A kind of steel for low-cost ribbed steel bars and its preparation method

Abstract

The present invention provides a low-cost steel for ribbed bars and a preparation method thereof. The chemical components in the steel are as follows by weight percentage: C 0.21% - 0.25%, Si 0.30% - 0.45%, Mn 0.40% - 0.65%, P ≤ 0.030%, S ≤ 0.030%, and Ceq ≤ 0.51%, with the rest being iron and inevitable impurities. The present invention adopts a production mode of "low-cost composition design + ultra-rapid cooling" to provide ribbed bar products that meet the physical quality requirements of ribbed bars with a yield strength ≥ 400 MPa and a strength ratio ≥ 1.08.

Description

Technical Field

[0001] The present invention relates to the technical field of iron and steel metallurgy, and particularly relates to a steel for low-cost ribbed steel bars and a preparation method thereof. Background Art

[0002] A certain steel plant exports ribbed steel bars to Myanmar. The product specifications of the ribbed steel bars are φ12 - 32mm, which are mainly used in the construction industry. Generally, the surface quality requirements for the ribbed steel bar products are few surface defects and high dimensional accuracy. The physical quality requirements for the ribbed steel bars are that the yield strength ≥ 400MPa and the strength ratio ≥ 1.08.

[0003] When domestic ribbed steel bars meeting the above quality requirements are produced, a microalloying process of adding vanadium and niobium is generally adopted. Although the quality requirements of the products can be obtained, the production cost is relatively high.

[0004] In response to the above quality requirements, a certain steel plant designs a production mode of "converter → ladle (deoxidation, alloying) → argon station (argon blowing refining) → small billet continuous casting → heating furnace heating → rolling → ultra-fast cooling → cooling bed cooling", and cooperates with a low-cost alloy composition design to produce ribbed steel bars for export to Myanmar that meet the physical quality requirements of "yield strength R eL ≥ 420Mpa, strength ratio ≥ 1.10. After natural aging, the yield strength ReL ≥ 400Mpa, strength ratio ≥ 1.08". Summary of the Invention

[0005] The purpose of the present invention is to provide a steel for low-cost ribbed steel bars and a preparation method thereof. The present invention adopts a production mode of "low-cost composition design + ultra-fast cooling" to provide ribbed steel bar products that meet the physical quality requirements of ribbed steel bars with a yield strength ≥ 400MPa and a strength ratio ≥ 1.08.

[0006] To achieve the above purpose, the present invention is realized by adopting the following technical solutions:

[0007] A steel for low-cost ribbed steel bars, the chemical components in the steel are calculated by weight percentage as follows: C 0.21% - 0.25%, Si 0.30% - 0.45%, Mn 0.40% - 0.65%, P ≤ 0.030%, S ≤ 0.030%, and Ceq ≤ 0.51%, and the rest is iron and inevitable impurities.

[0008] The yield strength of the ribbed steel bars is 440 - 535MPa, the tensile strength is 506 - 668MPa, the cross-sectional elongation rate is 20% - 28%, and the strength ratio is 1.15 - 1.25.

[0009] The metallographic structure in the steel is pearlite + ferrite.

[0010] The decarburized layer thickness of the steel bar is 1.8 - 1.9 mm.

[0011] The reasons for the component design of the present invention are as follows:

[0012] C: Carbon can improve the strength and hardness of steel. However, with the increase of carbon content, the plasticity and toughness of steel will decrease. The C content is controlled at 0.21% - 0.25%.

[0013] Si: Silicon can also improve the strength and hardness of steel, reduce the plasticity and toughness of steel, and has the function of deoxidizing in the steelmaking process. The Si content is controlled at 0.30% - 0.45%.

[0014] Mn: Manganese can improve the strength of steel, can combine with sulfur to form manganese sulfide to eliminate the "hot brittleness" of steel, and can improve the hardenability and wear resistance of steel. The Mn content is controlled at 0.40% - 0.65%.

[0015] P, S: Phosphorus and sulfur are harmful elements in steel. Phosphorus dissolves in ferrite. Although it can improve the strength and hardness of steel, the biggest harm is serious segregation, increasing temper brittleness, significantly reducing the plasticity and toughness of steel, resulting in easy brittle fracture during cold working of steel, that is, the so-called "cold brittleness" phenomenon; sulfur reduces the plasticity of steel, increases the brittleness of steel, and deteriorates the quality of steel. Among them, P ≤ 0.030%, S ≤ 0.030%.

