Steel plate and producing method thereof

Abstract

To provide a steel plate excellent in intrinsic and mechanical properties and a producing method thereof.SOLUTION: A steel plate characterized by having a component composition composed of at mass%, C: 0.03-0.18%, Si: 0.03-0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.0200% or less, Al: 0.001-0.100%, O: 0.0100% or less and N: 0.0100% or less, and the remainder of a component composition consisting of Fe and unavoidable impurities, and in a region of 30 mm thickness including a plate thickness center, an average area of porosity is not more than or equal to 4.0 mm2, the standard deviation of the area of porosity is not more than 4.0 mm2, the porosity area ratio is not more than 2.0%, the tensile strength is not less than 430 MPa, and the Charpy absorption energy is not less than or equal to 27 J at -50°C.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a steel sheet having excellent internal properties and a method for producing the same. [Background technology]

[0002] In recent years, as structures have become larger, the steel plates used have become thicker to support them. Residual porosity is an issue when manufacturing steel plates. It is known that porosity is inevitably generated when molten steel solidifies and shrinks during slab manufacturing. Such porosity is closed and compressed in the rolling process, but porosity that remains without being compressed significantly impairs the mechanical properties of the steel plate, so there is a demand for further reduction in porosity in the rolling process. Patent Document 1 proposes a rolling method in which the residual porosity coefficient after the final pass is 0 or less. Patent Document 2 proposes a technique for reducing the remaining porosity by suppressing the porosity volume during continuous casting and then performing rolling with a rolling shape ratio of 0.5 or more. In Patent Document 3, the number of porosity of 200 μm or more is reduced to 0.1 pieces / mm by controlling the rolling reduction rate per pass at each temperature. 2 The following techniques have been proposed: Patent Document 4 proposes a manufacturing method in which a forged slab is rolled at a reduction rate of 20% or more and less than 60%, thereby reducing the number of porosity in the center of the plate thickness. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 3-44417 [Patent Document 2] International Publication No. 2021 / 182618 [Patent Document 3] International Publication No. 2018 / 216665 [Patent Document 4] JP 2014-189886 A Summary of the Invention [Problem to be solved by the invention]

[0004] In order to satisfy the demand for larger structures in recent years, thicker steel plates are required, which means that for a given slab thickness, a reduction in the reduction in the rolling process is required. If the total reduction is reduced, it becomes difficult to close or press the porosity during rolling, so many porosity remain inside the steel plate. These remaining porosity not only cause a decrease in ductility, but also become the starting point of brittle fracture and fatigue fracture in structures, so further reduction in rolling is required. In particular, an increase in the area of ​​porosity increases the cross-sectional defect area, resulting in a significant deterioration in the elongation and reduction of the steel plate. In addition, the variation in porosity area increases the probability of the existence of porosity that can be the starting point of brittle fracture and fatigue fracture, increasing the risk of brittle fracture and fatigue fracture.

[0005] The technologies described in Patent Documents 1 and 2 propose rolling methods that result in a porosity coefficient calculated using the shape factor being 0, and rolling methods that include passes with a shape factor of 0.5 or more. The shape factor is a parameter consisting of the plate thickness and roll diameter, and in order to increase this value, it is necessary to either reduce the product thickness or increase the reduction amount per pass. Therefore, in order to increase the shape factor without reducing the product thickness, there is a problem in that it is necessary to introduce large diameter rolls or rolling rolls with high load capacity limits or upper torque limits.

[0006] The technology proposed in Patent Document 3 requires rolling at 1,050°C or higher with an average reduction rate of 10% or more per pass. However, since the slab needs to be stably bitten into the rolls in the early stage of rolling, it is difficult to ensure the specified reduction rate.

[0007] Although Patent Document 4 calls for forging to reduce porosity, the implementation of forging is not preferable from the viewpoints of productivity and manufacturing costs.

[0008] As described above, there is a demand for reducing porosity in order to improve the mechanical properties and further the internal properties of steel sheets. However, conventional techniques are not yet sufficient for reducing the porosity of steel sheets.

[0009] An object of the present invention is to solve the above problems and to provide a steel sheet having excellent internal properties and mechanical properties, and a method for manufacturing the steel sheet.

[0010] Here, "excellent internal properties" refers to a reduction in area of ​​35% or more in a tensile test based on JIS Z 2241 (2011). In addition, excellent mechanical properties means that the tensile strength is 430 MPa or more in a tensile test based on JIS Z 2241 (2011) and the Charpy impact test at -50°C based on JIS Z 2242 (2018) is 27 J or more. [Means for solving the problem]

[0011] The present invention was developed by conducting research to solve the above problems, and has gained knowledge on how to more effectively close and bond porosity and produce a steel sheet with excellent internal properties.

