JP3261556B2 - Continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reduction in toughness of a welded portion at the center of a sheet thickness of a hot-rolled material manufactured by a continuous casting method, and a hydrogen-induced cracking of a line pipe material used in a poor environment. The present invention relates to a method for supplying a continuously cast slab which eliminates defects such as segregation and center porosity of the slab which cause the slab.

[0002]

2. Description of the Related Art Conventionally, C, Si, M are formed at the center of the thickness of a slab subjected to hot rolling produced by a continuous casting method.
It is inevitable that n, P, S and other various elements are inevitably concentrated, and voids called center porosity are inevitably generated. The deterioration of steel quality due to accumulation of hydrogen in steel was inevitable.

[0003] Therefore, when hot rolling is performed using a continuous cast slab having these defects, refining is performed in order to substantially reduce the influence of elements such as phosphorus and sulfur which are particularly easily segregated during the solidification process. In the process, processing such as low phosphorous reduction and low sulfurization, addition of a step to perform dehydrogenation processing to reduce hydrogen that easily accumulates in the center porosity part, and soaking even before hot rolling In addition to performing a heat treatment to promote the diffusion of segregated elements and hydrogen, etc., referred to as a diffusion treatment, a preliminary treatment for pressing the center porosity is performed by high-pressure reduction rolling, referred to as a high shape ratio rolling, to remove the defects. Means for preventing deterioration of the quality of the steel material due to this have been used.

[0004] Against such a background, the steel material manufactured through these steps is necessarily a costly and economically inferior manufacturing method.

Accordingly, a technique for eliminating segregation and center porosity at the center of the thickness of the continuous cast slab having such defects has been strongly demanded, and new methods have been devised.

For example, two or more pairs of reduction rolls are arranged near the crater end where solidification is completed, and the reduction ratio per unit length in the casting direction is defined within a certain range as a function of the roll pitch and the crater end position. A method for preventing such defects is disclosed in JP-A-52-56017.

[0007] Further, the solidification end position is sandwiched by using a surface member, and in a region having a solidification rate of 40% or more, the rolling reduction per operation is 1.5% or less, and the total rolling reduction is 0.5 to 5.0%. Japanese Patent Laid-Open No. 59-59139 describes a method of completely solidifying while intermittently reducing the pressure in the range of
-202145.

[0008]

Since the slab produced by such a method is compensated for solidification shrinkage and heat shrinkage, segregation caused by agglomeration and accumulation due to the flow of molten steel resulting from these is prevented. In addition to the fact that the flow of molten steel caused by bulging, etc. is also prevented when the surface member is lightly reduced, segregation and center porosity can be further reduced. ing.

However, in the region where the solidification rate at the end of solidification is high, the viscosity of the molten steel is too high to flow easily. It is impossible to compensate for
Therefore, the fact is that the segregation and the center porosity have not been completely prevented.

[0010] In the light rolling method using a roll, the amount of reduction directly under the roll is inevitably increased due to the deformation characteristics of the solidified shell of the slab, and it is possible to secure a uniform gradient over the entire length of the slab. Not only is it difficult, but if the solidification is completed directly under the reduction roll, it is not necessary to reduce the bulging, but if the solidification is completed between the rolls, it also prevents the accompanying flow. Since there is no means to accurately grasp the solidification completion position, it is extremely difficult to control the reduction during casting. It was inevitable that large fluctuations existed in the segregation and center porosity at the center of the piece.

[0011] Accordingly, even though the slabs produced by applying these methods do not require the soaking process of soaking for a long time as before, the load of the refining process and the pretreatment such as heat treatment and high shape ratio rolling are still required. Therefore, there is a problem that the manufacturing cost is still high and the product thickness cannot be increased.

[0012]

SUMMARY OF THE INVENTION In order to solve the above-mentioned various problems, the present invention provides a method for producing a slab for hot rolling by continuous casting, wherein a solidification rate is 85% or more and 99% or less. Depending on the surface, light reduction of 1 mm or more and 25 mm or less is performed intermittently, and at the time of light reduction, the relationship between the unsolidified thickness on the entry side of the reduction band and the first reduction amount and the reduction speed satisfies the relationship of equation (1). A method for producing a continuous cast slab having no defect at the center of the slab thickness.

[0013]

## EQU2 ## δ · V / d ≧ 10 (1)

Here, δ is the first rolling reduction (mm),
d is the unsolidified thickness (mm) at the entrance side of the reduction zone, and V is the reduction speed (mm / sec) of the reduction terminal.

[0015]

Hereinafter, the present invention will be described in detail.

