JP4543980B2 - Manufacturing method of extra-thick steel plate with excellent internal properties - Google Patents

Description

  The present invention relates to a method for producing a very thick steel plate (product plate thickness of 150 to 200 mm or more) in thick plate rolling, and in particular, by effectively eliminating (closing-crimping) the center porosity and the zaku defect of the continuous cast slab, We propose a method to advantageously manufacture extra-thick steel plates with very good quality.

  In general, a thick steel plate is manufactured by using a continuous casting slab or an ingot slab obtained by split-rolling a mold-cast ingot and rolling them thick. Comparing the two, the former is more attractive in terms of manufacturing cost. However, in the case of continuous casting slabs, the thickness is thinner than the ingot slabs. The ratio of the remaining portion is high, and with the current capacity of the thick plate rolling mill, it is difficult to stably eliminate the center porosity that inevitably occurs at the central position of the continuous cast slab. Therefore, when an ultrasonic flaw detection test is performed on an extremely thick steel plate manufactured from a continuously cast slab, the quality of the steel plate is increased as the rolling reduction decreases.

  Therefore, in the manufacture of conventional heavy steel plates, when a lower limit is set for the rolling ratio (slave thickness / product thickness) and the heavy steel plates must be manufactured under processing conditions that fall below that lower limit, It is common to use a slough. For example, ASTM A20, which defines general production conditions for steel plates for pressure vessels, sets the lower limit of this reduction ratio to 3 or more, and the JIS standard does not provide such rules, but the steel plate manufacturer manages it voluntarily. The actual situation is that it is done and manufactured.

  However, in the case of ingot-making slabs, there is a problem in that the production process requires intermediate steps such as truncation of unsteady portions of the feeder and precipitation crystal parts and ingot-rolling, leading to increased costs and reduced productivity. is there.

Considering the above, it is desirable to produce an extra-thick steel plate excellent in steel sheet quality characteristics even if the reduction ratio from the continuous casting slab is small, and there are several proposals as described below.
Non-Patent Document 1 discloses that a steel sheet having a good steel sheet quality is manufactured by increasing the numerical value defined by the rolling shape ratio. Patent Documents 1 and 2 disclose that the center porosity of the continuous casting slab is eliminated by applying a roll or flat metal squeezing on the exit side of the continuous casting machine. Furthermore, in Patent Document 3, when manufacturing an extra-thick steel plate with a total reduction ratio of 20 to 60% and a thickness of 125 mm or more, the end of the continuous cast slab is forged from the width direction prior to thick plate rolling. It is disclosed that the center porosity is eliminated by reducing and reducing the original slab width by 150 mm or more and increasing the thickness and forging reduction in the slab thickness direction.

JP-A-55-114404 JP-A 61-273201 Japanese Patent No. 3528504 Iron and Steel 66th (1980) No.2, pages 201-210, published by Japan Iron and Steel Institute

However, in the technique of Non-Patent Document 1, the rolling shape ratio necessary for obtaining an extra-thick steel plate with good steel plate quality exceeds the upper limit of the range constrained by the equipment specifications of the rolling mill. There is a problem. Moreover, in the technique of the said patent document 1 and the patent document 2, there exists a problem that it takes too much cost for the equipment improvement of the continuous casting machine for thick plate materials. Furthermore, in the technique of the above-mentioned Patent Document 3, in the manufacture of extra heavy steel sheets with processing conditions where the total rolling reduction is less than 20%, the extinction effect of the center porosity is not sufficient, and the extra heavy steel sheets with a healthy steel sheet quality are obtained. There is a problem that it is difficult to manufacture.

  In view of these points, the main object of the present invention is to make a continuous cast slaved ultra-thick steel sheet with excellent quality of steel sheet under processing conditions within a range of 16% to 20% of total rolling reduction with relatively simple equipment. The idea is to propose steel plate manufacturing technology. Another object of the present invention is to investigate forging conditions effective for eliminating void defects more effectively by a slight reduction ratio in the thickness direction of a continuous casting slab.

