700MPa-grade hot-rolled twisted steel and production method thereof

Abstract

The invention relates to 700MPa-grade hot-rolled twisted steel and a production method thereof. The 700MPa-grade hot-rolled twisted steel comprises the following components in percentage by weight: C, Si, Mn, V, Nb, Cr, N and the balance of iron and impurities. The method comprises the following steps: adding molten iron, scrap steel and pig iron into an oxygen converter, carrying out conventional top-bottom combined blowing, and adding lime, dolomite and magnesite balls for slagging; after tapping is completed, molten steel is sent to an LF furnace for refining, and alloy is added after refining is completed; blowing nitrogen to the molten steel for 10 minutes, and continuously casting into a steel billet; the steel billet is fed into a heating furnace, rough rolling is conducted through a rough rolling machine, intermediate rolling is conducted through an intermediate rolling machine, water cooling is conducted before finish rolling, finish rolling is conducted, VN particles are induced to be separated out through controlled rolling deformation, weak water penetration is conducted after finish rolling, and air cooling is conducted through a cooling bed, so that the high-strength steel bar can be obtained. According to the twisted steel produced by adopting the components and the method, the lower yield strength is greater than 700MPa, the percentage elongation after fracture is greater than 16%, the microscopic structure is ferrite and pearlite, and the mechanical property is stable.

Description

Technical Field

[0001] The invention relates to the fields of iron and steel metallurgy and steel rolling, and in particular to a 700MPa hot-rolled threaded steel bar and a production method thereof. Background Art

[0002] Steel bars for construction account for a large proportion of the total steel production, and the proportion of low-strength steel bars is relatively high, which will have an adverse impact on the safety level of buildings and energy conservation and consumption reduction in the steel industry. With the development of the economy, steel bars will develop in the direction of higher strength levels, multifunctionality and higher quality. For example, the newly revised national standard has added 600MPa steel bars, making the existing national standard reach the international advanced level. But at the same time, it should be noted that foreign countries have long been mainly using 400MPa and above steel bars, while promoting 500MPa steel bars. The promotion and application of high-strength steel bars has reached a mature level, and the development of higher-level steel bars has also been in the forefront. At present, Japan has developed USD685MPa-USD1275MPa series of high-strength steel bars for concrete buildings above 60m. South Korea added SD600 and SD700 steel bars to the standard in 2007, and is developing 700MPa and above, and even 1000MPa steel bars in recent years. Australia will also take 800MPa grade steel bars as its main research and development direction in the future. It can be seen that there is still a certain gap between domestic high-strength steel bars and foreign countries in terms of application and development, and there is a lot of room for development.

[0003] The highest grade applied in China is 600MPa grade steel bar, and there is no report on the application of 700MPa grade steel bar. As we all know, high-strength steel bar plays a huge role in improving building safety, saving steel bar usage and expanding building space. At the same time, replacing low-grade threaded steel bar can reduce the cost of steel bar use and reduce environmental pollution caused by SO2, CO2 and other gas emissions. Therefore, the development of high-strength grade steel bar is of great significance. Threaded steel bar is the main supporting structure of the building, with large consumption and wide application range, which requires the cost of high-strength threaded steel bar to be controlled within a reasonable range for large-scale promotion and application. At present, there are a small number of 700MPa grade steel bars studied in laboratories in China, but there are problems such as high alloy cost and great welding difficulty, which greatly restrict the promotion of high-strength steel bars. For example, the chemical composition of "seismic and weather-resistant high-strength YS700MPa grade hot-rolled steel bars" applied by the Iron and Steel Research Institute is: C: 0.20-0.25%, Si: 0.40-0.80%, Mn: 1.20-1.60%, P≤0.035%, S≤0.035%, V: 0.06-0.20%, N: 0.010-0.030%, Ni: 0.20-2.0%, Cr: 0.20-2.0%, Ti: 0.01-0.03%. Combined with the Cr and Ni contents in the embodiment, which are in the middle limit of the composition range, it can be seen that the alloy elements such as Cr and Ni are relatively high, and the cost and carbon equivalent are difficult to control at an appropriate level. At the same time, its maximum force total elongation is only 5.5%-9.5%, and the plasticity is poor. However, residual heat treated steel bars and precision rolled threaded steel bars also have the limitations of difficult welding, high cost and poor plasticity. Therefore, it is of great significance to develop high-strength and plastic hot-rolled threaded steel bars with low cost and easy welding. Summary of the invention

[0004] In view of this, an object of the present invention is to provide a 700MPa grade hot-rolled threaded steel bar and a production method thereof.

