JP2000263103A - Manufacturing method of extra heavy steel plate using continuous cast slab - Google Patents

Abstract

(57) [Summary] [Purpose] With a relatively simple facility, a sufficient forging effect is exerted on the center even at a reduction ratio of less than 3, and not only the center porosity but also the center segregation zone of the continuous cast slab is crushed. We propose a method for manufacturing extremely thick steel plates with extremely homogeneous and excellent internal properties. SOLUTION: In producing an extremely thick steel plate by subjecting a continuous cast slab to forging and thick plate rolling at a total reduction rate of 29 to 66%, the continuous cast slab is heated to 1000 ° C. or more, and then reduced in the forging step. Cross forging at a rate of 20 to 56% is performed with a B / H ratio of 0.7 to 1.0, and thereafter, finish forming and rolling are performed in a rolling step. B is the contact length between the forging anvil and the continuous casting slab during forging reduction, and H is the continuous casting slab thickness during forging reduction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a heavy steel plate, and more particularly to a method for manufacturing a heavy steel plate using a continuously cast slab as a raw material.

[0002]

2. Description of the Related Art An extremely thick steel sheet is produced by using a slab produced by slab-rolling a large ingot or a continuously cast slab produced by continuously casting molten steel (hereinafter simply referred to as a continuously cast slab). It is manufactured by plate rolling. Among them, the former has a problem that the concentrated segregation part of the feeder part and the negative segregation part of the steel ingot bottom must be cut off, so that the yield is low, and the cost and time involved in slab rolling are required. On the other hand, although the latter method using a continuous cast slab does not have such a problem, the reduction ratio until reaching the product after casting is small due to the thickness of the continuous cast slab, so the center porosity and segregation existing in the continuous cast slab There was a problem that it was difficult to crush the belt. Therefore, the lower limit of the rolling reduction ratio is set when the ductility or toughness of the product center is required, and when it is lower than that, a lumped slab is used.

[0003] However, when a slab is used as a raw material, the production cost becomes extremely high for the above-mentioned reason. For this reason, conventionally, an attempt has been made to obtain a very thick steel plate having high ductility and toughness at the center by pressing center porosity during the manufacturing process while using a continuously cast slab. For example, "Iron and Steel" No. 201, No. 201
On pages 210 to 210, there is proposed a method for producing a thick steel plate having good internal quality by increasing the numerical value defined by the rolling shape ratio. In Japanese Patent Application Laid-Open Nos. 55-114404 and 61-273201, rolls or planar pressing means are provided on the exit side of a mold of a continuous casting machine to close a center porosity immediately after casting. Means for achieving pressure bonding have been proposed. Further, Japanese Patent Laid-Open No. 7-2
Japanese Patent No. 32201 discloses that a thick steel plate is manufactured from a continuous cast slab under a condition of a reduction ratio of 70% or less (a reduction ratio of 3 or less).
A method is disclosed in which a predetermined reduction is applied to a continuously cast slab by forging prior to roll rolling, and the center porosity is pressed.

[0004]

However, each of the above proposals has problems. First, “Iron and Steel” No. 6
In the method proposed on page 201-210 of the 6th year,
Since the rolling shape ratio is restricted by the equipment specifications of the rolling mill, it is practically impossible to reduce the rolling reduction to less than 3. Also,
The method proposed in Japanese Patent Application Laid-Open No. 55-114404 requires modification of a continuous casting machine and cannot be easily implemented. Further, the method disclosed in Japanese Patent Application Laid-Open No. Hei 7-232201 shows a compression effect of center porosity, but an insufficient effect of pulverizing the segregation zone at the center of the continuous casting slab.

[0005] The present invention solves the problems found in the above proposals and provides a sufficient forging effect at the center even at a reduction ratio of less than 3, while using relatively simple equipment, and provides not only center porosity but also center porosity. It is an object of the present invention to propose a method for manufacturing an extremely thick steel plate having a very homogeneous and excellent internal property, in which a central segregation zone of a continuously cast slab is also pulverized.

[0006]

Means for Solving the Problems The inventor of the present invention has conducted various studies to solve the above-mentioned problems. If the contact length between the continuous casting slab and the forged metal sheet is set to a certain value or more at the time of forging reduction, the sheet thickness center is reduced. That a large composition strain can be applied to the part,
By conducting the forging and stretching directions in both the longitudinal direction and the width direction of the continuous casting slab, it was found that the pressing effect of the center porosity of the continuous casting slab and the pulverizing effect of the segregation zone were simultaneously obtained, and the present invention was completed. .

More specifically, the present invention relates to a method of manufacturing an extra-thick steel plate, which is performed by subjecting a continuously cast slab to forging and rolling at a total reduction ratio of 29 to 66% to produce an extra-thick steel plate. Is heated to 1000 ° C. or higher, and in the forging step, the cross forging with a reduction ratio of 20 to 56% is performed at a B / H ratio of 0.1%.
7 to 1.0, and then finish forming rolling is performed in the rolling process. In addition, B is the contact length between the forged metal anvil and the continuous casting slab during forging reduction, and H is the continuous casting slab thickness during forging reduction.

