JPH0572448B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing a Cr-based stainless steel thin plate. (Prior art) Stainless steel thin plates are manufactured by, for example,
As disclosed in Publication No. 97430, slabs with a thickness of around 200 mm obtained by a continuous casting process are directly rough rolled or heated to a temperature of about 1200°C and then hot rolled. The hot-rolled sheet is then annealed in a bell-shaped annealing furnace, followed by cold rolling and final annealing to produce a product. As mentioned above, in the conventional process, because the thickness of the slab as a starting material is around 200 mm, it is difficult to process it to make a hot-rolled sheet of a predetermined thickness, for example, 2.3 mm. Various energies such as rolling energy and rolling power are required, and the hot-rolled sheet obtained by this process has a martensitic phase formed in the as-hot-rolled state, which is decomposed and cooled. In order to improve ductility, there is a problem in that long-time annealing is required in a bell-shaped annealing furnace. (Problems to be Solved by the Invention) The present invention provides a method for manufacturing Cr-based stainless steel thin plates, which eliminates the hot rolling process and the hot rolled plate annealing process using a bell-type annealing furnace, and yet prevents cracking in the cold rolling process. The object of the present invention is to provide a method for producing a thin Cr-based stainless steel plate that does not cause (Means for Solving the Problems) The gist of the present invention is as follows. (1) Cr: 8-30%, C: 0.001-0.5%, by weight%
Si: 0.1~1.0%, Mn: 0.1~0.5%, Al: 0.001~
0.5%, N: 0.001-0.5% and the remainder substantially
A process in which a molten Fe alloy is cast into a band with a thickness of 10 mm or less, and then cooled at a cooling rate of 100°C/sec or more from the solidification temperature to the end temperature of γ phase precipitation, and a temperature range of 150°C or more and 1000°C or less. A method for producing a Cr-based stainless steel thin plate, comprising the step of subjecting the cast strip to at least one pass of rolling. (2) In weight%, Cr: 8-30%, C: 0.001-0.5
%, Si: 0.1~1.0%, Mn: 0.1~0.5%, Al:
After casting a molten alloy consisting of 0.001 to 0.5% N, 0.001 to 0.5% N, and the balance substantially Fe into a strip with a thickness of 10 mm or less, it is heated at 100°C/sec or more from the solidification temperature to the γ phase precipitation termination temperature. A process of cooling at a cooling rate, followed by a process of precipitation treatment for 10 seconds or more in a temperature range of 700°C or more and 1000°C or less, and 150°C
A method for manufacturing a Cr-based stainless steel thin plate, comprising the step of subjecting the cast strip to at least one pass of rolling in a temperature range of 1000°C or less. This invention will be explained in detail below. The first object of the present invention is to improve the hot-rolled sheet annealing step, which is performed in the conventional manufacturing process of Cr-based stainless steel thin sheets, in order to improve the cold rollability when cold-rolling the hot-rolled sheet. It lies in omission. Among Cr-based stainless steels, steel types such as SUS410 contain a large amount of hard phase generated by γ-phase transformation in the as-hot-rolled state, so they cannot be cold-rolled in the as-hot-rolled state. This causes problems such as material breakage (cold rolling breakage), uneven rolling reduction, and large variations in thickness in the longitudinal direction of the material (strip). , it is necessary to perform hot-rolled sheet annealing to separate the above-mentioned hard phase into ferrite + carbide. The inventors of the present invention have repeatedly researched the configuration of a process for manufacturing Cr-based stainless steel thin sheets that can omit this hot-rolled sheet annealing process, and as a result, they have discovered that γ phase, which is the cause of the formation of hard phases in hot-rolled sheets, is produced. After solidifying the molten steel so that the molten steel is free from precipitation, the molten steel is cooled to the temperature at which the precipitation of the γ phase ends, within the α phase region, and a hot rolled sheet without the γ phase is produced. Furthermore, it has been found that the hot-rolled sheet annealing process can be omitted. In other words, in the case of slabs with a thickness of around 200 mm, after casting,
Even if the slab is cooled by a conventional method, the cooling rate is limited by the thermal conductivity of the slab itself, and the cooling rate is not high enough to prevent precipitation of the γ phase during the solidification process. When the thickness of the slab is reduced to 10 mm or less, preferably 4 mm or less, which is about the same as the thickness of a hot-rolled sheet (hot gauge), it is possible to use normal cooling means (for example, water cooling) after casting.
