JP3111853B2 - Method of manufacturing cold rolled stainless steel sheet - Google Patents

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 Cr-based stainless steel cold-rolled steel sheet containing Si and Cr, and more particularly, to a method for producing a scale generated during an annealing treatment of a cold-rolled stainless steel sheet. The present invention relates to a method for manufacturing a cold-rolled stainless steel sheet that can be easily removed.

[0002]

2. Description of the Related Art Generally, a cold-rolled stainless steel sheet is a process of cold-rolling a hot-rolled strip-shaped stainless steel sheet (hereinafter referred to as a steel strip), and removing work strain generated during the cold rolling. , An annealing process (atmospheric atmosphere) for the purpose of improving formability and workability, imparting predetermined mechanical properties, etc., a descaling process and a surface for removing scale generated on the surface of the steel sheet by this annealing process It is manufactured by a pickling process to finish cleanly. The annealing step is called bright annealing, and there is also a method of performing an annealing treatment in a non-oxidizing atmosphere. In this case, the descaling step and the pickling step can be omitted.

[0003] Annealing in an atmosphere in the atmosphere employs a method of heating in an oxidizing atmosphere using a hydrocarbon-based gas or the like as a fuel and an oxygen-rich combustion flame in order to prevent accelerated oxidation by coexisting steam. ing. Also, the annealing temperature is 80
In most cases, the temperature is as high as about 0 to 1100 ° C. Therefore, scale is generated on the surface of the stainless steel plate in the annealing step. Especially in the case of Cr stainless steel,
Since Cr, Si, and the like in steel are easily oxidized, generation of scale containing these oxides as a main component is inevitable. Therefore, descaling treatment is necessary following annealing.

Recently, a neutral salt electrolysis method has been adopted as a descaling method in a continuous annealing and pickling treatment of a stainless steel strip. The neutral salt electrolysis method is a method of performing electrolysis in a neutral aqueous solution such as sodium sulfate using a stainless steel strip as an anode. The descaling at this time proceeds by dissolving Cr oxide (Cr ₂ O ₃ ), which is the main component in the scale, as dichromate ions (Cr ₂ O ₇ ²⁻ ) by the following chemical reaction.

[0005] _{Cr 2 O 3 + 4H 2 O} → Cr 2 O 7 2-
As a descaling method of + 8H ⁺ + 6e stainless steel, an alkali molten salt immersion method is known in addition to the neutral salt electrolysis method. In the alkali molten salt immersion method, a mixture of sodium hydroxide (NaOH) and sodium nitrate (NaNO ₃ ) is heated at 450 to 520 ° C.
This is a method in which the steel sheet is heated to about a high temperature and melted, and a stainless steel plate is immersed in the molten salt bath. Cr in scale
₂ O ₃ is oxidized in a molten salt bath to change to a water-soluble chromate, and is dissolved in water in the course of the subsequent washing treatment, and is removed from the surface of the steel sheet.

In any of the above descaling methods,
Since the scale cannot be completely removed only by the descaling treatment, the metallic luster characteristic of stainless steel cannot be obtained. Therefore, an operation of completely removing the scale by performing a pickling treatment such as a method of dipping in a mixed acid of nitric acid and hydrofluoric acid or a method of electrolysis in nitric acid is employed.

As described above, when a cold-rolled stainless steel sheet is manufactured, the cold-rolled steel sheet is usually applied to the steel strip.
Annealing, descaling, and pickling are continuously performed. Annealing conditions and ease of descaling during this process (descalability)
There is a close relationship between For example, if the heating rate at the time of annealing is increased, the annealing time is shortened, the thickness of the scale is reduced, and the composition of the scale layer has a high Cr content. It is known that such a scale has good descaling properties (JP-A-4-254524, JP-A-5-306413, JP-A-5-331554). In particular, JP-A-5-306413, JP-A-5-306413
JP-A-331554 discloses that if annealing is performed at a temperature of 750 ° C. or higher, the Cr content in the scale increases, so that scale can be easily removed in a descaling step by a neutral salt electrolysis method following the annealing. ing.

[0008]

SUS430 (Cr: 1), which is most often used as a Cr-based stainless steel, is used.
6 to 18% by weight, Si: 0.75% by weight or less)
The scale can be relatively easily removed by annealing at a temperature of 0 to 850 ° C. and descaling by a neutral salt electrolysis method. However, if the descaling property can be further improved, the equipment such as annealing, descaling, and pickling can be downsized and the productivity can be improved. As described above, the descaling method includes the alkali molten salt immersion method. However, this method is excluded from the study because it has problems such as productivity and surface quality.

