JPH09157754A - Method for producing austenitic stainless cold-rolled steel sheet - Google Patents

Abstract

(57) 【Abstract】 PROBLEM TO BE SOLVED: To obtain a sound metal structure free from the influence of the casting structure, that is, a striped structure, from thin cast pieces of austenitic stainless steel produced by a continuous casting method, Moreover, the manufacturing method of the cold-rolled steel plate provided with the metal structure with fine crystal grains is provided. SOLUTION: A continuously cast thin slab is cast at 1100 ° C or higher,
And, the first step of heating at a temperature T (° C.) satisfying the following formula (1), and subsequently to the first step, cold rolling with a reduction rate of 10% or more and heat treatment with a heating temperature of 1100 to 1200 ° C. are performed. A method for producing an austenitic stainless cold-rolled steel sheet, comprising a second step which is repeated at least once. (1132-3.5 × t) ≦ T ≦ 1250 (1) Here, t: heating time (hour) at temperature T (° C.)

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention produces austenitic stainless cold-rolled steel sheets having good properties such as mechanical properties and pitting corrosion resistance from a thin piece of austenitic stainless steel produced by a continuous casting method. On how to do.

[0002]

2. Description of the Related Art Usually, cold-rolled steel sheet of austenitic stainless steel such as SUS304 has a molten steel thickness of 100 to 25.
It is manufactured by casting 0 mm continuously cast slab (thick slab), hot rolling the slab to form a hot rolled steel sheet, and then repeating cold rolling and annealing treatment once or several times. .
After the hot rolling and the annealing treatment, cleaning such as pickling is carried out if necessary. In the case of this conventional manufacturing method, since a thick cast slab having a thickness of about 200 mm is rolled to a cold-rolled steel sheet having a thickness of 1 mm or less, the workability (reduction rate) during that period is large. Therefore, the cold-rolled steel sheet has almost no structure or characteristics existing in the cast slab, and the cold-rolled steel sheet is manufactured so that its quality is sufficiently durable for general use.

On the other hand, in recent years, for the purpose of reducing the manufacturing cost, a thin cast piece having a thickness of about 1 to 5 mm is manufactured by a continuous casting method, and hot rolling is omitted to perform cold rolling. Development of a technique for manufacturing a cold-rolled steel sheet having a thickness of 1 mm or less has been advanced. In the method of producing an austenitic stainless cold-rolled steel sheet from such a continuously cast thin slab, the reduction rate is smaller than that in the case of producing a cold-rolled steel sheet from a thick slab. Therefore, even in the cold rolled steel sheet of the product, the influence of the casting structure of the thin slab remains, which may cause a problem in quality. Several measures have been taken as a solution.

JP-A-3-254336 discloses that a thin solidified shell is formed on the surface of each roll by a twin roll type continuous casting method for the purpose of producing an austenitic stainless steel sheet having a good surface property. A thin slab is continuously manufactured by crimping the solidified shells, and the thin slab is subjected to solution treatment at 1050-1150 ° C.,
A method of manufacturing a cold rolled steel sheet by the steps of cold rolling-intermediate annealing-cold rolling-finish annealing is disclosed. According to this method, since coarse crystal grains at the time of casting disappear and fine crystal grains are obtained, it is said that a cold-rolled steel sheet with good surface quality without surface roughness can be manufactured.

In addition to this, a method for producing a cold-rolled steel sheet having no anisotropy from a thin cast piece (for example, Japanese Patent Publication No. 4-24413) has been studied. However, when a cold-rolled steel sheet is manufactured from a continuously cast thin slab, the influence of the casting structure tends to remain on the cold-rolled steel sheet. For example, continuous casting thin slab 1050
When cold-rolled steel sheet is manufactured by heating it to ℃ and then cold-rolling it, and from conventional continuous cast slabs (thick slabs), cold-rolled steel sheets are processed by heating, hot rolling, cold rolling and annealing. Comparing the metallographic structures of the cold-rolled steel sheets in the case of production, there are the following differences. That is, in the cold rolled steel sheet manufactured from the thin cast piece, a striped structure which is not found in the cold rolled steel sheet manufactured from the thick cast piece is recognized, and the crystal grains tend to be coarse. The two points of the striped structure and coarse crystal grains are considered to be evidence that the influence of the cast structure has not disappeared.

