JPH02133522A - Production of cr-ni stainless steel sheet having excellent surface quality and material quality - Google Patents

Abstract

PURPOSE:To produce the stainless steel sheet having excellent surface quality and material quality by casting a Cr-Ni stainless steel to a thin strip-like ingot under specific conditions then to cooling to adjust the grain sizes, and subjecting the ingot adequately to hot, warm and cold working. CONSTITUTION:The Cr-Ni stainless steel represented by 18% Cr-8% Ni steel is continuously cast to the thin strip-like ingot having <=10mm thickness by a continuous casting machine in which the wall surfaces of the casting mold moves in synchronization with the ingot while the cooling rate at the time of solidifying is kept at >=100 deg.C/sec. This ingot is cooled at >=100 deg.C/sec cooling rate down to 1200 deg.C by starting the cooling from the solidification temp. as far as possible while the recuperation of the ingot is suppressed to prevent the growth of gamma grains or gamma grains, by which the gamma grain sizes are reduced to <=50mu in average over the entire part of the thickness. Further, the ingot is cooled at >=10 deg.C/sec average cooling rate in a 1200 to 550 deg.C temp. region to suppress the growth of the gamma particles and to prevent the precipitation of carbide; thereafter, the ingot is subjected to one or >=2 kinds of the hot working, warm working and cold working, and is thereby made into the product.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention uses a so-called synchronous continuous casting process in which there is no relative speed difference between the slab and the inner wall surface of the mold, so that the thickness of the slab can be reduced to a size close to the product thickness. The present invention relates to a method for manufacturing a Cr-Ni stainless steel thin plate having excellent surface properties by refining the structure from the slab stage.

(Prior art) Conventionally, in order to manufacture a stainless steel thin plate using a continuous casting method, a mold is vibrated in the casting direction and a thickness of 100 mm is produced.
After the surface of the obtained slab is treated and heated to 1000°C or higher in a heating furnace, it is hot rolled by a hot strip mill consisting of a rough rolling mill and a finishing rolling mill row. It was applied as a hot strip several mm thick.

When cold rolling the Horatos L-lip obtained in this way, the hot strip that has undergone intense hot processing is softened to ensure the shape (flatness), material quality, and surface quality required for the best product. In addition to annealing the hot-rolled sheet to improve the quality of the steel, scale and the like on the surface were removed by grinding after the pickling process.

This conventional process requires a large amount of energy to heat and process JrA #'4 in a long hot rolling facility, and is hardly an excellent manufacturing process in terms of productivity. . In addition, because the final product is manufactured from a cast slab with a thickness of 100 mm or more and subjected to many processes, a clustered Mi weave is developed, and when the product is subjected to press processing etc. by the user, the anisotropic There were many restrictions on its use, such as the need to consider gender.

1 f) In order to solve the problem of requiring a long hot rolling equipment and a large amount of energy and rolling power in order to roll a slab with a thickness of 0 mm or more into a real strip, we have recently developed an i ! ! Research is underway on a process for obtaining slabs (thin strips) with a thickness equal to or close to that of real strips during continuous casting.

For example, "Tetsu to Hagane" °85, A197-'85, A2
The article featured in 56 discloses a process for obtaining hot strip directly by continuous casting. In such a continuous casting process, the twin tram method is used when the gauge of the strip to be obtained is 1 to 10 mm, and the twin tram method is used when the gauge of the strip is 2 mm.
At the level of 0 to 50 cylinders, a twin belt system is being considered.

However, in these continuous casting processes, there are still problems at the casting stage, and the problems regarding the material and surface quality of the product have not yet been resolved.

(Problem B to be solved by the invention) In a process that is being developed as a new process and is based on the premise of obtaining a slab (thin strip) with a thickness equal to or close to that of hot strip by continuous casting. Because the process from casting to products is simplified,
The surface characteristics of stainless steel button crystals are sensitively affected by the properties of the slab. That is, in order to obtain a product with excellent surface properties, it is necessary to obtain an excellent slab.

The present invention has been made with the object of providing a simple manufacturing process capable of obtaining a Cr-Ni stainless steel plate free of surface defects called uneven gloss and roving phenomenon that are characteristic of thin stainless steel plate products.

(Failure to solve the problem) The gist of the invention is as follows.