By cooling the slab so that it is in the α-phase region after solidification, it is possible to bring the γ-phase to the region where α-phase + carbides exist without precipitating the γ-phase. In the α phase+carbide region, C, N, S, etc. supersaturated as a solid solution in the α phase require precipitation treatment to form carbon, nitride, or sulfide. If such precipitation treatment is not performed, the material (strip) suffers from severe work hardening during cold rolling, and the final product has low elongation and a high drop point. Therefore, in order to improve the cold rollability of the material, instead of hot-rolled plate annealing to decompose the α' phase formed from the γ phase into α+ carbides, supersaturated α is used to process C, N,
If heat treatment is performed only for precipitation treatment to precipitate S etc., cold rollability will be improved.
Compared to the α+ carbide reaction, this reaction can be carried out in a shorter time and can be carried out during the normal strip winding process after hot rolling, and a dedicated heat treatment step as in conventional processes is unnecessary. The second object of this invention is to produce a Cr-based stainless steel thin plate with improved workability, especially ridging properties.
The object of the present invention is to provide a technology for manufacturing thin cast slabs as a starting material. In order to improve the ridging properties of Cr-based stainless steel sheets, it is necessary that the size of colonies (a group of crystal grains with similar orientations) be small and randomly distributed, and that the crystal grain size be relatively small. It is. For this purpose, it is necessary to reduce the crystal grain size of the slab and to prevent the colony size from expanding due to grain boundary movement during the cooling process after solidification. Since the slab in the conventional process has 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 colony size expansion due to grain boundary movement in the high temperature range during cooling. It is necessary to recrystallize during the process to reduce the colony size. In contrast, in the case of thin-walled slabs as in the present invention, by setting the thickness of the slab to 10 mm or less, preferably 4 mm or less, solidification after casting can be speeded up. It is possible to make the particles smaller, and furthermore, the cooling rate after solidification can be significantly increased, so the colony size can be reduced. On the other hand, Cr-based stainless steels generally have low toughness, and particularly when left solidified, the toughness decreases significantly. Hot-rolled sheets manufactured using conventional processes undergo recrystallization during the hot-rolling process and hot-rolled plate annealing process, and the solidified structure is destroyed, so the toughness at room temperature is lower than that of as-solidified slabs. However, if a solidified slab is subjected to cold working, cracks will occur. The present inventors have discovered that in a process that omits the hot rolling process and hot-rolled sheet annealing process, even if the slab is solidified, heating it above the transition temperature prior to cold rolling will cause cracking. We have discovered that this can be prevented. In the present invention, the thin cast slab is heated at a temperature of 150°C or higher to 1000°C.
The reason why the rolling process is carried out in the following temperature range is mainly to prevent cold rolling cracks due to insufficient toughness, and also to compress the cracks produced during the solidification process. The amount of reduction in this rolling process is at least 5% per pass, and the rolling process is performed in one or more passes. Here, the temperature range for rolling processing is 150℃ or higher and 1000℃.
The following is the reason for the above-mentioned reason, but the lower limit is
The reason for setting the temperature to 150°C is that if the temperature is lower than this, the toughness as solidified is low and cracks occur during rolling. If the slab is heated above the transition temperature, the toughness will improve, but if the temperature exceeds 1000°C, the ridging properties will deteriorate, so the upper limit was set at 1000°C. In this case, the rolling reduction amount may be 5% or more per pass, preferably 10% or more, and it is desirable to perform the processing multiple times. In addition, during rolling, the temperature of the slab after casting is 200°C.
