JP3091418B2 - Hot rolled stainless steel sheet and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled stainless steel sheet and a method for producing the same, and more particularly, to a method suitable for performing processing by omitting a scale removing step such as pickling, or descaling during pickling. TECHNICAL FIELD The present invention relates to a hot-rolled stainless steel sheet having excellent surface properties after descaling and a method for producing the same.

[0002]

2. Description of the Related Art Stainless steel is a material that contains a large amount of alloying elements such as Cr and Ni and has a significantly higher corrosion resistance than ordinary steel. In general, stainless steel is formed by solidifying molten steel refined by continuous casting to form a steel slab. None, this steel slab is 1100-1300 ℃
After being heated to a degree, hot rolling is performed to form a hot-rolled steel sheet (also referred to as a hot-rolled steel strip or hot-rolled coil depending on the state at that time). The hot-rolled steel sheet is annealed continuously or batchwise, and further, a sulfuric acid bath and After pickling through a nitric acid / hydrofluoric acid mixed acid bath, it is cold-rolled to form a cold-rolled steel sheet (also referred to as a cold-rolled steel strip or a cold-rolled coil depending on the state at that time). This cold-rolled steel sheet is further annealed and pickled. It is manufactured in the process of doing.

[0003] Usually, cold-rolled steel sheets are used for various applications, but in some cases, annealed-pickled steel sheets after hot rolling are used without being cold-rolled. Since stainless steel contains a large amount of Cr, a Fe-Cr-based oxide layer mainly composed of (Fe, Cr) ₂ O ₃ and (Fe, Cr) ₃ O ₄ An intermediate oxide layer of SiO ₂ is formed at the interface between the Fe-Cr-based oxide layer and the base metal by the Si inside. If a hot-rolled steel sheet having such an oxide layer (scale) is annealed and then cold-rolled, the scale peels off during the rolling and damages the cold-rolled rolls, and causes bites in the rolled material to cause surface defects.

[0004] To remove such harmful scales, a pickling step is provided after hot rolling. However, the scale having the above-mentioned composition peculiar to stainless steel is very dense and has a pickling descalability (pickling). ), The rate of the pickling line is limited and productivity is impaired. To reduce the pickling load and increase the speed of pickling, so-called shot blasting, in which hard fine particles (shot particles) such as steel balls or sand are injected at high pressure onto the steel sheet surface to introduce cracks into the scale, has been conventionally used. Widely used. However, irregularities called shots are formed on the surface of the steel plate subjected to the shot blasting, the surface roughness increases, and the surface quality does not reach the level of the cold rolled steel plate. Therefore, there is a problem that a relatively inexpensive hot-rolled stainless steel pickled plate cannot be widely supplied as a substitute for a cold-rolled steel plate.

Another measure for reducing the pickling load is to reduce the thickness of the scale. Techniques for suppressing scale formation in the hot rolling process include, for example, JP-A-58-53323 and JP-A-59-53323. No. 97710, as disclosed in Japanese Patent Application Laid-Open No. 61-123403, etc., in the section from the exit side of the final rolling mill to the winder of hot rolling, the inside can be controlled to an inert or reducing gas atmosphere. A method is known in which a box is provided so that a hot-rolled steel strip after rolling can pass through the box.

[0006] This is because the scale generated before each pass of the hot rolling is removed, and only the scale generated after the final pass remains after winding, and the scale from the final rolling mill discharge side to the winder is removed. Is based on the technical idea that the scale thickness can be suppressed by maintaining the steel strip passing section in a non-oxidizing atmosphere to prevent scale formation in the section. However, this method is not practical because a large amount of gas must be supplied by gas-sealing the entire long section from the exit side of the final rolling mill to the winding machine.

As a thinning technique other than the above, for example, Japanese Patent Application Laid-Open No. Hei 4-238620 discloses a descaling method in which high-pressure water is sprayed onto a steel strip surface at a specific range of collision pressure before hot finish rolling. Proposed. However, when this method is applied to stainless steel, the metal is seized between the finish rolling roll surface and the steel strip surface, causing surface defects.

