Part 1 Comprehensive Building Materials
Chapter 1 Structural Materials
Section 2 Steel
★★Test Point 1 Steel Structure What are the main applicable steel materials? What are the thickness and main uses of steel plates?
Section steel is the main steel used in steel structures. Steel plates are divided into thick plates (thickness > 4mm) and thin plates (thickness ≤ 4mm). Thick plates are mainly used for structures, while thin plates are mainly used for roof panels, floor slabs and wall panels.
★★★Test Point 2 What standards should steel pipe welds for steel pipe concrete structures meet?
Steel pipe welding must use butt welds and meet the requirements of being as strong as the base metal. The quality of welds should meet the requirements of the "Construction Quality Acceptance Code for Steel Structure Engineering" GB50205-2001 secondary weld quality standards.
★★★Test point 3 What are the types and strength standards of hot-rolled steel bars and the applicable parts of plain round and ribbed steel bars?
Hot-rolled plain round steel bars have low strength and low bonding strength with concrete. They are mainly used as stress-bearing steel bars, stirrups and structural steel bars for slabs. The gripping force between hot-rolled ribbed steel bars and concrete is large, and the simultaneous working performance is good. Among them, HRB335 and HRB400 grade steel bars are the main stress-bearing steel bars used in reinforced concrete.
Note: HRBF belongs to fine-grained hot-rolled steel bars, HRBF400, HRBF500
★★★Test point 4: Steel bars with seismic resistance requirements, strength yield ratio, super yield ratio, maximum force total extension Long term requirements?
National standards stipulate that the steel bar grades suitable for seismic structures with higher requirements are: steel bars with ribbed steel bar grades followed by E (for example: HRB400E, HRBF400E) in Table 1A412012-1. In addition to meeting the following requirements, this type of steel bars has the same requirements as the corresponding existing brands of steel bars:
(1) The ratio of the measured tensile strength to the measured yield strength of the steel bar is ≥1.25;
(2) The ratio of the actual measured yield strength of the steel bar to the yield strength characteristic value specified in Table 1A412012-1 is ≤ 1.30;
(3) The maximum force total elongation of the steel bar is ≥ 9%.
★★Test Point 5 Requirements for Steel Bar Rolling Marks
The national standard also stipulates that hot-rolled ribbed steel bars should be rolled with a brand mark and production enterprise serial number on their surface (after the license 3 digits) and nominal diameter in millimeters, and can also be rolled with the registered factory name (or trademark). The steel bar grade is represented by Arabic numerals or Arabic numerals plus English letters, HRB400 HRB500 HRB600 is represented by 4, 5, and 6 respectively, HRBF400, HRBF500 is represented by C4 and C5 respectively, HRB400E and HRB500E are represented by 4E and 5E respectively, HRBF400E and HRBF500E are represented by C4E and C5E respectively. The factory name is represented by the Chinese pinyin prefix and the nominal diameter in millimeters is represented by Arabic numerals.
★Test Point 6 What is stainless steel? What is the thickness of the sheet?
Stainless steel refers to iron-based alloy steel with a chromium content of more than 12%. Stainless steel materials used for building decoration mainly include thin plates (thickness <2mm) and pipes and profiles made of thin plates.
Stainless steel is usually defined as an iron-based alloy with corrosion resistance containing more than 12% chromium. According to the type of corrosion-resistant alloy elements contained, it can be divided into chromium stainless steel, nickel-chromium stainless steel, and nickel-chromium-titanium stainless steel. According to the structural characteristics of stainless steel, it can be divided into martensitic stainless steel, ferritic stainless steel and austenitic stainless steel. Austenitic stainless steel is a nickel-chromium stainless steel and is the most widely used stainless steel variety.
★★★Test point 7 What are the mechanical properties and process properties of steel bars?
The main properties of steel include mechanical properties and process properties. Among them, mechanical properties are the most important performance of steel, including tensile properties, impact properties, fatigue properties, etc. Process properties represent the behavior of steel during various processing processes, including bending properties and welding properties.
★★★Test point 8 Tensile performance index of steel bars?
Indicators reflecting the tensile properties of construction steel include yield strength, tensile strength and elongation.
★Test Point 9 What is the impact of the impact properties of steel bars? How should steel bars be selected at negative temperatures?
The impact properties of steel are greatly affected by temperature, and the impact properties decrease as the temperature drops; when it drops to a certain temperature range, the impact value drops sharply, which can cause brittle fracture of the steel. This This property is called the cold brittleness of steel, and the temperature at this time is called the critical brittleness temperature. The lower the value of the brittle critical temperature, the better the low-temperature impact properties of the steel. Therefore, for structures used at negative temperatures, steel with a lower brittleness critical temperature than the service temperature should be selected.
