JPS5821009B2 - Kurokawa Scale Seijiyouno Suguretanetsuenkoutaino Seizouhouhou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a hot-rolled steel strip that is free from surface scales generated during hot rolling.

Conventionally, in order to manufacture hot-rolled steel strips with a tensile strength of 40 kg/mA or more and a plate thickness of 6 yen or more, killed steel is used to ensure the strength and low-temperature rolling is performed. It often goes through a cold unwinding and leveling process.

For example, in the process of manufacturing steel sheets by shearing hot-rolled steel strips, there is also an unwinding and leveling process, and during this unwinding and leveling, the black scale generated during hot rolling peels off (baldness) (occurs on the edges of the copper strip). Copper scale is generated, which adheres and accumulates on the leveler roll and pinch roll, which tends to cause scale intrusion defects on the surface of the copper strip.

In addition, especially during hot rolling of killed steel, island-like scale flaws present in killed steel, that is, remaining scale that could not be removed by the descaler during hot rolling, are pushed into the steel strip surface during rolling and stretched, forming island-like defects. This causes scale defects.

In order to eliminate such drawbacks, the present invention has been developed to reduce the Si content in killed steel including continuously cast slabs to 0.20% or less, to set the final phase roll exit temperature to 1000°C or higher, and to adjust the temperature during low-temperature rolling. This is done before the finish rolling mill to prevent island-like scale flaws, and the cooling rate after winding is reduced to 5℃/h.
It is characterized by improving scale adhesion by slowing down to less than r, and reducing the occurrence of scale baldness and powder scale in the unwinding and leveling special cold processing process, such as island scale, powder scale, scale baldness, etc. This aims to improve the surface of the product by reducing the number of scratches, and to reduce the occurrence of product defects due to the scale flaws and scale intrusion flaws.

The present inventors have discovered that S in killed steel including continuously cast slabs is
The experimental results of an investigation into the relationship between i content (%) and island scale flaws, edge scale baldness, powder scale occurrence, etc. revealed that when the Si content is 0.20% or less, the properties of melasma scale are We found that the condition was extremely good.
We also investigated and examined the relationship between the occurrence of island scales during low-temperature rolling performed to obtain the strength of hot-rolled killed steel sheets, the rough roll exit temperature, and the Si content.
It was confirmed that the occurrence of island scales was reduced at 0.20% or less and at a rough roll exit temperature of 1000° C. or higher.

Furthermore, we investigated the effect of the cooling rate after winding the coil on scale peeling, and found that the relationship between the number of cooling days from 600°C to 50°C, scale peeling, and powder scale generation decreased when the cooling rate was 5 days or more. I found out.

Based on this knowledge, the present inventor used the above-mentioned guild steel ingot, regulated the rough roll exit temperature, adjusted the temperature during low temperature rolling before the finishing mill, hot rolled it, and after coiling. By slowing down the cooling rate, we were able to successfully obtain a hot coil with extremely smooth black scale surface conditions.

Next, the present invention will be explained in more detail.

First, island-like scale occurs when the primary scale during heating is not removed by descaling during hot rolling, and some of the remaining scale is pushed into the rolled material during rolling and is stretched to form islands, resulting in island-like scale defects. This occurs significantly when the Si content in the steel is high.

This is because semi-molten Fe25i04 (feyarite), which is difficult to remove with a descaler during rolling, is produced, and as a result of experiments, as shown in Figure 1, the Si content in the steel is 0.
When the Si content is 0.20% or less, the scale properties are extremely good and the occurrence of island-like scale defects is drastically reduced, but when the Si content exceeds 0.20%, island-like scale defects occur. It was found that the occurrence of

In addition, in the case of hot rolling of killed steel strip, low-temperature rolling is performed to obtain the strength of the hot-rolled steel strip, but conventionally, the temperature of the material for this low-temperature rolling was adjusted before the final rough roll. Adjustments are being made, but in this case the coarse roll exit temperature is 10
Scale (Fe2S) is in a semi-molten state at around 00°C4
A portion of i04) remains and is pushed into the rolled material and stretched to form island-like scale flaws.

Therefore, the temperature of the material in the low-temperature rolling is adjusted before the finishing mill, and the rough roll rolling temperature is set the same as in normal rolling.

The mouth temperature is set to 1000°C or higher, and the low-temperature finish rolling temperature is adjusted to 940 to 980°C in front of the finish rolling mill.

During this adjustment process, scale swelling occurs during cooling and oxidation, that is, a peeling phenomenon occurs at the interface between the rolled material and the scale, resulting in pinch rolls and scale blurring before the finishing mill.

- It is possible to remove scale that is difficult to remove at high temperatures in a car, thereby preventing island-like scale defects from occurring in rolled products.

Next, we will discuss scale baldness and powder scale occurrence.

Relationship between the occurrence of scale baldness and powder scale that occurs during unwinding or leveling in the next process (shearing process to steel plate, etc.) in a coil wound after hot rolling with Si content and winding. We investigated the relationship between the coil cooling rate after winding, and the relationship between the Si content, scale baldness, and powder scale generation status is shown in Figure 3.The relationship between the coil cooling rate after winding, scale baldness, and powder scale generation status is shown in Figure 3. As shown in Figure 4, when the Si content in the steel is 0.20% or less, the edge scale baldness is about 20 mm, and the generation of powder scale is hardly a problem in use or processing. However, when the Si content exceeds 0.20%, the surface condition such as edge scale bald spots increases to 30 mm or more, and a large amount of powder scale is generated, impairing the quality of black leather products. resulting in deterioration.

