JPH0159330B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing thin-walled Cr-based stainless steel slabs. (Prior art) Stainless steel thin steel plates are, for example, disclosed in Japanese Patent Application Laid-open No. 55-97430.
As described in the publication, slabs continuously cast to a thickness of around 200 mm are roughly rolled or heated to 1200°C, then hot finish rolled to form hot rolled sheets, and hot rolled sheets are produced in a bell-shaped annealing furnace. The product is annealed, cold rolled, and finished annealed. (Problems to be solved by the invention) As mentioned above, in the known technology, the thickness of the slab is large;
It takes a lot of energy to form a hot-rolled sheet into a specified shape, and since the α' phase is formed in the slab as hot-rolled, it is necessary to form a bell-shaped slab to break it down and improve cold-rollability. Requires long annealing in an annealing furnace. (Means for solving the problems) The present invention provides the following features in the ingot making of Cr-based stainless steel:
To produce a slab in which the γ phase does not precipitate by continuously casting molten steel into a thin slab and cooling the slab to a temperature within the α phase region at which the precipitation of the γ phase ends. The main points are:
By using such a slab as a starting material, there is no need for various energy and equipment to make a specified hot-rolled sheet, and even if the process equivalent to hot-rolled sheet annealing is omitted, it can be made with good cold rollability and ridging properties. The effect of being able to manufacture thin steel sheets is achieved. The present invention will be explained in detail below. The first object of the present invention is to omit the step equivalent to hot-rolled sheet annealing, which is performed to improve cold rolling properties, in the production of Cr-based stainless steel thin sheets. Among Cr-based stainless steel sheets, steel types such as SUS410 contain a large amount of hard phase generated by the transformation of the γ phase in the as-hot-rolled state, so if they are cold-rolled in this state, , hot-rolled sheet annealing causes defects such as cold-rolling breakage, uneven rolling reduction during cold rolling, and large fluctuations in the thickness in the longitudinal direction of the coil.
It is necessary to perform hot-rolled sheet annealing to separate this hard phase into ferrite + carbide. As a result of research into ways to omit this hot-rolled sheet annealing step, the present inventors have found that, in order to prevent the formation of the γ phase, which causes the formation of hard phases, the present inventors Cool it so that γ
It has been discovered that if a phase-free hot rolled sheet is produced, no hard phase is formed, and therefore the process corresponding to the hot rolled sheet annealing described above becomes unnecessary. i.e. thickness
In the case of a slab of around 200 mm, even if the surface of the slab is cooled by the usual method after casting, the cooling rate is limited by the thermal conductivity of the slab itself, and the precipitation of the γ phase during the solidification process may occur. Although it cannot be prevented, by limiting the thickness of the slab to a predetermined thickness and cooling it so that it is within the α phase region after solidification, the α phase can be reduced without precipitating the γ phase.
This makes it possible to bring it to regions where phases and carbides exist. In the α phase + carbide region, C, N, S, etc. supersaturated in solid solution in the α phase are
Precipitation treatment is required to form carbonitrides and sulfides. If such precipitation treatment is not performed, there will be drawbacks such as significant work hardening during cold rolling, and low elongation and high yield point in the final product. That is, in order to improve cold rollability, instead of hot-rolled sheet annealing to decompose the α' phase formed from the γ phase into α+ carbide, heat treatment is performed only for precipitation treatment to precipitate C, N, S, etc. from supersaturated α. If this is carried out, the cold rollability will be improved, and this precipitation treatment can be done in a shorter time than the reaction of α′ → α + carbide, and can be done in the normal rolling process, so no special heat treatment process is required. be. A second object of the present invention is to provide a technique for producing a thin plate having improved workability, particularly ridging properties, using a thin slab as a starting material. In order to improve ridging properties, it is necessary that the size of colonies (a group of crystal grains having similar orientations) is small, that the colonies are randomly distributed, and that the crystal grain size is also relatively small. For this purpose, it is necessary to reduce the crystal grain size of the slab and to prevent an increase in locony size due to grain boundary movement during the cooling process after solidification. Since conventional slabs have a large thickness of 100 mm or more, it is difficult to control the solidification rate during casting, and it is impossible to reduce the crystal grain size in the as-cast state. In addition, the cooling rate of the slab after casting is difficult due to the large thickness of the slab, which causes an increase in colony size due to grain boundary movement in the high temperature range during cooling, and the hot rolling process It is necessary to recrystallize it to reduce the colony size. However, in the case of a thin slab as in the present invention, by limiting the thickness of the slab to 10 mm or less, preferably 3 mm or less, the solidification rate after casting can be increased, so it is possible to reduce the crystal grains of the slab. This is possible, and further, by increasing the cooling rate after solidification as in the present invention, the colony size can be reduced. SUS430 with components that form α+γ2 phase at high temperatures