[0012] Porosity present in a steel sheet reduces strength and deteriorates elongation in the case of ductile fracture, and serves as the starting point of fracture in the case of brittle fracture or fatigue fracture.

[0013] In order to close or press porosity by hot rolling (hereinafter, also simply referred to as rolling), it is important to increase strain at the center of the plate thickness where the porosity exists. However, the strain distribution in the plate thickness direction obtained by rolling is highest on the surface and lowest at the center of the plate thickness. Therefore, it is necessary to increase the strain at the center of the plate thickness by devising the rolling conditions. The present inventors have found that it is effective to carry out rolling after ensuring a certain or higher temperature difference between the slab surface and the center of the plate thickness during rolling. The obtained findings are explained in detail below.

[0014] (1) It was found that porosity can be further reduced by performing rolling that satisfies certain parameters calculated from the temperature difference between the center of thickness and the surface. Specifically, in hot rolling, the integrated value of X in each pass of the following formula (1) calculated from the roll entry thickness, roll exit thickness, absolute temperature at the thickness center before each hot rolling pass, and absolute temperature at the plate surface is set so that in passes where the plate thickness center temperature is 950°C or more and 1200°C or less, the integrated value of X (hereinafter also referred to as X1 value) is 1.0 or more and 10.0 or less, and in passes where the plate thickness center temperature is 800°C or more and less than 950°C, the integrated value of X (hereinafter also referred to as X2 value) is 3.0 or more. It has been found that this further reduces porosity.

[0015]

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[0016] (2) As described above, in the hot rolling pass where the temperature at the center of the plate thickness is 950°C or more and 1200°C or less, the X1 value is set to 1.0 or more, and in the pass where the temperature at the center of the plate thickness is 800°C or more and less than 950°C, the X2 value is set to 3.0 or more. As a result, the average area of ​​porosity with an echo height of 25% or more by ultrasonic inspection is set to 4.0 mm 2The standard deviation of the porosity area is 4.0 mm 2 It has been found that it is possible to manufacture a steel sheet having a porosity area ratio of 2.0% or less.

[0017] Regarding the temperature at the center of the plate thickness, the temperature range of 950°C to 1200°C is an area where the deformation resistance of the slab is relatively low, so it is possible to ensure a higher reduction rate per pass. Therefore, by making the X1 value 1.0 or more, the strain effectively introduced at the center of the plate thickness promotes the reduction of porosity. In addition, the temperature range of 800°C or higher and lower than 950°C is the non-recrystallization temperature range, and by performing rolling to set the X2 value to 3.0 or higher, ferrite transformation nuclei can be effectively introduced into the crystal grains, and the mechanical properties can be ensured by refining the ferrite grain size.

[0018] The present invention has been completed through extensive investigations based on the above findings. That is, the gist of the present invention is as follows.

[0019] [1] In mass percent, C: 0.03 to 0.18%, Si: 0.03 to 0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.0200% or less, Al: 0.001 to 0.100%, O: 0.0100% or less and N: 0.0100% or less The remainder of the composition is Fe and unavoidable impurities. In the 30mm thick area including the center of the plate, The average porosity area is 4.0 mm 2 is as follows: The standard deviation of the porosity area is 4.0 mm 2 is less than or equal to, and The porosity area ratio is 2.0% or less, The tensile strength is 430 MPa or more, A steel plate having a Charpy absorbed energy of 27 J or more at -50°C. [2] The composition further comprises, in mass%, Cu: 2.00% or less, Ni: 2.50% or less, Cr: 1.50% or less, Mo: 1.00% or less Nb: 0.100% or less, Ti: 0.100% or less, V: 0.30% or less, B: 0.0100% or less, W: 0.50% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Sn: 0.0500% or less and REM: 0.0500% or less The steel sheet according to the above [1], which contains one or more selected from the group consisting of: [3] A method for producing a steel plate according to the above [1] or [2], comprising heating a slab to 1050°C or more and 1200°C or less, and then subjecting the slab to one or more passes of hot rolling; In the hot rolling, In the pass where the plate thickness center temperature is 950 ° C or more and 1200 ° C or less, the integrated value of X represented by the following formula (1) in each pass satisfies 1.0 to 10.0, A manufacturing method for a steel sheet, in which, in a pass in which the sheet thickness center temperature is 800°C or higher and lower than 950°C, an integrated value of X represented by the following formula (1) in each pass satisfies 3.0 or higher.