  The inventors of the present invention have made various studies to achieve the above object, and the shape of the starting slab to be used for the slab thickness direction forging according to the width direction reduction amount of the continuous casting slab and the upper and lower forged anvils during the slab thickness direction reduction. If the desired relationship is specified for the shape of the steel, we know that the center porosity of the continuous casting slab can be more effectively eliminated more efficiently, and that a continuous cast slaved extra heavy steel plate with excellent steel quality can be obtained. The present invention has been made.

  That is, in a method of producing a very thick steel plate with a thickness of 125 mm or more using a continuous casting slab, reduced in thickness by combining forging and thick plate rolling, and under a processing condition in which the total rolling reduction is in the range of 16% to 20%. After forging down the end of the continuous casting slab from the width direction, reducing the slab width by 300 mm or more from the original slab width and increasing the thickness, the contact length between the forged anvil and the continuous casting slab may differ by about 3 times or more. It is desirable that forging reduction in the slab thickness direction is performed by 6% or more by using a forging anvil that is asymmetrical in the vertical direction. Here, the effect of improving the inner quality of the forging using an asymmetrical forged anvil is that the tensile stress at the center of the slab thickness that occurs when the slab is squeezed symmetrically is intentionally shifted in the slab thickness direction. It is generally known as an FM (Free from Mannesmann effect) forging method using the increase in hydrostatic pressure. In order to eliminate the center porosity, in the present invention, when the contact length of the upper anvil with the continuous casting slab is set to 1 in the forging method using the upper and lower asymmetric forging anvil, By performing forging reduction with a contact length of 3 or more with the continuous casting slab, the quality improvement effect in the thickness direction of the slab is enhanced, and the complete disappearance of the center porosity is achieved.

  Furthermore, the present inventors have studied the shape of a slab that is desirable for subjecting a continuous cast slab to forging pressure in the slab thickness direction. As a result, the three-surface cooling part at both width ends of the continuous casting slab, especially in the region called the triple point inside the slab thickness from the end, is the center porosity size generated at the center in the width direction of the continuous casting slab. When a large internal defect area exists and the amount of plastic strain at the center position of the slab thickness in forging or rolling is not sufficient, it has been found that an uncompressed portion of the center porosity remains in this portion, resulting in poor quality of the steel sheet.

Therefore, in the present invention, as shown in FIG. 1, the forging reduction in the width direction of the continuous casting slab is performed for a certain amount or more, both width end portions of the continuous casting slab are thickened, and the so-called cross-sectional shape becomes a dockbone shape, During forging reduction in the slab thickness direction, as described above, by using a forged anvil that is asymmetrical in the vertical direction where the contact length between the forged anvil and the continuous cast slab differs by about 3 times or more, the above-mentioned center porosity is reduced to the total pressure. It was found that even when the processing conditions were within the range of 16% to 20%, the rate disappeared completely.

  According to the present invention, even when the total reduction ratio is 20% or less, the thickening of the slab thickness associated with an appropriate amount of reduction in the slab width direction is combined with the reduction in the slab thickness direction using an asymmetric forged anvil. As a result, the center porosity of the continuous casting slab has completely disappeared, and it has become possible to produce extra-thick steel plates with extremely superior quality characteristics.

  In the present invention, both ends in the width direction of the continuous casting slab (1) are formed on the upper and lower anvils for forging (2, 3) as shown in FIG. Therefore, the thickness of both ends (E) of the continuous casting slab (1) is increased by reducing from the width direction, but according to various tests conducted by the inventors, the reduction allowance (r ) Is 300mm or more / both sides, and in combination with the use of an asymmetrical forged anvil in the reduction in the thickness direction of the continuous casting slab, the center porosity disappears even when the total reduction rate is 20% or less. I found out that there was enough effect.

  The present invention is a method for producing a continuous cast slab ultra-thick steel plate by combining forging and thick plate rolling in order to effectively eliminate the above-described center porosity. In particular, to ensure sound steel sheet quality characteristics even under processing conditions where the total rolling reduction is in the range of 16% to 20%, the slab width during forging, the thickness direction rolling amount, and the procedure during thickness direction rolling are specified. Stipulate.

