JP2018034197A - Steel continuously cast piece and continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuously cast piece having particularly small center porosity at a center and particularly small austenite particle diameter on a surface, and a continuous casting method capable of manufacturing such a continuously cast piece.SOLUTION: This steel continuously cast piece has a prescribed component of C:0.3% or less, and the number of porosities having circle equivalent diameters of 0.5 mm or more at a cast piece thickness center is 10/10mm. The austenite particle diameter on a cast piece surface is 0.05 mm or less. When molten steel having a prescribed component is continuously cast, a cast piece can be manufactured by executing drafting so as to achieve rolling reduction of 5-50% at a cast piece thickness center temperature of 1300-900°C and a cast piece surface temperature of 600-300°C.SELECTED DRAWING: None

Description

  The present invention relates to a continuous casting slab of steel and a continuous casting method.

  From the viewpoint of energy saving and productivity improvement, the demand for larger size and higher strength of structures has become strict, and the improvement of quality for extra-thick steel plates that are the materials of structures has become an issue along with cost reduction. Yes. In particular, extra-thick steel sheets have been manufactured by producing a slab of a slab by rolling the large steel ingot once produced by an ingot casting method and rolling the slab. However, when this slab is used, it is necessary to remove the feeder part provided at the top of the large steel ingot, segregation generated at the bottom, coarse inclusions and shrinkage nests, and the yield is significantly reduced. There was a problem to do. In addition, a process such as block rolling is necessary to manufacture the steel sheet, and a process of heating the block slab whose temperature has been lowered into a heating furnace is required, greatly increasing the manufacturing cost and the manufacturing process. However, the production efficiency was lowered.

  In order to solve this problem, a continuous casting method is being applied to the production of slabs for extra-thick steel sheets, and improvements in yield and production efficiency have been attempted. However, in this case, since the continuous cast slab is also made extremely thick, the generation of porosity becomes significant in the center of the slab, particularly with the coarsening of the solidified structure in the center. Since the generation of the porosity hinders the improvement of the quality of the entire slab, many proposals have been made on the technology for reducing the porosity in the continuous cast slab.

  In Patent Document 1, if the contact length between the continuous cast slab and the forged anvil is larger than a certain value during forging reduction, a large plastic strain can be applied to the center of the plate thickness, and the forging direction is set to the continuous cast slab. By implementing both in the longitudinal direction and in the width direction, a technique is disclosed in which the effect of pressing the center porosity of the continuously cast slab and the effect of pulverizing the segregation zone can be obtained simultaneously. In producing a very thick steel sheet by subjecting a continuous cast slab to forging and rolling with a total rolling reduction of 29 to 66%, the continuous casting slab is heated to 1000 ° C. or higher, and then the forging process is performed. In this, cross forging with a rolling reduction of 20 to 56% is performed with a B / H ratio of 0.7 to 1.0, and then finish forming rolling is performed in a rolling process. B is the contact length between the forged anvil and the continuous cast slab during forging reduction, and H is the thickness of the continuous casting slab during forging reduction. However, since the continuous cast slab is squeezed using a forged anvil, the slab is intermittently squeezed, and surface cracks occur at the boundary between the squeezed region due to the anvil and the unsqueezed regions on both sides of the slab, resulting in a decrease in product yield. Further, since the continuous casting slab is reheated, the manufacturing cost increases.

  In Patent Document 2, bulging is performed by intermittently rolling down a steel ingot at a predetermined time interval when sandwiching an unsolidified end portion of the steel ingot using a surface member in continuous casting of steel. When the steel ingot is completely solidified within the range that is prevented and simultaneously sandwiched by the surface member, a technique is disclosed in which a solidified structure in which macro segregation and point segregation are remarkably improved can be efficiently obtained with a small pressure. However, at the end of solidification, the strength of the solidified shell in the vicinity of the liquid phase interface is small, and if the force due to surface pressure exceeds the strength of the solidified shell, internal cracks occur, and temperature fluctuations and casting speed during continuous casting operations It is difficult to satisfy the desired surface pressure reduction condition due to the variation of the thickness and temperature of the solidified shell at the surface pressure reduction position due to the change of the.