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[0020] According to the present invention, there are provided a steel sheet having excellent internal properties and mechanical properties, and a method for manufacturing the steel sheet. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The steel plate (thick steel plate) of the present invention has a composition containing, by mass%, C: 0.03 to 0.18%, Si: 0.03 to 0.70%, Mn: 0.30 to 2.50%, P: 0.030% or less, S: 0.0200% or less, Al: 0.001 to 0.100%, O: 0.0100% or less, and N: 0.0100% or less, with the balance being Fe and unavoidable impurities; In the 30 mm thick area including the center of the plate thickness, the porosity detected by ultrasonic testing had an average area of ​​4.0 mm 2 and the standard deviation of the porosity area is 4.0 mm 2 The porosity area ratio is 2.0% or less, the tensile strength is 430 MPa or more, and the Charpy absorbed energy is 27 J or more at -50°C.

[0022] The reasons for limiting each of the constituent elements of the present invention will be explained below. First, the reason for limiting the composition of the steel plate and slab (steel billet) to the above range in the present invention will be explained. Note that "%" regarding the composition means "mass %" unless otherwise specified.

[0023] C: 0.03 to 0.18% C is an element that improves the strength of steel and must be contained in an amount of 0.03% or more. On the other hand, if the C content exceeds 0.18%, the weldability and toughness decrease. Therefore, the C content is set to 0.03 to 0.18%. The C content is preferably 0.05% or more. Also, the C content is preferably 0.17% or less.

[0024] Silicon: 0.03 to 0.70% Silicon is an element that is inevitably contained in steel as an impurity, and since it has the effect of increasing strength, a content of 0.03% or more is required. However, if the Si content exceeds 0.70%, the toughness decreases. Therefore, the Si content is set to 0.03 to 0.70%. The Si content is preferably 0.04% or more. Also, the Si content is preferably 0.60% or less.

[0025] Mn: 0.30-2.50% Mn is an element that improves the hardenability of steel, thereby improving its strength. From the viewpoint of strength, the Mn content must be 0.30% or more. On the other hand, if the Mn content exceeds 2.50%, there is a concern that the toughness may decrease. Therefore, the Mn content is set to 0.30 to 2.50%. The Mn content is preferably 0.50% or more. Also, the Mn content is preferably 2.20% or less.

[0026] P:0.030% or less P is an element that is inevitably contained in steel as an impurity and deteriorates the toughness of the steel. Therefore, the P content is set to 0.030% or less, and preferably 0.025% or less. On the other hand, since the less P, the better, the lower limit of the P content is not particularly limited and may be 0%. However, excessively low P content leads to an increase in refining time and an increase in costs. Therefore, the P content is preferably 0.001% or more.

[0027] S: 0.0200% or less S is an element that is inevitably contained in steel as an impurity and reduces the toughness of the steel. Therefore, the S content is set to 0.0200% or less. The S content is preferably set to 0.0100% or less. On the other hand, since the less S, the better, the lower limit of the S content is not particularly limited and may be 0%. However, excessively reducing the S content increases the refining time and the cost. Therefore, the S content is preferably 0.0001% or more.

[0028] Al: 0.001 to 0.100% Al is an element effective for deoxidization. In addition, Al is an element that has the effect of forming nitrides and reducing the austenite grain size. In order to obtain such effects, the Al content is set to 0.001% or more. On the other hand, if the Al content exceeds 0.100%, the cleanliness of the steel decreases. As a result, the ductility and toughness decrease. Therefore, the Al content is set to 0.001 to 0.100%. The Al content is preferably 0.005% or more. Moreover, the Al content is preferably 0.080% or less.

[0029] O: 0.0100% or less O is an element that is inevitably contained in steel as an impurity and reduces the ductility and toughness of steel, so the O content is set to 0.0100% or less. On the other hand, since the less O, the better, the lower limit of the O content is not particularly limited and may be 0%. However, excessive reduction in O content leads to an increase in refining time and an increase in costs. Therefore, the O content is preferably 0.0005% or more.

[0030] N: 0.0100% or less N is an element that is inevitably contained in steel as an impurity and reduces the ductility and toughness of steel, so the N content is set to 0.0100% or less. On the other hand, since the less N, the better, the lower limit of the N content is not particularly limited and may be 0%. However, excessively low N content leads to an increase in refining time and an increase in costs. Therefore, the N content is preferably 0.0005% or more.

