Method for producing aluminium-killed steel

Abstract

To provide a method for producing a slab excellent in surface crack resistance and corner crack resistance.SOLUTION: This invention relates to a method for producing an aluminium-killed steel having a C equivalent Cp calculated by the following formula within a range of 0.10 to 0.20 mass%, Cp=C-0.0022×Si+0.019×Mn-0.179×P+2.258×S-0.123×Al-0.002×Cr-0.035×Mo-0.438×Nb-0.058×V+0.025×Ni+0.378×N+0.019×Cu, wherein: a mass ratio Ti / N of a Ti content to an N content satisfies 3.5 or more; a surface temperature of a slab in an upper portion of a secondary cooling zone in continuous casting, where a solidified shell thickness of the slab is within a range of 10 to 30 mm, is controlled to be 1050°C or more; and the slab is cooled so that a temperature at a time of bending straightening or unbending straightening maintains a temperature at which hot ductility is 50% or higher. In the formula, each element symbol represents a content (mass%) of the corresponding element, and is set to 0 when the element is not contained.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a slab having excellent surface crack resistance and corner crack resistance when continuously casting aluminum-killed steel. In this specification, the term "x to y" representing a range of numerical values ​​means not less than x and not more than y, and includes the boundary value. [Background technology]

[0002] Generally, surface cracks that occur in aluminum-killed steel continuously cast using a curved or vertical bending type continuous casting machine tend to occur due to stress concentration in the cast slab caused by corrective distortion during straightening or unbending in the continuous casting machine.

[0003] Surface cracks caused by bending stress of this cast slab occur when nitrides such as AlN or carbides such as NbC precipitate at the austenite (hereinafter also referred to as γ) grain boundaries, generating voids (gaps), and cracks propagate from these voids along the γ grain boundaries.

[0004] It is known that the above-mentioned surface cracks occur when the slab surface temperature reaches the temperature at which the γ phase transforms into the ferrite (hereinafter also referred to as α) phase, the so-called embrittlement temperature range, during bending and straightening of the slab. Conventional countermeasures for preventing surface cracks that occur in this embrittlement temperature range include controlling the slab surface temperature during bending and straightening so as to avoid the temperature range (embrittlement temperature range) where high-temperature ductility decreases, either to the lower or higher side. However, the slab surface temperature changes due to changes in the casting speed and cooling water temperature, among other factors. Therefore, it is difficult to completely avoid the embrittlement temperature range by controlling only the slab surface temperature.

[0005] For example, Patent Document 1 discloses a technology for preventing surface cracks that occur during straightening or unstraightening of low-carbon aluminum-killed steel by suppressing the precipitation of AlN at γ grain boundaries and preferentially precipitating TiN. In Patent Document 1, Ti is added in an amount of more than 0.010 mass% but not more than 0.025 mass% when continuously casting low-carbon aluminum-killed steel. Patent Document 2 also discloses a continuous casting method for steel in which, without adding Ti, the Al and Nb contents are specified, the cooling rate and cooling stop temperature of the slab surface are specified, and the steel is straightened after recuperation. The technology in Patent Document 2 is said to refine γ grains to reduce crack susceptibility. Patent Document 3 discloses a technology for suppressing surface cracks in continuously cast slabs with a trace amount of Al by adding B in addition to Ti. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-022498 [Patent Document 2] Japanese Patent Application Publication No. 11-033688 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-112590 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the above-mentioned conventional techniques have the following problems to be solved. Specifically, the examples in Patent Documents 1 and 2 address cracking in slabs with a low carbon content of 0.08 to 0.09 mass%, which is relatively low in cracking susceptibility. These techniques rely on the refinement of γ grains and the suppression of AlN precipitation at γ grain boundaries. On the other hand, aluminum-killed steels with a carbon equivalent in the range of 0.10 to 0.20 mass%, particularly those in the so-called hypoperitectic region with a carbon equivalent of 0.14 to 0.16 mass%, develop deep oscillation marks on the surface of the slab in the mold. Stress then concentrates in the valleys of the oscillation marks, causing cracking. Therefore, the techniques in Patent Documents 1 and 2 have difficulty suppressing surface and corner cracking. Furthermore, the technique in Patent Document 3 is applicable to steels with a sol.Al (acid-soluble Al) content of less than 0.005 mass%, and is therefore not applicable to aluminum-killed steels.

