Cause Analysis of Unqualified Cold Bending of SS400 Steel Coil

The causes of unqualified cold bending of SS400 steel coils were analyzed through macroscopic inspection, mechanical property test, chemical composition analysis and metallographic inspection . The results show that the main reason for the cracks in the cold-bending specimens is that the aluminum oxide inclusions in the steel plate are too much, and the measures to reduce the cracks in the cold-bending are put forward.
Key words : SS400 steel coil; cold bending; crack; alumina inclusion

SS400 steel is a common structural hot-rolled strip steel commonly used in Japanese standard production. It is mainly used to manufacture structural parts such as buildings, bridges, ships, and vehicles. According to GB/T 228 - 2002, the author 's unit conducts mechanical inspection on the produced SS400 steel , and the obtained indicators such as Rm , Ra . =1.5a , d is the diameter of the bending center, a is the thickness of the sample) during the test, a large number of cold-bending unqualified samples appeared, as shown in Figure 1 , small cracks parallel to the rolling direction appeared on the surface of the sample. In order to find out the cause of the small cracks, the author checked the unqualified samples of 5. 75, 7. 75, and 13. 5 cm commonly used in the same batch (numbered 1, 2, and 3 respectively ) and the unqualified samples with a size of 13. 5 cm random sample of the same batch of qualified samples (No. 4 ) were subjected to physical and chemical inspection and comparative analysis.
1 physical and chemical test
1.1 Macro inspection
The parallel cracks on the surface of the sample are all gathered in the middle of the cold bending, parallel to the rolling direction and continuously appearing in the rolling direction . Measured by optical microscope, the crack length is 23 ~ 44 μm, of which crack 1 is the deepest, with a depth of 3 "m, and crack 5 is the longest, with a length of 44 #m .
1.2 Mechanical property test
Samples were taken from samples 1, 2, and 3 for mechanical performance tests. The results are shown in Table 1. It can be seen that the yield strength, tensile strength, and elongation after fracture are all within the required range, which is not much different from the qualified cold-formed samples. .
1.3 Chemical composition analysis
during the smelting process of No. 1, 2, 3 , and 4 samples for chemical composition analysis, and the results are shown in Table 2 .
Table 1 Mechanical properties of cold-formed cracked specimens
Tab. 1 Mechanical properties of the samples with cold bending cracks

Sample No	KeL/MPa	Rn/MPa	Rpo.2/MPa	A/%
1	340	445	345	26.0
2	300	425	300	2&0
3	305	435	315	26.5
Q/SGZGS	>245	400〜		$21
316-2008		510

It can be seen that after smelting in the LF furnace , the acid-soluble aluminum contents of No. 1, No. 2, and No. 3 samples are respectively 0.014 %, 0.023 %, and 0.012 % ; High, acid-soluble aluminum decreased to 0.006 %, 0.008 %, 0.004 % respectively, and the loss reached 57%, 65%, 66% respectively .
1.4 Metallographic examination
After the samples 1, 2, 3 , and 4 were chamfered, pre-grinded, and polished, they were etched with a 4% nitric acid alcohol solution, and their microstructures were observed and inclusions, grain sizes, and bands were evaluated. shape organization level, the results are shown in Figures 2 to 7 and Table 3 . It can be seen from Figure 2 and Figure 3 that the microstructures are fine ferrite and pearlite. It can be clearly seen from Figure 4 and Figure 5 that thick and long chain-like inclusions appear in the cold-bending cracked sample along the rolling direction. There is no chain inclusion in the middle of the crack shown in Figure 6 , but there are obvious alumina inclusions on the side of the same sample shown in Figure 5 , while the qualified sample shown in Figure 7 is only very small and scattered The spherical inclusions have little effect on the cold bending performance.

Tab. 2 Chemical compositions of SS400 steel in outbound LF furnace and tundish (mass)

smelting process	Sample No	C Si		mn	P	the s	AIS	AIT	Ca
	1	0. 17	0. 23	0.41	0. 023	0.004	0. 014	0.016	0.002 2
	2	0. 10	0. 19	0.50 _	0. 014	0.009	0. 023	0.026	0. 002 6
LF furnace outbound	3	0. 17	0.21	0.49	0.015	0.006	0.012	0. 013	0. 001 6
	4	0. 17	0. 16	0.42 _	0.012	0.004	0.006	0.008	0.002 0
	1	0. 17	0.23	0.42 _	0.022	0.004	0.006	0.008	0.001 1
	2	0. 13	0.20 _	0.50 _	0.014	0.003	0.008	0. 013	0. 003 9
Tundish	3	0. 18	0.20 _	0.50	0.016	0.005	0.004	0.008	0. 003 8
	4	0.18	0. 17	0.44	0.012	0.003	0.004	0.004	0. 001 3

Tab. 3 Inclusion, grain size and banded strucure grade of cold bending crack samples and qualified sample grade

