JP4609330B2 - Continuous casting method of ultra-thick steel plates with excellent internal quality and slabs for ultra-thick steel plates - Google Patents

Description

  The present invention provides a method for producing an ultra-thick steel plate excellent in internal quality, in which no defects are detected in an ultrasonic flaw detection test, and a slab that is the raw material, and a slab that is the raw material, when continuously produced from molten steel. The present invention relates to a continuous casting method for casting a slab having a small internal porosity and a good internal quality.

  In general, a steel plate is manufactured using a slab cast by a continuous casting method as a raw material. In the center of the thickness of this slab, small voids, so-called central porosity, are formed near the center of the thickness, which is the final solidification position, due to solidification shrinkage when the molten steel solidifies and heat shrinkage due to cooling after solidification. .

  When the center porosity exists in the slab, hydrogen dissolved in the molten steel diffuses during solidification and accumulates in the center porosity. When the slab in which hydrogen is accumulated in the central porosity is hot-rolled, even if the central porosity is pressed by rolling, the hydrogen accumulated in the central porosity is re-dissolved in the steel sheet and remains. In such a case, if the amount of remaining hydrogen is large, the steel sheet is cracked (hereinafter, this phenomenon is referred to as “hydrogen cracking”).

  In recent years, high carbon steel (SC materials) for molds and machine parts has been reduced in cost by changing the material from forged products to rolled materials, steel materials for industrial machines and construction machinery, marine structures and various pressure vessels. Also for steel materials, with the enlargement of facilities, etc., the opportunity to use extra-thick steel plates with a plate thickness exceeding 100 mm is increasing.

  When a slab is used for the production of the above ultra-thick steel plate, if the center porosity is present in the slab, the center porosity generated at the center of the slab is stabilized by the ability of the currently used thick plate mill. It is difficult to make them crimped. Therefore, when an ultrasonic flaw detection test (hereinafter referred to as “UST”) is performed on an extremely thick steel plate, the unbonded central porosity is often detected as a defect (hereinafter referred to as “UST defect”). For this reason, various methods have been proposed for the purpose of eliminating the central porosity generated in the slab.

  For example, in Patent Document 1, when producing a slab for hot rolling by continuous casting, light reduction of 1 mm or more and 25 mm or less is intermittently performed by a surface member at a position where the solidification rate is 85% or more and 99% or less. Thus, a method for producing a continuous cast slab having no segregation or central porosity in the center part of the slab thickness is disclosed.

  Further, in Patent Document 2, an unsolidified end portion of a slab is solidified while being reduced using a member that substantially constitutes a surface, and an average rolling true strain per pass is 0.2% or less, and And the method of manufacturing the thick steel plate excellent in the toughness of a plate | board thickness center part and the internal quality by performing rolling whose cumulative reduction rate is 30 to 99% is disclosed.

  However, in the methods disclosed in Patent Documents 1 and 2, in order to reduce the slab, it is necessary to install a large-scale surface reduction equipment in the continuous casting machine, which requires equipment area and equipment cost. There is a problem.

  In Patent Document 3, in continuous casting of steel, a light pressure of 5 to 15 mm is applied to the slab in the range where the solid phase ratio in the center of the slab cross section is 0.3 to 0.7, and the slab In the range where the solid phase ratio in the central portion of the cross section is 0.8 to 1.0, or the central temperature of the cross section of the slab after solidification is 1200 ° C. or higher, the reduction ratio per stage in at least one direction is 30. A method for improving the quality of slabs by applying a reduction of at least% is disclosed.

  Further, in Patent Document 4, at least a pair of rolls is installed at a slab position where the temperature of the slab center corresponds to a solid phase ratio of 0.05 to 0.7, and the slab is reduced by 4 to 20 mm. Continuous casting that reduces the center porosity by installing at least a pair of rolls at the position of the slab where the temperature of the slab center is 0.8 or more and reducing the slab at a reduction rate of 5 to 20%. A method is disclosed.

However, in the methods disclosed in Patent Documents 3 and 4, a large reduction is performed in which the solidification rate at the end of solidification of the slab, that is, the central portion of the thickness is 0.8 or more and the reduction rate is 5% or more. The slab for steel plates has a reduction force of 9.8 × 10 ³ kN (1000 ton) or more, and a reduction roll and a reduction facility become large-scale, which requires an equipment area and an equipment cost. There is.

  In Patent Document 5, in a method for producing an extremely thick steel plate using a continuous casting method, an unsolidified thickness of 1.1 is obtained in a region where the solid phase ratio is 0.6 or more in the thickness center portion of a continuous cast slab. There is disclosed a method for producing an extremely thick steel plate having excellent toughness and inner quality at the center of the plate thickness by using a continuous cast slab to which a reduction of 2 to 2.0 times is applied. However, this method is insufficient as a technique for eliminating the center porosity because the definition of the unsolidified thickness is not clear and the required amount of reduction is not clear.

  In Patent Document 6, a molten steel having C ≦ 0.18 mass% is continuously cast, and a portion having a solid phase ratio of 90 to 98% at the center of the slab at the end of solidification of the slab is 2 to 5%. A method for producing a continuous cast slab excellent in internal quality by reducing once at a reduction rate of% is disclosed. In this method, since the rolling reduction is small, it is not necessary to install a large-scale rolling reduction equipment in the continuous casting machine, and the equipment area and equipment cost are not required. However, since the rolling reduction is small, all the central porosity is crimped. In some cases, they may remain in the slab. In addition, there is a problem in that the center porosity remaining after rolling the slab cannot be crimped depending on the size, and remains as a defect in the extra-thick steel plate.