★The fatigue performance of test point 10 steel bars? Directly proportional to what?
The fatigue limit of steel is related to its tensile strength. Generally, the higher the tensile strength, the higher the fatigue limit.
★★Test Point 11 What is the most important element of steel? Main plus element?
Carbon is the most important element that determines the properties of steel.
Silicon is the main alloying element in steel for steel bars in my country.
★★Test point 12: What is the impact of manganese on steel?
Manganese can reduce the thermal brittleness caused by sulfur and oxygen
★★★Test point 13 What are the harmful elements in steel? Increase cold brittleness? Cause thermal brittleness?
(1) Phosphorus: Phosphorus is one of the most harmful elements in carbon steel. As the phosphorus content increases, the strength and hardness of the steel increase, but the plasticity and toughness decrease significantly. In particular, the lower the temperature, the greater the impact on plasticity and toughness, thereby significantly increasing the cold brittleness of the steel and significantly reducing the weldability of the steel. However, phosphorus can improve the wear resistance and corrosion resistance of steel, and can be used as an alloying element in combination with other elements in low alloy steel.
(2) Sulfur: Sulfur is also a very harmful element. It exists in steel as non-metallic sulfide inclusions and reduces various mechanical properties of steel. The low melting point caused by sulfide makes steel prone to thermal cracks during welding, resulting in a phenomenon called thermal brittleness. Sulfur reduces the weldability, impact toughness, fatigue resistance and corrosion resistance of steel.
(3) Oxygen: Oxygen is a harmful element in steel and will reduce the mechanical properties of steel, especially its toughness. Oxygen has the effect of promoting aging tendency. The low melting point caused by oxides also worsens the weldability of steel.
★★Test point 14 What is the impact of carbon, nitrogen and phosphorus on steel?
The effect of nitrogen on the properties of steel is similar to that of carbon and phosphorus. It will increase the strength of steel and significantly reduce the plasticity, especially the toughness.
★★Integration point 1 What index is used to express the weldability of steel?
Weldability of steel: refers to the degree to which a material is easy to weld and meets structural performance under appropriate design and working conditions. Weldability is often affected by factors such as the chemical composition of the steel, rolling method and plate thickness. In order to evaluate the influence of chemical composition on weldability, it is generally expressed by carbon equivalent (Ceq). The smaller the Ceq table, the smaller the hardening tendency of the steel and the better the weldability; conversely, the larger the Ceq table, the better the quenching tendency of the steel. The greater the hard tendency, the worse the weldability.
★★★Integration point 2 What is the difference between the plasticity indicators of steel and steel bars?
The ductility of steel bars is usually expressed by the elongation measured by a tensile test.
The plasticity of steel is usually expressed by the elongation measured by a tensile test.
The plasticity of steel bars is expressed by cold bending.
★★★Integration point 3 re-inspection and hidden inspection of steel
1. What is the content of the acceptance of hidden steel projects?
(1) The grade, specification, quantity, location, etc. of longitudinal steel bars;
(2) The grade, specification, quantity, spacing, location, and The bending angle of the stirrup hook and the length of the straight section;
(3) The connection method of the steel bar, the joint position, the joint quality, the joint area percentage, etc.; increase the overlap length, anchoring method and anchorage length.
(4) Specifications, quantity, location, etc. of embedded parts.
2. What is the content of steel bar entry acceptance?
(1) When steel bars enter the site, test specimens should be taken for yield strength, tensile strength, elongation, bending performance and weight deviation inspection in accordance with the relevant national standards (formed steel bars are not inspected for bending performance), the inspection results should comply with the provisions of the corresponding standards.
(2) For formed steel bars made of hot-rolled steel bars, when a representative from the construction unit or supervision unit is stationed at the factory to supervise the production process and provide a third-party inspection report on the mechanical properties of the raw steel bars, it can Only the maximum deviation test is performed.
3. Under what circumstances should all steel products be sampled and re-inspected?
1) Steel imported from abroad;
2) Mixed batches of steel;
3) Thick plates with plate thickness ≥40mm and designed with Z-direction performance requirements ;
4) The safety level of the building structure is Level 1, and the steel used in the main stress-bearing components of the long-span steel structure;
5) The steel is designed with re-inspection requirements;< /p>
6) Steel with doubtful quality.