JP2003306745A - Hot rolled steel sheet having excellent surface property and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet which has high scale adhesion in a final stage and has no rough spots in the surface after pickling, thus has excellent appearance, and to provide a production method therefor which is advantageous operationally and also cost-wise. <P>SOLUTION: In the hot rolled steel sheet, Si and O-containing, branched, beltlike oxides are formed by ≥2 μm in a thickness direction on the grain boundaries of the interfaces between scale and ferrite in the surface. The hot rolled steel sheet is obtained by removing scale in the surface of the steel sheet by pickling. Preferably, the surface mean roughness (Ra) after the pickling is 1.5 to 2.5 μm, and Rmax is 10 to 20 μm. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet having excellent surface properties and a method for producing the same, and in particular, it has high scale adhesion at the final stage after hot rolling and has no roughness unevenness even after scale removal. The present invention relates to a hot-rolled steel sheet having an appearance and a manufacturing method thereof.

[0002]

2. Description of the Related Art Generally, when manufacturing a hot-rolled steel sheet, a steel slab (slab) having a thickness of about 250 mm is first heated to a high temperature (1100 to 1400 ° C.) in a heating furnace and then formed on the surface. After descaling the scale layer, it is sent to the hot rolling process. In hot rolling, first rolling in a rough rolling mill to a thickness of about 40 mm, rolling it down to a thickness of 2 to 8 mm in a tandem finishing mill, finishing hot rolling, and then transporting it on a runout table. While being cooled with water, it is wound with a coiler to obtain a hot rolled steel sheet.
Scale is generated on the surface of the steel sheet when wound on a coiler.It is rewound and sent to the pickling process to remove the scale layer on the surface before shipment as a product. It may be shipped as a product without being processed.

Usually, when obtaining a hot-rolled steel sheet having excellent surface properties, it is considered important that the scale layer formed on the surface of the steel sheet is peeled off as much as possible by descaling means during hot rolling. Various measures have been taken. For example, in the past, Japanese Patent Laid-Open No. 2000-
As disclosed in Japanese Patent No. 254724, heating /
A method has been proposed in which descaling conditions are set to a high degree and scales generated in a heating furnace or the like are removed to obtain a steel sheet without surface flaws. Certainly, the above-mentioned JP-A-2000-25472
It can be said that the method of Japanese Patent No. 4 is effective to some extent in order to obtain a desired steel sheet having excellent surface properties, but the economical load due to high pressure is relatively large, and it is not necessarily practical. There was a point. Further, in particular, in the high Si content steel which is also targeted in the present invention, Fe ₂ SiO
_A large amount of scale mainly composed of ₄ (fire light) is generated, and this creates a state that it penetrates into the base metal, so peeling is not easy, and it cannot be completely peeled off under normal descaling conditions, and it has not peeled off. Parts may remain. This unpeeled portion causes unevenness of the base metal after pickling, and creates roughness unevenness, deteriorating the surface properties of the steel sheet.

On the other hand, as in the present invention, the scale layer is adhered to the base metal as much as possible until it is pickled without peeling, and after the pickling, a steel sheet having a relatively large roughness and an excellent appearance without unevenness is obtained. There are few proposals to make, and it is slightly disclosed in JP-A-3-79718.
The gazette can be mentioned. The method for producing a hot rolled steel sheet disclosed in this publication aims to stably produce a composite high-strength hot rolled steel sheet having an excellent surface texture without a cloud pattern, a steel sheet component, a casting temperature, a heating temperature, and The content is that the rolling-descaling conditions are finely defined, Si scale is positively and uniformly generated on the entire surface of the steel sheet, and the adhesion with the base steel is enhanced so as not to separate during hot rolling.

[0005]

However, the invention described in Japanese Patent Application Laid-Open No. 3-97718 has no problem in obtaining a hot-rolled steel sheet having desired properties, but is constrained by each constituent requirement. And Si is 1.1
Since it is as high as wt% or more and the maximum temperature is also high, the actual situation is that a steel material that satisfies the necessary conditions cannot be obtained.