[0008] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method for producing a cast slab having excellent resistance to surface cracking and corner cracking when continuously casting aluminum-killed steel located in the hypoperitectic region, which is highly susceptible to cracking. [Means for solving the problem]

[0009] The method for producing aluminum-killed steel according to the present invention, which advantageously solves the above-mentioned problems, is a method for producing aluminum-killed steel having a carbon equivalent Cp in the range of 0.10 to 0.20% by mass, calculated by the following formula (1), wherein the mass ratio of the Ti content to the N content, Ti / N, is 3.5 or more, the surface temperature of the slab in the upper part of the secondary cooling zone, where the solidified shell thickness of the slab in continuous casting is in the range of 10 to 30 mm, is controlled to be 1050°C or higher, and the temperature of the slab at the time of performing straightening or unbending straightening is cooled so that the high-temperature ductility is maintained at a temperature of 50% or higher. Cp=C-0.0022×Si+0.019×Mn-0.179×P+2.258×S-0.123×Al-0.002×Cr-0.035×Mo-0.438×Nb-0.058×V+0.025×Ni+0.378×N+0.019×Cu (1) Here, the element symbols in formula (1) represent the content of each element expressed in mass %, and are set to 0 when the element is not contained.

[0010] The method for producing aluminum-killed steel according to the present invention is as follows: (a) The surface temperature of the slab in the upper part of the secondary cooling zone in continuous casting, where the solidified shell thickness of the slab is in the range of 10 to 30 mm, is set to 1050°C or higher and 1400°C or lower, and the temperature of the slab at the time of bending straightening or unbending straightening is set to 850°C or higher. (b) The mass ratio of Ti / N to Ti content is 4.5 or more, and the surface temperature of the slab in the upper part of the secondary cooling zone where the solidified shell thickness of the slab in continuous casting is in the range of 10 to 30 mm is 1050°C or more and 1400°C or less, and the temperature of the slab at the time of bending straightening or unbending straightening is 820°C or more. (c) the carbon equivalent Cp calculated by the formula (1) is in the range of 0.14 to 0.16% by mass; (d) The hot ductility of a cast slab is measured by cutting a 100 mm long sample from a cast slab of the same composition, heating the sample to 1420°C for 60 seconds, and then holding it in the range of 1200 to 600°C, and conducting a tensile test every 100°C. This is a more preferred embodiment. [Effects of the Invention]

[0011] According to the present invention, by optimizing the Ti / N ratio of aluminum-killed steel and narrowing the embrittlement temperature range in the high-temperature region, it is possible to suppress surface cracking of slabs without changing the cooling pattern of the slabs. Furthermore, the addition of Ti causes TiN to precipitate within γ grains, thereby refining the crystal grains and improving ductility, thereby suppressing cracking. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a graph showing the temperature change of hot ductility of aluminum-killed steels having a carbon equivalent Cp in the range of 0.14 to 0.16. [Figure 2]1 is a graph showing the relationship between the Ti / N ratio and the reject rate caused by corner cracks in a slab, stratified into a hypo-peritectic region and other regions within the range of carbon equivalent Cp. [Figure 3] 1 is a graph showing the relationship between the carbon equivalent Cp and the arithmetic mean height (Sa) of waviness, which is an index of slab surface roughness. [Figure 4] 1 is a graph showing the relationship between the Ti / N ratio and the reject rate caused by corner cracks in a slab, stratified within the hypoperitectic region within the range of carbon equivalent Cp. DETAILED DESCRIPTION OF THE INVENTION

[0013] The following describes in detail the embodiments of the present invention. Note that the following embodiments are merely examples of methods for realizing the technical idea of ​​the present invention, and are not intended to limit the configuration to the following. In other words, the technical idea of ​​the present invention can be modified in various ways within the technical scope described in the claims.

[0014] The present invention is a method for suppressing surface cracks that occur at the bending straightening and unbending straightening points in a curved continuous caster or a vertical bending continuous caster by controlling the Ti / N ratio.

[0015] Among aluminum-killed steels, cast slabs of high-Si steels and high-Mn steels require solid solution strengthening and structural strengthening by bainite or martensite. High-Si steels and high-Mn steels have a carbon equivalent Cp, calculated by the following formula (1), in the range of 0.10 to 0.20 mass%. Cp=C-0.0022×Si+0.019×Mn-0.179×P+2.258×S-0.123×Al-0.002×Cr-0.035×Mo-0.438×Nb-0.058×V+0.025×Ni+0.378×N+0.019×Cu (1) Here, the element symbols in formula (1) represent the content of each element expressed in mass %, and are set to 0 when the element is not contained.

