JPH0417623A - Production of cold rolled ferritic stainless steel having excellent surface characteristic and moldability - Google Patents

Abstract

PURPOSE:To prevent the erosion of crystal grain boundaries by pickling and to stably produce a stainless steel sheet having excellent surface characteristics by subjecting the slab of a ferritic stainless steel to hot rolling and annealing under specific temp. conditions, then pickling the surface to remove scale, then subjecting the slab to cold rolling and annealing. CONSTITUTION:The slab of the ferritic stainless steel consisting of the compsn. contg., by weight %, <0.055% C, 16.0 to 18.0% Cr, 0.05 to 0.20% Al, and 0.025 to 0.070% Ni and having >=2 ratio of Al/N is hot rolled to a hot rolled sheet, which is then coiled at <=650 deg.C. This hot rolled sheet is annealed in a temp. range of 800 to 90O deg.C and is cooled at 25 deg.C/sec cooling rate down to <=600 deg.C; in succession, the steel sheet is pickled by an acid mixture composed of nitric acid and hydrofluoric acid, by which the surface scale is removed. The steel sheet is thereafter worked to the sheet of the final sheet thickness by cold rolling and is then subjected to the final finish rolling. The ferritic stainless steel sheet having the excellent surface characteristics and moldability is produced.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for manufacturing a cold-rolled ferritic stainless steel sheet having extremely excellent surface properties and formability.

<Prior Art> A ferritic stainless steel sheet represented by 5US430 is produced by hot rolling, hatch annealing or continuous annealing, and then cold rolling and final annealing. In this manufacturing process, the annealing treatment performed subsequent to the hot rolling is an essential step for improving ridging resistance and formability, but it also has an important purpose of restoring the sensitization of the hot rolled sheet.

By the way, following the annealing process, descaling is generally carried out by pickling using a mixed acid, but if the recovery of sensitization in this annealing process is insufficient, the mixed acid will cause grain boundary erosion, resulting in descaling after cold rolling. The appearance of surface defects such as gold dust or glitter significantly impairs the surface quality.

In the case of conventional batch annealing, the cooling rate during annealing is extremely low at 5 to 30°C/h, and sensitization does not occur after annealing, but in continuous annealing processes, the cooling rate during annealing is extremely low, and sensitization does not occur after annealing. Since it is cooled at a high cooling rate, it is difficult to recover the sensitization sufficiently.

However, since the process of Bacchi type annealing takes several tens of hours and has low productivity, in recent years, techniques have been implemented to shorten the processing time by performing continuous annealing in this process. For example, in Japanese Patent Publication No. 59-43978, a hot rolled sheet of ferritic stainless steel sheet containing M is annealed to 850 to 110
The temperature is 0°C, and during the cooling process, cooling is performed at 15°C/s to a temperature of 700 to 900°C.
A method for further cooling according to the amount of N without holding the temperature at 2 or more times is disclosed.

However, in the case of such conventional technology, although the cooling during hot-rolled sheet annealing is controlled to prevent sensitization, the formability, in-plane anisotropy, and ridging resistance are lower than that of conventional batch annealed materials. Not only is it inferior, but it also has the problem of easily causing grain boundary erosion when pickling with a mixed acid is performed in the next step.

<aS to be solved by the invention> This invention advantageously solves the problems of conventional continuous annealing technology for hot-rolled sheets, such as deterioration of formability and ridging resistance and occurrence of grain boundary erosion due to mixed acids. The object of the present invention is to provide a method for producing a cold-rolled ferritic stainless steel sheet with excellent surface properties and formability.

<! Means for Solving Problem I> That is, the present invention provides carbon of 0.055% or less, Cr of 16.0 to 18.0%, and A of 10.05 to 0.2% by weight.
0%, N0.025-0.070%, and AI7N≧2
A ferritic stainless steel slab containing is hot-rolled and coiled at a temperature of 650°C or less, followed by hot-rolled plate annealing at a temperature range of 800°C or more and less than 900°C, and cooling after annealing at 600"C. Production of cold-rolled ferritic stainless steel steel sheet with excellent surface texture and formability, characterized by cooling to the following temperature at a rate of 25°C/s or less, followed by pickling treatment, cold rolling, and annealing treatment. It's a method.

<Function> As described above, the present invention improves formability and ridging resistance after cold rolling and finish annealing, and improves the pickling process after continuous annealing even if a continuous annealing process is adopted for hot rolled sheets. The main purpose is to prevent grain boundary erosion caused by mixed acids.

