Improving Rate of Fixed-length for Φ 18 × 3 mm High Speed Rebar in Xinyu Steel

Xinyu Steel’s high-speed bar production line was officially put into operation in September 2013. Its main products are Φ10 ~ Φ 40 mm ribbed steel bars and Φ 18 ~ Φ50 mm round steel bars. The main steel types produced include ordinary hot-rolled steel bars and fine-grained hot-rolled steel bars. Rolled steel bars, hot-rolled steel bars for prestressed concrete and high-quality carbon structural steel. The delivery status is in straight strips and bundles, with a fixed length of 6 ~ 15 m, a bundle diameter of Φ 150 ~ Φ 300 mm, and a bundle weight of 2 ~ 3. 5 t. The production line has a designed annual production capacity of 1 million tons. After more than three years of process optimization, the actual annual production capacity has reached 1.5 million tons.

 

1. Equipment and process flow

 

The Xinyu Steel high bar production line consists of 18 rolling mills, which are divided into rough rolling, intermediate rolling and finishing rolling mills. They are each composed of 6 short stress line rolling mills arranged alternately horizontally and vertically. The 16th and 18th stands are flat-vertical. Convertible rolling mill, each stand of the rolling mill is driven by a DC motor independently. The designed maximum final rolling speed is 18 m/s, and the actual maximum rolling speed of the finished product is 15. 5 m/s.

 

The process flow is as follows

Loading platform → Furnace loading roller table → Hydraulic furnace loading pusher → Double regenerative heating furnace → Furnace discharge roller table → Roughing mill unit (4 × Φ 650 mm + 2 × Φ 550 mm) → No. 1 crank cutting head Tail cutting shear → Intermediate rolling mill unit (2 × Φ 550 mm + 4 × Φ 500 mm) → Pre-water penetration device (2 sections) → No. 2 accident breaking shear → Finishing rolling mill unit (2 × Φ 500 mm + 4 × Φ 390 mm) → Water penetration device (3 sections) → No. 3 double-length flying shear → Input variable frequency roller table → (120 × 12) m stepping cooling bed → Cold shearing to length → Counting and bundling → Weighing and tagging → Unloading Warehousing.

 

2. Factors affecting the cut-to-length rate and improvement measures

 

The cut-to-length rate is an important technical indicator for measuring bar production. Currently, the rolling speed of the Xinyu Steel high bar Φ 18 × 3 mm screw is 2 m/s, and the machine-hour output is about 235 t, which is the highest specification for shift production. Starting from improving the length qualification rate of continuous casting billets, accurately controlling the negative tolerance of finished products, improving the assembly accuracy of rolling mills, and controlling the cutting and rolling line differences, a series of measures have been taken, and effective methods for improving the length-to-length ratio have been summarized, and significant results have been achieved. Effect.

 

2.1 Stable blank length

 

At present, the Xinyu Steel high bar production line has a single specification and length, a wide range of product specifications, different compression ratios for each specification, and different free section lengths of the finished products. Therefore, in order to reduce the number of non-cut lengths after cutting the free section to length and improve the cut-to-length ratio, products of different specifications must have blanks of different lengths.

 

2.1.1 Theoretical calculation of the blank length

 

Take the main product specification Φ 18 mm × 3 as an example for calculation instructions. According to GB 1499. 2-2007, the nominal diameter of ribbed steel bars is 18 mm and the cross-sectional area is 254. 5 mm2. The cross-sectional area of the raw material 160 mm × 160 mm is 25 600 mm2 and the length is 12 520 mm. The length of the rough rolling cutting head is about 100 mm, and the cross-section is a circle with a radius of 76. 5 mm. The length of the cutting head and tail of the intermediate rolling is about 150 mm, and the cross-section is a circle with a radius of 51 mm. The mass of the primary iron oxide scale generated when the steel billet is heated accounts for about 2% to 3% of the total mass of the steel billet. Considering that the use of hot charging and hot feeding reduces burning loss, the burning loss is taken as 2% of the total mass of the billet. The specific algorithm for determining the theoretical length of the finished product is as follows:

 

According to the law of constant volume, we get:

L1 = (25 600 × 12 520 × (1-2%)-(76. 5 × 76. 5 × 100 × 3. 14) ÷ 4-(51 × 51 × 150 × 3. 14 × 2) ÷ 4) ÷ 3 ÷ 254. 5 = 410 m.

Considering that our factory adopts negative tolerance rolling and delivers goods according to theoretical quality, the current negative tolerance control standard is – 4% ~ – 4. 5%. Since the negative tolerance of each line of split rolling fluctuates, the negative tolerance is taken as – 4. 2%.

