Continuous casting method of ingot slab

Abstract

To reduce center segregation and porosity with simple constitution.SOLUTION: A continuous casting method for a cast slab includes a first reduction step and a second reduction step. The first reduction step slightly reduces a cast slab (10) at a rolling reduction rate of 0.5 mm / min or more and 1.0 mm / min or less until a central solid phase rate of the cast slab reaches 0.8 after the central solid phase rate reaches 0.3. The second reduction step largely reduces the cast slab (10) having the central solid phase rate of more than 0.8 and less than 1.0 with a rolling reduction quantity of 2.0 mm or more and 10.0 mm or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a method for continuous casting of a slab.

Background Art

[0002] In the continuous casting of steel, defects such as central segregation and porosity occur in the slab. In order to improve the internal quality due to these defects, the slab is usually subjected to reduction in thickness direction in a continuous casting machine.

[0003] As a technique for reducing central segregation, a technique of lightly reducing the slab by rolls in a continuous casting machine is known. This technique compensates for solidification shrinkage by light reduction and suppresses the flow at the end of solidification. As a technique for reducing porosity, a technique of heavily reducing the slab in a continuous casting machine is known. This technique applies a heavy reduction greater than solidification shrinkage to the slab.

[0004] For example, Japanese Patent Application Laid-Open No. 8-164460 (Patent Document 1) discloses a technique of lightly reducing a slab at the end of solidification and further heavily reducing it after complete solidification. In the technique of Patent Document 1, the light reduction is continuously performed by small-diameter reduction rolls. The heavy reduction is performed by heavy reduction rolls installed downstream of the small-diameter reduction rolls.

[0005] Japanese Patent No. 5929836 (Patent Document 2) discloses a technique of expanding the thickness of a slab by bulging and lightly reducing the slab with an increased thickness by a reduction roll.

[0006] Japanese Patent Application Laid-Open No. 2016-22531 (Patent Document 3) discloses a technique of reducing a slab by two pairs of reduction rolls arranged at intervals in the casting direction. In the technique of Patent Document 3, the slab with a solid fraction of 0.2 or less at the center of thickness is reduced by the first pair of rolls. Further, the completely solidified slab is reduced by the second pair of rolls.

Prior Art Documents

Patent Documents

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 8-164460 [Patent Document 2] Japanese Patent No. 5929836 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-22531 [Summary of the Invention] [Problems to be Solved by the Invention]

[0008] The slab obtained by continuous casting is rolled into a product. When the product is a steel plate, the slab is a slab. In recent years, the thickness of steel plates has been increasing, and along with this, the requirements for the internal quality of slab have become even higher. For this reason, it is required to further reduce the central segregation and porosity in the center of the slab.

[0009] In refining plants such as those for oil and natural gas, pressure vessels (tanks) are used. Steel plates for pressure vessels are required to be thick, high-strength, and have HIC resistance characteristics. HIC (Hydrogen Induced Cracking) is hydrogen-induced cracking in a corrosive environment containing wet hydrogen sulfide, and it occurs starting from inclusions such as manganese sulfide, carbonitrides of Nb and Ti, alumina, or porosity. The higher the strength around the inclusions or porosity, the easier it is for HIC to occur. That is, the higher the base metal strength, the easier it is for HIC to occur.

[0010] During continuous casting, in the central segregation part in the solidification process of the slab, due to component segregation, Mn, S, P, Nb, and Ti are concentrated, inclusions are likely to occur, and the strength increases compared to the base metal. Therefore, the influence on the HIC resistance characteristics of the central segregation part is extremely large. Also, when the thickness of the steel plate manufactured from the slab becomes 80 mm or more, the rolling reduction ratio required for the compaction of porosity becomes small. As a result, porosity remains in the steel plate, and this becomes the starting point for the occurrence of HIC. Therefore, in order to manufacture a thick, high-strength steel plate with HIC resistance characteristics, it is necessary to reduce both central segregation and porosity in the slab.

[0011] In this regard, in the technology of Patent Document 1, heavy reduction is performed on the slab after complete solidification. Therefore, the load required for heavy reduction becomes excessive, and a reduction device with excessive capacity is required. In the technology of Patent Document 2, a special device for bulging the slab is required. In the technology of Patent Document 3, heavy reduction is performed on the slab after complete solidification by the second pair of rolls. Therefore, similar to the technology of Patent Document 1, a reduction device with excessive capacity is required.

[0012] An object of the present disclosure is to provide a continuous casting method for slab slabs that can reduce both central segregation and porosity with a simple configuration.

