Negative tolerance rolling control of bar and rebar

This article discusses the production of bar and rebar at Shaogang Steel. Rebar is subject to national mandatory standards. Once a certain dimension fails to meet the standard requirements, it will be judged as a substandard product, causing serious losses to the company. , so when designing the hole pattern, it is particularly important to reasonably determine the dimensions. The influence of various factors must be fully considered to control the negative deviation of rebar within the qualified range, improve the product qualification rate and yield rate, and at the same time improve the product market competitiveness.

National Construction Steel Quality Supervision and Inspection Center: The product standard for hot-rolled ribbed steel bars “Hot-rolled ribbed steel bars for reinforced concrete” (GB1499.2-2007) regarding weight and allowable deviations is as follows:

 

Nominal diameter/mm The deviation between actual weight and theoretical weight/%
6~12 ±7
14-20 ±5
22-50 ±4

1. Factors affecting the negative difference of rebar:

 

1) The roller wears quickly, resulting in poor material control. I checked the “Roll Turning Inspection Record” in the work area and measured the hardness of the new roller and the off-line roller. The hardness of the new roller was consistent with the product quality certificate, and no unqualified rollers were found. The off-line rolls have some surface thermal cracks and severe thermal corrosion, as well as a small amount of chipping and meat loss.

2) The material type of alternating cold and hot billets is difficult to control. The alternating cold and hot steel will cause large fluctuations in steel temperature and a wide range of fluctuations in the weight of the finished product.

3) The functional accuracy of cold shearing and length baffle equipment is poor, the additional length of cold shearing is inaccurate, the weight of the bundles varies greatly, and the negative deviation loses accuracy.

4) The rolling mill has a large bounce, most of the rolls do not have effective elastic damping support, the material shape control is inaccurate, and it is difficult to adjust the material shape of each pass.

2. Process control measures

The increase in the content of Si and Mn in the rebar will increase the deformation resistance of the rolled piece, and the bounce value of the rolling mill will also increase accordingly. The roughing and intermediate rolling mill of our factory is a semi-opening mill with a large bounce value. According to the process parameters we have tracked and collected, the rolling mill has a large bounce value. When making rebar, the bounce value of the rough rolling mill is 0.4~0.5mm, and the bounce value of the intermediate rolling mill is 0.2~0.3mm; in addition, the plasticity and thermal conductivity of HRB500E steel is worse than HRB400E, the elongation performance is not good, and the width is larger. In order to ensure the size of the red blanks of semi-finished products and finished products during the rolling process and reduce size fluctuations, we adopt negative tolerance rolling for semi-finished products. In particular, we require that the rolled pieces cannot be too full. We strictly adopt the following measures for process control:

 

1) Reduce rolling groove wear

 

Optimize the finishing rolling cooling water pipeline, change the original round cooling water rake nozzle into an oblate shape, increase the speed of cooling water spraying from the nozzle, and adjust the angle and distribution of the nozzle near the exit of the rolling mill. The exit of the rolling groove is where the rolls have been in contact with the high-temperature rolling stock for the longest time, have the highest temperature, and need the most cooling water. The distance between the nozzles there is reduced, and the angle of the nozzles that was originally completely vertical to the roll is changed to that of the exit part. The nozzle angle with the rolling groove as the centre can effectively increase the cooling water volume of the turning groove at the exit of the turning machine and reduce the temperature of the roll.

At the same time, the water passing through the finishing rolling cooling waterway is used. After the waterway modification, the cooling water pressure can reach 1.3MPa and the water flow rate is 614, while the rolling mill water pressure is 0.9MPa and the rolling mill water flow rate is 450. It acts on the rolling groove at the same time as the shape of the cooling water nozzle is modified. , can effectively break the water vapour film generated by high temperatures, and the cooling water directly acts on the rolling groove, greatly increasing the cooling speed of the rolling groove.

 

2) Optimize hole design

 

If the size of the rebar does not meet the standard requirements, it will be judged as a substandard product, causing serious losses to the company. Therefore, it is particularly important to reasonably determine the dimensions when designing the hole pattern.

When rolling rebar using the smooth bar pass system of a bar rolling mill, the finished product is often not fully filled and the height of the transverse rib is lower than the minimum tolerance limit of the GB1499.2-2007 standard. This is because the transverse rib is an ellipse in the front hole of the finished product. When the rolled piece enters the bottom circle of the finished hole, the metal is extruded and formed. When the rolled piece comes out of the groove, the raised transverse ribs are often cut by the transverse rib grooves, which affects the width and height of the transverse ribs to a certain extent.

The purpose of using flat oval holes is to enable the finished holes to be filled with transverse ribs and longitudinal rib dimensions to meet the requirements of the GB1499.2-2007 standard. Therefore, when determining various dimensions in hole design, the influence of various factors must be fully considered.

