Layer-rolled steel sheet and its manufacturing method

Abstract

To provide: an overlapped rolled steel plate that can secure brittle crack propagation stop performance with a plate thickness of more than 70 mm or more, specifically, Kca at a service temperature of -10°C can stably secure 8000 N / mm1.5 or more, as well as, high strength overlapped rolled steel plate excellent in toughness; and a producing method thereof.SOLUTION: An overlapped rolled steel plate having a plate thickness of 70 mm or more comprises integrated two steel plates, which each have a specific composition, a specific surface integration degree, a Charpy fracture surface transition temperature (vTrs) at a center of a plate thickness being -50°C or less, the Charpy fracture surface transition temperature (vTrs) at a 1 / 4 position of the plate thickness of being -70°C or less, and the Charpy fracture surface transition temperature (vTrs) at a steel sheet surface being -80°C or less, and a position of a joining interface thereof is included within a positional range of 30 to 70% of a total plate thickness of the integrated two steel plates in a plate thickness direction.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a lap-rolled steel plate having a thickness of 70 mm or more, which is used for large structures such as ships, marine structures, cryogenic storage tanks, and architectural and civil engineering structures, and a method for manufacturing the same. [Background technology]

[0002] In large structures such as ships, marine structures, cryogenic storage tanks, and architectural and civil engineering structures, accidents caused by brittle fracture have a significant impact on society, the economy, and the environment. For this reason, improvements in safety are constantly being sought for these large structures, and the steel plates used as materials for these structures are required to have a high level of brittle crack arrestability at operating temperatures.

[0003] In ships such as container ships and bulk carriers, high-strength extra-thick steel plates are used for the outer hulls due to their structure. Recently, as ships have become larger, even higher strength is required, and the thickness of the extra-thick steel plates used as materials has been increasing.

[0004] Generally, the brittle crack arrestability of steel plates tends to deteriorate as their strength or thickness increases. For this reason, the requirements for brittle crack arrestability of extra-thick steel plates used in large structures are becoming increasingly stringent.

[0005] Increasing the Ni content in steel has been known as a means of improving the brittle crack arrestability of steel plates. For example, 9% Ni steel is used on a commercial scale in liquefied natural gas (LNG) storage tanks.

[0006] However, an increase in the Ni content in steel inevitably leads to a significant increase in manufacturing costs, making it difficult to apply 9% Ni steel to applications other than LNG storage tanks.

[0007] On the other hand, for relatively thin steel plates with a thickness of less than 50 mm, such as those used in ships and line pipes, which do not reach the extremely low temperatures of LNG, the TMCP (Thermo Mechanical Control Process) method can be used to refine the microstructure of the steel plate and improve low-temperature toughness, thereby achieving excellent brittle crack propagation arrest properties.

[0008] Furthermore, Patent Document 1 proposes a steel sheet having an ultra-fine structure in the surface layer in order to improve brittle crack propagation arrestability without increasing alloy costs.

[0009] The steel sheet with excellent brittle crack arrestability described in Patent Document 1 was developed by focusing on the fact that shear lips (plastic deformation regions) that occur in the surface layer of the steel sheet when a brittle crack propagates are effective in improving brittle crack arrestability, and is characterized by absorbing the propagation energy of the propagating brittle crack by refining the crystal grains in the shear lip portion. Patent Document 1 also describes a process in which the surface layer is cooled to below the Ar3 transformation point by controlled cooling after hot rolling, and then the controlled cooling is stopped and the surface layer is heated to above the Ar3 transformation point. This process is repeated one or more times, and during this process, a rolling reduction is applied to the steel sheet, causing repeated transformation or work recrystallization, thereby generating an ultrafine ferrite structure in the surface layer.

[0010] Patent Document 2 describes that in order to improve the brittle crack propagation arrestability of a steel plate having a microstructure mainly composed of ferrite-pearlite, it is important to reduce the variation in ferrite grain size by forming a ferrite structure on both surfaces of the steel plate having ferrite grains with a circular equivalent grain size of 5 μm or less and an aspect ratio of 2 or more in at least 5% of the surface layer region in the plate thickness direction, and that a method of reducing this variation is to suppress local recrystallization by setting the maximum reduction rate per pass during finish rolling to 12% or less.

