JP5994712B2 - Manufacturing method of ultra-thick martensitic stainless steel plate with excellent internal properties - Google Patents

Description

  The present invention is a method for producing an extremely thick martensitic stainless steel plate (JIS G4304 (SUS410 / SUS410S) ASTM A240TP410S having a product plate thickness of 80 mm or more and 230 mm or less) having very good internal properties by continuous casting slabs, and is produced thereby. The present invention relates to a very thick martensitic stainless steel plate with excellent internal properties.

  In general, a thick steel plate is manufactured using a continuous cast slab or an ingot slab obtained by ingot casting a cast ingot. Comparing the two, the former is more attractive in terms of production cost, but in the case of continuous cast slabs, the thickness is thinner than the ingot slab, so in the production of extra-thick steel plates, the unbonded part of the center porosity is The remaining ratio is high.

  With the current capacity of the plate mill, it is difficult to stably eliminate the center porosity that inevitably occurs at the center of the plate thickness of the continuous cast slab. When an ultrasonic flaw detection test is performed on an extremely thick steel plate manufactured from a continuously cast slab, the number of defects detected due to poor quality of the steel plate increases as the rolling reduction decreases.

  Therefore, in the production of extra-thick steel plates using continuous cast slabs, it is common practice to set a lower limit for the rolling ratio (slab thickness / product thickness), and for example, the general production conditions for steel plates for pressure vessels are defined. In ASTM A20, the lower limit value of this reduction ratio is defined as 3 or more. Although there are no such rules in the JIS standard, the steel sheet manufacturer performs the management and manufactures independently.

  When extremely thick steel plates had to be manufactured under processing conditions that were below the lower limit, ingot slabs were used, but in the manufacturing process, unsteady portions of feeders and precipitated crystal parts were cut off or split rolled. , Leading to increased costs and reduced productivity.

  Even though the rolling ratio is small, it is desirable to produce an ultra-thick steel plate that is excellent in the quality of the steel plate, and several proposals have been made for this purpose.

  Non-Patent Document 1 discloses a technique for manufacturing a steel plate with good steel sheet quality by increasing the numerical value defined by the rolling shape ratio. Patent Documents 1 and 2 disclose a technique for eliminating the center porosity of a continuous casting slab by applying a roll or flat metal squeezing on the exit side of the continuous casting machine.

  In Patent Document 3, when producing an extremely thick steel plate having a total reduction ratio of 20 to 60% under a thickness of 125 mm or more, prior to thick plate rolling, the end of the continuous cast slab is forged and reduced from the width direction, A technique for eliminating the center porosity by reducing the width of the original slab by 150 mm or more and increasing the thickness and forging reduction in the slab thickness direction is disclosed.

JP-A-55-114404 JP-A 61-273201 Japanese Patent No. 3528504

Iron and steel 66 (1980) No. 2, pages 201-210, published by Japan Iron and Steel Institute

  However, in the technique of Non-Patent Document 1, since the rolling shape ratio necessary for obtaining a very thick steel plate with good steel plate quality exceeds the upper limit of the range restricted by the equipment specifications of the rolling mill, the practicality is poor. In the techniques of Patent Documents 1 and 2, there is a problem that it is too costly to improve the equipment of the continuous casting machine for thick plate material. The technology of Patent Document 3 is a martensitic stainless steel represented by SUS410, which is different from these steel types in general 40 kg class steel, general 50 kg class steel, and high carbon steel. The effect with thick steel plates is unknown.

  The present invention has been made in order to solve the above-mentioned problems, and is an extremely thick martens manufactured by using existing equipment from a continuous casting slab and having extremely good steel sheet quality and having a product thickness of 80 mm or more and 230 mm or less. It is an object of the present invention to provide a method for manufacturing a site-based stainless steel plate (for example, JIS G 4304 (SUS410 / SUS410S) ASTM A240 TP410S) and an extra-thick austenitic stainless steel plate manufactured thereby.