First, the influence of segregation on the quality characteristics of products will be described as a representative of the segregation ratio (= concentration of element having segregated portion / concentration of molten steel, here, segregation ratio of phosphorus (abbreviated as P segregation ratio). .

FIG. 3 shows the relationship between the P segregation ratio in the slab and the toughness value of the heat affected zone at -60 ° C. in the thick plate product.
FIG. 4 shows the relationship between the maximum diameter of the center porosity and the tensile strength in the tensile test in the Z direction.

As shown in FIG. 3, when the P segregation ratio exceeds 2.0, the toughness value of the heat affected zone at -60 ° C. is extremely lowered, and the variation greatly depends on the standard of the steel material. Although the values do not satisfy the values or do not satisfy the standard values, there are problems that stable quality cannot be ensured.

On the other hand, when the P segregation ratio is less than 0.8, -6
It was found that the toughness value at 0 ° C. again showed a low value. In other words, they have found that the condition for obtaining a high toughness value can be obtained only in the range of 0.8 to 2.0 in P segregation ratio.

On the other hand, as shown in FIG.
It was also found that if a center porosity exceeding 1 mm is present in the cast slab, even if the P segregation ratio is 2.0 or less, characteristics satisfying the standard value in the tensile test in the Z direction cannot be secured.

The present inventors have further studied and studied a method for simultaneously controlling the segregation ratio of P to 2.0 or less and the maximum diameter of the center porosity at the center of the sheet thickness to 0.1 mm or less. In examining the solidification morphology in the laboratory, the solidification morphology near the final solidification part in the continuous casting process was reproduced in a laboratory using a simulated rolling device for solidification end shown in FIG. The relationship of unsolidified thickness was investigated.

As a result, as shown in FIG. 1, as the unsolidified thickness increases, in a region where the P segregation ratio can be controlled to 2.0 or less, the rolling reduction δ must be increased. On the other hand, if too large, there is a region with a P segregation ratio of less than 0.8 that generates a low value or there is a reduction amount at which internal cracks are generated, and the critical reduction amount at which this internal crack occurs varies depending on the solidification rate, The experiment found that the solidification rate was 25 mm or more in the range of 85% to 99%.

That is, it has been found that the P segregation ratio cannot be maintained in an appropriate range of 0.8 to 2.0 unless the relationship between the unsolidified thickness d and the reduction amount δ is maintained within a certain range.

However, even under such conditions, the P segregation ratio may be less than 0.8 or more than 2.0 in some cases, and it is not always possible to stably eliminate segregation and center porosity.

The present inventors further studied the cause of this, and as a result, found that the speed at the time of rolling was greatly affected. As a result of conducting experiments in which the rolling speed was variously changed in addition to the above conditions, FIG. As shown in Equation (1), if the equation (1) is not satisfied, the P segregation ratio that satisfies the above-described product quality characteristics can be 0.8 to 2.0 even if only the unsolidified thickness d during rolling and the rolling reduction δ are controlled.
It was found that it could not be controlled stably within the range.

[0026]

## EQU3 ## δ · V / d ≧ 10 (1)

Here, δ is the first rolling reduction (mm),
d is the unsolidified thickness (mm) at the entrance side of the reduction zone, and V is the reduction speed (mm / sec) of the reduction terminal.

The present invention has been made based on the above findings.

[0029]

EXAMPLES Examples of the present invention and comparative examples will be described below in detail.

6 and 7 using steels having the components shown in Table 1.
And the continuous cast slab produced by the rolling equipment shown in FIG. 8 under the conditions shown below was manufactured by applying the conditions shown in Table 2 to the strength and toughness of the thick steel plate, the Z-direction tensile test, and the hot-rolled steel plate. Table 2 shows the strength and toughness of the sour line pipe material and the crack area ratio (CAR) after the hydrogen-induced cracking test, which is divided into examples of the present invention and comparative examples.

No. 2 in Tables (1) to (3) 1 to N
o. No. 7 is an example of the present invention. 8 to No. 15 is a comparative example.

No. 2 in Table 2. 1 to No. As shown in FIG. 7, the P segregation ratio of the slab produced in the present invention example is in the range of 0.9 to 2.0 necessary for securing the quality characteristics of the product,
In addition, the results obtained were such that the center porosity was completely absent, and all the properties of the product obtained by hot rolling were sufficiently satisfied with the standard values.

In the comparative examples, no. 8 to No. 12 is (1)
It is an example that did not satisfy the condition shown by the equation, the P segregation ratio in the slab is 2.0 or more, and those having a center porosity maximum diameter of more than 0.1 mm frequently occur,
Various properties of the product obtained by hot rolling this product did not satisfy the standard values.