  In Patent Document 7, the slab is bulged so that the maximum thickness of the slab is 20 to 100 mm thicker than the short side length of the mold, and at least a pair of squeezing rolls is used to reduce the slab by 20 mm or more immediately before completion of solidification. A continuous casting method is disclosed in which plastic deformation due to the reduction of both ends of a cast slab having a large deformation resistance is unnecessary by reducing the amount corresponding to the bulging amount. However, this method is performed as a measure for preventing the center segregation, and there is no description about the center porosity, and the effect is unknown. Even if the slab short side (end) is not crushed by bulging, a large squeezing force is required at the end of solidification due to the large squeezing amount, which requires a large squeezing equipment and equipment area. .

In Patent Document 8, in order to prevent a UST defect that occurs in an extremely thick steel plate, a slab having a central porosity thickness of d ₀ is heated to 900 to 1300 ° C., and the following equation (a) is satisfied. A method of stably producing an extremely thick steel plate having excellent internal quality by rolling at a rolling reduction ratio r and satisfying the condition of the following formula (b) in the final rolling pass is disclosed.

r ≧ 0.2 × d ₀ +1.0 (a)
1.67 × ((t ₀ −t ₁ ) × R) ^1/2 / t ₀ + 0.5 ≧ 1.1 (b)
Here, t ₀ represents the thickness (mm) of the material to be rolled before the final rolling pass, t ₁ represents the thickness (mm) of the material to be rolled after the final rolling pass, and R represents the rolling roll radius (mm).

The method disclosed in Patent Document 8 is intended to eliminate the large central porosity existing in the slab by pressure bonding only by thick plate rolling. In this method, d ₀ in the formula (a) is obtained by extracting five from the specimens having a large central porosity thickness, and taking the average as d ₀ .

  However, as a result of detailed investigations by the present inventors, the larger the center porosity, the lower the generation ratio, so that the average value may be far from the maximum value, and the larger the center porosity, the more difficult it is to press-bond during rolling. Therefore, it has been found that it is likely to remain as a UST defect. For this reason, this method that attempts to eliminate the central porosity by rolling alone cannot completely eliminate the UST defect particularly for the needs of high-strength ultra-thick products.

Furthermore, the thickness center compressive stress (σ _max ) in the final rolling pass is defined by the above formula (b), but the reduction amount (corresponding to t ₀ -t ₁ ) is the temperature and width of the material to be rolled. However, it is limited by the deformation resistance and the rolling ability of the rolling mill. On the other hand, in order to increase the roll radius (R), it is necessary to enlarge the rolling mill itself, which is not a practical technique.

  Thus, in order to manufacture a continuous cast slab and a very thick steel plate having a small central porosity volume and excellent internal quality, there remains a problem that must be solved.

Japanese Patent Application Laid-Open No. 07-276020 Japanese Patent Laid-Open No. 02-156022 JP 05-69099 A JP-A-10-58106 Japanese Patent Laid-Open No. 06-106316 Japanese Patent Application Laid-Open No. 07-80615 JP 09-57410 A JP 2000-288604 A (Claims and paragraph [0030])

  As described above, the conventional technology aiming to eliminate the central porosity generated in the slab requires a large-scale reduction equipment and equipment area, which necessitates equipment costs. When used, there is a technical problem that the central porosity remaining when rolling the slab remains as a defect in the extra-thick steel plate.

  The present invention has been made in view of the above-mentioned problems, and is manufactured using an existing thick plate rolling mill, and is a high-strength extra-thick steel plate excellent in internal quality without UST defects and a casting serving as a material thereof. An object of the present invention is to provide a continuous casting method for casting a slab having a good internal quality in which the volume of the central porosity generated at the center of the slab thickness is small when the slab is continuously manufactured from molten steel.

  As a result of repeated tests on 400 MPa class steels, the present inventors have produced a very thick steel plate excellent in internal quality without UST defects, in order to reduce rolling during continuous casting and during hot rolling. It has been found that the combined use of reduction is effective, and the formation of the center porosity can be suppressed in advance by continuously casting the slab while reducing the slab under a constant reduction condition at the end of solidification.

Furthermore, it has been found that it is possible to quantitatively grasp the central porosity volume per unit mass of the slab and the extra-thick steel plate, and the same test as the 400 MPa class steel is performed with respect to the 500 MPa class steel and the 600 MPa class steel, As a result of repeated studies, the following findings (a) to (c) were obtained.
(A) Based on the measurement of the specific gravity of the center part of the slab and other parts, the central porosity is obtained as a volume ratio per unit mass, and this value is evaluated to improve the quality of the extra heavy steel sheet. It can be.
(B) By continuously casting the slab while reducing the slab under a constant reduction condition at the end of solidification, the effect of reducing the central porosity required for improving the quality of the extra-thick steel sheet can be obtained.
(C) The effect of (b) is effectively exhibited not only in 400 MPa class steel but also in high strength 500 MPa class steel and 600 MPa class steel.