[0008] In the present invention, the crushing effect of the segregation zone is surely obtained by setting the ratio of the reduction amount in the longitudinal direction to the reduction amount in the width direction of the continuous casting slab in the cross forging in the above invention to 30 to 70%. Further, the center segregation of the continuously cast slab can be more reliably improved by setting the finish forming rolling after the forging step to 30% or less of the total draft.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the manufacturing process of an extremely thick steel sheet according to the present invention, a method of forging and rolling a continuously cast slab is employed. In this case, the total draft is 29 to 66%.
And If the total reduction ratio is less than 29%, it is effective for pressure bonding of center porosity, but the pulverizing effect of center segregation is insufficient. On the other hand, if it exceeds 66%, it corresponds to the case where the reduction ratio exceeds 3. This is because the effect of improving the structure including the narrowing of the central segregation zone is sufficiently recognized by simple rolling, and it is not necessary to use forging together. In the present invention, the extremely thick steel sheet refers to a steel sheet having a product thickness exceeding 100 mm according to the general definition, but is not necessarily limited to this, and even if the product thickness is smaller than that, the reduction ratio is 3 or less. Refers to a steel plate to be manufactured.

The continuous cast slab is heated and soaked at a temperature of 1000 ° C. or more. In the case of the present invention, compression of center porosity by forging and grinding of center segregation are performed,
To further improve center segregation, perform high-temperature heating and soaking,
It is desirable to diffuse C, Mn, P, S, and the like. Therefore, in the present invention, the heating temperature of the continuous casting slab is set to 1000 ° C.
It was above.

In the present invention, a forging step is performed prior to roll forming. At that time, B shown in FIG. 1, that is, the contact length between the forged anvils 2, 2 'and the continuous casting slab 1 during forging reduction. And H, that is, the ratio of the thickness of the continuous casting slab during forging reduction, that is, the B / H ratio is set to 0.7 to 1.0.
FIG. 2 is a diagram showing the results of analysis of the effect of the B / H ratio on the maximum plastic strain in the thickness direction at the center of the thickness at the same rolling reduction. As the size increases, the maximum amount of plastic strain in the thickness direction increases. From this viewpoint, the B / H ratio is preferably set so that the maximum amount of plastic strain is maximized. However, the forging effect of forging,
In particular, the effect of pulverizing the center segregation of the slab is more uniform when the reduction amount is the same when the forging reduction position is increased, and therefore, the B / H ratio has an upper limit. In the present invention,
These effects and the effect of the B / H ratio on the effect of pulverizing and improving the maximum amount of plastic strain in the thickness direction and the center segregation in the longitudinal direction of the slab are comprehensively considered, and are considered as 0.7 to 1.0. And

In the above forging, the present invention employs cross forging. Here, the cross forging refers to forging in which the forging process is divided into two processes and the forging direction is different between the first process and the second process. This will be described below with reference to FIG. That is, in the first forging process, the anvil 2 in which the slab 1 is arranged in a direction perpendicular to the slab longitudinal direction
After forging by 2 'to stretch the slab 1 in the longitudinal direction, in the second forging step, forging is performed by using the slab longitudinal forgings 3, 3' arranged in parallel with the slab longitudinal direction to spread the slab in the width direction. The process is performed. In this case, the first forging process is a longitudinal forging performed so that the slab is forged and stretched in the longitudinal direction, and subsequently, it is normal to perform a width forging in a direction perpendicular to this, but even if these are reversed. Well, there is no need to cross exactly at right angles. In short, it suffices if forging effects in different directions are provided in the slab plane.

FIG. 3 is a relational diagram showing the distribution of the amount of plastic strain in the three axial directions of the continuous cast slab when the above-mentioned lengthwise forging is performed at a reduction ratio of 16% and a B / H ratio of 0.74. However, as shown here, only in the longitudinal direction forging, plastic strain occurs in the compression direction in the plate thickness direction due to shrinkage of the slab cross section, and in the slab longitudinal direction by the rounded part of the forged metal sheet Since the slab is forged, plastic strain occurs in the tensile direction, but in the slab width direction, it is almost neutral and almost no plastic strain occurs. In such a state, the pulverizing effect of center segregation cannot be sufficiently obtained. On the other hand, when the above-mentioned cross forging is performed, not only the compression of the center porosity but also the pulverization of the center segregation are sufficiently performed. This is a finding obtained by the present inventors and is the most important point.