It may be carried out before the temperature drops to below .degree. C., or it may be carried out after cooling to room temperature and then reheating. The thin slab that has been rolled in such a predetermined temperature range may be subsequently rolled to the final thickness and then annealed, or it may be wound into a coil, cold rolled, and annealed to form a thin plate. However, it goes without saying that 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 a ferrite single-phase steel, but it is preferably a steel with a composition containing a γ amount calculated based on the following formula, in which case the γ amount is 100 It is necessary to adjust the component system so that it does not exceed. γ%=420×[C%]+470×[N%]+7×[Mn%]−11.5×[Cr%+Si%] −49×[Ti%]−52×[Al%]+189 In implementing the present invention Since it is practical to perform the rolling process in the predetermined temperature range using a heating process before the cold rolling process, a temperature range of 150°C or higher and 500°C or lower is preferable from the perspective of energy saving. . 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 higher amount of Cr is not preferable, so 30% is set as the upper limit. The reason for setting the C content to be 0.001% or more is that it is difficult to melt with a starting material having a C content lower than this using conventional methods. 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%. Adding a large amount of N improves ridging properties, but adding more than 0.5% causes blistering, 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 thickness of the slab as a starting material in the present invention is 10
mm or less, preferably 4 mm or less. In other words, cooling at a high cooling rate is necessary in order to reduce the crystal grain size in the as-solidified slab and to prevent grain growth of the α phase and precipitation of the γ phase during the cooling process after solidification. From this point of view, it is better for the slab thickness to be as thin as possible. In the present invention, the upper limit of slab thickness is set to 10 mm because slabs with a thickness exceeding this thickness have large crystal grains in the as-cast state, and slabs with such thickness cannot be cooled by normal methods. However, the cooling rate is not high, and the precipitation of γ phase is prevented during the cooling process.
This is because it becomes difficult to prevent grain growth. In addition, in the present invention, the γ phase precipitation completion temperature (usually about
The cooling rate of slabs up to 1000°C (1000°C) is set at 100°C/sec or higher to prevent the precipitation of the γ phase and grain growth.If the cooling rate is 100°C/sec or higher, By being preventable. In addition, the precipitation treatment for 10 seconds or more in the temperature range of 1000°C to 700°C uses rapid cooling to remove C, N, S, etc., which are supersaturated solid solutions in the α phase, into carbonitrides,
The purpose is to release it in the form of sulfides, increase cold rollability and elongation of the final product, and lower the precipitation point. At temperatures above 1000°C, the solubility is large and it is not effective, and grain growth occurs, conversely impairing the ridging properties. The upper limit was set at 1000°C because this would cause deterioration, and the lower limit was set at 700°C because at temperatures lower than this, the precipitation rate is slow and ineffective. Furthermore, the reason why the duration was set at 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 this amount should be reduced to 10% of the total amount of γ.
The object of the present invention can also be achieved as follows. That is,
This is because 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. Examples Example 1 SUS430 steel having the components shown in Table 1 was manufactured and subjected to a Charpy test. A steel is cast into a 4mm thick slab using an iron mold, then immediately cooled in water and heated to 100mm.
The sample was cooled to 800°C at a cooling rate of approximately °C/sec, and then left to cool. Steels B and C are made into hot-rolled sheets with a thickness of 3mm from CC slabs with a thickness of 250mm using the normal manufacturing process, and then steel B is heated as hot-rolled and steel C is heated to 840℃, 4
The sample material was annealed for several hours. Figure 1 shows the relationship between temperature and impact value. In steel A, the transition temperature is around 150°C, but in steel B and C, the transition temperature is below room temperature. At room temperature, A steel becomes B, C
Impact value is significantly lower than steel. Figure 2 is a scanning electron micrograph showing the metallographic structure of the fracture surface of a Charpy specimen of A steel, where a is 25°C;
b is 200°C. At 25°C, the fracture surface exhibits cleavage cracks, indicating that it is brittle. However, at 200℃, the fracture surface exhibits a ductile fracture surface, indicating that it is highly tough. Based on this result, when steel A was heated to room temperature and 200℃ and rolled, cracks occurred on the second pass in the one rolled at room temperature, but cracks occurred in the one heated to 200℃. It was possible to roll it to the specified thickness without any problems. Example 2 SUS430 steel having the composition shown in Table 2 was cast into a thin slab with a thickness of 4 mm using an iron mold, and then immediately cooled with water to 800 °C at a cooling rate of about 100 °C/sec.
