JPH0349972B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a method for producing steel containing 5% Cr or more, ferritic stainless steel, and martensitic stainless steel, without reducing the Cr concentration on the surface of the product and without covering the surface. The present invention relates to a method for manufacturing Cr-containing thin steel sheets that have excellent corrosion resistance and oxidation resistance.

[Prior Art] Cr-containing steel and stainless steel are widely used as corrosion-resistant and oxidation-resistant materials. These Cr-containing
Steel may rust when used in a corrosive environment, for example in the atmosphere. As a result of investigating these causes of rusting, we found that a Cr-free layer had formed on the surface of the steel sheet, and the amount of Cr on the surface was significantly lower than the amount of Cr on the steel sheet. Often due to defects. All of these are conventional Cr-containing
This is caused by a fundamental flaw in the method of manufacturing thin steel sheets. Conventionally, in the production of Cr-containing steel and stainless steel, hot-rolled steel strips are annealed at about 800°C for a long time or are annealed at about 1000°C for a short time before being sent to a descaling process. During this hot-rolled sheet annealing, scales containing Cr as a main component are formed on the surface of the hot-rolled steel strip, and Cr is particularly concentrated in the lower layer of the scales. As a result, on the steel plate surface directly below the scale,
There is a Cr-free layer with a lower Cr concentration than the inside of the steel sheet, which is several μm thick.
generate. The degree of this Cr reduction becomes more severe as the amount of Cr in the base metal increases, the temperature at which it is heated is higher, and the longer it is heated. If this Cr-free layer remains in the product,
The Cr concentration on the surface of the product is several percent lower than that inside the steel sheet, significantly degrading the most important corrosion resistance. Therefore, these Cr-free layers are
It is removed during pickling and coil grinding processes, but if the removal is not sufficient, the product is manufactured with the Cr-free layer remaining on the surface. Furthermore, there are cover-like defects on the surface of stainless steel products, but these defects are caused by intergranular corrosion due to the use of HNO ₃ or HNO ₃ HF during the pickling process, and are not removed sufficiently during the coil grinding process, so they are removed during the cold rolling process. It was huge and remained until the final surface.

As described above, it is thought that the conventional manufacturing method does not ensure sufficient corrosion resistance of the product unless the coil grinding step is sufficient. However, it is desirable to simplify or omit the coil grinding process from the viewpoint of cost and yield, and there is a need to reform the conventional manufacturing method.

[Problems to be solved by the invention] In order to inexpensively manufacture Cr-containing thin steel sheets with excellent corrosion resistance, (1) development of a manufacturing method that does not cause a decrease in the amount of Cr on the surface; (2) development of a manufacturing method that does not cause a decrease in the amount of Cr on the surface of the steel sheet; Development of a manufacturing method that does not cause defects (3) It was found that the challenge was to develop a manufacturing method that omitted the hot-rolled plate annealing and coil grinding processes as much as possible.

The present inventors researched a method for producing Cr-containing steel and stainless steel without a surface Cr-free layer, but as a result, the Cr-free layer
It was found that the hot-rolled plate annealing process and final annealing process are the processes that greatly affect layer formation.
In other words, we clarified the conditions for omitting the hot-rolled sheet annealing step, controlling the annealing atmosphere during final annealing to prevent surface oxidation, and preventing the formation of a Cr-free layer.

On the other hand, as a result of repeated research aimed at preventing surface overburden, we found that the cause of these overburdens is that the especially sensitized coils are exposed to HNO ₃ and HF during pickling of hot-rolled sheets.
If intergranular corrosion occurs on the surface due to contact with water containing trace amounts of HNO ₃ or HF, it will cause corrosion during cold rolling.
It was found that the residue formed and remained on the product. In order to prevent this overlapping, it is necessary to use pickling conditions for hot rolled sheets to prevent the occurrence of intergranular corrosion, and the present invention was completed by clarifying these conditions.

