JPH10324985A - Chromium-containing hot-rolled steel strip and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a Cr-containing hot-rolled steel strip excellent in the corro sion resistance as hot-rolled and acid pickled and the surface characteristic and grindability without shotting trace by treating the hot-rolled steel strip containing a specific content of Cr so as not to develop the trace of shot blast ing and de-Cr layer. SOLUTION: The steel slab containing 6.0-25.0 wt.% Cr is hot-rolled and thereafter, wound at <=700 deg.C or water-quenched immediately after winding. By this method, the reduction of Fe base scale and the development of Cr base scale, are restrained. This hot-rolled steel strip in annealed in the reducing atmosphere, desirably composed of >=1 vol.% H2 and the balance non-oxidizing gas and having -60-0 deg.C dew point to reduce the oxide scale. Successively, after executing a brushing treatment with brush rolls having grinding function to the steel strip, the acid pickling is executed in nitric and hydrochloric acid solution. This acid pickling is desirable to be executed by an electrolytic dipping treatment in the nitric and hydrochloric acid solution having 10-300 g/l nitric acid and 1-50 g/l hydrochloric acid at 35-65 deg.C solution temp. and 1-30 A/dm<2> current density.

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 strip of a Cr-containing steel typified by stainless steel (including a steel sheet; the same applies hereinafter).
In particular, the present invention relates to a hot-rolled steel strip and a method for producing the same obtained by efficiently removing a surface scale of a Cr-containing hot-rolled steel strip in a short time without impairing the surface properties and corrosion resistance. is there.

[0002]

2. Description of the Related Art Since stainless steel contains Cr and Ni and has excellent corrosion resistance, its use tends to be further expanded. However, this stainless steel is a high alloy containing a large amount of expensive elements such as Cr and Ni,
The manufacturing process goes through a complicated process such as slab → hot rolling → (hot rolled steel strip annealing) → hot rolled steel strip pickling → cold rolling → cold rolled steel strip annealing → cold rolled steel strip pickling. Since it is manufactured, there is a problem that the manufacturing cost is high. Accordingly, recently, some of these manufacturing processes, particularly hot-rolled steel strips in which the steps after the cold rolling are omitted, have been attracting attention in view of reducing the manufacturing cost and the manufacturing period. .

In general, the scale formed on the surface of a hot-rolled steel strip of a Cr-containing steel represented by stainless steel is as follows:
Unlike the scale formed on the annealed surface of the cold-rolled steel sheet, which will be described later, the thickness of the cold-rolled steel sheet is several μm, and it is dense and difficult to remove as compared with carbon steel. For this reason, generally, for example, the stainless steel handbook 3rd edition (published by Nikkan Kogyo Shimbun, (1
988), p. 840), etc., it is removed by a pretreatment by immersion in a sulfuric acid bath and a finish pickling treatment by immersion in a mixed acid of nitric acid and hydrofluoric acid. Also, pickling using only acid immersion requires a long time for descaling,
Before such acid immersion, mechanical destruction and removal treatment of the scale layer, such as shot blasting, is often used in combination.

[0004] Annealing treatment of a stainless steel cold-rolled steel strip is performed by a method of treating in a strongly reducing atmosphere (bright annealing: B
A) In general, the treatment is performed in a combustion atmosphere. The surface film formed during the former annealing is extremely thin, and can obtain almost as-rolled gloss. In the latter annealing, scale is generated, and if it is left as it is, it has an adverse effect on corrosion resistance and die seizure at the time of press working, so that pickling is required. As this pickling, a pretreatment such as a salt treatment immersed in a molten alkali salt mainly composed of NaOH and Na ₂ CO ₃ or an electrolytic treatment in a neutral salt solution such as Na ₂ SO ₄ or NaNO ₃ is performed. After that, a method of immersing in an aqueous solution of sulfuric acid, nitric hydrofluoric acid, nitric acid or the like, or adding an electrolytic treatment step is used. These specific methods are disclosed, for example, in JP-B-38-12162, JP-A-59-59900, and the like.

In recent years, some ferritic stainless steel cold-rolled steel strips are provided with several percent by volume of reducing H ₂ gas and N ₂ , such as a continuous annealing line (CAL) of ordinary steel. A method of high-speed sheet passing in a mixed atmosphere with an inert gas such as _two gases has been implemented. This atmosphere is a reducing atmosphere in ordinary steel and is not oxidized. On the other hand, in stainless steel containing Cr that is easily oxidized, heat treatment v
As described in ol28 No. 6 (1988) pp. 373 to 378, a thin oxide film mainly composed of Fe and Cr and having a thickness of about several hundreds of mm is formed on the surface, so that a descaling treatment is required. This descaling method is described in, for example, Japanese Patent Application Laid-Open No. 63-2169.
No. 99, JP-A-1-147100 and the like.

