JP3661460B2 - Method for producing martensitic and ferritic stainless steel slabs - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing martensitic and ferritic stainless steel round billet slabs on a continuous casting machine, and more specifically, after casting, slab surface cracks due to thermal stress or transformation stress during slab cooling. The present invention relates to a production method that can be efficiently prevented.
[0002]
[Prior art]
When manufacturing slab slabs of martensitic stainless steel and ferritic stainless steel with a continuous casting machine, if the cast slab is cooled in the air, surface cracks will occur in the slab slab due to the cooling conditions. Occur. This occurs due to the tensile thermal stress on the slab surface caused by the slab temperature distribution during solidification and, further, in martensitic stainless steel, transformation stress associated with martensitic transformation. Conventionally, many methods for preventing this surface crack have been proposed in order to improve the yield of cast slabs and expand the applicable steel types in continuous casting machines.
[0003]
For example, in Japanese Patent Application Laid-Open No. 55-164029, the surface temperature of a martensitic stainless steel slab is encapsulated with a heat insulating material without being cooled to 650 ° C. or less after casting, and the retained heat of the slab is retained. A method for preventing surface cracks associated with martensitic transformation by slowly cooling while keeping the temperature is disclosed. In JP-A-55-33801, the surface temperature of a continuous cast slab of ferritic stainless steel is immediately heated without cooling to 150 ° C. or lower after casting, and then hot-rolled to obtain a slab. A method of cooling to room temperature after improving the material strength is disclosed.
[0004]
Conventionally, in the case of martensitic stainless steel and ferritic stainless steel small-section round billet casts, in order to prevent surface cracking, the round billet cast is cooled to room temperature after continuous casting as described above. Instead, it was immediately charged in a heating furnace and hot-rolled, or slowly cooled using a slow cooling furnace or heat insulation equipment.
[0005]
[Problems to be solved by the invention]
However, there are the following problems in the method of immediately inserting into a heating furnace after casting and performing hot rolling or the method of slow cooling in a slow cooling furnace or the like. That is, when the slab in a high temperature state is conveyed to a rolling process and heated and rolled, a consistent schedule management from casting to rolling is necessary, and the flexibility of operation of the continuous casting process and the rolling process is lost. In the case of slow cooling, it is necessary to install a dedicated slow cooling furnace and heat insulation equipment in the vicinity of the continuous casting equipment. Furthermore, when a hot slab is lifted and moved, a transfer device using magnetic force (hereinafter referred to as “the riff mug”) that is optimal for transporting round billet slabs with a small cross section cannot be used. It is necessary to use the tongs that are present, and there are problems such as extending the conveyance time by gripping with the tongs.
[0006]
The present invention has been made in view of the above circumstances, and its purpose is to provide flexibility in operation when manufacturing a small-section round billet slab of martensitic and ferritic stainless steel in a continuous casting machine. It is an object of the present invention to provide a manufacturing method capable of efficiently preventing slab surface cracking due to transformation stress or thermal stress during slab cooling without damaging and without requiring a dedicated annealing furnace or the like.
[0007]
[Means for Solving the Problems]
A method for producing a martensitic and ferritic stainless steel slab according to the first invention for solving the above-mentioned problems is that a martensitic stainless steel or a ferritic stainless steel has a sectional size of 350 mm or less in diameter in a continuous casting machine. After casting into a round billet slab, the round billet slab is allowed to cool in the atmosphere until the surface temperature of the slab becomes 100 ° C. or lower, and then A _C1 at a temperature rising rate of 10 to 200 ° C./Hr. The round billet slab is heated from a temperature 200 ° C. lower than the point to a temperature 100 ° C. higher than the AC ₁ point. After heating, the round billet slab is cooled in the atmosphere or in a furnace until the slab surface temperature becomes 100 ° C. or lower. It is a feature.
[0008]
The method for producing martensitic and ferritic stainless steel slabs according to the second invention is the method according to the first invention, wherein after the round billet slab is heated, the surface temperature of the round billet slab becomes at least 500 ° C. Slow cooling is performed at a cooling rate of 30 ° C./Hr or less , and then cooling is performed in the air or in a furnace until the slab surface temperature becomes 100 ° C. or less.
[0009]
In the present invention, after round billet cast pieces having a cross-sectional size of 350 mm or less in diameter of martensitic stainless steel and ferritic stainless steel are cast in a continuous casting machine, at least until the surface temperature reaches room temperature of 100 ° C. or less in the atmosphere. Cool with. Conventionally, if the surface of a round billet slab is cracked when cooled to room temperature in the atmosphere, the slab is heated immediately after casting and before the surface temperature of the slab reaches room temperature, without cooling in the air. Although slow cooling was performed, the present inventors found that cracks were not generated in the cast slab even when cooled in the atmosphere only for round billet slabs having a diameter of 350 mm or less.
[0010]
This is because the slab has a small cross section with a diameter of 350 mm or less, so that the thermal stress caused by the slab temperature distribution during solidification is reduced, and similarly the stress associated with phase transformation such as martensitic transformation is reduced. This is because the slab has a round shape and there is no portion where stress concentrates on the slab, that is, the stress that causes cracking is small and there is no portion that easily breaks.
