Method for improving performance stability of high-strength deformed steel bar

Abstract

The application discloses a method for improving the performance stability of high-strength reinforced bar, and relates to the technical field of steel smelting; the method comprises the following steps: S1, converter smelting: the molten iron and scrap steel are loaded into a converter, slag material is added, oxygen is blown for decarburization and dephosphorization, and after tapping, the molten iron enters a refining process; S2, LF refining: slagging is performed for desulfurization, and alloy raw materials are added into the converter; S3, LF bottom blowing nitrogen: bottom blowing nitrogen is performed according to the nitrogen content in the molten iron, and the molten iron is prepared; S4, the molten iron is sequentially subjected to continuous casting, heating and rolling. In the application, the heterogeneous nucleation mechanism is reasonably utilized, and proper Nb is added into the steel, so that Nb(C, N) with a relatively high precipitation temperature is first generated in the cooling process of the steel, nucleation base points are provided for the subsequent V(C, N) precipitation in the cooling process of the steel, and the stable precipitation of V(C, N) is promoted. Nitrogen-containing compounds such as Nb(C, N) and V(C, N) in the high-strength reinforced bar can be effectively promoted to precipitate, and the nitrogen-containing compounds have the effects of stabilizing and improving the performance.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting technology, specifically a method for improving the performance stability of rebar. Background Technology

[0002] HRB400E threaded steel bars refer to hot-rolled ribbed threaded steel bars with a yield strength greater than 400 MPa and seismic resistance. Their smelting method typically involves adding microalloying elements to grade II HRB335 steel bars and using controlled rolling and cooling processes to achieve high yield strength, good plasticity, weldability, and a relatively low ductile-brittle transition temperature. Due to its relatively low price, good mechanical properties and elongation, and a tensile strength greater than 400 MPa, HRB400E threaded steel bars have become a major construction material in current engineering projects. With the continuous increase in infrastructure construction, the annual demand is enormous, and the reliability of this product has a significant impact on the safety of engineering structures.

[0003] The unstable mechanical properties of HRB400E rebar caused by yield strength fluctuations are a significant problem frequently encountered during its production and service. Due to factors such as smelting levels and the supply of continuously cast billets, many steel enterprises experience varying degrees of fluctuation in the chemical composition of their continuously cast billets, mixed loading of cold and hot billets into the furnace, uneven cooling at different cutting lines during slitting and rolling, and ultimately, significant fluctuations in the yield strength of the finished rebar. In severe cases, these fluctuations can reach 100 MPa, severely impacting the product's performance stability and service reliability. While related technologies typically use large amounts of expensive alloying elements to ensure the rebar's performance, excessive addition of these elements can drastically increase production costs. Summary of the Invention

[0004] The purpose of this invention is to provide a method for improving the performance stability of rebar, so as to solve at least one aspect of the problems and defects mentioned in the background art.

[0005] Specifically, the present invention provides a method for improving the performance stability of rebar, comprising the following steps:

[0006] S1. Converter smelting:

[0007] Molten iron and scrap steel are loaded into a converter, slag is added, oxygen is blown to decarburize and dephosphorize, and the steel is then tapped into the refining process.

[0008] S2, LF Refining:

[0009] Slag formation and desulfurization are carried out simultaneously, and alloy raw materials are added to the furnace.

[0010] S3, LF bottom-blowing nitrogen:

[0011] Nitrogen is blown from the bottom of the molten steel to produce molten steel based on the nitrogen content of the molten steel.

[0012] S4. Molten steel is continuously cast, heated, and rolled in sequence to produce rebar;

[0013] The mass fraction of nitrogen (N) in the rebar is 0.012% to 0.02%.

[0014] The mass fraction of Nb in the rebar is 0.003% to 0.01%.

[0015] According to one technical solution of the method of the present invention, at least the following beneficial effects are achieved:

[0016] The method of this invention makes reasonable use of the high formation temperature of Nb (C, N) and utilizes the heterogeneous nucleation theory to significantly improve the stability of V (C, N) precipitation, thereby ensuring the stability of precipitation strengthening and effectively stabilizing the yield strength.

[0017] In V-Nb-N microalloyed rebar, excessively low nitrogen (N) levels can hinder the formation of Nb(C,N) and V(C,N) precipitates, thus reducing precipitation strengthening. Conversely, excessively high N levels can increase the steel's brittleness and negatively impact its usability.