Sample No	A fine line	A thick	B fine line	B thick	C line	c class thick	D line	D thick line	DS	grain size	Ribbon
1	0.5	0	>3.0	>3.0	2.5	0	0.5	0	1.0	10.0	1.0
2	0	1.0	2.0	>3.0	0	0	1.0	0	1.0	9.5	1.0
3	0.5	0	1.5	2.5	>3.0	0	1.0	0	1.0	9.5	1.0
4	0	0	0	0	0	0	1.0	0	0.5	9.5	1.0

2 Analysis and Discussion
2.1 Effect of chemical composition on cold bending cracks
Analysis of Table 2 clearly shows that the casting process is not well protected, causing the aluminum in the steel to be oxidized, resulting in alumina inclusions. The first reason is that aluminum is not added or the amount of addition is insufficient during the smelting process of steel; the second is that the amount of soft blowing nitrogen gas in the late stage of LF furnace smelting is not well controlled. However, the mass fraction of sulfur in the steel is 0.003 % ～ 0.006 %, which shows that the deoxidation of the steel in the LF furnace is better, and the reducing slag can be effectively desulfurized. So the second reason is more likely.
According to the analysis, there are two main sources of inclusions in the smelted steel: ① poor protection during the casting process, resulting in secondary oxidation of molten steel. The reason may be that the seal between the ladle and the long nozzle is not good, or the nitrogen cleaning is not good before the tundish is used; The shape of the inside of the nozzle causes deflected flow of the steel stream and excessive impact velocity, which increases the inclusion of mold powder, or directly accumulates on the inner surface of the submerged nozzle. Large-grained inclusions are formed, which are washed into the crystallizer by the steel flow and are drawn into the slab to form alumina inclusions.
2.2 Influence of microstructure on cold-bending cracks
Analysis of Table 3 and Figure 2 shows that the matrix of the cold-formed cracked steel plate is composed of ferrite and pearlite, and no obvious Widmanstatten structure was found. The grain size is mainly 9.5 ~ 10 grades, and there is no obvious surface band structure. And the banded structure is located in the center of the plate thickness, which is unfavorable to the transverse toughness of the steel plate, but it will not have an adverse effect on the cold bending performance of the steel plate (1).
2.3 Influence of inclusions on cold bending cracks
Non-metallic inclusions in steel are mainly caused by the deoxidation and desulfurization products in the smelting process that remain in the steel when the molten steel solidifies. During the tapping and pouring process, as the temperature drops, the impurity elements such as oxygen, sulfur, and nitrogen in the steel The solubility of the steel decreases, and it precipitates out from the molten steel to form non-metallic inclusions (2). Coarse alumina inclusions appear more in SS400 steel plates, and the grade reaches coarse 2.5 ~3 grades, followed by silicate inclusions. Although silicate inclusions have good plasticity when deformed in the high temperature range of 1 150 ~ 1 250 C, their deformation index is low when they are cold-formed at room temperature, which damages the continuous steel matrix . It is easy to generate cracks, and the thicker the inclusions, the easier and more serious the cracks are. The aluminum oxide inclusions in the steel are located on one side of the steel surface, and there are fine inclusions of aluminum oxide around the surface microcracks, which will deteriorate the cold bending performance of the steel. There are too many inclusions, the level reaches level 3 , and these inclusions are distributed along the rolling direction and tend to the upper surface of the steel plate, which is the main reason for the unqualified cold bending performance of the steel plate.
The effect of inclusions on cold bending properties depends on their number, size and distribution. When the alumina inclusions are large in number, large in size and concentrated in distribution, they can be used as a crack source and directly cause cold bending cracking of the steel plate (3). Therefore, alumina inclusions are the main factors affecting the cold bending properties of SS400 steel plates.
Conclusions and precautions
( 1 ) The main reason for the cracks in the SS400 steel plate cold-bending sample is the aggregation of alumina inclusions near the surface of the steel plate, which leads to the crack O
( 2) Preventive measures include: ① It is difficult to eliminate and reduce alumina and silicate inclusions in steelmaking, but it is feasible to change the harmful distribution of inclusions in steel. By adding electromagnetic stirring to the crystallizer, the harmful silicate inclusions can be dispersed and uniform without tending to the surface. During refining in LF furnace, deep desulfurization of reducing slag is made, aluminum wire is fed, composition is fine-tuned, and calcium wire is fed for inclusion denaturation treatment. In the later stage of LF furnace smelting, ammonia blowing or vacuum stirring is used to promote the floating of inclusions, but the nitrogen flow rate of soft blowing should be controlled, neither too large nor too small. From the ladle to the crystallizer, the whole process of protection casting is adopted, and the temperature of the tundish, the fluctuation of the liquid level of the crystallizer and the stable casting speed are strictly controlled to prevent slag rolling; Billets with unqualified quality are loaded into the furnace;

---