The inventors of the present invention have made various studies to achieve the above object, and have obtained the following knowledge. In the present invention, the center porosity and the zaku defect are collectively referred to as center porosity.
1. A center porosity with a larger size than the center porosity generated in the center in the width direction of the slab is generated in the three-surface cooling part at both width ends of the slab by continuous casting, particularly in the region corresponding to the slab thickness from the end. If there is an internal defect area and the amount of plastic strain at the center of the slab thickness in forging or rolling is not sufficient, it remains, resulting in poor quality of the steel sheet.
2. The internal defective area can be extinguished by applying a forging method in which the reduction ratio is properly defined after forging the both ends in the width direction of the slab from the width direction and thickening the both ends. .
3. If the center porosity of the internal defective area disappears, the center porosity generated at the center in the width direction of the slab also disappears at the same time.

The present invention has been made by further study based on the obtained knowledge. That is, the present invention is (1) a method for producing an extra-thick martensitic stainless steel sheet having a thickness of 80 mm or more, comprising the following steps. 1. A step of heating a slab by continuous casting in a range of 1010 ° C. to 1150 ° C., forging down both ends in the width direction of the slab, reducing the slab width by 300 mm or more, and thickening the both ends.
2. The forging die vertically symmetric, A _C1 transformation point (830 ° C.) or more from the step of forging the reduction in beyond a reduction ratio of 10% slab thickness direction at a temperature.
3. Thereafter, the slab is heated in the range of 1090 ° C. to 1150 ° C., and the rolling is performed under the processing conditions in which the total reduction ratio of the forging and the rolling reduction ratio in 2 is 20% or more and less than 60%.
(2) The number of porosity of 0.1 mmφ to 1.0 mmφ diameter is 10/100 mm or less at 80 mm thickness or more in the slab rolling vertical direction full width, 1/2 thickness of plate thickness ± 10 mm position Extremely thick martensitic stainless steel sheet with excellent internal properties.
(3) An extra-thick martensitic stainless steel sheet excellent in internal properties, in addition to claim 2, having a martensite fraction of less than 10%.

  According to the present invention, an ultra-thick martensitic stainless steel plate that has conventionally been manufactured using a block slab as a raw material is used to completely eliminate the center porosity of a continuous cast slab using an existing forging press. It can be manufactured and is extremely useful industrially.

The schematic diagram explaining a mode that a continuous casting slab is forged in the width direction.

  In the present invention, the three-surface cooling portion at both width end portions of the slab, particularly the inner quality defective portion in the vicinity of the inner portion of the slab corresponding to the slab thickness (hereinafter, this region is referred to as triple point) is eliminated. After forging the both ends in the direction from the width direction, forging is performed so that the reduction rate in the slab thickness direction is 10% or more.

  In the present invention, first, in order to make the both ends in the width direction of the continuous cast slab thickened, as shown in FIG. 1, the ends (E) in the width direction of the continuous cast slab (1) are The anvil (2) and the lower anvil (3) are pressed down from the width direction to increase the thickness of both ends (E) of the continuous cast slab (1).

  The slab by continuous casting is heated in the range of 1010 ° C to 1150 ° C. If it is less than 1010 ° C., deformation due to forging cannot be obtained sufficiently, and if it exceeds 1150 ° C., the high-temperature strength decreases and the effect due to forging cannot be obtained sufficiently, so that the temperature is set to 1010 ° C. to 1150 ° C.

  The reduction margin (r) of the slab width due to the reduction in the width direction is set to 300 mm or more. When the reduction allowance (r) of the slab width is set to 300 mm or more and then forging is performed, the center porosity in the vicinity of the triple point and in the center portion of the slab width can be eliminated by taking into account the effect of rolling down the plate. It is.

Forging, slab performed at a temperature higher than A _C1 transformation point (830 ° C.), the forging die is rolling at a reduction ratio of 10% or more slab thickness direction.