The present invention has been completed on the basis of the above findings, and the gist of the present invention is the following (1) extra heavy steel sheet , (2) continuous casting method and (3) extra heavy steel sheet manufacturing method . However, the extra-thick steel plate (1) below is an invention as a reference example of the present invention.
(1) Using a continuously cast slab as a raw material, it is obtained by hot rolling under a condition where the rolling reduction ratio r until finish rolling is 1.5 to 4.0, and the level of tensile strength X is 400 MPa class to 600 MPa class. An inner thickness characterized in that the central porosity volume Vp (cm ³ / g) at the thickness center of the steel sheet satisfies the relationship of the following formulas (1) and (2): Extremely thick steel plate.

Vp <Vp ₀ / r (1)
Vp ₀ = (0.020 × X / 10−0.085) × 10 ⁻⁴ (2)
Here, the tensile strength X of the extra-thick steel plate is a value selected in the range of 400 MPa to 600 MPa, and is a basis for calculating the converted center porosity volume Vp ₀ (cm ³ / g).
(2) A method of continuously casting a slab used as a raw material for manufacturing the extra-thick steel plate according to (1) above by hot rolling, wherein the solid phase ratio at the thickness center of the slab is 0 In the range of 0.8 or more and less than 1.0, the width central portion of the slab including the unsolidified portion is reduced by 3 to 15 mm with a pair of reduction rolls , and the central porosity volume Vp (per unit mass of the extra- thick steel plate) cm ³ / g) and the central porosity volume Vp ₀ ′ (cm ³ / g) per unit mass of the slab as an index for improving the quality of the extra-thick steel sheet, whether or not the following formula (a) is satisfied A continuous casting method for slabs for extra-thick steel plates characterized by evaluation (hereinafter, this continuous casting method may be referred to as “end-of-solidification reduction method”).
Vp <Vp ₀ '/ r (a)
(3) The slab for extra-thick steel plate produced by the continuous casting method described in (2) above is hot-rolled under a condition where the rolling reduction r until finish rolling is 1.5 to 4.0, a) Evaluating whether the equation (1) obtained by replacing Vp ₀ ′ in the equation (2) with Vp ₀ in the equation (2) is satisfied as an index for improving the quality of the extra heavy steel sheet, A method for producing an extra-thick steel sheet, characterized in that an extra-thick steel sheet having excellent internal quality is satisfied.

  In the present invention, “excellent in quality” means that the volume of the unbonded central porosity of the steel sheet is small enough not to be detected as a defect when UST is performed.

  The “extremely thick steel plate” means a steel plate having a thickness of 80 mm or more obtained by rolling a slab cast by a continuous casting method.

  The extra-thick steel plate of the present invention is an extra-thick steel plate with excellent internal quality that has no UST defects and can be manufactured using an existing thick plate rolling mill, and is widely used as a low-cost and high-quality steel plate. Can be used.

  Further, according to the continuous casting method of the present invention, when continuously producing the slab, which is the raw material of the extra-thick steel plate, from the molten steel, the central porosity is reduced by using a relatively simple rolling equipment. Since the reduced slab can be cast, it can greatly contribute to the production of extra-thick steel plates.

As described above, the extra-thick steel plate of the present invention is obtained by hot rolling using a continuously cast slab as a raw material under a condition where the reduction ratio r until finish rolling is 1.5 to 4.0, and has a tensile strength. A very thick steel sheet having a level of X of 400 MPa class to 600 MPa class, and the central porosity volume Vp (cm ³ / g) at the center of the thickness of the steel sheet is represented by the following expressions (1) and (2): It is characterized by satisfying.

Vp <Vp ₀ / r (1)
Vp ₀ = (0.020 × X / 10−0.085) × 10 ⁻⁴ (2)
Here, the tensile strength X of the extra-thick steel plate is a value selected in the range of 400 MPa to 600 MPa, and is a basis for calculating the converted center porosity volume Vp ₀ (cm ³ / g). Below, the reason and preferable range which prescribed | regulated this invention as mentioned above are demonstrated.
(1) Indicators for improving the quality of extra heavy steel sheets As shown in the above-mentioned prior art, many proposals have been made to specify the solid phase ratio and reduction conditions in order to reduce the center porosity. Nothing quantitatively defines the central porosity volume per unit mass of the product. The present inventors consider that it is indispensable to set an index for improving the inner quality of the extra-thick steel plate in order to produce a super-thick steel plate having a good quality without UST defects. The quantification of the central porosity volume per mass was studied.

  The present inventors set the steel grades to three levels of 400 MPa class steel, 500 MPa class steel, and 600 MPa class steel, and squeeze the slab with various reduction amounts in continuous casting. A sample was collected from the center of the thickness.

The evaluation of the central porosity is based on the specific volume of the central porosity calculated from the specific gravity at the center of the thickness, based on the average specific gravity at the 1/4 thickness position of the slab that is estimated to have little generation of central porosity. I took it. That is, the central porosity volume Vp (cm ³ / g) defined by the following equation (3) was determined from the average specific gravity ρ _{0 at the} 1/4 thickness position and the average specific gravity ρ at the thickness center.

Vp = 1 / ρ−1 / ρ ₀ (3)
Further, in the case of an extremely thick steel plate, the center porosity was evaluated in the same manner as in the case of a slab.