As described above, regarding the method for producing a cold-rolled steel sheet from a thin slab, despite some technical improvements, a cold-rolled steel sheet produced from a conventional thick slab It is the actual situation that a sound metallographic structure such as is not obtained. Therefore, for cold-rolled steel sheets manufactured from thin cast pieces, since the metallographic structure cannot be said to be sound, general mechanical properties are not always sufficient, and pitting corrosion resistance may be a problem. is there. Therefore, there is a demand for a cold-rolled steel sheet having at least the same metallographic structure and properties as those of a cold-rolled steel sheet manufactured by a process including hot rolling from a conventional continuously cast slab (thick slab).

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has excellent properties from a thin piece of austenitic stainless steel produced by a continuous casting method. Cold-rolled steel sheet, that is, a striped structure is not recognized, and the grain is a cold-rolled steel sheet having a fine metallographic structure, in other words, a cold-rolled steel sheet having a sound metallographic structure in which the influence of the cast structure does not remain. It is intended to provide a manufacturing method.

[0008]

In order to solve the above-mentioned problems, the present inventors have considered the cause of the striped structure observed in the cold-rolled steel sheet produced from the continuously cast thin cast austenitic stainless steel, and Research and development was carried out on measures to prevent residual stripes and measures to reduce the grain size. As a result, the following findings were obtained.

(Origin of Striped Structure) In the cast structure of thin slab, that is, in the structure composed of dendrites and their dendrites, Ni, Cr, Fe, etc. are segregated between the trees, and the segregation is caused. However, it remains in the cold-rolled steel sheet without being eliminated by the subsequent cold rolling and heat treatment. This causes a striped texture.

(Prevention of Remaining Striped Structure and Countermeasures for Miniaturization of Crystal Grains) This can be solved by a combination of the following.

A thin cast piece before cold rolling is subjected to a treatment for reducing segregation of alloy components under appropriate conditions.

Subsequently, cold rolling and heat treatment necessary for causing recrystallization and completely eliminating segregation are performed plural times.

The present invention is based on the above-mentioned findings. "In a method for producing a cold-rolled steel sheet from a continuously cast thin cast austenitic stainless steel, the continuous cast thin cast is 1100 ° C. Above, and following the first step of heating at a temperature T (° C.) satisfying the following formula (1), and subsequently to the first step, cold rolling with a rolling reduction of 10% or more and heating temperatures of 1100 to 1200 ° C. A method for producing an austenitic stainless cold-rolled steel sheet, comprising a second step of repeating heat treatment twice or more.

(1132-3.5 × t) ≦ T ≦ 1250 (1) where, t: heating time (hour) at temperature T (° C.) ”
Is the gist.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The conditions of the above (first step) and (second step) and the embodiments of the present invention will be specifically described below. The manufacturing method and chemical composition of the test material (thin cast piece) used in the following tests and investigations are as follows.

The thin slab (test material) was manufactured by a method of melting in a high-frequency induction melting furnace and casting molten steel adjusted to the chemical composition of JIS SUS304 by a twin roll lateral pouring method. The sample material has a thickness of 1.4 mm and a width of 300 m.
m coil.

Table 1 shows the chemical composition of the test materials.

[0018]

[Table 1]

First Step (Heat Treatment of Thin Cast Piece) In order to find the most appropriate condition, the following test was conducted.

From the test material, thickness 1.4 mm, width 150 m
A test piece having a length of m and a length of 250 mm was cut out, and each test piece was subjected to heat treatment in the atmosphere at a temperature of 900 ° C. to 1200 ° C. and a holding time of 0.5 to 20 hours. The cross-section structure of each test piece after the heat treatment was observed with an optical microscope. In the observation, it was investigated whether or not a cast structure (dendritic crystal) was observed. In addition, this observation
The degree of reduction of segregation of alloy components was investigated, and even if dendrites are not observed, segregation is not always completely eliminated.

FIG. 1 shows the results of observation of the cast structure.
◯: Not observed, Δ: somewhat retained, x: significantly retained From FIG. 1, it can be seen from an optical microscope that the cast structure is not observed at a temperature of 1100 ° C. or higher when the heating time is as long as 10 hours or longer, and even when the heating time is short at 1150 ° C. or higher. , Found that it was not observed. From the above test results, the lower limit of the heating condition of the thin cast piece for reducing segregation of alloy components as much as possible is 110 when the heating time is less than 10 hours.
When the temperature is 0 ° C. and the heating time is 10 hours or more, it depends on the heating temperature and the heating time, and it can be said that the following conditions must be satisfied.

