JP2000328142A - Production of stainless steel strip small in unevenness of gloss - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a producing method capable of stably reducing the unevenness of the surface gloss in the production of a steel strip of austenitic, martensitic or duplex stainless steel. SOLUTION: At the time of subjecting a slab of austenitic, martensitic or ferritic austenitic duplex stainless steel to hot rolling, annealing and pickling in succession to produce a steel strip, in the hot rolling stage, the coiling temp. is controlled to <700 deg.C, preferably to <=600 deg.C, in the annealing stage, the maximum arrival material temp. Tm is controlled to <=1100 deg.C, the average temp. rising rate from 700 deg.C to Tm is controlled to >=10 deg.C/s, preferably to>=10 deg.C/s, the soaking time at Tm is controlled to 0 to 1 min, and the average cooling rate from Tm to 700 deg.C is controlled to >=10 deg.C/s, preferably to >=15 deg.C/s, in the pickling stage, the concn. of nitric acid is controlled to 20 to 50g/L, and also, the concn. of hydrofluoric acid is controlled to 5 to 15 g/L, and the variation in the value of the glossiness Gs (20 deg.) in the same steel strip surface in the steel strip after the pickling is controlled to <=10.

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 steel strip having less uneven surface gloss in the production of austenitic stainless steel, martensitic stainless steel or ferrite + austenite duplex stainless steel. .

[0002]

2. Description of the Related Art In the production of austenitic stainless steel, martensitic stainless steel or ferrite + austenite duplex stainless steel strip, usually in a hot rolling process, a temperature of 750 to 850 ° C. is applied after a final pass is reduced. And then subjected to annealing and pickling. However, unevenness in appearance is often observed on the surface of the thus-obtained ordinary "hot-rolled annealed pickled steel strip". This glossy unevenness can be obtained by further cold rolling to form a "cold rolled unannealed steel strip", or by performing cold rolling → annealing → pickling to form a "cold rolled annealed pickled steel strip", or by cold rolling. → Bright annealing →
Even when it is subjected to pickling to obtain a "bright annealed pickled steel strip", it usually remains without disappearing. Such gloss unevenness is problematic in applications where appearance is important.

[0003] Various investigations have been conducted on the cause of the occurrence of uneven gloss in a hot-rolled annealed pickled steel strip. As a result, when the interface between the oxide scale and the metal base (hereinafter referred to as oxide scale / metal interface) generated on the steel strip surface during hot rolling or annealing is an interface with large unevenness, the unevenness of the interface is changed. It has been found that the gloss is reflected on the surface gloss after annealing and pickling, which causes uneven gloss.

Therefore, various means for removing irregularities on the surface of the stainless steel strip caused by irregularities on the oxide scale / metal interface have been studied. For example, a method for lengthening the pickling time to smooth the interface irregularities, A method of grinding and removing defects on the surface of a pickled steel strip and then performing cold rolling, and a method of increasing the rolling reduction per pass in cold rolling to reduce the surface roughness are employed. Japanese Patent Application Laid-Open No. 2-73918 discloses a mechanical descaling method in which an abrasive such as iron sand is sprayed onto a hot-rolled steel strip together with high-pressure water immediately after hot-rolling to smooth the scale / metal interface. .

[0005]

However, a method of increasing the pickling time, a method of grinding the surface of the steel strip, and a method of increasing the rolling reduction per cold rolling pass all involve increasing the process load and increasing the production load. Properties are significantly reduced. The mechanical descaling method also has a problem in non-uniformity depending on the location of the descaling. The present invention has been devised in order to solve the above-described problems. As has been conventionally done, not only the idea of removing the already generated unevenness of the oxide scale / metal interface, but also the oxide scale has been developed. An austenitic system with less surface gloss unevenness, by taking into account the idea of making the unevenness of the oxide scale / metal interface as smooth as possible from the stage where it occurs, and by making it possible to save labor in the annealing and cold rolling processes or to omit the polishing process. It is intended to produce a steel strip of martensitic or duplex stainless steel.