  In Patent Document 3, the ratio of the concentration of various elements such as C, Si, Mn, P, S, etc. in the region of at least 5 mm or more and 30 mm or less of the slab thickness central portion to the thickness of the portion excluding the central portion is 0. A technology for producing a continuous cast slab for hot rolling in which a center porosity having a maximum diameter of 0.1 mm does not exist at all in the center portion is disclosed. However, in order for the concentration ratio to be 0.9 to 1.0, it is necessary to suppress the concentration of solute elements accompanying solidification, which requires the liquid phase at the center to flow. In addition, although the walking bar method is adopted to lightly reduce the slab, it is difficult to perform uniform light reduction in the casting direction because it is intermittently lightly reduced in the casting direction. As a result, the flow of the liquid phase remaining in the center part of the slab thickness becomes non-uniform, and it is difficult to satisfy a desired concentration ratio range.

According to Patent Document 4, a continuous cast slab is rolled immediately after solidification, and the slab is formed based on the dendrite primary arm interval λ ₀ at the center of the slab in the thickness direction when it is manufactured without reduction. A technique is disclosed in which the ratio λ / λ ₀ of the dendrite primary arm interval λ and the λ ₀ at the center in the thickness direction is 0.1 to 0.9. In particular, if the reduction is performed immediately after the center of the slab in the thickness direction is solidified, the amount of reduction in the center of the slab in the thickness direction increases, and the amount of change in the solidified structure increases. It is said that it has been found effective. Although the diffusion of segregation can be promoted by this technique, it may be difficult to completely eliminate the porosity. Further reduction is necessary to completely eliminate the porosity.

  For the purpose of reducing the center porosity, a method is known in which the slab is reduced after the slab is completely solidified during continuous casting. When a roll having a uniform thickness in the width direction (hereinafter referred to as a flat roll) is used as the reduction roll, the entire width in the slab width direction is reduced. Considering the slab temperature at the center of the slab thickness immediately after the completion of solidification, the temperature at the both ends of the slab including the center of the slab has decreased, so that both ends of the slab width have deformation resistance over the entire thickness. Is large, and a large rolling force is required to roll down using a flat roll.

  As a method for reducing the porosity of the slab center, Patent Document 5 discloses that the surface temperature of the slab is 700 ° C. or more and 1000 ° C. or less after the slab is completely solidified, and the temperature between the internal temperature of the slab and the surface. In a region where the difference is 250 ° C. or more, the width of the slab is 5% or more and 40% or less using a roll (hereinafter referred to as “middle-thick roll”) in which the center of the width of the roll is thicker than the width end. A method of suppressing the porosity at the center of the slab by carrying out a large reduction in the range of 2% to 20% of the thickness of the slab is disclosed. Since only the center of the width of the slab is rolled down using the middle thickness roll, the porosity at the center of the slab can be efficiently suppressed.

  When using a middle-thick roll as described in Patent Document 5, it is possible to reduce without using excessive reduction force because the both ends of the width of the slab are not reduced, but when the reduction amount of the slab is large, Surface flaws occur due to dents in the slab formed on the slab in the subsequent rolling process. Therefore, the upper limit of the rolling reduction is limited in the post-solidification large rolling method using a middle thick roll.

  In a continuous cast slab after continuous casting, a solidified structure resulting from component segregation during solidification is formed, and at the same time, a crystal structure formed during cooling after solidification can be observed. In particular, the smaller the austenite grain size (hereinafter also referred to as “γ grain size”) observed in the slab, the smaller the grain coarsening during heating when the slab is heated for subsequent hot rolling. Therefore, it is preferable. Here, the austenite grain size (γ grain size) is the process of cooling the slab that has been solidified during continuous casting, and the grain size of the austenite phase formed when the slab temperature is in the austenite region. means. Since the slab after casting is cooled via the ferrite transformation region, the austenite phase is often not observed, but the austenite grain size of the slab has a cross section perpendicular to the growth direction of the dendritic solidified structure. The region where the direction of dendrite is the same can be observed and evaluated as the crystal grain size.

