800MPa-grade online heat treatment anti-fatigue high-strength steel bar and production method thereof

Abstract

The invention discloses a 800MPa-grade online heat treatment anti-fatigue high-strength steel bar and a production method thereof, and belongs to the technical field of hot-rolled ribbed steel bars for concrete, the steel bar comprises the following chemical components in percentage by weight: 0.40-0.50% of C, 0.80-1.20% of Si, 1.00-1.20% of Mn, 0.15-0.25% of V, 0.010-0.030% of Nb, 0.80-1.0% of Cr, 0.015-0.025% of N, and the balance of Fe and inevitable impurities. The metallographic structure of the surface of the steel for the reinforcing steel bar is tempered sorbite, the metallographic structure of the core is ferrite and pearlite, the steel for the reinforcing steel bar has excellent comprehensive mechanical performance and anti-fatigue performance, the tensile strength of the steel is larger than or equal to 950 MPa, the percentage elongation after fracture is larger than or equal to 14%, the total elongation under the maximum force is larger than or equal to 7.5%, and the steel for the reinforcing steel bar is qualified after a one-way fatigue tension test is conducted for 5 million times.

Description

Technical Field

[0001] The invention belongs to the technical field of hot-rolled ribbed steel bars for concrete, and particularly relates to an 800MPa-grade online heat-treated fatigue-resistant high-strength steel bar and a production method thereof. Background Art

[0002] High-strength steel bars offer higher strength and superior performance than ordinary steel bars, making them particularly suitable for high-rise buildings, long-span bridges, and large-scale space structures. They significantly save materials and achieve lightweight construction steel, aligning with my country's national policy of sustainable development. For engineering construction, high-strength steel bars contribute to energy conservation and fuel conservation, promoting the scientific development of engineering construction, and aiding the adjustment of product mix and the elimination of outdated production capacity within the steel industry.

[0003] Compared to the currently dominant 400MPa grade, 800MPa high-strength steel bars can further address the problem of "fat beams and columns" in building structures, increasing the usable area of buildings, improving their functionality, and enabling more flexible and rational structural design. Currently, construction steel bars account for approximately 20% of my country's total steel production. Improving the grade, quality, and stability of rebar products is crucial for easing energy conservation and emissions reduction in the steel industry and accelerating the shift from a focus on scale expansion to one focused on quality and efficiency.

[0004] Chinese patent CN110760735A discloses a PSB830 fine-rolled threaded steel bar and its production method. Its chemical composition and mass percentages are as follows: C: 0.43% to 0.48%, Si: 1.60% to 1.80%, Mn: 0.90% to 1.00% (Cr+Ni+Cu) ≤ 0.10%, Nb: 0.01% to 0.025%, V: 0.125% to 0.155%, P ≤ 0.020%, S ≤ 0.015%, N: 75 to 110 ppm, the remainder being Fe and unavoidable impurities. The steel has a yield strength of Rel ≥ 830 MPa, a tensile strength of Rm ≥ 1030 MPa, an elongation after fracture of ≥ 6%, and a total elongation at maximum force of ≥ 3.5%. Although the PSB830 fine-rolled threaded steel bar disclosed in this patent has high mechanical strength, it has poor plasticity. Summary of the Invention

[0005] The object of the present invention is to provide an 800MPa grade online heat-treated fatigue-resistant high-strength steel bar and a production method thereof. The metallographic structure of the steel bar surface is tempered troostite, and the metallographic structure of the core is ferrite and pearlite, and the steel bar has excellent comprehensive mechanical properties and fatigue resistance.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] The invention provides an 800MPa-level online heat-treated fatigue-resistant high-strength steel bar. The 800MPa-level online heat-treated fatigue-resistant high-strength steel bar comprises the following chemical components in weight percentage: C 0.40-0.50%, Si 0.80-1.20%, Mn 1.00-1.20%, V 0.15-0.25%, Nb 0.010-0.030%, Cr 0.80-1.0%, N 0.015-0.025%, P≤0.020%, S≤0.010%, and the remainder is Fe and unavoidable impurity elements.