[0006]

Means for Solving the Problems In order to achieve the above object, the invention of claim 1 is directed to an austenitic stainless steel, a martensitic stainless steel or a ferrite +
In producing a steel strip by sequentially performing hot rolling, annealing, and pickling on an austenitic duplex stainless steel slab, the winding temperature is set to less than 700 ° C in the hot rolling process, and the maximum attained material temperature Tm is 1100 ° C in the annealing process. Hereinafter, the average heating rate from 700 ° C to Tm is controlled at 10 ° C / s or more, the soaking time at Tm is controlled to 0 to 1 minute, and the average cooling rate from Tm to 700 ° C is controlled at 10 ° C / s or more. In the pickling process, the nitric acid concentration is controlled to 20 to 50 g / L, and the hydrofluoric acid concentration is controlled to 5 to 15 g / L, and the gloss Gs (20 °) in the same steel strip surface in the steel strip after the pickling is controlled. This is a method for producing a stainless steel strip with less gloss unevenness, which makes the value variation less than or equal to 10.

Here, the soaking time is the time during which the material is kept at the temperature Tm, and the soaking time of 0 minutes means that the temperature is started at the same time when the material temperature reaches Tm.

The gloss Gs (20 °) is an index specified in JIS Z 8741. “Variation in the value of gloss Gs (20 °) within the same steel strip surface” (hereinafter sometimes referred to as ΔGs (20 °)) refers to the value defined below. At least three or more measurement areas having a length of 1 m in the rolling direction are provided in the steel strip rolling direction at intervals of at least the minimum distance L (m) defined below, and at least 10 points are provided at any position in each measurement area. The measurement points are determined so that each measurement point has an interval of 10 cm or more, and the gloss Gs (20 °) of each measurement point is measured, and all the measured values of Gs (20 °) (at least 30 points or more) )
The difference between the maximum value and the minimum value is determined, and this is used as the variation in the value of the gloss Gs (20 °) within the same steel strip surface. When the total length of the steel strip is 201 m or more, L = 100, and when it is less than 201 m, L = (length of steel strip (m) −1) / 2. However, the vicinity of the end of the steel strip (near the top and tail) is usually cut and removed in any process for manufacturing reasons. The length of the steel strip.

According to a second aspect of the present invention, in the first aspect of the invention, the winding temperature is set to 600 ° C. or less in the hot rolling step, and the maximum attained material temperature Tm is set to 1100 ° C. or less in the annealing step, and from 700 ° C. to Tm. Heating rate 20 ° C / s or more, soaking time at Tm 0 ~
It specifies that the average cooling rate from Tm to 700 ° C for 1 minute is controlled to 15 ° C / s or more.

[0010] A third aspect of the present invention is the invention according to the first and second aspects, wherein the steel is an austenitic stainless steel containing 12 to 30% by mass of Cr and 5 to 30% by mass of Ni. It is.

[0011] The invention of claim 4 provides that in the invention of claims 1 and 2, the steel is a martensitic stainless steel containing 12 to 20% by mass of Cr and 10% by mass or less of Ni. Things.

The invention of claim 5 is characterized in that in the invention of claims 1 and 2, the steel is a ferrite + austenite duplex stainless steel containing 20 to 30% by mass of Cr and 2 to 15% by mass of Ni. It is specified.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of detailed studies, the present inventors
In order to make the oxide scale / metal interface formed in the steel strip manufacturing process of austenitic, martensitic or ferrite + austenite duplex stainless steel as smooth as possible in the forming stage, the winding temperature in hot rolling is required. It has been found that it is very effective to lower the temperature and to increase the heating rate and the cooling rate in annealing after hot rolling. In addition, after making the oxide scale / metal interface smooth, the acid concentration is strictly controlled so that over pickling does not occur during pickling, so that a process such as grinding is added or a path under high pressure is incorporated. It has become clear that a stainless steel strip of excellent quality with less surface gloss unevenness can be stably manufactured without performing cold rolling.