Means for Solving the Problems

[0013] The continuous casting method for slab slabs according to the present disclosure includes a first reduction step and a second reduction step. In the first reduction step, the slab is lightly reduced at a reduction rate of 0.5 mm / min or more and 1.0 mm / min or less from when the central solid fraction of the slab reaches 0.3 until it reaches 0.8. In the second reduction step, the slab having a central solid fraction greater than 0.8 and less than 1.0 is heavily reduced with a reduction amount of 2.0 mm or more and 10.0 mm or less.

Effects of the Invention

[0014] According to the continuous casting method for slab slabs according to the present disclosure, central segregation and porosity can be reduced with a simple configuration.

Brief Description of the Drawings

[0015]

Figure 1

Embodiments for Carrying Out the Invention

[0016] In order to solve the above problems, the present inventors have conducted intensive studies and as a result, obtained the following findings.

[0017] In order to produce a steel plate with thick thickness, high strength, and HIC resistance properties, it is necessary to reduce both central segregation and porosity in the slab. The thickness of the steel plate is assumed to be 80 mm or more, and the strength of the steel plate is assumed to be 550 MPa or more. The thickness of the slab is assumed to be 200 mm or more, and the width of the slab is assumed to be 1600 mm or more. Hereinafter, the slab is also simply referred to as a casting.

[0018] First, the generation mechanisms of central segregation and porosity in continuous casting are considered.

[0019] In the region where the central solid fraction of the casting is 0.3 or more, negative pressure due to bulging and solidification shrinkage occurs at the center of the casting. Due to this negative pressure, the pressure at the center of the casting decreases, and concentrated molten steel accompanying microsegregation during solidification of the steel is sucked and accumulated in this pressure reduction part. Central segregation is formed by the concentrated molten steel accumulated in the pressure reduction part at the center of the casting.

[0020] In this specification, the central solid fraction of the casting means the ratio of the solid phase occupied at the center of the thickness of the casting.

[0021] Porosity is formed when concentrated molten steel does not flow into the pressure reduction part at the center of the casting. In particular, in the region where the central solid fraction of the casting is greater than 0.8, the flow of molten steel is unlikely to occur. This is because the viscosity of the molten steel is high and the flow resistance due to the solid phase part is large. Therefore, porosity is likely to occur.

[0022] Subsequently, the rolling reduction method of the casting in continuous casting and the correlation between central segregation and porosity are considered.

[0023] In the region where the central solid fraction of the slab is 0.3 or more, compared with porosity, central segregation is formed over a wide range. To reduce central segregation, it is effective to gradually reduce the thickness of the slab using a plurality of reduction rolls so that negative pressure does not occur at the center of the slab. This reduction is called soft reduction. Soft reduction is performed in segment units composed of a plurality of reduction rolls. That is, soft reduction is performed in a soft reduction zone where a plurality of reduction rolls are arranged in the casting direction.

[0024] In the region where the central solid fraction of the slab is less than 0.3, the formation of central segregation does not start. Therefore, even if the slab is subjected to soft reduction in this region, there is no effect of reducing central segregation. On the other hand, if the slab is subjected to soft reduction in the region where the central solid fraction of the slab is greater than 0.8, a certain effect on central segregation occurs. However, in this case, it is inevitable to press down even the completely solidified part with high deformation resistance. Therefore, in order to properly perform soft reduction on the slab in the region where the central solid fraction of the slab is greater than 0.8, a reduction device having excessive capacity is required. Or an extension of the soft reduction zone is required. This leads to an increase in cost.

[0025] The reduction rate when performing soft reduction on the slab needs to be 0.5 mm / min or more in order to compensate for solidification shrinkage. If the reduction rate is less than 0.5 mm / min, the reduction amount cannot sufficiently compensate for the solidification shrinkage amount, so excessive negative pressure is generated at the center of the slab. As a result, central segregation increases. On the other hand, if the reduction rate is greater than 1.0 mm / min, the reduction amount exceeds the solidification shrinkage amount, so the molten steel flows backward from the downstream side to the upstream side in the casting direction due to soft reduction. As a result, central segregation deteriorates instead.

[0026] In this specification, the reduction rate of soft reduction means the value obtained by dividing the reduction amount of the slab in the soft reduction zone by the time it takes for the slab to pass through the soft reduction zone. The reduction amount of the slab in the soft reduction zone means the difference between the reduction amount by the roll at the exit of the soft reduction zone and the reduction amount by the roll at the entrance of the soft reduction zone. The time it takes for the slab to pass through the soft reduction zone means the value obtained by dividing the distance from the entrance to the exit of the soft reduction zone (that is, the total length of the soft reduction zone) by the casting speed.

[0027] From the above considerations, appropriate conditions for soft reduction to reduce central segregation are derived. The conditions are as follows. If the slab is subjected to soft reduction at a reduction rate of 0.5 mm / min or more and 1.0 mm / min or less from the time when the central solid fraction of the slab reaches 0.3 until it reaches 0.8, central segregation can be sufficiently reduced.