Therefore, the present inventors first conducted research on the influence of components on formability and ridging resistance after cold rolling finish annealing in the continuous annealing process, and as a result, actively added M and N.
It has been found that reducing C is extremely effective.

It is generally known that addition of about 0.1% of N is effective in continuous annealing of ferritic stainless steel represented by 5US430.

That is, since At is a strong ferrite-forming element, the addition of At increases the A transformation temperature, which is 850-90°C, whereas conventional batch annealing is 800-850°C.
Since annealing can be performed at a higher temperature such as 0° C., the effect of annealing can be easily obtained in a short time. However, the addition of M causes a serious problem of reducing the amount of austenite at high temperatures and deteriorating the ridging resistance.On the other hand, although high-temperature and short-time annealing can obtain the same level of Y value as conventional methods, the hot-rolled structure deteriorates. Because the material is not sufficiently homogenized, elongation, which is the most important factor in press workability, deteriorates significantly compared to conventional punch annealed materials. For these problems, N
The addition of C and the reduction of C are extremely effective.

Since N is an austenitic element, its addition increases the γ phase during hot rolling and homogenizes the hot rolling texture, thereby improving ridging resistance. However, if only N is added, although the ridging resistance is improved, the steel plate becomes hard and the press formability deteriorates. On the other hand, in the method of the present invention, A
By adding I and 0.03% or more of N in combination,
It improves press formability and also makes it possible to improve ridging resistance. In other words, the addition of N increases the γ phase during hot rolling and improves the ridging resistance, but in the temperature range of about 1000°C or less, by setting A77N≧2, N does not dissolve in solid solution but precipitates as AIN. Does not cause deterioration of press formability.

Further, in order to obtain press formability equivalent to or higher than that of conventional bunch annealed materials, it is effective to reduce C to 0.055% or less.

Therefore, we next conducted a more detailed study on prevention of grain boundary erosion by mixed acids, keeping in mind the above-mentioned findings regarding the effects of ingredients on press formability and ridging resistance in continuous annealing processes.

As a result, the ferritic stainless steel steel sheet is generally wound into a coil at a temperature of about 700 to 750° C. during the hot rolling process, and a Cr-free layer is formed on the surface layer of the steel sheet due to the self-annealing effect after winding. The thickness of the Cr-free phase generated here reaches approximately 1100 Ir, which causes grain boundary erosion by the mixed acid. That is, 17C represented by 5US430
Rtli is sensitized by heating to a temperature of about 900°C or higher and then cooling, but it can be decarbonized by winding after hot rolling.
When an R-layer is formed, the Cr concentration in the surface layer of the steel sheet decreases to about 10%, so a continuous annealing process with a relatively high cooling rate even at temperatures as low as 800°C is necessary due to the lowering of the solid solution temperature of Cr carbides. In this way, the present inventors have elucidated the mechanism of grain boundary erosion caused by mixed acids.

Here, the experiments that served as the basis for finding the appropriate process conditions for the present invention will be explained.

Small vacuum high frequency furnace, C0.01-0.07%, MO7
Various small steel ingots containing 15% N, 0.035% N, and 16.5% Cr were melted, heated to 1230°C, hot rolled to a thickness of 4 mm, and immediately heated from 800°C to room temperature. 3 in those temperature ranges to simulate the winding operation.
After holding at 0 m, it was air cooled. These were hot-rolled sheets annealed at 870°C and 1 hour. At this time, the cooling rate after annealing was set to 10 to 4
It was varied in the range of 0°C/s.

For these, mixed acid pickling (HNOx 100 g/
l' + HF20g/I! , 50° C. for 160 seconds), and grain boundaries were observed using a scanning electron microscope. The results are shown in FIG.

By lowering the coiling temperature, the sensitization temperature range is reduced, and lowering the C and lowering the cooling rate after annealing is also effective in preventing grain boundary erosion.The cooling rate after annealing is 25℃/s.
If the carbon content is smaller, grain boundary erosion does not occur at a winding temperature of 650° C. or less, but if the C content exceeds 0.055%, grain boundary erosion is rapidly likely to occur. In addition, the cooling rate after annealing is 25
If it increases beyond ℃/s, grain boundary erosion tends to occur, and it is difficult to prevent it. This is thought to be because a slight amount of Cr carbide dissolved in solid solution during annealing re-precipitates during cooling, causing sensitization, but when cooling at a rate lower than -25°C/s, Cr diffuses and recovers the sensitization. It will be done.

It is not necessary to cool down to room temperature at a rate lower than 25°C/s, and cooling control up to 600'C, where Cr atoms are actively diffusing, is effective.Sensitization does not occur even if rapidly cooled from a temperature below that temperature. do not have.