[0005] In order to achieve the above purpose, the technical solutions adopted are as follows:

[0006] A 700MPa hot-rolled threaded steel bar comprises the following components in weight percentage:

[0007] C: 0.26~0.30wt%; Si: 0.40~0.80wt%;

[0008] Mn: 1.20~1.60wt%; V: 0.14~0.20wt%;

[0009] Nb: 0.02~0.06wt%; Cr: 0.16-0.20wt%;

[0010] N: 200-260 ppm; the rest is Fe and unavoidable impurities.

[0011] A method for producing 700MPa grade hot-rolled threaded steel bars comprises the following steps:

[0012] S1: After adding molten iron, scrap steel and pig iron into an oxygen converter, conventional top and bottom composite blowing is performed, and lime, dolomite and magnesite balls are added for slag making;

[0013] S2: Control the endpoint carbon>0.06wt%, and the tapping temperature is less than 1680℃;

[0014] S3: After the steel is tapped, the molten steel is sent to the LF furnace for refining and blown for 5 minutes. After the refining is completed, micro-alloying elements with a good ratio are added;

[0015] S4: Blow nitrogen into the molten steel for 10 minutes and continuously cast it into a steel billet;

[0016] S5: The steel billet is fed into a heating furnace with a heating temperature of 1150-1250°C, the heating time is ≮120 minutes, the steel billet rolling temperature is 1080-1150°C, the rough rolling mill is used for rough rolling, the intermediate rolling mill is used for intermediate rolling, and the steel billet is water-cooled before finishing rolling, and then enters finishing rolling, and VN particles are induced to precipitate through controlled rolling deformation, which are used as nucleation positions to further refine the ferrite grains. After finishing rolling, weak water penetration is performed, and the steel billet is air-cooled on a cooling bed to obtain 700MPa grade high-strength steel bars.

[0017] Preferably, in step S5, the finishing rolling temperature is controlled at 850-900°C, and the upper cooling bed temperature is 850-900°C.

[0018] The beneficial effects of the present invention are:

[0019] By adding micro-alloy elements in combination, the compound addition increases the solid solubility, reduces the dissolution temperature, facilitates precipitation, refines the grains and improves the strength and plasticity. Low-cost high-strength threaded steel products with good plasticity and strength are produced. The strength can reach 700MPa. The smelting process does not require special processes and is simple to operate. Compared with low-strength threaded steel, the rolling process has no complex requirements and can be promoted and used on a large scale in various environments.

[0020] The threaded steel bar produced by the above composition and method has a lower yield strength of >700MPa, an elongation after fracture of >16%, a microstructure of ferrite and pearlite, stable mechanical properties, and is a low-cost 700MPa grade threaded steel bar that meets the needs of large-scale production and use. DETAILED DESCRIPTION

[0021] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application may be combined with each other.

[0022] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiment of the present application will be clearly and completely described below in combination with the embodiment of the present application. Obviously, the described embodiment is only a part of the embodiment of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work should fall within the scope of protection of the present application.

[0023] A 700MPa hot-rolled threaded steel bar comprises the following components in weight percentage:

[0024] C: 0.26~0.30wt%; Si: 0.40~0.80wt%;

[0025] Mn: 1.20~1.60wt%; V: 0.14~0.20wt%;

[0026] Nb: 0.02~0.06wt%; Cr: 0.16-0.20wt%;

[0027] N: 200-260 ppm; the rest is Fe and unavoidable impurities.

[0028] A method for producing 700MPa grade hot-rolled threaded steel bars comprises the following steps:

[0029] S1: After adding molten iron, scrap steel and pig iron into an oxygen converter, conventional top and bottom composite blowing is performed, and lime, dolomite and magnesite balls are added for slag making;

[0030] S2: Control the endpoint carbon>0.06wt%, and the tapping temperature is less than 1680℃;

[0031] S3: After the steel is tapped, the molten steel is sent to the LF furnace for refining and blown for 5 minutes. After the refining is completed, micro-alloying elements with a good ratio are added;

[0032] S4: Blow nitrogen into the molten steel for 10 minutes and continuously cast it into a steel billet;

[0033] S5: The steel billet is fed into a heating furnace with a heating temperature of 1150-1250°C, the heating time is ≮120 minutes, the steel billet rolling temperature is 1080-1150°C, the rough rolling mill is used for rough rolling, the intermediate rolling mill is used for intermediate rolling, and the steel billet is water-cooled before finishing rolling, and then enters finishing rolling, and VN particles are induced to precipitate through controlled rolling deformation, which are used as nucleation positions to further refine the ferrite grains. After finishing rolling, weak water penetration is performed, and the steel billet is air-cooled on a cooling bed to obtain 700MPa grade high-strength steel bars.

[0034] Wherein, in step S5, the finishing rolling temperature is controlled at 850-900°C, and the upper cooling bed temperature is 850-900°C.