[0011] Patent Document 3 describes a technology that is an extension of TMCP, which not only refines ferrite grains but also controls the subgrains formed within the ferrite grains to improve brittle crack propagation arrestability.

[0012] Another known method is to improve the brittle crack arrestability by applying a reduction to the transformed ferrite during controlled rolling to develop a texture. This method increases the resistance to brittle fracture by generating separation on the fracture surface of the steel sheet in a direction parallel to the sheet surface and alleviating the stress at the tip of the brittle crack.

[0013] For example, Patent Document 4 describes that brittle fracture resistance can be improved by controlling the (110) plane X-ray intensity ratio to 2 or more through controlled rolling and by controlling the area ratio of coarse grains with a circle-equivalent diameter of 20 μm or more to 10% or less.

[0014] Patent Document 5 discloses a steel plate for welded structures that has an X-ray plane intensity ratio of 1.5 or more for the (100) plane on the rolled surface within the plate thickness, as a steel plate with excellent brittle crack propagation arrestability at joints, and describes that the excellent brittle crack propagation arrestability is achieved due to the angular deviation between the stress load direction and the crack propagation direction caused by the texture development.

[0015] Furthermore, Patent Document 6 describes a method for manufacturing a high-strength thick steel plate with excellent brittle crack propagation arrestability, in which a texture is developed in each part in the plate thickness direction (e.g., 1 / 3 or more of the total plate thickness including the plate center) by specifying the average reduction rate in controlled rolling.

[0016] Furthermore, recent large container ships exceeding 6,000 TEU use extra-thick steel plates with a plate thickness of 70 mm or more. Non-Patent Document 1 evaluates the brittle crack arrest properties of a steel plate with a plate thickness of 65 mm, and reports that brittle cracks were not arrested in a large brittle crack arrest test of the base material. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] Special Publication No. 7-100814 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-256375 [Patent Document 3] Patent No. 3467767 [Patent Document 4] Patent No. 3548349 [Patent Document 5] Patent No. 2659661 [Patent Document 6] Patent No. 5733425 [Non-patent literature]

[0018] [Non-Patent Document 1] Long and Large Brittle Crack Propagation Behavior in Thick Shipbuilding Steel, Proceedings of the Japan Society of Naval Architects and Ocean Engineers, No. 3, 2006, pp. 359-362 Summary of the Invention [Problem to be solved by the invention]

[0019] In the techniques described in Patent Documents 1 and 2, a specific structure is obtained by cooling only the surface layer of the steel sheet, then reheating it, and processing it during reheating. This makes it difficult to control on an actual production scale, and the process places a heavy load on rolling and cooling equipment, especially in the production of thick steel sheets with a thickness of 70 mm or more.

[0020] Furthermore, based on the manufacturing conditions and disclosed examples, the steel plates described in Patent Documents 1 to 6 are all primarily intended for plate thicknesses of approximately 50 mm to 70 mm. It is unclear whether the specified properties can be obtained when applied to thicker materials of 70 mm or more, and the crack propagation properties in the plate thickness direction, which are necessary for large structures, have not been verified at all.

[0021] Furthermore, in Non-Patent Document 1, the ESSO test of the test material showed that the Kca at a use temperature of -10°C was 3000N / mm 1.5This suggests that the technology of Non-Patent Document 1 cannot ensure sufficient safety in the case of large structures that use steel plates with a thickness of more than 50 mm.