[0006]

However, the pickling method using sulfuric acid and nitric hydrofluoric acid immersion, which has been conventionally used for hot-rolled stainless steel strip, has a poor pickling ability even when mechanical descaling is used in combination. Not only is it weak and low productivity, but the scale layer can be removed, but the dissolution of the de-Cr layer is not sufficient, so the corrosion resistance is inferior to that of a steel strip of the same composition where the de-Cr layer is mechanically removed by surface polishing There was a problem.
The de-Cr layer in the hot-rolled steel strip is generated near the surface of the base steel under the scale, and if cold rolling is subsequently performed, it will be expanded by rolling and become extremely thin, which will adversely affect corrosion resistance. However, when the final product is formed in a hot-rolled state, such an effect by cold rolling cannot be expected, and the corrosion resistance is lowered. In addition, when a mechanical scale removing method such as shot blasting is used, there is a problem that even after pickling, a process called “shot” remains on the surface and deteriorates the surface properties. Such shots not only reduce the surface gloss after cold rolling, but also deteriorate the abrasion, increase the polishing cost, and reduce the productivity when the hot-rolled sheet is polished and used. I was invited.

Therefore, an object of the present invention is to solve these problems of the above-described known technology in the production of a Cr-containing steel strip represented by stainless steel, and to eliminate the surface-dechromized layer,
Hot-rolled-To provide a hot-rolled steel strip having excellent corrosion resistance even when used as pickled, having no shots and the like, and having excellent surface properties and polishing properties, and an efficient production method thereof. It is in.

[0008]

Means for Solving the Problems In order to realize the above-mentioned objects, the present inventors have, in particular, proposed a method of producing a steel sheet, in particular, including a Cr content in a steel composition, hot rolling conditions, annealing conditions of a hot-rolled steel strip, and hot rolling. After diligent research on the pickling conditions of the steel strip, it was found that it is possible to produce a Cr-containing hot rolled steel strip having excellent corrosion resistance after pickling and having no shots with high efficiency. In other words, the Cr-containing steel hot-rolled steel strip is first reduced by annealing in a reducing atmosphere to reduce the scale generated during hot rolling, and then pickled in a nitric-hydrochloric acid solution to obtain the Cr-containing steel having the above-described properties. The present inventors have found that the hot-rolled steel strip can be manufactured significantly more efficiently than before, and that the pickling time can be reduced by appropriately controlling the coil winding conditions, annealing atmosphere, and pickling conditions in hot rolling. Was completed.

That is, the gist of the present invention is as follows. (1) A hot-rolled steel strip containing 6.0 to 25.0 wt% of Cr, wherein the surface of the steel strip has no trace of shot blast and has no Cr removal layer. Cr-containing hot rolled steel strip with excellent corrosion resistance.

(2) A steel material containing 6.0 to 25.0 wt% of Cr is hot-rolled, then annealed in a reducing atmosphere, and then pickled in a nitric-hydrochloric acid solution. Manufacturing method of rolled steel strip.

(3) A steel material containing 6.0 to 25.0 wt% of Cr is hot-rolled and then wound at 700 ° C. or lower, or water-quenched immediately after winding, and then reduced in a reducing atmosphere. A method for producing a Cr-containing hot-rolled steel strip, comprising annealing and then pickling in a nitric-hydrochloric acid solution.

(4) A steel material containing 6.0 to 25.0 wt% of Cr is hot-rolled, annealed in a reducing atmosphere, and then brushed with a brush roll having a grinding function. A method for producing a Cr-containing hot rolled steel strip, which comprises pickling.

(5) The atmosphere of the above annealing is carried out at a hydrogen concentration of 1 vol.
% Or more and the rest are non-oxidizing gas, and the dew point is
The method for producing a Cr-containing hot-rolled steel strip according to any one of the above (2) to (4), wherein the temperature is not more than ° C.

(6) The above pickling nitric acid / hydrochloric acid solution is mixed with a nitric acid concentration:
The composition is 10-300 g / l and hydrochloric acid concentration: 1-50 g / l.
(2) to (5), characterized in that the liquid temperature is up to 65 ° C.
The method for producing a Cr-containing hot-rolled steel strip according to any one of the above.