[0014] In the hot rolling process, it is important to reduce as much as possible the oxide scale generated while the steel strip (coil) wound after the hot rolling finishing pass is still exposed to a high temperature. This is the most effective means when considered. Under general hot rolling conditions for austenitic, martensitic, and duplex stainless steels, the rolled steel strip is 500-8
It is cooled while being exposed to a temperature range of 00 ° C. for a long time. However, this temperature range is a temperature range in which Cr carbides precipitate and generate a Cr-deficient layer near the grain boundaries. The generated Cr deficient layer locally deteriorates the oxidation resistance particularly in the vicinity of the grain boundary in the subsequent reheating (annealing or the like). For this reason, during the cooling process after winding
If a large number of Cr-deficient layers are generated, oxidation near the grain boundaries is promoted in annealing heat in a later step, and as a result, the oxide scale / metal interface after annealing has a form with extremely large irregularities.

For this reason, it is desirable to reduce the hot-rolling winding temperature as much as possible. If the operation can be carried out at a winding temperature of 500 ° C. or less, the problem of the Cr-deficient layer is eliminated as far as the hot rolling process is concerned. However, such operations have many other limitations and are not always feasible. As a result of detailed studies, if the post-process is performed according to the annealing conditions and pickling conditions described below, the oxide scale required for the hot-rolling process is set by setting the hot-rolling winding temperature to less than 700 ° C. It has been confirmed that the smoothing of the metal / metal interface can be sufficiently achieved, that is, a high-quality steel strip with little uneven gloss can be obtained after pickling.
Therefore, in the present invention, the hot-rolling winding temperature is set to at most 700.
It was required that the temperature be lower than ° C.

In particular, when the hot-rolling winding temperature is limited to 600 ° C. or lower, a steel strip of excellent quality with less uneven gloss can be produced more stably. A winding temperature of about 600 ° C. can be implemented in actual operation without much difficulty. Therefore, in the present invention, the hot rolling winding temperature 60
0 ° C. or lower was set as a preferable condition.

In the annealing step applied to the steel strip after hot rolling, it is important to pass through the temperature range in which the Cr-based carbide is generated in the shortest possible time for the same reason as described above. It is also important to suppress and reduce the amount of oxide scale formation. However, in addition to this, the difference in thermal expansion between the oxide scale and the metal during the heating and cooling process and the cooling and cooling process is used to cause "interface separation" between the oxide scale and the metal interface. The inventors' research has shown that it is very effective in reducing unevenness.

In the annealing performed after hot rolling of austenitic, martensitic or duplex stainless steel, one of the important purposes is generally to form a solid solution of Cr carbide.
Usually, it is often heated to a high temperature exceeding 1100 ° C. in order to achieve its purpose in a short time. However, in the present invention, since the hot-rolling winding temperature is defined as a temperature at which carbide precipitation is unlikely to occur, annealing at a temperature of 1100 ° C. or less is sufficient.
Therefore, in the present invention, the highest reached material temperature Tm of annealing is 1100 ° C.
The amount of oxidized scale generated in the annealing step was reduced by setting the temperature so as to be as follows and setting the soaking time as short as 0 to 1 minute.

In order to effectively cause interfacial separation of the oxide scale, it is effective to increase the rate of temperature rise and the rate of cooling. In particular, increasing the rate of temperature rise / cooling in the temperature range from 700 ° C. to the highest reached material temperature Tm is advantageous for interfacial separation. As a result of various studies, the interface peeling phenomenon can be effectively used by setting the average temperature rising rate from 700 ° C. to Tm to 10 ° C./s or more and the average cooling rate from Tm to 700 ° C. to 10 ° C./s or more. Was recognized. Further, by increasing the heating / cooling rate in the temperature range of 700 ° C. or more, the formation of Cr carbide can be sufficiently suppressed. In order to obtain these effects more effectively, it is necessary to set Tm from 700 ° C.
It is desirable that the average heating rate up to 20 ° C / s or more and the average cooling rate from Tm to 700 ° C be 15 ° C / s or more.
From the above, in the present invention, the heat pattern of the annealing performed after hot rolling is performed at an average heating rate from 700 ° C. to Tm of 10 ° C./s.
As described above, the control point is specified so that the soaking time at Tm is 0 to 1 minute and the average cooling rate from Tm to 700 ° C is 10 ° C / s or more. As a preferred heat pattern,
Control so that the average heating rate from 700 ° C to Tm is 20 ° C / s or more, the soaking time at Tm is 0 to 1 minute, and the average cooling rate from Tm to 700 ° C is 15 ° C / s or more. Embodiments have been defined.

