JPH06335706A - Production of hot rolled steel sheet excellent in surface quality - Google Patents

Abstract

(57) [Summary] [Object] To provide a method for producing a hot-rolled steel sheet having good surface quality. [Structure] C: 0.1% or less by weight%, Mn: 0.0
5 to 0.4%, S: 0.025% or less, Al: 0.00
Steel containing 1 to 0.1% and N: 0.01% or less with the balance Fe and unavoidable impurities is melted to have a plate thickness of 10 mm.
Continuously cast into the following thin slabs, and after casting, cool to the hot rolling start temperature in an atmosphere in which the oxygen concentration is controlled to 0.1 to 5%, and hot-roll at 10% or more at 1200 to 900 ° C. And wind at 650 ° C or lower.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot rolled steel sheet having excellent surface quality. In recent years, a technology for directly casting thin slabs with a plate thickness of 10 mm or less from molten steel has been developed.
Testing is being done on an industrial scale. This new technology is a process to simplify or omit the hot-rolling process and produce hot-rolled sheet or thin slab with the same thickness as the hot-rolled sheet, which is expected to save energy and cost. I am collecting.

Thereafter, the above-mentioned process is referred to as STC process (St
rip casting process), a process of casting a slab having a plate thickness of 100 mm or more by continuous casting and performing hot rolling to manufacture a hot rolled steel plate having a plate thickness of several millimeters is called a current process.

[0003]

2. Description of the Related Art In the current process, a cast slab is heated to 1200 to 1250 ° C. in a heating furnace to obtain 900 to 125
A hot rolled steel sheet was obtained by rolling in a temperature range of 0 ° C. A descaler using high-pressure water was installed in front of each hot rolling stand, and hot rolling was performed immediately after removing the scale. On the other hand, ST
The thin slab cast by the C process has a surface roughness Rmax of 60 to 120 μm in the as-cast state and cannot be used as a substitute for the hot rolled sheet in the current process. Therefore, a hot rolling process for adjusting the surface roughness is necessary. However, in the method of hot-rolling by performing descaling using high-pressure water similar to the current process, the thickness of the hot-rolled material is extremely thin unlike the current process, so the slab is cooled by the high-pressure water used in the descaler. And could not be hot rolled at high temperature.

In the method of hot rolling without using a descaler, a slab with scale is hot rolled. The slab cast by the STC process has a high temperature of about 1400 to 1300 ° C. when it is separated from the mold, and a thick scale is produced when cooled in the atmosphere. Therefore, hot rolling is performed through a thick scale, which is a problem because good surface quality cannot be built in. As a method of descaling and hot rolling without using high-pressure water, Japanese Unexamined Patent Publication (Kokai) No. 61-219448 discloses a descaling method using a brush roll, and JP-A-1-138.
Japanese Patent No. 013 discloses a descaling method using multi-needle chisel.

However, these methods have a problem in terms of equipment durability, and it has been difficult to carry out the method on an industrial scale.

[0006]

By controlling the scale before hot rolling in the STC process, the present invention prevents surface roughening caused by scale that occurs in hot-rolled steel sheets of thin cast slabs, and provides hot rolling with excellent surface quality. It is an object to provide a method for manufacturing a steel sheet.

[0007]

SUMMARY OF THE INVENTION The gist of the present invention is that, in% by weight, C: 0.1% or less, Mn: 0.05.
~ 0.4%, S; 0.025% or less, Al; 0.001
~ 0.1%, N; 0.01% or less, steel with the balance Fe and unavoidable impurities is smelted, continuously cast into thin slabs with a plate thickness of 10 mm or less, and oxygen concentration after casting. 0.
Cooling to a hot rolling start temperature in an atmosphere controlled to 1 to 5%, hot rolling of 10% or more at 1200 to 900 ° C, and winding at 650 ° C or less, excellent heat of surface quality. It is in the method of manufacturing rolled steel sheets.

[0008]

The present invention will be described in detail below. The cause of deteriorating the surface quality of the hot rolled steel sheet manufactured by the STC process is as follows. First, the scale thickness is about 20 to 1
This is a change in deformation behavior during hot rolling due to the thickness of 00 μm. Even if hot rolling is performed to smooth the unevenness (60 to 120 μm) of the slab, the porous and thick scale becomes a lubricant to sufficiently smooth the slab to fill the unevenness of the slab. I couldn't.

Second, the scale generation behavior is non-uniform. The scale that grows rapidly in a high temperature region has a non-uniform thickness. When hot rolling is performed without descaling, roughening of the hot-rolled sheet corresponding to uneven scale thickness occurs. The third is nonuniform scale peeling. The scale generated in the high temperature range causes cracking and peeling of the scale due to heat shrinkage during the cooling process. When hot rolling is performed in a state where the scale is partially peeled off, surface roughness corresponding to the scale thickness is generated on the surface of the hot rolled sheet.