T ≧ (1132−3.5 × t) where t: heating time (hour) at temperature T (° C.) The upper limit of the heating temperature is preferably about 1250 ° C. 125
When the temperature exceeds 0 ° C, the degree of segregation reduction increases, but the oxidation of the surface of the thin cast piece proceeds excessively, resulting in a decrease in yield. In addition, a burner having a large heating capacity is required, and a refractory for the furnace is also required to withstand high temperatures, which is not preferable in terms of equipment and cost.

Therefore, it is desirable that the heating temperature of the thin slab suitable for industrial production, which is the first step of the present invention, be 1100 ° C. or higher and the temperature obtained from the following equation (1).

(1132-3.5 × t) ≦ T ≦ 1250 (1) By the above heat treatment, segregation between dendrites of thin cast slabs is reduced, and the combination with the second step described below is performed. Can be completely eliminated by the processing by.

In the above test, a test piece having a thickness of 1.4 mm was used. Some thin slabs are about 5 mm, but the lower limit of heating temperature is
It was confirmed that the formula (1) should be satisfied.

Second Step (Cold Rolling and Heat Treatment of Thin Slab) In order to understand the conditions of cold rolling and heat treatment of the thin slab suitable for the second step, the following test was conducted.

A test piece (thickness: 1.4 m, taken from the test material
m, width 150 mm, length 250 mm)
In the air atmosphere, heat treatment (first step) was performed at a temperature of 1150 ° C. for 0.5 hours. This condition is a heat treatment condition within the range of the first step of the present invention confirmed by the above-mentioned test. Next, the oxide scale generated on the surface of the test piece during heating was removed by pickling with a mixed acid of hydrofluoric acid and nitric acid (3% by weight of hydrofluoric acid, 6% by weight of nitric acid). Then, using a four-high cold rolling mill with a work roll diameter of 150 mm, the reduction rate is 1
The test piece was rolled once in the range of 0 to 80%. Furthermore, with respect to this test piece, in an air atmosphere, 900 to 1200 ° C.
Heat treatment was performed by holding each temperature for 1 minute. With respect to the test piece subjected to the above-mentioned treatment, the metallographic structure of the cross section was observed with an optical microscope and the segregation of alloy components was investigated with an electron probe microanalyzer (EPMA). In observation with an optical microscope, the presence or absence of residual cast structure (striped structure) was examined. Further, in the segregation investigation by EPMA, Ni was selected as an element, and a line analysis was performed for analysis to evaluate the degree of segregation. In addition, in the case of the segregation investigation by EPMA, a cold-rolled steel sheet (material: JIS SUS304) manufactured by a process including hot rolling was also investigated based on a conventional thick slab for comparison. .
The survey results were evaluated by classifying them into 3 stages shown in Table 2.

[0028]

[Table 2]

FIG. 2 shows the survey results collectively.

As is clear from FIG. 2, in one cold rolling and heat treatment, a striped structure due to the cast structure was observed in the test piece, and segregation of alloy components did not disappear. It was found that even a heat treatment under the conditions of a cold rolling reduction of 80% and a heating temperature of 1200 ° C. did not yield a cold rolled steel sheet having a sound metallographic structure corresponding to ◯ in Table 2.

Next, a test was conducted in which cold rolling and subsequent heat treatment were performed twice. The heat treatment and pickling of the test piece before cold rolling are the same as in the above test. This test piece was subjected to reduction and heat treatment in the order of cold rolling → heat treatment → cold rolling → heat treatment. Cold rolling was on the same mill as in the above test. Moreover, the total reduction rate in the first and second cold rolling was set to 10 to 80%. The heat treatment conditions after cold rolling are as follows: the temperature is 900 to 1200 ° C. (the same temperature for the same test piece for both the first and second times) in the atmosphere, even after the first and second cold rolling. It was 1 minute. With respect to the test piece after the above treatment, the cross-sectional metallographic observation by an optical microscope and the segregation investigation of the alloy components by EPMA were performed. The results were evaluated in the same manner as in the above test (three-stage classification in Table 2).

In FIG. 3, the horizontal axis represents the total rolling reduction and the vertical axis represents the heat treatment temperature.

As is clear from FIG. 3, when the cold rolling and the heat treatment are repeated twice, the total rolling reduction is 19% or more (1
Under the conditions of a rolling reduction of 10% or more) and a heat treatment temperature of 1100 ° C. or more, a cold-rolled steel sheet having a sound metallographic structure without segregation of alloy components and no residual cast structure (striped structure) was obtained. . That is, based on the conventional continuous casting thick slab, it was confirmed that it is possible to obtain a metal structure equivalent to that of the cold-rolled steel sheet manufactured in a process including hot rolling, and thus a cold-rolled steel sheet of equivalent quality. .

