JPH07331330A - Manufacture of chromium-nickel stainless steel sheet excellent in surface quality and manufacturing equipment for cast strip - Google Patents

Abstract

PURPOSE:To produce a cold rolled sheet excellent in surface quality from a cast Cr-Ni stainless steel strip cast by a thin-wall casting method. CONSTITUTION:A Cr-Ni stainless steel, such as 18%Cr-8%Ni steel as a typical example, is continuously cast into a cast strip of <=10mm thickness by a continuous casting machine in which the wall surface of a mold moves synchronously with a cast slab. Successively, hot rolling is done at 900-1200 deg.C at 10-50% draft, followed by heat treatment at 900-1200 deg.C for >=5sec. Then, the resulting hot rolled plate is coiled at <=600 deg.C, descaled, and cold-rolled and then annealed and pickled or bright-annealed. If necessary, temper rolling is done.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for casting a Cr-Ni type stainless steel into a thin-walled slab having a plate thickness of 10 mm or less, and cold-rolling the thin-walled slab to obtain a thin plate product.
The present invention relates to a method for producing a Cr-Ni-based stainless steel thin plate having excellent surface quality.

In recent years, a technique for directly obtaining a thin cast piece having a plate thickness of 10 mm or less from molten steel by casting has been developed, and an actual scale test has been conducted. With this new technology, the hot rolling process can be simplified or omitted. Conventionally, a slab having a plate thickness of more than 100 mm is hot-rolled by a hot-rolling mill by spending a great deal of energy, and the merit of simplifying or omitting the hot-rolling process not only lowers the manufacturing cost but also reduces the environmental impact. Is also desired. Hereinafter, a process including a step of casting a thin strip having a plate thickness of 10 mm or less from molten steel is referred to as a new process, and a process including a step of hot rolling a slab into a thin strip is referred to as a current process.

[0003]

2. Description of the Related Art Conventionally, when a Cr-Ni type stainless steel cold-rolled thin sheet represented by 18% Cr-8% Ni steel is manufactured by a new process, the surface of the product is roughened (called orange peel or roping). Was a problem.

For example, a paper described in "Materials and Processes" Vol. 1 (1990), p770 of the Iron and Steel Institute of Japan describes a phenomenon that the surface quality of SUS304 thin plate products manufactured by the new process deteriorates. ing. In this paper, it is said that orange peel-like roughening (roping) occurs on the surface of the cold-rolled sheet due to the coarse grains of the material before finish cold rolling. In order to prevent this, it is effective to refine the crystal grains of the material before finish cold rolling by the following two measures. 1) Perform hot rolling-hot rolled sheet annealing on the slab. For example, 1
16% hot rolling is performed at 200 ° C., and solution treatment is performed at 1150 ° C. for 1 minute. 2) The slab is cold-rolled twice with intermediate annealing. For example, rolling is performed at room temperature with a reduction ratio of 10%, intermediate annealing is performed, and then finish rolling is performed.

Also, "Materials and Processes" Vol. 4 (199)
1), p996 describes that surface roughening (roping) of cold-rolled sheets is improved by temper rolling at a high-pressure reduction rate, but γ-phase instability is caused because the material, especially elongation, decreases. It is stated that the components need to be adjusted so that they have a component system, that is, a high Md30. For example, Md30 to 30
It is said that roping and workability (elongation) can be made comparable to those of current process materials by performing temper rolling at a temperature of ℃ and a reduction rate of 1%.

Also, "Materials and Processes" Vol. 4 (199)
1), p997 describes that the surface roughness (roping) of the cold-rolled sheet is improved by increasing the amount of δ-ferrite and designing the components such that the γ-phase becomes unstable.