[0028] Porosity is likely to be formed in a relatively narrow region at the end stage of solidification. That is, porosity is likely to be formed in the region where the central solid fraction of the slab is greater than 0.8 and less than 1.0. This is because the molten steel becomes less fluid in this region. This region is at a very high temperature, and in this region, if a large-diameter reduction roll is used to apply a large reduction to the slab, the compressive strain concentrates on the porosity. As a result, the porosity can be reduced. This reduction is called large reduction.

[0029] Even if the slab is subjected to large reduction in the region after the central solid fraction of the slab reaches 1.0, the effect of reducing porosity is small. This is because after complete solidification, when large reduction is applied, the deformation resistance of the slab increases and the porosity is difficult to shrink. On the other hand, if a reduction larger than the solidification shrinkage amount is applied to the slab in the region where the central solid fraction of the slab is 0.8 or less, the porosity is reduced. However, in this case, the unfrozen molten steel flows backward from the downstream side to the upstream side in the casting direction. As a result, central segregation is promoted.

[0030] From the above considerations, appropriate conditions for large reduction to reduce porosity and central segregation are derived. The conditions are as follows. If a slab having a central solid fraction greater than 0.8 and less than 1.0 is subjected to large reduction with a reduction amount of 2.0 mm or more in the thickness direction, the porosity can be sufficiently reduced. The larger the reduction amount, the more the porosity can be reduced. However, if the reduction amount is too large, the reduction force required for large reduction becomes excessive. In this case, a reduction device with excessive capacity is required, which leads to an increase in cost. Therefore, from the perspective of equipment, it is preferable that the upper limit of the reduction amount is about 10.0 mm.

[0031] In short, by accurately setting the conditions of light reduction and heavy reduction that are mutually correlated, it becomes possible to reduce the center segregation and porosity with a simple configuration without requiring a reduction device with excessive capacity. In particular, in the region where the center solid fraction of the slab is 0.3 or more and center segregation is formed, light reduction is carried out from when the center solid fraction reaches 0.3 until it reaches 0.8, and since the reduction rate is set within an appropriate range, the effect of reducing center segregation is significant.

[0032] The continuous casting method according to the embodiment of the present disclosure has been completed based on the above findings.

[0033] The continuous casting method of a slab according to the embodiment of the present disclosure includes a first reduction step and a second reduction step. In the first reduction step, from when the center solid fraction of the slab reaches 0.3 until it reaches 0.8, the slab is lightly reduced at a reduction rate of 0.5 mm / min or more and 1.0 mm / min or less. In the second reduction step, a slab having a center solid fraction greater than 0.8 and less than 1.0 is heavily reduced with a reduction amount of 2.0 mm or more and 10.0 mm or less (the first configuration).

[0034] As described above, in continuous casting, center segregation is formed in the region where the center solid fraction of the slab is 0.3 or more. In the region from when the center solid fraction reaches 0.3 until it reaches 0.8, if the slab is lightly reduced at an appropriate reduction rate, it is possible to reduce the center segregation without requiring a reduction device with excessive capacity. Also, in the region where the center solid fraction of the slab is greater than 0.8 and less than 1.0, porosity is likely to occur. In this region, if the slab is heavily reduced with an appropriate reduction amount, it is possible to reduce the porosity while suppressing the formation of center segregation without requiring a reduction device with excessive capacity.

[0035] Such conditions of light reduction and heavy reduction are defined in the continuous casting method of the first configuration. Therefore, according to the continuous casting method of the first configuration, both center segregation and porosity can be reduced with a simple configuration. In this case, the slab casting obtained by the continuous casting method of the first configuration has reduced center segregation and porosity. For this reason, this slab casting is suitable for manufacturing a steel plate that is thick, high-strength, and has HIC resistance characteristics.

[0036] In the continuous casting method of the first configuration, the casting preferably has, by mass%, C: 0.03 to 0.10%, Si: 0.05 to 0.45%, Mn: 0.8 to 1.6%, P: 0.020% or less, S: 0.0010% or less, Nb: 0.005 to 0.040%, Ti: 0.003 to 0.040%, V: 0.01 to 0.07%, Al: 0.010 to 0.060%, N: 0.001 to 0.006%, Ca: 0.0005 to 0.0040%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 1.50%, Mo: 0 to 0.50%, B: 0 to 0.0020%, and the balance consists of Fe and impurities (second configuration).

[0037] According to the continuous casting method having the chemical composition defined in the second configuration, a slab casting that serves as a material for a high-strength and HIC-resistant steel plate can be manufactured.