Recently, high-purity Nb-containing ferritic stainless steel has been used for roof materials for buildings, exterior materials for vehicles and the like, and parts for automobiles. For these stainless steels, the current annealing,
Since the scale is not easily removed under the neutral salt electrolysis condition, short-time descaling treatment is performed to avoid insufficient descaling. The reason for the low descalability is SU
Since the annealing temperature is higher than S430 and is about 900 ° C. or more, the thickness of the scale generated at the time of annealing is large,
The chemical composition of the scale is also different from SUS430.

[0010] The present invention has been made to solve the above-mentioned problems, and is intended to provide a stainless steel cold-rolled steel sheet capable of easily removing scale generated during annealing of a Cr-based stainless steel cold-rolled steel sheet. It is intended to provide a manufacturing method.

[0011]

Means for Solving the Problems In order to develop a method for solving the above-mentioned problems, the present inventors first developed a scale layer formed on the surface of a Cr-based stainless steel sheet during annealing. Survey was conducted.

FIG. 1 shows a Cr-based stainless steel SUS430.
For LX (Cr 16.5, Si 0.6, Mn 0.5, Cu 0.4, Nb 0.6 wt%), the results of analyzing the surface layer after annealing by secondary ion mass spectrometry (SIMS) are shown.
The horizontal axis represents the primary ion irradiation time (min) representing the depth from the surface, and the vertical axis represents the atomic% (percentage of the number of atoms) of each element.

Although the scale layer may be in the range shown in FIG. 1 from the oxygen (O) concentration distribution pattern, it can be seen that oxides of Cr, Fe and Si are present in this scale layer. In particular, it has been found that Si in the scale layer has a low distribution at the surface layer and exhibits the highest distribution pattern at the boundary between the scale layer and the metal layer. In addition, including investigations on other Cr-based stainless steels,
The ratio of the number of atoms of r to Si, Cr / Si, is determined by the ratio of Cr to S in the steel sheet.
It was also confirmed that it was affected by the content of i.

The present inventors considered the results of these investigations and focused on the following points regarding the scale of the steel sheet surface generated during annealing of Cr-based stainless steel and the descalability by neutral salt electrolysis. .

Chromium oxide (Cr ₂ O ₃ ), which is the main component of the scale, is easily dissolved by the neutral salt electrolytic treatment, and the higher the proportion of Cr ₂ O ₃ in the scale layer, the more the solubility (descalability) ) Is good.

The silicon oxide (SiO ₂ ) contained in the scale is determined by the potential of Pourbaix—pH
As can be seen from the figure, it is presumed that the neutral salt electrolysis method does not dissolve at all.

In the neutral salt electrolysis method, the lower the atomic ratio Cr / Si in the scale layer, the harder the scale becomes soluble. In particular, when the atomic ratio Cr / Si at the boundary between the scale layer and the metal layer is less than 1, the solubility of the scale is poor.

The Si in the steel sheet has a temperature of 600 during annealing.
Oxidation at or above ° C is significant. Therefore, the longer the heating time at 600 ° C. or higher, the more Si (Si
O ₂ ) also increases (Cr / Si decreases).

As a result of examining the basic conditions for solving the problem based on the above basic investigation results and facts, it is necessary to consider the following two factors simultaneously for descalability. I understood.

(1) Descalability (easiness of descaling) determined from the chemical composition of the steel sheet: Cr and Si in the Q ₁ steel sheet
The higher the weight% ratio (Cr / Si) of the sample, the better the descaling property.

(2) Descalability determined by annealing conditions (hardness of descaling): The higher the temperature at a temperature of 600 ° C. or higher during Q ₂ annealing and the longer the heating time at that temperature, the more the descaling property. Is bad.

That is, the descaling property of the scale formed by annealing includes “good descaling property determined by the chemical composition of the steel sheet: Q ₁ ” and “bad descaling property determined by the annealing conditions: Q ₂ ”. It is necessary to consider two factors, and it is possible to improve descalability by selecting a condition where Q ₁ exceeds Q ₂ .

The present invention has developed a specific method for realizing the above-described basic idea. When manufacturing a Cr-based stainless steel cold-rolled steel sheet, the steel sheet is subjected to the following formula ( The gist is that the annealing treatment is performed under the conditions satisfying the relationship of 1), and the descaling treatment is further performed by a neutral salt electrolysis method.

[0024]

(Equation 2)

Here, Cr: Cr content of steel sheet (% by weight) Si: Si content of steel sheet (% by weight) t: Heating time (second) t ₁ : From the start of heating until the temperature of steel sheet reaches 600 ° C. Time (seconds) t ₂ : Time from the start of heating to the time when the temperature falls to 600 ° C. after holding at the annealing temperature (seconds) T: Temperature of steel sheet (° C.)

