Analysis of Causes of Edge Cracks in SPHC Cold Rolled Strip

in the SPHC cold-rolled strip of line 1580 were analyzed, and it was considered that the grains at the edge of the strip were coarser and the presence of tertiary cementite was the main reason for the edge cracks. The finishing temperature was increased from 850 t to 890 t , the coiling temperature was decreased from 660 t to 610 t , and the temperature difference in the width direction was reduced by adjusting the cooling water of the rolls , which greatly reduced the edge crack defects of the strip.
Key words : cold rolling ; strip ; edge crack ; cause ; analysis

The cold rolling factory reported that the SPHC strip produced by the 1580 hot rolling line has edge cracking after cold rolling . The edge crack defect refers to the crack on one or both sides of the edge of the strip along the length direction . Opening , or cracking after cold rolling , hot rolling edge cracking has a great impact on cold rolling , and quality accidents such as broken strips are prone to occur. Therefore , the control in the steel coil quality inspection and release process generally does not enter the post-cold rolling process. However, the hot-rolled strips with intact edges sometimes have edge cracks after cold rolling , and the entire length of the severe strip edges are jagged. In this study, the reason for the defects of the strip was explored , and the cold rolling cracking of the strip was reduced to a certain extent by adjusting the parameters of the hot rolling process .
2 Reasons for edge cracking 2 . 1 Reasons for cold rolling
Cold-rolled edge cracks are usually caused by edge inclusions, poor edge mechanical properties, or
Caused by abnormal cold rolling equipment, etc. , mainly include two categories.
( 1 ) The structure and composition of hot-rolled raw materials , the edge of the strip is coarse or the content of inclusion elements is high ; the plasticity of the strip is low , and tertiary cementite is precipitated on the grain boundary ( increase the coiling temperature of the steel plate , reduce the layer flow cooling intensity is an effective measure to solve this problem ) ; the hot rolling temperature system is not properly controlled.
( 2 ) The problem of cold rolling is mainly due to the uneven distribution of cold rolling load. Aiming at the edge cracks generated in the strip during the continuous cold rolling process , the cause was firstly found from the state of the acid rolling equipment and the control system of the rolling mill . When the state of the acid rolling equipment was relatively stable , no obvious abnormality was found in the rolling parameters. The 1700 production line of Tangshan Iron and Steel Co., Ltd. without edge cracks was selected for comparison with the 1580 production line with edge cracks to analyze the cause of the edge cracks in the strip , because the material supplied in the rolling 1700 line and the same specification ( 0. 38 mm X 1 000 mm ) When the product was produced , the rolling procedure and other parameters adopted were unchanged , and there was no edge cracking. Therefore , strip edge cracks caused by the state of the pickling rolling equipment and the control of the rolling mill are excluded. The chemical composition of steel coils with edge cracks is shown in Table 1 .

It can be seen from Table 1 that there is no obvious abnormality in the composition of steel coils with serious edge cracks. In order to further study the influence of chemical composition on edge cracking , 300 coils , 1700 lines and 1580 lines were respectively selected for comparison of the components C , Si , Mn , P , S of the raw materials for acid rolling through the MES system , so the histogram shown in Figure 1 picture.
Table 1 Chemical composition of edge-cracked steel coils

Item number	parent roll number	C	Si	mn	P	S	Item number	parent roll number	C	Si	mn	P	S
1WY0014	1C00130B03	0.04	0.01	0. 17	0.015	0. 011	1WY0054	1C00147B04	0.05	0.02	0. 18	0. 019	0.007
1WY0017	1C00139B03	0.05	0.01	0. 18	0. 017	0. 009	1WY0480	1C00326B03	0. 05	0. 03	0. 16	0. 013	0. 007
1WY0018	1C00149B04	0. 05	0. 02	0. 17	0. 009	0. 003	1WY0640	1B00330A03	0. 04	0. 01	0. 21	0. 009	0. 005
1WY0053	1C00128B04	0. 04	0. 02	0. 17	0. 015	0. 006

 

1700 and line 1580 is relatively similar , and the inclusion elements of raw materials for acid rolling for line 1580 are lower . The contents of Mn , P , S and Al elements in the two are almost the same , but the The content of C and Si is lower , for example, the content of C in more than 50% of the steel coils in the 1700 line is 0 . 04% or less , while 80% of the 1580 line coil C content is 0 . 04% or more ; the Si content of almost all steel coils in the 1580 line is below 0. 022% , and most of the 1700 lines are at 0 . 022% or more. From this point of view , the chemical composition is not the reason for the edge cracking of the 0. 38 mm x 1 000 mm thin product produced by the 1580 line for acid rolling raw materials , and even helps to reduce the hardness of the product in the cold rolling plant.

2. 2 Unstable hot rolling temperature leads to mixed crystals at the edge of the strip. The mixed crystals at the edge of the strip are unavoidable for hot-rolled coils. The main reason is that the heat dissipation at the edge of the high-temperature strip is faster , so that the phase transition at the edge occurs before the middle part , the proeutectoid ferrite produced by the phase transformation grows to form mixed crystals. Hot rolling should minimize the mixed crystal area at the edge , and measures should be taken to control the temperature difference between the edge and the middle within a reasonable range.