[0016] When the carbon equivalent Cp is in the range of 0.10 to 0.20 mass%, the solidified shell tends to be generated non-uniformly in the mold during continuous casting, which causes the oscillation marks generated by mold oscillation during casting of the slab to become deeper, and stress concentrates in the valleys of the oscillation marks, causing cracks.

[0017] Figure 1 shows the temperature dependence of hot ductility (RA) (%) for a specific Ti / N mass ratio (Ti / N) for a high-alloy aluminum-killed steel with a carbon equivalent (Cp) of 0.14-0.16 mass%. It is generally believed that when the slab surface temperature is below 50% of the hot ductility (RA) (%), surface cracking occurs when the slab enters the bending straightening zone. The results in Figure 1 demonstrate that hot ductility (RA) (%) improves at high temperatures as the Ti / N ratio increases. Hot ductility (RA) (%) was measured by cutting a 100 mm long sample from the cast slab, heating the sample to 1420°C for 60 seconds, then holding it in the temperature range of 1200-600°C, and conducting a tensile test every 100°C.

[0018] The reason for the improvement in hot ductility RA (%) is thought to be that Ti, which has a higher affinity for N than Al, preferentially bonds with N, suppresses AlN precipitation at the γ grain boundaries, and causes TiN to precipitate within the γ grains, thereby narrowing the width of the embrittlement temperature range. In other words, it is preferable to adjust the amount of Ti added, or to make low-N steel by denitrification treatment, with the mass ratio of Ti / N to Ti content set at a lower limit of 3.5, where Ti / N is the mass ratio of Ti to N, assuming that the atomic ratio of Ti to N reacts at approximately 1:1.

[0019] Furthermore, as can be seen from Figure 1, by increasing the Ti / N ratio, further improvements in cast slab cracking can be expected. Figure 1 shows that within this range of chemical composition, when the mass ratio of Ti to N, Ti / N, is 3.5 or more, hot ductility RA can be ensured to be 50% or more at temperatures of 850°C or higher.

[0020] The present invention is particularly suitable for application to the case where the oscillation marks are deep and the carbon equivalent Cp is 0.14 to 0.16 mass%, and further, it is preferable that the mass ratio of the Ti content to the N content Ti / N is 4.5 or more. From Fig. 1, it can be seen that within this range of composition, when the mass ratio of the Ti content to the N content Ti / N is 4.5 or more, a hot ductility RA of 50% or more can be ensured at 820°C or higher.

[0021] Furthermore, by controlling the surface temperature of the slab in the upper secondary cooling zone, where the solidified shell thickness is in the range of 10–30 mm, between the TiN precipitation start temperature (approximately 1400°C) and the AlN precipitation start temperature (approximately 1050°C), TiN precipitation is prioritized within the γ grains, particularly in the slab surface layer, while AlN precipitation at the γ grain boundaries is suppressed. Regarding the surface temperature of the slab, the amount of heat removal was calculated from the temperature measured by the in-mold thermocouples installed at positions ±500 mm from the center of the slab width, and the surface temperature of the slab was calculated by heat transfer calculations estimated from this amount of heat removal. Similarly, the heat removal amount was calculated from the temperature measured by the in-mold thermocouples, and the solidified shell thickness was calculated by heat transfer calculations. The surface temperature and solidified shell thickness in the secondary cooling zone were calculated by heat transfer calculations using the water flow rate in the secondary cooling zone. In addition, when the slab is a slab, the surface temperature at the center of the width was used.

[0022] Furthermore, the slab temperature at the time of straightening or unstraightening is cooled so as to maintain a high-temperature ductility of 50% or more. This allows aluminum-killed steel to be produced without complex operations such as reheating or excessive reduction in secondary cooling water, i.e., without reducing the casting speed and maintaining productivity. From the results shown in Figure 1, when the mass ratio (Ti / N) of the Ti content to the N content is 3.5 or more, it is preferable to control the slab surface temperature so as to maintain 850°C or higher, at which the high-temperature hot ductility RA (%) is 50% or more. Furthermore, when the mass ratio (Ti / N) of the Ti content to the N content is 4.5 or more, it is preferable to control the slab surface temperature so as to maintain 820°C or higher, at which the high-temperature hot ductility RA (%) is 50% or more. From the viewpoint of suppressing surface cracking, it is preferable to maintain a high slab surface temperature. On the other hand, from the viewpoint of productivity, it is preferable to increase the secondary cooling, i.e., to lower the slab surface temperature. Therefore, it is preferable to control the temperature of the slab at the time of straightening or unbending straightening to within a range of +100°C from the temperature at which the hot ductility is 50% or more, since this achieves both slab surface quality and productivity. This slab surface temperature can be measured using a thermocouple or a radiation thermometer. When the slab is a slab, the surface temperature is taken as the temperature at the center in the width direction. It is also preferable to measure the hot ductility of a slab with a carbon equivalent close to the chemical composition of the steel to be produced in advance, and to control the slab temperature at the time of straightening based on this hot ductility.

