JPH11277104A - Manufacture of copper-containing austenitic stainless steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the generation of two-ply crack at the time of hot-rolling a copper-containing austenitic stainless steel strip. SOLUTION: A slab of the austenitic stainless steel containing 1.0-5.0 wt.% Cu and 0.004-0.05 wt.% B is cut into a predetermined length and an oxidation inhibitor is applied on the cut surfaces of one side and the other side of the cut slab (S4). After that, the salb is heated (S5) and hot rolling is executed (S6). In this way, the oxidation in the austenitic grain boundary which is easy to be oxidized by microsegregation of the copper and boron is restrained and the generation of the two-ply crack at the time of hot rolling is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a copper-containing austenitic stainless steel strip having excellent antibacterial properties.

[0002]

2. Description of the Related Art Hot and cold-rolled steel strips made of copper-containing austenitic stainless steel (hereinafter sometimes abbreviated as copper-containing stainless steel) have recently attracted attention as materials having excellent antibacterial properties. Demand is expanding for applications that make use of the characteristics of hygiene, such as kitchen equipment. The copper-containing hot-rolled stainless steel strip is usually manufactured by the following manufacturing process. Smelting-continuous casting-flaw removal-heating-hot rolling. The copper-containing cold-rolled stainless steel strip is manufactured, for example, by adding the following manufacturing process after the hot rolling. Annealing pickling-cold rolling-annealing pickling-refinement. Hereinafter, a slab manufactured by continuous casting is referred to as a slab.

[0003] In the hot rolling, a heated slab is subjected to rough hot rolling, a head and tail portion (hereinafter referred to as a crop) of the rough hot rolled material is cut, and then the rough hot rolled material is subjected to finish hot rolling. This is done by: The reason why the crop is cut in this manner is to adjust the shape of the head and tail portions that have become irregular due to the rough hot rolling and to smoothly bite the rough hot rolled material into the finishing mill. The crop is usually cut to a predetermined standard cutting length, for example 250 mm. Hereinafter, the cut length of the crop is referred to as a crop cut amount.

With respect to copper-containing stainless steel having excellent antibacterial properties, several prior arts have been disclosed. For example,
Japanese Patent Application Laid-Open No. 9-176800 discloses a copper-containing stainless steel in which a copper-containing stainless steel is hot-rolled and then heat-treated to uniformly disperse a second phase mainly composed of copper in a matrix, and a method for producing the same. ing. Although this copper-containing stainless steel has excellent antibacterial properties, according to the investigations of the present inventors, microsegregation of copper is likely to occur, and a defect called double fracture may occur during hot rolling. is there. Two-piece cracking is a phenomenon in which a hot-rolled sheet material is separated into two layers in the sheet thickness direction. The location of the occurrence of double fractures in copper-containing stainless steel is almost limited and occurs at the head and tail of the hot-rolled steel strip. Therefore, in order to solve this problem, measures are taken to increase the crop cut amount.

[0005]

The increase in the crop cut amount during hot rolling lowers the yield of the copper-containing stainless steel hot-rolled steel strip. Further, since there is a limit to the crop cut amount that can be cut at a time, if the crop cut amount is increased, it is necessary to perform the cutting operation a plurality of times. For this reason, the efficiency and productivity of hot rolling decrease.

An object of the present invention is to solve the above problems and to provide a method for producing a copper-containing austenitic stainless steel strip capable of preventing the occurrence of splitting during hot rolling without increasing the amount of crop cut. It is to provide.

[0007]

According to the present invention, a copper-containing austenitic stainless steel strip is cut into a predetermined length, and the cut slab is heated and hot-rolled. In the manufacturing method,
Copper-containing austenitic stainless steel contains 1.0% copper.
A method for producing a copper-containing austenitic stainless steel strip, characterized in that the cut surface of one and the other side of a slab is coated with an antioxidant before heating.

According to the present invention, the slab is heated after coating one and the other cut surfaces of the slab made of copper-containing austenitic stainless steel with an antioxidant. Microsegregation of copper is apt to occur at the austenite grain boundaries, and the austenite grain boundaries with the segregated copper are easily oxidized during heating, but the oxidation is suppressed by the coating with the antioxidant. As a result, the oxidized region having a small deformability is reduced, so that the separation of two layers in the thickness direction of the plate material, which is called a double crack, is prevented. Further, since the upper limit value of copper is properly determined, it is possible to avoid a decrease in hot workability.

