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Low-carbon and efficient rebar production process practice(Ⅱ)

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Description:This paper introduces the main energy saving and consumption reduction technologies of rod and wire production in China, and analyzes the deficiencies of these technologies. The new low carbon, high efficiency rebar production process has fully achieved the low carbon and efficient production target.

Keyword: Rebar steel production process, low-carbon

Control rolling and cold process under multi-line cutting production mode (h2)

Reple control control principle (h3)

The new national standard number of rebar products is: GB/T1499.2-2018, the standard metallographic organization requirement of steel bar is ferrite and bead light body, and the tempering martensite tissue shall not appear on the base circle. That is to say, the building reinforcement meeting the standard should also meet the following requirements: the organization form of the product is mainly ferrite plus beads, and no other tissues affecting the performance; the lower limit of grain size is level 9.

Control rolling (h4)

The control rolling of rebar is through the reasonable control of deformation temperature and cumulative deformation, so as to increase the ferrite phase change core when rolling, combined with the graded cooling after rolling, to obtain the microrefinement of phase change organization and improve the comprehensive mechanical properties of steel.

The first stage of control rolling: the rolling piece is in the first finishing rolling unit, and the temperature is controlled between 900~950℃. At this time, the rebar is in the uncrystallized area of austenitic phase, and its cumulative deformation is 60~75%;

In the second stage of control rolling, when the rolling piece is finally rolled through the second finishing rolling unit, the surface temperature of the rolling piece is controlled near Ar 3 (800~ 850℃), and the cumulative deformation is 35~50%. Deformation of austenite boundary is austenite to ferrite nucleation [9], in the control of the second stage of rolling process, rolling temperature control in austenite not recrystalstallization area, after multiple rolling, austenite grain is stretched, the number of deformation zone increased and uniform distribution, provides more nucleation site, for the subsequent phase transition produces small uniform ferrite grain created good conditions.

Cold control (h4)

After rolling, the graded cooling method [10] is adopted to control the cooling of the rolling pieces to ensure that the section of steel base circle is pearl and ferrite, so as to avoid the occurrence of martensitic tissue or other low temperature tissue in the section of steel base circle. The grain size and the proportion of ferrite tissue is controlled within the appropriate range to ensure that the mechanical properties and tissue form of steel bars meet the requirements of national standards.

In conclusion, there are two key factors to control the rolling system:

One is the cumulative deformation amount, and the other is the deformation temperature.

The accumulated deformation is mainly related to the hole system, rolling road quantity and the deformation of each channel; the deformation temperature control is mainly related to the arrangement of water cooling line and the cooling capacity of water tank.

Control rolling process under traditional polysegmentation process (h3)

The precision rolling hole type of the traditional multi-line cutting process is flat roll, vertical box hole, pre-cut hole type, cutting hole type, flat roller and finished hole type. The schematic diagram of the controlled rolling hole system is shown in Figure 3. The water tank is arranged before finishing rolling, so the deformation after water cooling is finishing rolling 6 times, and the cumulative deformation is far more than 50%. At the same time, the finishing rolling hole type includes pre-cut hole type and cut hole type, which is irregular deformation, and the rolling parts are difficult to cool evenly, which is not conducive to the control effect of rolling making.

Control rolling process under improved multi-cutting process (h3)

Improvement Plan (I) (h4)

The improved precision rolled hole type is the pre-cut hole elliptical hole elliptical hole of the water tank, and the schematic diagram of the control rolling hole system is shown in Figure 4. The water tank is arranged between K3 and K2, and the control rolling is mainly carried out in the last two channels. The cumulative deformation is between 35 and 50%, and it is an elliptical hole system with uniform deformation and fully meets the control rolling theory.

The material type of the cooling mill is round, so the cooling nozzle is designed to be an annular nozzle, the cooling water into a ring shape, sprayed on the rolling piece, the annular water seam can be adjusted by the gasket. Three channels are designed in the tank, and the left and right channels are used for simultaneous cooling of two lines (φ 20mm 2, φ 22mm 2, φ 25mm 2); the middle channel is used for cooling of single line (φ 28~ φ 50mm).

Improvement Plan (II) (h4)

The improved refined rolled hole type is the cut-cut hole type of the water tank and the finished flat roller. The water tank is arranged between K4 and K3 times, which basically meets the requirements of deformation temperature and accumulated deformation amount. The cooling nozzle is designed as a rectangular nozzle, and the rolling pieces are cooled through each set of rectangular nozzles to ensure the cooling uniformity of the rolling parts.

Post-rolling grading cooling process (h3)

The graded cooling method is to add the intermediate air cooling process on the basis of the traditional section of control cooling method. The main purpose of air cooling is to reduce the temperature difference between the surface and the heart after water cooling. The graded cooling control effectively reduces the temperature difference between the surface and the heart of the bar during the cooling control process, thus creating conditions for obtaining uniform room temperature tissue to avoid low temperature tissue on the surface.

Practical Application of Improved Control Rolling Process (h2)

Area layout of production line mill (h3)

The area layout of the rolling mill takes into account the improvement scheme (1) and the improvement scheme (2) two kinds of control rolling and cooling processes, forming the control rolling and cooling process with full specifications and high yield capacity. The rolling area layout of the production line is shown in Figure 9.

Mill arrangement (h4)

The rolling mill is divided into 4 units: 6 rolling mills, 4 rolling mills, 6 rolling mills of rod finishing-1 unit, 3 rolling mills of rod finishing-2 units, and 19 rolling mills in the whole line.

Low-carbon and efficient rebar production process practice(Ⅱ)

Arrangement of water penetration cooling device (h4)

The arrangement of the water-through cooling device is divided into three parts: the first part is located between the center line of the last mill and the center line of the first mill of the mill-1 unit; the second part is located between the center line of the last mill of the mill-1 unit and the center line of the mill-2 unit;

The third part is located between the center line of the last mill of the finishing-2 unit and the center line of the flying shear, which is composed of water tank and recovery section guide groove.

Metallographic organization status of new process production bar (h3)

All finished products are rolled from the last two or the last three frames of the finishing rolling unit. A pre-water cooling device is provided before and after the finishing rolling-1 unit to control the rolling, providing more core sites for the post-rolling phase change. After the finishing rolling-2 unit, there is a graded water-through cooling device to control the cooling speed of the rolling parts, keeping the surface of martenite and bainite transition area and the base circle, and preventing the austenite grain from growing rapidly.

Conclusion

This paper introduces the main energy saving and consumption reduction technologies of rod and wire production in China, and analyzes the deficiencies of these technologies. At the same time, to meet the production demand of high production capacity and low consumption of the production plant, a new low carbon and high efficiency rebar production process is put forward, which has been successfully applied in the new production line of a steel plant, completely achieving the production goal of low carbon and high efficiency.

(1) The new process solves the technology of direct rolling in the rebar production line with high yield. The daily output of the practical production line is between 5500 and 5800 t, the direct rolling rate is 75%, and the emission of CO2 ton of steel is 63.2 kg.

(2) Improved multi-cutting control rolling process proposed the rebar control cooling control technology under the multi-cutting production mode, which realized the production of 400E rebar (below 25mm specification) without adding microalloy elements.

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