[0026]

According to the present invention, as described above, the descaling property of the scale formed by annealing is evaluated as follows: Good descaling property determined by the chemical composition of the steel sheet: Q
₁ determined from the annealing conditions descaling of poor: considering two elements Q _2, Q ₁ is a basic to choose the condition exceeds the Q _2.

Good descalability determined by the chemical composition of the steel sheet: Q ₁ is evaluated by the above equation (2). Equation (2) shows that Si to Cr ₂ O ₃ in the scale
The ratio of O ₂ depends on the ratio of Si to Cr (Cr
/ Si, weight% ratio). That is, the larger the Cr / Si, the higher the descalability, and therefore Q ₁ is Cr / Si
As a function of It should be noted that Cr of the formula (2)
The coefficient 2 × 10 ⁵ of / Si is a value empirically obtained from the relationship with the value of Q ₂ .

Poor descaling property determined by the annealing conditions: Q ₂ depends on the temperature and for how long the steel in the steel sheet is exposed to a temperature of 600 ° C. or more, where it is easily oxidized. You. Focusing on this point, as a result of examining an index suitable for expressing poor descalability, it was confirmed that the index obtained by Expression (3) was optimal.

That is, when the temperature of the steel sheet is T ° C., the poor descaling property can be accurately expressed by the product of the square of (T−600) and the time (dt) held at that temperature. .

FIG. 2 shows the relationship between the heating time and the temperature T of the steel sheet and the value of (T-600) ² calculated from the relationship. The shaded portion surrounded by the curve of (T-600) ^{2 in} FIG. 2 indicates that it corresponds to the value of Q ₂ obtained by Expression (3).

If the steel sheet is annealed under the condition that Q ₁ is larger than Q ₂ as the annealing condition, descaling can be performed very easily by the subsequent neutral salt electrolysis method. Incidentally, Q ₁ is Q ₂ greater than condition, i.e., the Q ₂ chooses Q ₁ smaller conditions would annealing time to select a short condition. Since it is a necessary condition that a sufficient annealing effect is obtained, the time must be secured. It is necessary to secure about 5 to 10 seconds as the lower limit of the annealing time (holding time at the annealing temperature).

The stainless steel targeted in the method of the present invention is a ferritic Cr stainless steel. This C
r-based stainless steel usually contains about 1% by weight or less of Si as a deoxidizing agent. The method of the present invention is applicable to all Cr-based stainless steels,
This is particularly effective for high Cr stainless steel having an r content of about 16% to 30% by weight. Among them, a further effect is exerted on a material containing Nb. As described above, even when the Cr content is high, when the Si content is low, the Cr content in the scale generated by annealing is low.
The ratio of the oxide is large, it tends to be descaled in neutral salt electrolysis method, but if Si content is high, since SiO ₂ is often in the scale, difficulty is descaled in a neutral salt electrolysis method That's why. Further, when Nb is contained in stainless steel, the scale tends to be thick because the annealing temperature is high, and a scale having a composition that is difficult to be dissolved by the neutral salt electrolysis method is generated.

In the neutral salt electrolysis method, sodium sulfate (Na ₂
In a neutral aqueous solution such as a SO ₃ ) solution or a sodium nitrate (NaNO ₃ ) solution, in principle, electrolysis is performed using stainless steel as an anode. In the case of continuous treatment of a steel strip or the like, an alternating electrolysis method is adopted in which, for example, an anode plate and a cathode plate are alternately arranged in the traveling direction of a steel plate in an electrolytic tank containing an aqueous solution and indirectly energized. In this method, the pole of the steel sheet repeats the anode and the cathode, and when it becomes the anode, the scale is dissolved.

The concentration of the aqueous solution must be determined according to conditions such as descalability of the scale, productivity, and surface smoothness required for the steel sheet of the product.
The neutral salt concentration is preferably about 30% by weight. The conditions for the electrolysis were a current density of 50 to 100 mA / cm ² ,
A voltage of 5 to 20 V, an aqueous solution temperature of 50 to 90 ° C., and an electrolysis time of about 10 to 60 seconds are appropriate. In the case of the present invention, since descaling is possible even if a relatively weak condition is selected among these conditions, the present invention has a feature that the surface roughness of the steel sheet is small.

Following descaling, the stainless steel plate is generally subjected to a pickling treatment to impart a surface gloss. The pickling treatment may be a usual treatment method, for example, a method of dipping in a mixed acid of nitric acid and hydrofluoric acid, a method of electrolysis in a nitric acid aqueous solution, and the like may be applied.

[0036]

EXAMPLES As test materials, Cr-based stainless steels having four kinds of chemical compositions shown in Table 1 were used.

