JPH0670253B2 - Method for producing Cr-Ni type stainless steel thin plate having excellent surface quality and material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention uses a so-called synchronous continuous casting process in which there is no relative speed difference between a slab and an inner wall surface of a mold, and the slab thickness is close to the product thickness. The present invention also relates to a method for producing a Cr-Ni-based stainless steel sheet, in which the structure is refined from the cast piece stage to produce a Cr-Ni-based stainless steel sheet having excellent surface properties.

(Prior Art) Conventionally, in order to produce a stainless steel thin plate by using a continuous casting method, a casting having a thickness of 100 mm or more is cast while vibrating the mold in the casting direction, and the surface of the obtained casting is cleaned. After heating to 1000 ° C. or higher in a heating furnace, hot rolling was performed using a hot strip mill consisting of a rough rolling mill and a finishing rolling mill row to obtain a hot strip having a thickness of several mm.

When cold-rolling the hot strip thus obtained, the shape (flatness), material,
In order to secure the surface texture, hot-rolled sheet annealing is performed to soften the hot strip that has undergone strong hot working, and the surface scale and the like are removed by grinding after the pickling step. This conventional process requires a large amount of energy for heating and processing the material in a long hot rolling facility, and it is difficult to say that the manufacturing process is excellent in terms of productivity. In addition, the final product develops a texture because it is manufactured by applying many processes from a slab with a thickness of 100 mm or more, and when the product is pressed by the user, its anisotropy will change. There were also many restrictions on use, which required consideration.

In order to roll a slab with a thickness of 100 mm or more into a hot strip, a long hot rolling facility and a large amount of energy,
Recently, in order to solve the problem of requiring rolling power,
Research on a process for obtaining a slab (thin band) having a thickness equal to or close to that of a hot strip in the process of continuous casting is under way. For example, "iron and steel"'85, A197 to '85, A2
A paper featured at 56 discloses a process for directly obtaining hot strip by continuous casting.
In such a continuous casting process, when the gauge of the slab (strip) to be obtained is at a level of 1 to 10 mm, the twin drum system is used, and the gauge of the slab is 20 to 50 mm.
The twin-belt method is being considered when the level is.

However, it is said that there is still a problem in the casting stage in these continuous casting processes, and it has not reached the stage where the problem regarding the material and surface properties of the product has been solved.

(Problems to be Solved by the Invention) In a process that is being developed as a new process and is premised on obtaining a slab (thin band) having a thickness equal to or close to that of a hot strip by continuous casting, Since the process from casting to product is simplified,
The surface characteristics of stainless steel products will be sensitively affected by the slab properties. That is, in order to obtain a product having excellent surface properties, it is necessary to obtain an excellent slab.

The present invention has been made for the purpose of providing a simple manufacturing process capable of obtaining a Cr-Ni-based stainless steel sheet free from surface defects called uneven luster and roping phenomenon peculiar to stainless steel sheet products.

(Means for Solving the Problems) The gist of the present invention is as follows.

(1) The cooling rate during solidification of Cr-Ni stainless steel represented by 18% Cr-8% Ni steel is 100 ℃ / sec or more by the continuous casting machine in which the mold wall surface moves in synchronization with the slab. As a continuous casting to a thin strip-shaped slab with a thickness of 10 mm or less, the obtained slab is cooled from the solidification temperature as much as possible while suppressing the reheat of the slab and a cooling rate of 100 ° C / sec or more. To prevent the growth of δ grains or γ grains, reduce the γ grain size to an average of 50 μm or less over the entire plate thickness, and then average the temperature range of 1200 ° C to 550 ° C to 10 ° C / sec or more. It is characterized by cooling at a cooling rate to suppress the growth of γ grains and preventing the precipitation of carbides, and then subjecting the product to one or more of hot working, warm working and cold working to obtain a product. A method for manufacturing Cr-Ni-based stainless steel sheet with excellent surface quality and material.