In the present invention, the total reduction including the forming and rolling by thick plate rolling is 29 to 66%. Of these, the reduction by cross forging is large, and the total of longitudinal forging and width forging is 20%. To 56%. If it is less than 20%, the effect of improving the structure cannot be sufficiently obtained even when cross forging is performed. On the other hand, if it exceeds 56%, the number of heating times of forging is required to be two or more, which increases the manufacturing cost, which is undesirable. . Forming and rolling is 30% of the total reduction.
The following fully accomplishes that goal.

The ratio of the amount of reduction in the longitudinal direction to the amount of reduction in the width direction of the continuous casting slab in the cross rolling is 30 to 70%.
It is good to do. When the amount of reduction in the width direction is extremely small relative to the amount of reduction in the longitudinal direction, or vice versa, the segregation and pulverization effect by cross rolling cannot be obtained. Basically, it is preferable to divide these reduction amounts equally, but they can be appropriately determined within the above range in consideration of required characteristics of the product and restrictions on the product shape after rolling.

[0016]

EXAMPLE Molten steel having a composition with a tensile strength of 50 kg / mm ² was continuously cast into a slab having a thickness of 310 mm. The obtained continuous cast slab was processed by changing the heating temperature, forging conditions, and plate rolling conditions as shown in Table 1. Obtained product (test material)
The effect of the present invention was confirmed by investigating the microstructure at the center of the sheet thickness mainly for the presence or absence of center porosity and the degree of pulverization of the segregation zone.

[0017]

[Table 1]

[0018]

[Table 2]

The results are shown in Table 2. As is clear from Tables 1 and 2, when all the conditions of the present invention are satisfied (Sample Nos. 2, 5, and 8), the center porosity disappears and the segregation zone is also pulverized to show a uniform structure. However, the test material No. When the heating temperature was insufficient as in 1 and 9, the effect of diffusion was insufficient, and many segregation bands remained. In addition, sample material No. When the B / H ratio was small as in 3 and 6, even if the total draft was large, the pulverizing effect of the segregation zone was insufficient. Similarly, sample N
o. Even when forging was performed only in one direction as in 4, 7, and 10, the effect of grinding the segregation zone was insufficient. In addition, sample No. When the total reduction is extremely low as in 11,
Despite cross-forging, the center porosity remained. FIG. 5 shows a microstructure at the center of the thickness of a thick steel plate manufactured according to the present invention, and FIG. 6 shows a metal micrograph of a microstructure in which a segregation zone microstructure shown as a comparative example remains.

As can be understood from the above results, when the conditions of the present invention are satisfied, the center porosity disappears and the segregation zone is pulverized and vanished by the forging effect and diffusion. The material properties of were improved.

[0021]

According to the present invention, although the total draft ratio is less than 3.0, the forging effect is sufficiently exerted even in the center of the thickness of the extremely thick steel plate, and the segregation zone in the center is reduced. As a result, it is possible to manufacture a steel sheet having a more uniform material.
In addition, according to the present invention, such a steel sheet can be manufactured using an existing forging press machine, so that the manufacturing cost can be reduced as compared with the related art.

[Brief description of the drawings]

FIG. 1 is an explanatory view schematically showing a dimensional relationship between a continuous casting slab and an anvil used for forging in a forging process.

FIG. 2 is a diagram showing the influence of the B / H ratio on the maximum plastic strain in the thickness direction at the center position of the thickness.

FIG. 3 is a relationship diagram showing a distribution of a plastic strain amount in a three-axis direction of a slab when a lengthwise forging is performed on a continuously cast slab.

FIG. 4 is an explanatory diagram showing a positional relationship between a slab and an anvil in cross forging.

FIG. 5 is a metallurgical micrograph of a central part of a thick steel plate manufactured according to the present invention.

FIG. 6 is a metal micrograph of a thick steel plate having a segregated structure remaining as a comparative example and showing a central portion of the thickness of the steel plate.

[Explanation of symbols]

 1: Continuous cast slab 2, 2 ': Forged anvil 3, 3': Anvil at the time of forging and elongation in slab width H: Thickness of continuously cast slab under forging reduction B: Contact length

Claims (3)

[Claims] In producing a very thick steel sheet by subjecting a continuous cast slab to forging and rolling at a total draft of 29 to 66%, the continuous cast slab is heated to 1000 ° C. or more, and then the draft is reduced in the forging step. A method for producing an extra-thick steel sheet, comprising performing a cross forging of 20 to 56% with a B / H ratio of 0.7 to 1.0, and then performing finish forming rolling in a rolling step. Here, B: contact length between forged anvil and continuous casting slab during forging reduction H: thickness of continuous casting slab during forging reduction 2. The method according to claim 1, wherein the ratio of the amount of reduction in the longitudinal direction to the amount of reduction in the width direction of the continuous cast slab in the cross forging is 30 to 70%. 3. The method for producing an extremely thick steel sheet according to claim 1, wherein the finish forming and rolling is performed at 30% or less of the total draft.