JP 2000-263103 A JP 59-202145 A JP-A-6-297090 Japanese Patent Laying-Open No. 2015-6680 JP 2009-279651 A

The present invention is used for producing a steel plate used for an offshore structure having a product thickness after hot rolling of 30 mm or more using a slab produced by a continuous casting method, before hot rolling. An object of the present invention is to provide a continuous cast slab of steel having a particularly low center porosity at the center of the slab and a continuous casting method capable of producing such a continuous cast slab. To do.
Furthermore, since the austenite crystal grain size is growing on the surface of the continuous cast slab as cast, if the austenite crystal grain size on the slab surface can be refined, it may occur during rolling in the next process. This is preferable because a certain surface crack can be improved.

  By the technique disclosed in Patent Document 4, the temperature of the central portion of the slab is higher than that of the slab surface layer by reducing the temperature of the continuous cast slab immediately after the solidification is completed. It is possible to increase the strain applied to a region having a low strength. In addition, as described in Patent Document 5, in the region where the surface temperature of the slab is 700 ° C. or more and 1000 ° C. or less, and the temperature difference between the internal temperature of the slab and the surface is 250 ° C. or more, the middle thick roll is used. The porosity at the center of the slab can be suppressed by performing a large reduction of the slab thickness of 2% to 20%. However, it has been difficult to completely eliminate the porosity. In order to completely crush, it is necessary to apply a larger strain to the center portion of the slab thickness.

  Based on the knowledge of Patent Document 3 and Patent Document 4, in order to increase the strain applied to the center part of the slab thickness, the difference between the temperature of the center part of the slab and the temperature of the surface is increased, depending on the temperature. It can be seen that the difference in strength of the changing steel should be increased.

  During continuous casting, the thickness center portion temperature is the solidus temperature at the position where the slab thickness center portion has been solidified, and then the slab thickness center portion gradually decreases toward the downstream side. The central temperature cannot be changed directly. On the other hand, the surface temperature of the slab is governed by the radiant heat transfer of the slab and the conduction heat transfer caused by contact with the rolls. By increasing these heat transfer amounts, the surface temperature can be lowered, and conversely the heat transfer amount. Decreasing the surface temperature can suppress a decrease in surface temperature. Therefore, in order to give a large strain to the central part of the slab thickness by performing the reduction at the position where the temperature of the central part of the thickness becomes a predetermined temperature after complete solidification during continuous casting, the temperature of the central part is required. By lowering the temperature of the slab surface at the position where the temperature reaches the temperature, the temperature difference between the central part and the surface at the reduction position can be increased, and accordingly, the difference in strength between the central part and the surface can be increased. become. However, in order to make the temperature difference between the center of the slab and the surface larger than a certain value, it is necessary to forcibly cool the slab by water spray, mist spray, or the like.

This invention is made | formed based on these knowledge, The place made into the summary is as follows.
(1) By mass%, C: 0.05% to 0.3%, Si: 0.05% to 0.4%, Mn: 0.2% to 2.0%, P: 0.02% or less , S: 0.003% or less, Al: 0.1% or less, N: 0.001% to 0.01%, the balance is Fe and inevitable impurities, and the slab thickness is 100 mm or more. The continuous cast slab is characterized in that the number of porosity having an equivalent circle diameter of 0.5 mm or more at the center of the slab thickness is 10/10 ⁵ mm ³ or less.
(2) Further, by mass%, Mo: 1.5% or less, Ni: 1.5 to 3.0%, Cr: 5% or less, Cu: 1.5% or less, Ti: 0.1% or less, Nb The continuous cast slab according to (1), containing one or more of 0.1% or less and V: 0.1% or less.
(3) The continuous cast slab according to (1) or (2), wherein the austenite crystal grain size on the slab surface is 0.05 mm or less.
(4) C: 0.05% to 0.3%, Si: 0.05% to 0.4%, Mn: 0.2% to 2.0%, P: 0.02% or less, S: 0 0.003% or less, Al: 0.1% or less, N: 0.001% to 0.01%, the balance Fe and the temperature at the center of the slab thickness when continuously casting the molten steel which is an unavoidable impurity The continuous casting method is characterized in that the slab is rolled down so that the rolling reduction is 5 to 50% at 1300 to 900 ° C and the slab surface temperature is 600 to 300 ° C.
(5) The molten steel is further mass%, Mo: 1.5% or less, Ni: 1.5 to 3.0%, Cr: 5% or less, Cu: 1.5% or less, Ti: 0.1% The continuous casting method according to (4), wherein one or more of Nb: 0.1% or less and V: 0.1% or less are contained.