(2) In the manufacturing method defined in claim (1), δ-
Fe.cal (%) = 3 (Cr + 1.5Si + Mo + Ti + Nb) -2.8 (Ni + 0.5Mn +
Δ-Fe.ca defined by 0.5Cu) -84 (C + N) -19.8 (%)
l (%) is -2 to 10% and the δ phase is set as the primary crystal or eutectic of solidification to suppress the growth of γ grains during solidification and complete solidification to lower the starting temperature of crystallization and precipitation of γ phase. The average slab γ grain size is 50 μm or less over the entire plate thickness
-Method for manufacturing Ni-based stainless steel sheet.

(3) 100 ° C / in the manufacturing method defined in claim (1)
Cooling to 1200 ° C after solidification at a cooling rate of sec or more is applied to the slab with a reduction of 5% or less by one or more sets of a combination of an internal cooling roll, a water-cooled roll from the outside, and an air-cooled roll. A method for producing a Cr-Ni-based stainless steel thin plate with excellent surface quality and material that is made under the conditions.

(4) A Cr-Ni-based stainless steel thin plate excellent in surface quality and material, which is obtained by using the component system of claim (2) and the cooling method of claim (3) in the manufacturing method defined in claim (1). Production method.

(5) 100 ° C / in the manufacturing method defined in claim (1)
Cooling to 1200 ° C after solidification at a cooling rate of sec or more is performed using gas and / or liquid. A method for producing Cr-Ni-based stainless steel thin plate with excellent surface quality and material.

(6) A Cr-Ni-based stainless steel thin plate excellent in surface quality and material, which is obtained by using the component system of claim (2) and the cooling method of claim (5) in the manufacturing method defined in claim (1). Production method.

(7) 100 ° C / in the manufacturing method defined in claim (1)
One or more sets of internal cooling rolls, water-cooled rolls from the outside, air-cooled rolls, etc. are used under the condition that a cooling rate of less than 5% is applied to the slab after solidification at a cooling rate of sec or more to 1200 ° C. A method for producing a Cr-Ni-based stainless steel thin plate excellent in surface quality and material, which is made by a cooling method that combines cooling performed by a roll and cooling performed by using a gas and / or a liquid.

(8) A Cr-Ni-based stainless steel thin plate excellent in surface quality and material, which is obtained by using the component system of claim (2) in the manufacturing method defined in claim (1) and using the cooling method of claim (7). Production method.

The present invention will be described in detail below.

Molten steel containing SUS 304 steel as a basic component was cast by a twin roll (twin drum) continuous casting tester of an internal water cooling system to form a ribbon having a thickness of 2 to 4 mm, which was cooled and wound.

The cast piece (thin band) thus obtained was descaled, then directly cold-rolled, finally annealed, and pickled to obtain a 2B product. The surface texture of these products was obtained by continuous casting of molten steel into a slab with a thickness of 100 mm or more, reheating, hot rolling with a hot strip mill, and cold rolling. The surface properties of the products were compared and examined in detail.

As a result, molten steel was cast by an internal water-cooled twin roll (twin drum) continuous casting tester into a ribbon of 2 to 4 mm in thickness, which was cooled and wound, followed by descaling and cold rolling, It was found that the following surface defects may occur in the 2B product obtained by final annealing and pickling.

(1) Roping or orange peel ... Fine irregularities are generated on the surface during cold rolling or during product processing.

(2) Uneven luster: Uneven luster occurs due to sensitization of the structure of the material during winding of the cast slab (thin band), oxidation of grain boundaries, or coarsening of γ grains.

These problems related to the surface properties of products are process-specific problems that are not found in the conventional process and include a process of directly obtaining a thin slab (strip) by continuous casting.

The inventors, as a result of detailed examination of the cause of these problems relating to the surface properties of the product, when the γ grain size of the material before cold rolling is large or when the cooling of the slab in the Cr carbide precipitation temperature range is insufficient. It was clarified that these surface defects occur remarkably in the.

Thus, as a roping measure, the γ grain size of the material before cold rolling is set to a grain size No. 6 or more, that is, 50 μm or less, and as a measure for uneven gloss, control the cooling of the slab in the high temperature range. Have found that it is desirable when taking a process including a process of directly obtaining a thin slab by continuous casting.

These measures will be described in more detail below.

As a material for cold rolling, there are various ideas as follows as means for making the material having a γ particle diameter of 50 μm or less. That is, (1) the γ grain of the thin cast piece itself is reduced, (2) the thin cast piece is subjected to hot working subsequent to casting to be recrystallized into fine grains, (3) the thin cast piece is cold Process, anneal, recrystallize and refine.