[0038] In the continuous casting method of the second configuration, the chemical composition is more preferably Cu: 0.10 to 0.50%, Ni: 0.10 to 0.50%, Cr: 0.10 to 1.50%, and Contains one or more elements selected from the group consisting of 0.10 to 0.50% of Mo (third configuration).

[0039] In the continuous casting method of the third configuration, the lower limits of the Cu content, Ni content, Cr content, and Mo content are limited to 0.10%. Therefore, according to the continuous casting method of the third configuration, a slab casting that can be used as a material for a higher-strength steel plate can be manufactured.

[0040] In the continuous casting method of the second configuration or the third configuration, the chemical composition is more preferably Contains 0.0005 to 0.0020% of B (fourth configuration).

[0041] In the continuous casting method of the fourth configuration, the lower limit of the B content is limited to 0.0020%. Therefore, according to the continuous casting method of the fourth configuration, a slab casting that can be used as a material for a higher-strength steel plate can be manufactured.

[0042] The slab casting obtained by any one of the continuous casting methods of the first to fourth configurations is a material for a steel plate having a thickness of 80 mm or more (fifth configuration).

[0043] The slab casting obtained by the continuous casting method of the fifth configuration is rolled into a steel plate. The thickness of this steel plate is 80 mm or more, which is extremely large. When the thickness of the steel plate is large, the rolling reduction ratio when rolling from the casting to the steel plate becomes small. If the rolling reduction ratio is small, the porosity in the casting is not crimped by rolling and remains in the steel plate. According to the continuous casting method of the fifth configuration, a casting with reduced porosity can be manufactured. Therefore, this casting is suitable for manufacturing a thick steel plate with excellent internal quality.

[0044] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The same or corresponding components in each figure are denoted by the same reference numerals, and the same description will not be repeated.

[0045] [Continuous casting machine] FIG. 1 is a schematic view of a continuous casting machine 1 used in the continuous casting method according to the embodiment. The continuous casting machine 1 includes a tundish 2, a mold 4, a plurality of support rolls 5, a plurality of soft reduction rolls 6, and a large reduction roll 7.

[0046] Molten steel M is supplied to the tundish 2 from a ladle (not shown). The molten steel M in the tundish 2 is supplied to the mold 4 through a submerged nozzle 3. A secondary cooling spray nozzle (not shown) is disposed below the mold 4. The molten steel M in the mold 4 is cooled by the mold 4 and further cooled by the cooling water from the secondary cooling spray nozzle. Thereby, a solidified shell S is formed, and a slab 10 including the solidified shell S and the molten steel M in an unsolidified state is obtained.

[0047] The slab 10 including the molten steel M in an unsolidified state is guided downstream in the casting direction by a plurality of support rolls 5. In this process, the molten steel M in the unsolidified state gradually decreases, and the slab 10 in a completely solidified state is obtained. In such continuous casting, the slab 10 is sent in the casting direction by a pinch roll (not shown). That is, the casting speed of the slab 10 is controlled by the pinch roll.

[0048] Here, a plurality of soft reduction rolls 6 are arranged on the downstream side in the casting direction of the support rolls 5. The soft reduction rolls 6 are paired with each other and perform soft reduction on the slab 10. More specifically, a soft reduction zone 61 is formed by a series of soft reduction rolls 6. The soft reduction zone 61 is divided into a plurality of segments, and a plurality of soft reduction rolls 6 are provided for each segment. The reduction amount of the soft reduction rolls 6 is controlled for each segment. The plurality of soft reduction rolls 6 are arranged at substantially equal intervals between the inlet 61i and the outlet 61o of the soft reduction zone 61. Here, in the soft reduction zone 61, the position of the inlet 61i coincides with the position of the soft reduction roll 6 arranged at the most upstream in the casting direction, and the position of the outlet 61o coincides with the position of the soft reduction roll 6 arranged at the most downstream in the casting direction.

[0049] On the downstream side in the casting direction of the plurality of soft reduction rolls 6, a heavy reduction roll 7 is arranged. The heavy reduction rolls 7 are paired and perform heavy reduction on the slab 10. The heavy reduction point 7a coincides with the position where the heavy reduction roll 7 is arranged. The heavy reduction roll 7 has, for example, a diameter of 400 mm or more and 600 mm or less.

[0050] [Continuous casting method] The continuous casting method of the slab in this embodiment includes a first reduction step and a second reduction step. The slab obtained by this continuous casting method becomes a material for products such as steel plates. When the product is a steel plate, the slab is heated, rolled, and cooled, and then annealed. Thereby, for example, a thick steel plate having high strength and HIC resistance characteristics can be obtained. This thick steel plate has, for example, a thickness of 80 mm or more and a tensile strength of 550 MPa or more.