[Means and actions to solve the problem] As already mentioned, 5% Cr steel and 7% Cr steel of Cr-containing steel
For stainless steels based on steel, 11% Cr steel, and 17% Cr steel, Cr-rich scales are formed on the steel plate surface due to high temperature and long-term heating during hot-rolled plate annealing, and Cr-rich scales are formed directly below the scales. It was found that a Cr-free layer was formed at a depth of about 3 to 5 μm depending on the amount of Cr in the steel, and the Cr concentration was about 5% lower at the surface. On the other hand, when hot-rolled sheet annealing is omitted, almost no Cr-free layer is observed directly under the scale, and only a slight Cr-free layer is observed in high-Cr alloys.
Omitting hot-rolled sheet annealing in this way is extremely effective in reducing the surface Cr layer. To omit hot-rolled sheet annealing, it is necessary to coil the sheet at 600 to 800℃ after hot rolling and soften the sheet during slow cooling, but in this process, Cr-containing steel has Cr ₂₃ C ₆ in the grain boundaries. Precipitates carbides and creates Cr-deficient regions at grain boundaries. Therefore, in the pickling process, it is necessary to select a pickling solution that does not cause intergranular corrosion. If intergranular corrosion occurs, it will be covered during the cold rolling process and remain as an overlapping defect on the product surface.
Corrosion resistance is significantly reduced.

In particular, for ferritic and martensitic Cr-containing steels, hot-rolled sheet annealing is omitted and HNO ₃ / HF or
The use of HNO ₃ is dangerous, and in addition to descaling by selecting an active dissolving pickling solution containing mainly H ₂ SO ₄ or HCl, the Cr-free layer immediately below the scale that is slightly formed by cutting the surface is removed. It is necessary to completely remove it. 5Cr steel, 7Cr steel without hot-rolled plate annealing,
150 to 400 for 13Cr steel, 17Cr steel, 19Cr steel, etc.
Complete descaling and removal of surface Cr layer can be completed by immersion or spray pickling in 80 to 90°C of g/H ₂ SO ₄ for about 20 to 90 seconds. In HCl
Completely descale and remove surface by soaking 100 to 300g/in 60 to 90℃ for 20 to 90 seconds or spray pickling.
It is possible to complete the cutting of the Cr layer. Mechanical descaling prior to pickling is effective, such as light shot blasting or spraying with high-pressure water containing an abrasive.

Pickling with H ₂ SO ₄ or HCl produces smut on the surface. Conventionally, HNO ₃ was used for this removal.
and HNO ₃ HF have been considered effective, but these liquids are harmful because they cause intergranular corrosion if the hot rolled sheet is sensitized by omitting annealing. Therefore, mechanical methods are effective for removing smut, and wet scrubber methods are preferred. In this case, if HF is mixed into the water used, intergranular corrosion is likely to occur, so by regulating HF to 0.5g/or less, intergranular corrosion can be completely prevented, and overlapping defects in cold-rolled sheets can be removed by coil grinding. This can be sufficiently prevented even if the process is omitted.

As described above, a steel plate without a Cr-free layer on the surface is cold-rolled and finally annealed, but in this case as well, it is necessary to perform annealing under conditions that do not generate scale mainly composed of Cr on the surface. For this purpose, it is necessary to perform annealing in an atmosphere that prevents the formation of a Cr-depleted layer at the required annealing temperature determined by the Cr content of the alloy, and as a result of repeated tests, the conditions shown in Figure 1 were found.

The shaded area in FIG. 1A is the annealing temperature range necessary to carry out the present invention, and the temperature C ₁ or C ₂ is expressed by the following formula.

When 5%≦[Cr]<13%, 3.8[Cr]+651≦C ₁ ≦3.8[Cr]+801 13%≦[Cr]≦23% 22.2[Cr]+441C ₂ 16.6[Cr]+634 However, [Cr]: Chromium content of steel plate (weight%), C ₁ , C ₂ : (℃) If the annealing temperature C ₁ or C ₂ is higher than above,
The scale layer on the surface of the steel plate becomes thicker, impairing the corrosion resistance of the steel plate.

The shaded area in FIG. 1B is the dew point range of the annealing atmosphere required at that time, and the temperature D ₁ or D ₂ is expressed by the following formula.

