JPH07276020A - Continuous casting method - Google Patents

Abstract

PURPOSE:To provide a continuous casting method which effectively prevents the segregation and the center porosity in a cast slab causing lowering of the strength and the toughness of a steel plate and does not damage the productivity and the profitability. CONSTITUTION:A light rolling reduction is applied so that a value dividing the product of a first rolling reduction delta and a rolling reduction velocity V with an unsolidified thickness (d) at the input side of the rolling reduction zone becomes >= at least 10 at the time of intermittently applying the rolling reduction to the surface in the range of 85-99% solidified ratio in the casting time. Then, the segregation at the center part in the thickness and the center porosity having >=0.1mm the max. diameter in the cast slab are not fully developed. By this method, the cast slab having drastically excellent strength and toughness at the center part in the thickness in comparison with the conventional hot-rolled steel plate can be obtd. without lowering the profitability.

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 plate thickness of a hot-rolled material produced by a continuous casting method, hydrogen-induced cracking of a line pipe material used in a bad environment, etc. The present invention relates to a method for supplying a continuously cast slab that eliminates defects such as segregation of the slab and center porosity that cause the above.

[0002]

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

Therefore, when performing hot rolling using a continuously cast slab having these defects, refining is carried out in order to substantially reduce the influence of elements such as phosphorus and sulfur, which are particularly likely to segregate during the solidification process. Process such as low phosphorus and low sulfurization in the process, dehydrogenation process to reduce hydrogen that tends to accumulate in the center porosity part, and soaking before hot rolling. In addition to performing a heat treatment for promoting the diffusion of segregation elements and hydrogen, which is called diffusion treatment, a pretreatment for pressure bonding of the center porosity is performed by high-pressure reduction rolling, which is called high shape ratio rolling, and the defects are A means for preventing the deterioration of the quality of the steel material due to this has been used.

From such a background, the steel material manufactured through these steps is inevitably high in cost and inferior in economic efficiency.

Therefore, a technique for eliminating segregation and center porosity in the center portion of the thickness of continuously cast slab having such defects has been strongly demanded, and a new method has 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 in a certain range as a function of roll pitch and crater end position. Japanese Patent Application Laid-Open No. 52-56017 discloses a method for preventing such defects.

Further, by sandwiching the coagulation end position by using a surface member, the reduction ratio per time of the region where the coagulation ratio is 40% or more is 1.5% or less, and the total reduction ratio is 0.5 to 5.0%. A method for completely solidifying while intermittently reducing the pressure in the range of JP
-202145 gazette.

[0008]

Since the slabs produced by such a method are compensated for solidification shrinkage and heat shrinkage, segregation caused by agglomeration / accumulation due to molten steel flow caused by these is prevented. Not only that, but also when the surface member is lightly pressed down, the flow of molten steel due to bulging is also prevented, so it is possible to further reduce segregation and center porosity. ing.

However, in the region where the solidification rate in the final stage of solidification is high, the viscosity of the molten steel is also high and it cannot flow easily. Therefore, even if the above-mentioned light reduction method is used, even the solidification shrinkage in the vicinity of the final solidification part Is also impossible to compensate,
Therefore, the actual situation is that the segregation and the center porosity have not been completely prevented.

Further, in the light reduction method using a roll, the amount of reduction immediately below the roll is inevitably increased due to the deformation characteristics of the solidified shell of the slab, and a uniform gradient can be secured throughout the length of the slab. Not only is it difficult, but when solidification is completed just below the roll, it is not necessary to reduce the amount of bulging, but when solidification is completed between rolls, even the flow that occurs with it is prevented. It is necessary to apply a reduction amount for this purpose, and therefore there is no means for accurately grasping the solidification completion position, and it is also extremely difficult to control the reduction amount during casting. It was unavoidable that there was a large variation in the segregation and center porosity of the center of the piece.

Therefore, even with a slab produced by applying these methods, the soaking and diffusion treatment for a long time as in the conventional case is unnecessary, but the load of the refining process and the pretreatment such as heat treatment and high shape ratio rolling are still required. However, the manufacturing cost is still high and the product thickness cannot be increased.