Further, in the case of a hot-rolled stainless steel sheet for applications where surface characteristics such as glossiness are not important, if it can be used with a scale, it is possible to omit the pickling step and to expect a significant cost reduction. When a hot-rolled stainless steel sheet with a scale manufactured in the step is subjected to processing such as bending and drawing with a mold, the scale is partially peeled off, shortening the mold life, and There is a problem that the working environment deteriorates.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art, and to provide a level of scale adhesion that does not peel or generate dust even when processed with a scale.
Another object of the present invention is to provide a hot-rolled stainless steel sheet having a level of pickling property that does not require a shot blast treatment before pickling, and a method for producing the same.

[0010]

Means for Solving the Problems To achieve the above object, the inventors of the present invention have developed a scale structure of a hot-rolled stainless steel sheet with a scale having a scale adhesion (in other words, peeling property) at the time of processing the hot-rolled stainless steel sheet. ) And its effect on pickling,
In addition, intensive experiments and examinations were repeated on the relationship between the scale structure and the hot rolling conditions. As a result, a hot rolled stainless steel sheet having a specific composition and a specific scale structure has characteristics satisfying the above requirements, and the hot rolled stainless steel sheet leads to a new finding that it can be obtained under specific hot rolling conditions. Based on such knowledge, the present invention described below has been completed.

The present invention is a hot-rolled stainless steel sheet containing 10 wt% or more of Cr and having a scale thickness of a surface layer of 2.5 μm or less.
A hot-rolled stainless steel sheet containing 1.0 wt% or less and having a thickness of a Si-containing oxide layer formed at an interface between a scale layer and a base alloy of 0.1 μm or less. The above-described hot-rolled stainless steel sheet according to the present invention is a hot-rolled stainless steel sheet producing a steel strip coil by hot rolling a slab containing 10 wt% or more of Cr, preferably 1.0 wt% or less of Si. It is the gist that hot rolling is performed so that the elongation ratio defined in the above becomes 150 or more.

[0012] Elongation / ratio = (area of rolled surface of steel strip coil) / (area of rolled surface of slab)

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The hot-rolled stainless steel sheet of the present invention
Contains Cr at 10wt% or more, and the surface scale thickness is 2.5μm
Because of the following, it is particularly excellent in scale adhesion during processing. The present inventors have repeated many experiments to obtain a hot-rolled stainless steel sheet with good scale adhesion,
We have newly found that scale adhesion shows a remarkably high value in a hot-rolled stainless steel sheet having a scale structure with an unprecedented scale thickness of 2.5 μm or less.

That is, in order for the scale not to peel off during processing, the thickness of the surface layer scale must be 2.5 μm or less. However, the Cr content in stainless steel
If less than 10wt% , heat the steel slab to about 1100-1300 ℃
After rough rolling, finish rolling and winding at 600 to 1050 ° C
Hot rolled steel manufactured with an elongation ratio ≥150 by a simple hot rolling method
It is not only difficult to obtain a scale structure with a thickness of 2.5 μm or less, but also the corrosion resistance that stainless steel should originally have is insufficient.
Limit to 10 wt% or more. There is no particular reason for setting the lower limit because the smaller the scale thickness, the better. Also, Cr
The upper limit of the content may be set as the upper limit (about 20 wt%) of the Cr content of ordinary stainless steel.

The hot-rolled stainless steel sheet of the present invention further comprises:
Contain not more than 1.0 wt% of Si, and reduce the thickness of the Si-containing oxide layer formed at the interface between the scale layer and the base alloy by 0.1 μm
It is preferable to set the following. This hot-rolled stainless steel sheet has excellent adhesiveness during processing and, at the same time, has excellent pickling descaling properties (pickling properties). The present inventors have conducted intensive studies to find out the controlling factors of the pickling property of a hot-rolled stainless steel sheet. As a result, the pickling property was found to be higher than that of the Si-containing oxide layer (SiO ₂ layer) formed at the interface between the surface scale and the base alloy. When the thickness exceeds 0.1 μm, the pickling properties deteriorate significantly and it is necessary to remove mechanical scale such as shot blasting before pickling.
It was found that the pickling property was improved to the extent that it was not required to be less than μm. In other words, in order to obtain a hot-rolled stainless steel sheet that does not require mechanical scale removal before pickling and therefore has excellent surface quality after pickling without cutting, for example, the thickness of the Si-containing oxide layer is 0.1 μm. The following is preferred.