[0008] The surface metallographic structure of the 800MPa grade online heat-treated fatigue-resistant high-strength steel bar is tempered sorbite, and the core metallographic structure is ferrite and pearlite.

[0009] The 800MPa grade online heat-treated fatigue-resistant high-strength steel bar has a yield strength of ≥800MPa, a tensile strength of ≥950MPa, an elongation after fracture of ≥14%, a total elongation under maximum force of ≥7.5%, and passes the unidirectional fatigue tensile test for 5 million times.

[0010] The present invention also provides a production method for the 800MPa grade online heat-treated fatigue-resistant high-strength steel bar, which comprises the following steps: smelting, continuous casting of square billets, hot rolling, online heat treatment, and slow cooling on a cooling bed; wherein the temperature of the upper cooling bed is controlled at 700-750°C.

[0011] The smelting steps specifically include: converter or electric furnace smelting, and LF furnace refining.

[0012] In the converter smelting step, the converter end point C ≥ 0.05%, P ≤ 0.015%, S ≤ 0.015%, and the steel tapping temperature ≥ 1670°C.

[0013] In the billet continuous casting step, full-process protected pouring is adopted.

[0014] In the hot rolling step, the heating temperature is controlled at 1080-1200° C. and the heating time is ≥60 min, which fully ensures the solid solution of alloy elements such as V, Nb, and Cr in the steel to exert their precipitation strengthening effect.

[0015] No additional temperature control equipment is required during the hot rolling process, and all existing bar rolling mills in China can achieve this rolling.

[0016] In the online heat treatment step, the rolled steel bars immediately enter the online heat treatment water cooling device with a cooling water pressure of 2.4-2.8 MPa. When the steel bar surface temperature is ≤400°C, the steel bars are transferred to the cooling bed via a conveyor roller for slow cooling.

[0017] Further, the product is placed on a cooling bed and slowly cooled until it is completely cooled.

[0018] The functions and controls of the various components of the steel bars provided by the present invention are as follows:

[0019] Carbon is a primary alloying element in steel. It forms a solid solution in both austenite and ferrite, exerting a solid solution strengthening effect. It can also form carbides with Nb and V, which precipitate and exert a strengthening effect. However, increasing the carbon content decreases plasticity and toughness. Therefore, to ensure that the steel's strength meets the required performance, the carbon content should be minimized. The carbon content in the present invention is 0.40-0.50%.

[0020] Si forms a solid solution in steel, plays a role in solid solution strengthening, and can improve the strength of steel. However, when it exceeds 2%, it will significantly reduce the plasticity and toughness of steel. The Si content of the present invention is 0.80-1.20%.

[0021] At low concentrations, Mn can dissolve in steel, improving hardenability and significantly increasing its strength. However, at higher concentrations, temper brittleness can be more pronounced and grain size can be increased. When the mass fraction of manganese exceeds 1.2%, the weldability of the steel deteriorates. The Mn content of the present invention is between 1.00 and 1.20%.

[0022] Both V and Nb have precipitation strengthening and grain refining strengthening effects, which can significantly improve the strength of steel while ensuring the elongation of steel. V mainly plays a precipitation strengthening role, while Nb mainly plays a grain refining strengthening role. In the present invention, the V content is 0.15-0.25%, and the Nb content is 0.010-0.030%.

[0023] Nitrogen was once considered a harmful element, increasing steel's tendency to harden and making it brittle. Later research revealed that adding nitrogen to steel can improve its strength and corrosion resistance. In particular, in V-containing steels, nitrogen promotes V precipitation and refines grains. The precipitation strengthening effect of VN steel is twice that of V steel. In the present invention, the nitrogen content is 0.015-0.025%.

