Properties of steel

Mechanical properties meet the functional requirements of the structure (strength, stiffness, fatigue, etc.) 

Process performance – meet the requirements of processing process 

(1) Mechanical properties of steel

Strength tensile strength, yield strength;

Plasticity elongation, reduction of area

Toughness impact energy absorption

Before the proportional limit (elastic limit), there is a linear relationship between stress and strain (Hooke’s law), – ob is the elastic deformation stage of the material 

After exceeding the elastic limit, the relationship between stress and strain is no longer linear. When the material enters the elastic-plastic region, the deformation increases rapidly, and the curve fluctuates in zigzag shape. When the stress does not increase, the deformation continues to develop. The material enters the plastic deformation stage. In the stress-strain curve, there is an up-down fluctuation stage, in which the highest point of fluctuation is the upper yield point, and the lowest point is the lower yield point (material)Yield strength); 

After the plastic deformation, the grains in the material rearrange, the strength increases, and enters the strain hardening stage, reaching the highest point d-tensile strength. Then the specimen necks and the specimen breaks.

The criterion of steel structure design is that the maximum stress of the component reaches the yield point of the material as the limit state, and the ultimate strength of the steel is regarded as the safety reserve of the local stress peak, which can meet the requirements of the strength and stiffness of the component at the same time. Therefore, for the material selection of the bearing structure, it is required to ensure the strength index of the tensile strength and yield point at the same time.

The toughness of steel is the ability of absorbing mechanical energy and resisting fracture of steel under load, which reflects the performance of steel under dynamic load.At present, the impact toughness of Charpy Specimens with V-notch is generally measured by the value of impact energy consumed in the impact test at home and abroad (in the past, u-notch specimens were used in China). The impact energy in Joule (J) should not be less than 27j.In the impact test, the greater the work consumed to break the specimen, the higher the impact toughness, the better the material toughness, and it is not easy to brittle fracture.

Technological properties of steel

1) . processability

The processing of steel into the required structural components requires a series of processes, including various machining (milling, planing, hole making), cutting, cold, hot correction and welding, etc. the process performance of steel should meet the needs of these processes, and the phenomenon of steel cracking or material damage in the process of processing should not occur.The good plasticity of low carbon steel and low alloy steel can meet the processing requirements to a great extent.

2) . cold bending performance

The cold bending property of steel is a kind of comprehensive performance judged by 180 ° bending test.The steel is processed into plate shape according to the original thickness. After bending 180 ° at normal temperature, if there is no cracking on the outer surface and side and no delamination, it is considered as qualified; when bending, different bending center diameter D is allowed according to the steel grade and plate thickness (it can be changed in the range of 0.5-3 plate thickness).

3) . weldability

Weldability refers to the steel’s adaptability to the welding process, including two requirements: one is to ensure that the welded joint has good mechanical properties through certain welding process; the other is that the sensitivity of hot (cold) cracks in weld metal and steel heat affected zone can be avoided after selecting appropriate welding materials and welding process parameters in the construction process.The weldability evaluation of steel can be divided into two methods: chemical composition identification and process test evaluation.The determination of chemical composition means that the weldability of steel is determined by the content of carbon equivalent, that is, all the elements in the chemical composition of steel which have significant influence on welding are converted into carbon content.Carbon is not only the main element to form the strength of steel, but also the primary element affecting the weldability. The steel with more than a certain content of carbon can not be welded.The higher the carbon equivalent, the worse the weldability.

Common steel structure numbering method

In the current national standards “carbon structural steel”, “low alloy high strength structural steel” and “steel for building structure”, the Chinese pinyin letter “Q” of yield point is taken as the first letter of structural steel number.The numbering principle (for example) is as follows:

Introduction of steel grades for common building structures

Carbon structural steel (GB / t700-2006)

Grade and chemical composition of steel

Low alloy high strength structural steel (GB / t1591-1994)

Grade and chemical composition of steel

High weathering structural steel (GB / t4171-2000)

According to the chemical composition, high weathering structural steel can be divided into copper phosphorus steel and copper phosphorus chromium nickel steel.There are q295gnh, q295gnh, q345gnh, q345gnh and q390gnh.Among them, “GNH” stands for “high weathering resistance” and “L” stands for high weathering steel containing chromium and nickel.

This kind of steel is suitable for atmospheric corrosion-resistant building structures. The products are usually used in the delivery state, and can be used for bolt connection, riveting and welding structures. However, when used as welding structure materials, the plate thickness should not be greater than 16mm.

Weathering steel for welded structures (GB / t4172-2000)

The weathering steel used for welding structure has good welding performance, the thickness can reach 100 mm, and its grades are q235nh, q295nh, q355nh and q460nh.Among them, “NH” stands for “weathering resistance” and “L” stands for high weathering steel containing chromium and nickel, and the quality grade is divided into C, D and E.

Thickness direction performance steel plate (gb5313-1985)

1) . practical scope

In some important stress components, not only the steel plate should have certain mechanical properties along the length direction and width direction, but also the good lamellar tearing resistance along the thickness direction; 

It is applied to killed steel plate with thickness of 15-150 mm and yield strength of no more than 500 MPa.Steel plates with thickness less than 15mm and more than 150mm or yield strength greater than 500MPa shall be negotiated separately when ordering; 

The lamellar tearing resistance of the steel plate was evaluated by the reduction of area in the thickness direction tensile test. 

2) . steel grade and technical requirements

3) Applicable scope and brand of steel

Generally, it is suitable for the production of high-rise building steel plate with the thickness of 6-100 mm, which is divided into 2 strength levels and 4 grades.The grade is composed of Pinyin letter (q) representing yield point, yield point value, Chinese phonetic alphabet (GJ) of high-rise buildings and quality grade (C, D, e) symbols. The suffix letter “Z” is added to the thickness direction performance steel plate.The four brands are q235gj, q235gjz, q345gj and q345gjz.

Steel plate for bridge structure (GB / t714-2000)

There are three grades of steel for bridge structure: Q235q, q345q, q370q, q420q, and the quality grade is divided into C, D and E.

Hot rolled H-beam and split T-beam

Classification code

H-beam with wide flange — HW 

H-section steel for flange of medium section — HM 

H-beam with narrow flange — HN 

T-beam with wide flange — tw 

T-section steel for flange split in middle part — TM 

T-section steel with narrow flange — TN 

Specification marking method for hot rolled H-beam and part of T-beam

Height h × width b × web thickness t1 × flange thickness T2

For example: hw100 × 100 × 6 × 8; tn125 × 60 × 6 × 8.