Further, in JP-A-63-421, a thin slab having a plate thickness of 10 mm or less is hot-rolled at 800 ° C. or more and 50% or less and wound at 650 ° C. or less to cause anisotropy. It is stated that small sheets (small earrings) and excellent corrosion resistance can be produced. However, this technique is intended to prevent the reduction of anisotropy due to hot rolling. For that reason, the hot rolling reduction rate is 50% or less, and hot rolling is performed to adjust the shape. Therefore, it is considered that roping occurs in the thin plate manufactured by this technique.

Further, in Japanese Patent Laid-Open No. 2-133528, it is stated that hot rolling at a rolling reduction of 60% or less in a temperature range of 900 ° C. or higher recrystallizes the structure of the slab and improves roping. ing. It is stated that this technique improves roping regardless of the presence or absence of annealing after hot rolling, and the temperature history of the slab after hot rolling is 900-
It only mentions cooling the temperature range of 550 ° C. at a cooling rate of 50 ° C./s or more. The present inventors have disclosed in
Using the technique of Japanese Patent No. 33528, a Cr-Ni type stainless steel thin plate was experimentally manufactured, but the surface quality could not be made excellent. Therefore, it was found that a complete recrystallized structure cannot be obtained only by the hot rolling conditions of 900 ° C. or higher and 60% or lower.

[0009]

In order to produce a Cr--Ni type stainless steel sheet having a good surface quality by the new process, the above-mentioned known means are applied to improve productivity, equipment, and product quality stability. In terms of aspects, various problems occur. For example, when hot rolling and solution heat treatment are performed on a slab, an extra solution heat treatment step is required. If the slab is cold-rolled twice with intermediate annealing, the time required for cold rolling-annealing is doubled. If the temper rolling ratio is increased, the material (elongation) is reduced. If the composition is controlled, the types of steel that can be manufactured are limited. Even if hot rolling is performed at 900 ° C or higher and 60% or lower,
Stable surface quality cannot be obtained. And other problems will arise.

The present invention provides a new process for Cr-Ni
The purpose of the present invention is to solve the surface quality problem, which is a problem when manufacturing stainless steel thin plates, without lowering the productivity as compared with the current process.

[0011]

The present invention is configured as follows to achieve the above object. The feature is Cr
-Continuous casting of molten Ni-based stainless steel into a thin-walled slab with a plate thickness of 10 mm or less by a continuous casting machine in which the wall surface of the mold moves in synchronization with the slab, and then a rolling rate of 10 to 50% in a temperature range of 900 to 1200 ° C. Hot rolling, followed by 900-12
A heat treatment is carried out in the temperature range of 00 ° C. for 5 seconds or more, followed by winding in the temperature range of 600 ° C. or less, descaling of this steel strip, cold rolling, and annealing pickling or bright annealing. Further, if necessary, temper rolling may be performed subsequent to the above process. When cooling to the winding temperature after the heat treatment, it is desirable to cool the temperature range of 900 to 600 ° C. at a cooling rate of 10 ° C./s or more, preferably 20 ° C./s or more. The rolling temperature is preferably 1150 to 1000 ° C, and the heat treatment temperature after rolling is preferably 1150 to 1050 ° C.