The unpickled annealed steel strip having the oxide scale / metal interface smoothed in this way is controlled by reliably preventing over pickling in the subsequent pickling step. Thus, a steel strip having very little gloss unevenness can be obtained. As a result of various studies, it has been found that strict control of the acid concentration is most effective for preventing over-acid washing. Specifically, it has been found that by controlling the nitric acid concentration to 20 to 50 g / L and the hydrofluoric acid concentration to 5 to 15 g / L, it is possible to sufficiently prevent peracid washing in a range of a normal line speed and a liquid temperature. .

It is not always easy to quantitatively evaluate the visual "gloss uniformity" of the stainless steel strip surface. The degree of gloss at an arbitrary portion of the steel strip surface can be evaluated, for example, by the gloss Gs (20 °) specified in JISZ 8741. However, in order to evaluate the uniformity of the glossiness within the same steel strip surface, the glossiness must be strictly measured for the entire steel strip surface. It is not practically possible to control surface quality in such a way.

The present inventors have eagerly developed a control index capable of appropriately evaluating the uniformity of the surface gloss of a stainless steel strip in a mass production process in which a steel strip is continuously processed in a process of “hot rolling → annealing → pickling”. Have been considering. As a result, by using a method of sampling the value of gloss Gs (20 °) at several points on the surface of the steel strip, it is possible to sufficiently evaluate the uniformity of the gloss over the entire surface of the steel strip, and to obtain an index that can be managed. I found it. The index is the “variation in the value of the gloss Gs (20 °) within the same steel strip surface” defined above, that is, the ΔGs (20 °) value.

When determining the ΔGs (20 °) value, by measuring the gloss Gs (20 °) at 10 or more points at intervals of 10 cm or more in a measurement area having a length in the rolling direction of 1 m, For example, the uniformity of visual gloss in a product using the steel strip can be evaluated. In addition, by providing at least three measurement areas in the rolling direction of the steel strip at a sufficient interval, for example, uniformity of glossiness between products using the steel strip, that is, glossiness between products, The fluctuation of the average absolute value can be evaluated. When manufacturing in a continuous line with manufacturing conditions controlled within a certain range,
It is unlikely that the glossiness fluctuates randomly in the longitudinal direction of the steel strip. Therefore, by setting three or more measurement areas at a sufficient interval in the longitudinal direction in accordance with the above-mentioned rules, it is possible to accurately estimate the fluctuation tendency of the glossiness over the entire length of the steel strip in the longitudinal direction.

In the present invention, in the “hot-rolled annealed pickled steel strip” at the stage where hot-rolling → annealing → pickling is completed, ΔGs (20 °)
Is required to be 10 or less. This ΔGs (2
(0 °) A stainless steel strip having a value of 10 or less throughout one steel strip exhibits a very uniform surface appearance, and can be said to be of extremely high quality in surface properties. In a normal austenitic, martensitic or duplex stainless steel hot rolled annealed pickled steel strip manufactured according to general hot rolling → annealing → pickling conditions, the Δ
In most cases, the Gs (20 °) value is 15 or more, and even if a value of 10 or less appears, such a case is rare and accidental. That is, conventionally, a method of stably producing a stainless steel strip whose quality is controlled so that the ΔGs (20 °) value becomes 10 or less in the process of hot rolling → annealing → pickling has not been known. In the present invention, austenitic, martensitic or 2
This was made possible by controlling the conditions of hot rolling, annealing, and pickling in the production of a duplex stainless steel as described above.

