First, the production process of steel
rebar steel surface is ribbed, also called ribbed steel bar, usually with two longitudinal ribs and transverse ribs uniformly distributed along the longitudinal direction. The shape of the transverse ribs is spiral, herringbone, and crescent. It is expressed in millimeters of the nominal diameter. The nominal diameter of ribbed steel bars is equivalent to the nominal diameter of smooth round steel bars with equal cross-sections. The nominal diameter of the steel bars is 8-50 mm, and the recommended diameters are 8, 12, 16, 20, 25, 32, and 40 mm. Ribbed steel bars mainly bear tensile stress in concrete. Ribbed steel bars have greater bonding capacity with concrete due to the action of ribs, so they can better withstand external forces. Ribbed steel bars are widely used in various building structures, especially large, heavy, light thin-walled and high-rise building structures.
Rebar is produced by small rolling mills. The main types of small rolling mills are divided into continuous, semi-continuous and horizontal. At present, most of the newly built and in use in the world are full continuous small rolling mills. Today’s popular steel bar rolling mills include general-purpose high-speed rolling steel bar rolling mills and 4-splitting high-yield steel bar rolling mills.
The billet used in the continuous small rolling mill is generally a continuous cast billet, the side length is generally 130-160mm, the length is generally about 6-12 meters, and the unit weight of the billet is 1.5-3 tons. Most of the rolling lines are arranged alternately between horizontal and vertical to realize full-line rolling without torsion. According to different billet specifications and finished product sizes, there are 18, 20, 22, and 24 small rolling mills, with 18 being the mainstream. At present, bar rolling adopts new technologies such as walking heating furnace, high-pressure water descaling, low-temperature rolling, and endless rolling. Rough rolling and intermediate rolling are developing in the direction of adapting to large billets and improving rolling accuracy. Mainly to improve accuracy and speed (up to 18m/s). The product specifications are generally ф10-40mm, and there are also ф6-32mm or ф12-50mm. The steel grades produced are low, medium and high carbon steels and low alloy steels that are in large demand in the market; the maximum rolling speed is 18m/s. The production process is as follows:
Walking heating furnace→rough rolling mill→intermediate rolling mill→finishing rolling mill→water cooling device→cooling bed→cold shear→automatic counting device→baler→discharging stand
Second, the characteristics and quality of the steel
1, the classification of steel
rebar commonly classified in two ways: First, the geometry of classification, according to the cross-sectional shape of rib and rib spacing different type or classification, such as the British In the standard (BS4449), rebar is divided into type I and type II. This classification mainly reflects the gripping performance of rebar. The second is the performance classification (level). For example, in the Chinese standard (GB1499.2-2007), the rebar is divided into 3 grades according to the strength level (yield point/tensile strength); in the Japanese Industrial Standard (JI SG3112) , According to the comprehensive performance, the rebar is divided into 5 types; the British Standard (BS4461) also stipulates several grades of the performance test of the rebar. In addition, rebar can be classified according to its purpose, such as ordinary steel bars for reinforced concrete and heat-treated steel bars for reinforced concrete.
The hot-rolled ribbed steel bar for reinforced concrete in China is composed of HRB and the minimum yield point of the grade according to national standards. H, R, and B are the first letters of the three words: Hotrolled, Ribbed, and Bars respectively. Hot-rolled ribbed steel bars are divided into three grades: HRB335 (the old brand is 20MnSi), HRB400 (the old brand is 20MnSiV, 20MnSiNb, 20Mnti), and HRB500.
2, rebar specifications and quality
ordering principles rebar is in general meet the design performance requirements of the desired grip on the base, mechanical strength or mechanical process performance metrics based.
Specifications: The specifications of rebar should be specified in the import and export trade contract. Generally, it should include the standard grade (type code), the nominal diameter of the steel bar, the nominal weight (quality), the specified length and the allowable difference of the above indicators. Our standard recommends the rebar series with nominal diameters of 8, 10, 12, 16, 20, and 40mm. All quality requirements should meet the requirements of GB1499.2-2007 “Hot Rolled Ribbed Steel Bars”.
Appearance quality: ①Surface quality. Relevant national standards stipulate the surface quality of the rebar, requiring that the end should be cut straight, without cracks, scars and folds on the surface, and no harmful defects in use; ②Allowed value for deviation of external dimensions. The bending degree of the rebar and the requirements of the geometric shape of the steel bar are stipulated in the relevant national standards. For example, China’s national standards stipulate that the curvature of straight bars is not more than 6mm/m, and the total curvature is not more than 0.6% of the total length of the steel bars.
Grade and chemical composition of hot-rolled ribbed steel bars for reinforced concrete:
Grade chemical composition (%)
C Si Mn PS Ceq
HRB335 0.25 0.8 1.6 0.045 0.045 0.52
HRB400 0.25 0.8 1.6 0.045 0.045 0.54
HRB500 0.25 0.8 1.6 0.045 0.045 0.55
Chemical composition inspection:
(1) Inspection method: The standard inspection methods commonly used in the inspection and analysis of the above chemical composition are as follows: GB/T223, JISG1211-1215, BS1837, BS Manual 19, etc.
(2) Composition index: The main indexes for assessing the composition content of rebar are: C, Mn, P, S, Si, etc. The content is different for different grades, and the approximate range is: C (0.10～0.40%), Mn ＜1.80%, P＜0.050%, S＜0.050%, Si (0.60～1.00%).
Mechanical performance inspection:
(1) Performance indicators: The inspection items for evaluating the mechanical performance of rebar include tensile test (tensile strength, yield strength, elongation), bending test (one-time bending and reverse bending).
(2) Inspection method: ①Tensile test method: commonly used standard inspection methods are GB/T228-87, JISZ2201, JI SZ2241, ASTMA370, ГОСТ1497, BS18, etc.; ②Bending test method: commonly used standard inspection methods are GB/T232 -88, JISZ2248, ASTME290, ГОСТ1401 9 and so on.
Rebars are generally delivered in bare packaging and bundled, so please pay attention to moisture-proof during storage. Corrosion will adversely affect the performance of rebars.
Third, the use of
rebar rebar is widely used in civil engineering construction such as houses, bridges, and roads. From highways, railways, bridges, culverts, tunnels, flood control, dams and other public facilities, to the foundations, beams, columns, walls and slabs of housing construction, rebar are all indispensable structural materials.
With the continuous deepening of China’s urbanization, infrastructure construction and the vigorous development of real estate have a strong demand for rebar. Reinforced concrete structure is still the main structural form of our country’s architecture at present and for a long period of time in the future. Therefore, it can be expected that the demand and output of rebar will remain at a relatively high level.
According to statistics, the amount of steel used in China’s construction industry accounts for about 50% of the total steel consumption. As one of the industries with large resource consumption, the construction industry must adjust the structure of building material consumption to achieve sustainable development, vigorously apply high-strength steel bars and high-performance concrete, and take the road of economical development. If the steel and concrete currently used can be increased to a level of strength, it can bring huge savings to the society.
According to calculations, if the strength of the main stressed steel bars of reinforced concrete can be increased to 400-500N/mm2 in accordance with the requirements of the specification, about 10% of the current level of steel consumption can be saved.