[0016] Ceq represents the carbon equivalent. The chemical composition of steel has an impact on weldability. In the present invention, Ceq ≤ 0.51% is controlled.

[0017] A preparation method of low-cost steel for threaded steel bars is mainly to meet the mechanical property requirements during the use of exported threaded steel bars, while taking into account low-cost operation to ensure that the yield strength R of the exported threaded steel bars after rolling eL ≥ 420 Mpa, and the ratio of yield strength to tensile strength ≥ 1.10. After natural aging, the yield strength ReL ≥ 400 Mpa, and the ratio of yield strength to tensile strength ≥ 1.08. The following process is adopted: "converter + ladle (deoxidation, alloying) + argon blowing in argon station + continuous casting of 150 mm × 150 mm small square billets + heating in heating furnace + controlled rolling and controlled cooling rolling + ultra-fast cooling → cooling on cooling bed → sizing and shearing → sampling and inspection → bundling → hoisting and warehousing", which specifically includes the following steps:

[0018] 1) Converter smelting: Scrap steel accounts for 10% - 15%, and hot metal accounts for 85% - 90%; The smelting cycle is 26 - 32 min, with top-bottom combined blowing of oxygen, the oxygen blowing time is 13 - 15 min, and the oxygen blowing volume is 2100 - 2500 m 3 , the tapping temperature is 1680 - 1700 °C, the tapping end point C is 0.10% - 0.15%, P ≤ 0.010%, and slagging is strictly prohibited;

[0019] 2) Ladle alloying: When 1 / 4 to 1 / 3 of the molten steel is tapped, lime, pre-deoxidizer and ferroalloy are added for deoxidation and alloying, and the argon blowing time is ≥ 5 min; Target amount of alloy added per ton of steel: 5.7 - 7.1 kg of ferromanganese-silicon, 3.3 - 4.4 kg of ferrosilicon, 1.7 - 2.2 kg of steelmaking recarburizer; Amount of auxiliary materials added per ton of steel: 21 - 24 kg of quicklime, 12.2 - 13.2 kg of dolomite, 17.5 - 22 kg of limestone, 18 - 20 kg of return scrap (as coolant).

[0020] 3) Stationary argon blowing: The stationary argon blowing time is ≥ 8 min, the temperature before treatment is 1600 - 1635 °C, and the temperature after treatment is 1585 - 1600 °C;

[0021] 4) Continuous casting: Platform temperature is 1590 - 1600 °C, tundish temperature is 1535 - 1560 °C, superheat is maintained at 25 - 35 °C, and casting speed is 2.4 - 2.8 m / min; Protective casting is carried out throughout the process, and the mold powder used is the bloom medium carbon steel mold powder; The billet cutting adopts a combination of automatic and manual methods.

[0022] 5) Heating process: Pusher-type heating furnace, two-stage heating is adopted during the heating process, the temperature control of the heating section is 1170 - 1280 °C, and the temperature control of the soaking section is 1180 - 1250 °C.

[0023] 6) Rolling process: The starting rolling temperature is 990 - 1080 °C; The strong water-cooling process is adopted: The water-passing pressure of the second bar is 1.0 - 1.2 Mpa, and the water-passing pressure of the third bar is 1.3 - 1.7 Mpa; The tempering temperature is 600 - 640 °C.

[0024] 7) The product is air-cooled after being taken off the production line.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] The present invention adopts the production mode of "low-cost composition design + ultra-fast cooling" to achieve yield strength of 440 - 535 MPa, tensile strength of 506 - 668 MPa, cross-sectional elongation of 20% - 28%, and strength ratio of 1.15 - 1.25 for the threaded steel products. Description of the Drawings

[0027] Figure 1 It is the metallographic structure diagram of the product in Example 1.

[0028] Figure 2 It is the decarburized layer thickness diagram of the product in Example 1. Detailed Embodiments

[0029] The following further describes the specific embodiments of the present invention in conjunction with the embodiments.

[0030] The chemical components of the embodiments of the present invention are shown in Table 1; the manufacturing process parameters of the embodiments of the present invention are shown in Table 2; the addition amounts of ladle smelting auxiliary materials in the embodiments of the present invention are shown in Table 3; the macrostructure defects of the continuous casting billets in the embodiments of the present invention are shown in Table 4; the mechanical properties of the products in the embodiments of the present invention are shown in Table 5; the microstructure and non-metallic inclusions of the products in the embodiments of the present invention are shown in Table 6.