The copper-containing austenitic stainless steel according to the present invention is characterized in that it contains 0.004 to 0.05% by weight of boron.

According to the present invention, the copper-containing austenitic stainless steel contains an appropriate amount of boron, so that hot workability is improved. Thereby, the occurrence of edge cracks during hot rolling can be prevented.

[0011] The antioxidant of the present invention is characterized by containing silicon carbide as a main component.

According to the present invention, since the antioxidant contains silicon carbide as a main component, oxidation of austenite grain boundaries of the slab can be effectively suppressed.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a process diagram showing a process for producing a copper-containing austenitic stainless steel strip having excellent antibacterial properties according to a first embodiment of the present invention. With reference to FIG. 1, a method for producing a copper-containing austenitic hot-rolled stainless steel strip will be described. In step 1, melting of copper-containing austenitic stainless steel (hereinafter, sometimes referred to as copper-containing stainless steel) is performed. Melting is performed by finishing and refining molten steel roughly refined in a converter by a vacuum decarburization method (VOD method).

The copper-containing stainless steel according to the present embodiment contains C: 0.1% by weight or less, Si: 2% by weight or less, M:
n: 5% by weight or less, Cr: 10 to 30% by weight, Ni: 5
-15% by weight, 1.0-5.0% by weight of copper (Cu), and 0.004-0.05% by weight of boron (B). The reason why the range of Cr is limited to 10 to 30% by weight is that if it is less than 10%, the corrosion resistance as austenitic stainless steel cannot be secured, and if it exceeds 30%, the manufacturability and workability deteriorate. Because. The reason why the range of Ni is limited to 5 to 15% by weight is that if it is less than 5%, it becomes difficult to stabilize the austenite phase, and if it exceeds 15%, the cost increases significantly.

Cu is an element that improves antibacterial properties.
The range of Cu is limited to 1.0 to 5.0% by weight,
If it is less than 1%, the antibacterial property cannot be sufficiently secured, and if it exceeds 5%, the hot workability is impaired as follows. That is, when the Cu content exceeds 5%, a Cu-rich phase is precipitated, and the deposited Cu-rich phase is heated at a temperature equal to or higher than the melting point at the time of heating described below. by this,
Since the Cu-enriched phase is hot-rolled in a liquid phase at the time of rough hot rolling of hot rolling described later, liquid film embrittlement occurs and hot workability is impaired.

B is an element that improves hot workability.
The reason that the range of B is limited to 0.004 to 0.05% by weight is that if it is less than the lower limit, the effect of improving hot workability of B is poor.
This is because ear cracks are likely to occur during hot rolling, and if the upper limit is exceeded, liquid film embrittlement occurs during hot rolling as in the case of Cu, and hot workability is impaired.

In step 2, a continuous casting of the molten copper-containing stainless steel melt is performed. Continuous casting is performed by pouring molten steel into a water-cooled mold and continuously drawing solidified slab of the surface layer. The drawn slab (hereinafter, referred to as a slab) is cut into a predetermined length, for example, 8 m, after solidifying to the inside. The copper-containing stainless steel slab has the following metal structure.

An optical microscope sample (hereinafter referred to as a microscopic sample) was collected from the slab, and the optical microscope structure was observed. The speculum sample 14 is sampled from one cut surface 15 of the slab as shown in FIG. 2A, and the sampling position is the center of the cut surface 15 in the plate thickness direction and the plate width direction. The speculum sample 14 is shown in FIG.
As shown in the figure, the surface 16 to be observed by the optical microscope is a surface parallel to one side surface 17 of the slab. As shown in FIG. 3, the optical microscope structure of the microscopic sample 14 includes a Cr—B, P, S precipitate phase 19 and a Cu—Mn precipitate phase 2.
0 and pores 21 are recognized. These are all present along the austenite grain boundaries. Each of these precipitation phases 1
9, 20 are Cu, due to micro-segregation during solidification.
B is produced by enrichment. The identification of the components of the precipitated phase in the optical microscope structure was performed using a fluorescent X-ray analyzer.

The reason why micro-segregation is clearly recognized in the copper-containing stainless steel is that the diffusion of elements is slower in the austenitic stainless steel than in the ferritic stainless steel. In addition, near the center of the thickness of the cut surface 15, which is the final solidification position of the slab, there is a macro-segregation region 23, which is an aggregate of the micro-segregation, as shown by hatching in FIG. The region 23 extends over the entire length of the slab in the length direction.