The present invention particularly relates to (1) a method for reducing the γ grains of the slab itself.

First, as a method of reducing the γ grain itself of the thin cast piece such as the twin roll method or the single roll method, the γ grain at the time of solidification is made small, and in order to suppress the growth of the γ grain thereafter, it is cooled from a high temperature. This is very important.

According to the above idea, the present inventors
Casting molten steel based on −8Ni with small twin rolls and single rolls in the laboratory, quenching immediately below casting, and paying attention to the surface quality of stainless steel, especially roping that causes surface waviness. Carried out. As a result, it was found that it is desirable to set the γ grain size before cold rolling to γ average grain size No. 6 or more, that is, 50 μm or less as the average grain size. The γ-grains of thin cast pieces cast by the twin roll method or the single roll method grow rapidly after solidification. In order to investigate the cooling rate required to suppress the growth of γ grains in the slab after solidification, the cooling rate and cooling temperature range after solidification were changed using molds of various materials, and the γ grain size was measured before direct cold rolling. And evaluated roping.

As a result, it was found that the grain growth remarkably progressed at 1400 to 1200 ℃ after solidification, and that it was necessary to rapidly cool to 1400 to 1200 ℃.

Fig. 1 is a diagram showing the relationship between the average cold speed from 1400 to 1200 ° C and the slab γ grain size by casting and solidifying small samples of 18.3% Cr-8.3 to 9% Ni alloy in molds of various materials. But 10
At a cooling rate of less than 0 ° C./sec, the particle size was more than 50 μm and the roping was poor.

FIG. 2 shows the relationship between the average cold speed from 1400 to 1200 ° C. and the roping height when the slab is directly cold-rolled. The roping height has decreased due to the increase in cold speed, and is 100 ° C / sec.
At the above-mentioned cold speed, the roping height is less than 0.2 μm, which is equivalent to the current cold rolled SUS304 proper sheet. Ie 140
It was found that the average cooling rate from 0 to 1200 ℃ must be 100 ℃ / sec or more.

Further, in addition to the above-mentioned cooling, it has been found that it is important to take an optimum cooling condition suitable for the alloy composition in relation to the alloy composition.

Figure 3 shows Creq + in the equilibrium diagram of the Fe-Cr-Ni ternary system.
Nieq ≈ 30% Corresponding cross-sectional state diagram (Transaction o
f JWRI.Vol 14.No.1.1985.p125). Creq and Nieq are as follows and are calculated from the components.

Creq = Cr (%) + 1.5 × Si (%) + Mo (%) + Nb (%) + Ti (%) Nieq = Ni (%) + 1 / 2Mn (%) + 1 / 2Cu (%) + 30 (C (%) + N
(%)) Firstly, in the case of small Creq, in the case of, Creq = 17.3% and the primary crystal solidifies in γ and is a perfect γ phase. In this case, the γ phase crystallizes at 1450 ° C or higher just below the liquidus line and then grows. On the other hand, in the case where Creq becomes large, when Creq = 19.5% or more, the primary crystal completes solidification in the δ phase, and the solid phase reaction starts at about 1370 ° C and becomes γ.
Although the phase starts to precipitate and then the growth starts, the growth of γ grains is greatly suppressed as compared with the case where Creq is small as described above. This is fully considered because the high temperature region immediately after casting controls the growth of γ grains. Creq is complicated in these intermediate regions by the addition of a peritectic eutectic reaction, but a component system that causes δ solidification is advantageous for suppressing the growth of γ grains. In particular, a combination of component selection that delays the initiation of γ precipitation by utilizing δ solidification and a method of rapidly cooling the high temperature region is effective for suppressing the growth of γ grains and making them finer.

As a result of experiments with many component systems, δ−Fe.cal (%) = 3 (Cr + 1.5Si + Mo + Nb + Ti) -2.8 (Ni + 1 /
2Mn + 1 / 2Cu) -84 (C + N) -19.8 (%) has been found to be effective when δ-Fe.cal (%) is -2% or more and up to 10%.