[0022] In view of the above circumstances, the present invention provides a steel sheet having a thickness of 70 mm or more, excellent brittle crack propagation arrestability, specifically, a steel sheet having a stable Kca of 8000 N / mm at a service temperature of -10°C. 1.5 The object of the present invention is to provide a lap-rolled steel sheet that can ensure the above and has high strength and excellent toughness, and a method for manufacturing the same. [Means for solving the problem]

[0023] In order to solve the above-mentioned problems, the inventors conducted extensive research into high-strength steel plates having excellent brittle crack arrestability even at thicknesses of 70 mm or more, and a manufacturing method for reliably obtaining such steel plates. As a result, they discovered that extra-thick steel plates having a texture in which the (211) plane integration degree on the rolled surface at the center of the thickness of the steel plate is 1.2 or more and the (200) plane integration degree on the rolled surface at the steel plate surface (sometimes simply referred to as the "surface") is 1.7 or more, and in which the Charpy fracture appearance transition temperature (vTrs) at the 1 / 4 thickness position, which are indicators of toughness, is -70°C or less and the Charpy fracture appearance transition temperature (vTrs) on the steel plate surface is -80°C or less, have extremely excellent brittle crack arrestability.

[0024] Furthermore, when attempting to obtain the above properties with steel plates 70 mm or thicker, if a conventional continuously cast slab is used as the raw material, the initial thickness of the slab must be large to ensure a sufficient rolling reduction, but there are cases where it is not possible to cast slabs with such large initial thicknesses due to equipment limitations on the continuous casting machine. Furthermore, slabs with large initial thicknesses generally have poor internal quality, and after rolling, internal defects such as lumps, in which unsolidified areas from the casting stage remain in the center of the plate thickness, can occur, which can adversely affect brittle crack propagation arrest properties.

[0025] In order to solve the above problems, in the present invention, two continuously cast slabs are reheated in advance, then rolled, and these slabs are then stacked and rolled. This makes it possible to achieve the following: (1) refinement of the steel sheet microstructure by reheating and rolling twice; (2) refinement of the steel sheet microstructure and development of texture by ensuring the rolling reduction rate by ensuring the initial thickness of the stacked slabs; and (3) ensuring internal quality by avoiding the location of center segregation and cracks in the center of the thickness of the stacked rolled steel sheet, and preventing the deterioration of brittle crack propagation arrestability that accompanies the deterioration of internal quality.

[0026] The present invention has been completed based on the above findings and further investigations. The gist and configuration of the present invention are as follows. [1] A steel sheet having two integrated steel plates and a joining interface between the two steel plates, Two steel plates each have the following mass%: C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.2%, P: 0.01% or less, S: 0.005% or less, Ti: 0.005 to 0.03%, Al: 0.005 to 0.080%, and N: 0.0050% or less and has a component composition in which Ceq defined by the following formula (1) is 0.36 or more and 0.43 or less, and the balance is Fe and unavoidable impurities, The (211) plane concentration on the rolled surface at the center of the thickness of the two integrated steel plates is 1.2 or more, The (200) plane concentration on the rolled surface of the steel sheet surface is 1.7 or more, the Charpy fracture appearance transition temperature vTrs at the center of the plate thickness of the two integrated steel plates is -50°C or less, the Charpy fracture appearance transition temperature vTrs at a 1 / 4 position of the plate thickness of the two integrated steel plates is -70°C or less, and the Charpy fracture appearance transition temperature vTrs at the surface of the steel plate is -80°C or less, A lap-rolled steel plate having a thickness of 70 mm or more, wherein the position of the joining interface is included in a position that is 30 to 70% of the total thickness of the two integrated steel plates in the plate thickness direction. Ceq=C+Mn / 6+Cu / 15+Ni / 15+Cr / 5+Mo / 5+V / 5...(1) Here, C, Mn, Cu, Ni, Cr, Mo, and V in formula (1) represent the content (mass%) of each element, and are set to 0 when not contained. [2] The component composition is further expressed in mass% as follows: Nb: 0.005 to 0.05%, Cu: 0.05 to 1.0% Ni: 0.05 to 1.5% Cr: 0.01 to 0.5%, Mo: 0.01 to 0.5%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, REM: 0.0100% or less The laminated rolled steel sheet according to [1], containing one or more of the following: [3] Two slabs having the composition described in [1] or [2] are heated to a temperature range of 950 to 1100 ° C., then hot-rolled, and cooled to room temperature; The surfaces of one side of the two slabs are polished, and the two slabs are stacked so that the polished surfaces of the two slabs are in contact with each other, and the joints of the two slabs are fillet welded to form a single stacked slab. A method for producing a stack-rolled steel plate with a thickness of 70 mm or more, comprising heating the stacked slabs to a temperature range of 1000 to 1150°C, and then hot rolling the stacked slabs under conditions where the cumulative reduction in the austenite recrystallization temperature range at the temperature at the center of the thickness of the stacked slabs is 10% or more, the cumulative reduction in the austenite non-recrystallization temperature range at the temperature at the center of the thickness of the stacked slabs is 60% or more, and the cumulative reduction in the temperature range where the temperature at the surface of the stacked slabs is equal to or lower than the Ar3 transformation point and the temperature at the center of the thickness of the stacked slabs is equal to or higher than the Ar3 transformation point is more than 20%, and then cooling the stacked slabs at an average cooling rate of 0.5°C / s or more to a cooling stop temperature of 500°C or less. [4] The method for manufacturing an overlap-rolled steel sheet according to [3], wherein after cooling to the cooling stop temperature of 500°C or less, tempering is performed at a temperature at which the temperature at the center of the sheet thickness is lower than the Ac1 transformation point. [Effects of the Invention]