[0023] Figure 2 shows the relationship between the Ti / N ratio and the reject rate due to corner cracking in high-alloy aluminum-killed steel with Ti added. For slabs with a carbon equivalent Cp in the range of 0.02% by mass or more but less than 0.10% by mass, or greater than 0.20% by mass but less than 0.35% by mass, no clear difference in the reject rate for corner cracking was observed (Figures 2(a) and (b)). On the other hand, for slabs with a carbon equivalent Cp in the range of 0.10 to 0.20% by mass, the reject rate for corner cracking improved as the mass ratio of Ti to N content (Ti / N) increased. The reason for this is that in the range of carbon equivalent Cp in the range of 0.02% by mass or more but less than 0.10% by mass, or greater than 0.20% by mass but less than 0.35% by mass, the depth of oscillation marks was very shallow, and the incidence of corner cracking itself was very low.

[0024] Figure 3 shows the relationship between the carbon equivalent Cp and the arithmetic mean height Sa, which is an index of surface roughness and indicates the depth of oscillation marks. Figure 3 shows that the oscillation mark depth is high when the carbon equivalent Cp is in the range of 0.10 to 0.20 mass%, and that this correlates well with the rate of corner crack defects. In particular, the oscillation mark depth is high in slabs with a carbon equivalent Cp in the range of 0.14 to 0.16 mass%. The reason for the deeper oscillation mark depth in Figure 3 is believed to be that the volumetric shrinkage due to the δ-γ transformation increases in the carbon equivalent Cp range of 0.14 to 0.16 mass%.

[0025] In the present invention, the preferred ranges of the chemical composition of elements that affect the carbon equivalent Cp and the reasons for specifying Ti / N for aluminum-killed steel will be explained below. Hereinafter, "mass %" will be simply referred to as "%".

[0026] C: 0.01 to 0.20% C is an essential element for imparting the necessary strength to steel materials. However, if the content is less than 0.01%, the desired strength may not be obtained. Furthermore, attempting to control the concentration to less than 0.01% may result in high costs. On the other hand, if the content exceeds 0.20%, the strength of the steel increases, reducing the workability of the product, and there is a risk that the product may not satisfy the properties required. Therefore, the C content is preferably 0.01 to 0.20%. More preferably, it is 0.08% or more and 0.18% or less.

[0027] ·Si:3.0% or less Si is an element that contributes to improving the strength of steel through solid solution strengthening and is also an effective deoxidizer. However, adding more than 3.0% of Si may result in excessive solid solution strengthening, resulting in a decrease in deformability and poor ductility. Therefore, the Si content is preferably 3.0% or less. If adding Si has adverse effects, it is more preferable to keep it at 0.03% or less.

[0028] Mn: 0.25 to 2.5% Mn is an element that is effective not only in deoxidation and desulfurization but also in improving resistance to softening caused by quenching and tempering during heat treatment after cold working. It is preferable to add 0.25% or more. However, if added in excess of 2.5%, MnS is likely to precipitate, which may increase cracking sensitivity. Therefore, the Mn content is preferably set to 0.25 to 2.50%, and more preferably 0.50% or less.

[0029] P: 0.005~0.030% P is effective as an element for improving hardenability. However, if added in an amount exceeding 0.030%, microsegregation occurs during solidification, and grain boundary segregation occurs during hot rolling, embrittling the grain boundaries and potentially causing cracking during cold working. On the other hand, if the amount added is less than 0.005%, the load on the dephosphorization process may increase. Therefore, the P content is preferably set to 0.005 to 0.030%.

[0030] ·S: 0.03% or less S mainly forms sulfide-based inclusions, such as MnS. If the S content exceeds 0.03%, it segregates in the ingot during hot rolling, embrittling the ingot and potentially causing cracking during cold working. Therefore, the S content is preferably 0.03% or less. On the other hand, if machinability is required, an S content of less than 0.01% makes it difficult for MnS sulfide-based inclusions to precipitate, which may result in a decrease in the machinability of the product. Therefore, the S content is preferably 0.01 to 0.03%.