As described above, if gloss unevenness occurs at the stage of "hot-rolled annealed pickled steel strip", the "cold-rolled unannealed steel strip" or "cold-rolled annealed pickled steel strip" manufactured using the same is used. In the “steel strip” or “bright-annealed pickled steel strip”, gloss unevenness usually remains without disappearing. The effect of the ΔGs (20 °) value on the visual uniformity of gloss varies depending on the type of surface finish. For example, in the case of a `` hot-rolled annealed pickled steel strip '' having pickling skin, the appearance of a product utilizing its surface gloss by reducing the ΔGs (20 °) value to 10 or less as described above is within the same product and in the product The feeling of uniformity is very high in between.
In the case of the “cold rolled unannealed steel strip”, since the average absolute value of the glossiness is high, the ΔGs (20 °) value is 20 or less, and it is judged that the uniformity of the glossiness is good. In the case of "Cold rolled annealed pickled steel strip",
A ΔGs (20 °) value of 10 or less is judged to be good. In the case of the “bright annealed and pickled steel strip”, since the average absolute value of the glossiness is considerably high, the ΔGs (20 °) value of 35 or less is judged to be good.

The inventors have found that Δ in the hot-rolled annealed pickled steel strip
The relationship between the Gs (20 °) value and the ΔGs (20 °) value in the cold-rolled unannealed steel strip, cold-rolled annealed pickled steel strip, and bright annealed pickled steel strip manufactured using the steel strip is also considered. Investigated in detail. As a result, by setting the ΔGs (20 °) value in the hot-rolled annealed pickled steel strip to 10 or less, the ΔGs (20 °) value of the cold-rolled unannealed steel strip manufactured by a general method in the post-process is set to 20. It is possible to reduce the ΔGs (20 °) value of the cold-rolled annealed pickled steel strip to 10 or less, and to reduce the ΔGs (20 °) value of the bright annealed pickled steel strip to 35 or less. I knew I could do it. That is, the provision of the present invention that the ΔGs (20 °) value at the stage of the hot-rolled annealed and pickled steel strip is 10 or less is to ensure the uniformity of the surface gloss in a product utilizing the surface skin of the steel strip as it is. Cold-rolled unannealed steel strip produced through post-processing,
This is also a requirement for ensuring uniform surface gloss in the cold-rolled annealed and pickled steel strips.

The austenitic stainless steel to be used in the present invention is, for example, a steel containing 12 to 30% by mass of Cr and 5 to 30% by mass of Ni. SUS316 and the like.

The martensitic stainless steel to be used in the present invention is a steel containing, for example, Cr: 12 to 20% by mass and Ni: 10% by mass or less. , SUS420J2, and the like.

The ferrite + austenite duplex stainless steel specified in the present invention is, for example, Cr: 20 to 30% by mass, N
i: Steel containing 2 to 15% by mass, and typical steel types corresponding thereto include SUS329J1, SUS329J3L and the like.

[0030]

EXAMPLES Three types of austenitic stainless steels (steel types A, B and C), two types of martensitic stainless steels (types D and E), and one type of ferrite + austenite duplex stainless steel shown in Table 1 A slab was obtained by melting steel (steel type F) in the steps of electric furnace, converter, degassing, and continuous casting.

[0031]

[Table 1]

Each of the slabs was hot-rolled to 3.8 mm and wound up, and subsequently subjected to annealing and pickling in a continuous line to form a hot-rolled annealed pickled steel strip. At this time, the hot rolling coiling temperature, the annealing conditions of the hot rolled steel strip, and the pickling conditions were varied as shown in Table 2. The hot-rolled annealed pickled steel strip is cold-rolled to 1.0 mm, partly subjected to finish annealing and pickling to form a cold-rolled annealed pickled steel strip, and the remaining part is subjected to bright annealing and pickling. Brightly annealed and pickled steel strip. Note that finish annealing or bright annealing after pickling and pickling were performed under conditions within a range usually performed in a mass production line.