  The ultra-thick steel sheet after rolling was evaluated for unbonded central porosity by the UST evaluation method described later. When the measured number of defects, maximum indicated length per defect, density, space factor, etc. are below the values specified in the JIS, the heavy steel plate is accepted and there is no UST defect It was judged.

The present inventors consider that the central porosity is hollow, and that the amount of crimping and volume changes if the rolling reduction ratio changes, considering the reduction ratio, and the relationship between the central porosity volume of the slab and extra heavy steel sheet. Considered. As a result, the central porosity volume Vp (cm ³ / g) per unit mass of the extra-thick steel plate is changed from the value of the central porosity volume Vp ₀ ′ (cm ³ / g) per unit mass of the slab to the rolling reduction ratio r. Knowing that it was inversely proportional, the following equation (4) was obtained.

Vp <Vp ₀ '/ r (4)
The present inventors have further studied, and the ultra-thick steel plate made of a slab to which the end-solid-phase reduction method of the present invention is applied has no defect detected in the above-mentioned UST evaluation, and under the same end-solid-state reduction conditions. Even if it exists, it turned out that the effect on the center porosity reduction of a slab changes with strength levels.

FIG. 1 is a diagram showing the correlation between the central porosity volume Vp ₀ of a slab to which the final solidification reduction method of the present invention is applied and the tensile strength level of a steel type. By expressing this relationship approximately by the tensile strength level X (MPa), the following (specify the central porosity volume Vp ₀ (cm ³ / g) of the slab required for improving the quality of the extra-thick steel plate) 2) The formula was obtained.

Vp ₀ = (0.020 × (X / 10) −0.085) × 10 ⁻⁴ (2)
Here, since the equation (2) is a regression analysis at a tensile strength level of 400 MPa to 600 MPa and represents the relationship between the tensile strength X of the extra-thick steel plate and the central porosity volume Vp ₀ of the slab, the tensile strength X Can apply values selected within the same range.

The above formula (2) is a formula that defines the central porosity volume of the slab required for improving the quality of the extra-thick steel plate from the value of the tensile strength of the extra-thick steel plate, and the volume determined from the formula (2) It is defined as a reduced central porosity volume Vp ₀ (cm ³ / g).

Further, by substituting Vp ₀ ′ and the converted central porosity volume Vp ₀ in the above formula (4), the formula (1) that defines the central porosity volume of the extra heavy steel sheet required for improving the quality of the extra heavy steel sheet is obtained. It was.

Vp <Vp ₀ / r (1)
The above formulas (1) and (2) can serve as an index for improving the inner quality of the extra-thick steel sheet. If the above formula (1) is satisfied, the thickness is excellent without any UST defects and excellent in the inner quality. A steel plate can be obtained.
(2) Reduction ratio 1.5 to 4.0
The rolling reduction ratio r in the case of rolling continuously cast slabs is defined by the rolling reduction ratio = (cast piece thickness after completion of casting / steel plate rolled finish thickness). In the normal continuous casting, this reduction ratio is set to 2.5 to 4.0 for the following reason.

  If the reduction ratio is less than 2.5, even a small central porosity (thickness less than about 1 mm) remaining in the slab may not be crimped or eliminated during rolling, and a UST defect is found in the manufactured extra-thick steel plate. There is a case.

  In addition, when an extremely thick steel sheet with a thickness of 80 mm or more is to be manufactured, the reduction ratio exceeding 4.0 is required to ensure the thickness of the slab by a continuous casting machine with a large mold thickness. Means you have to do. For this reason, a huge continuous casting machine with a long captain is required, and a large equipment cost is required. Alternatively, there is a problem that the casting speed during operation must be extremely slow, and the productivity becomes extremely poor.

  In the extra-thick steel plate of the present invention, the rolling reduction ratio is set to 1.5 to 4.0 on the premise that the end-solidification reduction method is applied.

  FIG. 2 is a diagram showing the central porosity volume of the slab and the extra heavy steel plate with or without the application of the end-of-solidification reduction method of the present invention when rolling reduction with a reduction ratio of 3.0 is performed. (A) is a figure which shows the case where the tensile strength level of an ultra-thick steel plate is a 500 MPa class, (b) is a figure which shows the case where the tensile strength level of an ultra-thick steel plate is a 600 MPa class.

  As shown in the figure, by applying the end-solid-state reduction method of the present invention and suppressing the central porosity formation in advance, no batch reduction is performed by the reduction roll used in the present invention during continuous casting. The central porosity volume can be reduced to about 1/3 that of a very thick steel plate made of a slab (hereinafter referred to as “normal slab”).

  Further, Table 1 shows a comparison of porosity volumes after rolling in the case where a very thick steel plate having the same thickness of 200 mm is manufactured by rolling using a slab and a normal slab to which the final solidification reduction method of the present invention is applied.

  In the comparative material 1 for producing a 200 mm-thick extra-thick steel plate from a 300 mm-thick normal slab, the reduction ratio is 1.5, which is higher than that of the comparative material 2 in which the reduction ratio is increased to 3.0 with a 600 mm-thick slab. The central porosity volume is increased. On the other hand, the ultra-thick steel plate made of 300 mm-thick slab to which the end-solidification reduction method of the present invention is applied has a central porosity volume reduced to 1/3 or less compared to the comparative material 1 having the same reduction ratio. In addition, the central porosity volume is reduced as compared with the comparative material 2 having a reduction ratio of 3.0.