[0051] In the first reduction step, the slab 10 is softly reduced using a plurality of soft reduction rolls 6 provided in the soft reduction zone 61. The soft reduction is performed from when the central solid fraction of the slab 10 reaches 0.3 until the central solid fraction reaches 0.8. That is, at the inlet 61i of the soft reduction zone 61, the central solid fraction of the slab 10 is 0.3, and at the outlet 61o of the soft reduction zone 61, the central solid fraction of the slab 10 is 0.8. The reduction rate of the slab 10 in the soft reduction zone 61 is 0.5 mm / min or more and 1.0 mm / min or less.

[0052] In the second reduction step, the slab 10 is heavily reduced using the heavy reduction roll 7. The heavy reduction is performed on the slab 10 having a central solid fraction greater than 0.8 and less than 1.0. That is, the central solid fraction of the slab 10 at the heavy reduction point 7a is greater than 0.8 and less than 1.0. From another perspective, at the end of solidification before the slab 10 is completely solidified, the slab 10 is heavily reduced. The reduction amount of the slab 10 at this time is 2.0 mm or more and 10.0 mm or less.

[0053] [Chemical composition] An example of the chemical composition of the slab 10 obtained by the continuous casting method of this embodiment is shown below. “%” regarding the contained elements means mass% unless otherwise specified.

[0054] C: 0.03 to 0.10% Carbon (C) is an element that is extremely effective in increasing the strength of the steel plate. If the C content is less than 0.03%, the desired strength cannot be ensured, so it is necessary to contain 0.03% or more. However, if it exceeds 0.10%, inclusions that become the starting points for HIC generation, such as NbC, are likely to be generated. Therefore, the C content is set to 0.03 to 0.10%. The C content is preferably 0.04 to 0.08%.

[0055] Si: 0.05 to 0.45% Silicon (Si) is an element necessary as a deoxidizer in the deoxidation process. It is necessary to contain 0.05% or more to obtain a sufficient deoxidation effect. However, if it exceeds 0.45%, island-shaped martensite is generated in the heat-affected zone of welding and becomes the starting point of fracture, leading to a decrease in joint toughness. Therefore, the Si content is set to 0.05 to 0.45%. The Si content is preferably 0.08 to 0.20%.

[0056] Mn: 0.8 to 1.6% Manganese (Mn) is an element that increases the hardenability of steel and enhances the strength and toughness of the steel plate. If the Mn content is less than 0.8%, these effects cannot be obtained. On the other hand, if the Mn content exceeds 1.6%, the central segregation deteriorates and the MnS inclusions formed with S increase, resulting in a decrease in HIC resistance characteristics. Therefore, the Mn content is set to 0.8 to 1.6%. The Mn content is preferably 1.0 to 1.4%.

[0057] P: 0.020% or less Phosphorus (P) is generally contained as an impurity. If P is contained in an amount exceeding 0.020%, it segregates at the grain boundaries and reduces the grain boundary strength, resulting in deterioration of the low-temperature toughness. Therefore, the P content is set with 0.020% as the upper limit, and it is desirable to reduce it as much as possible. The P content is preferably 0.015% or less.

[0058] S: 0.0010% or less Sulfur (S) is generally contained as an impurity. S combines with Mn in steel to form MnS, deteriorating the HIC resistance property. Therefore, the S content is capped at 0.0010% and it is desirable to reduce it as low as possible. The S content is preferably 0.0008% or less.

[0059] Nb: 0.005 - 0.040% Niobium (Nb) is an element effective for expanding the austenite non-recrystallized region. It further contributes to the refinement of crystal grains, improving strength and toughness. To obtain such effects, it is necessary to contain Nb at 0.005% or more. However, when the Nb content exceeds 0.040%, coarse carbides are generated and toughness decreases. Therefore, the Nb content is set at 0.005 - 0.040%. The Nb content is preferably 0.010 - 0.030%.

[0060] Ti: 0.003 - 0.040% Titanium (Ti) is an element that combines with N in steel to form TiN, enhancing the cleanliness of the slab surface and the steel plate surface. It also has the effect of suppressing the coarsening of austenite crystal grains. To obtain such effects, Ti needs to be contained at 0.003% or more. On the other hand, when Ti is contained in excess of 0.040%, the precipitates may coarsen and the toughness of the base metal may deteriorate. Therefore, the Ti content is set at 0.003 - 0.040%. The Ti content is preferably 0.005 - 0.020%.

[0061] V: 0.01 - 0.07% Vanadium (V) has the effect of forming carbonitrides and precipitation-strengthening the steel plate. To obtain this effect, it is necessary to contain V at 0.01% or more. On the other hand, when the V content exceeds 0.07%, the effect saturates and the economic rationality deteriorates. Therefore, the V content is set at 0.01 - 0.07%. The V content is preferably 0.02 - 0.05%.