The calculated actual finished product length is L2 = 410 ÷ (1-4. 2%) = 428 m = multiple of fixed length × fixed length + X.

In the above formula: The upper limit of this length range refers to 4 000 ~ 9 000 mm, which is a non-fixed length, and the lower limit refers to the minimum shearing length necessary to ensure product quality. If it exceeds the upper limit, it means that the length of the billet is unreasonable, which will lead to serious waste; if it exceeds the lower limit, it is impossible to roll qualified products with the current production technology level.

From the above calculation results, it can be concluded that when producing a 9 m fixed length of Φ 18 mm, X = 5000 mm, which exceeds the upper limit of the shearing allowance length of 300 ~ 4000 mm, its length is unreasonable.

When producing a 12 m fixed length of Φ 18 mm, X = 8 000 mm. From this, we can know that the length is unreasonable because it exceeds the upper limit of the shear allowance length of 300 ~ 4,000 mm.

Assuming X = 300 mm, the length of the raw material can be calculated. Assume that the length of the raw material is Y. According to the law of constant volume when producing a 9 m fixed length of Φ 18 mm: (25 600 × Y × (1-2%)-(76. 5 × 76. 5 × 100 × 3. 14) ÷ 4-(51 × 51 × 150 × 3. 14 × 2) ÷ 4) ÷ 3 ÷ 254. 5 ÷ (1-4. 2%) = multiple of fixed length + 300

The fixed length multiple here is 48, and the calculation result is Y = 12 646 mm.

When producing a 12 m fixed length of Φ 18 mm: (25 600 × Y × (1-2%)-(76. 5 × 76. 5 × 100 × 3. 14) ÷ 4-(51 × 51 × 150 × 3. 14 × 2) ÷ 4) ÷ 3 ÷ 254. 5 ÷ (1-4. 2%) = multiple of fixed length + 300

The multiple of the fixed length here is 36, and the calculation result is Y = 12 646 mm. Taking into account factors such as burning loss, the fluctuation and uncertainty of negative tolerances, as well as output and yield, 12 650 mm was finally determined as the reasonable raw material length for rolling Φ 18 mm (actually it is about 12 520 mm).

 

2.1.2 Add a weighing system for billets entering the furnace

 

At present, the high rod production line has added a furnace billet weighing system. This system can feedback on the quality signal in real-time, and then transmit the corresponding weight data to the billet-cutting control system. The cutting system can adjust the No. 2 continuous casting in time based on the received data. The blank length of the machine is adjusted to achieve the purpose of stabilizing the blank length. The workflow diagram of the blank weighing system is shown in Figure 1.

 

Billet weighing system, billet

Figure 1 Schematic diagram of the workflow of the blank weighing system

 

2.2 Stabilize negative tolerances and line differences of finished products

 

During slitting and rolling, the stable control of the negative tolerance and line difference of the finished product determines the stability of the length of the finished product, which in turn affects the sizing rate. Therefore, during the actual production process, through exploration and analysis, it was found that the following factors will affect the negative tolerance and line difference of the finished product.

 

2.2.1 The influence of stack-pull relationship

 

During continuous rolling, it is quite difficult or even impossible to keep the theoretical flow rate per second equal and the continuous rolling constant constant. In actual production, in order to ensure that the rolling process can proceed smoothly, the operation technology of steel stacking or steel drawing is often consciously adopted, which ultimately leads to uneven changes in the material shape of each pass. This accumulates into the finished rolling groove, causing the rolling groove to be filled from time to time. , sometimes not full, eventually leading to the unstable length of the finished product.

Since it is impossible to maintain an absolute equal flow rate per second, the method of micro-stacking and micro-pull between rough rolling mills, intermediate rolling mills, and finishing rolling mills is used to adjust the stacking relationship of the entire rolling line so that the rolled pieces can be The flow within the hole pattern is smooth, and the looper remains stable after the entire steel is rolled, and the length of the finished product is basically stable.