(1) Cr-Ni represented by 18%Cr-8%Nj steel
A continuous casting machine, in which the mold wall surface moves in synchronization with the slab, cools the stainless steel at a cooling rate of 100 °C during the second solidification.
Continuously cast into thin strip slabs with a thickness of 10 mm or less at 1/sec or more, and solidify the obtained slabs as much as possible/starting cooling from 'Jr degrees to suppress reheating of the slabs to 100 mm. ℃/s
The growth of δ grains or γ grains is prevented by cooling to 1200°C at a cooling rate of ec or higher, and the γ grain size is reduced to an average of 50μ throughout the plate thickness.
m or less, and then cooled in the temperature range of 1200 ° C to 550 ° C at an average cooling rate of 10 ° C / sec or more to suppress the growth of one grain and prevent carbide precipitation, and then hot working, C with excellent surface quality and material quality, characterized by products that have been subjected to one or more types of warm processing and cold processing.
Method for manufacturing r'-Ni stainless steel thin plate.

(2) In the manufacturing method defined in claim (1), δF
e, cal(χ)=3(Cr+1.5Si+Mo+Ti
+Nb)-2,8(Ni+0.5Mn+0.5Cu)-
δ-Fe defined as 84(C+N)-19,8(X)
.

Cal (χ) is set to -2 to 10% to make the δ phase primary or eutectic during solidification, suppress the growth of γ grains during solidification, complete solidification, and lower the starting temperature for crystallization and precipitation of the γ phase. A method for producing a thin plate of Cr-Ni stainless steel axe having excellent surface quality and material quality in which the grain size of each slab is 50 μm or less on average throughout the plate thickness.

(3) In the manufacturing method defined in claim (1), 100
Cooling to 1200℃ after solidification at a cooling rate of ℃/sec or higher is achieved by applying a reduction of 5% or less to the slab using one or more sets of rolls that combine internal cooling rolls, externally water-cooled rolls, air-cooled rolls, etc. A method for manufacturing a Cr-Ni stainless steel tAgJ plate with excellent surface quality and material quality under applicable conditions.

(4) Manufacture of a Cr-Ni stainless steel thin plate with excellent surface quality and material quality by using the cooling method of claim (3) with the component system of claim (2) in the manufacturing method defined in claim (]). Method.

(5) In the manufacturing method defined in claim (1), 100%
A method for manufacturing a CrNi stainless steel thin plate having excellent surface quality and material quality, in which cooling to 1200'C after solidification is performed at a cooling rate of at least C/sec using gas and/or liquid.

(6) Cr-Ni stainless steel with excellent surface quality and material quality, which is produced by using the cooling method of claim (5) with the component system of claim (2) in the manufacturing method defined in claim (1). Manufacturing capacity for thin fabric;

(7) In the manufacturing method defined in claim (1), 100%
'(: Cooling to 1200℃ after solidification at a cooling rate of 1/sec or more is applied to the slab with a reduction of 5% or less, using a combination of internal cooling rolls, externally water-cooled rolls, air-cooled rolls, etc. Cr whose surface quality and material quality are improved by a cooling method that combines cooling performed with one or more sets of rolls and cooling performed using gas and/or liquid.
- A method for producing a Ni-based stainless steel thin plate.

(8) The l specified in claim (1)! ! A method for manufacturing a thin plate of "r-Ni stainless steel 1" in which surface quality and material quality are met using the cooling method of claim (7) with the component system of claim (2).

The present invention will be explained in detail below.

Molten steel containing SO5304 steel as a basic component was cast into a thin strip of 2 to 4 bars in length using an internal water-cooled twin-roll continuous casting machine, which was cooled and wound.

The slabs (thin strips) thus obtained were descaled, then directly cold rolled, final annealed, and pickled to obtain 2B products. Continuously cast to obtain a slab with a thickness of 100 mm or more, reheated, hot rolled using a strip mill, and cold rolled to determine the surface properties and details of the resulting product. I did a comparative study.

As a result, the molten steel was cast into a thin strip with a thickness of 2 to 4 mm using an internal water-cooled twin-roll (twin-drum) continuous casting machine, which was cooled and wound, which was then descaled and cold-rolled. It has been found that the following surface defects may occur in the final annealed and pickled 2B product.

(1) Roving and orange beer: Fine irregularities occur on the surface during cold rolling or product processing.

(2) Uneven gloss...Uneven gloss occurs due to the sensitization of the structure of the material during winding of the slab (thin ribbon), grain boundary oxidation, or T-grain).

These problems concerning the surface properties of products are unique to the process, which does not occur in conventional processes, and involves the process of directly obtaining thin slabs (thin strips) by continuous casting.

As a result of a detailed study of the causes of problems related to the surface properties of these products, the inventors found that the single grain size of the material n before cold rolling was large, or insufficient cooling of the slab in the Cr carbide precipitation temperature range. It was found that these surface defects occur significantly in some cases.