[0031] Al: 0.0063~0.12% Al is an effective deoxidizing element. However, it easily forms nitrides (AlN), and excessive AlN content leads to grain refinement. Furthermore, Al acts as a nitrogen-immobilizing element, bonding with N (described below) to solve the age-hardening problem caused by free N in steel. However, during continuous casting, if stress is applied during bending straightening or reduction of the cast slab while AlN is precipitated at the γ grain boundaries, stress concentration occurs at the γ grain boundaries, potentially leading to surface cracks. Therefore, it is best to minimize AlN formation. To minimize AlN formation, the Al content should be set below the median of the composition range, at 0.0063% to 0.12%.

[0032] Cr: 0.1 to 1.0% Cr is a ferrite stabilizing element. When added to steels containing carbon and manganese, it acts as a solute inhibitor, slowing the expansion of carbon and influencing grain refinement, thereby improving performance. In high-alloy aluminum-killed steels, the effect of adding chromium is minimal when it is less than 0.1%. On the other hand, when it is more than 1.0%, the weldability or toughness of the heat-affected zone may be reduced. A more preferred range is 0.2 to 0.5%.

[0033] ·Mo: More than 0 and less than 0.5% Mo is used as an element that increases hardenability, and can improve the strength, toughness, and hardenability of steel. Specifically, the molybdenum content is more than 0% and not more than 0.5%. If the Mo content exceeds 0.5%, weldability may be reduced. More preferably, the content is 0.01 to 0.10%.

[0034] Nb: 0.005 to 0.2% Nb is an element that forms carbides to refine the structure, improve toughness, and also contribute to high strength. Therefore, adding 0.005% or more Nb can be effective as needed. On the other hand, adding more than 0.2% Nb reduces toughness. For these reasons, the range is limited to 0.005 to 0.2%.

[0035] ·V: More than 0 and less than 0.2% V has a stronger carbide-forming ability than chromium and refines crystal grains, so it is also used to improve stainless steels and cutting tool steels. It also forms compounds with other metal elements, resulting in a significant precipitation hardening effect, so it is also used in precipitation hardening shaped steels and permanent magnets. Specifically, V is contained in an amount of more than 0% and not more than 0.2%. If V is contained in an amount exceeding 0.2%, the production cost increases, reducing economic viability, and low-temperature impact toughness may decrease. More preferably, V is contained in an amount of 0.08 to 0.15%.

[0036] ·Ni: More than 0 and less than 0.25% Ni increases the strength of steel and ensures low-temperature impact value. In a specific example, Ni is contained in an amount of more than 0% and 0.25% or less. If Ni is contained in an amount of more than 0.25%, the room-temperature strength may become excessively high, which may deteriorate the weldability and toughness. More preferably, Ni is contained in an amount of 0.0001 to 0.005%.

[0037] ·N:0.0060% or less When present in a free state in steel, N is an element that causes age hardening of the product, and therefore, for products requiring workability, such as low-carbon steel, it is preferable that the N content be as low as possible. Therefore, it is preferable that the N content be 0.0060% or less. However, if the N content is reduced, excessive sealing is required to suppress N pickup from the atmosphere, which leads to reduced operational costs. More preferably, the N content is 0.0005% or more and 0.0050% or less.

[0038] ·Cu: more than 0 and less than 0.4% Cu exhibits a solid solution strengthening effect, which slightly improves strength and hardness, but may reduce elongation. If the Cu content exceeds 0.4 wt%, there is a risk that hot workability and elongation may decrease and red shortness may occur. The preferred Cu content is 0.1 to 0.3%.

[0039] ·Ti / N:3.5 or more Ti combines with N in the steel and precipitates as TiN, thereby suppressing the precipitation of AlN and preventing surface cracking at the austenite grain boundaries. From this perspective, Ti is added to achieve a stoichiometric atomic ratio of Ti to N of 1:1, i.e., a mass ratio of Ti to N (Ti / N) of 3.5 or more, or to produce low-nitrogen steel. Preferably, Ti / N is 4.5 or more, and the carbon equivalent Cp calculated by the above formula (1) is preferably in the range of 0.14 to 0.16%. [Example]

[0040] The results of tests conducted to confirm the effects of the present invention will be described below. In the test, Al-killed steel was cast into slabs measuring 225 mm thick and 1400 mm wide using a curved continuous casting machine. The surface of the resulting slab was inspected for cracks, particularly in the corners. The cracks were judged as having a clear opening in the slab corner, a width of 1 mm or more, and a length of 5 mm or more. Continuous casting was performed at a slab withdrawal speed of 1.0 to 1.3 m / min, and the specific flow rate of the secondary cooling water was controlled to 0.8 to 1.2 liters / kg-steel. The surface temperature of the slab was measured using a thermocouple and a radiation thermometer.