[0062] Al: 0.010 - 0.060% Aluminum (Al) acts as a deoxidizer and is most commonly used in the deoxidation process of molten steel for steel plates. Also, by fixing the dissolved N in the steel to form AlN, it has the effect of suppressing the coarsening of crystal grains. If the Al content is less than 0.010%, this effect cannot be obtained. On the other hand, when the Al content exceeds 0.060%, it mixes into the welded metal part during welding and deteriorates the toughness of the welded metal. Therefore, the Al content is set to 0.010 - 0.060%. The Al content is preferably 0.015 - 0.040%.

[0063] N: 0.001 - 0.006% Nitrogen (N) combines with Ti and Al to form TiN and AlN. The same effects as those described for Ti and Al can be obtained. To obtain such effects, N needs to be contained at 0.001% or more. On the other hand, when the content exceeds 0.006%, the nitrides coarsen and the toughness decreases. Therefore, the N content is set to 0.001 - 0.006%. The N content is preferably 0.002 - 0.004%.

[0064] Ca: 0.0005 - 0.0040% Calcium (Ca) has the effect of spheroidizing non-metallic inclusions such as MnS and suppressing the occurrence of HIC. However, when its content is excessive, inclusions such as CaO and CaS are generated in large quantities and hydrogen is likely to accumulate around the inclusions, deteriorating the HIC resistance. Therefore, the Ca content is set to 0.0005 - 0.0040%. The Ca content is preferably 0.0010 - 0.0030%.

[0065] [Optional element] The above chemical composition may further contain one or more elements selected from the group consisting of Cu, Ni, Cr, and Mo in place of a part of Fe. All of these elements are optional elements and increase the strength of the steel plate.

[0066] Cu: 0 - 0.50% Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu is an element effective for improving hardenability and increasing strength through precipitation hardening during tempering. To effectively obtain such an effect, it is preferable to add 0.10% or more of Cu. A more preferable lower limit of the Cu content is 0.20%. On the other hand, when contained in excess, there is a concern of high-temperature cracking due to Cu checking. Therefore, when containing Cu, it is preferably 0.50% or less. A more preferable upper limit of the Cu content is 0.40%.

[0067] Ni: 0 to 0.50% Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, Ni is an element that contributes to improving the toughness of the steel sheet and increasing the hardenability to improve the strength of the steel sheet. To effectively obtain such an effect, it is preferable to add 0.10% or more of Ni. On the other hand, when added in excess of 0.50%, there is a concern that the stress corrosion cracking characteristics deteriorate in a hydrogen sulfide environment and fine cracks occur on the surface. Therefore, the Ni content is preferably 0.10 to 0.50%. The Ni content is more preferably 0.12 to 0.30%.

[0068] Cr: 0 to 1.50% Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When contained, Cr is an element effective for improving strength because it suppresses ferrite transformation and increases hardenability. To effectively obtain such an effect, it is preferable to add 0.10% or more of Cr. On the other hand, when added in excess of 1.50%, there is a concern that the strength of the steel sheet becomes too high and the toughness decreases. Therefore, the Cr content is preferably 0.10 to 1.50%. The Cr content is more preferably 0.20 to 1.00%.

[0069] Mo: 0 to 0.50% Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When contained, Mo is an element that increases the hardenability of the steel sheet and improves the base metal strength. In order to effectively obtain such an effect, it is preferable to add Mo in an amount of 0.10% or more. On the other hand, when the addition exceeds 0.50%, the weldability significantly deteriorates. For this reason, the Mo content is preferably 0.10 to 0.50%. The Mo content is more preferably 0.20 to 0.40%.

[0070] The above chemical composition may further contain B in place of a part of Fe. B is an optional element and increases the hardenability of the steel sheet.

[0071] B: 0 to 0.0020% Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B can significantly improve the hardenability by segregating at the austenite grain boundaries and suppressing the formation of ferrite. In order to obtain such an effect, it is preferable to contain B in an amount of 0.0005% or more. However, when the B content exceeds 0.0020%, the toughness deteriorates. For this reason, the B content is preferably 0.0005 to 0.0020%.

[0072] [Effect] In the continuous casting method of the present embodiment, in the first soft reduction step, soft reduction is performed from the time when the central solid fraction of the slab 10 reaches 0.3 until it reaches 0.8. The reduction rate of the slab 10 by soft reduction is 0.5 mm / min or more and 1.0 mm / min or less. Further, in the second heavy reduction step, the slab 10 having a central solid fraction greater than 0.8 and less than 1.0 is subjected to heavy reduction. The reduction amount of the slab 10 by heavy reduction is 2.0 mm or more and 10.0 mm or less. By such soft reduction and heavy reduction, it is possible to reduce central segregation and porosity without requiring a reduction device having excessive capacity. In short, with a simple configuration, both central segregation and porosity can be reduced. In this case, the slab cast piece obtained by the continuous casting method of the present embodiment has reduced central segregation and porosity. Therefore, this slab cast piece is suitable for manufacturing a steel plate having a thick thickness, high strength, and HIC resistance characteristics.