[0024] Cr can significantly increase the hardenability of steel and has a high strengthening effect on low alloy steel, improving the strength, hardness and wear resistance of steel. However, as the Cr content increases, the temper brittleness tendency of steel increases. The Cr content of the present invention is 0.8-1.0%.

[0025] The present invention provides a method for producing 800MPa-grade online heat-treated fatigue-resistant high-strength steel bars. The method employs a process route consisting of smelting, continuous billet casting, hot rolling, online heat treatment, and slow cooling on a cooling bed. During the online heat treatment step, after the steel bars exit the online heat treatment water-cooling device, the steel bar surface is rapidly cooled to a temperature of ≤400°C by water quenching. At this point, a martensite structure with a thickness of ≤2.5mm forms on the surface. However, since the steel bar surface is rapidly cooled while the core temperature is higher, heat from the core of the steel bar is transferred to the surface on the subsequent conveyor roller. When the steel bar reaches the cooling bed, the upper cooling bed temperature is controlled within the range of 700-750°C, causing the martensite on the steel bar surface to self-temper and transform into tempered bainite, significantly improving the strength of the steel bar while still maintaining good plasticity. The final structure of the steel bar is a dual-phase structure of tempered bainite on the surface and ferrite + pearlite in the core.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] This invention, using existing process equipment, employs a V and Nb composite microalloying method, combined with steelmaking and in-line bar heat treatment. The resulting 800MPa-grade high-strength steel bar exhibits excellent comprehensive mechanical properties and fatigue resistance. Its yield strength is ≥800MPa, its tensile strength is ≥950MPa, its elongation after fracture is ≥14%, its total elongation at maximum force is ≥7.5%, and it passes 5 million cycles of unidirectional fatigue tensile testing. This makes it suitable for large-scale industrial production and offers significant benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The metallographic structure of the steel bar surface in Example 1 is a tempered bainite structure;

[0029] Figure 2 This is the metallographic structure of the core of the steel bar in Example 1, which is ferrite and pearlite. DETAILED DESCRIPTION

[0030] The invention provides an 800MPa-grade online heat-treated fatigue-resistant high-strength steel bar, the chemical composition and weight percentage of which are: C 0.40-0.50%, Si 0.80-1.20%, Mn 1.00-1.20%, V 0.15-0.25%, Nb 0.010-0.030%, Cr 0.80-1.0%, N 0.015-0.025%, P≤0.020%, S≤0.010%, and the remainder is Fe and unavoidable impurity elements.

[0031] The production method of the 800MPa grade online heat-treated fatigue-resistant high-strength steel bar comprises the following steps: smelting, billet continuous casting, hot rolling, online heat treatment, and slow cooling on an upper cooling bed; wherein the temperature of the upper cooling bed is controlled at 700-750°C.

[0032] The smelting steps specifically include: converter or electric furnace smelting, and LF furnace refining. The converter endpoint C is ≥0.05% to avoid overoxidation of molten steel; P and S are ≤0.015%, and slag is blocked during tapping to reduce the return of P and S to the molten steel; the tapping temperature is ≥1670°C to ensure that Nb is fully dissolved in the steel. Argon is blown into the ladle to promote the floating and removal of non-metallic inclusions. The LF furnace fine-tunes the chemical composition to the target range to obtain molten steel with qualified composition. Argon blowing and weak stirring make the composition and temperature of the molten steel in the upper and lower parts of the ladle uniform, which is conducive to pouring. The continuous casting of square billets adopts full-process protection casting to prevent secondary pollution of the molten steel and ensure the production of high-quality billets.

[0033] In the hot rolling step, the heating temperature is controlled at 1080-1200° C. and the heating time is ≥60 min, which fully ensures the solid solution of alloy elements such as V, Nb, and Cr in the steel to exert their precipitation strengthening effect.

[0034] No additional temperature control equipment is required during the hot rolling process, and all existing bar rolling mills in China can achieve this rolling.