[0018] In conclusion, nitrogen blowing and niobium addition can significantly enhance the stability of steel yield strength, thereby facilitating lean production and cost reduction.

[0019] According to some embodiments of the present invention, the mass ratio of molten iron to scrap steel is 3 to 4:1.

[0020] According to some embodiments of the present invention, the mass fraction of C in the rebar is 0.2% to 0.25%.

[0021] According to some embodiments of the present invention, the mass fraction of P in the rebar is 0 to 0.03%.

[0022] According to some embodiments of the present invention, the mass fraction of Si in the rebar is 0.35 to 0.5%.

[0023] According to some embodiments of the present invention, the mass fraction of Mn in the rebar is 1.1% to 1.5%.

[0024] According to some embodiments of the present invention, the mass fraction of sulfur in the rebar is 0 to 0.03%.

[0025] According to some embodiments of the present invention, the mass fraction of V in the rebar is 0.015 to 0.03%.

[0026] According to some embodiments of the present invention, the casting speed of the continuous casting is 2.5 to 3.0 m / min.

[0027] According to some embodiments of the present invention, the heating temperature for tapping steel is 900–950°C.

[0028] According to some embodiments of the present invention, the temperature of the upper cooling bed after rolling is 850°C to 900°C.

[0029] According to some embodiments of the present invention, the thickness after rolling is 12 to 28 mm.

[0030] According to some embodiments of the present invention, the rolling time is 85 to 95 minutes.

[0031] According to some embodiments of the present invention, the rebar is composed of the following elements in parts by weight:

[0032] C 0.2-0.25%, Si 0.35-0.5%, Mn 1.1-1.5%, V 0.015-0.03%, Nb 0.005-0.01%, P 0-0.03%, S 0-0.03%, O 0-0.002%, N 0.012-0.018%, and the balance being Fe.

[0033] According to some embodiments of the present invention, the rebar is composed of the following elements in parts by weight:

[0034] C 0.2-0.25%, Si 0.35-0.45%, Mn 1.1-1.3%, V 0.015-0.03%, Nb 0.005-0.006%, P 0-0.03%, S 0-0.03%, O 0-0.002%, N 0.012-0.018%, and the balance being Fe.

[0035] According to some embodiments of the present invention, the rebar is composed of the following elements in parts by weight:

[0036] C 0.2-0.25%, Si 0.35-0.45%, Mn 1.1-1.3%, V 0.015-0.03%, Nb 0.005-0.006%, P 0-0.03%, S 0-0.03%, O 0-0.002%, N 0.015-0.018%, and the balance being Fe.

[0037] According to some embodiments of the present invention, the standard deviation of the yield strength of the rebar is 5 MPa to 10 MPa. Attached Figure Description

[0038] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0039] Figure 1 Typical precipitate morphology for nitrogen-blown niobium-coated rebar. Detailed Implementation

[0040] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0041] In the description of this invention, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0042] Unless otherwise specified in the examples, the procedures should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0043] To better explain and understand the content of this invention, a more detailed description of the invention will be provided through specific embodiments.

[0044] In the embodiments of the present invention described below, the smelting equipment is BOF-LF-CC, the average weight of molten steel per furnace is 105t, and the steel grade smelted is HRB400E rebar (composition control is shown in Table 1).

[0045]

[0046] Example 1

[0047] This embodiment describes a method for improving the performance stability of rebar, which consists of the following steps:

[0048] S1. Converter smelting:

[0049] 85 tons of molten iron and 25 tons of scrap steel were loaded into the converter, along with slag materials: 2000 kg of lime and 500 kg of magnesium balls. Oxygen was blown in for decarburization and dephosphorization at a flow rate of 25000 Nm. 3 / h, total oxygen blowing volume approximately 5000 Nm³ 3 .

[0050] After tapping, the steel enters the refining process;

[0051] S2, LF Refining:

[0052] Slag formation and desulfurization are carried out simultaneously, and silicon manganese, carbon powder, ferrosilicon, vanadium nitrogen, and niobium ferroalloy materials are added to the furnace.