The present invention has been made in view of the above points, and provides a hot-rolled steel sheet which has high scale adhesion at the final stage and which has no roughness unevenness on the surface of the steel sheet after pickling and has an excellent appearance. It is an object of the present invention to provide a method for manufacturing a hot-rolled steel sheet which is advantageous in terms of operation and cost as well as providing the same.

[0007]

The gist of the present invention for solving the above problems is as follows. (1) A hot-rolled steel sheet having excellent surface properties, characterized in that a branched band-shaped oxide containing Si and O is formed in a grain boundary at the scale-base iron interface on the surface of the steel sheet in a thickness direction of 2 μm or more. (2) The hot-rolled steel sheet according to (1), which is obtained by removing the scale on the surface of the steel sheet by pickling. (3) The hot-rolled steel sheet according to (1) or (2), which is obtained by completing finish rolling of a steel material containing 0.8 mass% or more of Si at 850 ° C. or more. (4) 0.8 mass% or more of Si and 0.02 mass% of Ni
As described above, the steel material having S of 0.01 mass% or less is obtained by hot rolling after keeping the dew point of 55 ° C. or more and the oxygen concentration of 2% or more and the surface temperature of 1173 ° C. or more for 2 hours or more (1) Alternatively, the hot rolled steel sheet according to (2). (5) Prior to heating the steel material for rolling, SiO is formed on the surface of the steel material.
The hot-rolled steel sheet according to any one of (1) to (4), which is obtained by applying ₂ solutions and performing heating-hot rolling. (6) The hot-rolled steel sheet according to any one of (1) to (5), which is obtained by colliding high-speed droplets accelerated to 200 m / s or more with a steel sheet surface to remove scale before hot rough rolling. . (7) Surface average roughness (Ra) is 1.5 μm or more and 2.5 μm
The hot-rolled steel sheet according to any one of (1) to (6), wherein the Rmax is 10 μm or more and 20 μm or less when m or less. (8) A surface characterized in that a branched band-shaped oxide containing Si and O is generated in a thickness direction of 2 μm or more at a grain boundary of a scale-base iron interface on the surface of a steel sheet and the surface scale is removed by pickling. A method for producing a hot rolled steel sheet having excellent properties. (9) The method for producing a hot-rolled steel sheet according to (8), wherein finish rolling of a steel material containing 0.8 mass% or more of Si is completed at 850 ° C or higher. (10) A steel material containing 0.8 mass% or more of Si, 0.02 mass% or more of Ni, and 0.01 mass% or less of S with a dew point of 55 ° C. or more and an oxygen concentration of 2% or more and a surface temperature of 1173 ° C. or more. The method for producing a hot-rolled steel sheet according to (8), wherein hot rolling is performed after maintaining the state for 2 hours or more. (11) Prior to heating the steel material for rolling, Si is applied to the surface of the steel material.
The method for producing a hot-rolled steel sheet according to any one of (8) to (10), which comprises applying an O ₂ solution and performing heating-hot rolling. (12) The heat according to any one of (8) to (11), characterized in that prior to the hot rough rolling, high-speed droplets accelerated to 200 m / s or more are collided with the steel sheet surface to remove scale. Manufacturing method of rolled steel sheet.

[0008]

DETAILED DESCRIPTION OF THE INVENTION The details of the present invention will be described below.
In the hot-rolled steel sheet according to the present invention, a branched band-shaped oxide containing Si and O was formed in the thickness direction of 2 μm or more at the grain boundary of the scale-base iron interface on the surface at the final stage after winding. As a feature, the branched band-shaped oxides here are those in which crystal grains on the iron side and grain boundary oxides spreading in a plane along the interface of these crystal grains are developed and connected. Point to. As shown in the sketch of FIG. 2, the oxide is not only branched, but also spreads in a plane with a certain width to form a strip.
The steel sheet has a uniform scale layer that enhances the adhesion of scale adhering to the surface of the base metal and is resistant to peeling. Moreover, since scale peeling in the steps prior to the pickling step (during transportation, rewinding, etc.) is eliminated, uniform pickling can be realized, and macro unevenness is less likely to occur, and microscopically In this case, the base metal-scale interface is appropriately roughened by the grain boundary oxidation, and the interface roughness after scale removal is relatively large.