In Cr-based stainless steels, such as Cr stainless steels, it is not possible to refine the colony size in the as-cast state.In order to refine the colony size in slabs of normal thickness, it is necessary to finely disperse the γ phase. γ
Techniques for finely dispersing phases include, for example, a technique in which a cast slab is reheated to an α single phase region, then subjected to a large pressure reduction, and then cooled to an α + γ two phase region (Japanese Patent Laid-Open Publication No. 11827/1982); Technology for heating hot-rolled sheets to the α+γ2 phase region,
Further, there are prior art techniques such as a technique (Japanese Unexamined Patent Publication No. 83724/1983) of heating to the α+γ2 phase region between rough rolling and finish rolling. In the case of the method of reheating to the α single phase region, the crystal grains of the α phase are coarsened due to reheating to a high temperature, so that the effect of improving the ridging properties, which is the final objective, is small. The method of reheating a hot-rolled sheet to the α+γ2 phase region is as follows:
Although the effect of improving ridging properties is significant, cold rollability deteriorates and surface defects such as sparkling scratches are likely to occur. The method of heating to the α+γ2 phase region between rough rolling and finish rolling has a large effect on improving ridging properties.
This is the most excellent method among the above three methods, as there is no deterioration in cold rollability or occurrence of surface defects such as sparkling scratches. The present inventors improved and developed these measures and found a method for finely dispersing the γ phase. That is, after solidification is completed, the slab is cooled in the α phase region to prevent the precipitation of the γ phase, and then reheated to the α+γ phase region to cause the precipitation of the γ phase. Compared to the case where the γ phase is precipitated during the solidification process, the α phase is
Since the γ phase precipitates after the fine precipitation of precipitates such as Cr carbides, these precipitates become sites for γ precipitation, and since γ phase precipitation occurs at low temperatures, the growth rate of the γ phase is slow. Therefore, compared to the case where the γ phase is precipitated during the cooling process from a high temperature, it becomes possible to precipitate and disperse the fine γ phase at a higher density. In this case, by applying processing to the α+γ2 phase region before reheating, the number of precipitation sites for the γ phase increases, so that the fine dispersion of the γ phase is performed more effectively. In the present invention, another purpose of adding processing is to press the dents produced by rapid solidification to prevent cold rolling breakage, and to promote recrystallization at high temperatures by processing. In this case, although it is effective to apply a reduction of 5% or more to compress the Zaku, a reduction of 25% or more, preferably 35% or more is required to promote recrystallization by processing. Next, the reasons for limiting the components of the starting materials of the present invention will be explained. The reason why Cr is set to 8% or more is because corrosion resistance is poor if the Cr content is less than this. Corrosion resistance improves as the amount of Cr added increases, but if it exceeds 30%, the effect is small and cold rollability deteriorates. Considering economic efficiency, a larger amount of Cr is not preferable, so 30% was set as the upper limit. The reason for setting the C content to be 0.001% or more is because it is difficult to melt a starting material with a C content lower than this using a normal method. The greater the amount of C added, the better the ridging properties will be, but if added in excess of 0.5%, cold rollability and r-value will deteriorate, so the upper limit should be set at 0.5%.
%. The r value improves as the amount of Al added increases, but 0.5%
The upper limit was set at 0.5% because the effect would be saturated and it would be uneconomical to add more than this amount.The lower limit was set at 0.001% because if the amount of Al is less than this, O ₂ will increase significantly.
Since this is not preferable, the lower limit is set at 0.001%. The higher the amount of N added, the better the ridging properties will be, but adding more than 0.5% will cause blisters, etc., so the upper limit is set at 0.5%.
The lower the value is 0.004% or less, the better the r value improves, but
Less than 0.001% cannot be melted using normal methods.
The lower limit is 0.001%. The components of the starting material in the present invention have a Cr of 8 to 30
% range, any element that becomes α+ (carbide) at room temperature is within the scope of the present invention, and α+ (carbide) in all temperature ranges.