(7) The method according to any of (2) to (6), wherein the pickling is performed by electrolytic immersion in a nitric acid solution at a current density of 1 to 30 A / dm ² . The method for producing a Cr-containing hot-rolled steel strip according to one of the aspects.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below. Cr content: 6.0 to 25.0 wt% Cr is an indispensable element for imparting corrosion resistance, and is contained according to the corrosion resistance level for the purpose of use. If the Cr content is less than 6.0 wt%, the improvement of corrosion resistance by Cr is not remarkable, so the lower limit of the content is set to 6.0 wt%. On the other hand, Cr
If the content exceeds 25.0% by weight, the scale remains after pickling also according to the present invention, and the corrosion resistance decreases. The reason is,
If the content of Cr exceeds 25.0 wt%, the amount of Cr in the scale of the hot-rolled sheet increases, and the pickling property decreases, and the oxidation of Cr proceeds simultaneously with the reduction of the hot-rolled scale during annealing. it is conceivable that. Therefore, the Cr content is set to 6.0 to 25.0 wt%, preferably 9.0 to 25.0 wt%. Regarding steel components other than Cr, JIS G4304, G4305, G4306,
The components of stainless steel specified in G4307 and the like can be contained according to the required characteristics.

After hot-rolling, the coil is wound at a temperature of 700 ° C. or less, or the water-cooled hot-rolled coil is rolled at a temperature of 700 ° C. or less, or water-cooled. Reduction and the accompanying Cr
The formation of a Cr-based scale due to the oxidation of aluminum can be suppressed. If the coil winding temperature exceeds 700 ° C, the pickling properties will decrease. This is considered to be because a scale layer having a high Cr concentration is generated in the inner layer of the scale during coil cooling, making it difficult to reduce or pickle the scale during annealing. By setting the winding temperature at 600 ° C. or lower, the pickling property is further improved. Therefore, it is preferable to wind at 600 ° C. or lower.

Annealing atmosphere: 1 vol% or more of hydrogen, dew point: -60 to 0 ° C. The annealing treatment in the present invention has a role as an annealing for removing strain and recrystallization of a hot-worked steel strip, It has the role of reducing hot-rolled scale and suppressing the formation of Cr-based scale. Regarding the conditions of the annealing atmosphere, if the hydrogen concentration is less than 1 vol% or the dew point exceeds 0 ° C., the reducibility of the hot-rolled scale decreases. On the other hand, when the dew point is lower than −60 ° C., Cr is selectively oxidized during annealing, and the pickling property is reduced. If the hydrogen content exceeds 20 vol%,
May be selectively oxidized and the pickling properties may be reduced. Therefore, the preferable range of the annealing atmosphere is a hydrogen concentration of 1 to 20 vol%,
Dew point: -60 to 0 ° C. The annealing temperature may be determined in consideration of the recrystallization temperature of the steel. For example, 800 ° C. to 1000 ° C. is preferable for ferritic stainless steel, and 1000 ° C. to 1200 ° C. is preferable for austenitic stainless steel.

These conditions were determined by the present inventors in investigating the effects of the atmosphere of annealing on the scale formation process, and the results were as follows: hot-rolled steel strip (with scale on the surface) and cold-rolled steel strip (without scale on the surface). ) Is based on the finding that the behavior of scale formation is different. That is, due to the annealing under the above-mentioned atmospheric conditions, the cold-rolled steel strip containing Cr
While the main oxide scale is generated, in the hot-rolled steel strip of the present invention, the hot-rolled scale is reduced during annealing,
A reduced iron layer is formed on the surface of the steel strip. This reduced iron layer suppresses contact between the annealing atmosphere gas and Cr in the alloy and the accompanying oxidation reaction. As a result, during annealing, not only does a new Cr-free layer form due to the oxidation of Cr,
It is considered that the effect of reducing the Cr-free layer immediately below the scale formed in the hot rolling process due to the diffusion and homogenization of Cr in the alloy during annealing was brought about. In any case, if annealing is performed in an atmosphere within the above range, the hot-rolled scale can be reduced sufficiently and sufficiently, and the oxidation of Cr can be suppressed.