Thereafter, it was left to cool to form a thin slab. For comparison, we also produced thin slabs that were left to cool to room temperature after casting. After pickling the thin slab produced in this way,
The material was heated to 200°C, rolled to a predetermined thickness, and further annealed. The cast slab according to the present invention is
There was no precipitation of the γ phase during the cooling process, and there was little edge cracking or thickness variation during cold rolling, showing good cold rollability. Ta. Example 3 SUS430 steel having the composition shown in Table 2 was cast into a thin slab with a thickness of 4 mm using an iron mold, and then immediately cooled with water to 800 °C at a cooling rate of about 100 °C/sec.
After that, it was subjected to a precipitation treatment for 20 seconds and then allowed to cool to form a thin slab. For comparison, a thin slab was produced which was left to cool to room temperature after casting. The thus produced thin slab was pickled, heated to 200°C, rolled to a predetermined thickness, and further annealed. In the slab made according to the present invention, supersaturated C and N were precipitated as carbonitrides, so there were few edge cracks during cold rolling, and good cold rollability was exhibited. In the case of rolled steel, martensite was formed and cold rolling was poor.

【table】

[Table] (Effects of the Invention) As detailed above, 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 hot rolling process and the hot rolled plate annealing process.
In addition, it can be manufactured without cold rolling cracking, which is the most problematic problem with thin slabs, and Cr-based stainless steel thin plates can be produced in the manufacturing process of ordinary steel, which has great industrial effects.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the relationship between temperature and impact value in Cr-based stainless steel hot gauge materials (including as-cast materials), and Fig. 2 a and b show Example 1 of the present invention.
1 is a scanning electron micrograph (magnification: 200) showing the metallographic structure of the fracture surface of a Charpy specimen of Steel A.

Claims (1)

[Claims] 1% by weight, Cr: 8-30%, C: 0.001-0.5
%, Si: 0.1~1.0%, Mn: 0.1~0.5%, Al: 0.001
~0.5%, N: 0.001~0.5% and the remainder substantially
After casting the molten alloy consisting of Fe into a strip with a thickness of 10 mm or less, from the solidification temperature to the temperature at which γ phase precipitation ends,
A process of cooling at a cooling rate of 100℃/sec or more,
A method for manufacturing a Cr-based stainless steel thin plate, comprising the step of subjecting the cast strip to at least one pass of rolling in a temperature range of 150°C or higher and 1000°C or lower. 2. The Cr-based stainless steel according to claim 1, wherein at least one pass of rolling is performed on the band in a temperature range of 150°C or higher and 1000°C or lower, following a cooling process after casting. Method for manufacturing thin steel sheets. 3 At least one pass of rolling is performed on the strip in a temperature range of 150°C or higher and 1000°C or lower, followed by cooling from the solidification temperature to the γ phase precipitation termination temperature at a cooling rate of 100°C/sec or higher, and then Claim 1, which is made by cooling to room temperature and then heating to a temperature range of 150°C to 1000°C.
Method for manufacturing Cr-based stainless steel thin plate. 4 In weight%, Cr: 8-30%, C: 0.001-0.5
%, Si: 0.1~1.0%, Mn: 0.1~0.5%, Al: 0.001
~0.5%, N: 0.001~0.5% and the remainder substantially
After casting the molten alloy consisting of Fe into a strip with a thickness of 10 mm or less, from the solidification temperature to the temperature at which γ phase precipitation ends,
A process of cooling at a cooling rate of 100°C/sec or more, followed by a process of precipitation treatment for 10 seconds or more in a temperature range of 700°C or more and 1000°C or less, and a step of cooling the cast band in a temperature range of 150°C or more and 1000°C or less. characterized by having a process of applying at least one rolling pass to the
Method for manufacturing Cr-based stainless steel thin plate. 5 At least one pass of rolling applied to the strip at a temperature range of 150°C or higher and 1000°C or lower is performed at a temperature of 700°C or higher.
5. The method for producing a Cr-based stainless steel thin plate according to claim 4, which is performed following a precipitation treatment process for 10 seconds or more in a temperature range of 1000° C. or lower. 6 At least one pass of rolling applied to the strip at a temperature range of 150°C or higher and 1000°C or lower is performed at a temperature of 700°C or higher.
After precipitation treatment for 10 seconds or more in a temperature range of 1000℃ or less, after cooling to room temperature, 150℃ or more and 1000℃
The method for manufacturing a Cr-based stainless steel thin plate according to claim 4, which is performed by heating to the following temperature range.