[0012]

In order to solve the above-mentioned various problems, the present invention, in producing a slab for hot rolling by continuous casting, at a position where the solidification rate is 85% or more and 99% or less, The surface is intermittently subjected to a light reduction of 1 mm or more and 25 mm or less, and at the time of the light reduction, the relation of the formula (1) is satisfied in the relation between the unsolidified thickness on the side of the reduction zone and the first reduction amount and the reduction speed. A method for producing a continuously cast slab having no defects in the central portion of the thickness of the slab, which comprises:

[0013]

[Formula 2] δ · V / d ≧ 10 (1)

Where δ is the first reduction amount (mm),
d is the unsolidified thickness (mm) on the side of entering the rolling band, and V is the rolling speed (mm / sec) of the rolling terminal.

[0015]

The present invention will be described in detail below.

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

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

As shown in FIG. 3, when the P segregation ratio exceeds 2.0, the toughness value of the weld heat affected zone at -60 ° C. is extremely lowered, and the variation is large, which depends on the standard of the steel material. Although the value is not satisfied or the standard value is not satisfied, there is a problem that stable quality cannot be secured.

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. That is, the inventors have found that the condition for obtaining a high toughness value can be obtained only in the P segregation ratio range of 0.8 to 2.0.

On the other hand, as shown in FIG. 4, the maximum diameter is 0.
It was also found that when center porosity exceeding 1 mm is present in the slab, characteristics satisfying the standard value cannot be secured in the Z direction tensile test even when the P segregation ratio is 2.0 or less.

The present inventors further researched, and simultaneously controlled the above-mentioned P segregation ratio to 2.0 or less and the maximum diameter of the center porosity at the center of the plate thickness to 0.1 mm or less. In studying, the laboratory method was used to reproduce the solidification morphology in the vicinity of the final solidification part in the continuous casting process using the simulated reduction device at the solidification end shown in FIG. The relationship between the non-solidified thickness of the was investigated.

As a result, as shown in FIG. 1, as the unsolidified thickness increases, the reduction amount δ must be increased in the region where the P segregation ratio can be controlled to 2.0 or less. On the other hand, if it is too large, there is a region with a P segregation ratio of less than 0.8 that produces a low value, or there is a reduction amount that causes internal cracking. Although the critical reduction amount that causes this internal cracking varies depending on the solidification rate, It was found from this experiment 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 region of 0.8 to 2.0 unless the relationship between the unsolidified thickness d and the rolling reduction δ is maintained within a certain range.

However, even under such conditions, the P segregation ratio sometimes becomes less than 0.8 or exceeds 2.0, and segregation and center porosity are not always completely eliminated.

As a result of further investigations by the present inventors regarding this cause, it was found that the speed at the time of reduction greatly influences, and as a result of conducting experiments in which the reduction speed was variously changed in addition to the above conditions, the results shown in FIG. As shown in (1), if only the unsolidified thickness d during rolling and the rolling amount δ are controlled, the P segregation ratio satisfying the above-mentioned product quality characteristics is 0.8 to 2.0.
It was discovered that stable control was not possible within the range of.

[0026]

[Formula 3] δ · V / d ≧ 10 (1)

Here, δ is the first reduction amount (mm),
d is the unsolidified thickness (mm) on the side of entering the rolling band, and V is the rolling speed (mm / sec) of the rolling terminal.

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

[0029]

EXAMPLES The present invention and comparative examples will be described in detail below.

6 to 7 using steels having the components shown in Table 1.
And the strength and toughness of the thick steel plate manufactured by applying the conditions shown in Table 2 to the continuously cast slabs manufactured under the conditions shown below by the reduction equipment shown in FIG. 8 and the Z direction tensile test and the hot rolled steel plates. Table 2 shows the strength / toughness of the sour-resistant line pipe material and the crack area ratio (CAR) after the hydrogen-induced cracking test are divided into the present invention example and the comparative example.

