JP2000297329A - Manufacture of stainless steel strip free from deterioration in corrosion resistance due to grain boundary precipitate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deterioration in corrosion resistance due to grain boundary precipitates by an easy method, in the manufacture of a strip of austenitic, martensitic or duplex stainless steel. SOLUTION: This steel strip can be manufactured by applying, in succession, hot rolling and annealing to a slab of austenitic, martensitic or ferrite + austenite duplex stainless steel. At this time, coiling temperature is regulated to <600 deg.C in the hot rolling stage. In the annealing stage, the maximum ultimate material temperature Tm is regulated preferably to 800-1000 deg.C; the average temperature rise rate from 600 deg.C to Tm is regulated to >=10 deg.C/s; the soaking time at Tm is regulated preferably to 0-1 min; and the average cooling rate from Tm to 600 deg.C is controlled to >=10 deg.C/s. By this procedure, the occupancy of precipitates in the grain boundaries of the annealed hot rolled steel strip can be reduced to <=10%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing a stainless steel strip of austenitic stainless steel, martensitic stainless steel or ferrite + austenite duplex stainless steel in which corrosion resistance is prevented from deteriorating due to grain boundary precipitates. How to do it.

[0002]

2. Description of the Related Art In the production of steel strips of austenitic stainless steel, martensitic stainless steel or ferrite + austenite duplex stainless steel, usually, in a hot rolling process, a temperature of 750 to 850 ° C. is applied after a reduction in a final pass. Winding of the steel strip is performed at the temperature, followed by annealing. The main purpose of this annealing is to achieve recrystallization of the material and to form a solid solution of a Cr-based carbide which causes deterioration of corrosion resistance.

In the production of a stainless steel strip, when Cr-based carbides precipitate at grain boundaries, a so-called Cr-deficient layer having a lower Cr concentration than the surroundings is generated in the vicinity thereof. It is well known that when a Cr-deficient layer is formed, the corrosion resistance of the steel material is significantly degraded from the corrosion resistance originally possessed by the steel, since the portion is easily corroded locally. For austenitic, martensitic, or ferrite + martensitic two-phase stainless steels, precipitation of Cr carbide at grain boundaries is promoted when exposed to a temperature range of approximately 500 to 800 ° C for a long time. It is known that in normal steel strip production, during the cooling process after winding in the hot rolling process, the steel is exposed to the above-mentioned temperature range for a relatively long time, and in this process Cr-based carbide precipitation at grain boundaries is likely to occur. Has been confirmed.

Conventionally, in the production of austenitic, martensitic, or ferrite + martensitic two-phase stainless steels, in order to avoid deterioration of corrosion resistance due to precipitation of Cr-based carbides at grain boundaries, usually, precipitation is usually performed. Under the concept of dissolving the Cr-based carbide almost completely in solid solution, a method has been adopted in which annealing is performed on the steel strip after hot rolling and winding under conditions that emphasize the solid solution of the Cr-based carbide. .
As such an annealing method, if the soaking time is extended, or if the soaking time is shortened to about 1 minute, for example, 1
A method of heating at a high temperature such as 100 ° C. or higher has been adopted.

[0005]

However, in the annealing after hot rolling, it is not preferable to lengthen the soaking time because the line speed is lowered and the production efficiency is lowered. In addition, increasing the soaking temperature increases the amount of oxide scale generated on the material surface, increases the burden on post-processes and decreases the quality of the material, and is not preferable from the viewpoint of equipment maintenance and energy cost. . Therefore, it is desired to establish a technology capable of sufficiently dissolving a Cr-based carbide by annealing at a temperature as low as possible and in a short time.

[0006] On the other hand, a quantitative study in relation to manufacturing conditions has not yet been sufficiently conducted on how much the grain boundary precipitation of Cr-based carbides can be suppressed to maintain the corrosion resistance level inherent in steel. It is true that it has not been done. For this reason, it cannot be denied that the conventional annealing conditions based on the idea of almost completely dissolving the Cr-based carbides resulted in often excessive conditions in suppressing deterioration of corrosion resistance.