  The continuous cast slab of the present invention is a slab having a very small porosity at the center of the slab thickness and a fine crystal grain size on the slab surface, while the thickness of the slab is 100 mm or more. It is. Further, according to the continuous casting method of the present invention, it is possible to produce a continuous cast slab having such quality.

  In the slab pressure reduction during continuous casting, the reduction ratio (%) is defined as “(slab thickness before reduction−slab thickness after reduction) / slab thickness before reduction × 100”.

  In order to eliminate the pinhole at the center part of the slab thickness by crushing the slab at a predetermined reduction rate, solidification of the center part of the slab thickness is completed, and the surface of the slab and the thickness center It is better that the temperature difference is large and the temperature at the center is high. The larger the temperature difference between the surface and the central part, the smaller the deformation because the surface has a large deformation resistance, and the central part has a small deformation resistance because the deformation resistance is small, and the distortion at the central part increases even at the same rolling reduction. Because it can. Although the surface temperature of the slab can be measured with a radiation thermometer or a contact-type thermocouple, it is difficult to measure the temperature in the center area of the slab. And the relationship between the temperature in the central region. Then, when the slab was reduced at a predetermined reduction rate immediately after completion of solidification, the influence of the surface temperature and the central temperature during reduction on the reduction of the center porosity was evaluated. As a result, the following points became clear.

In addition, about the temperature of the slab center part in a reduction position, it is computable as follows. Solidification analysis is performed in advance, taking into account the concentration of solute elements by microsegregation, using a slab heat transfer simulation program. Thereby, at each position in the casting direction, the liquidus position and the solidus position in the thickness direction can be calculated. Furthermore, the center temperature at the reduction position can be calculated by heat transfer analysis after completion of solidification. Here, the slab surface temperature during casting was measured, the measured slab surface temperature was substituted, and solidification analysis was performed by a computer to calculate the temperature at the center of the slab at the reduction position. Here, the density ρ was ρ = 7.27 + 0.25 × solid phase rate (g / cm ³ ). (Iron and Steel, vol. 94 (2008), p. 507: Hideo Mizukami, Akihiro Yamanaka)

  If the temperature at the center of the slab thickness exceeds 1300 ° C., solidification of the center of the slab may not be completed, and if it is reduced in this state, an internal crack will occur. Also. When the temperature at the center is less than 900 ° C., the reduction force for crushing the porosity becomes too large, and the equipment cost increases. there. It was decided to reduce the temperature at the center of the slab in the range of 1300 ° C to 900 ° C.

  When the surface temperature is lower than 300 ° C., the strength of the slab is high and a large force is required to reduce the slab, resulting in an increase in equipment cost. For this reason, by setting the surface temperature to 300 ° C. or higher, the force required to reduce the slab is reduced, and the equipment cost can be reduced. Further, if the surface temperature of the slab is higher than 600 ° C., the temperature gradient inside the slab is lowered, the strain applied to the central portion is reduced, and it is difficult to completely crush the porosity. Further, if the surface temperature is higher than 600 ° C., the effect of refining the surface crystal grains is small, which is not desirable. Therefore, the reduction was performed at a surface temperature of the slab of 300 ° C. or more and 600 ° C. or less.