With respect to the obtained hot-rolled annealed pickled steel strip, cold-rolled unannealed steel strip, cold-rolled annealed pickled steel strip or bright-annealed pickled steel strip, samples of 1 mx 1 m were each taken in the longitudinal direction of the steel strip.
Three samples are taken every 100m and the glossiness specified in JIS Z 8741
Gs (20 °) at intervals of 10 cm or more in the rolling direction
Points, a total of 30 points were measured. The maximum value of Gs (20 °) at this time is ma
xGs (20 °), minimum value is minGs (20 °), ΔGs (20 °) = ma
xGs (20 °) −minGs (20 °) was calculated. Table 2 shows the results.

[0034]

[Table 2]

From Table 2, it can be seen that hot rolling under the conditions specified in the present invention →
In the hot-rolled annealed pickled steel strip subjected to annealing → pickling, Δ
The Gs (20 °) value was 10 or less, and the uniformity of surface gloss was extremely good. Further, in the cold-rolled unannealed steel strip, cold-rolled annealed pickled steel strip, and bright annealed pickled steel strip manufactured using the hot-rolled annealed pickled steel strip having a ΔGs (20 °) value of 10 or less, The ΔGs (20 °) value was 20 or less, 10 or less, and 35 or less, respectively, and the surface gloss uniformity was extremely good.

[0036]

According to the present invention, a high-quality steel strip with less uneven surface gloss can be stably produced in austenitic, martensitic or duplex stainless steel.

 ──────────────────────────────────────────────────の Continued on front page (72) Inventor Kenichi Morimoto 4976 Nomura Minamicho, Shinnanyo-shi, Yamaguchi Prefecture Nissin Steel Engineering Co., Ltd. New Steel Co., Ltd. Technical Research Laboratory F term (reference) 4K032 AA04 AA13 AA16 AA19 AA20 AA21 AA24 AA25 AA31 BA01 CE01 CE02 CF03 CG02 4K037 EA05 EA12 EA15 EA17 EA18 EA21 EA27 EB05 EB09 EB03 FE01 FE03 FE01 FE03

Claims (5)

[Claims] 1. A method for producing a steel strip by sequentially subjecting a slab of austenitic stainless steel, martensitic stainless steel or ferrite + austenite duplex stainless steel to hot rolling, annealing and pickling to form a steel strip. The temperature is set to less than 700 ° C, and the maximum reached material temperature Tm in the annealing process
1100 ° C or less, average temperature rise rate from 700 ° C to Tm at 10 ° C / s
As described above, the soaking time at Tm is controlled to 0 to 1 minute, the average cooling rate from Tm to 700 ° C. is controlled to 10 ° C./s or more, and in the pickling step, the nitric acid concentration is 20 to 50 g / L and the hydrofluoric acid concentration is Is controlled to 5 to 15 g / L to manufacture a stainless steel strip with less uneven gloss to make the variation of the value of gloss Gs (20 °) within the same steel strip surface in the pickled steel strip to 10 or less. Law. 2. In the hot rolling step, the winding temperature is set to 600 ° C. or less, and in the annealing step, the maximum reached material temperature Tm is set to 1100 ° C. or less, 700 ° C.
The average temperature rise rate from Tm to Tm is 20 ° C / s or more, the soaking time at Tm is 0 to 1 minute, and the average cooling rate from Tm to 700 ° C is 15 ° C.
The production method according to claim 1, wherein the production rate is controlled to not less than / s. 3. The method according to claim 1, wherein the steel is an austenitic stainless steel containing 12 to 30% by mass of Cr and 5 to 30% by mass of Ni. 4. The steel is a martensitic stainless steel containing 12 to 20% by mass of Cr and 10% by mass or less of Ni.
The method according to claim 1. 5. The method according to claim 1, wherein the steel is a ferrite + austenite duplex stainless steel containing 20 to 30% by mass of Cr and 2 to 15% by mass of Ni.