[0031] The basic composition of the steel sheet according to one embodiment of the present invention has been described above. The balance of the composition of the steel sheet according to one embodiment of the present invention other than the above elements is Fe and unavoidable impurities. From the viewpoint of further improving strength and weldability (toughness of welded parts, weldability, etc.), one or more of the following optional additive elements may be appropriately contained. Cu: 2.00% or less, Ni: 2.50% or less, Cr: 1.50% or less, Mo: 1.00% or less, Nb: 0.100% or less, Ti: 0.100% or less, V: 0.30% or less Bottom, B: 0.0100% or less, W: 0.50% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Sn: 0.0500% or less, REM: 0.0500% or less

[0032] Cu:2.00% or less Cu is an element that improves the strength of steel without significantly deteriorating toughness. However, if the Cu content exceeds 2.00%, hot cracking due to the Cu-enriched layer formed just below the scale becomes a problem. Therefore, when Cu is contained, the Cu content is preferably 2.00% or less. The Cu content is more preferably 1.50% or less. The Cu content is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.10% or more.

[0033] Ni: 2.50% or less Ni is an element that can increase the strength of steel plate without significantly deteriorating toughness. Ni is also an element that has the effect of improving toughness. However, if the Ni content exceeds 2.50%, an increase in manufacturing costs becomes a problem. Therefore, when Ni is contained, the Ni content is preferably 2.50% or less. The Ni content is more preferably 2.00% or less. The Ni content is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.10% or more.

[0034] Cr:1.50% or less Cr is an element that improves the hardenability of steel, thereby improving the strength of the steel. However, if the Cr content exceeds 1.50%, the weldability decreases. Therefore, if Cr is contained, the Cr content is preferably 1.50% or less. The Cr content is more preferably 1.20% or less, further preferably 0.90% or less, and further preferably 0.60% or less. The Cr content is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.10% or more.

[0035] Mo: 1.00% or less Mo is an element that improves the hardenability of steel, thereby improving the strength of the steel. However, if the Mo content exceeds 1.00%, the weldability decreases. Therefore, when Mo is contained, the Mo content is preferably 1.00% or less. The Mo content is more preferably 0.80% or less. The Mo content is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.10% or more.

[0036] Nb: 0.100% or less Nb is an element that suppresses recrystallization when strain is applied to the austenite structure by solid solution Nb and finely precipitated NbC. Nb is also an element that has the effect of raising the temperature of the non-recrystallization temperature range. However, if the Nb content exceeds 0.100%, the weldability decreases. Therefore, when Nb is contained, the Nb content is preferably 0.100% or less. The Nb content is more preferably 0.075% or less, and further preferably 0.050% or less. The Nb content is more preferably 0.001% or more, and further preferably 0.005% or more.

[0037] Ti:0.100% or less Ti is an element that has the effect of pinning the movement of grain boundaries and suppressing grain growth by precipitating as TiN. However, if the Ti content exceeds 0.100%, the cleanliness of the steel decreases. As a result, the ductility and toughness decrease. Therefore, when Ti is contained, the Ti content is preferably 0.100% or less. The Ti content is more preferably 0.080% or less. The Ti content is further preferably 0.060% or less, and even more preferably 0.050% or less. The Ti content is more preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.010% or more.

[0038] V: 0.30% or less V is an element that improves the hardenability of steel and generates carbonitrides to improve the strength of steel. However, if the V content exceeds 0.30%, the weldability decreases. Therefore, when V is contained, the V content is preferably 0.30% or less. The V content is more preferably 0.25% or less. The V content is more preferably 0.20% or less, and further preferably 0.15% or less. The V content is preferably 0.01% or more.

[0039] B: 0.0100% or less B is an element that improves the hardenability of steel, thereby improving the strength of the steel. However, if the B content exceeds 0.0100%, the weldability decreases. Therefore, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0070% or less. The B content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.

[0040] W: 0.50% or less W is an element that improves the hardenability of steel, thereby improving the strength of the steel. However, if the W content exceeds 0.50%, the weldability decreases. Therefore, when W is contained, the W content is preferably 0.50% or less. The W content is more preferably 0.40% or less. The W content is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.10% or more.

[0041] Ca:0.0200% or less Ca is an element that improves weldability by forming oxysulfides that are highly stable at high temperatures. However, if the Ca content exceeds 0.0200%, the cleanliness of the steel decreases, and the toughness of the steel decreases. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. The Ca content is more preferably 0.0180% or less. The Ca content is further preferably 0.0150% or less, and even more preferably 0.0120% or less. The Ca content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.