1700 line sample and the 1580 sample have mixed crystals. The crystal grains on the operating side and the driving side of the 1580- line sample are relatively coarse . It is confirmed by transmission electron microscopy that there are three cementite precipitations in the structure , and the grains of the 1700- line sample are relatively fine.
In order to verify the influence of tertiary cementite on the mechanical properties of steel coils , hardness tests in the width direction of the 1580 - line and 1700- line samples were carried out , and the test results are shown in Table 2 and Table 3 .

table 2 directio	Hardness distribution in the width direction of 1580- line SPHC Tab . 2 Hardness distribution along width n of SPHC steel coil of 1580 production line	HRB HRB
Location	Hardness value	average value
Operation side	55 ． 752 . 0 50 . 5	52 . 7
middle	62 . 663 ． 863 ． 1	63 . 2
drive side	60 . 0 61 . 0 61 . 5	60 . 0
Table 3	Hardness distribution in the width direction of 1700- line SPHC	HRB
	Tab ． 3 Hardness distribution along width
direction of SPHC steel coil of 1700 production line		HRB
Location	Hardness value	average value
Operation side	63 . 0 64 . 0 63 . 0	63 . 3
middle	59 ． 0 55 ． 0 58 ． 5	57 . 5
drive side	63 . 5 62 . 5 58 ． 5	61 . 5

It can be further confirmed from Table 2 and Table 3 that there are mixed crystals in the edge structure of the 1580- line sample , and the grain size is coarser than that of the 1700- line sample. From this, it can be determined that the reason for the cracks in the strip is the abnormal structure , which requires improving the rolling reduction and the cooling method of the strip. 2 . 3. Analysis of edge mixed crystals
Since hot-rolled edge mixed crystals are unavoidable for hot-rolled coils , it is necessary to control the size of the edge mixed crystal area as much as possible , and take measures to control the temperature difference between the edge and the middle of the strip within a reasonable range. The control of the mixed crystal area at the side is that the mixed crystal area at the driving side is 0 ~ 25 mm ; the mixed crystal area at the operating side is 0 ~ 28 mm .
3 Improvement measures
Under slow cooling conditions , tertiary cementite forms on the phase boundary between ferrite and cementite. In order to reduce the precipitation of tertiary cementite and control the formation of mixed crystals at the edge, the finish rolling temperature was raised from 850 T to 890 T , the coiling temperature was lowered from 660 T to 610 T , and the water volume of cooling water of rolling mill rolls was adjusted to reduce The temperature difference in the width direction of the SPHC strip and the adjustment of the reduction rate distribution of the rolling mill after cold rolling , the edge crack of the strip has been greatly improved.

	Table 1 Composition of LF furnace final slag under different refining modes of 20Mn2A Tab ． 1 Composition of LF end - slag in 20Mn2A under different refining models				% %
Slagging method	Ca0	Al 2 0 3	Si02	Mg0	Fe0 + Mn0
LF Furnace Added	54 . 45 ~ 62 . 53	12 . 83 ~ 18 . 69	14 . 28 ~ 19 . 31	6 . 99 ~ 10 . 50	0 . 29 ~ 1 . 64
tapped to join	49 ． 50 ~ 58 . 96	22 . 71 ~ 30 . 92	6 . 46 ~ 12 . 76	4 . 86 ~ 8 . 69	0 . 21 ~ 0 . 98

After the slagging system is optimized , white slag can be formed quickly in the early stage of LF treatment , and the average retention time of white slag in the refining process is relatively improved.
The advance time was increased by 3 min , and it can be seen from Figure 2 that after the optimization of the slagging system, the average soft Ar blowing time was increased by 2 min .
The TO ] of the steel out of the furnace is 25 X10 -6 , which is 35.9 % lower than that before the improvement , and the TO ] of the slab is 21 X10 -6 , which is 32.9% lower than that before the improvement .
3% ; From the point of view of desulphurization effect , after optimizing the composition of refining slag and improving the slagging system, the agglomeration phenomenon of ladle slag disappeared , and the desulphurization rate increased by 20.47% . Quality is improved.

epilogue
( 1 ) When designing the final slag of aluminum-containing steel refining , the primary molten steel should be considered
O ] high results in a large amount of de- zero products , and the high-melting-point solid phase inclusions that are not fully floating remain in the molten steel, which is more harmful , and the molten steel C Relatively low is unfavorable to desulphurization and other factors , ensuring the needs of purifying molten steel and desulphurization .
( 2 ) Taking into account the dual requirements of desulfurization and absorption of Al2 03 inclusions , Ca0 - Al 2 0 3 -CaF 2 _ The slag system is suitable as aluminum-containing steel refining slag , the optimum composition is Ca0 = 50% ~ 55% , SiO2 = 6% ~ 10% , Al 2 0 3 =26% ~ 29% , Mg0 =4% ~ 8% , Ca0/ Al 2 0 3 =1.7 ~ 1.9 .
( 3 ) the slag washing process is adopted in the tapping process , and large
Partial refining slag, control the amount of slag , effectively use the hot refining slag coming down from the continuous casting rotary table , and quickly form slag during LF refining , obtain sufficient white slag retention time and soft Ar blowing time, and achieve good refining Effect. When using this process to refine 20Mn2A , the desulfurization efficiency reaches 77.13% , the slab T 0 ] is 21 X10 -6 , and the Als ] in the slab is 0.026% .

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