  About the content component of the continuous cast slab of this invention, and the content component of the molten steel used with the continuous casting method, the component range is limited as mentioned later. As a result of limiting to such a component range, the high temperature strength of the slab does not become excessively high after completion of solidification during continuous casting. And, as a result of numerical analysis of the high temperature deformation characteristic by the finite element method reflecting the high temperature strength in the component range of the present invention, it has been found that the slab surface temperature is sufficiently rollable at 600 ° C. or lower and 300 ° C. or higher. .

  During normal continuous casting, after the solidification is completed, the cooling capacity of secondary cooling is reduced as much as possible to prevent the slab surface temperature from decreasing. This is to maintain a high sensible heat of the cast slab after casting and to save energy in the subsequent hot rolling. On the other hand, the present invention is characterized in that the slab surface temperature is reduced to 600 ° C. or lower at the reduced position after complete solidification. In order to change the surface temperature of the slab as in the present invention, it is effective to change the amount of cooling water in the secondary cooling zone. When the amount of cooling water is increased, the surface temperature of the slab can be lowered, and the surface temperature is adjusted to the range of 300 to 600 ° C. at the reduction position where the temperature at the center of the slab is in the range of 1300 ° C. to 900 ° C. Can do. By increasing the amount of secondary cooling water so that the slab surface temperature is cooled at a cooling rate of 50 ° C./min or more upstream from the reduction position, the surface temperature during reduction may be within the range of the present invention. it can.

  When the reduction ratio of the slab is less than 5%, it is difficult to completely crush the porosity at the center of the thickness of the slab and to improve the toughness index. On the other hand, when the rolling reduction exceeds 50%, the force required for rolling down increases and the equipment cost increases. For this reason, the rolling reduction was set to 5 to 50%. Here, the definition of the rolling reduction is as described above.

  In the present invention, a slab having a thickness of 100 mm or more used for an offshore structure having a plate thickness of 30 mm or more is targeted. This is because the reduction ratio during hot rolling is usually 3.0 or more.

In order to decrease the strength and toughness of the steel material obtained by rolling the slab, the influence of the porosity of 0.5 mm or more in diameter on the slab is large, and it is necessary to reduce the number of the steels. If the porosity exceeds 10 pieces / 10 ⁵ mm ³ , the strength and toughness are drastically reduced. Therefore, in the continuous cast slab of the present invention, the number was 10 pieces / 10 ⁵ mm ³ or less. By applying the continuous casting method of the present invention, the density of the porosity can be set in the above range.

  If the austenite grain size on the slab surface exceeds 0.05 mm, surface cracks may occur during rolling in the next step. Therefore, in the continuous cast slab of the present invention, the average austenite grain size on the slab surface was set to 0.05 mm or less.

  Hereinafter, the components contained in the continuous cast slab of the present invention and the components contained in the molten steel used in the continuous casting method will be described. % Means mass%.

C: 0.05-0.3%
C is an element effective for securing strength and toughness. If the content is less than 0.05%, the above effects cannot be obtained sufficiently. On the other hand, if the content exceeds 0.3%, the toughness of the base material and the HAZ part decreases. Moreover, since the C content is limited to 0.3% or less and the other components are also limited as described below, the high temperature strength of the steel does not become excessively high. In spite of maintaining the slab surface temperature at a low temperature of 600 to 300 ° C., the slab can be reduced by the reduction roll. Therefore, the appropriate range of C is set to 0.05 to 0.3%.

Si: 0.05-0.4%
If the Si content is less than 0.05%, the strength of the base material cannot be secured, so the lower limit was made 0.05%. Moreover, since weldability will fall when it exceeds 0.4%, an upper limit was made into 0.4%. For the above reason, the appropriate range is set to 0.05 to 0.4%.

Mn: 0.2 to 2.0%
Mn is an element effective for increasing the strength of the steel sheet and ensuring toughness. In order to acquire said effect, the content needs to be 0.2% or more. On the other hand, if the content exceeds 2.0%, the toughness is impaired. For this reason, the suitable range of Mn content was made into 0.2 to 2.0%.