The apparatus for carrying out the method of the present invention is constructed as follows. That is, a twin-drum type continuous casting machine that casts a Cr-Ni-based stainless molten steel into a slab having a plate thickness of 10 mm or less, a conveyor band that conveys the slab obtained by the casting machine,
A hot rolling mill that rolls the conveyed slab at a reduction rate of 10% or more, a heat treatment device that heats and / or keeps the heat of the hot rolled slab, and the heat treated slab at 10 ° C. /
A Cr-Ni-based stainless steel thin strip slab production apparatus, characterized in that a cooling device for cooling at a cooling rate of at least 2 seconds and a winding device for winding up the slab are sequentially arranged. The carrier zone may be provided with a heating function for heating the slab and controlling the inlet temperature of the hot rolling mill. The transport belt may be covered with a cover so as to be controlled to be in a degassing atmosphere. A bridle roll may be arranged upstream of the hot rolling mill. A heat insulating cover is provided between the hot rolling mill and the heat treatment furnace downstream thereof, and the heat treatment furnace is
You may comprise so that it may be hold | maintained in the oxygen atmosphere of% or less. Further, in the cooling zone following the heat treatment furnace, the slab may be cooled by a slit cooling header. That is,
The present invention relates to a twin-drum type continuous casting machine for casting molten Cr-Ni-based stainless steel into a slab having a plate thickness of 10 mm or less, and a hot rolling machine for heating the slab obtained while conveying the slab obtained by the casting machine. A heat transfer belt having a heating function for controlling the inlet side temperature of the steel and being covered with a cover to control the degassing atmosphere, and a bridle roll arranged upstream to heat the rolled slab with a rolling reduction of 10% or more. Hot rolling mill, having the function of heating and / or keeping the hot rolled slab 6
% Of oxygen atmosphere, a heat treatment device for cooling the heat-treated slab with a slit cooling header at a cooling rate of 10 ° C./sec or more, and a winding device for winding the slab. And a Cr-Ni-based stainless steel thin strip caster manufacturing apparatus.

[0013]

In order to refine the crystal grains of the thin slab, it is most efficient to carry out hot rolling by means of a hot rolling machine directly connected to the casting machine. However, it is inefficient to carry out solution heat treatment after cooling to room temperature after hot rolling as in the above literature. Therefore, by performing hot rolling with a hot rolling machine directly connected to the casting machine and subsequently performing heat treatment to sufficiently advance recrystallization, by performing low temperature winding,
A method of promoting recrystallization and performing solution treatment is desirable.

The inventors of the present invention have investigated a method for obtaining a sufficient recrystallized structure by a process of omitting annealing of a hot rolled sheet, and have revealed that a sufficient recrystallized structure can be obtained by performing heat treatment immediately after hot rolling. I chose The feature of this heat treatment is that the heat treatment necessary for recrystallization is performed without lowering the temperature of the slab after hot rolling to 800 ° C or lower.

FIG. 3 shows a twin-drum type thin-wall continuous casting and rolling heat treatment line according to the present invention. A thin-walled slab 2 cast from a twin-drum type thin-wall continuous casting machine 1 is heated in an atmosphere control zone 3 to generate heat. The inlet temperature of the inter-roll mill 6 is controlled. It is conveyed by the pinch rolls 4 under the drum, and the bridle rolls 5 on the downstream side secure the inlet side tension of the hot rolling mill 6 to prevent meandering at a high reduction rate. In the hot rolling mill 6,
It is rolled down by about 10 to 50%. Because of continuous rolling, the shape of the slab that has been rolled down due to the thermal expansion of the rolling roll deteriorates over time into medium elongation. Therefore, the hot rolling mill can bender shape control and cross the roll between runs. Further, there is a possibility that roll wear and heat cracks may occur due to continuous rolling, and rolls can be recombined during running. A plate thickness gauge 13 that performs feedback to shape control is arranged downstream of the hot rolling mill 6, and a heat insulating cover 15 is provided to suppress the temperature drop of the hot steel plate 2 on the delivery side of the hot rolling mill 6. It is provided. In the heat treatment furnace 7 following this, the temperature of the slab is controlled by a jet burner or an open flame burner, and the oxygen concentration of the atmosphere is maintained at about 2 to 6% or less. A cooling zone 8 is provided on the exit side of the heat treatment furnace 7, and the slab is cooled by a slit cooling header and cut by a shearing machine 11 to a predetermined hot steel plate length. The coil 14 is continuously wound around the coil 14 while being switched.