Thus, as a countermeasure against roving, it is necessary to set the gamma grain size of the material before cold rolling to a grain size number of 6 or more, that is, 50 ttrn or less, and as a countermeasure against gloss unevenness, it is necessary to control the cooling of the slab in the high temperature range. However, it has been shown that it is desirable to employ a process that involves obtaining thin slabs directly by continuous casting.

These measures will be explained in more detail below.

There are various ways of thinking as follows as means for making a material for cold rolling with a grain size of 50 um or less. That is, (1) reducing the T-grain of the thin slab itself; (2) hot working the thin slab after casting to make it recrystallized; (3) cold working the thin slab. Processing, annealing, recrystallization to refine grains, etc.

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

First, as a method to reduce the T grains themselves in thin slabs such as the twin roll method and single roll method, in addition to reducing the γ grains during solidification, cooling from a high temperature is used to suppress the subsequent growth of T grains. This is very important.

Based on the above idea, the present inventors have developed various compositions.
Molten steel based on 8Cr-8Ni is formed into S7η using a small twin roll or single roll in a laboratory, and then rapidly cooled immediately after casting to improve the surface quality of stainless steel I/S steel, especially the roving that causes surface waviness. The research was conducted with a focus on As a result, the grain size before cold rolling mentioned earlier was determined to be T's average grain size No.
It has been found that it is desirable to have an average particle diameter of 50 μm or less. T-grains in thin slabs cast by the twin-roll method, single-roll method, etc. grow rapidly after rust hardens. In order to investigate the cooling rate necessary to suppress the growth of T grains in the slab after solidification, we used molds made of various materials to vary the cooling rate and cooling temperature range after solidification, measured the single grain size, and then directly cold-rolled the slab. and evaluated roving.

As a result, it was found that grain growth significantly progresses between 1i4oo and 1200°C during solidification.
It was found that it was necessary to rapidly cool the

Figure 1 shows that small samples of 18.3% Cr-8, 3-9% Ni alloy were cast and solidified into molds made of various materials.
It is a diagram showing the relationship between the average 6 speeds from 0 to 1200°C and the grain size of a slab. At 6 speeds below 100°C/sec, the grain size exceeded 50 μffl, and roving was poor.

FIG. 2 shows the relationship between the average 6 speeds from 1400 to 1200°C and the roving height when the slab is directly cold rolled. The roving height has decreased due to the increase in 6th gear and 1
The current roving height is 5 at 6 speeds over 00℃/sec.
The length is 0.211m or less, which is comparable to that of S304 proper cold-rolled product sheets. In other words, it was found that the average 6 speed from 1400 to 1200°C must be 100°C/sea or more.

Furthermore, in addition to the above-mentioned cooling, it has been found that in relation to the alloy composition, it is important to adopt optimal cooling conditions suitable for the alloy composition.

Figure 3 shows the phase diagram of the surface corresponding to the Creq mo N1 eq #30% equivalent part in the equilibrium phase diagram of the Fe-Cr-Ni ternary system, based on the literature (Transaction of JWRl, Vo
l 14. No. 1.1985. p! 25). Creq and N1eq are as follows and are calculated from the components.

Creq=Cr(χ)+1.5XSi(χ)+Mo(χ
)+Nb(χ)+Ti(X)Nieq=Ni(χ)+1
/2Mn(z)+1/2Cu(χ)+30(C(X)+
N(χ)) First, if Creq is small, in the case of ■, C
At the request of 17.3%, the initial product is solidified with T and has a complete γ phase. In this case, the γ phase crystallizes and grows at 1450° C. or higher, just below the liquidus line. On the other hand, when Cr6q increases and Cr exceeds ~19.5%, the initial product completes solidification in the δ phase, and as a solid phase reaction, the γ phase begins to precipitate at about 1370°C, after which growth begins. Compared to the case of small Creq described in 1., the growth of a single grain is greatly suppressed.

This is entirely conceivable because the high temperature region immediately after casting controls the growth of γ grains. When Creq is in these intermediate ranges, the peritectic reaction is added and becomes complicated, but in order to suppress the growth of a single grain, a component system that causes δ solidification is advantageous. In particular, the combination of component selection that utilizes δ6λ solids to delay the start of T precipitation and a method of rapidly cooling the high temperature region is effective for suppressing the growth of γ grains and making them fine.