  That is, in the rolling of a normal slab, the rolling ratio needs to be 2.5 to 4.0, but in the rolling using the extra-thick steel plate of the present invention, the conventional rolling ratio is about 3.0 at the slab stage. The center porosity reduction effect is previously given. For this reason, the conditions of the rolling ratio specified in the present invention of 1.5 to 4.0 are 4.5 (= 1.5 × 3) to 12.0 (= 4.0 ×) in the rolling ratio of a normal slab. This corresponds to 3).

  However, in order to improve the internal quality, not only the reduction of the central porosity but also the refinement of the solidified structure is an important factor, so the rolling ratio of the present invention is 1.5.

  In addition, the present invention uses a slab of about 300 mm thickness cast by a continuous casting method as a raw material to produce an extremely thick steel plate having a plate thickness of 80 mm or more, so the rolling ratio of the present invention is 4.0.

  According to the present invention, the small center porosity (thickness of less than about 1 mm) remaining in the slab can be eliminated by pressure bonding by rolling at a reduction ratio of 1.5 to 4.0, so that the reduction ratio cannot be ensured. It may be possible to produce a product of wood.

Furthermore, as shown in Table 1, in the case of producing a 200 mm thick extra-thick steel sheet, a reduction ratio of about 3 is required in the prior art. According to the above, it is possible to realize with a mold having a thickness of about 300 mm. That is, it is possible to manufacture by using a mold that is less than the mold thickness necessary for casting the extra-thick steel plate and more than half that thickness, and can be within the range that can be manufactured by an existing continuous casting machine. Therefore, it is possible to manufacture without the need for manufacturing from an ingot or a continuous casting machine dedicated to extra-thickness.
(3) Chemical composition Although the steel composition which this invention makes object does not limit a chemical composition, according to the mechanical characteristic as a thick steel plate which is a final product, weldability, a welding heat affected zone characteristic, etc. It may be desirable to combine alloying elements and may contain the following chemical composition. In the following description, “%” represents “mass%”.

C: 0.02 to 0.56%
C is an element effective for securing strength, but in order to obtain the effect, it is desirable to contain 0.02% or more. On the other hand, the base material such as a structural material and the toughness of the welded portion are required to be 0.18% or less from the viewpoint of securing the toughness (e.g., 0.about.20 ° C when low temperature toughness is required). It is desirable to use those having a content of 09% or less. In addition, it is desirable to use a material having a content of 0.56% or less in order to obtain the required hardness for the purpose of improving the wearability and the like by increasing the hardness.

Si: 0.04 to 0.60%
Si is an element necessary for deoxidation of molten steel, and 0.04% or more is desirable to obtain the effect. However, if it exceeds 0.60%, the weld heat-affected zone toughness deteriorates, so it is preferably 0.35% or less.

Mn: 0.50 to 2.00%
Mn, like C, is an element effective for ensuring the strength of the base material, and is desirably contained in an amount of 0.50% or more in order to effectively obtain the strength. However, if the Mn content is too large, deterioration of the base metal and weld heat affected zone toughness due to center segregation becomes significant, so it is desirable to use within a range of 2.00% or less.

P: 0.020% or less and S: 0.006% or less P and S are elements that remarkably deteriorate the toughness of steel, and it is desirable that the content is small, but it is expensive to reduce it extremely. Therefore, it is used within the above range.

  Furthermore, in order to ensure the intended strength, hardness, and weld heat affected zone toughness, Cu: 0.1 to 1.2%, Ni: 0.1 to 4.0%, Cr: 0 as necessary. 0.1-1.2%, Mo: 0.01-0.6%, Nb: 0.01-0.1%, V: 0.01-0.1%, Ti: 0.01-0.03 %, B: 0.0003 to 0.003%, Al: 0.003 to 0.10%, and N: 0.001 to 0.01% can be contained alone or in combination. If these elements are contained in excess of the above range, the characteristics are adversely deteriorated, or the effects of the inclusion are not commensurate with the alloy cost.

  In addition, for the purpose of improving the performance of the weld heat affected zone toughness, one or more of Ca, Mg, and REM can be contained, but the ranges are 0.0005% or more and 0.01% or less, respectively. desirable.

  In recent years, piers used on the Metropolitan Expressway often adopt a special shape due to restrictions on installation location, and there is a tendency to increase the thickness. In addition, since fatigue cracks are a problem in existing piers, UST of welds has been tightened. For this reason, an extra-thick steel sheet that cannot have a large reduction ratio has a slight porosity defect even if it is subjected to strong reduction rolling in addition to light reduction by continuous casting, and it is detected as a defect by high-sensitivity UST, which makes it difficult to manufacture. It was thought. The steel plate of the present invention is desirably applied to the steel types for the above uses.

  Next, the continuous casting method of the present invention, that is, the end-solidification reduction method will be described.