Next, the reason for limiting the ingredients will be explained.

Good elongation can be obtained with a C content of 0.055% or less, and if it exceeds 0.055%, elongation deteriorates and grain boundary erosion is likely to occur due to mixed acid, so 0.055% is selected.
Limited to the following.

C is a basic element in ferritic stainless steel, and must be at least 16.0% in order to obtain the desired corrosion resistance.
Although the corrosion resistance is improved by increasing the content, the moldability deteriorates when the content exceeds 18%, so the content is set in the range of 16.0 to 18.0%.

N is an effective austenite-forming element that improves ridging resistance by increasing the amount of austenite precipitated during hot rolling, but if it is less than 0.025%, it has no effect, and if it exceeds 0.07%, A7N Since it precipitates in large amounts and deteriorates the surface properties of the steel plate, it is limited to 0.025 to 0.07%.

A! is a strong ferrite-forming element that not only increases the A1 transformation temperature and enables annealing at a higher temperature, but also stabilizes N as An and improves press formability. If it is less than 0.05%, stabilization of N is insufficient, so the lower limit was set to 0.05%, and AI/N was limited to 2 or more. On the other hand, when it exceeds 0.2%, the grain size increases and r(
Since I deteriorates, the upper limit was limited to 0.2%.

In addition, the composition range of elements such as Si and Mn is not particularly limited, but Si is 1.0% from the viewpoint of ensuring workability.
Hereinafter, Mn is preferably 0.7% or less from the viewpoint of ensuring corrosion resistance.

Next, when the coiling temperature in hot rolling exceeds 650"C, the carbon is removed.
The temperature was limited to 650° C. or lower because the amount of r-layer formed increases and the mixed acid causes grain boundary erosion.

Although good press formability and ridging resistance can be obtained even when the hot-rolled sheet annealing temperature is lower than 800°C, the in-plane anisotropy increases, so the lower limit was limited to 800°C or higher. On the other hand, at temperatures above 900°C, the amount of Cr carbide solid-solved during annealing increases and becomes more sensitive.
The upper limit was set to less than 900°C.

The cooling rate after annealing was limited to 25°C/s or less because if it exceeded 25°C/s, grain boundary erosion would occur due to the mixed acid.

Also, cooling at a rate higher than 25°C/s from 600°C or higher will cause grain boundary erosion due to the mixed acid.
The cooling end temperature below 600° C. was limited to 600° C. or below.

Next, the present invention will be explained in more detail with reference to Examples.

<Example> After hot rolling a continuously cast slab containing the chemical components shown as A-G in Table 1 to a thickness of 4 mm, annealing and subsequent mixed acid pickling (8 mm
1103100g#! + IIF 20g/f, 50
℃, 60 s), and then cold rolled to a thickness of 0.7 mm, followed by final annealing at 850 ℃ for 30 s.

Table 2 shows the results of grain boundary erosion observation by SEM after hot-rolled FiM washing, the T value, Δγ, elongation, and ridging waviness height of the finished annealed sheet, together with the manufacturing conditions.

As is clear from Table 2, a ferritic stainless steel slab containing appropriate amounts of Cr, A7, and N was hot-rolled at an appropriate coiling temperature and hot-rolled plate annealed at an appropriate processing temperature and cooling. By applying this method, a cold-rolled sheet with extremely excellent press formability was obtained without causing grain boundary erosion due to mixed acid pickling.

<Effects of the Invention> According to the present invention, a ferritic stainless steel sheet with excellent surface properties and formability can be manufactured by short-time annealing of a hot-rolled sheet, resulting in a significant reduction in manufacturing costs and productivity compared to the conventional method. It is possible to realize an improvement in

[Brief explanation of drawings]

FIG. 1 is a graph showing the effects of C content, winding temperature, and cooling rate on the occurrence of grain boundary erosion.

Claims (1)

[Claims] In terms of weight ratio, C0.055% or less, Cr16.0-1
8.0%, Al0.05~0.20%, N0.025~
A ferritic stainless steel slab containing 0.070% and Al/N≧2 is hot rolled, coiled at a temperature of 650°C or lower, and then hot-rolled plate annealed at a temperature of 800°C or higher and 900°C.
A surface texture characterized by performing annealing in a temperature range of less than 600°C, cooling after annealing to a temperature of 600°C or less at a cooling rate of 25°C/s or less, and then subjecting it to pickling treatment, cold rolling, and annealing treatment. and a method for producing cold-rolled ferritic stainless steel steel sheet with excellent formability.