[0053] S3, LF bottom-blowing nitrogen:

[0054] The bottom blowing nitrogen process is based on the nitrogen content of the molten steel. The nitrogen blowing process is as follows: for N < 0.01%, nitrogen gas is blown in at a rate of 10m³. 3 0.01% ≤ N < 0.015% nitrogen gas was blown in for 6m 3 When N ≥ 0.015%, nitrogen is no longer blown in to obtain molten steel;

[0055] S4. The molten steel is sequentially cast (casting speed is 2.5m / min), heated (tap temperature is 900℃) and rolled (rolled thickness is 14mm, rolling time is 90min, upper cooling bed temperature is 875℃).

[0056] This embodiment is a method for improving the performance stability of rebar. The difference from Embodiments 1 and 2 is that the composition of the final rebar is shown in Table 2.

[0057] Comparative Example 1

[0058] This comparative example illustrates a method for preparing rebar, comprising the following steps:

[0059] S1. Converter smelting:

[0060] 85 tons of molten iron and 25 tons of scrap steel were loaded into the converter, along with slag materials: 2000 kg of lime and 500 kg of magnesium balls. Oxygen was blown in for decarburization and dephosphorization at a flow rate of 25000 Nm. 3 / h, total oxygen blowing volume approximately 5000 Nm³ 3 After tapping, the steel enters the refining process;

[0061] S2, LF Refining:

[0062] Slag formation and desulfurization are carried out simultaneously, and silicon manganese, carbon powder, ferrosilicon, and vanadium-nitrogen alloy materials are added to the furnace.

[0063] S3, LF bottom-blown argon:

[0064] Argon is blown into the bottom throughout the LF refining process;

[0065] S4. The molten steel is sequentially cast (casting speed is 2.5m / min), heated (tap temperature is 900℃) and rolled (rolled thickness is 14mm, rolling time is 90min, upper cooling bed temperature is 875℃).

[0066] Comparative Example 2

[0067] This embodiment describes a method for improving the performance stability of rebar, which consists of the following steps:

[0068] S1. Converter smelting:

[0069] 85 tons of molten iron and 25 tons of scrap steel were loaded into the converter, along with slag materials: 2000 kg of lime and 500 kg of magnesium balls. Oxygen was blown in for decarburization and dephosphorization at a flow rate of 25000 Nm. 3 / h, total oxygen blowing volume approximately 5000 Nm³ 3 After tapping, the steel enters the refining process;

[0070] S2, LF Refining:

[0071] Slag formation and desulfurization are carried out simultaneously, and silicon manganese, carbon powder, ferrosilicon, and vanadium-nitrogen alloy materials are added to the furnace.

[0072] S3, LF bottom-blowing nitrogen:

[0073] The bottom blowing nitrogen process is based on the nitrogen content of the molten steel. The nitrogen blowing process is as follows: for N < 0.01%, nitrogen gas is blown in at a rate of 10m³. 3 0.01% ≤ N < 0.015% nitrogen gas was blown in for 6m 3 When N ≥ 0.015%, nitrogen is no longer blown in to obtain molten steel;

[0074] S4. The molten steel is sequentially cast (casting speed is 2.5m / min), heated (tap temperature is 900℃) and rolled (rolled thickness is 14mm, rolling time is 90min, upper cooling bed temperature is 875℃).

[0075] Comparative Example 3

[0076] This embodiment describes a method for improving the performance stability of rebar, which consists of the following steps:

[0077] S1. Converter smelting:

[0078] 85 tons of molten iron and 25 tons of scrap steel were loaded into the converter, along with slag materials: 2000 kg of lime and 500 kg of magnesium balls. Oxygen was blown in for decarburization and dephosphorization at a flow rate of 25000 Nm. 3 / h, total oxygen blowing volume approximately 5000 Nm³ 3 After tapping, the steel enters the refining process;

[0079] S2, LF Refining:

[0080] Slag formation and desulfurization are carried out simultaneously, and silicon manganese, carbon powder, ferrosilicon, vanadium nitrogen, and niobium ferroalloy materials are added to the furnace.

[0081] S3, LF bottom-blown argon:

[0082] Argon is blown into the bottom throughout the LF refining process;

[0083] S4. The molten steel is sequentially cast (casting speed is 2.5m / min), heated (tap temperature is 900℃) and rolled (rolled thickness is 14mm, rolling time is 90min, upper cooling bed temperature is 875℃).

[0084] The average composition of the rebar obtained in Example 1 and Comparative Examples 1-3 of this invention is shown in Table 2, with data from 500 heats of each sample.