In the new process, the slab heating step before hot rolling, which was performed in the current process, is completely omitted. Further, since the plate thickness is thin, the cooling rate after solidification is significantly higher than that of the slab. Therefore, precipitates such as MnS and Cu ₂ S, which had been precipitated in the current continuous cast slab, are in solid solution.
Hot rolling is performed in this state, and 80
If it is cooled down to 0 ° C or less, fine precipitates are precipitated along the dislocations introduced by hot rolling, and in order to obtain a perfect recrystallized structure in the subsequent hot-rolled sheet annealing, heat treatment immediately after hot rolling is required. It takes a long time. Therefore, in order to efficiently recrystallize the structure of the hot rolled sheet, the slab temperature should be 800 ° C immediately after hot rolling.
It is effective to perform a heat treatment without lowering the temperature below to obtain a completely recrystallized structure.

Next, the reasons for limiting the constituent features of the present invention will be described. As the steel type, Cr-Ni type stainless steel represented by 18% Cr-8% Ni steel was targeted. Since general carbon steel undergoes two phase transformations after solidification, Cr-Ni
This is because the roping problem due to the coarsening of the structure is less likely to occur as compared with the system stainless steel.

On the other hand, if the thickness of the slab exceeds 10 mm, the crystal grains will become coarse during solidification, and in order to refine the crystal grains by hot rolling recrystallization, the slab with a plate thickness of 10 mm or less is used. A high-pressure reduction ratio is required rather than a hot rolling reduction ratio. for that purpose,
It becomes necessary to arrange a plurality of hot rolling machines or to make a single huge hot rolling machine, and the economic effect is lost. The preferable range of the thickness of the slab is 3 to 5 mm.

The conditions of the hot rolling temperature and the rolling reduction were determined by the following experiment. That is, SUS3 in the laboratory
04 steel was cast into a thin slab with a plate thickness of 4 mm, and 1250 to 850
After hot rolling 5 to 50% in the temperature range of ℃ to make a hot-rolled steel strip, it is passed through a heat treatment furnace kept at 1000 ℃ for 5 seconds, and then secondary cooling is performed at 600 ℃ or less. Rolled up at temperature. After descaling, the slab was cold-rolled at a reduction rate of 50% to evaluate the roping of the cold-rolled sheet surface.
The result is shown in FIG.

When the hot rolling temperature is higher than 1200 ° C., the recrystallized grains become coarse, so that the roping cannot be improved. Also,
If the temperature is lower than 900 ° C., recrystallization does not proceed due to precipitation of MnS and Cu ₂ S during hot rolling. When the hot rolling reduction was lower than 10%, a complete recrystallized structure could not be obtained and roping occurred. From the above results, hot rolling is performed at a rolling reduction of 10% or more in the temperature range of 900 to 1200 ° C. However, in order to carry out hot rolling of more than 50% on thin slabs, multiple huge hot rolling mills are required, and the characteristics of the new process cannot be utilized.
It was set to 0% or less. A desirable range is a reduction rate of 20 to 40%.
The hot rolling temperature is 1000 to 1150 ° C.

The heat treatment conditions after hot rolling were determined by the following experiment. That is, SUS304 steel was cast into a plate having a thickness of 4 mm, hot rolled at 1100 ° C. for 20%, and then heated by induction heating in a temperature range from 1250 to 850 ° C.
A heat treatment of holding for 50 seconds was performed. After descaling, the slab was cold-rolled at a reduction rate of 50% to evaluate the roping of the cold-rolled sheet surface. The result is shown in FIG. When the heat treatment temperature after hot rolling exceeds 1200 ° C., the recrystallized grains become coarse, and when the temperature is less than 900 ° C., recrystallization does not proceed and roping occurs. When the heat treatment time was shorter than 5 seconds, a complete recrystallized structure could not be obtained, so that roping occurred. From the above test results, the heat treatment after hot rolling is 900 to 1200 ° C.
The temperature range was maintained for 5 seconds or longer. The preferred range is 105
It is 10 to 30 seconds at 0 to 1150 ° C.

Thereafter, the slab is wound up at 600 ° C. or lower. If this condition is not met, carbides will precipitate at the grain boundaries, causing intergranular corrosion when the material is pickled and impairing the surface gloss of the product. A desirable range is 550 ° C or lower.