It is known that the Si-rich layer and the internal oxide layer at the interface of the base metal during high-temperature oxidation of Si-containing steel are produced by the content diffusion of oxygen. For the existence of the internal oxide layer, for example, " Iron and Steel ”(vol. 85 (199
9), No. 12, p. 878). However, this document does not mention at all the relationship between the grain boundary oxidation and the surface roughness in the final stage after winding the hot rolled steel sheet as described above, and the shape of the oxide is also described as spherical. However, there is no disclosure about the exact shape and the effect on the steel plate quality.

Further, in the present invention, the branched band-shaped oxide is formed over 2 μm or more in the thickness direction of the steel sheet, but if it is less than 2 μm, it is too short and the oxide spreads to the grain boundaries. Is insufficient, and it may not be possible to expect the desired scale adhesion improvement. Although the upper limit is not particularly specified, it is about 10 μm in terms of operation. The range of t of FIG. 2 shows the area | region which produced the strip | belt-shaped oxide in the steel plate thickness direction.

Conventionally, the interface roughness after scale removal is generally made as small as possible, and there are not many proposals for purposely increasing the interface roughness as in the present invention. However, even if it is attempted to easily peel off the scale to reduce the roughness after pickling, it is often technically difficult, and the peelability of the scale is not uniform and the peeled surface and the non-peeled surface are different. Since they were mixed, streak-like spots were formed on the surface of the steel sheet after pickling, which was an adverse effect on the coated surface. The present invention is characterized in that the scale adhesion is enhanced and the scale peeling is suppressed as much as possible before the pickling, and the scale is uniformly and peeled off by the pickling to obtain a steel plate surface having unevenness. The surface roughness of the steel sheet according to the present invention after this pickling is such that the average roughness (Ra) is 1.5 μm or more and 2.5 μm or less and Rmax is 10 μm or more and 20 μm or less. It is necessary to obtain a hot-rolled steel sheet with an excellent appearance without any appearance.

Next, various conditions in the manufacturing process of the hot-rolled steel sheet according to the present invention will be described below. FIG. 1 shows a schematic view of the manufacturing process of the present invention. In the figure, 1 is a heating furnace for heating a steel material to a required temperature, 2 is a descaling device for removing scale generated on the surface of the steel material (for example, a high-pressure water jet type), 3 is a roughing material for rolling the steel material. 63 rolling mill trains, 4 is a subsequent finishing rolling mill train, 5 is a runout table for cooling the rolled steel plate, and 6 is a coiler for winding the steel plate. Reference numeral 7 is a pickling tank for rewinding the once rolled steel plate to remove the surface scale layer thereof, and 8 is for applying a SiO ₂ solution to the steel material before charging it into the heating furnace 1 as described later. The application part for is shown.

In the hot-rolled steel sheet of the present invention obtained by the above manufacturing process, 0.8 mass% of Si is used as a raw material steel component.
A steel material containing the above is preferable. This is because if Si is less than 0.8% by mass, a sufficient amount of band-shaped oxide will not be generated at the grain boundaries. Although the upper limit of the Si content is not particularly specified, it may be about 2% by mass, and preferably less than 1.1% by mass is recommended because it does not impair the required workability. . In addition, final hot rolling is to be completed at 850 ° C. or higher during hot rolling. This is to ensure a temperature of 850 ° C. or higher in finish rolling even if Si is 0.8 mass%. In this case, it is possible to generate a sufficient amount of band-shaped oxides at the grain boundaries and finally smooth the surface roughness distribution.
If the temperature is lower than 50 ° C., it is too low to expect such a phenomenon. The finish rolling temperature is preferably 900 ° C. or higher in order to more effectively exert the above-mentioned effects.

