Analysis of cross-bending defects of Q235B hot-rolled steel strip

Abstract : Cross-folding refers to the occurrence of strip-like creases perpendicular to the rolling direction in a certain part of the coil during the uncoiling process of the hot-rolled coil. After straightening, the creases become denser, affecting the surface quality of the steel plate. This paper introduces the method of eliminating cross-bending defects of Q235B hot-rolled steel strip by changing the rolling process.
Key words : hot-rolled steel strip ; cross-bending ; hardness ; metallographic analysis

The transverse fold of hot-rolled coil is the crease or crease perpendicular to or approximately perpendicular to the rolling direction. Cross folds sometimes appear on the edge of the board, sometimes appear in the center of the board width, sometimes exist at the same time on the board side and the center, and sometimes run through the entire board width. When the horizontal fold is serious, there will be obvious concave-convex feeling when touched by hand, and users often raise quality objections to this, causing certain economic losses to both users and the company. Therefore, it is of great significance to analyze the causes of cross-bending defects in hot-rolled steel strip and take a series of measures to eliminate cross-bending defects.
1 Analysis of chemical composition of steel coil and hot rolling process parameters
11 Statistical Analysis of Chemical Composition
Statistical analysis was carried out on 254 steel strips with cross-bending defects , and it was found that these steel coils belonged to 78 heats . Some furnaces have multiple branches, and some appear single. The chemical composition of these steel coils with transverse folds was analyzed , It is found that there is a strong correspondence between the probability of cross-folding and the carbon content , The relationship between the carbon content of the steel strip and the cross-fold is shown in Figure 1 . It can be seen from Figure 1 that the lower the carbon content , the The higher the probability of cross-bending defects. However, if the carbon content is too high, it not only has a certain influence on the mechanical properties of the steel strip , but also is not conducive to welding , so it is particularly necessary to optimize its chemical composition. The internal control components are shown in Table 1 .
12 hot rolling process
The process currently implemented in the hot rolling mill is as follows :
(1) Furnace temperature control of heating furnace : preheating section W 1 150C , Heating section 1 240-1 32JC, soaking section 1 260-1 32JC, adjust furnace temperature according to different strip thickness , Thick specifications are controlled by the lower limit, and thin specifications are controlled by the upper limit.
(2) The starting temperature of rough rolling is 1 150 ~ 1 22CTC, and when rolling the specifications below 2.0mm , the upper limit is controlled at 1 250C; the finishing rolling temperature is (850±20)C , The coiling temperature is controlled at (600±20)C , The laminar flow cooling adopts the front stage cooling, and the cooling water temperature is W 32C .

Table 1 Chemical composition of Q235B steel ( mass fraction , % )
Table 1 Chan ica I can position of Q 235b steel (m^ ^ fracton % )

	C	Si	mn	P	S
GB/T700-2003	C0.20	C0. 35	C 1.40	C 0.045	C 0.045
Internal Control Standards	0 . 10- 0. 15	0 . 10- 0.30	0. 40 ~ 0. 70	C 0.030	C 0.025
target value	0. 12	0.20 _	0.55	C 0.020	C 0.020

After adjusting the chemical composition , the defect rate of cross-folding was improved , but not obviously. For horizontal folding and carbon content in 0 . 10 % - 0 . 12 % of the strips were analyzed for hot rolling process parameters ( see Table 2) . It can be seen from Table 2 that the final rolling temperature is controlled at (860±10)C, and the coiling temperature is controlled at (610±10)C, both within the required range and relatively stable.

Table 2 Process parameters of steel coils with cross-bending defects
Table 2 Process paraneters of coils occurring cross
break defects

Coil No.	Finishing temperature C	coiling temperature	User Feedback
80110243	856	609	There is a horizontal fold at 1/3 of the tail , no hand feeling
80204104	850	615	There are horizontal folds throughout the roll
80316024	851	618	There are horizontal folds throughout the roll
80304045	848	616	There are horizontal folds throughout the roll
80412053	862	610	There are horizontal folds throughout the roll