Further, the slab after the heat treatment is cooled in a temperature range of 900 to 600 ° C. at a cooling rate of 10 ° C./second or more. If the cooling rate is less than 10 ° C./sec, carbides are precipitated at the grain boundaries, causing grain boundary corrosion when the material is pickled, and impairing the surface gloss of the product. A desirable cooling rate is 20 ° C./second or more.

[0024]

[Example] As shown in Table 1, Cr-Ni-based stainless steel having a composition based on 18% Cr-8% Ni steel was melted, and a plate thickness of 2 to 2 was obtained by an internal water-cooled vertical twin roll continuous casting machine.
Slabs of varying thickness between 10 mm were cast. After casting, the temperature of the slab is controlled by a jet burner to 900-12
Hot rolling was performed in the temperature range of 00 ° C. Hot rolling reduction is 10
-50%. After hot rolling, the temperature of the slab was controlled by a jet burner, and heat treatment was performed in the temperature range of 900 to 1200 ° C. for 5 seconds or more and winding was performed at 600 ° C. or less. In the comparative material, hot rolling conditions, heat treatment conditions after hot rolling, and winding conditions are outside the scope of the present invention.

After that, the material was pickled, descaled, cold-rolled, and then subjected to normal annealing or bright annealing. The surface properties of the products thus obtained were investigated. Particular attention was paid to the roping height and gloss of the product surface. As shown in Table 1, the one shown in the present example has a fine recrystallized structure obtained by optimizing the hot rolling conditions and the heat treatment conditions after hot rolling, and the cooling rate control after that is also added. All showed good surface quality. On the other hand, in the comparative method, the hot rolling temperature, the rolling reduction, the heat treatment after the hot rolling were insufficient, and the cooling control was not performed thereafter, so that the roping was large and the surface gloss was also poor.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

INDUSTRIAL APPLICABILITY The present invention enables the production of a cold rolled steel sheet having good surface quality by controlling the hot rolling conditions and the subsequent heat treatment conditions in a new Cr-Ni system stainless steel process. Is. Therefore, its industrial effect is great.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a hot rolling temperature and a hot rolling reduction performed on a thin cast product and a roping height of a cold rolled product.

FIG. 2 is a diagram showing a relationship between a heat treatment temperature and time subsequent to hot rolling of a thin cast slab and a roping height of a cold rolled product.

FIG. 3 is a diagram showing an outline of an apparatus for producing a Cr—Ni-based stainless steel thin strip slab in the method of the present invention.

[Explanation of Codes] 1 twin drum type thin wall continuous casting machine 2 thin wall slab 3 atmosphere control zone 4 pinch roll 5 bridle roll 6 hot rolling mill 7 heat treatment furnace 8 cooling zone 9 winder 10 winding pinch roll 11 shearing machine (Shear) 12 Pinch roll before shearing machine 13 Plate thickness gauge 14 Coil 15 Heat insulation cover

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tetsuro Takeshita 20-1 Shintomi, Futtsu City, Chiba Shin Nippon Steel Co., Ltd.Technology Development Division (72) Inventor Shunji Shoda 3434 Shimada, Hikari City, Yamaguchi Prefecture Inside the Hikari Steel Works (72) Inventor Takashi Arai 3434 Shimada, Hitsu City, Yamaguchi Prefecture Shin Nippon Steel Inside the Hikari Works, Japan (72) Hideki Oka 3434 Shimada, Hikari City, Yamaguchi Prefecture Made in Japan (72) Inventor Yoshikatsu Nohara 20-1 Shintomi, Futtsu City, Chiba Nippon Steel Co., Ltd. Technology Development Division

Claims (11)