However, when the Si content in the base alloy is 1.0 wt%
Above, the steel slab is heated to about 1100-1300 ° C and then
General hot rolling, finishing and rolling at 600 to 1050 ° C
Manufactured by rolling method with Cr content ≧ 10 wt%, elongation ratio ≧ 150
In the hot-rolled coil products thus manufactured, the thickness of the Si-containing oxide layer was so large that it exceeded 0.1 μm, so the Si content in steel was 1.
0 wt% or less. Note that the lower the thickness of the Si-containing oxide layer, the better. On the other hand, Si
Is an element that is effective for deoxidation of steel and improves oxidation resistance at high temperatures and is relatively important.
It is preferable to contain about wt% or more.

The constituent elements in stainless steel include Cr,
In addition to Si, corrosion-resistant elements such as Ni, Cu, Mo, and S in steel
Fixed element Mn, deoxidized element Al, and C, N fixed element
Ti, Nb, Zr, V, and the like are often added, but these components may be added according to a conventional method, and the contents of the respective components are not particularly limited in the present invention. The hot-rolled stainless steel sheet of the present invention contains Cr in an amount of 10 wt% or more, preferably Si.
In a method for producing a hot-rolled stainless steel sheet in which a slab containing not more than 1.0 wt% is hot-rolled to form a steel strip coil, the expansion ratio [= (the area of the rolled surface of the steel strip coil) / (the Area)] is 150 or more.

Although the details of this reason are not clear, the point that the surface scale thickness is suppressed to 2.5 μm or less is that the surface scale also expands under hot rolling conditions with a large expansion ratio, and becomes thinner as rolling progresses. In addition to this, the base alloy was exposed to a large scale at a large elongation ratio of 150 or more, which had not been done before, causing cracks to partially occur in the scale in the latter half of hot rolling and newly exposing to the crack initiation part It is considered that a new scale is generated on the surface, and the new scale eventually becomes thinner than the scale before the crack.

The inventors conducted experiments on the point that the thickness of the Si-containing oxide layer was suppressed to 0.1 μm or less, and
Oxide layer during slab heating or early in hot rolling (during rough rolling)
Temperature of 1100 ° C or higher, but the temperature range in the latter half of hot rolling (during finish rolling) (generally 600 to 5050 ° C)
It has been found that almost no silicon oxide is formed, and from this, the existence of such a thin Si-containing oxide layer after winding is because the initial Si-containing oxide layer is expanded and thinned as described above. In addition, it is considered that a new Si-containing oxide layer is less likely to be generated on the surface of the underlying alloy exposed to the cracks generated in the latter half of hot rolling.

The upper limit of the stretching ratio is not particularly limited as long as it is within the range of the rolling capacity of the hot rolling equipment.

[0021]

EXAMPLES Stainless steel slabs having the components shown in Table 1 were adjusted to various thicknesses shown in Table 2, hot-rolled at various expansion ratios shown in the table, wound up into coils, and various types shown in the table. Hot-rolled sheet having a hot-rolled finished sheet thickness of Slab heating temperature is 11 for steel A
The temperature was 50 ° C., 1200 ° C. for steel B, 1100 ° C. for steels C, D and E, and the coil winding temperature was 800 ° C. for all.

[0022]

[Table 1]

Regarding the measurement of the thickness of the scale and the silicon-containing oxide layer, the polished surface of a sample cut out from a hot rolled plate and polished in cross section was observed by SEM, and the distance between the scale surface and the base alloy surface was determined from the photographed image. The value obtained by direct measurement is regarded as the scale thickness, and the composition of the scale layer is identified by AES analysis, and the value obtained by measuring the thickness of the layer where the Si peak is detected is used as the thickness of the Si oxide-containing layer. I did it. When the Si-containing oxide layer thus identified was further analyzed by X-ray diffraction, a SiO ₂ peak was observed.

The scale adhesion at the time of processing was evaluated by the scale peeling amount. This is 10 mm width x length in the rolling direction of the hot rolled sheet.
Cut out a 100mm tensile test piece, apply adhesive tape to the front and back of the marked part (10mm x 20mm) of the test piece, and
The tape was peeled off after the tensile processing was performed, and the weight increase of the tape before and after peeling was measured.

The pickling property is 100 × 100 m from the hot rolled sheet.
m test piece was cut out and mixed with sulfuric acid (H ₂ SO ₄ [200 g / l]) and mixed acid (H
Pickling was performed in a laboratory using NO ₃ [150 g / l] + HF [25 g / l]), and the plate surface after the pickling was visually observed and evaluated according to the following four grades. 1: No remaining scale (0% area ratio of remaining scale) 2: There is a dot scale residue (1% or less area ratio of remaining scale) 3: There is a lump scale residue (1% area ratio of remaining scale) 4: Less than 5%) 4: Striped scale remains (area ratio of remaining scale is 5% or more). These results are also shown in Table 2.