Brushing treatment after annealing Brushing treatment performed by a brush roll having a grinding function after annealing is performed to further improve the pickling property by grinding and removing a part of the reduced layer and a part of the residual scale layer on the surface of the steel strip. It is an effective means for increasing the rate and suppressing the deterioration rate of the pickling solution. Here, the brush roll used for the brushing treatment is, for example, abrasive grains such as alumina, silicon carbide, and tungsten carbide, and a base material serving as a binder for these abrasive grains can be elastically deformed according to pressure, for example, nylon. And a polymer roll, a non-woven fabric roll, and the like, and desirably follow a plane shape of the steel strip and perform a good grinding function.

A nitric acid solution having a nitric acid concentration of 10 to 300 g / l, a hydrochloric acid concentration of 1 to 50 g / l, and a liquid temperature of 35 to 65 ° C. in a short time if the nitric acid temperature is less than 10 g / l. It is difficult to perform passivation, while if it exceeds 300 g / l, the amount of NOx generated increases. If the concentration of hydrochloric acid is less than 1 g / l, descaling at high speed becomes difficult, while 50 g / l
If it exceeds 300, skin roughness will occur and the corrosion resistance will also deteriorate. For the above reasons, the liquid composition of nitric acid and hydrochloric acid is 10 to 300 g / l of nitric acid,
A mixed solution of 1 to 50 g / l of hydrochloric acid, preferably 50 to 200 g / l of nitric acid, and 3 to 30 g / l of hydrochloric acid. If the solution temperature is lower than 35 ° C., it is not possible to perform pickling treatment in a short time, and if the temperature is higher than 65 ° C., the amount of generated NOx increases and the skin becomes rough. For this reason, the temperature of the pickling solution is 35 to 65 ° C, preferably 40 to 60 ° C.

Electrolytic immersion treatment at a current density of 1 to 30 A / dm ^{2 By} electrolytic treatment in nitric acid and hydrochloric acid, a slight residual Cr-based scale can be dissolved and removed. it can. However, if the current density exceeds 30 A / dm ² , the amount of generated NOx increases, and the skin becomes rough. Therefore, the current density is 1 to 30 A / dm ² , preferably 5 to 25 A / dm ²
dm ² .

[0023]

The present invention will be specifically described below with reference to examples. Example 1 Five slabs of SUH409 steel (11% Cr-0.2% Ti) of the same tapping charge were heated to 1100 ° C, and then hot-rolled to a thickness of 1.5 mm,
, 700, 600, 540 ° C. at various temperatures. One of them was wound into a water tank immediately after winding at 780 ° C and cooled. Hot rolled sheet samples were collected from these coils, and were annealed and pickled under the conditions shown in Table 1. Here, the annealing pattern was heated to 900 ° C. in about 200 seconds, kept at 900 ° C. × 60 seconds, and allowed to cool. In addition, for some test materials, after annealing, a grinding brush (made by Hotani:
A brushing process was performed using Model No. 16S-100-3H). For comparison, after annealing by a conventional method (annealing in a combustion atmosphere), under some conditions, mechanical descaling by shot blasting was also used, followed by pickling (200 g).
/ L immersion in sulfuric acid tank for 40 seconds → nitric hydrofluoric acid (nitric acid 100g / l + hydrofluoric acid 20)
g / l) for 40 seconds.

The descaling property was evaluated by visual judgment (：: good descaling property, Δ: minute scale remaining, ×: scale remaining). The evaluation of corrosion resistance is 6cm
5% Na at 35 ° C on a × 8 cm test piece (test area 96 cm ² )
10 hours salt spray test (SST) using Cl solution (JI
SZ2371), and the following three levels were shown according to the number of rusting points of the entire test piece. ：: 0 pieces / test piece △: 1-2 pieces / test piece ×: 3 or more pieces / test piece Note that the surface of the steel plate of this tapping charge was ground and polished to completely remove the Cr-free layer. It was confirmed that no rust was observed after the SST test. The evaluation of the dechromized layer was made by quantifying the surface of the steel sheet after pickling with an EPMA (Electron Probe Microanalyzer), and those having a Cr content of 10% or less were judged as "with dechromized layer". The remaining shot was evaluated by SEM observation of the surface of the shot-blasted sample after pickling.