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-No. As shown in FIG. 7, the P segregation ratio of the slab produced in the example of the present invention is in the range of 0.9 to 2.0 necessary to secure the quality characteristics of the product,
In addition, the center porosity did not exist at all, and the various properties of the products obtained by hot rolling the center porosity also satisfied the standard values.

In the comparative example, No. 8 to No. 12 is (1)
This is an example in which the condition shown by the formula is not satisfied, the P segregation ratio in the cast piece is 2.0 or more, and the center porosity having a maximum diameter of more than 0.1 mm frequently occurs.
The various characteristics of the product obtained by hot-rolling this product did not satisfy the standard values.

No. 13-No. No. 14 is a case where the amount of reduction per one time is extremely large, and although segregation and center porosity were prevented from occurring, negative segregation with a segregation ratio of less than 0.8 was also described in the table. However, internal cracking was observed due to rolling, which was a major cause of material deterioration.

No. No. 15 is the case where no positive reduction was performed, but as is clear from the table, not only the segregation of the center of the thickness of the cast slab but also the level of center porosity is extremely poor, so the material of the obtained product is also satisfactory. There was a situation that was not reached.

As can be seen from the above results, the steel sheets produced from the continuous cast slab for hot rolling produced according to the present invention all showed excellent properties.

The method for producing the continuously cast slab is as follows. Continuous casting slab size; thickness 200/284 mm x width 19
00mm End-solidification segregation and center porosity control device;
(Device shown in Fig. 3) Model Walking bar system Configuration Inner bar (2) 3 Outer bars (1) 4 Shift amount 100 mm Rolling part length 2.5 m Rolling part inlet side slab thickness Max. 284 mm Rolling down amount in rolling zone 0 to max. 35 mm Rolling zone entry side unsolidified thickness 0 to max. 40 mm

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

Industrial Applicability As described above, the present invention stably achieves the extremely important cast quality, especially segregation / center porosity reduction, in order to secure the material properties conventionally required for the hot rolled material. Therefore, not only the pretreatments such as low sulfurization, low phosphorus and dehydrogenation treatments that were conventionally performed in the molten steel treatment process, but also the pretreatments such as high temperature aging heating and high shape ratio rolling in the hot rolling process are completely eliminated. It is not necessary and the product material can be stabilized, and it can be manufactured extremely economically. Therefore, the effect brought to this field is extremely large.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a relationship between an unsolidified thickness on a side of entering a rolling zone, a rolling amount and a rolling speed, which affect a segregation ratio.

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

FIG. 4 is a diagram illustrating the relationship between the maximum diameter of center porosity and the tensile strength in the Z direction.

FIG. 5 is an explanatory diagram of a surface pressure reduction simulation test device at a coagulation end portion.

FIG. 6 is a side view of one embodiment showing a means for surface-rolling down the unsolidified end of the slab in the continuous casting process of the present invention.

FIG. 7 is a front view taken along the line BB in FIG.

FIG. 8 is a cross-sectional view of a face member of a face reduction device used in an example of the present invention.

[Explanation of symbols] 1 ingot 2 unsolidified portion of ingot 3 cooling water 4 differential transformer 5 reduction jack 6 ingot lifting device 7 reduction terminal 8 load cell 9 mold 10 support roll 11 slab 12 face member 12-1 outside Bar 12-2 Inner bar 13 Eccentric cam / drive wheel 14 Reduction amount detection device 15 Unsolidified part

Claims (1)

[Claims] 1. When producing a slab for hot rolling by continuous casting, at a position where the solidification rate is 85% or more and 99% or less, a light reduction of 1 mm or more and 25 mm or less is intermittently performed by a surface, and a light reduction is performed. At this time, in the relationship between the unsolidified thickness on the side of entering the rolling band and the first rolling amount and rolling speed, the relationship of the formula (1) is satisfied, and the continuous slab with no defects in the thickness center portion of the slab. Casting slab manufacturing method. [Formula 1] δ · V / d ≧ 10 (1) where δ is the first reduction amount (mm) during intermittent reduction.
d is the unsolidified thickness (mm) on the side of entering the rolling band, and V is the rolling speed (mm / sec) of the rolling terminal.