[0027] According to the present invention, the texture in the plate thickness direction is appropriately controlled, so that even an extremely thick steel plate having a plate thickness of 70 mm or more has excellent brittle crack arrestability, excellent toughness, and high strength. Furthermore, according to the present invention, by optimizing the rolling conditions, it is possible to stably produce a lap-rolled steel plate using an industrially very simple process. For example, by applying the lap-rolled steel plate of the present invention to deck members joined to hatch side coamings in the strong deck structures of container ships and bulk carriers in the shipbuilding field, it contributes to improving the safety of ships and is extremely useful industrially. DETAILED DESCRIPTION OF THE INVENTION

[0028] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. <Steel plate composition> Each component will be explained below. Note that "%" representing the content of a component means "% by mass."

[0029] C: 0.03 to 0.20% C is an element that improves the strength of steel. In the present invention, the C content is set to 0.03% or more to ensure the desired strength. The C content is preferably 0.05% or more. On the other hand, if the C content exceeds 0.20%, not only does the weldability deteriorate but also the toughness is adversely affected. For this reason, the C content is set to a range of 0.20% or less. The C content is preferably 0.15% or less.

[0030] Si: 0.03 to 0.5% Si is effective as a deoxidizing element and as a steel strengthening element. If the Si content is less than 0.03%, these effects cannot be obtained. Therefore, the Si content is set to 0.03% or more. On the other hand, if the Si content exceeds 0.5%, not only will the surface properties of the steel be impaired, but toughness will also be significantly reduced. Therefore, the Si content is set to 0.5% or less. The Si content is preferably 0.4% or less.

[0031] Mn: 0.5 to 2.2% Mn is included as a strengthening element in steel. If the Mn content is less than 0.5%, its effect is insufficient. For this reason, the Mn content must be 0.5% or more. On the other hand, if the Mn content exceeds 2.2%, toughness and weldability deteriorate and the cost of the steel plate increases. For this reason, the Mn content must be 2.2% or less.

[0032] P: 0.01% or less, S: 0.005% or less P and S are unavoidable impurities in steel. If their content increases, toughness deteriorates. To maintain good toughness in steel plates with a thickness of 70 mm or more, the P content should be kept to 0.01% or less and the S content to 0.005% or less. It is more desirable for the P content to be 0.006% or less and the S content to be 0.003% or less.