If the slab temperature is less than the A _C1 transformation point (830 ° C.), the forging effect cannot be obtained sufficiently, so the A _C1 transformation point (830 ° C.) or higher is set. If the reduction ratio in the slab thickness direction is less than 10%, the center porosity in the vicinity of the triple point and the center part of the slab width cannot be eliminated, so the reduction is performed by 10% or more. After forging, the slab is heated to 1090 ° C to 1150 ° C. Thick plate rolling is performed under processing conditions in which the total reduction ratio of the forging reduction ratio and the rolling reduction ratio is 20% or more and less than 60%. When the slab heating temperature is less than 1090 ° C., the alloy element is not sufficiently dissolved, and when it exceeds 1150 ° C., the austenite grain size becomes too large and the toughness deteriorates, so that the temperature is set to 1090 ° C. to 1150 ° C.

  After forging the above-described forging to a continuous cast slab having a slab thickness of 110 to 350 mm, the ultrathick steel plate having a product plate thickness of 80 mm or more and 230 mm or less is rolled. Therefore, the total reduction ratio of forging and rolling reduction is totaled. The rolling reduction is 20% or more and less than 60%. If the total rolling reduction is less than 20% and 60% or more, an extremely thick steel plate having a desired product thickness cannot be obtained.

  According to the method of the present invention, the number of porosity with a diameter of 0.1 mmφ to 1.0 mmφ at a position of 80 mm thickness or more, full width in the vertical direction of the slab rolling, ½ thickness ± 10 mm of the plate thickness ± 10 mm is 10/100 mm It is possible to manufacture ultra-thick martensitic stainless steel sheets with excellent internal properties. If the martensite fraction is less than 10%, the material properties are further improved, and it is possible to produce an ultra-thick martensitic stainless steel plate having excellent internal properties and workability. If this invention is a martensitic stainless steel, the effect will be acquired and the component composition of steel will not be prescribed | regulated.

  Using a continuous cast slab having a thickness of 110 to 350 mm (JISG4304 (SUS410 / SUS410S) ASTM A240TP410S), the reduction ratio of forging and thick plate rolling was changed, and an extra-thick steel plate having a thickness of 79 to 228 mm was manufactured and obtained. The steel sheet was subjected to an ultrasonic flaw detection test according to JISG0801, and the internal properties were investigated. Table 1 shows the chemical composition of the test steel.

  The investigation of the porosity was conducted by observing the porosity of 0.1 mmφ to 1.0 mmφ with an optical microscope (X200) from the end of the slab to the full width of the vertical cross section of 100 mm from the end of the slab to 1/2 (center). And the number per unit length (pieces / 100 mm) was measured. In addition, the pass / fail standard was the number of porosity of 0.1 mmφ to 1.0 mmφ which has been applied in steel for molds (NKM3), and 10/100 mm or less was within the scope of the present invention.

  Table 2 shows the manufacturing conditions and test results together. The extremely thick steel plates (No. 6, 7, 11, 13, 15-21, 23-26, 28) manufactured by the method of the present invention each have a porosity of 10 mm / 100 mm with a porosity of 0.1 mmφ to 1.0 mmφ. It was the following. Comparative examples (Nos. 1 to 5, 8 to 10, 12, 22, and 27) out of the scope of the present invention are over any one of the rolling reduction ratio, the forging rolling reduction ratio, the total rolling reduction ratio, or a plurality of conditions of the slab width end. The ultrasonic flaw detection test result was unacceptable.

1 Continuous casting slab 2 Upper anvil 3 Lower anvil E Both ends r Reduced allowance

Claims (1)

The manufacturing method of the ultra-thick martensitic stainless steel plate of 80 mm thickness or more provided with the following processes.
1. A step of heating a slab by continuous casting in a range of 1010 ° C. to 1150 ° C., forging down both ends in the width direction of the slab, reducing the slab width by 300 mm or more, and thickening the both ends.
2. The forging die vertically symmetrical, the step of performing the forging of reduction in A _C1 transformation point (830 ° C.) or higher temperatures in the slab thickness direction reduction ratio of 10% or more.
3. Thereafter, the slab is heated in the range of 1090 ° C. to 1150 ° C., and the rolling is performed under the processing conditions in which the total reduction ratio of the forging and the rolling reduction ratio in 2 is 20% or more and less than 60%.

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