  That is, in the present invention, in the method for producing an extremely thick steel plate having a thickness of 125 mm or more under a processing condition in which the total reduction ratio is in the range of 16% to 20%, the end of the continuous casting slab is forged and reduced from the width direction. After reducing the slab width by 300 mm or more from the original slab width and increasing the thickness, the forging reduction in the slab thickness direction is achieved by using an asymmetrical forging anvil where the contact length of the forged anvil and continuous casting slab is different by about 3 times or more, for example. Process control of 6% or more.

  Here, the total rolling reduction is set to 16 to 20% for the following reason. In other words, if the total reduction ratio exceeds 20%, the forging and thick plate rolling can be used together without changing the continuous casting slab width / thickness reduction amount and the forging reduction procedure in the thickness direction as described above. According to the specified production conditions, it is possible to produce a very thick steel plate having sound steel quality characteristics (Japanese Patent No. 3528504). On the other hand, if the total rolling reduction is less than 16%, the center porosity of the continuous casting slab will not be sufficiently crimped even if the forging conditions specified here are applied, and it will be difficult to secure the center position of the plate thickness. .

  Next, the result of manufacturing an extremely thick steel plate using the forging / rolling method according to the present invention will be described with reference to a comparative example.

The inventors changed the rolling reduction ratio of forging and thick plate rolling using a continuous casting slab having a thickness of 310 mm, and conducted an ultrasonic flaw detection test according to JIS G0801 for the internal properties of the steel plate. The results are shown in Table 1.

  From Table 1, when the total reduction ratio is 16 to 20%, the forging reduction amount in the continuous casting slab width direction is 300 mm as in No. 3 and 7, and the upper and lower anvils during the slab thickness direction forging reduction have a slab contact length. Good results were obtained for the quality of extra-thick steel sheets produced with an asymmetry three times different and a forging reduction ratio of 6%. This result is not limited to the general 40kg steel, but the 50% steel (No.9,10), high carbon steel (No.11, Similarly, when applied to tempered 60kg steel (No.13, 14), good results were confirmed for the quality of the extra heavy steel plate.

On the other hand, in No. 1 and 2, when the forging reduction rate in the slab thickness direction is insufficient or when an anvil is symmetric, the quality of the steel sheet is unhealthy. Even when an asymmetrical anvil with a slab contact length ratio of 5 is used, the forging reduction in the continuous casting slab width direction is less than 300 mm, as in No. 4 and 5, or as in No. 6 On the other hand, when the forging reduction ratio in the slab thickness direction was less than 6%, the quality of the steel sheet was unsatisfactory. In addition, when using an asymmetrical anvil with a slab contact length ratio of 2 such as No. 8, a steel plate that is good even when the rolling reduction in the continuous casting slab width direction is 350 mm and the forging reduction ratio in the slab thickness direction is 9% The internal quality could not be secured.

According to the method of the present invention, by using an existing forging press machine, it is possible to reliably and easily manufacture an extremely thick steel plate that has been manufactured using a conventional slab as a raw material by using a continuous cast slab.

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

Explanation of symbols

1 Continuous casting slab 2 Upper metal forging machine 3 Lower metal forging machine E Thickening part r Reduction allowance

Claims (1)

    In a method for producing an ultra-thick steel plate having a thickness of 125 mm or more by using forging and thick plate rolling in combination with a continuous casting slab, both ends in the width direction of the continuous casting slab are forged and reduced from the width direction. After the width is reduced by 300 mm or more from the original slab width and thickened, when the contact length of the upper anvil with the continuous cast slab is 1, the contact length of the lower anvil with the continuous cast slab is 3 With the above-described asymmetrical forging die, forging reduction of 6% or more is performed in the slab thickness direction, and thereafter, plate rolling is performed under the processing conditions in which the total reduction ratio is 16% or more and 20% or less. A method for producing an extra-thick steel sheet.

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