q235 steel contains carbon, phosphorus, manganese, silicon and other components and impurities, and forms different solid solution forms such as ferrite, pearlite, and austenite. The components show different potentials, which can easily lead to electrochemical reactions, so Q235 steel is a steel that is easy to corrode.

8. According to the delivery status of the steel, Q235 steel is generally delivered in a hot-rolled state.

According to the definition of hot rolling, rolling above the recrystallization temperature of the metal is called hot rolling. The advantage of hot rolling is that it can refine the grains and improve the properties of the steel, but it is mainly reflected in the rolling direction. At the same time, hot rolling can eliminate the defects of the microstructure and improve the mechanical properties. The disadvantage of hot rolling is that it will deteriorate the mechanical properties of the steel along the rolling thickness direction, which may lead to laminar tearing and poor weldability; in addition, uneven cooling leads to residual stress, which can lead to deformation of steel components, affecting the accuracy, but also lead to poor stability and fatigue resistance of steel components.

9. According to the fire resistance, Q235 is a non-refractory steel.

When the temperature of the steel is below 350℃, the ultimate strength is slightly higher than normal temperature due to the phenomenon of blue brittleness; when the temperature exceeds 350℃, the strength decreases; when the temperature reaches 500℃, the strength decreases by about 50%, and when the temperature reaches 600℃, the decrease is about 70%. The yield point of the steel gradually decreases with the increase of temperature, and it is about 50% of the normal temperature at 500℃. The general trend of elongation and section shrinkage of steel increases with the increase of temperature, indicating that the plasticity of steel increases at high temperature and is prone to deformation. When the temperature is above 300 °C and there is a certain stress, the creep phenomenon of the steel becomes obvious, and the higher the temperature, the more obvious it is. Judging from the time of high temperature action, the steel will soften sharply after 15-24 minutes of fire, which can make the building lose its stability as a whole and destroy it, and the damaged structure cannot be repaired and reused.

Application of Q235 Steel in Modern Architecture

It is widely used in ordinary concrete structures as hot-rolled round bar Q235. Q235 steel is used as secondary reinforcement or erection reinforcement in ordinary concrete, which is carbon structural steel. In addition, a large number of hot-rolled ribbed steel bars are also used in ordinary concrete. According to "Reinforced Concrete Ribbed Steel Bars" and "Code for Design of Concrete Structures", it can be known that its code is HRB335 or HRB400.

In reinforced concrete construction projects, Q235 steel is also widely used in the formwork. The combined formwork uses the grade Q235 hot-rolled steel plate, and its basic material thickness is 2.5~3mm; the all-steel large formwork uses the grade Q235 hot-rolled plate, and the thickness is 5~6mm.

A large number of non-essential steel frame structures use Q235 steel. Most of the steel supports in the project are made of steel pipes with a diameter of 48mm and the grade of Q235, and a small amount of steel pipes with a diameter of 42mm are used to make door-shaped and square tower-shaped supports.



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