In the present invention, the process of performing the cold rolling and the heat treatment at least twice is the second step. As is clear from the above results, the necessary conditions in the second step are that the reduction ratio per cold rolling is 10% or more, and the heat treatment temperature after cold rolling is 1100 ° C. or more. The rolling reduction is 1
Since it needs to be 0% or more, the total reduction rate in the second step (difference in thickness before and after the second step / thickness before the second step × 100) needs to be 19%. become.
The upper limits of the rolling reduction and the total rolling reduction per one time are not particularly limited, but the thickness of the thin cast piece is about 1 to 5 mm, and the thickness of the cold rolled steel sheet of the product is about 0.1 to 1 mm. So this condition naturally limits us. Here, the upper limit of the heating temperature is preferably 1250 ° C. The reason is the same as the reason for selecting the desirable upper limit temperature of 1250 ° C. in the heat treatment in the first step.

It is desirable that the number of repetitions of cold rolling and heat treatment in the second step is large, but from the viewpoint of manufacturing cost or productivity, the number of repetitions is as small as possible (2 times).
Is better. For that purpose, it is necessary to select appropriate cold rolling conditions and heat treatment conditions so that a product having the target characteristics can be obtained by repeating twice. The holding time during the heat treatment in the second step was 1 minute in the above-mentioned examples. Since the heat treatment in this step has the main purpose of refining the crystal grains by recrystallization, the effect can be obtained by heating for a short time. However, in the second step as well, it is necessary to eliminate segregation of the alloy components, so it is effective to lengthen the holding time if necessary.

In the above test, SUS304 was investigated as the material of the austenitic stainless steel in both the first step and the second step. Among austenitic stainless steels, there is a certain range of chemical composition depending on the material. However, since the conditions of the method of the present invention are basically aimed at eliminating segregation of Ni, Cr, etc. in the austenite phase, they are applicable to most austenitic stainless steels.

[0037]

[Example] Melting in a high frequency induction melting furnace, JIS G 4
Molten steel adjusted to the chemical composition of 305 SUS 304 was cast by the twin roll side pouring method to produce a thin cast piece. The size of the thin slab is a coil having a thickness of 1.4 mm and a width of 300 mm. This thin slab was used as a test material of the present invention and comparative examples. In addition, as a conventional example, a cold rolled steel sheet (thickness 0.5 mm) manufactured by a process including hot rolling from a thick slab (thickness 150 mm) by a normal continuous casting method is used as a sample material of the conventional example. used.

Table 3 shows the chemical compositions of the test materials of the present invention and comparative examples and the chemical composition of the cold rolled steel sheet of the conventional example.

[0039]

[Table 3]

In the example of the present invention, a test piece having a thickness of 1.4 mm, a width of 150 mm and a length of 250 mm was cut out from a thin cast piece, and the temperature was 1150 ° C. and the holding time was 0.5 hours in an air atmosphere. Heat treatment was performed (first step).
Next, the oxide scale generated on the surface of the test piece during heating was treated with a mixed acid of hydrofluoric acid and nitric acid (3% by weight of hydrofluoric acid, 6% by weight of nitric acid).
It was removed by pickling with. In addition, the total rolling reduction is 61 by a four-high cold rolling mill with a work roll diameter of 150 mm.
% (First 45%, second 30%), heat treatment temperature 115
Cold rolling and heat treatment were repeated twice under the condition of holding at 0 ° C. (the same temperature for the first time and the second time) for 1 minute (second step). The heat-treated test piece was pickled with a mixed acid of hydrofluoric acid and nitric acid (hydrofluoric acid 3% by weight, nitric acid 6% by weight) to clean the surface of the test piece.

In the comparative example, the second step of the present invention example was subjected to one cold rolling (reduction of 45%) and one heat treatment (1
Hold at 150 ° C. for 1 minute) only.

For these test pieces, segregation of alloy components, metal structure and pitting corrosion resistance were investigated.

The segregation of alloy components was evaluated by performing a linear analysis of Ni by EPMA. Also,
Regarding pitting corrosion resistance, JIS G was used except that the test solution was a 0.1 molar sodium chloride solution (temperature: 60 ° C).
Evaluation was carried out by measuring the pitting potential (V _C '10, V _C '100) by the test according to the regulations of 0577.