The present inventors have conducted research on a method of controlling the cooling atmosphere of a slab after being separated from the mold as a method for drastically improving the cause of the surface quality deterioration of the hot rolled steel sheet as described above. The hot-rolled steel sheets targeted are SS300, SS400, SS500, SPHC, which are specified by JIS standards.
It is intended to manufacture SPHD and SPHE and presupposes the general ingredients in these products. That is, in% by weight, C: 0.1% or less, Mn: 0.05 to 0.4%,
S: 0.025% or less, Al: 0.001-0.1%,
N: 0.01% or less is included, and the balance is intended for a hot rolled steel sheet composed of Fe and unavoidable impurities. In the case of high Cr steel such as stainless steel, the amount of scale produced is small, so that roughening does not occur even if hot rolling is performed without descaling. It is only ordinary steel as defined in the present invention that hot rolling without roughening and roughening occurs.

After casting, the cooling rate is 20 to 30 in an atmosphere in which the oxygen concentration is controlled to 0.1 to 5% up to the hot rolling start temperature.
It was found that a hot rolled steel sheet with excellent surface quality can be produced by cooling at a cooling rate of 0 ° C./sec, rolling at a hot rolling reduction of 10% or more, and then winding at a temperature of 650 ° C. or less. That is, in the thin plate casting-hot rolling equipment having the equipment configuration shown in FIG. 1, the plate thickness is 4 mm and the surface roughness is about 80 μm in Rmax.
After casting, the thin slab of No. 1 was cast, the atmosphere was variously changed, the temperature was cooled to 1100 ° C, and hot rolling was performed at 1100 ° C.
After hot rolling, it was cooled with steam and wound at 500 ° C.

The results are shown in FIG. If the oxygen concentration exceeds 5%, the surface roughness is Rmax; 4 even if the hot rolling reduction is increased.
At 0 μm or more, the surface quality equivalent to that of the current process material was not obtained. On the other hand, when the oxygen concentration is 5% or less, the hot rolling reduction rate is 10
%, The smoothness became Rmax; 20 μm or less. However, when the oxygen concentration was less than 0.1%, roughening of the skin occurred when the hot rolling reduction was set to 20% or more, and the Rmax was more than 20 μm. That is, it was considered that if the scale was too thin, rough skin would occur due to poor lubrication during hot rolling.

The hot rolling temperature is set to 1,200 in consideration of the wear resistance of the roll and the influence of buildup of the scale on the roll.
It was set to ˜900 ° C. Since the hot rolling of the STC process is performed continuously for a longer time than the hot rolling of the current process,
If it exceeds 1200 ° C, defects due to roll wear occur on the hot rolled sheet. Further, it was found that if the temperature is lower than 900 ° C., the hot-rolled sheet will be flawed due to build-up of the scale on the roll.

Regarding cooling after hot rolling, it is desirable to cool the surface smoothed by hot rolling quickly so as not to be damaged by scale produced after hot rolling. As for the cooling condition, the influence of the coiling temperature is the largest, and if the coiling temperature is 650 ° C. or less, the amount of scale formation is small, and finally a hot-rolled steel sheet having good surface quality can be manufactured. That is, although the relationship between the winding condition after hot rolling and the surface roughness of the hot rolled steel sheet is shown in FIG. 3, it was found that good surface quality cannot be obtained when the winding temperature exceeds 650 ° C.

[0015]

EXAMPLE A common steel was melted and a thin cast piece having a plate thickness of 5 mm was cast by the twin drum type thin plate casting machine shown in FIG. After casting, it is cooled under the atmosphere shown in Table 1 and Table 2 of the present invention,
Perform hot rolling of 10 to 30%, and after hot rolling, cool with steam.
It was wound at 650 to 400 ° C. After pickling the hot rolled steel sheet,
The surface roughness was measured using a two-dimensional roughness meter.

As a comparative example, ordinary steel was cast into a thin slab having a thickness of 5 mm by the twin drum type thin plate casting machine shown in FIG. After casting, a hot rolled steel sheet was prepared under conditions outside the scope of the present invention (Table 2), pickled and the surface roughness was measured. As is clear from Tables 1 and 2, the hot rolled steel sheet having excellent surface properties could be produced by the method of the present invention, but the hot rolled steel sheet produced by the comparative method had poor surface properties.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

The hot rolled steel sheet having good surface quality can be produced by the STC process of the present invention. Therefore, the technical effect is extremely large in terms of economy.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of equipment for producing a hot-rolled steel sheet according to the present invention.

FIG. 2 is a diagram showing the influence of a cooling atmosphere (mold to hot rolling machine) and a hot rolling reduction rate on the surface quality of a hot rolled steel sheet.

FIG. 3 is a diagram showing an influence of a winding temperature after hot rolling on a surface quality of a hot rolled steel sheet.

[Explanation of symbols]

 1; Cooling roll (mold) 2: Atmosphere cooling layer 3: Hot rolling machine 4: Secondary cooling zone 5: Winding machine 6: Guide roll 7: Cast slab 8: Molten metal 9: Hot rolling coil

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C22C 38/00 301 W 38/06

Claims (1)

[Claims] 1. C. by weight%; 0.1% or less; Mn;
0.05-0.4%, S; 0.025% or less, Al;
Steel containing 0.001 to 0.1%, N; 0.01% or less, and the balance Fe and unavoidable impurities is melted and continuously cast into thin slabs with a plate thickness of 10 mm or less, and after casting Is cooled to a hot rolling start temperature in an atmosphere in which the oxygen concentration is controlled to 0.1 to 5%, hot rolled at 10% or more at 1200 to 900 ° C, and wound at 650 ° C or less. A method for producing a hot rolled steel sheet having excellent surface quality.