[0035] C: Carbon is an effective strengthening element. Carbon can be dissolved in the matrix to improve its yield strength and tensile strength. It is cheap and can form carbonitrides with vanadium, molybdenum and other elements in steel to improve the strength of the steel matrix. However, too much carbon will reduce the plasticity and toughness of the steel and reduce the welding performance. In order to ensure the performance of the steel and reduce the cost at the same time;

[0036] Si: A common reducing agent and deoxidizer in steel, it can significantly improve the elastic limit, yield point and tensile strength of steel, mainly through solid solution strengthening in ferrite, and can reduce the diffusion rate of carbon in austenite, delay the phase transformation of ferrite and pearlite, and improve the yield strength and tensile strength. However, silicon will increase the ductile-brittle transition temperature. Therefore, the reasonable range of silicon is 0.40-0.80wt%.

[0037] Mn: Manganese is a major solid solution strengthening element and an effective element for improving the hardenability of steel. In the present invention, manganese is used as a major solid solution strengthening element to improve the strength of the matrix. Too high a manganese content will reduce the ferrite formation temperature, making the precipitate phase too fine and affecting the strengthening effect. At the same time, a high manganese content will also increase the carbon equivalent and affect the welding performance. The reasonable range of manganese is 1.20-1.60wt%.

[0038] N: It can promote the precipitation of vanadium compounds, refine the size of the precipitated phase, give full play to the potential of V precipitation strengthening, improve the strength of threaded steel, reduce the use of V and reduce production costs. Combined with the optimal vanadium-nitrogen ratio, the nitrogen content is controlled at 200-260ppm;

[0039] V: It is a microalloying element that is easy to dissolve into steel and precipitate from steel. In addition to forming VC with C element in steel, it is easier to combine with N in steel to form VN. It exists in the form of V[C, N] compounds in the matrix and grain boundaries, playing a role in precipitation strengthening and inhibiting grain growth. In addition, when the microalloying element V is added to steel, it will be precipitated due to deformation in the rolling process to form V compound particles, which can prevent the growth of austenite grains on the one hand, and prevent or delay the recrystallization of austenite on the other hand, refine the austenite, thereby refining the ferrite grains.

[0040] Embodiment 1, the chemical composition of the steel bar is: C content 0.26%, Si content 0.4%, Mn content 1.2%, V content 0.14%, Nb content 0.02%, Cr content 0.16%, N content 200ppm, and the rest is Fe and unavoidable impurities.

[0041] Embodiment 2: The chemical composition of the steel bar is: C content 0.28%, Si content 0.70%, Mn content 1.54%, V content 0.165%, Nb content 0.031%, Cr content 0.19%, N content 209ppm, and the rest is Fe and unavoidable impurities; yield strength>700MPa, tensile strength>900MPa, elongation after fracture>16%, and the structure is ferrite and pearlite.

[0042] Embodiment 3

[0043] The chemical composition of the steel bar is: C content 0.3%, Si content 0.8%, Mn content 1.6%, V content 0.2%, Nb content 0.06%, Cr content 0.20%, N content 260ppm, and the rest is Fe and unavoidable impurities.

[0044] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A 700MPa hot-rolled threaded steel bar, characterized in that: The following components are included in weight percentage: C: 0.26~0.30wt%; Si: 0.40~0.80wt%; Mn: 1.20~1.60wt%; V: 0.14~0.20wt%; Nb: 0.02~0.06wt%; Cr: 0.16-0.20wt%; N: 200-260 ppm; the rest is Fe and unavoidable impurities.

2. A method for producing 700MPa grade hot-rolled threaded steel bars, characterized in that: The following steps are involved: S1: After adding molten iron, scrap steel and pig iron into an oxygen converter, conventional top and bottom composite blowing is performed, and lime, dolomite and magnesite balls are added for slag making; S2: Control the endpoint carbon>0.06wt%, and the tapping temperature is less than 1680℃; S3: After the steel is tapped, the molten steel is sent to the LF furnace for refining and blown for 5 minutes. After the refining is completed, micro-alloying elements with a good ratio are added; S4: Blow nitrogen into the molten steel for 10 minutes and continuously cast it into a steel billet; S5: The steel billet is fed into a heating furnace with a heating temperature of 1150-1250°C, the heating time is ≮120 minutes, the steel billet rolling temperature is 1080-1150°C, the rough rolling mill is used for rough rolling, the intermediate rolling mill is used for intermediate rolling, and the steel billet is water-cooled before finishing rolling, and then enters finishing rolling, and VN particles are induced to precipitate through controlled rolling deformation, which are used as nucleation positions to further refine the ferrite grains. After finishing rolling, weak water penetration is performed, and the steel billet is air-cooled on a cooling bed to obtain 700MPa grade high-strength steel bars.

3. The method for producing a 700MPa grade hot-rolled threaded steel bar according to claim 2, characterized in that: In step S5, the finishing rolling temperature is controlled at 850-900°C, and the upper cooling bed temperature is 850-900°C.