[0042] Mg: 0.0200% or less Mg is an element that improves weldability by forming oxysulfides that are highly stable at high temperatures. However, if the Mg content exceeds 0.0200%, the effect of adding Mg becomes saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less. The Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, and further preferably 0.0010% or more. Moreover, the Mg content is more preferably 0.0180% or less, further preferably 0.0150% or less, and further preferably 0.0120% or less.

[0043] Sn: 0.0500% or less Sn is an element that improves the corrosion resistance of steel. However, if the Sn content exceeds 0.0500%, the elongation of the steel decreases. Therefore, when Sn is contained, the Sn content is preferably 0.0500% or less. The Sn content is more preferably 0.0450% or less. The Sn content is further preferably 0.0400% or less, and even more preferably 0.0300% or less. The Sn content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.

[0044] REM: 0.0500% or less REM (rare earth metal) is an element that improves weldability by forming oxysulfides that are highly stable at high temperatures. However, if the REM content exceeds 0.0500%, the effect of adding REM becomes saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when REM is contained, the REM content is preferably 0.0500% or less. The REM content is more preferably 0.0450% or less. The REM content is further preferably 0.0400% or less, and even more preferably 0.0300% or more. The REM content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.

[0045] Regarding the elements related to the above-mentioned optional components, when the content of each element is less than the respective preferable lower limit value, the element is treated as an inevitable impurity. Here, the inevitable impurities include Zn, Pb, As, Sb, Bi, etc., which may be mixed in from scrap, etc. The mixing of these impurities is also permitted as long as it does not impair the purpose of the present invention.

[0046] In the steel plate of the present invention, the average area of ​​porosity detected by ultrasonic flaw detection in a 30 mm thick region including the plate thickness center (hereinafter, sometimes referred to simply as the plate thickness center region) is 4.0 mm 2and the standard deviation of the porosity area is 4.0 mm 2 and the porosity area ratio is 2.0% or less.

[0047] Porosity area ratio: 2.0% or less Porosity remaining in the steel plate not only reduces ductility but also becomes the starting point of brittle fracture and fatigue fracture, which should be avoided in structures. If the porosity area ratio in the plate thickness center region exceeds 2.0%, the number of porosity that can become the starting point of fracture increases, and the desired reduction in area cannot be obtained. Therefore, the porosity area ratio in the plate thickness center region is set to 2.0% or less. It is preferably 1.5% or less, and more preferably 1.0% or less. Although it is preferable that the porosity area ratio in the center region of the plate thickness is as low as possible, it is difficult to completely close and compress the porosity, and therefore the porosity area ratio in the center region of the plate thickness is preferably 0.01% or more.

[0048] Average porosity area: 4.0mm 2 below As mentioned above, porosity in steel plate causes a decrease in ductility and brittle fracture and fatigue fracture. 2 When the porosity exceeds 4.0 mm, the elongation is significantly reduced and the material becomes more susceptible to brittle fracture. 2 Less than or equal to 3.5 mm. 2 More preferably, it is 3.0 mm or less. 2 The average area of ​​the porosity in the central region of the plate thickness is preferably 2.5 mm or less. 2 and even more preferably 2.0 mm or less. 2 The following is the result. Although it is preferable that the average area of ​​the porosity in the center region of the plate thickness is as low as possible, it is difficult to completely close and compress the porosity. Therefore, the average area of ​​the porosity in the center region of the plate thickness is preferably 0.01 mm 2 That's all.

[0049] Standard deviation of porosity area: 4.0mm 2 below The larger the porosity size (area of ​​porosity), the lower the ductility, and the more likely brittle fracture and fatigue fracture will occur. Therefore, the standard deviation of the porosity area is set to 4.0 mm 2 By setting the standard deviation of the porosity area to 3.5 mm or less, it is possible to suppress large porosity and reduce the frequency of brittle fracture and fatigue fracture. 2 More preferably, it is 3.0 mm or less. 2 The standard deviation of the porosity area in the center region of the plate thickness is more preferably 2.5 mm or less. 2 and even more preferably 2.0 mm or less. 2 The following is the result. Although it is preferable that the standard deviation of the porosity area in the center region of the plate thickness is as small as possible, it is difficult to completely close and compress the porosity. Therefore, the standard deviation of the porosity area in the center region of the plate thickness is preferably 0.01 mm or less. 2 That's all.