Even in the case of single-phase component composition, grain growth can be prevented by rapidly cooling the slab, but in this case, C, N, S, etc. become supersaturated solid solutions and the slab becomes abnormal. Because it hardens, these supersaturated O,
By precipitating N and S, it is possible to manufacture slabs with small grain size and ductility, which is included in the scope of the present invention. However, the components that achieve the main purpose of the present invention include: Needless to say, it is a component system that forms α and γ2 phases at high temperatures. The thickness of the slab in the starting material of the present invention is 1 mm or more
It is 30 mm or less, preferably 3 mm or less. In other words, it is necessary to cool the slab as rapidly as possible in order to reduce the size of crystal grains in the as-solidified state and to prevent grain growth of the α phase and precipitation of the γ phase during the cooling process after solidification. The thinner it is, the more preferable it is, but when casting slabs less than 1 mm, productivity is low and it is economically unfavorable, so the lower limit is set to 1 mm. In addition, the upper limit is set to 30 mm or less, preferably 3 mm or less, because if the slab thickness exceeds this, the crystal grain size in the as-cast state will be large, and even if it is rapidly cooled by the usual method, the cooling rate will be too fast. This is because it is often difficult to prevent precipitation of the γ phase or grain growth during the cooling process. Slab thickness is 30
It goes without saying that if it is less than mm, the rough rolling step is completely unnecessary even when hot rolling is performed, and the rough rolling step can be omitted. Furthermore, if the thickness of the slab is 10 mm or less, preferably 5 mm or less, the finishing hot rolling process can also be omitted. In addition, in the present invention, the γ phase precipitation completion temperature (usually about
γ
This is aimed at preventing phase precipitation and grain growth; if the cooling rate is higher than air cooling, precipitation of the γ phase can be prevented, although not completely, and if the cooling rate is 100℃/sec or higher, it can be prevented substantially. By being preventable. In addition, the precipitation treatment for 10 seconds or more in the temperature range of 1000°C to 700°C is performed by rapid cooling, which removes C, N, S, etc., which are supersaturated solid solution in the α phase, into carbonitrides,
This is to precipitate in the form of sulfides, increase cold rollability and elongation of the final product, and lower the yield point, and at 1000℃
In the above, the upper limit was set at 1000℃ because the solubility is large and it is not effective, grain growth occurs, and the ridging properties deteriorate. Since the precipitation rate was slow and ineffective, the lower limit was set at 700°C. Furthermore, the reason why the duration was set to 10 seconds or more is that shorter durations have no retention effect. Usually, the above precipitation treatment is carried out at temperatures above 700℃, preferably
This can be achieved by rolling the slab at a high temperature of 800°C or higher. In the present invention, it is preferable that no amount of γ precipitates during the cooling process, but the object of the present invention can be achieved even if the amount of γ is 10% or less of the total amount of γ (maximum amount of equilibrium γ throughout the entire temperature). That is, if the amount of γ is at this level, there will be no problem in cold rollability even if hot-rolled sheet annealing is omitted, and there will be no problem in improving ridging properties. In order to improve ridging properties, it is preferable to reduce the crystal grain size as solidified and to finely disperse the γ phase as described above, but if more than 10% of the total γ amount precipitates during the cooling process after solidification, , even if the temperature is then reheated to the temperature range where the γ phase should precipitate, the already precipitated γ phase becomes the precipitation site for the newly precipitated γ phase, so the position of the γ phase that has been precipitated until now is The proportion of γ phase precipitated at different new sites decreases, and as a result, the effect of separate dispersion of γ phase decreases.