[0033] Ti: 0.005 to 0.03% Ti, when contained in trace amounts, forms nitrides, carbides, or carbonitrides, which refines crystal grains and improves the toughness of the base material. This effect can be achieved by making the Ti content 0.005% or more. Therefore, the Ti content is set to 0.005% or more. On the other hand, if the Ti content exceeds 0.03%, the toughness of the base material and the weld heat-affected zone decreases. Therefore, the Ti content is set to 0.03% or less. The Ti content is preferably 0.02% or less.

[0034] Al: 0.005 to 0.080% Al acts as a deoxidizer. To use Al as a deoxidizer, the Al content must be 0.005% or more. The Al content is preferably 0.020% or more. On the other hand, if the Al content exceeds 0.080%, the toughness decreases and, when welded, the toughness of the weld metal decreases. For this reason, the Al content is set to 0.080% or less. The Al content is preferably 0.060% or less.

[0035] N: 0.0050% or less N combines with Al in the steel, adjusts the grain size during rolling, and strengthens the steel. To achieve this effect, the N content is preferably 0.0010% or more. On the other hand, if the N content exceeds 0.0050%, toughness deteriorates. In the present invention, the N content is 0.0050% or less. The N content is preferably 0.0040% or less.

[0036] The above is the basic composition of the present invention, with the balance being Fe and unavoidable impurities.

[0037] In the present invention, in order to further improve the properties, in addition to the above-mentioned component composition, one or more of Nb, Cu, Ni, Cr, Mo, V, B, Ca, and REM may be contained.

[0038] Nb: 0.005 to 0.05% Nb precipitates as NbC during ferrite transformation or reheating, contributing to increasing the strength of steel. Furthermore, Nb has the effect of expanding the unrecrystallized region during rolling in the austenite region, contributing to the refinement of ferrite grains. Therefore, the inclusion of Nb is also effective in improving toughness. This effect is achieved by setting the Nb content to 0.005% or more. Therefore, when Nb is contained, the Nb content is set to 0.005% or more. Preferably, the Nb content is 0.01% or more. If the Nb content exceeds 0.05%, coarse NbC precipitates, which may result in a decrease in toughness. Therefore, when Nb is contained, the Nb content is set to 0.05% or less. The Nb content is preferably 0.04% or less.

[0039] Cu: 0.05 to 1.0% Copper (Cu) is an element that improves the hardenability of steel. Cu not only directly contributes to improving strength after rolling, but can also be added to improve functions such as toughness, high-temperature strength, and weather resistance. These effects are achieved by keeping the Cu content at 0.05% or more. Therefore, if Cu is included, the Cu content must be 0.05% or more. However, excessive Cu content degrades toughness and weldability. To maintain sufficient strength in steel plates with a thickness of 70 mm or more without degrading toughness or weldability, the Cu content must be 1.0% or less.

[0040] Ni: 0.05 to 1.5% Ni is an element that improves the hardenability of steel. Ni not only directly contributes to improving strength after rolling, but can also be added to improve functions such as toughness, high-temperature strength, and weather resistance. These effects are achieved by making the Ni content 0.05% or more. Therefore, if Ni is contained, the Ni content should be 0.05% or more. On the other hand, excessive Ni content deteriorates toughness and weldability. In steel plates with a thickness of 70 mm or more, the Ni content, if contained, should be 1.5% or less to maintain sufficient strength without deteriorating toughness or weldability.

[0041] Cr: 0.01 to 0.5% Cr is an element that improves the hardenability of steel. Cr not only directly contributes to improving strength after rolling, but can also be added to improve functions such as toughness, high-temperature strength, and weather resistance. These effects are achieved by keeping the Cr content at 0.01% or more. Therefore, if Cr is included, the Cr content must be 0.01% or more. However, excessive Cr content can deteriorate toughness and weldability. To maintain sufficient strength even in plate thicknesses of 70 mm or more without deteriorating toughness and weldability, the Cr content must be 0.5% or less.