[0035] In the online heat treatment step, the rolled steel bars immediately enter the online heat treatment water cooling device with a cooling water pressure of 2.4-2.8 MPa. When the surface temperature of the steel bars is ≤400°C, they are transferred to the cooling bed via a conveyor roller and slowly cooled until they are completely cooled.

[0036] The present invention is described in detail below with reference to the embodiments.

[0037] The chemical composition and weight percentage of the steel bars in each embodiment and comparative example are shown in Table 1, with the remainder being iron and unavoidable impurities.

[0038] Table 1

[0039]

[0040] The steelmaking operation of Example 1 is as follows:

[0041] (1) The converter end point is C0.25%, P0.009%, S0.013%, and the tapping temperature is 1700℃.

[0042] (2) The ladle was blown with argon for 6 minutes, with an argon flow rate of 115 L / min.

[0043] (3) The LF furnace was first blown through with a large argon flow rate of 200 L / min, then reduced to 108 L / min, and slagging was carried out at level 7 for 13 minutes.

[0044] (4) The argon blowing and weak stirring time is 15 minutes, the argon volume is 35L / min, the steel liquid surface is not exposed, and the slag surface fluctuates slightly.

[0045] (5) Billet continuous casting adopts full protection pouring, with a cooling water flow of 115m3 / h, secondary cooling water volume 1.3L / kg, continuous casting into 150 square billets.

[0046] The steelmaking operation of Example 2 is as follows:

[0047] (1) The converter end point is C0.30%, P0.007%, S0.013%, and the tapping temperature is 1690℃.

[0048] (2) The ladle was blown with argon for 7 minutes, with an argon flow rate of 110 L / min.

[0049] (3) The LF furnace was first blown through with a large argon flow rate of 200 L / min, then reduced to 102 L / min, and slagging was carried out at level 8 for 13 minutes.

[0050] (4) The argon blowing and weak stirring time is 20 minutes, the argon volume is 32L / min, the steel liquid surface is not exposed, and the slag surface fluctuates slightly.

[0051] (5) Billet continuous casting, using full protection pouring, one cooling water flow rate of 105m 3 / h, secondary cooling water volume 1.1L / kg, continuous casting into 140 square billets.

[0052] The steelmaking operation of Example 3 is as follows:

[0053] (1) The converter end point C0.35%, P0.010%, S0.012%, and the tapping temperature is 1710.

[0054] (2) The ladle was blown with argon for 7 minutes, with an argon flow rate of 105 L / min.

[0055] (3) The LF furnace was first blown through with a large argon flow rate of 200 L / min, then reduced to 104 L / min, and slagging was carried out at level 8 for 11 minutes.

[0056] (4) The argon blowing and weak stirring time is 18 minutes, the argon volume is 34L / min, the steel liquid surface is not exposed, and the slag surface fluctuates slightly.

[0057] (5) Billet continuous casting adopts full protection pouring, with a cooling water flow of 110m 3 / h, secondary cooling water volume 1.2L / kg, continuous casting into 165 square billets.

[0058] The steelmaking operations of each comparative example are the same as those of Example 1.

[0059] The rolling process parameters of the embodiments and comparative examples are shown in Table 2.

[0060] Table 2 Steel rolling process parameters

[0061]

[0062]

[0063] The three embodiments of the present invention have the same surface metallographic structure. Figure 1 、 2 The surface is tempered troostite and the core is ferrite + pearlite.

[0064] The mechanical properties of the embodiments and comparative examples are shown in Table 3.

[0065] Table 3 Mechanical properties

[0066]

[0067]

[0068] Where: R eL is the yield strength. If there is no yield strength, use R P0.2 Replace; R m is the tensile strength; A is the elongation after fracture; A gt is the total elongation at maximum force.

