Q235B steel coil cross-fold elimination method

Aiming at the cross-bending defects in the uncoiling process of hot-rolled Q235B steel coils, without changing the existing chemical composition of Q235B steel and without increasing, On the premise of adding production process, the relationship between cross-bending defects, coil shape and rolling process parameters was studied . The results show that cross-fold defects can be effectively eliminated by reducing the coiling temperature of Q235B to below 580°C .
Key words :transverse folding ; plate shape ; coiling temperature

When the company's hot rolling mill produced hot-rolled low-carbon steel coils , a large number of cross-bending defects occurred , causing great economic losses . It is a common phenomenon that hot-rolled low-carbon steel coils have cross-bending defects after uncoiling . Both domestic products and imported products have cross-bending defects to varying degrees , which brings a lot of inconvenience and loss to users . In order to solve this problem , many companies have done research , trying to analyze the causes of cross-bending defects , and put forward corresponding solutions , such as solving the cross-bending problem by optimizing the uncoiling process and controlling the shape of the plate [1-2] . According to the production characteristics of Xingang's hot rolling mill , this paper studies the main causes of cross-bending defects and seeks solutions . The results show that by optimizing the rolling process parameters, the cross-bending defects of Q235B are effectively eliminated .

1. Test conditions and methods

The steel plate used in the test is Q235B specification : 3mm X 1250mm . Basic rolling process: continuous casting slab with a thickness of 230 mm is heated to 1250C, the thickness is reduced to 40 mm after rough rolling out of the furnace, the temperature is cooled to 1000C by air cooling , continuous rolling is carried out in a seven-stand continuous rolling unit, and then rolled by a 145 m After cooling by laminar flow, coils are coiled into steel coils weighing about 20 t . After cooling in air for 2 days , they are uncoiled on an uncoiler and cut into plates . The final rolling temperature and coiling temperature selected for the test are shown in Table 1 .

1. test results

2.1 The effect of plate shape on cross-folding
Many scholars have conducted research on the relationship between plate shape and transverse fold ,

It is believed that there is a strong corresponding relationship between transverse fold and plate shape . Flatness is an important indicator to measure the shape of the coiled plate . The straightness of No. 1 and No. 2 steel coils is shown in Figure 1. Through the observation of the transverse break defects of No. 1 and No. 2 steel coils, it was found that the entire coil of No. 1 steel coil has transverse breaks, and the front of the head of No. 2 steel coil There is a horizontal fold at about 000 m , and a horizontal fold occurs at the tail after about 700 m , and there is no horizontal fold in the middle part . It can be seen from the No. 1 steel coil that when the flatness is within 10 1 , slight fluctuations will cause cross-bending defects; it can be seen from the No. 2 steel coil that no cross-bending occurs when the plate shape is controlled at 01 . It can be seen from this that the cross-bending can be controlled by adjusting the shape of the plate, and when the flatness of the steel coil is controlled at 0 1 , the cross-bending can be avoided . From the No. 1 steel coil and No. 2 steel coil, it can be known that when the coiling temperature is 670 °C , there is a certain relationship between the shape and the transverse fold .
Table 1 Mechanical properties under different rolling processes

 

Volume number	Specifications ／( mm )	Finishing temperature /(C)	Coiling temperature /(°C )	Rel/ ( MPa )	Rm /( MPa )	A/(%)	Horizontal folding effect
1	3X1 250	870	670	345	450	35.0	The whole roll has severe horizontal folds
2	3X1 250	870	670	350	460	35.5	There are horizontal folds at the head and tail
3	3X1250	870	600	380	495	34.0	Slight horizontal fold throughout the roll
4	3X1250	870	580	395	500	33.5	No horizontal fold
5	3X1250	890	580	384	490	33.0	No horizontal fold
6	3X1250	870	540	405	510	32.0	No horizontal fold

The straightness of No. 456 steel coil is similar to that of No.1 steel coil ( the plate shape fluctuates within 101 ) . According to the cross-folding effect in table 1 , it can be known that No.45 and No. No. 1 steel has a whole roll of horizontal folds . It can be seen that when the coiling temperature is below 580 C , even if the flatness of the steel is within 101 , it has no effect on cross-folding . According to the production situation, it is difficult to control the flatness of the steel coil at 0 1. Therefore, in order to eliminate the cross-fold, the only way to reduce the coiling temperature is to reduce the coiling temperature .

2- 2 Effect of final rolling and coiling temperature on structure and properties
Under different rolling process conditions, the microstructure of the steel coil is shown in Figure 2 . It can be seen from Figure 2 that the metallographic structure of No. 1 steel coil contains about 93.81% polygonal ferrite and a small amount of pearlite, and the ferrite grains are relatively coarse . No. 3 steel coil contains 80.34% ferrite, most of which are needle-shaped and accompanied by a small amount of polygonal ferrite and pearlite . The structure of No. 4 steel coil is 75.86% acicular ferrite, the content of pearlite is more than that of No. 13 steel coil, and there is a small amount of bainite . The structure of No. 6 steel coil is 76.97% acicular ferrite, with more pearlite content . It can be considered that the cause of cross-bending defects is the existence of coarse polygonal ferrite in the steel coil structure .
In order to study the influence of temperature on the strength index, tensile tests were carried out on the samples respectively, and their properties are shown in Table 1 . It can be seen from No. 1 and No. 3 steels that when the finishing temperature is 870 C and the coiling temperature is reduced from 670 C to 580 C , the yield strength and tensile strength increase accordingly; it can be seen from No. 4 and No. 5 steels that the final As the rolling temperature increases from 870 C to 890 C, the strength of the steel coil decreases accordingly . Therefore, by reducing the coiling temperature to 580 C and appropriately increasing the final rolling temperature, the mechanical properties of the steel can meet the requirements of the national standard, and the transverse bending defects of Q235B steel can also be eliminated . After repeated tests at the production site, it is determined that the final rolling temperature of the steel coil is 890C, and the coiling temperature is 580C , which can meet the requirements of on-site production .

 

In order to study the influence of stress-strain curve on cross-bending , tensile tests were carried out on test steel at different coiling temperatures , and the stress-strain curves are shown in Figure 3 . It can be seen from Figure 3 that when the coiling temperature is lower than 600 ° C, the yield platform is obviously shorter than that when the coiling temperature is 670 ° C . When the coiling temperature is higher than 600 C , the length of the yield plateau is basically the same . It can be seen that the lower and longer the yield plateau on the stress-strain curve , the easier and more serious the cross-bending .
By studying the effect of rolling process on cross-folding, the following conclusions can be drawn :
1)When the coiling temperature is 670 C , there is a certain relationship between the plate shape and the cross-folding; when the coiling temperature is less than 580 C, the plate shape has no relationship with the cross-folding .
2)The final rolling temperature is 890C, and the coiling temperature is 580C, which can effectively eliminate cross-bending defects, and its mechanical properties meet the requirements of the national standard .
3)There is a certain relationship between cross-bending defects and the length of the yield platform. The lower and longer the yield platform, the easier and more serious the cross-bending .

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