As a result of experiments with a component system of poly(δ-Fe, cal(χ)~3(Cr+1.5Si+Mo
+Nb+Ti)-2,8(Ni+1/2Mn+1/2C
u) −84(C+N)−19,8(χ) δ−
It has been found that it is effective to keep Fe and cal(χ) from -2% to 10%.

The microstructure photographs of metal B in Figures 5 (a), (b), and (C) are obtained by comparing the structures of 2 mm slabs cast with different compositions of δFe and cal(X) and cooled. show. As is clear from the figure, in the case of δ-Fe and cal(χ) of -2.3%, T solidification occurred and one grain grew. δ-Fe, ca
In the case where l('1) is 1.1%, δ ferrite remains and one grain is small. When δ-Fe, cal (X) is 3.0%, δ solidification clearly occurs, and γ grains remain extremely small. Furthermore, when δ-Fe, cal (χ) is large, γ grains, δ Both phases remain extremely small. In this way, in conjunction with the slab cooling mentioned earlier, Cr-Ni
The composition selection in the system has a big influence on the fineness of the γ grains in the slab.
It is extremely important to control δ-Fe, cal ($) from -2% to 10%. δ−
These effects are saturated when PC1cal(χ) exceeds 10%.

In this way, compared to T solidification, in δ solidification, the precipitation temperature of the γ phase is lower, so even if the start of cooling after solidification is delayed, a finer T grain structure can be obtained. The subsequent selection of cooling conditions is important.

The main point of the solution to the problem of the present invention is based on the following two ideas, and it is important to cool the slab after the cylinder a, especially a uniform cooling method. Another problem with thin-walled Cr-Ni casting is the embrittlement of the slab during solidification, but research by the present inventors has shown that 18Cr-8Ni is particularly susceptible to embrittlement in the temperature range of about 50"C below the freezing point. High temperature embrittlement is large, for example, in 18Cr 8Ni@gold, 139% at the center of the slab.
It has been found that the high-temperature ductility of the alloy is significantly recovered at temperatures below 0"C (Figure 4). Therefore, below these temperatures, internal cooling rolls are used to reduce the reduction by a small amount, for example, below 5%. An effective method is to cool the slab while rolling it down within a range of 1. By cooling one set or multiple sets of rolls, it is possible to prevent recuperation and effectively cool the slab across its width. It is possible to cool at an average cooling rate of 100 °C/sec or more up to 1200 °C.Of course, in combination with roll cooling, gas cooling such as high-pressure air or nitrogen, or mist cooling mixed with a small amount of liquid can be used. It is also effective to carry out uniform cooling, and it is also effective to use these cooling methods alone.

Examples of the present invention will be described below.

(Example) Stainless steel based on the 18Cr-8Ni system with mainly varying amounts of Ni was melted and produced into slabs with a thickness of 1 mm to 7.5 mm using an internal water-cooled twin roll casting machine. . Examples of ingredients are shown in Table 1. δFe, cal
(%) was changed from -3.6 to 7.8 (χ).

On the exit side of the casting machine, a cooling means for spraying high-pressure nitrogen gas was followed by a cooling means for internal water-cooled rolls to cool the slab and prevent heat recuperation. Some mistakes were made [the cooling means was also placed after the roll cooling. Thus, depending on the cast plate thickness and therefore the casting speed,
Average cooling rate up to 1200℃ from 2650℃ to 120℃
/sec. Thereafter, the film was cooled in the range of 1200 to 550°C by water cooling or air cooling at a cooling rate of 10°C/sec or more, and then wound up.

As a result of observing the structure of the obtained slab, it was found that δ-Fe, ca
When 1(χ) is less than about -2%, the γ particle size can be clearly recognized, and the cooling rate from 1400 to 1200 °C is 300 °C.
When the temperature is below ℃/sec, the average particle size of one particle is 40 to 50 J.
It was about nn. A slab with δ-Fe, cal (Z) of -2% or more, with a cooling rate of 1 from 1400 to 1200℃
When the temperature was 00° C./sec or higher, the δ ferrite river was also extremely wide, and the γ grain boundaries could not be discerned, and the locally observed γ grains were as fine as 20.1 or less. When these slabs were directly cold-rolled, no roving was observed on the surface of the slabs with a gamma grain size of 50 μm or less, and the results were good.

However, δ-Fe, eal (%) is -2% less than 71! ], the cooling rate from 1400℃ to 1200℃ is 300℃[/
sec or less, or when δ-Fe, cal(χ) is -2% or more, the cooling rate from 1400℃ to 1200℃ is 100℃.
If the temperature is less than ℃/sec, the particle size is 80! It exceeded the ura, and both surface gloss and roving were poor.