The present invention has been devised to solve the problem of the Cr-based carbide solid solution accelerating method in the conventional production method, and has a grain boundary precipitate sufficient to prevent deterioration of corrosion resistance. Austenitic stainless steel, martensitic stainless steel, or ferrite + austenite duplex stainless steel capable of preventing deterioration of corrosion resistance due to grain boundary precipitates by annealing at as low a temperature as possible and for a short time by grasping the amount of precipitation of steel It is intended to provide a belt manufacturing method.

[0008]

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 and annealing on a slab of austenitic duplex stainless steel, the winding temperature is set to less than 600 ° C in the hot rolling process, and the average temperature from 600 ° C to the highest attained material temperature Tm in the annealing process. Heating rate of 10 ° C / s or more and Tm
Control the average cooling rate from 10 ° C to 600 ° C to 10 ° C / s or more to increase the precipitate occupancy at the grain boundaries of the annealed hot-rolled steel strip.
This is a method for producing a stainless steel strip in which the corrosion resistance is prevented from deteriorating due to grain boundary precipitates, which is set to 10% or less.

Here, the annealed hot-rolled steel strip refers to a steel strip at a stage obtained by sequentially performing hot rolling and annealing.

The “precipitation occupancy at the grain boundary” is defined as a value obtained by observing the metal structure of the cross section of the steel strip in the following procedure. That is, a metal structure was observed with an optical microscope on a sample obtained by electrolyzing a cross section of a steel strip using a 10% aqueous oxalic acid solution (25 ° C.) at a current density of 1 A / cm ² for 10 seconds. Is measured, and this value is defined as a (μm). Next, this sample was further subjected to a hydrofluoric / nitric acid aqueous solution (hydrofluoric acid: 20%, nitric acid: 10%, temperature:
After immersion for 10 seconds at 25 ° C.), the total extension of the etched grain boundaries in the same observation field of view as described above is measured, and this is defined as b (μm). At this time, the value represented by (a / b) × 100 is defined as the precipitate occupancy (%) at the grain boundary.

According to a second aspect of the present invention, in the production method according to the first aspect, the maximum attained material temperature Tm is set to 800 to 800 in the annealing step.
It is specified that the soaking time at 1000 ° C. and Tm is 0 to 1 minute. Here, the soaking time is a 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.

A third aspect of the present invention is the method according to the first or second aspect, wherein the steel is an austenitic stainless steel containing 12 to 30% by mass of Cr and 5 to 30% by mass of Ni. Things.

According to a fourth aspect of the present invention, in the method of the first or second aspect, the steel is a martensitic stainless steel containing 12 to 20% by mass of Cr and 10% by mass or less of Ni. Things.

According to a fifth aspect of the present invention, in the method of the first or second aspect, 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. Is defined.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied austenitic,
Martensite or ferrite + austenite 2
Investigations have been made in detail on the relationship between the amount of grain boundary precipitates formed during the manufacturing process of the steel strip of duplex stainless steel and the deterioration of corrosion resistance due to the formation of the grain boundary precipitates. As a result, it has been found that the amount of the grain boundary precipitate sufficient to prevent the corrosion resistance deterioration can be quantitatively grasped by the “precipitation occupancy at the grain boundary” defined as described above. Hereinafter, matters for specifying the present invention will be described.

[Precipitate occupation ratio at grain boundaries] After being subjected to hot rolling and annealing, the stainless steel strip is used as an annealed hot rolled steel sheet in some applications. Annealed and used as a cold rolled steel sheet. As a result of the study by the inventors, in the production of austenitic, martensitic or duplex stainless steel strip, the amount of grain boundary precipitates precipitated at the stage of annealed hot-rolled steel strip was reduced to a certain level or less. By doing so, it was confirmed that even in a cold-rolled steel sheet subjected to cold-rolling and annealing by a usual method, deterioration of the corrosion resistance of the steel sheet due to grain boundary precipitates was prevented. In the present invention, based on this finding, the “occupation ratio of precipitates at grain boundaries” in the annealed hot-rolled steel strip is defined.