[0073] However, the chemical composition containing the above-described elements is a preferable chemical composition. Therefore, the chemical composition of the slab 10 obtained by the continuous casting method of the present embodiment is not limited to those containing the above-described elements.

Example

[0074] In order to confirm the effects of the continuous casting method of the present embodiment, the following tests were carried out and the results were evaluated. Specifically, the Mn segregation degree and the porosity volume of the slab cast piece obtained by continuous casting were evaluated.

[0075] The slab cast piece used in this test was manufactured using the continuous casting machine shown in FIG. 1. The mold was a water-cooled copper mold. The length of the mold was 800 mm. The cross-section of the mold was rectangular. The slab cast piece continuously cast using this mold had a thickness of 300 mm and a width of 2300 mm. The casting speed was 0.6 m / min.

[0076] The position of the entrance of the soft reduction zone was 16 m from the meniscus (the molten metal surface in the mold). The position of the exit of the soft reduction zone was 21 m from the meniscus. The position of the heavy reduction point where the heavy reduction rolls were arranged was 21.4 m. The heavy reduction rolls were driven by hydraulic cylinders, and the maximum load per roll was 600 tf. The central temperature and solid fraction of the slab were calculated by two-dimensional solidification analysis in the thickness and width directions of the slab. The specific water volume of the cooling water sprayed from the secondary cooling spray nozzles was set to 0.5 - 1.0 L / kg-steel.

[0077] The main chemical composition of the slab used in this test was C: 0.05%, Si: 0.1%, Mn: 1.3%, P: 0.009%, S: 0.0008%, Nb: 0.015%, Ti: 0.01%, V: 0.025%, Al: 0.02%, N: 0.0035%, Ca: 0.0015%, Cu: 0.1%, Ni: 0.1%, Cr: 0.2%, and Mo: 0.1%.

[0078] The Mn segregation degree of the slab was investigated by the following procedure. The slab was cut at the center in the width direction, and a sample was taken to include a region 40 mm in the casting direction and 40 mm in the thickness direction from the center in the thickness direction of the cut surface. For this sample, surface analysis of the Mn concentration was performed by EPMA (Electron Probe Micro Analyzer). The Mn concentrations within a 2-mm width along the casting direction were integrated with the position of the highest Mn concentration as the center and averaged, and the resulting value was defined as the maximum Mn concentration (Cmax). The value obtained by dividing the maximum Mn concentration (Cmax) by the Mn concentration (C0) of the bulk composition of the slab was defined as the Mn segregation degree. The Mn concentration (C0) of the bulk composition was determined by chemical analysis after taking an analytical sample from the slab.

[0079] The porosity volume of the slab casting was investigated by the following procedure. Samples with a length of 50 mm in the casting direction, a width of 100 mm, and a thickness of 7 mm were taken from the central part in the thickness direction of the slab casting. The samples were taken from 16 locations along the width direction of the slab casting. For each sample, the density ρ was measured by the method for measuring the density and specific gravity of solids specified in JIS Z 8807. Then, the porosity volume V (cm 3 / g) per unit weight was determined by the following formula (1). The density ρ0 in formula (1) was the density measured in the same manner as above by taking samples from the 1 / 4 thickness part of the slab.

[0080]

Equation

[0081] Here, the slab casting obtained by continuous casting was heated, rolled, and cooled, and then annealed to produce a thick steel plate with a thickness of 100 mm. Samples with a length of 100 mm in the rolling direction, a width of 20 mm, and a thickness of 20 mm were taken from the obtained steel plate, and an HIC test was conducted. The HIC test was carried out in accordance with NACE (National Association of Corrosion Engineers) TM00284-2003. Specifically, the taken samples were immersed in a normal temperature 5% sodium chloride + 0.5% acetic acid aqueous solution saturated with 1 atm of hydrogen sulfide gas for 96 hours, and then the crack area ratio in the samples was determined by ultrasonic flaw detection test. Note that 3 samples were prepared from the same steel plate, and the average value of their crack area ratios was taken as the HIC area ratio.

[0082] If the HIC area ratio is 5% or less, it can be said that the HIC resistance property is satisfied. From the previously conducted test results, if the Mn segregation degree of the slab casting is 1.4 or less and the porosity volume is 3.0×10 -4 cm 3 / g or less, the HIC area ratio of the thick steel plate was 5% or less. Therefore, in this test, the HIC resistance property was evaluated by the Mn segregation degree and porosity volume of the slab casting. Specifically, if the Mn segregation degree of the slab casting is 1.4 or less and the porosity volume is 3.0×10-4 cm 3 It was evaluated as passing if it was below / g.