Furthermore, if the cooling rate after winding the coil is slowed down to 5°C/hr or less, the adhesion of scale will be improved and edge scale baldness will be prevented.
It is possible to reduce the occurrence of powder scale, and if cooling is performed at a slower rate so that the number of days for cooling from about 600°C to about 50°C after winding the coil is 5 days or more, that is, 5°C/hr or less, As shown in the figure, the occurrence of edge scale baldness and powder scale is reduced, and good surface scale properties can be obtained.

The reason why scale adhesion improves in this way is that the stable phase of black scale at high temperatures is Fed (riceite), but FeO changes from 4FeO to αFe during the cooling process.
It is known that +Fe3O4 eutectoid reaction occurs,
Fe3O4 (magnetite) obtained by this decomposition is relatively dense and is said to have good adhesion to the base metal (Fe) due to the lattice spacing.

Therefore, in order to ensure sufficient decomposition, the cooling rate in the transformation reaction temperature range (maximum at 570-260°α480°C) is slowed down to improve scale adhesion and reduce scale baldness and powder scale generation. It is something that makes you

Normally, in order to cool the coil as quickly as possible, the side surface of the coil (perpendicular to the width direction) is placed horizontally so that it is in contact with the atmosphere to improve air exchange (No. 5
However, with this arrangement method, the cooling period from 600°C to 50°C for the coil is approximately 3.5 to 4 days, which is sufficient to prevent scale baldness and powder scale that may cause the above-mentioned transformation reaction. do not have.

Therefore, in the present invention, the coil cooling rate is approximately ℃/hr.
In order to reduce the cooling rate as much as possible, the side surfaces of the coils (the surfaces perpendicular to the plate width direction) are arranged vertically to minimize the influence of atmospheric ventilation and to slow down the cooling rate as much as possible (see Figure 6).

When the coil is cooled in this way, the
It takes more than 5 days to cool down to 0℃, and the cooling rate is 5℃.
C/hr or less, the transformation reaction of the scale can be sufficiently caused to obtain scale properties with good adhesion.

In this way, when cooling after winding, place it between other hot coils to keep it warm! -In addition to cooling, a heat insulating cover may be provided for cooling.

Next, examples of the present invention will be described.

C = 0.13%, Si2 ratio of 0.12, Takumi = 0.79 ratio,
P2 0.021%, S2 0.009%, 5olAt=0
．． Killed steel slab of composition 036%, thickness 220 Tomomi,
A width of 1,550 m and a length of 9.5 m was heated to a heating temperature of 1,230°C, and rough rolling was performed as usual until the final roll set exit temperature was 1.
The arrival temperature before the finishing mill was 1020°C at 060°C.

This is left to cool down to 970°C in front of the finishing mill to adjust the temperature, remove scales with pinch rolls and scale breakers in front of the finishing mill, and finish-roll the rolled material with a tandem rolling mill. The rolling mill outlet temperature was 830°C, and no island-like scale defects were observed.

The rolled steel strip was wound at 590°C and then placed in a vertical arrangement at a coil storage area, and after about 5.5 days the coil temperature was approximately.

Since the temperature reached 50°C, the steel plate was sheared in the cutting process, but there was no scale loss or powder scale, and a steel plate with a good surface scale condition could be obtained.

As described above, the method of manufacturing a hot rolled steel strip according to the present invention makes it possible to obtain a steel sheet with excellent surface black scale properties, making a great contribution to industry.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the occurrence of island-like scale defects depending on the Si content in steel. FIG. 2 is an explanatory diagram showing the Si content, the scale bald width at the edge of the copper band, and the occurrence of powder scale. FIG. 3 is an explanatory diagram showing the rough roll exit temperature and the occurrence of island scale defects depending on the Si content. FIG. 4 is an explanatory diagram showing the relationship between the coil cooling rate (number of days) after winding into a coil, scale growth, and the amount of powder scale generated. FIG. 5 is a schematic explanatory diagram showing a conventional coil arrangement method for coil cooling. FIG. 6 is a schematic explanatory diagram of the coil arrangement according to the coil cooling method according to the present invention. In the figure, 1... Coil. In Figures 2 and 4, grade 1 is a product with a small amount of powder scale that does not pose a problem as a black skin product, and grade 2 is a product with a small amount of powder scale that is considered a black skin product. Grade 3 indicates that it can be used, and Grade 3 indicates that there is a problem in using it as a black skin product due to the generation of powder scale. In Fig. 3, ○ indicates that there is no island scale, and × indicates that there is island scale. It is something that

Claims (1)

[Claims] I Killed steel with a Si content of 0.20% or less is heated at a rough roll exit temperature of 1000°C or higher, and rolled in front of a finishing mill.
A method for producing a hot-rolled steel strip with excellent black scale properties, characterized by hot rolling at a temperature of 40 to 980°C, and cooling temperature after coiling to 5°C/hr or less.