FIG. 4 shows the line analysis result of Ni by EPMA. In the example of (a) of the present invention, almost no segregation is observed in Ni. Therefore, it was confirmed that the cold-rolled steel sheet manufactured under the conditions of the present invention had almost no segregation of alloy components, and was equivalent to the cold-rolled steel sheet manufactured from the conventional thick cast piece of (c). On the other hand, in the comparative example manufactured under the condition of the present invention shown in (b), it was proved that the segregation was not sufficiently reduced because the Ni content varied greatly depending on the position.

FIG. 5 shows the segregation investigation (EP
The results of observing the metal structure of the test piece subjected to (MA ray analysis) with an optical microscope are shown. In the example of the present invention of (a), the striped structure due to the residual of the cast structure is not observed, and the metal structure equivalent to that of the cold-rolled steel sheet manufactured from the conventional thick cast piece of (c) is obtained. I understand. On the other hand, in the comparative example, a striped structure was observed as apparent from the metal structure shown in (b), and the influence of the cast structure was not eliminated.

Table 4 shows the measurement results of the pitting potential in the pitting resistance test.

[0047]

[Table 4]

As is apparent from Table 4, the pitting corrosion potentials of the examples of the present invention are the same as those of the conventional examples, and the pitting corrosion resistance is sufficiently improved. On the other hand, it was confirmed that the comparative example had a low pitting potential and had a problem in pitting resistance. As the cause thereof, as estimated from the segregation investigation results of the above alloy components, or the above investigation results of the metal structure, the elimination of the segregation of the alloy components, the refinement of the metal structure, etc. are insufficient, and the cast structure This is because the effect of is left on the cold rolled steel sheet.

As described above, in the cold-rolled steel sheet produced by the method of the present invention, the sound metallic structure targeted by the present invention can be obtained and segregation of alloy components does not occur. It was confirmed that the eating habit was also good.

[0050]

The method of the present invention, that is, the heat treatment (first step) and the treatment of repeating cold rolling and heat treatment twice or more (second step) on the continuously cast thin slabs.
In the austenitic stainless cold-rolled steel sheet produced by the method of applying a striped structure, segregation of alloy components, etc., a product having a sound metal structure without defects due to residual cast structure can be obtained. Cold-rolled steel sheet manufactured by a process including hot rolling based on conventional continuous cast slabs (thick slabs) such as specific mechanical properties, workability / formability, and corrosion resistance including pitting corrosion resistance. A product with properties comparable to is obtained.

As described above, according to the method of the present invention, it is possible to manufacture a cold-rolled steel sheet having no quality problem without performing hot rolling. Therefore, it becomes possible to manufacture an austenitic stainless cold-rolled steel sheet at low cost, which produces a great industrial effect.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a heating time and a heating temperature of a thin slab and a metal structure of a cross section after the heat treatment.

FIG. 2 shows the reduction ratio, heat treatment temperature, and the metallographic structure and alloy of the cross section of the test piece when cold rolling and heat treatment were performed once on the test piece heat-treated at 1150 ° C. for 0.5 hours. It is a figure which shows the relationship with the segregation of a component.

FIG. 3 shows the total reduction ratio, heat treatment temperature, and metallographic structure of the cross section of a test piece when cold rolling and heat treatment were performed twice on a test piece heat-treated at 1150 ° C. for 0.5 hours. It is a figure which shows the relationship with the segregation of an alloy component.

FIG. 4 is a result of investigating segregation of alloy components by performing a linear analysis of Ni using EPMA. (A) is a test piece manufactured under the conditions of the present invention based on a thin cast piece. , (B) is a test piece manufactured based on a thin cast piece under the conditions of the present invention, and (c) is a result of a cold rolled steel sheet manufactured from a conventional thick cast piece.

FIG. 5 is a result of observing a metallographic structure of a test piece investigated for segregation of alloy components by using EPMA with an optical microscope. (A) shows the condition of the present invention based on a thin cast piece. The manufactured test piece, (b) is the result of the test piece manufactured based on the thin cast piece under the conditions of the present invention, and (c) is the result of the cold rolled steel sheet manufactured from the conventional thick cast piece. .

Claims (1)

[Claims] 1. A method for producing a cold-rolled steel sheet from a continuously cast thin slab of austenitic stainless steel, wherein the continuously cast thin slab has a temperature T (° C.) of 1100 ° C. or higher and satisfying the following formula (1). ) In the first step, and subsequent to the first step, a second step of repeating cold rolling with a rolling reduction of 10% or more and heat treatment at a heating temperature of 1100 to 1200 ° C. two or more times. A method for manufacturing an austenitic stainless cold-rolled steel sheet. (1132-3.5 × t) ≦ T ≦ 1250 (1) Here, t: heating time (hour) at temperature T (° C.)