P: 0.02% or less P is an element that deteriorates the ductility, toughness, and workability of the steel sheet, so the content is limited to 0.02% or less.

S: 0.003% or less S has the effect of promoting the formation of ferrite in the crystal grains by reacting with Mn, but it contains coarse inclusions MnS to reduce the ductility of the steel material. Limit the amount to 0.003% or less.

Al: 0.1% or less Al is an element added to deoxidize steel. If it exceeds 0.1%, the size of the oxide inclusions increases, so the surface properties of the steel sheet also deteriorate. For these reasons, in the present invention, it is preferable that the appropriate range of the Al content is 0.1% or less.

N: 0.001 to 0.01%
N is an impurity inevitably contained in the steel, and from the viewpoint of the bendability of the steel sheet, the content is preferably as low as possible, but 0.001% or more is necessary to utilize the nitride. Therefore, in this invention, it is preferable that N content rate shall be 0.001 to 0.01%.

  The present invention may further contain the following elements as necessary.

Mo: 1.5% or less Mo, if contained, is an element that exhibits an effective action for improving hardenability and strength. However, if the Mo content exceeds 1.5%, coarse inclusions are formed and the toughness is lowered. Therefore, it is preferable that the appropriate range of the Mo content is 1.5% or less.

Ni: 1.5-3.0%
Ni is an element having an action of improving the toughness of the base material if contained. However, when the Ni content is less than 1.5%, there is no effect of improving the toughness of the base material. On the other hand, if the Ni content exceeds 3.0%, the hardenability becomes excessive and adversely affects the toughness of the steel. Therefore, the range of Ni content when Ni is contained is set to 1.5 to 3.0%.

Cr: 5.0% or less When Cr is contained, Cr is an element that exhibits an effective action for improving the hardenability and improving the base material strength by precipitation strengthening. When the Cr content is higher than 5.0%, the tendency of steel toughness and weldability to deteriorate is recognized. Therefore, the appropriate range of Cr content when Cr is contained is set to 5.0% or less.

Cu: 1.5% or less Cu, if contained, is an element having an effect effective in improving hardenability and precipitation strengthening. If the Cu content is higher than 1.5%, the hot workability of the steel is lowered. For the above reason, the range of Cu content when Cu is contained is set to 1.5% or less.

Ti: 0.1% or less Ti is an effective element that mainly precipitates carbonitride and contributes to improvement of the strength of the base metal by its precipitation strengthening action. When the Ti content exceeds 0.1%, coarse precipitates and inclusions are formed in the steel, and the toughness of the steel is reduced. For the above reason, the appropriate range of Ti content is set to 0.1% or less.

Nb: 0.1% or less Nb is an element having an action of generating carbides and nitrides to improve the strength of steel when contained. If the Nb content exceeds 0.1% and becomes high, coarse carbides and nitrides are formed in the steel, so the toughness is reduced. For the above reason, the range of Nb content when Nb is contained is set to 0.1% or less.

V: 0.1% or less V, if contained, is an element that has the effect of improving the strength of steel by forming carbides and nitrides. When the V content exceeds 0.1%, the toughness of the steel is reduced. For the above reason, the range of the V content when V is contained is set to 0.1% or less.

  In order to confirm the effect of the continuous casting method of the slab of the present invention, the following tests were performed and the results were evaluated. In Table 2, numerical values outside the scope of the present invention are underlined.

(1) Casting conditions Molten steel components: listed in Table 1
Molten steel temperature: 1570 ° C (temperature of molten steel in tundish)
Mold size: width 1600mm x thickness 240mm
Casting speed: 1.0 m / min Rolling rate: 5 to 50% (described in Table 2)
Roll diameter for reduction: 550 mm in diameter (flat roll)
Slab surface temperature during reduction: 600 to 300 ° C. (described in Table 2)

(2) Evaluation method The slab surface temperature during rolling and the surface cooling rate immediately before the rolling were measured by pressing a contact-type thermocouple against the slab surface. The slab center temperature at the time of reduction was obtained by calculation using the method described above. In Table 2, the surface cooling rate immediately before the reduction is in the “cooling rate” column, the slab surface temperature at the time of reduction is in the “surface temperature” column, and the slab center temperature at the time of reduction is in the “center temperature” column. It is described.