[0069] The fatigue properties of the examples and comparative examples are shown in Table 4. The stress loading method was pull-pull, the test waveform was a sine wave, and the test temperature was 26°C. Five parallel specimens were prepared for each specification. Any specimen that broke before 5 million cycles was considered unqualified for fatigue performance.

[0070] Table 4 Fatigue properties

[0071]

[0072] It can be seen from Table 3 and Table 4 that the mechanical properties and fatigue properties of Examples 1 to 3 all meet the requirements, while some properties of Comparative Examples 1 to 5 do not meet the requirements.

[0073] In Comparative Examples 1 and 3, the contents of C and V, the main elements for improving strength, were reduced. After stretching, the strength did not meet the mechanical property requirements of the present invention.

[0074] In Comparative Example 2, the Mn element was increased, which improved the hardenability and significantly increased the strength of the steel. However, the temper brittleness caused by manganese reduced the elongation and made the steel unqualified. The fatigue performance was also unqualified.

[0075] Comparative Example 4 lowered the upper cooling bed temperature, resulting in organizational changes, increased strength, insufficient toughness, reduced elongation, and failure. The fatigue performance was also unqualified.

[0076] In Comparative Example 5, the temperature of the upper cooling bed was increased, the hardened layer was thinned, and the strength was reduced, which did not meet the mechanical property requirements of the present invention.

[0077] The above-mentioned detailed description of an 800MPa grade online heat-treated fatigue-resistant high-strength steel bar and its production method with reference to the embodiment is illustrative rather than restrictive. Several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. An 800MPa grade online heat-treated fatigue-resistant high-strength steel bar, characterized in that: The 800MPa grade online heat-treated fatigue-resistant high-strength steel bar includes the following chemical components in weight percentage: C 0.40-0.50%, Si 0.80-1.20%, Mn 1.00-1.20%, V 0.15-0.25%, Nb 0.010-0.030%, Cr 0.80-1.0%, N 0.015-0.025%, P≤0.020%, S≤0.010%, and the rest are Fe and unavoidable impurity elements.

2. The 800MPa grade online heat-treated fatigue-resistant high-strength steel bar according to claim 1, characterized in that: The surface metallographic structure of the 800MPa grade online heat-treated fatigue-resistant high-strength steel bar is tempered sorbite, and the core metallographic structure is ferrite and pearlite.

3. The 800MPa grade online heat-treated fatigue-resistant high-strength steel bar according to claim 1, characterized in that: The 800MPa grade online heat-treated fatigue-resistant high-strength steel bar has a yield strength of ≥800MPa, a tensile strength of ≥950MPa, an elongation after fracture of ≥14%, a total elongation under maximum force of ≥7.5%, and passes the unidirectional fatigue tensile test for 5 million times.

4. The method for producing 800MPa grade online heat-treated fatigue-resistant high-strength steel bars according to any one of claims 1 to 3, characterized in that: The production method comprises the following steps: smelting, continuous casting of billets, hot rolling, online heat treatment, and slow cooling on an upper cooling bed; wherein the temperature of the upper cooling bed is controlled at 700-750°C.

5. The production method according to claim 4, characterized in that The smelting steps specifically include: converter or electric furnace smelting, and LF furnace refining.

6. The production method according to claim 5, characterized in that In the converter smelting step, the converter end point C ≥ 0.05%, P ≤ 0.015%, S ≤ 0.015%, and the steel tapping temperature ≥ 1670°C.

7. The production method according to claim 4, characterized in that In the billet continuous casting step, full-process protected pouring is adopted.

8. The production method according to claim 4, characterized in that In the hot rolling step, the heating temperature is controlled at 1080-1200° C., and the heating time is ≥60 min.

9. The production method according to claim 4, characterized in that In the online heat treatment step, the rolled steel bars immediately enter the online heat treatment water cooling device with a cooling water pressure of 2.4-2.8 MPa. When the steel bar surface temperature is ≤400°C, the steel bars are transferred to the cooling bed via a conveyor roller for slow cooling.

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