[0083] Table 1 shows the test conditions and test results.

[0084]

Table 1

[0085] In Table 1, the test conditions showed the conditions regarding soft reduction (first reduction process) and heavy reduction (second reduction process) in continuous casting. In the test results, if the Mn segregation degree of the obtained slab was 1.4 or less, it was shown as "excellent", otherwise "unacceptable". Similarly, if the porosity volume was 3.0×10 -4 cm 3 / g or less, it was shown as "excellent", otherwise "unacceptable".

[0086] As shown in Table 1, Invention Examples 1 to 5 all satisfied the conditions defined in this embodiment. Therefore, in Invention Examples 1 to 5, slab billets with good Mn segregation degree and porosity volume were obtained. That is, central segregation and porosity were reduced.

[0087] In Comparative Example 1, the reduction rate during soft reduction was small compared to the conditions defined in this embodiment. In this case, the solidification shrinkage amount could not be sufficiently compensated, and the Mn segregation degree and porosity volume deteriorated. On the other hand, in Comparative Example 2, the reduction rate during soft reduction was large compared to the conditions defined in this embodiment. In this case, due to excessive soft reduction, the molten steel flowed backward from the downstream to the upstream in the casting direction, and the Mn segregation degree deteriorated.

[0088] In Comparative Example 3, the central solid fraction at the inlet of the soft reduction zone was large compared to the conditions defined in this embodiment. In this case, the compensation for solidification shrinkage on the low solid fraction side was insufficient, and the Mn segregation degree deteriorated. In Comparative Example 4, the central solid fraction at the outlet of the soft reduction zone was small compared to the conditions defined in this embodiment. In this case, the compensation for solidification shrinkage on the high solid fraction side was insufficient, and the Mn segregation degree deteriorated.

[0089] In Comparative Example 5, the reduction amount under high pressure was small compared to the conditions defined in the present embodiment. In this case, the porosity volume deteriorated due to insufficient reduction amount.

[0090] In Comparative Example 6, the central solid fraction under high pressure was small compared to the conditions defined in the present embodiment. Accordingly, the central solid fraction at the outlet of the soft reduction zone was also small. Due to the small central solid fraction under high pressure, the molten steel flowed backward from the downstream to the upstream in the casting direction under high pressure, and as a result, the Mn segregation degree deteriorated. On the other hand, in Comparative Example 7, the central solid fraction under high pressure was large compared to the conditions defined in the present embodiment. That is, the central solid fraction under high pressure was 1.0. In this case, due to high pressure being applied after complete solidification, it became difficult for the porosity to shrink, and as a result, the porosity volume deteriorated.

[0091] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Description of Reference Numerals

[0092] 1: Continuous casting machine 2: Tundish 4: Mold 5: Support roll 6: Soft reduction roll 61: Soft reduction zone 7: High pressure roll 10: Cast slab

Claims

1. A method for continuous casting of a slab casting, comprising: a first soft reduction step of performing soft reduction on the casting at a reduction rate of 0.5 mm / min or more and 1.0 mm / min or less from when the central solid fraction of the casting reaches 0.3 until the central solid fraction reaches 0.8; a second heavy reduction step of performing heavy reduction on the casting having the central solid fraction greater than 0.8 and less than 1.0 with a reduction amount of 2.0 mm or more and 10.0 mm or less; A continuous casting method comprising the above.

2. The continuous casting method according to Claim 1, wherein the casting has, by mass%, C: 0.03 to 0.10%, Si: 0.05 to 0.45%, Mn: 0.8 to 1.6%, P: 0.020% or less, S: 0.0010% or less, Nb: 0.005 to 0.040%, Ti: 0.003 to 0.040%, V: 0.01 to 0.07%, Al: 0.010 to 0.060%, N: 0.001 to 0.006%, Ca: 0.0005 to 0.0040%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 1.50%, Mo: 0 to 0.50%, B: 0 to 0.0020%, and the balance consists of Fe and impurities, having a chemical composition.

3. The continuous casting method according to Claim 2, wherein the chemical composition contains at least one element selected from the group consisting of Cu: 0.10 to 0.50%, Ni: 0.10 to 0.50%, Cr: 0.10 to 1.50%, and Mo: 0.10 to 0.50%.

4. The continuous casting method according to Claim 2 or 3, wherein the chemical composition contains B: 0.0005 to 0.0020%.

5. The continuous casting method according to any one of Claims 1 to 4, wherein the obtained slab casting is a material for a steel plate having a thickness of 80 mm or more.

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