In addition to Si ≧ 0.8 mass%, N
i ≧ 0.02% by mass and S ≦ 0.01% by mass, the dew point is 55 ° C. or higher and the oxygen concentration is 2% or higher, and the surface temperature is 117.
The stipulation that the condition of 3 ° C or higher be maintained for 2 hours or longer is
For the following reasons. It is known that Ni, unlike Si, remains at the base iron interface without being oxidized and causes the unevenness of the interface to increase (“Iron and Steel” 1996, No 1.63).
In the present invention, the relationship between this phenomenon and the final product is examined, and even if the finish rolling temperature of the steel material is less than 850 ° C,
This is because it has been found that a branched band-shaped oxide can be produced if Ni ≧ 0.02 mass% and S ≦ 0.01 mass% and the above heating conditions are satisfied. Ni is 0.
If it is less than 02% by mass, it is insufficient to form irregularities on the interface, and if S is more than 0.01% by mass, a low melting point substance such as FeS is easily generated to facilitate scale peeling. It was suppressed to 0.01% by mass or less. A more preferable lower limit of S is 0.001% by mass.

In addition, the condition that the surface temperature is 1173 ° C. or more and the dew point is 55 ° C. or more and the oxygen concentration is 2% or more and is held for two hours or more and then subjected to hot rolling is based on the premise of each component defined above. Under the above conditions, it is necessary to produce the desired band oxide in the best condition.
3 ° C. is the melting point of Fe ₂ SiO ₄ , which is the main constituent of the band-shaped oxide, and maintaining the surface temperature above this temperature is a condition required for the production of the branched band-shaped oxide. Needless to say, it is not preferable for achieving the object of the present invention that the other conditions are also satisfied.

Furthermore, in the present invention, the SiO ₂ solution is applied to the surface of the steel material prior to heating the steel material for rolling so as to compensate for the shortage of the amount of Si in the steel material necessary for scale formation. It is also preferable to adjust the coating amount according to the Si amount in the steel material. Specifically, the coating means 8 is provided in front of the heating furnace shown in FIG. 1, and a predetermined amount of SiO ₂ solution is coated on the surface of the steel material. As the coating means, any means may be used as long as it can be coated uniformly and evenly. For example, well-known means such as brush coating and spraying may be appropriately adopted. The application of this SiO ₂ solution is effective for further enhancing the uniformity of the hot rolled steel sheet after pickling.

Further, in the present invention, it is desirable to remove the scale by colliding the high-speed droplets accelerated to 200 m / s or more with the surface of the steel sheet prior to the hot rough rolling. Since the steel material is heated to a high temperature in the heating furnace, oxide scale is formed and adheres to the surface of the steel material. If this scale is rolled while attached to the steel material surface, the scale bites into the surface of the rolled material and remains as scale flaws, which promotes unevenness in the roughness of the steel sheet surface when finally pickled, so rough rolling The scale is removed by high-pressure water jet before. To remove this scale to a satisfactory degree, 200m /
It is necessary to make high-speed droplets accelerated to s or more collide with the steel plate surface.

[0018]

[Examples] Using the materials of steel types A to D shown in Table 1, the steel materials were processed and rolled under various heating conditions, descaling conditions and hot rolling conditions shown in Table 2 to obtain hot rolled steel sheets. The average depth and surface roughness of the strip oxide of this steel sheet are shown together in Table 2. The average thickness (depth) of the strip oxide formed at the grain boundary of the scale-base iron interface is represented by the average of the measurements of embedded samples (diameter mm) sampled at several points in the longitudinal direction of the steel sheet. From Table 2, the average depth of the strip oxide of the present invention is 2 μm.
It can be seen that in the above cases, a hot rolled steel sheet having excellent surface properties can be obtained.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

According to the present invention of the present invention described above,
It is possible to obtain a hot-rolled steel sheet having high scale adhesion and a hot-rolled steel sheet with excellent surface properties without roughness unevenness after pickling, and to produce this steel sheet without strict production conditions. It is possible.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of a manufacturing process of the present invention.