[0085]

[0086] The HRB400E steels prepared in Example 1 and Comparative Examples 1-3 were processed according to GB / T228-2002 "Metallic Materials - Tensile Testing at Room Temperature" and then subjected to room temperature tensile tests at 25°C and a tensile speed of 2 mm·min. -1 The final statistical results are shown in Table 3.

[0087]

[0088] As shown in Table 3, the average tensile strength, yield strength, and elongation after fracture of Example 1 are almost the same as those of Comparative Examples 1-3. However, the standard deviation of the yield strength of Example 1 is 7 MPa, 4 MPa, and 3 MPa lower than that of Comparative Examples 1-3, respectively. This indicates that the performance fluctuation of Example 1 is significantly reduced, and the rebar exhibits higher yield strength stability under the condition of simultaneous nitrogen blowing and niobium addition. The reason is that the Nb and N content added simultaneously in Example 1 are controlled within a reasonable range. Since the formation temperature of Nb(C,N) is relatively high, adding a certain amount of Nb to the steel can form a certain amount of Nb(C,N) in the steel earlier. Utilizing the heterogeneous nucleation theory, the early-formed Nb(C,N) can significantly improve the stability of precipitation such as V(C,N), thereby ensuring the stability of precipitation strengthening and effectively stabilizing the yield strength. The N element can promote the formation of precipitates such as Nb(C,N) and V(C,N), improving the grain refinement and precipitation strengthening effect. Typical precipitates in the rebar smelted according to the method described in Example 1 are as follows: Figure 1 As shown, by Figure 1 It can be seen that the precipitate contains N and Nb, and also contains C and V. Under the condition that Nb (C, N) precipitates first, it is entirely possible for V (C, N) to undergo heterogeneous nucleation around it. Figure 1The elemental mass contents in (j) are: C: 34.02%; N: 20.45%; V: 3.55% and Nb: 41.98%; Figure 1 The nitrogen content in the steel (C: 35.77%; N: 28.34%; V: 21.31% and Nb: 14.58%) is as follows: Under conventional smelting conditions, the nitrogen content in steel fluctuates greatly. By controlling the nitrogen content in the steel to a stable range through bottom-blowing nitrogen blowing during refining, the stability of the yield strength of rebar can be significantly improved. In summary, nitrogen blowing and niobium addition can significantly enhance the stability of the yield strength of rebar.

[0089] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for improving the performance stability of rebar, characterized in that, Includes the following steps: S1. Converter smelting: Molten iron and scrap steel are loaded into the converter, slag is added, oxygen is blown to decarburize and dephosphorize, and the steel is then tapped into the refining process. S2. LF Refining: Slag formation and desulfurization, while adding alloy raw materials to the furnace, including silicon manganese, carbon powder, ferrosilicon, vanadium-nitrogen alloy and ferroniobium; S3. LF Bottom-blowing Nitrogen: Bottom-blowing nitrogen is performed based on the nitrogen content of the molten steel. The nitrogen blowing process is as follows: when the N content in the molten steel is less than 0.01%, 10m³ of nitrogen gas is blown in. 3 When 0.01%≤N<0.015%, nitrogen gas is blown in for 6m. 3 When N ≥ 0.015%, nitrogen blowing is stopped to obtain molten steel; S4. Molten steel is continuously cast, heated, and rolled in sequence to produce rebar; The mass fraction of nitrogen (N) in the rebar is 0.012%~0.018%; The mass fraction of Nb in the rebar is 0.005%~0.006%; The method improves the stability of the yield strength of rebar by controlling the content of Nb and N and utilizing the heterogeneous nucleation mechanism, so that the first precipitated Nb(C,N) provides nucleation sites for the subsequent V(C,N). The mass fraction of carbon in the rebar is 0.2% to 0.25%. The mass fraction of phosphorus (P) in the rebar is 0% to 0.03%. The mass fraction of Si in the rebar is 0.35% to 0.5%; The mass fraction of Mn in the rebar is 1.1% to 1.5%; The mass fraction of sulfur in the rebar is 0% to 0.03%. The mass fraction of V in the rebar is 0.015% to 0.03%.

2. The method for improving the performance stability of rebar according to claim 1, characterized in that, The casting speed is 3.0 m / min to 3.5 m / min.

3. The method for improving the performance stability of rebar according to claim 1, characterized in that, The temperature of the cooling bed after rolling is 850℃~900℃.

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