The term “precipitate occupancy at grain boundaries” in the present invention is as defined above. The oxalic acid electrolysis condition specified in the definition is a condition in which a grain boundary precipitate is etched, but a portion of a crystal grain boundary where no precipitate is formed is not etched. Therefore, the value of a (μm) obtained by measuring the length of the crystal grain boundary etched under the oxalic acid electrolysis condition is the total of the crystal grain boundary portion where the grain boundary precipitate exists in the metallographic structure observation visual field. This represents an extension. The etching conditions for immersion in the aqueous solution of hydrofluoric-nitric acid defined in the above definition are conditions for etching the entire grain boundaries of the annealed austenitic, martensitic, or duplex stainless steel. Therefore, after the oxalic acid electrolysis, the immersion in hydrofluoric-nitric acid is further performed to measure the length of the crystal grain boundary etched, thereby obtaining the b (μ
The value of m) indicates the total extension of the crystal grain boundaries in the metallographic structure observation visual field.

In the present invention, the value of (a / b) × 100 calculated using the values of a and b is defined as “precipitation occupancy (%) at the grain boundary”. Conceptually, it means that it represents the ratio of the total length of the portion of the grain boundary where the grain boundary precipitate exists to the total extension of the grain boundary existing within the metallographic observation field of view of the steel strip cross section. be able to. In addition, as a method of measuring the length of the etched grain boundary, a method of obtaining an optical microscope observation image by image processing with a computer can be adopted.

As a result of detailed studies by the present inventors, in austenitic, martensitic, or duplex stainless steels, the occupation ratio of precipitates at grain boundaries of annealed hot-rolled steel strip was reduced to 10% or less. It has been found that by controlling the production conditions so as to prevent the deterioration of corrosion resistance due to grain boundary precipitates (Cr-based carbides). In other words, it is not necessary to dissolve the grain boundary precipitates almost completely in solid solution in order to exhibit the inherent corrosion resistance of the various stainless steels described above. If the "occupation ratio of precipitates in
When used as an annealed hot-rolled steel strip or when further cold-rolled and annealed in a usual manner, the steel can exhibit its original corrosion resistance sufficiently. This point will be demonstrated in Examples described later.

[Hot Rolling Winding Temperature] In the hot rolling step, grain boundary precipitates generated while the steel strip (coil) wound after the hot rolling finishing pass is still exposed to a high temperature are reduced as much as possible. This is the most effective means when considering the influence on the post-process. Austenitic, martensitic,
Under general hot rolling conditions for a duplex stainless steel, the wound steel strip is cooled while being exposed to a temperature range of 500 to 800 ° C. for a long time. This temperature range is a temperature range in which Cr-based carbides precipitate and generate a Cr-deficient layer near the grain boundaries. For this reason,
It is desirable that the hot-rolling winding temperature be as low as possible.
If the operation can be carried out at a winding temperature of 500 ° C. or less, the problem of formation of grain boundary precipitates in the hot rolling process can be solved. However, such operations have many other limitations,
It is not always easy to implement. As a result of detailed studies, if the post-process is performed according to the annealing conditions described below, by setting the hot-rolling winding temperature to less than 600 ° C, the above-mentioned `` at the grain boundary of the annealed hot-rolled steel strip '' is used. It has been confirmed that it is possible to reduce the “precipitate occupancy of the steel” to 10% or less. Even if the temperature is lower than 600 ° C., carbides are formed, but precipitates formed in the temperature range are small in size and do not adversely affect corrosion resistance. Therefore, in the present invention, the hot rolling winding temperature is set to 60
It was required that the temperature be lower than 0 ° C.