[0042] Mo: 0.01 to 0.5% Mo is an element that improves the hardenability of steel. Mo not only directly contributes to improving strength after rolling, but can also be added to improve functions such as toughness, high-temperature strength, and weather resistance. These effects are achieved by setting the Mo content to 0.01% or more. Therefore, if Mo is included, the Mo content must be 0.01% or more. However, excessive content degrades toughness and weldability. To maintain sufficient strength even in plate thicknesses of 70 mm or more without degrading toughness and weldability, the Mo content must be 0.5% or less.

[0043] V: 0.001 to 0.10% V is an element that improves the strength of steel by precipitation strengthening, where it precipitates as V(CN). This effect is achieved by setting the V content to 0.001% or more. Therefore, when V is contained, the V content is set to 0.001% or more. However, if the V content exceeds 0.10%, toughness may decrease. Therefore, when V is contained, the V content is set to 0.10% or less.

[0044] B: 0.0030% or less B is an element that improves the hardenability of steel, and the above effect can be achieved even with a small B content of 0.0030% or less. Furthermore, if the B content exceeds 0.0030%, the toughness of the welded joint decreases. Therefore, if B is to be contained, the B content should be 0.0030% or less. From the viewpoint of obtaining the above effect, the lower limit of the B content is preferably 0.0006%.

[0045] Ca: 0.0050% or less, REM: 0.0100% or less Ca and REM refine the structure of the weld heat-affected zone and improve toughness. The effects of the present invention are not impaired even if these elements are included, so they may be included as needed. However, excessive content can lead to the formation of coarse inclusions, which can degrade the toughness of the base material. Therefore, when these elements are included, the upper limit of their content is set to 0.0050% for Ca and 0.0100% for REM.

[0046] Ceq: 0.36 or more and 0.43 or less In the lap-rolled steel sheet of the present invention, in addition to each component being within the above content range, Ceq, as expressed by the following formula (1), is adjusted to 0.36 or more and 0.43 or less. If Ceq is less than 0.36, it becomes difficult to increase the (211) plane concentration on the rolled surface at the center of the sheet thickness. For this reason, Ceq is set to 0.36 or more. Furthermore, to ensure toughness and weldability, Ceq is set to 0.43 or less. Ceq=C+Mn / 6+Cu / 15+Ni / 15+Cr / 5+Mo / 5+V / 5...(1) In formula (1), C, Mn, Cu, Ni, Cr, Mo, and V represent the content (mass%) of each element, and are set to 0 if the element is not contained.

[0047] <Collective organization> The lap-rolled steel sheet of the present invention has a texture in which the (211) plane integration degree on the rolled surface at the center of the thickness of two integrated steel sheets (lap-rolled steel sheets) is 1.2 or more, and the (200) plane integration degree on the rolled surface at the surface (within the range from the extreme surface to 1 mm below the surface) is 1.7 or more. The higher the (211) plane integration degree on the rolled surface at the center of the thickness, the easier it is to arrest a propagating brittle crack by finely branching it. To achieve this effect, the (211) plane integration degree on the rolled surface at the center of the thickness must be 1.2 or more, so the (211) plane integration degree on the rolled surface at the center of the thickness must be 1.2 or more. Furthermore, the higher the (200) plane integration degree on the rolled surface at the center of the thickness, the easier it is to arrest a propagating brittle crack by changing its propagation direction as it progresses. To obtain this effect, the (200) plane concentration degree must be 1.7 or more, so the (200) plane concentration degree on the rolled surface at the surface (within the range from the extreme surface to 1 mm below the surface) must be 1.7 or more. As will be described later, lap-rolled steel sheets are made by lap-rolling two slabs into one lap-rolled steel sheet.