The negative tolerance finished pass design of hot-rolled ribbed steel bars should control the inner diameter, transverse rib, and longitudinal rib dimensions of the steel bar within the negative deviation range allowed by the standard, and take into account the proportion of metal occupied by the transverse rib and longitudinal rib parts of the steel bar. Very few, have a great influence on the holding force of steel bars. The inner diameter part accounts for a large proportion of metal and is the main part that bears tensile force. The GB1499.2-2007 standard stipulates that the nominal cross-sectional area of the steel bar is used to calculate the strength of the steel bar. Therefore, the inner diameter of the steel bar should be controlled within the negative deviation range. As long as the engineering design is based on the nominal cross-sectional area of the steel bar and the strength value specified in the standard. By using it, the stress of the steel bars can be guaranteed.

Based on the above characteristics, the negative deviation control of steel bars is actually a negative deviation control of the inner diameter, and the transverse ribs and longitudinal ribs are controlled according to the nominal size. The maximum allowable negative tolerance value for ¢25~¢40 threaded steel bars is -4%. To calculate the inner diameter d, the negative tolerance utilization rate K value must first be determined. If the K value is too small, the negative deviation of the inner diameter will not be fully utilized. If the K value is too large, it will inevitably cause part of the inner diameter to exceed the lower limit specified by the standard, or cause a large loss of steel strength and fail to meet the standard. strength value and be judged as unqualified.

The K value is determined based on the existing process control level of our factory, including the average and standard deviation of the inner diameter, the average and standard deviation of the strength, and other factors. The K value is selected as 30%. The value of the inner diameter d of the finished product is determined by the square root of the square of the nominal inner diameter of the steel bar plus the empirical coefficient 1.274 and the square root of the sum of the steel bar negative deviation utilization rate, the steel bar negative deviation limit weight deviation, and the steel bar nominal cross-sectional area. In order to obtain rolled parts full of grooves and ensure the bonding strength of steel bars when used for concrete pouring, the height of the transverse rib cannot be selected according to the negative tolerance. However, if the height of the transverse rib is too deep, it will easily increase the consumption of the roller ring, so the height of the transverse rib h = standard Size + (0~0.4) mm.

 

3) Ensure that the spacing between transverse ribs is within standard tolerances

 

Not only must the transverse rib spacing be within the standard tolerance range, but in order to obtain the best economic benefits from negative tolerance rolling, it is hoped that L is at the upper limit of the positive deviation. However, forward slip during rolling will increase the transverse rib spacing. In order to avoid the transverse rib spacing exceeding the standard, the transverse rib spacing is L=standard size + (0~0.1) mm.

4) Strictly control the steel burning temperature and adopt real-time control to reduce the oxidation and burning loss of the steel billet, ensure the uniformity of temperature in the length direction of the steel billet, and reduce the phenomenon of large dimensional fluctuations caused by uneven temperature to ensure the quality of rolled products.

Strictly test the roll gap system. Before the start of rolling, in order to prevent the spring jump of the rolling mill from being too large, the rolling mill column pins of the rough and intermediate rolling mills are tightened. The roll gap values of the rough and intermediate rolling mills are adjusted 0.3~0.2mm smaller than the prescribed roll gap values respectively. The height of the unit is 0.1mm smaller than the specified rolling piece height, and fine-tuning will be performed after rolling starts normally. The K2 hole type is not easy to over-hollow. When the K2 over-empty main operator adjusts the tension after the steel temperature changes, the finished product ears will fluctuate greatly, resulting in large weight fluctuations.

5) The inner diameters of semi-finished products and finished products are controlled according to negative tolerances. During the rolling process, the operation is strictly carried out in accordance with the “Reduction Timing Compensation System” to reduce the negative impact of rolling groove wear on the quality of the finished product.

6) The quality worker pays attention to the negative tolerance value of the finished product weight and makes timely adjustments. The main operator pays attention to the rolling flow rate per second of the 1# rolling mill. Under the premise of the same speed, the flow rate per second of the 1# machine remains unchanged to ensure that the negative tolerance value is within The middle value of the national standard range.

By optimizing pass design, strengthening process control and other measures, a total of 77,212 tons of rebar will be rolled from 2021 to 2022, achieving large-scale and stable production of rebar, with product qualification rate and yield rate reaching 99.88% and 98.91% respectively. The cross-section and assembly dimensions of steel all meet international requirements, the weight PR rate reaches 100%, and the PR value is stably controlled at about -3, which has played an immeasurable role in expanding sales business, enhancing market competitiveness, and increasing the company’s economic benefits.

 

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