The number of porosity at the center of the slab thickness is measured with a plate-like test piece having a thickness of ± 5 mm in the thickness direction, ± 50 mm in the width direction of the slab, and ± 50 mm in the casting direction (volume: 10). ⁵ mm ³ ) were collected, transmission X-ray photographs were taken, and the number of porosity with an equivalent circle diameter of 0.5 mm or more was counted and listed in Table 2.

  The measurement of the austenite crystal grain size on the slab surface was performed by taking a sample of 50 mm in the casting direction, 50 mm in the width direction, and 10 mm in the thickness direction from the surface part of the slab, and grinding 0.1 mm in the thickness direction from the black skin surface. Mirror polishing was performed. Then, it was immersed in a saturated picric acid solution at room temperature for 60 s, and the crystal grain structure was observed. A region where the directions of the dendrite, which is a solidified structure, are the same as one austenite crystal grain, the circle equivalent diameter of the crystal grain was measured, and the crystal grain diameter on the surface of the slab was shown in Table 2.

(Evaluation results)
In Invention Examples 1 to 6 in Tables 1 and 2, the component composition and the reduction method were within the scope of the present invention, and the cast continuous cast slabs satisfied the quality defined in the present invention.

  In Comparative Example 1, no reduction after solidification was performed, and neither water cooling for rapidly cooling the slab surface after complete solidification was performed. Since no reduction was performed, the porosity number was out of the scope of the present invention. The surface crystal grain size was also outside the preferred range of the present invention.

  Although Comparative Examples 2 and 3 were subjected to reduction, Comparative Example 2 had a reduction rate outside the lower limit of the scope of the present invention, and Comparative Example 3 had a surface temperature during reduction outside the upper limit of the present invention. It was outside the scope of the present invention. The surface crystal grain size was also outside the preferred range of the present invention.

  According to the continuous casting method of the present invention, it is possible to produce a continuous cast slab which is a raw material for a steel plate having no porosity and good mechanical properties.

Claims (5)

In mass%, C: 0.05% to 0.3%, Si: 0.05% to 0.4%, Mn: 0.2% to 2.0%, P: 0.02% or less, S: 0.003% or less, Al: 0.1% or less, N: 0.001% to 0.01%, balance Fe and inevitable impurities, slab thickness is 100 mm or more, A continuous cast slab characterized in that the number of porosity having an equivalent circle diameter of 0.5 mm or more in the central part of the piece thickness is 10/10 ⁵ mm ³ or less.   Further, in terms of mass%, Mo: 1.5% or less, Ni: 1.5 to 3.0%, Cr: 5% or less, Cu: 1.5% or less, Ti: 0.1% or less, Nb: 0.0. The continuous cast slab according to claim 1, comprising one or more of 1% or less and V: 0.1% or less.   The continuous cast slab according to claim 1 or 2, wherein an austenite crystal grain size on the slab surface is 0.05 mm or less.   C: 0.05% to 0.3%, Si: 0.05% to 0.4%, Mn: 0.2% to 2.0%, P: 0.02% or less, S: 0.003% Hereinafter, when continuously casting the molten steel which contains Al: 0.1% or less, N: 0.001% to 0.01%, and remaining Fe and inevitable impurities, the temperature at the center of the slab thickness is 1300 A continuous casting method, wherein the slab is squeezed so that the reduction rate is 5 to 50% at 900 ° C. and a slab surface temperature of 600 to 300 ° C.   The molten steel is further in mass%, Mo: 1.5% or less, Ni: 1.5 to 3.0%, Cr: 5% or less, Cu: 1.5% or less, Ti: 0.1% or less, Nb The continuous casting method according to claim 4, comprising one or more of: 0.1% or less and V: 0.1% or less.