The continuous casting method of the present invention is a method of continuously casting a slab used as a raw material for producing the extra-thick steel plate by hot rolling, wherein the solid phase ratio at the center of the thickness of the slab is 0. In the range of 8 or more and less than 1.0, the width center portion of the slab including the unsolidified portion is reduced by 3 to 15 mm with a pair of reduction rolls , and the central porosity volume Vp (cm) per unit mass of the extra- thick steel plate ³ / g) and the central porosity volume Vp ₀ ′ (cm ³ / g) per unit mass of the slab are used as indices for improving the quality of the extra-thick steel sheet, and whether or not the following formula (a) is satisfied is evaluated. It is characterized by doing. As described above, the end-coagulation reduction method of the present invention sufficiently exhibits the effect of suppressing central porosity formation in advance, but the reasons and preferred ranges for defining the end-coagulation reduction method as described above are as follows. explain.
(4) Reduction time In the continuous casting method of the present invention, the central solid fraction is reduced to the end of solidification of 0.8 or more, that is, when the central solid fraction is 0.8 or more, it can be reduced by a reduction roll. Adjust operating conditions (casting speed, cooling water volume, etc.).

  If the center solid phase ratio is less than 0.8, a relatively large amount of molten steel at the end of solidification still remains in the center of the slab thickness. It is discharged and flows toward the mother molten steel.

  For this reason, the progress of solidification is not necessarily uniform, and the thickness of the solidified shell becomes non-uniform due to uneven cooling, etc., so the central solid phase ratio during rolling is strictly different depending on the position of the slab.

  Therefore, when the central solid phase ratio is 0.6 or more and less than 0.8, there may be a portion where the central solid phase ratio is 0.8 or more depending on the position of the slab. At this time, the molten steel discharged by reduction cannot flow at a portion where the central solid phase ratio is 0.8 or more, and as a result, it becomes difficult to flow and mix to the mother molten steel. For this reason, although the center porosity is reduced, the discharged molten steel remains as segregation in the slab as it is, and the center segregation situation is worsened.

  Further, when the central solid fraction is lower and the central solid fraction is less than 0.6, a large amount of molten steel remains inside the slab, so a large amount of reduction is required to discharge this molten steel. Must. For this reason, a large reduction force is required, and a large-scale reduction facility is required.

  On the other hand, when the solid phase ratio is 0.8 or more, there is little molten steel at the end of solidification in the slab, and even when a large reduction is applied, the molten steel hardly flows. For this reason, the center segregation situation does not deteriorate. Therefore, in the present invention, the reduction is performed when the central solid phase ratio at the end of coagulation is 0.8 or more.

  Thus, if reduction is applied in a region where the central solid phase ratio is 0.8 or more, that is, 0.8 to 1.0, it has an effect on the compression of the central porosity. However, after the central solid phase ratio is 1.0, that is, after solidification is complete, the temperature at the central part of the thickness of the slab decreases, and the deformation resistance increases rapidly. For this reason, if a large reduction is applied after the central solid phase ratio reaches 1.0, the thickness center of the slab where the central porosity is distributed is not effectively reduced, and the large central porosity is too small. It may not be possible.

The central solid phase ratio fs is calculated from fs = (T _L −T) / (T _L −T _S ) from the liquidus temperature T _L and the solidus temperature T _S of the molten steel and the temperature T at the thickness center. Can be sought. When the temperature T at the thickness center of the slab is equal to or higher than the liquidus temperature _{TL of the} molten steel, fs = 0, and when the temperature T at the thickness center is lower than the solidus temperature T _{S of the} molten steel. fs = 1.0. The temperature T at the center of the slab thickness can be obtained by unsteady heat transfer analysis calculation in the slab considering the casting speed, surface cooling of the slab, physical properties of the cast steel type, and the like.
(5) Amount of reduction In the continuous casting method of the present invention, the amount of reduction at the central portion in the width direction of the slab is 3 to 15 mm because the center porosity of the slab is surely reduced when the amount of reduction is less than 3 mm. It is because it is not possible. That is, when the amount of reduction is less than 3 mm, the large central porosity is only slightly reduced, and it cannot be crimped by subsequent rolling and remains as a defect.

On the other hand, when the central solid phase ratio is 0.8 or more, a very large reduction force is required to increase the reduction amount exceeding 15 mm. Therefore, a large-scale reduction including a hydraulic facility is required. This is because equipment is required.
(6) Reduction method In the present invention, since the required reduction amount is 3 mm to 15 mm, it is desirable to symmetrically reduce the upper and lower surfaces of the slab so that the reduction can be efficiently performed with a small reduction capacity. Sometimes it is desirable to have the lower roll protrude above the lower pass line of the slab. The slab bulging amount is not particularly defined. For example, when the required reduction amount is 3 mm, if the bulging amount is 2 mm, the reduction amount of the short side portion of the slab having a large deformation resistance during reduction may be 1 mm. Therefore, bulging may be used in combination as necessary.

In order to confirm the effect of the present invention, the following continuous casting test was performed, and the obtained slab was rolled into an extra-thick steel plate, and product evaluation of the extra-thick steel plate was performed by an ultrasonic flaw detection test.
(Test method)
1) Casting method FIG. 3 is a diagram schematically showing a vertical bending die continuous casting apparatus used for testing the continuous casting method of the present invention. The mold used for the test had a thickness of 311 mm and a width of 2300 mm. In the following description, a slab cast with this mold is referred to as a “300 mm thick slab”.

  The target steel types were three levels of 400 MPa class steel, 500 MPa class steel and 600 MPa class steel, and the chemical compositions shown in Table 2 below were used.

  The casting speed Vc varies in a range of Vc = 0.61 to 0.62 (m / min) so that the center solid phase ratio of the slab during reduction is 0.8 or more in advance by solidification heat transfer calculation. changed. The amount of secondary cooling water was 0.62 to 0.73 L / kg-steel.