In step 3, the slab is scraped, and surface defects of the slab are ground and removed by a grinder.
In step 4, an antioxidant is applied. This process is performed on one and the other cut surfaces of the slab,
Nothing else is done. Thereby, the cut surface of the slab is covered with the antioxidant. The application of the antioxidant may be performed over the entire cut surface, or may be performed only in the macro-segregation region 23. The antioxidant is in powder form, mixed with water or a solvent to form a slurry, and then applied with a brush. After the application, the antioxidant is air-dried, and after drying, the application and drying are repeated again. The antioxidant is applied to, for example, three layers, and the total applied thickness is 1 mm or less. The reason for applying the antioxidant and the component composition of the antioxidant will be described later.

In step 5, the slab is heated. This process is performed by inserting a slab into a heating furnace and heating the slab with combustion heat from a burner. The heating furnace is open to the atmosphere, and oxide scale is formed on the surface of the slab. The heating conditions for the slab in the heating furnace are, for example, a heating temperature of 1230 ° C. and a heating time of 1 hr. In step 6, hot rolling is performed in a hot rolling facility. The slab heated in the heating furnace is roughly hot-rolled by two rough rolling mills 26 as shown in FIG.
After passing through 7, finishing hot rolling is performed to a predetermined thickness by a finishing rolling mill 28 of seven stands, and then wound on a tension reel 30. Thus, the production of the copper-containing hot-rolled stainless steel strip is completed. In the crop shear 27, the crop which is the head and tail of the rough hot rolled material is cut. The crop cut amount is the standard cut length of 250 mm, which can cut off irregular shapes of the head and tail.

As described above, an antioxidant is applied to one and the other cut surfaces of the slab before heating. This processing is performed based on the following findings of the present inventors. FIG. 6 is a schematic diagram showing an optical microscopic structure near the cut surface of the slab after heating when the cut surface of the slab is heated without applying an antioxidant. The speculum sample shown in FIG. 6 is obtained by the sample collection method shown in FIG. 2, and corresponds to the speculum sample shown in FIG. Therefore, the region extending up and down on the right side of FIG. 6 represents the cut surface of the slab. From FIG. 6, it can be seen that the oxide scale 33 is formed on the cut surface of the slab by heating, and the oxidized phase 34 extending along the grain boundary is formed at the austenite grain boundary. As described above, austenite grain boundaries are enriched with Cu, B, and the like by microsegregation, and the heating temperature is as high as 1230 ° C., so that the Cu, B-enriched phase exhibits a semi-molten state during heating. Therefore, the diffusion of oxygen at the austenite grain boundaries during heating is accelerated, and the oxidized phase 34 proceeds rapidly and deeply from the cut surface toward the inside of the slab.

FIG. 7 is a cross-sectional view of the steel material after heating, rough hot rolling and finish hot rolling when the cut surface of the slab is heated without applying an antioxidant, as viewed from the side surface in the longitudinal direction. It is. Therefore, the vertical direction in FIG. 7 represents the thickness direction. After the heating, as shown in FIG. 7A, an oxidized phase 34 extending inward in the longitudinal direction of the slab is formed on one and the other cut surfaces 15 of the slab. After the rough hot rolling, as shown in FIG.
7 and the occurrence of cracks 39 in the tail portion 38 are observed.
The crack 39 is generated based on a difference in deformability between the copper-containing stainless steel material and the oxidized phase 34 during rough hot rolling. The head 37 and the tail 38
Is cut at the first position 40 and the second position 41, respectively. Due to this cutting of the crop, when the length of the crack 39 is short, the crack 39 is cut off together with the crop as in the tail 38. On the other hand, when the length of the crack 39 is long, the crack 39 remains without being cut off with the crop as in the head 37.

After the finish hot rolling, as shown in FIG. 7 (3), generation of a double crack 44 on the head of the finished hot rolled material 43 is observed. The two-piece crack 44 corresponds to the rough hot-rolled material 3
The crack 39 remaining in 6 is extended in the rolling direction by finish hot rolling. Therefore, the length of the two splits 44 is determined based on the sheet thickness ratio between the rough hot rolled material 36 and the finished hot rolled material 43, that is, the rolling ratio. However, the length of the two splits 44 generated at the head of the finished hot-rolled material 43 is formed longer than the length based on the rolling ratio. This is because, at the head of the finished hot-rolled material 43, the air existing in the cracks 39 is confined by the finish hot rolling, and the air compressed to a high pressure causes the crack to propagate downstream in the rolling direction. It is. On the other hand, in the tail portion of the finished hot-rolled material 43, even if the crack remains, the crack 39
Since the air existing inside is released downstream in the rolling direction,
The length of the double split 44 is extended to a length based on the rolling ratio.