From Table 1, according to the conventional example, in the step of shot blasting → sulfuric acid pickling → nitric acid hydrofluoric acid pickling, even when the pickling time was 80 seconds (Experiment Nos. 1 and 2), rust was found in the SST. In addition, when the pickling time was 40 seconds (Experiment No. 3) or when the shot blast was omitted (Experiment No. 4), the scale and the dechromized layer remained, and the corrosion resistance was significantly deteriorated. In contrast,
In the examples of the present invention, it can be seen that descaling can be completed by performing pickling for 40 seconds, and the corrosion resistance after pickling is also good. In addition, as a matter of course, the surface properties were good with no shot blast residue. further,
By regulating the coil winding temperature in hot rolling to 700 ° C or less, there is no scale residue even if the pickling time is shortened.
It was found that a hot-rolled steel strip having good corrosion resistance was obtained. Thus, by setting the annealing atmosphere, the pickling solution and the electrolysis conditions in appropriate ranges, a hot-rolled steel strip of stable quality could be manufactured in a short time. Further, the pickling property was further improved under the condition that the brush grinding step was added before the pickling.

[0026]

[Table 1]

Example 2 Similar investigations were made on ferritic stainless steels of SUS430 (16Cr steel), 25Cr steel and 30Cr steel. In the case of 16Cr steel, the temperature was raised to 85 ° C in about 200 seconds,
℃ × 60sec hold → Allow to cool, other steel 950 in about 200 seconds
The temperature was raised to ℃ C, and the temperature was maintained at 950 C for 60 seconds and then allowed to cool. The corrosion resistance test was SST for 20 hours. The evaluation of the de-Cr layer is
A material in which the Cr content of the material was reduced by 1% or more was regarded as "having a Cr-free layer". Table 2 shows the results. Example 1 for 16Cr to 25Cr
As in the case of, there was no scale residue and good corrosion resistance was exhibited. However, with 30Cr, scale residue was observed even when the method of the present invention was applied.

[0028]

[Table 2]

[0029]

As described above, according to the present invention,
A Cr-containing hot rolled steel strip having no shot residue, having good surface properties and polishing properties, having no Cr removal layer, and having excellent corrosion resistance can be provided. Therefore, according to the present invention, it is possible to provide a Cr-containing hot-rolled steel strip that can be used as an alternative even in applications where only cold-rolled steel sheets have been used. In addition, according to the present invention, the efficiency of annealing and pickling can be greatly improved, and this greatly contributes to the improvement of productivity. Therefore, the hot-rolled steel strip can be manufactured at low cost.

────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code FI C25F 1/06 C25F 1/06 B // C22C 38/00 302 C22C 38/00 302Z 38/18 38/18 (72) Inventor Kazuhide Ishii 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Prefecture, in the Technical Research Institute of Kawasaki Steel Co., Ltd. Person Hiroshi Yaginuma 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd.

Claims (7)

[Claims] 1. A hot-rolled steel strip containing 6.0 to 25.0 wt% of Cr, wherein the surface of the steel strip has no trace of shot blast and no de-Cr layer. Cr-containing hot rolled steel strip with excellent properties and corrosion resistance. 2. A steel material containing 6.0 to 25.0 wt% of Cr,
A method for producing a Cr-containing hot-rolled steel strip, comprising performing hot rolling, annealing in a reducing atmosphere, and then pickling in a nitric acid / hydrochloric acid solution. 3. A steel material containing 6.0 to 25.0 wt% of Cr,
Cr is characterized in that after hot rolling, it is rolled at a temperature of 700 ° C or less or quenched immediately after winding, then annealed in a reducing atmosphere, and then pickled in a nitric-hydrochloric acid solution. Method for producing hot-rolled steel strip. 4. A steel material containing 6.0 to 25.0 wt% of Cr,
After hot rolling, annealing in a reducing atmosphere, then brushing with a brush roll having a grinding function,
A method for producing a Cr-containing hot-rolled steel strip, comprising pickling in a nitric-hydrochloric acid solution. 5. The atmosphere of the above-mentioned annealing has a hydrogen concentration of 1 vol%.
The above and the rest are non-oxidizing gases, and the dew point is -60 ° C or higher and 0 ° C
The method for producing a Cr-containing hot-rolled steel strip according to any one of claims 2 to 4, characterized in that: 6. The nitric acid / hydrochloric acid solution obtained by pickling as described above, the nitric acid concentration: 10
~ 300g / l, hydrochloric acid concentration: 1 ~ 50g / l, 35 ~
The method for producing a Cr-containing hot-rolled steel strip according to any one of claims 2 to 5, wherein the liquid temperature is 65 ° C. 7. The method according to claim 2, wherein the pickling is performed by electrolytic immersion in a nitric acid solution at a current density of 1 to 30 A / dm ^2. Method for producing Cr-containing hot rolled steel strip.