 

2.2.2 Effect of rolling temperature

 

The opening rolling temperature of the billet has a great influence on the size and multiple lengths of the finished product. Uneven steel temperatures during production will cause a large increase in short lengths. The rolling temperature should be strictly controlled. Rolling of low-temperature steel should be strictly prohibited during the production process to ensure the quality of the steel. Cold shrinkage is consistent. Since 18 screws are currently rolled in three cuts, the uneven steel temperature is not only uneven between one piece of steel but also between different sections and different faces of the same piece of steel. This can lead to different lengths of different steels, different lengths of the same steel, or different cutting lengths of the same length of steel. When the steel temperature is low, the bounce amount of the material type in each pass will be larger, which will cause the longitudinal flow of the rolled piece to become smaller, that is, the forward slip will be reduced. At the same time, the oxide scale on the surface of the rolled piece will increase the friction between the rolled piece and the groove. Further reducing the forward slip; when the steel temperature is high, the oxide scale on the surface of the red steel will become the lubricant between the groove and the rolled piece, which is conducive to the increase of the forward slip. The cumulative result is that the looper is sometimes high and sometimes low, and the looper is sometimes high and sometimes low. The duration of the meeting is long and sometimes short, and if it is difficult to control, the length will increase.

In this regard, appropriate methods can be used to compensate for the problems caused by uneven steel temperature and stabilize the size and overall length of the finished product. The specific method is to control the soaking section temperature stably at (1 150 ± 20) ℃, and the finishing rolling temperature to be controlled at 1 040 ~ 1 080 ℃; manually adjust the looper stacking relationship; adjust the additional length of the double shear.

 

2.2.3 The influence of rolling mill, guide assembly accuracy and roll material

 

The springiness and working stability of the rolling mill, the assembly accuracy of the import and export guides, and the material of the rolls will all affect the stability of the finished product line difference and negative difference and then affect the finished product sizing rate. The following measures can be taken to improve the line difference and negative tolerance of finished products and increase the sizing rate.

1) Ensure that there are no wrong rolls in the rolling groove, and the guide assembly at the entrance and exit of each rolling mill must be accurately aligned.

2) The online rolling mill must use wire rods to brush the roll gaps on both sides of the roll to ensure that the roll gaps on both sides are consistent.

3) Inspect the rolling mill’s pressure adjustment system in a timely manner, and put the rolling mill with a rusty adjustment mechanism online to make the roll gap adjustment system flexible, stable and reliable.

4) Develop reasonable bearing oil clearance standards to improve the assembly accuracy of rolling mill bearings and reduce roll bounce.

5) The wear of the pre-slit (K4) and slit (K3) rolling grooves in finishing rolling directly affects the stability of the line difference, and the wear of the finished product (K1) and pre-finished product (K2) rolling grooves directly affects the stability of the size and length of the finished product. Therefore, high-speed steel rolls with high wear resistance must be used, which can not only stabilize the linear difference and negative difference but also improve the stability of rolling.

6) Ensure that the K3, K4 and K5 pass guides are aligned with the rolling line. During the production process, the “plank sliding edge” method is used, that is, using a plank to mark both sides of the material, observing whether the traces left by the rolled piece are symmetrical or ear-shaped, and making timely adjustments based on the observation.

 

2.3 Other methods and precautions

 

In the actual production process, the chemical composition of the continuous casting billet has a great influence on the performance of the rebar. If the composition fluctuates greatly, the performance of the steel bars at the lower limit of size may not meet the national standard. Therefore, when w (C) is less than 0. 21%, the negative tolerance of the finished product is controlled according to the upper limit. On the premise of ensuring that the performance meets the national standard requirements, the full fixed-length length is determined based on the weighing quality of the finished product and the fixed-length rate is improved.

The current delivery regulations for finished products mainly include fixed lengths of 9 m and 12 m. During the rolling 12 mm fixed length process, non-fixed lengths with lengths greater than 9 m but less than 12 m must be selected and then cut to 9 m. The fixed length rate can be appropriately increased.

 

3. Effect

 

Research on the fixed-length rate began in 2015. After the implementation of the measures, the average fixed-length rate increased from 99. 20% to 99. 31% and the fixed-length rate showed a steady upward trend.

By analyzing the process and equipment of Xinyu Steel’s high-speed bar production line, this article finds out the main factors that affect the bar sizing rate and formulates corresponding improvement measures. After the implementation of the measures, the sizing rate of the finished bar and wire products has been significantly improved, but there are still some problems, such as the length of the continuous casting billet still fluctuating to a certain extent, the steel temperature of the hot and cold billets is uneven, etc., which has a greater impact on the sizing rate. Therefore, there is still room for further improvement in the finished length ratio of Xinyu Steel’s high bar products. By bringing more intelligent equipment online to specifically monitor and solve the problem of billet length fluctuations, and monitoring billet temperature through thermal imaging systems, etc., the final length rate of finished bars can be further improved.

Article resource: Liu Yang. Exploration of Improving Rate of Fixed-length for Φ 18 × 3 mm High-Speed Rebar in Xinyu Steel[J]. Jiangxi Metallurgy, 2017, 37(4): 10-12,15.

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