The contents of Cr and Si are each 1
6.5 to 21.9% by weight, 0.15 to 0.63% by weight, and Cr / Si (weight% ratio) is 33 to 146;
The Q ₁ value is 6.3 × 10 ^{6 to} 29.1 × 10 ⁶ (see Table 2).

The test materials having the above four chemical compositions had a sheet thickness of 0.4 as shown in Table 1 in a state of being cold-rolled.
１．５1.5 mm. From this test material, the size is 150
A sheet sample of 200 mm was prepared.

[0039]

[Table 1]

The sheet-shaped sample was first subjected to an annealing treatment under the conditions shown in Table 2 in an electric furnace in an atmosphere of an oxygen-excess hydrocarbon combustion gas. Annealing conditions include an annealing temperature of 950 to 1050 ° C., a heating time at 600 ° C. or higher for 40 to 182 seconds, and a Q ₂ value of 4.3 × 10, including the present invention examples and comparative examples.
^{6 to} 32 × 10 ⁶ .

[0041]

[Table 2]

After the annealing treatment, descaling was performed by neutral salt electrolysis under the conditions shown in Table 3, and further, pickling was performed by nitric acid electrolysis and nitric hydrofluoric acid immersion under the same conditions as shown in Table 3. The electrolysis at the time of descaling and pickling is performed by an alternating electrolysis method, and the conditions are as shown in Table 3.

[0043]

[Table 3]

In order to evaluate the descalability of the sample subjected to the above treatment, the surface of the sample was observed with an optical microscope, and the presence or absence of scale was examined. Further, in the method of the present invention, conditions for restricting the heating time at 600 ° C. or more are selected, and thus there is a possibility that sufficient annealing may not be performed. In order to confirm that a predetermined annealing effect was obtained, the hardness of each sample was measured. Table 2 shows the results of these surveys.

As is clear from Table 2, the processing N of the present invention example
o. The 1-8, both because Q ₂ is Q ₁ or less, residual scale on the steel sheet surface after descaling was not observed. In addition, the surface was smooth and no rough skin was observed. On the other hand, in the processing No. of the comparative example. 9-1
For 2, because Q ₂ is greater than Q _1, the steel sheet surface after descaling and the scale remains and the surface texture was poor.

As shown in Table 2, the heating time at 600 ° C. or higher is shorter in the present invention examples than in the comparative examples for all of the test materials A to D. However, as can be determined from the measured values of the hardness of the steel sheet after descaling shown in Table 2, the Vickers hardness of the present invention example is 160 or less, equivalent to that of the comparative example, and a sufficient annealing effect is obtained. This was supported.

[0047]

The method for producing a Cr-based stainless cold-rolled steel sheet according to the present invention is characterized in that the descaling property is determined by the chemical composition of the steel sheet: Q
₁ determined from the annealing conditions descaling of poor: considering two elements Q _2, Q ₁ has a base of carrying out the annealing treatment of the steel sheet under conditions of greater than Q _2. Therefore,
The descaling property at the time of descaling by neutral salt electrolysis after annealing is extremely good, and no scale remains on the steel sheet surface after descaling.

Further, since a relatively low concentration of acid can be used at the time of descaling, there is no roughening of the surface of the steel sheet and the surface properties are good. Further, since the descaling process is easy, economical effects such as reduction of facility costs and reduction of running costs by downsizing the descaling facility and the pickling facility can be obtained.

As described above, the method for producing a cold-rolled stainless steel sheet of the present invention has an excellent effect that a cold-rolled stainless steel sheet having good surface properties can be economically produced.

[Brief description of the drawings]

FIG. 1 shows the results of measurement of the chemical composition of a surface layer of an annealed stainless steel sheet by secondary ion mass spectrometry.
It is a figure showing an example.

FIG. 2 is a heating time t for annealing a stainless steel plate.
Temperature T (° C.) (in seconds) and the steel sheet is a diagram showing an example of the relationship between (T-600) ² and Q _2.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-306413 (JP, A) JP-A-5-331554 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/46 C21D 9/52 101 C21D 1/76 C21D 6/00 102 C25F 1/06

Claims (1)

(57) [Claims] 1. After cold rolling a Cr-based stainless steel sheet,
A method for producing a cold-rolled stainless steel sheet, comprising: annealing under conditions satisfying the following formula (1); and descaling by a neutral salt electrolysis method. (Equation 1) Here, Cr: Cr content of steel sheet (% by weight) Si: Si content of steel sheet (% by weight) t: Heating time (second) t ₁ : Time from the start of heating until the temperature of steel sheet reaches 600 ° C. ( Seconds) t ₂ : Time from the start of heating to the time when the temperature is lowered to 600 ° C. after holding at the annealing temperature (seconds) T: Temperature of steel sheet (° C.)