Although the effect of improving the ridging properties is reduced, if the precipitation of γ phase during the cooling process is 10% or less, this problem does not occur and the effect of improving the ridging properties can be expected. According to the present invention, the purpose of cooling the Cr-based thin slab from the solidification temperature to below the γ-phase precipitation termination temperature so that it is in the α-phase region and then heating it again to the α+γ2 phase is to improve the ridging properties. The objective is to precipitate the γ phase in a finely dispersed manner, and in this case it is advantageous to maintain this temperature range until at least 30% of the total γ amount of the γ phase is precipitated. In order to improve the ridging properties, it is desirable that as much γ as possible is precipitated homogeneously and finely, but if at least 30% of the total γ amount is precipitated by such heat treatment, This is because the effect of improving ridging characteristics becomes remarkable. Further, according to the present invention, when rolling is applied before the heat treatment for precipitation of the γ phase, precipitation of the γ phase is promoted, and the number of precipitation sites is increased and refined, but the rolling reduction ratio in this case is preferably 10% or more. This is because if the rolling reduction is less than this, the effect of promoting the precipitation of the γ phase and refining is hardly expected, and the higher the rolling reduction, the more effective it is, but 90%
Above that, the effect is saturated. It goes without saying that this processing can be applied any time before the precipitation of the γ phase, immediately after solidification, or immediately before reheating to the γ phase after rapid cooling, which is effective. When reheating to the α + γ2 phase region, by rapid cooling,
Since the supersaturated solid solution of C and N in the α phase moves into the γ phase, the above-mentioned precipitation treatment is not necessarily required. However, if the precipitation treatment as described above is continued, γ→α transformation will occur partially, and solid solution C,
Since the amounts of N and S are reduced, cold rollability is improved, product elongation is improved, and yield strength can be reduced. In the present invention, for the purpose of preventing scratches, 5
% or more processing at 800° C. or higher, and in order to promote recrystallization by processing, processing of 25% or more, preferably 35% or more is necessary. The thin slab that has been processed and heat treated as described above may be subsequently hot rolled, then wound up as a coil, cold rolled, and annealed to form a thin plate, or it may be directly cold rolled without hot rolling. Annealing may be performed, but
Needless to say, cold rolling and annealing conditions must be determined according to the target quality of the thin plate. The steel composition targeted by the present invention may be ferrite single phase steel, but the γ calculated based on the following formula
(This γ amount is a value as an index indicating the maximum γ amount throughout the entire temperature range).In this case, the γ amount should not exceed 100. It is necessary to adjust the component system. γ%=420×[C%]+470×[N%]+7×[Mn%]−11.5×[Cr%+Si%]−49×[Ti%]−52×[Al%]+189 (Example) Implementation Example 1 Cr steel with the composition shown in Table 1 was processed by the twin roll method to a thickness of 4 mm.
After casting into a thin slab of mm, immediately cooled with water to 850℃.
It was wound into a thin-walled slab coil. For comparison, we also produced a coil that was air-cooled to room temperature after casting and then wound. Thin slabs produced in this way were cold-rolled, but those that were water-cooled immediately after casting and rolled at 850℃ showed no precipitation of γ phase during the cooling process, and no edge cracks or cold-rolling occurred during cold rolling. Thickness variation during rolling was small and good cold rollability was exhibited, but martensite was formed in the air-cooled steel sheet, resulting in poor cold rollability.

[Table] Example 2 SUS430 steel having the components shown in Table 2 was cast into a thin slab with a thickness of 10 mm by the twin roll method and immediately cooled with water. For comparison, a cast piece that had been air cooled after casting and a slab with a thickness of 200 mm were produced using a known method. The slabs cooled by water according to the present invention had a single ferrite phase without precipitation of the γ phase, whereas the slabs cooled by air or cast by a known method had an α+α′2 phase structure.
The prototype thin slab with a thickness of 10 mm was subjected to a 30% reduction and then reheated to α+γ2 phase, then hot-rolled into a hot-rolled plate with a thickness of 3 mm, and then cold-rolled according to a conventional method. , annealing was performed. In the thin plate according to the present invention, almost no ridging was observed, but in the air-cooled plate, ridging was significantly large.

[Table] Example 3 After casting 17Cr steel with the composition shown in Table 3 into a thin slab with a thickness of 7 mm, it was rolled to a thickness of 4 mm before the γ phase precipitated, and then it was further rolled in the α phase region. It was cooled and rolled up at 850°C to form a coil. For comparison,
A prototype slab with a thickness of 4 mm was also produced by casting into a slab of 7 mm and rolling it after the gamma phase started to precipitate.
These slabs were subsequently cold rolled and annealed into thin sheets. The slab made according to the present invention had no roughness defects and good cold rollability, and the final product had an r value of 1.10 in γ and a ridging property of 15 μm, both of which were good. A hard and brittle α' phase existed before cold rolling, and edge cracking occurred during cold rolling, deteriorating cold rollability.

[Table] Example 4 After casting 17Cr steel with the composition shown in Table 4 into a slab with a thickness of 15 mm, it was cooled to 800°C at a cooling rate of 150°C/sec in the α phase, and then cooled to 1100°C, which is the α+γ2 phase region. After reheating to a temperature of , rolling was performed to obtain a hot rolled sheet with a thickness of 4 mm. For comparison, a slab with a thickness of 15 mm was cast, then cooled in the air as it was, and after reaching a temperature of 1100°C, it was rolled into a hot rolled plate with a thickness of 4 mm. The hot-rolled sheet prototyped in this way was made into a thin sheet by a known method, and after cooling in the α phase according to the method of the present invention, α
The material that has been reheated and rolled to the +γ2 phase region has significantly better ridging properties (10 μm) than the comparative material.
showed that. The comparative material had a thickness of 28 μm.