2 Test plan and results
According to statistics, the steel strip with a thickness between 3 mm and 5 mm has a higher probability of cross-bending defects. Combined with the actual production situation on site, for the steel strip with a thickness of 375mm , the finishing temperature is controlled at 890C , 850C , 82JC, and the coiling temperature is controlled at 600C , 550C , 500C , and a cross-comparison test is done. In order to ensure that the final rolling temperature can reach the set target of 89OC , the cooling water between the stands is turned off. The carbon content of the test steel strips were all in the range of 0.10% to 0.01% . 12 % range. The process parameters and results of the test strip are shown in Table 3 . The typical structure of the surface and center of the steel strip is shown in Figure 2 .
Tensile test on steel strip , According to the stress - strain curve : When the final rolling temperature is 82CTC and 89OC , the yield plateau of the steel strip is smaller, while the yield plateau of the steel strip is longer at 85OC . At the same finishing temperature , The steel strip decreases with the coiling temperature , Elongation also decreases.
from Table 3 and Figure 2 that: ① When the finish rolling temperature is 89OC , as the coiling temperature decreases , The hardness difference between the upper and lower surfaces of the strip and the center is reduced. When the coiling temperature is 500C , The hardness difference between the upper and lower surfaces of the strip and the center is the smallest. When the coiling temperature is 600C , the hardness difference between the upper and lower surfaces of the steel strip and the center is the largest . And the 3- degree difference between the upper and lower surfaces of the steel strip is relatively small. The ferrite grains are relatively uniform, and as the coiling temperature decreases , The ferrite grains transform from equiaxed to acicular , And the surface grain size is smaller than the central grain size. ② When the final rolling temperature is 850C , there is no obvious corresponding relationship between the microhardness of the upper and lower surfaces and the center of the steel strip and the coiling temperature . The hardness difference between the upper and lower surfaces and the center and the hardness difference between the upper and lower surfaces are small. The ferrite grains tend to be needle-shaped as the coiling temperature decreases . There is not much difference between the surface of the steel strip and the central structure and grain. ③When the final rolling temperature is 820C , there are different degrees of mixed crystal structure and banded structure in the samples . The lower the coiling temperature, the more serious the mixed crystal and banded structure. When coiling at 600C , only the mixed crystal structure exists on the surface of the steel strip. When coiling at 500C , there is a serious band structure on the entire test section , the metallographic structure is fine , the size is uneven , and there is mixed crystal phenomenon . Large grains and Small grains differ by about 5 grades.

3 analysis
From the above test results, it can be seen that when the steel strip is at the same final rolling temperature and different coiling temperature, there is no obvious rule between the upper and lower surface hardness values and the central hardness value of the steel strip, but there is a difference between the central and surface hardness values as the coiling temperature decreases. closer trend. Generally speaking, considering the surface quality and output of the steel strip, the exit speed of the steel strip only depends on the specification thickness of the steel strip, and the cooling rate after rolling is basically determined by the difference between the final rolling temperature and the coiling temperature. At the same final rolling temperature, the lower the coiling temperature, the faster the cooling rate, and the thickness of the steel strip is constant, so the temperature drop of the central part of the steel strip with low coiling temperature is larger than that of the steel strip with high coiling temperature. Correspondingly, as the coiling temperature decreases, the organization also transforms from polygonal to needle-like.
the final rolling temperature is 820C , there is mixed crystal phenomenon on the surface of the steel strip, so the final rolling temperature must be higher than this temperature. It can be seen from the stress-strain curve that when the final rolling temperature is controlled at 890C and the coiling temperature is controlled at 6O0C , the yield platform length is the smallest.
Cross-comparison test results of finish rolling temperature and coiling temperature

serial number	Finishing temperature C	Coiling temperature /C	mechanical properties			Microhardness ( HV °.2 )
			R h MPa	R MPa	A(%)	upper surface	center	lower surface
1-1	890	500	300	430	35	166	158	163
1-2	890	550	285	370	38	152	145	159
1-3	890	600	300	385	44	156	134	160
2-1	850	500	330	420	37	146	138	140
2-2	850	550	315	405	42	135	134	148
2-3	850	600	300	395	45	131	128	132
3-1	820	500	345	400	32	154	145	150
3-2	820	550	330	405	37	147	141	154
3-3	820	600	290	395	39	154	150	164

At this time, the hardness difference between the upper and lower surfaces of the steel strip is small, while the hardness difference between the surface and the center of the steel strip is the largest. That is, increasing the hardness difference between the surface and the center of the steel plate is beneficial to reducing the size of the yield platform. Therefore, according to the test, it can be determined that the final rolling temperature of 80C and the coiling temperature of 6O0C are the best processes to eliminate cross-bending defects. Batch tests were carried out using this process, and it was found that the mechanical properties of the steel strip were good . The metallographic structure is ferrite and pearlite , The grain size is around 0 grade. The test steel strip was sent to the user for Kaiping test, and it was found that the steel strip had no cross-bending defects and the surface quality was good.
in conclusion

1. The final rolling temperature must be controlled above 82CTC to prevent deformation.
2. The final rolling temperature is controlled at (890±c0)C , The coiling temperature is controlled at (600±c0)C , The length of the yield platform of the steel strip can be less than 5mm , and the steel coil can be flattened without cross-bending defects.

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