As described above, when the cut surface of the slab is heated without applying an antioxidant, the oxidation progresses deeply at austenite grain boundaries which are easily oxidized by microsegregation of Cu, B, etc. 2 during hot rolling
Splitting occurs. Therefore, it is necessary to suppress the oxidation of the austenite grain boundaries in order to prevent splitting of the two pieces.
For that purpose, it is effective to apply an antioxidant to the cut surface of the slab. As described above, the reason why the antioxidant is applied to the cut surface of the slab in the present embodiment is as described above.

In this embodiment, Mg is used as an antioxidant.
It is preferable to use an O-based antioxidant, and
It is particularly preferable to use a system antioxidant. The reason is as follows. One and the other cut surfaces of the copper-containing stainless steel slab are coated with a SiC-based antioxidant and Mg.
An O-based antioxidant was applied, and the slab to which the antioxidant was not applied was placed in a heating furnace, and heat treatment was performed under the conditions of a heating temperature of 1300 ° C., a heating time of 1 hr, and an atmosphere of air. After the heat treatment, a microscopic sample was collected by the method shown in FIG. 2 and an optical microscopic structure was observed. Table 1 shows the component compositions of the SiC-based and MgO-based antioxidants, respectively.

[0027]

[Table 1]

The MgO-based antioxidant contains MgO as a main component, and further contains Al and the like. The SiC-based antioxidant contains SiC as a main component, and further contains Cr ₂ O ₃ and B ₂ O ₃ . The melting start temperature of the SiC-based antioxidant is 900 ° C.
At 00 ° C., it is in a semi-molten state. The antioxidant is applied in three layers by brush and the total applied thickness is 0.8 mm.

Table 2 shows the observation results of the optical microscope structure of each microscopic sample. Table 2 shows the measured values representing the degree of oxidation,
That is, the average oxidation depth and the maximum oxidation depth are shown.

[0030]

[Table 2]

From Table 2, it can be seen that the oxidation of the slab coated with the antioxidant is significantly suppressed as compared with the uncoated slab.
It can be seen that the slab coated with the iC-based antioxidant is further suppressed in oxidation as compared with the slab coated with the MgO-based antioxidant.

As described above, the present embodiment is configured so that the slab is heated after the antioxidant is applied to the cut surface of the slab made of copper-containing stainless steel. Can suppress oxidation of austenite grain boundaries that are easily oxidized by microsegregation of. Therefore, the oxidized region having a small deformability is reduced, and it is possible to prevent the occurrence of cracks during hot rolling. In addition, since an appropriate amount of Cu is contained, excellent antibacterial properties can be exhibited. Further, since B is contained in an appropriate amount, hot workability is improved, and occurrence of edge cracks can be prevented.

As described above, this embodiment relates to a method for producing a copper-containing austenitic hot-rolled stainless steel strip. In contrast, a copper-containing austenitic cold-rolled stainless steel strip can be manufactured as follows. That is, when producing a copper-containing austenitic cold-rolled stainless steel strip, subsequent to the hot rolling in the present embodiment, annealing pickling-cold rolling-annealing pickling-
It may be manufactured by adding a normal series of manufacturing steps consisting of refining, or may be manufactured by performing the heat treatment described in JP-A-9-176800 and then adding the normal series of manufacturing steps. Good. Further, in the present embodiment, although the slab is manufactured by continuous casting, instead of manufacturing a steel ingot of copper-containing stainless steel, the slab is manufactured by slab-rolling the steel ingot. Is also good.
Further, although the antioxidant is applied to the cut surface of the slab by a brush, the antioxidant may be configured to cover the cut surface of the slab by spraying the antioxidant.

According to a second embodiment of the present invention, the copper-containing austenitic stainless steel contains 0.1% by weight or less of C, 2% by weight or less of Si, 5% by weight or less of Mn, and Cr:
10 to 30% by weight, Ni: 5 to 15% by weight, Cu: 1.
It may be configured to contain 0 to 5.0% by weight. This is a component configuration excluding B from the first embodiment. Other configurations of the present embodiment are the same as those of the first embodiment. This embodiment has excellent antibacterial properties similarly to the first embodiment, and can prevent the occurrence of cracks during hot rolling.