[0050] How to calculate the porosity area ratio, average porosity area, and standard deviation of porosity area The porosity area ratio, size (average area of ​​porosity), and standard deviation of porosity area in the center of plate thickness were examined using an ultrasonic flaw detector. Steel plates were sampled from a 30 mm thick area (15 mm from the center of plate thickness to both sides of the plate surface) including the center of plate thickness over a length of 1000 mm in the rolling direction and across the entire width, and the porosity state was visualized by treating the area with an echo height of 25% or more as porosity. Image analysis was applied to the obtained scanned image to quantify the porosity (defect) area ratio, size (average area of ​​porosity), and standard deviation of porosity area. The ultrasonic flaw detection conditions were a 3-inch probe with a frequency of 10 MHz, a focal length of 15 mm, a transducer diameter of 0.375 inches, a flaw detection speed of 100.0 mm / s, and a flaw detection pitch of 0.1 mm. Size (average porosity area) is calculated by dividing the total measured porosity area by the number of porosities. A porosity is counted as one area surrounded by non-porosity areas. In addition, the standard deviation of the porosity area is calculated based on the area of ​​each porosity.

[0051] Plate Thickness The thickness of the steel plate is preferably 30 to 180 mm. The thickness of the steel plate is more preferably 40 mm or more, and further preferably 50 mm or more. The thickness of the steel plate is more preferably 170 mm or less, and further preferably 160 mm or less.

[0052] Next, a method for manufacturing a steel sheet according to an embodiment of the present invention will be described. In the manufacturing method of the steel plate of the present invention, a slab is heated to 1050°C or more and 1200°C or less, and then subjected to one or more passes of hot rolling. In the hot rolling, in passes where the plate thickness center temperature is 950°C or more and 1200°C or less, the integrated value of X represented by the following formula (1) in each pass satisfies 1.0 or more and 10.0 or less, and in passes where the plate thickness center temperature is 800°C or more and less than 950°C, the integrated value of X represented by the following formula (1) in each pass satisfies 3.0 or more. The temperature specified in the method for producing a steel plate of the present invention refers to the surface temperature of the slab or the steel plate.

[0053]

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[0054] Heating temperature: 1050℃ or higher and 1200℃ or lower Prior to hot rolling, the slab (steel billet) is heated to a heating temperature of 1050°C or more and 1200°C or less. If the heating temperature is lower than 1050°C, the deformation resistance increases, the hot rolling conditions described below cannot be satisfied, and a sufficient porosity reduction effect cannot be obtained. Therefore, the heating temperature is set to 1050°C or more, preferably 1070°C or more. On the other hand, if the heating temperature is higher than 1200° C., the austenite grains become coarse, causing a decrease in strength and toughness, so the heating temperature is set to 1200° C. or lower, preferably 1170° C. or lower.

[0055] Hot rolling The heated steel plate is hot-rolled. The rolling conditions are controlled in the following two stages based on the temperature at the center of thickness of the steel plate.

[0056]

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[0057] The reasons for limiting the above rolling conditions will be explained.

[0058] Accumulated value of X (X1 value) in the pass where the temperature at the center of the plate thickness is 950°C or more and 1200°C or less: 1.0 or more and 10.0 or less In order to adjust the initial austenite grain size, rolling is performed in a recrystallization temperature range of 950 ° C or more. In this temperature range, the deformation resistance of steel is low and the closure of porosity is likely to proceed. When the integrated value of X (hereinafter also referred to as X1 value) expressed by formula (1) in each pass where the plate thickness center temperature is 950 ° C or more and 1200 ° C or less satisfies 1.0 to 10.0, the closure of the porosity in the steel plate proceeds further, and the porosity with a small initial size is crimped after closure. Even for porosity that does not reach crimping, the closure progresses, reducing the height of the porosity and changing it to a shape that is more easily crimped. If the X1 value exceeds 10.0, rolling in the non-recrystallization region below 950 ° C becomes insufficient, coarse ferrite grains are generated, and both strength and toughness may deteriorate. Therefore, in the temperature range of 950 ° C or more and 1200 ° C or less, the upper limit of the X1 value is 10.0. The X1 value is preferably 8.0 or less, more preferably 7.5 or less. On the other hand, if the X1 value is less than 1.0, it is difficult to close and press the porosity, and the strength and toughness may deteriorate due to the presence of coarse porosity remaining in the steel sheet. Therefore, the X1 value is set to 1.0 or more. The X1 value is preferably 1.2 or more, and more preferably 1.4 or more. Here, the temperature at the center of the sheet thickness being 950°C or higher and 1200°C or lower in each pass means that the temperature at the center of the sheet thickness being 950°C or higher and 1200°C or lower before each pass (at the roll entry side of each hot rolling pass).