[Claims] 1. A continuous casting machine in which the wall surface of a Cr-Ni stainless steel melt moves in synchronism with the slab to obtain a plate thickness of 10
Continuous casting into thin-walled slabs of mm or less, then 900-120
Hot rolling with a rolling reduction of 10 to 50% in a temperature range of 0 ° C.
Subsequently, heat treatment is carried out in the temperature range of 900 to 1200 ° C. for 5 seconds or more, followed by winding in the temperature range of 600 ° C. or less, descaling of this thin strip-shaped slab, cold rolling, and subsequent annealing pickling or A method for producing a Cr-Ni-based stainless steel sheet having excellent surface quality, characterized by performing bright annealing. 2. A method for producing a Cr—Ni-based stainless steel sheet having excellent surface quality, which is characterized by performing temper rolling subsequent to the method according to claim 1. 3. 900 to 60 following the heat treatment of claim 1.
The method for producing a Cr-Ni-based stainless steel sheet having excellent surface quality according to claim 1, wherein the cooling rate in the temperature range of 0 ° C is 10 ° C / sec or more. 4. A continuous casting machine in which the wall surface of a Cr-Ni stainless steel melt moves in synchronism with the cast slab to obtain a plate thickness of 10
Continuously cast into thin-walled slabs of mm or less, then 1000 to 11
Hot rolling with a reduction rate of 10 to 50% is performed in a temperature range of 50 ° C., heat treatment is performed for 5 seconds or more in a temperature range of 1050 to 1150 ° C., and then winding is performed in a temperature range of 600 ° C. or less. A method for producing a Cr-Ni-based stainless steel sheet having excellent surface quality, which comprises descaling the strip-shaped slab, cold rolling, and subsequently performing annealing pickling or bright annealing. 5. A Cr-Ni system stainless steel melt having a plate thickness of 10
Twin-drum continuous casting machine for casting slabs of mm or less, a conveyor belt for transporting the slabs obtained by the casting machine, a hot rolling machine for rolling the transported slabs at a rolling reduction of 10% or more. ,
A heat treatment device for heating and / or keeping the hot rolled slab heat-retaining, a cooling device for cooling the heat treated slab at a cooling rate of 10 ° C./sec or more, and a winding device for winding the slab. Cr characterized by being sequentially arranged
-Ni-based stainless steel ribbon casting device. 6. A Cr—Ni type stainless steel thin strip slab according to claim 5, which has a heating function of heating the slab in the conveying zone to control the inlet temperature of the hot rolling mill. Manufacturing equipment. 7. The Cr- according to claim 6, wherein the conveying belt is covered with a cover and is controlled to be in a degassing atmosphere.
Ni-stainless steel ribbon casting device. 8. The apparatus for producing a Cr—Ni-based stainless steel thin strip slab according to claim 5, wherein a bridle roll is arranged upstream of the hot rolling mill. 9. The Cr according to claim 5, wherein a heat insulating cover is provided between the hot rolling mill and the heat treatment furnace downstream thereof, and the heat treatment furnace is maintained in an oxygen atmosphere of 6% or less. -Ni-based stainless steel ribbon casting device. 10. The apparatus for producing a Cr—Ni type stainless steel thin strip slab according to claim 5, wherein the cooling strip following the heat treatment furnace cools the slab by a slit cooling header. 11. A Cr-Ni system stainless steel melt having a plate thickness of 1
Twin-drum type continuous casting machine for casting into a slab of 0 mm or less, having a heating function of heating the slab obtained by conveying the slab obtained by the casting machine and controlling the inlet temperature of the hot rolling mill A conveyor belt that is covered with a cover and controls to a degassing atmosphere,
A bridle roll is arranged upstream and the conveyed slab is
A hot rolling mill that rolls at a rolling reduction of 0% or more, a heat treatment apparatus that has a function of heating and / or heat-retaining the hot rolled slab and that holds an oxygen atmosphere of 6% or less, the heat treated casting 10 pieces by slit cooling header
An apparatus for producing a Cr-Ni-based stainless steel thin strip slab, which comprises a cooling device for cooling at a cooling rate of not less than C / sec and a winding device for winding the slab.