(Example) An invention example in which an antioxidant is applied to one and the other cut surfaces of a slab made of a copper-containing austenitic stainless steel and heated, and a comparative example in which heating is performed without applying an antioxidant. About 2 at the time of hot rolling
The splitting rate was compared. The invention example was manufactured by the manufacturing process shown in FIG. 1, and the comparative example was manufactured by the manufacturing process of FIG. 1 except for the antioxidant application process. The component of the copper-containing austenitic stainless steel is C: 0.
037%, Si: 0.62%, Mn: 1.77%, N
i: 9.33%, Cr: 18.07%, Cu: 3.75
%, B: 0.007%. As the antioxidants of the invention examples, the SiC-based antioxidants shown in Table 1 were used. The crop cut amount before finish hot rolling in the hot rolling process is
Both the invention example and the comparative example had a standard cutting amount of 250 mm. FIG. 8 shows a comparison between the occurrence rate of double cracks of the invention example and the comparative example. From FIG. 8, it can be seen that the double crack occurrence rate of the invention example is 0%, which is much lower than the comparative example.

As described above, according to the present invention, it is possible to reliably prevent the occurrence of splitting during hot rolling under a standard crop cut amount. Therefore, the yield of the copper-containing hot-rolled stainless steel strip is improved. Further, since the crop cutting amount is standard, it is not necessary to repeat the crop cutting work. Therefore, the efficiency and productivity of hot rolling are greatly improved.

[0037]

As described above, according to the first aspect of the present invention, after the cut surface of the copper-containing austenitic stainless steel slab is coated with the antioxidant, the slab is heated. Oxidation of austenite grain boundaries is suppressed, and the occurrence of cracks during hot rolling is prevented. Therefore, the yield of the copper-containing austenitic stainless steel strip is improved.

According to the second aspect of the present invention, since the copper-containing austenitic stainless steel contains an appropriate amount of boron, hot workability is improved. Thereby, the occurrence of edge cracks during hot rolling can be prevented.

According to the third aspect of the present invention, since the antioxidant contains silicon carbide as a main component,
Oxidation of austenite grain boundaries of the slab can be effectively suppressed.

[Brief description of the drawings]

FIG. 1 is a process diagram showing a process for producing a copper-containing austenitic stainless steel strip having excellent antibacterial properties according to a first embodiment of the present invention.

FIG. 2 is a schematic view showing a method for collecting a sample for an optical microscope.

FIG. 3 is a schematic diagram showing an optical microscope structure of a slab before heating.

FIG. 4 is a schematic view showing a macro-segregation region of a slab before heating.

FIG. 5 is a simplified side view showing the configuration of a hot rolling facility.

FIG. 6 is a schematic diagram showing an optical microscopic structure near the cut surface of the slab after heating when the cut surface of the slab is heated without applying an antioxidant.

FIG. 7 is a cross-sectional view of each steel material after heating, after rough hot rolling and after finish hot rolling when heated without applying an antioxidant to a cut surface of the slab, as viewed from a side surface in the longitudinal direction.

FIG. 8 is a graph showing the incidence of double cracks of the invention example and the comparative example in comparison.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Melting 2 Continuous casting 3 Flaw removal 4 Application of antioxidant 5 Heating 6 Hot rolling 14 Sample for optical microscope 15 Cutting surface 26 Rough rolling mill 27 Crop shear 28 Finishing rolling mill 30 Tension reel 33 Oxidation scale 34 Oxidation phase 44 2 sheets Crack

Claims (3)

[Claims] 1. A method for producing a copper-containing austenitic stainless steel strip, comprising cutting a copper-containing austenitic stainless steel slab into a predetermined length, heating the cut slab, and hot rolling the copper-containing austenitic stainless steel strip. For stainless steel, copper is 1.0
A method for producing a copper-containing austenitic stainless steel strip, comprising coating an antioxidant on one and the other cut surface of a slab before heating. 2. The method according to claim 1, wherein the copper-containing austenitic stainless steel contains 0.004 to 0.05% by weight of boron. 3. The method for producing a copper-containing austenitic stainless steel strip according to claim 1, wherein the antioxidant contains silicon carbide as a main component.