[0059] Accumulated value of X (X2 value) in passes with a thickness center temperature of 800°C or more and less than 950°C: 3.0 or more This temperature range includes a partial recrystallization region and a non-recrystallization region, and suppresses recrystallization after rolling. The strain introduced by rolling functions as a transformation nucleus during ferrite transformation, thereby refining the structure of the steel sheet and ensuring strength and toughness. The temperature is lower than 950°C, and the deformation resistance of the steel is high. Therefore, the integrated value of X (hereinafter also referred to as X2 value) represented by formula (1) in the pass with a plate thickness center temperature of 800°C or more and less than 950°C needs to be 3.0 or more. The X2 value is preferably 3.3 or more, and more preferably 3.6 or more. Moreover, the X2 value is preferably 10.0 or less, and more preferably 7.0 or less. Here, the temperature at the center of the sheet thickness being 800°C or higher and lower than 950°C in each pass means that the temperature at the center of the sheet thickness being 800°C or higher and lower than 950°C before each pass (at the roll entry side of each hot rolling pass).

[0060] How to create a temperature difference between the center of thickness and the surface The larger the temperature difference between the center of thickness and the surface, the larger X in formula (1) becomes, and the more effectively the strain caused by rolling can be imparted to the center of thickness. Methods for providing a temperature difference include forced cooling by air or water cooling, and a temperature difference of 50°C or more is expected to increase the compressive stress imparted to the center of thickness more effectively, which will exceed the deformation resistance of the steel and contribute to the closure of defects. The temperature difference is preferably 100°C or more. Other conditions than those mentioned above are not limited and may be in accordance with conventional methods. For example, the rolling temperature is preferably 700 to 1200°C. In addition, the cooling conditions after the rolling step are not particularly limited and may be in accordance with conventional methods. For example, it is preferable to cool in the range of 0.1 to 30°C / s. The cumulative rolling reduction is preferably 40 to 95%. The number of passes in the hot rolling is preferably 10 to 30. The rolling reduction in each pass is preferably 3 to 50%.

[0061] In addition, in the hot rolling method of the steel sheet manufacturing method of the present invention, it is preferable to perform multiple rolling operations at a temperature at the center of thickness of 950°C or more and 1200°C or less, wait until the temperature decreases, and then perform multiple rolling operations at a temperature at the center of thickness of 800°C or more and less than 950°C. In passes with a plate thickness center temperature of 950°C or more and 1200°C or less, the cumulative reduction is preferably 20 to 90%, the number of passes in hot rolling is preferably 4 to 20, and the reduction in each pass is preferably 3 to 35%. In addition, in passes where the plate thickness center temperature is 800°C or more and less than 950°C, the cumulative reduction is preferably 50 to 95%, the number of passes in hot rolling is preferably 2 to 20, and the reduction in each pass is preferably 3 to 35%.

[0062] In formula (1), t0 is the plate thickness (mm) at the roll entry side of each hot rolling pass, and t1 is the plate thickness (mm) at the roll exit side of each hot rolling pass. Moreover, R indicates the radius (mm) of the circular cross section of a pair of cylindrical rolls that clamp and roll the steel sheet used in hot rolling. Also, T mid is the absolute temperature (K) at the center of thickness at the roll entry side of each hot rolling pass (before each hot rolling pass), and T sur is the absolute temperature (K) of the plate surface at the roll entry side of each hot rolling pass (before each hot rolling pass). The sheet thickness t1 at the roll exit side of each hot rolling pass can be calculated from the rolling reduction rate. Also, T sur can be measured before each rolling pass, and T mid can be calculated based on the measurement results. Regarding X in formula (1), the thickness of the steel sheet to be finally obtained during manufacturing is taken into consideration. sur , T mid The rolling reduction rate can be controlled to adjust the rolling reduction rate to a desired value.