When 5%≦[Cr]<13% -40℃≦D ₁ ≦-0.75[Cr]+4.75 13%≦[Cr]≦23% -40℃≦D ₂ ≦-2.8[Cr]+31.1 However, [Cr]: chromium content (weight %) of the steel sheet, D ₁ , D ₂ : (°C) The temperature for cold rolling annealing is determined according to the Cr content of the alloy. Of course it affects the annealing time, but the time is
This was determined as a short period of time within 30 seconds. If the dew point of the annealing atmosphere is less than -40°C, the manufacturing cost of the atmosphere gas increases, but there is no sufficient effect.

When the required annealing temperature is determined by the amount of Cr, the dew point of the annealing atmosphere that does not generate scale on the surface is also determined. The annealing atmosphere has a gas composition of 0 to 75% hydrogen gas and the remainder substantially nitrogen.

By selecting the annealing temperature and dew point in this way, after annealing,
You can pickle it as usual to create a 2D surface, or add a skin pass to create a 2B surface, or you can use it as is because there is no bluing even if it is completely annealed.
There is almost no Cr deficiency, ensuring excellent corrosion resistance. Of course, if the BA surface is annealed under bright annealing conditions in which the dew point is significantly lowered, a Cr-free layer will not be formed.

Next, reasons for limiting the constituent elements of the present invention will be described. Hot-rolled sheet annealing is omitted because it produces a Cr-free layer. To do this, the hot-rolled coil is wound up at a temperature of 600 to 800°C, and the steel sheet is softened during slow cooling. 600
Below ℃, softening is insufficient and edge cracking occurs during cold rolling. Moreover, at temperatures above 800°C, a Cr-free layer begins to form during winding.

Pickling should be done with H ₂ SO ₄ or HCl, with HNO ₃ or
Do not use _HNO3 /HF. This is because materials omitted from annealing are susceptible to intergranular corrosion during pickling, and the intergranular corrosion areas become covered during cold rolling and remain on the product surface.
This is because it deteriorates corrosion resistance. For the same reason, the permissible amount of HF in hot water is 0.5g/or less.

The atmosphere at the time of final annealing is as shown in Fig. 1, with dew points below _three points A ₁ , A ₂ , and A. This is because if it exceeds this range, scale will occur during annealing, and a Cr-free layer will occur on the surface.

In this way, products ranging from Cr-containing steel to stainless steel are produced without a Cr-free layer on the surface of the product, but the corrosion resistance properties are determined mostly by the amount of Cr in the alloy, but other components also affect the corrosion resistance of the steel sheet. . Therefore, the action of the steel components is also important, and each component of the steel plate is selected in consideration of the following points and in accordance with the intended use of the finished product.

The lower the value of C<0.1%, the better. Si<3
%, and the higher the value, the better, especially for oxidation resistance. Especially when Mn<1%, the lower the corrosion resistance, the better the corrosion resistance. The lower the value of P<0.05%, the more desirable it is. S＜0.008%
In particular, the lower the value, the better the corrosion resistance. Cr is the main component of Cr-containing steel, and exhibits corrosion resistance under mild corrosion conditions when it exceeds 5%, and the higher the Cr content, the better the corrosion resistance, and when it exceeds 10%, it exhibits excellent corrosion resistance as stainless steel.
It is especially good when it exceeds 17Cr and 19Cr. When it exceeds 22%, the Cr-free phenomenon becomes particularly large, and it is difficult to prevent the Cr-free layer from forming on the product surface. Ni does not particularly affect corrosion resistance, but does affect toughness. However, it can be selectively added within 10%. Mo is extremely effective in improving corrosion resistance and can be selectively added within 6%.
Cu is extremely effective in improving corrosion resistance and can be selectively added within 3%. Corrosion resistance and toughness are improved by selectively adding Ti, Nb, Zr, W, etc. within a range of 1%. N is effective in improving corrosion resistance and can be selectively added within 0.5%, but it impairs toughness. A lower content of O is desirable for improving toughness, and is 0.01% or less. Al can be selectively added within 0.2% as a deoxidizing agent, but if the amount is too much, it will impair toughness. The rest consists essentially of Fe and unavoidable impurities.

[Example] Examples of the present invention will be described below. Contains Cr
As for steel, low C type 7Cr as shown in Table 1,
11Cr, 17Cr, 19Cr, and 21Cr alloys were melted and Mo, Ti, Nb, Al, etc. were selectively added depending on the purpose of use.