5 (a), (b), and (c) are micrographs of metallographic structures, in which δ-Fe.cal (%) was used to cast a 2 mm ingot and the cooled ingots were compared. Show. As is clear from the figure, when δ-Fe.cal (%) is -2.3%, γ solidification occurs and γ grains grow. If δ−Fe.cal (%) is −1.1%, δ
The ferrite remains and the γ grains are small. δ−F
When e.cal (%) is 3.0%, δ is clearly solidified and γ grains remain extremely small, and when δ-Fe.cal (%) is large, both γ grains and δ phase are extremely small. There is. Thus, in combination with the cooling of the slab described above, Cr-Ni
The composition selection in the system has a great influence on the refinement of γ grains of the cast slab, and it is extremely important to control δ-Fe.cal (%) to be not less than -2% and not more than 10%. δ-Fe.cal (%)
Above 10%, these effects saturate.

In this way, since the precipitation temperature of the γ phase is lower in δ solidification than in γ solidification, even if the cooling start after solidification is delayed, a finer γ grain structure can be obtained. Subsequent selection of cooling conditions is important.

The main points of the solution to the problem of the present invention are based on the above idea, but cooling of the slab immediately after solidification, particularly a uniform cooling method, is important. Cr-Ni-based thin-walled casting is another problem that brittleness of the slab during solidification is another problem, but from studies by the present inventors, in the 18Cr-8Ni-based, the temperature range of about 50 ° C below the freezing point is particularly high. It has been found that the brittleness is large, and for example, in the 18Cr-8Ni alloy, the hot ductility of the alloy remarkably recovers at 1390 ° C or lower at the center of the slab (Fig. 4). Therefore, below these temperature ranges, it is effective to use a roll of an internal cooling type and perform roll cooling while performing a slight reduction, for example, in a range of 5% or less. By cooling one set or multiple sets of rolls, it is possible to prevent double heat in the width direction of the slab and effectively cool it.
It is possible to cool at an average cooling rate of up to 00 ° C at 100 ° C / sec or more. Of course, it is also effective to perform uniform cooling by using gas cooling such as high-pressure air or nitrogen in combination with roll cooling, or mist cooling mixed with a small amount of liquid, and these cooling methods are used alone. That is also effective.

Examples of the present invention will be described below.

(Example) A stainless steel in which the amount of Ni was changed mainly based on 18Cr-8Ni system was melted and produced into a cast piece having a thickness of 1 mm to 7.5 mm by using an internal water-cooled twin roll casting machine. Component example is first
It is as shown in the table. δ-Fe.cal (%) was changed from -3.6 to 7.8 (%).

Following the cooling means for blowing high-pressure nitrogen gas on the outlet side of the casting machine, a cooling means by means of an internal water-cooled roll was arranged to cool the slab to prevent recuperation and cooling. Part of the mist cooling means was also placed after the roll cooling. Thus, depending on the cast plate thickness and thus the casting speed,
The average cooling rate up to 1200 ° C was set to 2650 ° C to 120 ° C / sec for cooling. Then, in the range of 1200 to 550 ° C, it was cooled by water cooling or air cooling at a cooling rate of 10 ° C / sec or more and wound up.

As a result of observing the structure of the obtained slab, δ-Fe.cal (%)
Is clearly less than -2%, the γ particle size can be clearly recognized.
When the cooling rate up to 1200 ° C was 300 ° C / sec or more, the average grain size of γ grains was about 40 to 50 µm. δ-Fe.cal (%)
Is -2% or more, it is a slab, and the cooling rate from 1400 to 1200 ° C is
At 100 ° C./sec or more, the δ ferrite phase was also extremely fine, the γ grain boundaries could not be discriminated, and the locally recognized γ grains were as fine as 20 μm or less. When these slabs were directly cold-rolled, no roping was observed on the surface of the slabs having a γ particle size of 50 μm or less, which was good.

However, when δ-Fe.cal (%) is less than -2%, 1400 ℃ ～ 1200
When the cooling rate to ℃ is less than 300 ℃ / sec or δ-Fe.cal
(%) Is -2% or more, the cooling rate from 1400 ℃ to 1200 ℃ is 1
When it was less than 00 ° C / sec, the γ particle diameter exceeded 80 μm, and the surface gloss and roping were poor.