[0041] ·Carbon equivalent Cp:0.10~0.13% The chemical composition of the aluminum-killed steel was set to C: 0.08-0.18%, Si: 1.6% or less, Mn: 1.00-2.20%, P: 0.006-0.025%, S: 0.00-0.02%, Al: 0.01-0.05%, and N: 0.000-0.005%, and the amount of Ti added was changed to adjust the Ti / N mass ratio. ·Carbon equivalent Cp:0.14~0.16% The chemical composition of the aluminum-killed steel was set to C: 0.08-0.18%, Si: 1.6% or less, Mn: 1.00-2.20%, P: 0.006-0.025%, S: 0.00-0.02%, Al: 0.01-0.05%, and N: 0.000-0.005%, and the amount of Ti added was changed to adjust the Ti / N mass ratio. ·Carbon equivalent Cp:0.17~0.20% The chemical composition of the aluminum-killed steel was set to C: 0.08-0.18%, Si: 1.6% or less, Mn: 1.00-2.20%, P: 0.006-0.025%, S: 0.00-0.02%, Al: 0.01-0.05%, and N: 0.000-0.005%, and the amount of Ti added was changed to adjust the Ti / N mass ratio. During continuous casting, the slab surface temperature at the bending back straightening point was 850 to 900°C. Figure 4 shows the relationship between the Ti / N mass ratio and the reject rate due to corner cracks in the slab. The reject rate is expressed as a percentage of the number of slabs judged to have corner cracks according to the above crack judgment criteria.

[0042] From FIG. 4, it can be seen that the defect rate due to corner cracking is high when the carbon equivalent Cp is 0.14 to 0.16%, and that the defect rate due to corner cracking is significantly improved when the Ti / N mass ratio is 3.5 or more, especially 4.5 or more.

[0043] In this specification, the unit of volume "liter" is 10 -3 m 3 Let's say.

Claims

1. A method for producing aluminum-killed steel having a carbon equivalent Cp calculated by the following formula (1) in a range of 0.10 to 0.20% by mass, comprising: The mass ratio Ti / N of the Ti content to the N content is 3.5 or more, The surface temperature of the slab in the upper part of the secondary cooling zone where the solidified shell thickness of the slab in continuous casting is in the range of 10 to 30 mm is controlled to be 1050°C or higher, A method for manufacturing aluminum-killed steel, wherein the temperature of the slab at the time of bending straightening or unbending straightening is cooled so as to maintain a high-temperature ductility of 50% or more. Cp=C-0.0022×Si+0.019×Mn-0.179×P+2.258×S-0.123×Al-0.002×Cr-0.035×Mo-0.438×Nb-0.058×V+0.025×Ni+0.378×N+0.019×Cu (1) Here, the element symbols in formula (1) represent the content of each element expressed in mass %, and when no element is contained, it is set to 0.

2. The surface temperature of the slab in the upper part of the secondary cooling zone where the solidified shell thickness of the slab in continuous casting is in the range of 10 to 30 mm is set to 1050°C or more and 1400°C or less, 2. The method for producing aluminum-killed steel according to claim 1, wherein the temperature of the slab at the time of performing straightening or unbending straightening is 850°C or higher.

3. The mass ratio Ti / N of the Ti content to the N content is 4.5 or more, The surface temperature of the slab in the upper part of the secondary cooling zone where the solidified shell thickness of the slab in continuous casting is in the range of 10 to 30 mm is set to 1050°C or more and 1400°C or less, 2. The method for producing aluminum-killed steel according to claim 1, wherein the temperature of the slab at the time of performing straightening or unbending straightening is 820°C or higher.

4. The method for producing an aluminum-killed steel according to any one of claims 1 to 3, wherein the carbon equivalent Cp calculated by the formula (1) is in the range of 0.14 to 0.16% by mass.

5. The hot ductility of the cast slab was measured by cutting a 100 mm long sample from a cast slab of the same chemical composition, heating the sample to 1420°C for 60 seconds, and then holding it in the range of 1200 to 600°C, and conducting a tensile test every 100°C. The method for producing the aluminum-killed steel according to claim 1.

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