(Effects of the Invention) According to the present invention, a thin strip of Cr-Ni stainless steel with excellent surface quality and material quality is produced by a simple process of directly obtaining a thin strip with a thickness close to the product thickness by continuous casting. Obtainable.

[Brief explanation of the drawing]

Part 1 is a diagram showing the relationship between the 6 speed of a slab just below the melting point and the γ grain size, and Figure 2 is a diagram showing the relationship between the 6 speed of a slab just below the melting point and the cold rolled plate when the slab is directly cold rolled. Figure 3 is a diagram showing the relationship between the roving heights of
The circle is a diagram showing the relationship between positive load just below the melting point of SUS304m and crack occurrence, and Figures 5 (a), (b), and (C) are δ-
It is a metallurgical microscopic structure photograph showing a comparison of the assembled fibers of 2++ unl' Roving height (Am) Fig. 3 Cre < (%) Diameter j' of the convex (Ag) Distortion load ('c) ＞ '+-Pa,qai之X)w-1,il;:K100'
1 Figure 6/CX

Claims (1)

[Claims] (1) Cr-Ni steel represented by 18% Cr-8% Ni steel
A continuous casting machine, in which the mold wall surface moves in synchronization with the slab, cools the stainless steel at a cooling rate of 100°C/s during solidification.
Continuous casting is carried out into thin strip-shaped slabs with a thickness of 10 mm or less at EC or more, and cooling of the obtained slabs is started from the solidification temperature as much as possible to suppress reheating of the slabs and cast at a rate of 100 ° C / sec or more. The growth of δ grains or γ grains is prevented by cooling to 1200°C at a cooling rate, and the γ grain size is refined to an average of 50 μm or less throughout the plate thickness, and then the temperature range from 1200°C to 550°C is reduced by 10°C/
Cooling at an average cooling rate of sec or more to suppress the growth of γ grains and prevent carbide precipitation, followed by hot working,
C: Excellent surface quality and material cost, characterized by products that have been subjected to one or more types of warm processing and cold processing.
Method for producing r-Ni stainless steel thin plate. (2) In the manufacturing method defined in claim (1), δ-
Fe. cal(%)=3(Cr+1.5Si+Mo+T
i+Nb)-2.8(Ni+0.5Mn+0.5Cu)
δ-F defined as -84(C+N)-19.8(%)
e. By setting the cal (%) to -2 to 10%, the δ phase becomes the primary crystal or eutectic of solidification, suppresses the growth of γ grains during solidification, completes the solidification, and lowers the starting temperature of crystallization and precipitation of the γ phase. A method for manufacturing a Cr--Ni stainless steel thin plate having excellent surface quality and material quality, in which the γ grain size of the cast slab is made to be 50 μm or less on average throughout the plate thickness. (3) In the manufacturing method defined in claim (1), 100
Cooling to 1200℃ after solidification at a cooling rate of ℃/sec or higher is achieved by applying a reduction of 5% or less to the slab using one or more sets of rolls that combine internal cooling rolls, externally water-cooled rolls, air-cooled rolls, etc. A method for producing a Cr-Ni stainless steel thin plate with excellent surface quality and material quality under applicable conditions. (4) In the manufacturing method defined in claim (1), using the component system of claim (2) and the cooling method of claim (3), Cr-
A method for manufacturing a Ni-based stainless steel thin plate. (5) In the manufacturing method defined in claim (1), 100
A method for producing a thin Cr--Ni stainless steel sheet with excellent surface quality and material quality, in which cooling to 1200° C. after solidification is performed using gas and/or liquid at a cooling rate of at least 0° C./sec. (6) A thin Cr-Ni stainless steel sheet with excellent surface quality and material quality produced by using the cooling method of claim (5) with the composition system of claim (2) in the manufacturing method defined in claim (1). Production method. (7) In the manufacturing method defined in claim (1), 100%
One set that combines internal cooling rolls, externally water-cooled rolls, air-cooled rolls, etc. under conditions where the slab is cooled to 1200°C after solidification at a cooling rate of ℃/sec or higher and a reduction of 5% or less is applied to the slab. Cr-Ni with excellent surface quality and material quality achieved by a cooling method that combines cooling performed with the above rolls and cooling performed using gas and/or liquid.
A method for manufacturing thin stainless steel sheets. (8) A thin Cr-Ni stainless steel sheet with excellent surface quality and material quality produced by the manufacturing method defined in claim (1) using the component system of claim (2) and the cooling method of claim (7). Production method.