[0031] Table 1 Chemical Components of Embodiments

[0032]

[0033] Table 2 Manufacturing Process Parameters of Embodiments (I)

[0034]

[0035] Table 2 Manufacturing Process Parameters of Embodiments (II)

[0036]

[0037] Table 3 Addition Amounts of Ladle Smelting Auxiliary Materials in Embodiments

[0038]

[0039]

[0040] Table 4 Inspection Results of Macrostructure Defects of Continuous Casting Billets in Embodiments

[0041] Embodiment Central porosity Central segregation Shrinkage cavity Corner crack Edge crack Intermediate crack Central crack Subsurface blowhole Non-metallic inclusion 1 1.0 0.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5 2 0.5 0.5 0.5 0.5 0.5 0.5 1.0 0.5 0.5 3 0.5 0.5 0.5 0.5 0.5 0.5 1.0 0.5 0.5 4 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 5 0.5 0.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5 6 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 7 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 8 0.5 0.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5 9 0.5 0.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5 10 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

[0042] Table 5 Mechanical Properties of Products in Embodiments

[0043]

[0044] Table 6 Microstructure and Non-Metallic Inclusions of Products in Embodiments

[0045]

Claims

1. A steel for low-cost ribbed steel bars, characterized in that, The chemical components in the steel are by weight percentage: C 0.21% - 0.25%, Si 0.30% - 0.45%, Mn 0.53% - 0.60%, P ≤ 0.030%, S ≤ 0.030%, and Ceq ≤ 0.51%, with the rest being iron and inevitable impurities; The product specification of the ribbed steel bar is φ25mm; A preparation method of low-cost steel for ribbed steel bars, characterized by comprising the following steps: 1) Converter smelting: The smelting cycle is 26 - 32 min, with top and bottom combined blowing of oxygen. The oxygen blowing time is 13 - 15 min, and the oxygen blowing volume is 2100 - 2500 m 3 , the tapping temperature is 1680 - 1700 °C, the C content at the end of tapping is 0.10% - 0.15%, and P ≤ 0.010%; 2) Ladle alloying: Deoxidizing alloying is carried out when 1 / 4 - 1 / 3 of the steel is tapped, and the argon blowing time ≥ 5 min; 3) Stationary argon blowing: The stationary argon blowing time ≥ 8 min, the temperature before treatment is 1600 - 1635 °C, and the temperature after treatment is 1585 - 1600 °C; 4) Continuous casting: The platform temperature is 1590 - 1600 °C, the tundish temperature is 1535 - 1560 °C, the superheat is maintained at 25 - 35 °C, and the casting speed is 2.4 - 2.8 m / min; 5) Heating process: Pusher-type heating furnace, the heating process adopts two-stage heating, the temperature control in the heating section is 1170 - 1280 °C, and the temperature control in the soaking section is 1180 - 1250 °C; 6) Rolling process: The starting rolling temperature is 990 - 1080 °C; Adopt a strong water-cooling process: The water-passing pressure of the second bar is 1.0 - 1.2 Mpa, and the water-passing pressure of the third bar is 1.3 - 1.7 Mpa: The tempering temperature is 600 - 640 °C; In the converter smelting in the above step 1), the scrap steel accounts for 15% and the hot metal accounts for 85%; In the above step 2), the addition amount of auxiliary materials per ton of steel is 21 - 24 kg of quicklime, 12.2 - 13.2 kg of dolomite, 17.5 - 22 kg of limestone, and 18 - 20 kg of return scrap.

2. The steel for low-cost ribbed steel bars according to claim 1, characterized in that, The yield strength of the ribbed steel bar is 440 - 535 MPa, the tensile strength is 506 - 668 MPa, the elongation is 20% - 28%, and the strength ratio is 1.15 - 1.

25.

3. A steel for low-cost ribbed steel bars according to claim 1, characterized in that, The metallographic structure of the steel is pearlite + ferrite.

4. A steel for low-cost ribbed steel bars according to claim 1, characterized in that, The decarburized layer thickness of the steel bar is 1.8 - 1.9 mm.

5. The preparation method of a kind of steel for low-cost ribbed steel bars according to claim 1, characterized in that, In the above step 3), 1 kg - 1.2 kg / t of pure calcium cored wire is fed after stationary argon blowing, and the wire feeding speed is 2.8 - 3.2 m / s.

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