  The molten steel 4 injected into the mold 3 from the tundish (not shown) through the immersion nozzle 1 is cooled by spray water sprayed from the mold 3 and a group of secondary cooling spray nozzles (not shown) below it, A solidified shell 5 is formed to become a slab 8. The slab 8 is pulled out by the reduction roll 7 through the guide roll 6 group while the unsolidified portion is held inside the slab 8.

A pair of rolling rolls 7 was installed at a position 21.5 m below the molten steel surface (meniscus) 2 formed inside the mold 3. The diameter of the reduction roll 7 was 450 mm, and the maximum reduction force was 5.88 × 10 ³ kN (600 ton). Although the continuous casting machine used for the test is a vertical bending type continuous casting machine, it goes without saying that a curved type continuous casting machine may be used.

  The central solid fraction during rolling is one-dimensional heat transfer calculation at various casting speeds according to the casting speed and the thickness of the central part of the slab. Conditions were sought.

Moreover, the molten steel temperature in the tundish was made substantially constant between ΔT (degree of superheat) = 27 ° C. to 50 ° C. ΔT is the difference between the molten steel temperature and the liquidus temperature.
2) Evaluation of central porosity of cast slab and extra-thick steel plate The obtained slab was sampled from a part for investigation of central porosity, then heated to 950-1170 ° C, and in the range of 1050-750 ° C. Finished rolling was performed to produce a very thick steel plate. The finish rolling mill used had a work roll diameter of 1040 mm and a maximum rolling force of 6.17 × 10 ⁴ kN (6300 ton). Samples were taken from some of the thick steel plates for the investigation of central porosity.

  About the slab, the sample was extract | collected from the width direction 7 places of the 1/4 thickness position of a slab, and the width direction 16 places of the thickness center. The sample shape is 50 mm long x 100 mm wide x 7 mm thick considering the accuracy of specific gravity measurement, and the surface processing accuracy is about JIS-based finishing (triangle symbol ▽▽▽: maximum surface roughness 3.2μ) did.

Evaluation was made based on the specific volume of the central porosity calculated from the specific gravity at the center of the thickness with reference to the average specific gravity at the 1/4 thickness position of the slab where the generation of the central porosity is considered to be almost absent. From the average specific gravity ρ _{0 at the} ¼ thickness position and the average specific gravity ρ at the thickness center, the central porosity volume Vp (cm ³ / g) defined by the following equation (3) was determined.

Vp = 1 / ρ−1 / ρ ₀ (3)
The extra thick steel plates were also sampled under the same conditions as described above, but because the reduction ratio was different for each extra thick steel plate, the thickness of the sample shape was unified at about 1/20 of the thickness of the extra thick steel plate.

The extremely thick steel sheet after rolling was evaluated for unbonded central porosity by the following UST evaluation method. The UST apparatus was an A scope display type flaw detector, and a vertical flaw detector having a transducer diameter of 30 mm and a nominal frequency of 2 MHz was used. When the measured number of defects, maximum indicated length per defect, density, space factor, etc. are below the values specified in the JIS, the heavy steel plate is accepted and there is no UST defect It was judged.
(UST evaluation method)
The purpose of the present invention is to produce a high-strength ultra-thick steel plate having no internal defects and excellent in quality. Therefore, the UST defects were evaluated by a test method stricter than the UST specified in JIS G0801 (1993). . In the vertical ultrasonic flaw detection method employed in the present invention, JIS G0801 is applied, and the flaw echo height (hereinafter referred to as “F ₁ ”) exceeds 25% in a line flaw detection with a vertical or horizontal 100 mm pitch, and the defect indication length. Is defined as 1 piece / square meter or less.

  JIS G0801 is a regulation for steel plates for pressure vessels, but this method was also applied to this steel material for convenience. However, it is not sufficient to set the sensitivity as it is in JIS G0801, and after setting the flaw detection sensitivity based on JIS G0801, the sensitivity is further increased by +12 dB (approximately 4 times the sensitivity), and it is displayed on the A scope display device. It evaluated in 0-100% display in. In the case of using a digital flaw detector, this was followed.

In normal JIS G0801, F ₁ or F ₁ / B ₁ is mainly subject to medium and heavy defects exceeding 50%, and F ₁ or F ₁ / B ₁ is 25 to 50%. In many cases, it is not regarded as a problem, but in the evaluation of the inside of the steel plate of the vertical ultrasonic flaw detection method adopted in the present invention, the defect was also evaluated. Moreover, since the evaluation method employed in the present invention increases the flaw detection sensitivity by +12 dB (approximately 4 times the sensitivity), the defect to be evaluated means a defect of 6 to 12.5%, Such a minute defect requires extremely strict internal soundness of steel in which the number of defects having a defect indication length exceeding 5 mm within the steel sheet is 1 piece / square meter or less.
(Test results)
Table 3 shows the test conditions of the examples, and Table 4 shows the test results. Test numbers T1 to T4 of the present invention example are tests on a very thick steel plate using a slab manufactured by applying the final solidification reduction method of the present invention with a reduction amount of 12 mm, and a test number of a comparative example. T5 to T8 are tests on a very thick steel plate that did not apply the end-solidification reduction method, that is, a slab manufactured with a reduction amount of 0 mm as a rolling material.