Further, No. 13-No. No. 14 is a case where the amount of reduction per operation is extremely large, and although segregation and generation of center porosity are prevented, in addition to the generation of negative segregation having a segregation ratio of less than 0.8, it is described in the table. However, internal cracks were generated due to the reduction, which was a major cause of material deterioration.

No. No. 15 is a case in which no active reduction was performed, but as is clear from the table, not only the segregation at the center of the slab thickness, but also the level of the center porosity was extremely poor, so that the material of the obtained product was satisfactory. It was in a situation that had not been reached.

As can be seen from these results, all of the steel sheets manufactured from the continuous cast slabs for hot rolling manufactured according to the present invention exhibited excellent properties.

The method for producing the continuous cast slab is as follows. Continuous cast slab dimensions; thickness 200/284 mm x width 19
00mm late solidification segregation and center porosity controller;
(Apparatus shown in Fig. 3) Model Walking bar system Configuration Inner bar (2) 3 Outer bar (1) 4 Shift amount 100mm Rolling down length 2.5m Rolling down casting side slab thickness Up to 284mm Rolling down amount in rolling band 0 to 35 mm, unsolidified thickness at the time of reduction band entry 0 to 40 mm

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

As described above, the present invention stably achieves the reduction of cast material quality, especially segregation and center porosity, which are extremely important in securing the material properties required for conventional hot rolled materials. In order to be able to do this, not only pretreatments such as low sulfuration, low phosphorus reduction and dehydrogenation treatments that were conventionally performed in the molten steel treatment process, but also pretreatments such as high-temperature ripening heating and high shape ratio rolling in the hot rolling process are completely unnecessary. Since it becomes unnecessary and can be manufactured very economically as well as stabilizing the material of the product, the effect brought to this field is extremely large.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between the segregation ratio, the limit of the occurrence of internal cracks, the rolling reduction of an ingot, and the solidification rate (the region indicated by S in the drawing is an appropriate rolling reduction region).

FIG. 2 is a graph showing the relationship between the unsolidified thickness at the entry side of the reduction zone, the reduction amount, and the reduction speed, which affect the segregation ratio.

FIG. 3 is a diagram for explaining a relationship between a segregation ratio and a toughness value in a steel material.

FIG. 4 is a view for explaining the relationship between the maximum diameter of the center porosity and the tensile strength in the Z direction.

FIG. 5 is an explanatory view of an apparatus for simulating a surface pressure reduction of a solidification end portion.

FIG. 6 is a side view of one embodiment showing a means for reducing the surface of an unsolidified end of a slab in the continuous casting step of the present invention.

FIG. 7 is a front view in a BB section of FIG. 6;

FIG. 8 is a cross-sectional view of a surface member of the surface reduction device employed in the embodiment of the present invention.

[Explanation of Signs] 1 Ingot 2 Unsolidified part of ingot 3 Cooling water 4 Differential transformer 5 Depressor jack 6 Ingot elevating device 7 Depressor terminal 8 Load cell 9 Mold 10 Support roll 11 Cast piece 12 Surface member 12-1 Outside Bar 12-2 Inner bar 13 Eccentric cam / drive wheel 14 Roll-down amount detection device 15 Unsolidified part

Continuation of the front page (56) References JP-A-6-297112 (JP, A) JP-A-5-305395 (JP, A) JP-A-3-138056 (JP, A) JP-A-59-202145 (JP) JP-A-60-82257 (JP, A) JP-A-3-281048 (JP, A) JP-A-63-183765 (JP, A) JP-A-3-275259 (JP, A) 2-15857 (JP, A) JP-A-2-224856 (JP, A) JP-A-3-281050 (JP, A) JP-A-62-181255 (JP, A) JP-A-4-231157 (JP, A A) JP-A-63-108955 (JP, A) JP-A-52-56017 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/128 350 B22D 11/20

Claims (1)

(57) [Claims] When producing a slab for hot rolling by continuous casting, light reduction of 1 mm or more and 25 mm or less is performed intermittently at a position where a solidification rate is 85% or more and 99% or less, and light reduction is performed. In this case, the relationship between the unsolidified thickness on the entry side of the reduction zone and the first reduction amount and the reduction speed satisfies the relationship of the formula (1), characterized in that there is no defect in the center of the slab thickness. A method for producing cast slabs. Δ · V / d ≧ 10 (1) where δ is the first reduction amount (mm) during intermittent reduction,
d is the unsolidified thickness (mm) at the entrance side of the reduction zone, and V is the reduction speed (mm / sec) of the reduction terminal.