(Effects of the Invention) According to the present invention, it is possible to obtain a Cr-Ni-based stainless steel sheet excellent in surface quality and material by a simple process of directly obtaining a ribbon having a thickness close to the product thickness by continuous casting. it can.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the cold speed of the cast just below the melting point and the γ grain size, and FIG. 2 is the cold speed of the cast directly below the melting point and the cold rolled sheet when the cast was directly cold rolled. Fig. 3 shows the relationship between the roping heights of Fe and Cr in the equilibrium diagram of the ternary Fe-Cr-Ni system.
q ≈ 30% equivalent cross-sectional state diagram, Fig. 4 is a diagram showing the relationship between strain load and cracking just below the melting point of SUS304 steel, and Figs. 5 (a), (b) and (c) are δ- It is a metallographic micrograph showing a comparison of the microstructures of molten steels of various compositions with various Fe.cal (%) continuously cast into 2mm-thick slabs.

Claims (8)

[Claims] Claims: 1. A Cr-Ni-based stainless steel represented by 18% Cr-8% Ni steel is cooled by a continuous casting machine in which the wall surface of the mold moves in synchronism with the slab and the cooling rate during solidification is 100 ° C / Continuously cast into a thin strip-shaped slab with a thickness of 10 mm or less as sec or more, 100 ℃ / sec or more while suppressing cooling of the obtained slab by starting cooling from the solidification temperature as much as possible Cool to 1200 ℃ at a cooling rate to prevent the growth of δ grains or γ grains, reduce the γ grain size to an average of 50 μm or less over the entire plate thickness, and then 1200 to 550 ° C.
The temperature range of 10 ℃ / sec or more to suppress the growth of γ grains and prevent the precipitation of carbides, and then one or two of hot working, warm working and cold working. A method for producing a Cr-Ni-based stainless steel thin plate having excellent surface quality and material, which is obtained by performing the above. 2. The manufacturing method as defined in claim 1, wherein δ-Fe.cal (%) = 3 (Cr + 1.5Si + Mo + Ti + Nb) -2.8 (N
i + 0.5Mn + 0.5Cu) -84 (C + N) -19.8 (%) defined as δ-Fe.cal (%) of -2 to 10% and the δ phase as a solidification primary crystal or eutectic Suppresses the growth of γ grains during solidification, completes solidification, and lowers the starting temperature of crystallization and precipitation of the γ phase.
A method for producing a Cr-Ni-based stainless steel thin plate with excellent surface quality and material that reduces the average grain size to 50 μm or less over the entire plate thickness. 3. A manufacturing method defined in claim (1)
Cooling up to 1200 ° C after solidification at a cooling rate of 100 ° C / sec or more is an internal cooling roll or a water-cooled roll from the outside.
5 with one or more pairs of air-cooled rolls
A method for producing a Cr-Ni-based stainless steel thin plate having excellent surface quality and material, which is performed under a condition in which a rolling reduction of not more than 10% is applied to a slab. 4. A Cr-Ni type stainless steel excellent in surface quality and material which is produced by using the component system of claim (2) in the manufacturing method defined in claim (1) by using the cooling method of claim (3). Method for manufacturing thin plate. 5. A manufacturing method defined in claim (1)
Cooling at a cooling rate of 100 ° C / sec or more to 1200 ° C after solidification is performed using a gas and / or liquid. 6. A Cr-Ni-based stainless steel excellent in surface quality and material, which is produced by using the component system of claim (2) in the manufacturing method defined in claim (1) by using the cooling method of claim (5). Method for manufacturing thin plate. 7. A manufacturing method defined in claim (1)
A combination of an internal cooling roll, a roll that is water-cooled from the outside, an air-cooling roll, etc. under the condition that a cooling rate of 5% or less is applied to the slab after the solidification at a cooling rate of 100 ° C / sec or more until 1200 ° C A method for producing a Cr-Ni-based stainless steel thin plate having excellent surface quality and material, which is achieved by a cooling method that combines cooling performed by more than one set of rolls and cooling performed by using gas and / or liquid. 8. A Cr-Ni type stainless steel excellent in surface quality and material, which is produced by using the component system of claim (2) in the manufacturing method defined in claim (1) by using the cooling method of claim (7). Method for manufacturing thin plate.