[0011]
However, since the slab is high in hardness while being cooled in this way, the load at the time of cutting the slab is large and the work efficiency is lowered. Furthermore, when left as it is cooled, cracks occur in the slab in proportion to the passage of time due to residual thermal stress and transformation stress. Therefore, the slab is heat-treated for the purpose of reducing the slab hardness and removing the residual stress.
[0012]
The heating rate of the heat treatment is 10 to 200 ° C./Hr. It is not preferable to make it slower than this, and the efficiency is lowered, and if it is made faster than this, the temperature difference between the inside and the surface of the slab becomes remarkable. The heating temperature is in a range from a temperature 200 ° C. lower than the A _C1 point of the slab to a temperature 100 ° C. higher than the A _C1 point. Heating within this range removes residual stress and lowers the hardness of the slab.
[0013]
After heating, no new thermal or transformation stress is generated by cooling in the air or in the furnace until the slab surface temperature becomes 100 ° C. or lower. However, it is preferable to cool slowly in the furnace at a cooling rate of 30 ° C./Hr or less until the surface temperature of the round billet slab reaches at least 500 ° C. during cooling. This is because by gradually cooling to 500 ° C. at a cooling rate of 30 ° C./Hr or less, generation of thermal stress and transformation stress can be prevented, and occurrence of warping of the round billet can be prevented.
[0014]
In addition, the martensitic stainless steel and the ferritic stainless steel shown in the present invention conform to the classification according to JIS. For example, martensitic stainless steel is SUS410, SUS416, etc., and ferritic stainless steel is SUS405, SUS430, etc. It is.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
A martensitic stainless steel or a ferritic stainless steel melted in a steelmaking factory is cast into a round billet cast piece having a diameter of 350 mm or less in a round billet continuous casting machine. Then, the slab that is continuously manufactured is cut into a predetermined length by a synchronous cutter of a round billet continuous casting machine, and the slab cooled by connecting the cut slab to the synchronous cutter by a conveying roller table or the like. It is transported to the field and cooled in the atmosphere, and the cooling is continued until at least the surface temperature of the slab becomes 100 ° C. or lower. In the present invention, the time when it is cut with a synchronous cutter is defined as the end of casting.
[0016]
When cooling in the atmosphere, the slabs may be stacked and cooled, or the slabs may be arranged side by side and cooled, but it is not necessary to cover the slab with a cover or the like. However, in the slab cooling station, it is necessary to prevent the slab from being exposed to water including rain water. In addition, when there is no slab cooling place connected with a synchronous cutter, a slab is conveyed to the place which can be cooled in air | atmosphere using a tongue, a trolley | bogie, etc., and it cools there.
[0017]
After the surface temperature of the slab drops to 100 ° C. or lower, the slab is transported to a facility capable of heat-treating the slab, such as a heating furnace or a soaking furnace, and the slab is heat-treated. The heat treatment is preferably performed within 10 days from the end of casting in order to prevent cracking of the slab due to residual stress.
[0018]
In the heat treatment, the temperature of the slab is raised at an arbitrary value of 10 to 200 ° C./Hr, and the surface temperature of the slab is 200 ° C. lower than the AC ₁ point of the slab. The temperature is raised to a temperature range 100 ° C. higher than the A _C1 point, and the temperature is maintained. The holding time depends on the rate of temperature rise, and may be several minutes when the rate of temperature rise is slow in the above range, and if it is fast, the temperature is raised to the inside of the slab as 4 to 5 hours. However, it is not necessary to hold until the temperature of the inside and the surface of the slab becomes equal.
[0019]
After holding for a predetermined time, it may be cooled in the furnace as it is, or may be taken out from the furnace and cooled in the air, and cooled until the slab surface temperature becomes 100 ° C. or less. When cooling in the furnace, the cooling may be performed while controlling the cooling rate, or the heating may be stopped and the cooling may be performed. However, until the surface temperature of the slab reaches at least 500 ° C., it is preferable to control the cooling rate at 30 ° C./Hr or less and gradually cool in the furnace. Thus, it cools to normal temperature and complete | finishes heat processing.
[0020]
FIG. 1 is a schematic diagram showing an example of the transition of the surface temperature from the end of casting to the end of heat treatment of a slab produced according to the present invention described above, in which curve A is a slab cooling curve after casting, curve B Indicates a temperature rise curve in heat treatment, and curve C and curve D indicate cooling curves in heat treatment.
[0021]
By treating the slab in this way, it is possible to prevent surface cracking of the slab due to thermal stress and transformation stress, reduce the hardness of the slab, and cut the slab to a desired length with a cold saw Slab yield is improved. In addition, since it is not necessary to match the operation of a heating furnace or the like for performing heat treatment with a continuous casting machine, both operation schedules can be made flexible, and a dedicated furnace for performing heat treatment is not required. It is possible to perform a heat treatment using a free time and the like, and it is possible to use a riff mug, and the slab can be conveyed quickly.