[0063] The present invention has been described above. The present invention is a method for rolling a steel plate by providing a temperature difference between the surface and the center of thickness, which allows for effective introduction of strain at the center of thickness and further reduction of porosity. In addition, by rolling in a temperature range where the center of thickness is high temperature, deformation resistance is reduced and porosity can be sufficiently closed and compressed, further reducing the area ratio, average area, and size standard deviation of porosity in the steel plate, and excellent mechanical properties are obtained. Thus, according to the steel plate (thick steel plate) of the present invention, it is possible to improve the internal properties and mechanical properties without requiring the introduction of new equipment such as rolling rolls and without impairing productivity. EXAMPLES

[0064] Next, the present invention will be described more specifically based on examples. Steel plates were manufactured under the manufacturing conditions shown in Table 2 using steel slabs with the chemical composition shown in Table 1. The surface temperature of the slab during rolling was measured using a radiation thermometer, etc. Additionally, the temperature at the center of the slab's thickness was calculated by heat transfer calculation based on the surface temperature in this case, but it is also possible to measure it directly using a thermocouple. In addition, in Table 2, the X1 value and the X2 value are the sum of X for each path obtained from the following formula (1).

[0065]

number

[0066] A tensile test was conducted in the thickness direction for each steel plate obtained, and the internal properties were evaluated. The results are shown in Table 2. For the tensile test, two tensile test pieces were taken from the 1 / 4 width position, where there tend to be many defects in the steel plate, and one from the center of the width, and then evaluation was performed. The shape of the tensile test piece conformed to JIS Z 2241 (2011). The reduction in area was obtained by a tensile test based on JIS Z 2241 (2011), and steel grades with a reduction in area of ​​35% or more, with the minimum value of each value taken as the representative value, were evaluated as having excellent internal properties when rolled. In addition, the tensile strength was measured by a tensile test based on JIS Z 2241 (2011). In addition, Charpy impact test specimens were taken in the rolling direction, and the toughness of each steel plate was evaluated by a Charpy impact test based on JIS Z 2242 (2018). In the Charpy impact test, three Charpy test pieces were taken from the 1 / 4 width position, where defects tend to be more frequent, and three from the center of the width, as in the tensile test, and the test was carried out at -50°C, with the minimum value of each value being adopted as the representative value. The shape of the Charpy test piece was a V-notch based on JIS Z 2242. Steel plates that showed a tensile strength of 430 MPa or more in a tensile test and a Charpy impact strength of 27 J or more were evaluated as having excellent mechanical properties. The results are shown in Table 2. As shown in Table 2, in this development example, it was confirmed that excellent internal quality properties and mechanical properties were obtained by reducing the porosity size (average area), area ratio, and standard deviation to the desired range. On the other hand, the steel plate in the comparative example was unable to satisfy at least one of the internal quality properties and mechanical properties.

[0067] [Table 1]

[0068] [Table 2]

Claims

1. In mass percent, C: 0.03-0.18%, Si: 0.03-0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.0200% or less, Al: 0.001-0.100%, O: 0.0100% or less and N: 0.0100% or less The balance is Fe and unavoidable impurities, In the 30 mm thick region including the center of the plate thickness, The average area of ​​porosity is 4.0 mm ２ is as follows: The standard deviation of the porosity area is 4.0 mm ２ is less than or equal to, and The porosity area ratio is 2.0% or less, The tensile strength is 430 MPa or more, A steel plate having a Charpy absorbed energy of 27 J or more at -50°C.

2. The composition further comprises, in mass%, Cu: 2.00% or less, Ni: 2.50% or less, Cr: 1.50% or less, Mo: 1.00% or less, Nb: 0.100% or less, Ti: 0.100% or less, V: 0.30% or less, B: 0.0100% or less, W: 0.50% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Sn: 0.0500% or less; REM: 0.0500% or less The steel sheet according to claim 1 , comprising one or more selected from the group consisting of:

3. A method for producing a steel sheet according to claim 1 or 2, comprising heating a slab to 1050°C or more and 1200°C or less, and then subjecting the slab to one or more passes of hot rolling; In the hot rolling, In the pass where the plate thickness center temperature is 950 ° C. or more and 1200 ° C. or less, the integrated value of X represented by the following formula (1) in each pass satisfies 1.0 to 10.0, A method for producing a steel sheet, wherein in a pass in which the sheet thickness center temperature is 800° C. or higher and lower than 950° C., an integrated value of X represented by the following formula (1) in each pass satisfies 3.0 or higher. [0010] However, in formula (1), t ０ : Plate thickness (mm) at the roll entry side of each hot rolling pass, t １ : Plate thickness (mm) at the roll exit side of each hot rolling pass, R: roll radius (mm), T ｍｉｄ : Absolute temperature (K) at the center of thickness on the roll entry side of each hot rolling pass, T ｓｕｒ : Absolute temperature (K) of the plate surface on the roll entry side of each hot rolling pass, A: A constant determined by the following formula (2). A=2851+2968C-1120C ２ ・・・Formula (2) In formula (2), C is the carbon content (mass%).

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