[Heat Pattern in Annealing Step] In the annealing step applied to the steel strip after hot rolling, it is important to pass through the temperature range for generating the Cr-based carbide in the shortest possible time for the same reason as described above. As a result of various investigations, the hot rolling coiling temperature was set to 60
600 ° C for hot rolled austenitic, martensitic or duplex stainless steel strips below 0 ° C
At the grain boundary by annealing the heat pattern to control the average temperature rise rate from Tm to the highest attained material temperature Tm to 10 ° C / s or more, and the average cooling rate from Tm to 600 ° C to 10 ° C / s or more. The precipitate occupancy can be reduced to 10% or less.

In the annealing performed after hot rolling of austenitic, martensitic or duplex stainless steel, usually,
In order to achieve a solid solution of the Cr-based carbide in a short time, it is often heated to a high temperature exceeding 1100 ° C. However, in the present invention, since the hot-rolling winding temperature is defined as a temperature at which carbide precipitation is unlikely to occur, it is not necessary to heat to such a high temperature. Even if the maximum temperature Tm of the annealing reaches 1000 ° C. or less, the occupation ratio of precipitates at the grain boundaries can be reduced to 10% or less, and deterioration of corrosion resistance can be prevented. However, it is desirable that the highest attained material temperature Tm be 800 ° C. or higher.

Even when Tm is 1000 ° C. or lower, the soaking time at Tm can be as short as 0 to 1 minute. As described above, the passing speed of the continuous annealing line is increased by adopting a heat pattern that increases the temperature raising / cooling speed, lowers the maximum attained material temperature Tm lower than usual, and shortens the soaking time at Tm. Productivity can be improved. In addition, even with such a heat pattern, deterioration of corrosion resistance due to grain boundary precipitates can be prevented.
Therefore, in the annealing step performed on the steel strip after hot rolling, the maximum attained material temperature Tm is set to 800 to 1000 ° C, the average heating rate from 600 ° C to Tm is 10 ° C / s or more, and the soaking time at Tm is It is desirable to adopt a heat pattern that controls the average cooling rate from 0 to 1 minute and from Tm to 600 ° C to 10 ° C / s or more.

[Target Steel] The austenitic stainless steel targeted 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. Examples of suitable steel types include SUS304 and SUS316.

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

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

[0027]

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. Each slab was hot-rolled and wound up to a thickness of 3.8 mm, and subsequently annealed and pickled in a continuous line to obtain an annealed hot-rolled steel strip. A part was further cold-rolled to a thickness of 1.0 mm, and then subjected to finish annealing and pickling to obtain a cold-rolled annealed steel strip.

[0028]

[Table 1]

The hot rolling coiling temperature and the annealing conditions of the hot rolled steel strip were variously changed as shown in Table 2, and the precipitate occupancy (%) at the grain boundaries was determined for each annealed hot rolled steel strip. . In addition, a corrosion resistance test was performed on the cold-rolled annealed steel strip.

The precipitate occupancy (%) at the grain boundaries was determined according to the method defined above. Here, an optical microscope is used to observe the metal structure, and the length of the etched grain boundary is measured by using an image processing apparatus (manufactured by Interquest) to measure the length of the etched grain boundary.
(LA1125 type).

The corrosion resistance test was performed by a pitting potential measurement method in accordance with JIS G 0577. That is, using a test solution of 350 ppm Cl ⁻ , 40 ° C., the noblest potential Vc ′ ₁₀₀ (V) corresponding to the pitting potential density of 100 μA / cm ² was measured.

[0032]

[Table 2]

As can be seen from Table 2, the occupation of precipitates at the grain boundaries of the annealed hot-rolled steel strip was 10% for all steel types.
In the following invention examples, since there is almost no difference in corrosion resistance among the same steel types, all of them can be considered to exhibit the original corrosion resistance of steel. On the other hand, when the occupation ratio of precipitates at the grain boundaries exceeds 10%, the corrosion resistance of each steel type is significantly deteriorated as compared with those of the invention examples. As described above, in the austenitic, martensitic, or ferrite + austenite two-phase stainless steel types, the precipitate occupation ratio at the grain boundaries of the annealed hot-rolled steel strip is 10%.
It has been confirmed that the following can stably prevent deterioration of corrosion resistance due to grain boundary precipitates.