FIG. 2 is a sketch drawing based on an electron micrograph (× 30000) of the surface structure of the hot rolled steel sheet according to the present invention.

[Explanation of symbols]

1 Heating Furnace 2 Descaling Device 3 Rough Rolling Machine 4 Finishing Rolling Machine 5 Runout Table 6 Coiler 7 Pickling Tank 8 Coating Section

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/08 C22C 38/08 (72) Inventor Takehiro Nakamoto Oita-shi, Oita Prefecture Nishinosu No. 1 New Shin Made in Japan Oita Steel Co., Ltd. (72) Tetsuya Yamada, Inventor Tetsuya Yamada 1 Nishinosu, Oita-shi, Oita Prefecture Shin-Nippon Steel Co., Ltd. F-term in Oita Steel Co., Ltd. (reference)

Claims (12)

[Claims] 1. A hot-rolled steel sheet having an excellent surface property, characterized in that a branched band-shaped oxide containing Si and O is formed in a grain boundary of a scale-base iron interface on the surface of a steel sheet in a thickness direction of 2 μm or more. . 2. The hot-rolled steel sheet according to claim 1, obtained by removing the scale on the surface of the steel sheet by pickling. 3. A steel material containing 0.8 mass% or more of Si is 85
The hot rolled steel sheet according to claim 1 or 2, which is obtained by completing finish rolling at 0 ° C or higher. 4. Si is 0.8 mass% or more and Ni is 0.0
Dew point 5 for steel materials containing 2% by mass or more and 0.01% by mass or less of S
The hot rolled steel sheet according to claim 1 or 2, which is obtained by hot rolling after maintaining a state where the surface temperature is 1173 ° C or more for 2 hours or more at an oxygen concentration of 5% or more and an oxygen concentration of 2% or more. 5. The hot-rolled steel sheet according to claim 1, which is obtained by applying a SiO ₂ solution to the surface of a steel material prior to heating the steel material for rolling and performing heat-hot rolling. 6. The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet is obtained by colliding high-speed droplets accelerated to 200 m / s or more with a steel sheet surface to remove scale before hot rough rolling. . 7. The hot-rolled steel sheet according to claim 1, wherein the surface average roughness (Ra) is 1.5 μm or more and 2.5 μm or less, and the Rmax is 10 μm or more and 20 μm or less. 8. A branched band-shaped oxide containing Si and O is produced in a thickness direction of 2 μm or more at a grain boundary of a scale-base iron interface on the surface of a steel sheet, and the surface scale is removed by pickling. A method for producing a hot rolled steel sheet having excellent surface properties. 9. A steel material containing 0.8 mass% or more of Si is 85
The method for manufacturing a hot-rolled steel sheet according to claim 8, wherein finish rolling is completed at 0 ° C or higher. 10. Si in an amount of 0.8 mass% or more and Ni in an amount of 0.
A steel material containing 02% by mass or more and 0.01% by mass or less S has a dew point of 55 ° C. or more and an oxygen concentration of 2% or more, and a surface temperature of 1173 ° C.
The method for manufacturing a hot-rolled steel sheet according to claim 8, wherein hot rolling is performed after the above state is maintained for 2 hours or more. 11. The hot-rolled steel sheet according to claim 8, wherein an SiO ₂ solution is applied to the surface of the steel material prior to heating the steel material for rolling, and heating-hot rolling is performed. Manufacturing method. 12. The hot rolling according to claim 8, wherein prior to hot rolling, high-speed droplets accelerated to 200 m / s or more are collided with the surface of the steel sheet to remove scale. Steel plate manufacturing method.