[Table] Example 5 17% Cr steel with the composition shown in Table 5 was cast into a slab with a thickness of 3 mm, cooled to 1000°C at a cooling rate of 100°C/sec, and then rolled at a temperature of 850°C. . For comparison, slabs were also produced that were rapidly cooled to room temperature at a cooling rate of 100°C/sec. The material rolled at 850°C was then subjected to 80% cold rolling and annealed at 850°C for 2 minutes to form a finished product. This product has a ridding of approximately 15 μm and an r value of 0.8.
It was good. In contrast, when the material was rapidly cooled to room temperature, although it was processed in the same process, cold rolling breakage occurred and the yield was greatly reduced.

[Table] Example 6 After casting 17% Cr steel with the composition shown in Table 6 into a 3 mm slab, it was heated at 20°C/sec up to 1200°C and from 1200°C to 1000°C.
After cooling up to 900℃ at a cooling rate of 100℃/sec
It was rolled up and cooled with a heat insulating cover so that it cooled at a slower rate than air cooling. For comparison, we also made specimens that were air-cooled to room temperature after casting. The thus cast slab was cold rolled to 0.4 mm and then final annealed at 900°C for 40 seconds to produce a finished product. The product manufactured according to the present invention has a ridging property of 17 μm, a total elongation of 27%, and a yield strength of 35 kg/
It was good at ^mm2 . In contrast, those that were air-cooled to room temperature after casting had a total elongation of 18% and a ridging of 28 μm, and the ridging properties were also poor.

[Table] (Effects of the Invention) According to the present invention, a Cr-based stainless steel thin plate with good cold rollability and ridging properties can be produced by omitting the rough rolling process, finish rolling process, and hot rolled sheet annealing process. In addition, since large-scale equipment such as a continuous hot rolling mill is not required, the economic effect is extremely large, and therefore, the present invention is extremely useful industrially.

Claims (1)

[Claims] 1. By weight, Cr: 8-30%, C: 0.001-0.5%,
A method for producing thin-walled Cr-based stainless steel slabs whose main components are N: 0.001 to 0.5% and Al: 0.001 to 0.5%, with the remainder being substantially Fe, the method comprising: Up to the end temperature of γ phase precipitation, 100
Cool at a cooling rate of ℃/s or higher, then 700℃ or higher
A method for producing thin-walled Cr-based stainless steel slabs, characterized by performing precipitation treatment for 10 seconds or more in an α single-phase temperature range of 1000°C or less. 2 By weight, Cr: 8-30%, C: 0.001-0.5%,
A method for producing thin-walled Cr-based stainless steel slabs whose main components are N: 0.001 to 0.5% and Al: 0.001 to 0.5%, with the remainder being substantially Fe, the method comprising: Up to the end temperature of γ phase precipitation, 100
Cooling at a cooling rate of ℃/s or more, and rolling the slab during or after the cooling process, followed by precipitation treatment for 10 seconds or more in the α single phase temperature range of 700℃ or higher and 1000℃ or lower. A method for manufacturing thin-walled Cr-based stainless steel slabs, characterized by subjecting them to the following steps: 3 By weight, Cr: 8-30%, C: 0.001-0.5%,
A method for producing thin-walled Cr-based stainless steel slabs whose main components are N: 0.001 to 0.5% and Al: 0.001 to 0.5%, with the remainder being substantially Fe, the method comprising: Up to the end temperature of γ phase precipitation, 100
A method for producing a thin-walled Cr-based stainless steel slab, characterized by cooling at a cooling rate of ℃/s or more, and then heating to an α+γ two-phase region to precipitate the γ phase. 4 By weight, Cr: 8-30%, C: 0.001-0.5%,
A method for producing thin-walled Cr-based stainless steel slabs whose main components are N: 0.001 to 0.5% and Al: 0.001 to 0.5%, with the remainder being substantially Fe, the method comprising: Up to the end temperature of γ phase precipitation, 100
cooling at a cooling rate of ℃/s or more; rolling the slab during or after the cooling process; and heating the thus obtained thin slab to an α+γ two-phase region to remove the γ phase. A method for producing thin-walled Cr-based stainless steel slabs, characterized by precipitation.