[0022]
【Example】
[Example 1]
An embodiment of the present invention in a round billet slab of martensitic stainless steel having a diameter of 300 mm, containing 0.2 wt% C and 0.5 wt% Mn will be described.
[0023]
The slab cast by the round billet continuous casting machine was cooled in the slab cooling field. The surface temperature of the slab became 25 ° C. at room temperature when 15 hours had elapsed from the end of casting. The cooling rate during this period was calculated to be about 60 ° C./Hr. Then, when about 72 hours passed from the end of casting, heat treatment was performed using the free time of the soaking furnace. In the heat treatment, the temperature rising rate was 70 ° C./Hr, the slab surface temperature was increased to 820 ° C., and then the slab surface temperature was controlled at 820 ° C. and held for 2 hours. At this rate of temperature increase, the A _C1 point of the martensitic stainless steel is about 805 ° C. Thereafter, the temperature is controlled to a cooling rate of 25 ° C./Hr and cooled to 500 ° C. Then, the heating is stopped and cooled in a soaking furnace, and when the slab surface temperature reaches 150 ° C., The slab was taken out. The cooling rate in cooling in the soaking furnace was about 40 ° C./Hr.
[0024]
The slab at room temperature was inspected by a penetrant flaw test to investigate surface cracks, but there were no surface cracks due to thermal stress and transformation stress.
[0025]
Further, the above-described martensitic stainless steel having two other casting opportunities was left in the slab cooling field after casting, and the occurrence of cracks due to residual stress during the time left was examined. Surface cracks were inspected by penetrant testing. The survey results are shown in FIG. In the two lots examined, no cracks were observed until 10 days after the end of casting, but cracks were observed after 13 days or more. From this result, it was found that the occurrence of cracks can be prevented by performing a heat treatment within 10 days from the end of casting. In addition, the crack incidence shown in FIG. 2 is shown by the number ratio of the round billet cast pieces in which cracks occurred.
[0026]
[Example 2]
An embodiment of the present invention in a ferritic stainless steel round billet slab having a diameter of 300 mm, containing 0.01 wt% C and 0.3 wt% Mn will be described.
[0027]
The slab cast by the round billet continuous casting machine was cooled in the slab cooling field. The surface temperature of the slab became 25 ° C. at room temperature when 15 hours had elapsed from the end of casting. The cooling rate during this period was calculated to be about 60 ° C./Hr. Then, when about 144 hours passed from the end of casting, heat treatment was performed using the free time of the soaking furnace. In the heat treatment, the temperature rising rate was 70 ° C./Hr, the slab surface temperature was raised to 600 ° C., and then the slab surface temperature was controlled at 600 ° C. and held for 1.5 hours. At this rate of temperature increase, the A _C1 point of the ferritic stainless steel is about 630 ° C. Thereafter, the cooling rate is controlled to 25 ° C./Hr to cool to 500 ° C., then the heating is stopped and cooled in a soaking furnace, and when the slab surface temperature reaches 150 ° C., The slab was taken out. The cooling rate in cooling in the soaking furnace was about 40 ° C./Hr.
[0028]
The slab at room temperature was inspected by a penetrant flaw test to investigate surface cracks, but there were no surface cracks due to thermal stress and transformation stress.
[0029]
【The invention's effect】
According to the present invention, it is possible to prevent slab surface cracking due to thermal stress and transformation stress, reduce the hardness of the slab, and use it by cutting it to a desired length with a cold saw. The slab yield is improved. Furthermore, since it is not necessary to match the operation of a heating furnace or the like that performs heat treatment with the continuous casting machine, both operation schedules can be made flexible, and a dedicated furnace for performing heat treatment is not required, and the heating furnace It is possible to perform a heat treatment using a free time and the like, and it is possible to use a riff mug, so that a slab can be conveyed quickly, and the industrial effect is exceptional.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an example of the transition of the surface temperature of a slab manufactured according to the present invention.
FIG. 2 is a diagram showing the results of investigating the relationship between the cooling time in the atmosphere and surface cracks in an example of the present invention.

Claims (2)

Martensitic stainless steel or ferritic stainless steel is cast into a round billet slab having a cross-sectional size of 350 mm or less in a continuous casting machine. allowed to cool to, then, at a heating rate of 10 to 200 ° C. / Hr, heating the round billet cast strip from 200 ° C. lower temperature than the _C1 point a to 100 ° C. temperature range higher than point _C1 a, heating Then, it cools in air | atmosphere or in a furnace until slab surface temperature becomes 100 degrees C or less, The manufacturing method of the martensitic type and ferritic stainless steel slab characterized by the above-mentioned. After the round billet slab is heated, it is gradually cooled at a cooling rate of 30 ° C./Hr or less until the surface temperature of the round billet slab reaches at least 500 ° C., and then the slab surface temperature in the air or in a furnace It cools until it becomes 100 degrees C or less, The manufacturing method of the martensitic and ferritic stainless steel slab of Claim 1 characterized by the above-mentioned.

Images