[0026]

[Table 2]

For example, a slab of steel A (SUS430 [16C
r-0.06C]; No. 1 to 15), the relationship between the draw ratio of hot-rolled sheet, scale thickness, Si-containing oxide layer thickness, scale peeling amount, and pickling properties 1, 2, 3, and 4 respectively. Regardless of the slab thickness and the hot-rolled finish thickness, the scale thickness and the thickness of the Si-containing oxide layer become thinner as the elongation ratio becomes higher.
2.5μm or less is achieved and the thickness of the Si-containing oxide layer is
It can be controlled to 0.1μm or less. (See FIGS. 1 and 2). Along with that, No. satisfying an expansion ratio of 150 or more. 5, 10, 11, 1
In Nos. 2 and 15, the scale peeling amount was very small at 0.1 mg / cm ² or less (see FIG. 3), and no scale residue after pickling was observed (see FIG. 4). In other words, this hot-rolled sheet has a level of scale adhesiveness that does not cause mold deterioration or dust pollution even if it is processed as black scale (common name of hot-rolled tight scale) without removing scale and at the same time, A scale structure with excellent pickling properties is provided without mechanical scale removal before pickling.

This tendency is evident from the fact that austenitic stainless steel slab B containing a large amount of Ni (SUS304;
20) and slab C with relatively small Cr content of about 11 wt%,
The same applies to D (No. 21 to 26). However, the Si content
In the hot-rolled sheet (No. 27) produced with slab E of 1.4 wt% at an elongation ratio of 200.0, the scale thickness was suppressed to 2.0 μm, and the scale peeling amount after processing was as small as 0.02 mg / cm ^2, which was excellent. Despite exhibiting scale adhesion, the thickness of the Si-containing oxide layer in the scale layer was 0.21 μm, which exceeded 0.1 μm, and dotted scale residues were observed after pickling.

As described above, the hot-rolled stainless steel sheet rolled at an elongation ratio of 150 or more in accordance with the method of the present invention has a scale obtained by processing with black scale regardless of the original slab thickness and the hot-rolled finished sheet thickness. It is clear that the amount of peeling is small. Also,
Hot-rolled stainless steel sheet excellent in pickling and descaling properties without mechanical pretreatment such as shot blasting, if hot-rolling is performed according to the method of the present invention using a stainless steel material having a Si content limited to 1.0 wt% or less. It is clear that is obtained.

[0030]

According to the present invention, a hot-rolled stainless steel sheet excellent in scale adhesion can be obtained, so that it can be used for processing with a scale without fear of mold deterioration and dust pollution.
Furthermore, since the scale can be efficiently pickled and removed without performing mechanical scale destruction treatment such as shot blasting, there is no unevenness such as shots even for the hot rolled product required specification omitting the cold rolling step. A stainless steel plate having good surface properties can be provided.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between an expansion ratio and a scale thickness.

FIG. 2 is a graph showing the relationship between the elongation ratio and the thickness of a silicon-containing oxide layer.

FIG. 3 is a graph showing the relationship between the elongation ratio and the amount of scale peeling after processing.

FIG. 4 is a graph showing a relationship between an elongation ratio and pickling properties.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-204703 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B21B 3 / 02,45 / 00

Claims (4)

(57) [Claims] 1. A hot-rolled stainless steel sheet containing 10 wt% or more of Cr and having a surface thickness of 2.5 μm or less. 2. The hot-rolled stainless steel according to claim 1, further comprising 1.0 wt% or less of Si and a thickness of a silicon-containing oxide layer formed at an interface between the scale layer and the base alloy being 0.1 μm or less. steel sheet. 3. A method for producing a hot-rolled stainless steel sheet in which a slab containing Cr of 10% by weight or more is hot-rolled to form a steel strip coil, wherein a hot-rolled stainless steel sheet is formed so that an elongation ratio defined below becomes 150 or more. A method for producing a hot-rolled stainless steel sheet, characterized by rolling. Elongation ratio = (Area of rolled surface of steel strip coil) / (Area of slab rolled surface) 4. The method according to claim 3, wherein said slab further contains 1.0 wt% or less of Si.