  FIG. 4 is a diagram showing specific gravity measurement results of 500 MPa class steel slabs (1/4 thickness and 1/2 thickness) of the present invention and comparative examples. As shown in the figure, in the slab of the present invention to which the end-solidification reduction method of the present invention was applied, the center (1/2 thickness) porosity was a slab of the comparative example in which the end-solidification reduction method was not applied Than it was clearly improved.

  The other steel types were measured in the same manner, and the central porosity volume per unit mass calculated by the above equation (3) was obtained.

  FIG. 5 is a diagram showing the central porosity volume per unit mass of the slabs of the present invention example and the comparative example. As shown in the figure, when the end-solidification reduction method of the present invention is applied, in any steel type of 400 MPa class steel, 500 MPa class steel, and 600 MPa class steel, compared to the case where the end-solidification reduction method is not applied. It was also confirmed that there is an effect of reducing the central porosity volume to 1/3 or less.

FIG. 6 is a graph showing the distribution of the central porosity volume per unit mass of the extra-thick steel plates of the present invention and the comparative example, with the rolling ratio r on the horizontal axis and the central porosity volume per unit mass on the vertical axis. . As shown in the figure, when the end-coagulation reduction method of the present invention is applied, the central porosity volume is distributed in a region below the curve Vp ₀ / r, and the above formula (1) defined in the present invention is used. Satisfied with the relationship. On the other hand, in the case of the comparative example, the central porosity volume was all distributed in the region above the curve Vp ₀ / r.
Furthermore, as shown in Table 4, the test numbers T1 to T4 satisfying the relationship of the expression (1) defined in the present invention obtained good results that passed in the above-mentioned UST evaluation.

  The extra-thick steel plate of the present invention is an extra-thick steel plate with excellent internal quality that has no UST defects and can be manufactured using an existing thick plate rolling mill, and is widely used as a low-cost and high-quality steel plate. Can be used.

  Further, according to the continuous casting method of the present invention, when continuously producing the slab, which is the raw material of the extra-thick steel plate, from the molten steel, the central porosity is reduced by using a relatively simple rolling equipment. Since the reduced slab can be cast, it can greatly contribute to the production of extra-thick steel plates.

  Thereby, there exists an advantage that the manufacturable range of the ultra-thin high sensitivity UST object material considered that manufacture is difficult can be expanded.

Is a graph showing the correlation of the coagulation end reduction method of the slab to which the center porosity volume Vp ₀ and steels of a tensile strength level of the present invention. It is the figure which showed the center porosity volume of the slab and the extra-thick steel plate by the presence or absence of application of the end-of-solidification reduction method of the present invention when rolling reduction of a reduction ratio of 3.0 was performed. It is a figure which shows the case where the tensile strength level of a very thick steel plate is a 500 MPa class, and the figure (b) is a figure which shows the case where the tensile strength level of a very thick steel sheet is a 600 MPa class. It is the figure which showed typically the perpendicular | vertical bending type continuous casting apparatus used in order to test the continuous casting method of this invention. It is a figure which shows the specific gravity measurement result of the 500 MPa class steel slab (1/4 thickness and 1/2 thickness) of the example of this invention and a comparative example. It is a figure which shows the center porosity volume per unit mass of the slab of the example of this invention and a comparative example. It is a figure which shows the reduction ratio r on a horizontal axis | shaft and the center porosity volume per unit mass on a vertical axis | shaft, and shows the distribution of the center porosity volume per unit mass of the ultra-thick steel plate of the example of this invention and a comparative example.

Explanation of symbols

1. Immersion nozzle Molten steel surface (meniscus)
3. Mold 4. Molten steel 5. Solidified shell 6. Guide roll 7. Rolling roll 8. Slab

Claims (2)

Reduction ratio r to the finish rolling to obtain hot rolled materials under conditions of 1.5 to 4.0, in order to level the tensile strength is manufacturing a very thick steel plate is 400MPa class to 600MPa class, a method of continuous casting a slab to be used as the material,
In the range where the solid phase ratio of the thickness center portion of the slab is 0.8 or more and less than 1.0, the width center portion of the slab including the unsolidified portion is reduced by 3 to 15 mm by a pair of reduction rolls ,
The central porosity volume Vp (cm ³ / g) per unit mass of the extra-thick steel plate and the central porosity volume Vp ₀ ′ (cm ³ / g) per unit mass of the slab are indicators for improving the quality of the extra-thick steel plate And evaluating whether or not the following expression (a) is satisfied .
Vp <Vp ₀ '/ r (a)
  The ultra-thick steel plate slab manufactured by the continuous casting method according to claim 1 is hot-rolled under a condition that the reduction ratio r until finish rolling is 1.5 to 4.0, and the formula (a) Vp ₀ ′ Is Vp in the following equation (2) ₀ Using the following formula (1) obtained by substituting as an index for improving the quality of extra-thick steel plates, whether or not the formula is satisfied is evaluated, and what is satisfied is to make the extra-thick steel plate excellent in inner quality. A method for producing a very heavy steel sheet.
  Vp <Vp ₀ / R (1)
  Vp ₀ = (0.020 * X / 10-0.085) * 10 ^-Four       ... (2)
  Here, the tensile strength X of the extra-thick steel plate is a value selected in the range of 400 MPa to 600 MPa, and the converted center porosity volume Vp. ₀ (Cm ^Three / G)

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