The alloy was subjected to a casting process and then hot-rolled through hot-charging or direct rolling to form a steel strip with a thickness of 3-4 mm, which was then coiled and cooled at 640-770°C. Then, as mechanical descaling, fine iron sand grains were added as an abrasive to 100 kg/cm ² of high-pressure water and sprayed with 150 to 400 g/cm of H ₂ SO ₄ at 90°C to 70°C.
℃ for 30 to 120 seconds to descale and coat the surface with 35μ of 21Cr type, 10μ of 17Cr type, and 10μ of 7Cr type.
After cutting about 1μ, the smut on the surface was removed using a wet scrubber. HF in water was regulated to 0.2g/or less to prevent intergranular corrosion. Thereafter, the coil grinding step was omitted and the steel sheets were cooled with a large diameter roll to obtain a 1.0 mm cold rolled steel sheet, and final annealing was performed depending on the Cr content.
The annealing conditions are shown in Table 2. Some of it was pickled and made into 2B products. In addition, some of the sheets were cold-rolled to 0.4 mm using small-diameter Sendzimir cold rolling, and then subjected to bright annealing as shown in Table 3 as final annealing.

As comparison materials, SUS409 and SUS430 were used, hot-rolled sheets were annealed as usual, descaled under the same conditions as 11A and 17A above, and the coil grinding process omitted, and after cold rolling, annealed under normal atmospheric annealing conditions and pickled ( Table 4).

The degree of surface Cr removal of the materials of the above examples was examined at the product stage, and an accelerated salt water spray test was further conducted. The results are shown in Table 5. Thus, according to the method of the present invention, even 7Cr and 9Cr steel exhibits considerable corrosion resistance, and 17Cr steel or higher exhibits excellent corrosion resistance. These correspond well to the behavior of the Cr-free layer on the surface, and this is because the extent of the Cr-free layer has been significantly reduced compared to the conventional method, and there is no longer an overlapping rusting starting point on the surface. .

[Effects of the present invention] The present invention improves the Cr-free layer on the surface, which has been overlooked in the conventional manufacturing methods of stainless steel and Cr-containing steel.
By elucidating the cause of surface overburden and revealing a method to prevent it, we developed a manufacturing method based on the corrosion resistance of the surface, and succeeded in manufacturing products with excellent corrosion resistance by omitting processes. . In addition, the method of the present invention does not involve hot-rolled plate annealing or coil grinding, which are conventionally special methods for manufacturing chromium-containing thin steel sheets, and allows for a highly efficient process using a tandem cold rolling mill, which is extremely effective. big.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the annealing conditions of the present invention.

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Claims (1)

[Claims] 1. In Cr-containing steel or stainless steel containing 5% or more Cr by weight, a hot-rolled coil is
Mechanical descaling and pickling descaling with H ₂ SO ₄ or HCl are performed without coiling at a temperature of 600 to 800 °C and hot-rolled sheet annealing, and the Cr-free layer on the surface is completely dissolved, and HF Contamination is 0.5g/
Desmat treatment is performed using hot water and a scrubber containing the following, the intermediate coil grinding process is simply performed or omitted, and after cold rolling, the following formula (1) or (3) is applied.
A surface characterized by final annealing at an annealing temperature of the formula and an annealing atmosphere with a dew point of the following formula (2) or (4).
A method for producing a chromium-containing corrosion-resistant thin steel sheet that does not reduce the Cr concentration and has excellent corrosion resistance. When 5%≦[Cr]<13%, 3.8[Cr]+651≦C ₁ ＜3.8[Cr]+801 …(1) −40℃≦D ₁ ≦−0.75[Cr]+4.75 …(2) When 13%≦[Cr]≦23%, 22.2[Cr]+411≦C ₂ ≦16.6[Cr]+634 …(3) −40℃≦D ₂ ≦−2.8[Cr]+31.1 …(4) However, [Cr]: Chromium concentration in steel (weight%) C ₁ , C ₂ : Annealing temperature (°C) D ₁ , D ₂ : Dew point of annealing atmosphere (°C)