For reference, FIGS. 1, 2 and 3 show:
Test No. A2 (Invention), B3 (Comparative) and D4 (Comparative) in Table 2
About 10% aqueous oxalic acid (25 ° C) at a current density of 1 A / cm ²
The optical microscope observation photograph at the time of electrolyzing for 10 seconds is shown. FIG. 4 shows a transmission electron micrograph of the crystal grain boundary portion of D3 (Comparative Example) in Table 2. According to FIG. 4, the grain boundaries have a completely grooved structure.

[0035]

According to the present invention, in the production of an austenitic, martensitic or duplex stainless steel strip, the metal structure which causes deterioration of corrosion resistance due to grain boundary precipitates is quantitatively grasped. By doing so, the conditions of hot rolling and annealing that can impart the inherent corrosion resistance of steel could be optimized. If the production method of the present invention is carried out, the annealing conditions after hot rolling can be made lower in temperature and in a shorter time than usual conventionally, which is very advantageous in terms of both quality and cost.

[Brief description of the drawings]

Fig. 1 When anodized 10% oxalic acid aqueous solution (25 ° C) at a current density of 1 A / cm ² for 10 seconds is applied to the cross section of the annealed hot-rolled steel strip (the center of the plate) of test number A2 (inventive example) in Table 2 3 is an optical microscopic observation photograph of FIG.

FIG. 2 A case where a 10% oxalic acid aqueous solution (25 ° C.) is used for electrolysis at a current density of 1 A / cm ² for 10 seconds with respect to a cross section (plate center portion) of an annealed hot-rolled steel strip of test number B3 (comparative example) in Table 2. 3 is an optical microscopic observation photograph of FIG.

FIG. 3 shows a case where the section of the annealed hot-rolled steel strip (the center of the plate) of test number D4 (comparative example) in Table 2 is electrolyzed at a current density of 1 A / cm ² for 10 seconds using a 10% oxalic acid aqueous solution (25 ° C.). 3 is an optical microscopic observation photograph of FIG.

FIG. 4 is a transmission electron micrograph of a crystal grain boundary portion of D3 (Comparative Example) in Table 2.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C21D 9/52 101 C21D 9/52 101 C22C 38/00 302 C22C 38/00 302Z 302H 38/40 38/40 (72) Inventor Kenichi Morimoto 4976 Nomura Minami-cho, Shinnanyo-shi, Yamaguchi Prefecture F-term in Nisshin Steel Co., Ltd.Technical Research Laboratories (reference) AB15 AB18 AB20 AB23 AB24 AB25 AB26 AB27 BA05 BA06 DA05 EA01 FA03 FA12 FA13

Claims (5)

[Claims] A slab of austenitic stainless steel, martensitic stainless steel or ferrite + austenite duplex stainless steel is subjected to hot rolling and annealing in order to produce a steel strip.
It should be less than 600 ° C, and in the annealing process, the material temperature T from 600 ° C to the maximum reached
The average heating rate up to 10 m / s is 10 ° C / s or more, and from Tm to 600 ° C
Control the average cooling rate up to 10 ° C / s or more,
A method for manufacturing a stainless steel strip, wherein deterioration of corrosion resistance due to grain boundary precipitates is prevented, wherein the occupation ratio of precipitates in grain boundaries of annealed hot-rolled steel strips is 10% or less. 2. The maximum reached material temperature Tm in the annealing step is 800.
The method according to claim 1, wherein the soaking time at Tm is from 0 to 1 minute. 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 method according to claim 1, wherein the steel is a martensitic stainless steel containing 12 to 20% by mass of Cr and 10% by mass or less of Ni. 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.