[0048] <Position of the joining interface: A position 30 to 70% of the total thickness of the two steel plates that have been integrated in the plate thickness direction> In the present invention, two slabs are stacked to form a single stacked slab, which is then rolled to form a stack-rolled steel sheet. The two integrated steel sheets refer to the stack-rolled steel sheet described above, which has a joining interface between the two steel sheets. The total thickness refers to the thickness of the stack-rolled steel sheet obtained by rolling a single stacked slab. In ordinary steel sheets, center segregation or pitting of Mn, P, etc. may occur in the center of the thickness, which can deteriorate the internal quality. However, in the stack-rolled steel sheet of the present invention, the center segregation or pitting can be avoided in the center of the thickness, ensuring internal quality and, as a result, preventing deterioration of brittle crack propagation arrestability. If the bonding interface of overlap-rolled steel plates, which is created by stacking two slabs, is less than 30% or more than 70% of the total plate thickness in the thickness direction of the overlap-rolled steel plate, central segregation or cracking of one slab will be located near the center of the plate thickness of the overlap-rolled steel plate, and the toughness at the center of the plate thickness will deteriorate, thereby deteriorating the brittle crack propagation arrest properties, so the position should be 30 to 70% of the plate thickness.The position of the bonding interface is the distance from the upper surface of the overlap-rolled steel plate to the bonding interface divided by the total plate thickness of the overlap-rolled steel plate.

[0049] In the lap-rolled steel sheet of the present invention, the Charpy fracture transition temperature (vTrs) at the center of the thickness of the two integrated steel sheets (lap-rolled steel sheet) is −50°C or lower, the Charpy fracture transition temperature (vTrs) at a quarter-thickness position of the two integrated steel sheets is −70°C or lower, and the Charpy fracture transition temperature (vTrs) at the surface of the steel sheet is −80°C or lower. Specifically, the Charpy fracture transition temperature (vTrs) at the center of the thickness of −50°C or lower can be achieved by suppressing the segregation of elements such as Mn and P at the center of the thickness compared to conventional materials (steel sheets manufactured from a single slab) and by controlling the manufacturing conditions. The Charpy fracture transition temperature (vTrs) at the quarter-thickness position of the two steel sheets and the Charpy fracture transition temperature (vTrs) at the surface of the steel sheet of −70°C or lower can be achieved by controlling the manufacturing conditions described below.

[0050] As described above, in the present invention, as long as the texture satisfies the above ranges by controlling the chemical composition and the manufacturing conditions described below, the lap-rolled steel sheet of the present invention has the effect of being excellent in strength, toughness, and brittle crack propagation arrestability even if the sheet thickness is 70 mm or more.

[0051] <Manufacturing method> Molten steel with the above-mentioned chemical composition is melted in a converter or the like, formed into two slabs by continuous casting or the like, heated to a temperature of 950 to 1100°C (this temperature is the material heating temperature), hot rolled to the target thickness, cooled to room temperature, polished on one side of the two slabs, stacked so that the polished surfaces are in contact, and fillet welded at the joint to form a single stacked slab. Note that the above room temperature refers to the ambient temperature (-10°C to 50°C) in a factory, etc.

[0052] If the heating temperature of the raw material (two slabs) is less than 950°C, abnormal grain growth of austenite occurs during reheating after manufacturing one stacked slab, resulting in mixed grains and a deterioration in toughness. Therefore, the heating temperature of the raw material (two slabs) is set to 950°C or higher. On the other hand, if the heating temperature of the raw material (two slabs) exceeds 1100°C, the austenite grains become coarse, and the austenite also becomes coarse during reheating after manufacturing one stacked slab, resulting in a decrease in toughness. Therefore, the heating temperature of the raw material (two slabs) is set to 1100°C or lower. From the viewpoint of improving the toughness of the steel plate, the preferable heating temperature of the raw material (two slabs) is 970°C or higher. Furthermore, the preferable heating temperature of the raw material (two slabs) is 1050°C or lower. Note that the temperature of the slab means the temperature at the center of the thickness of the steel plate.

[0053] After heating, the sheet is hot rolled to a target thickness and then cooled to room temperature, but the conditions for this are not particularly specified. For example, accelerated cooling may be performed after hot rolling.

[0054] One side of each of the two slabs is polished, and the polished surfaces are overlapped and fillet welded at the joint to form a single stacked slab. There are no specific fillet welding methods specified, but to prevent oxidation of the polished surfaces, it is preferable to use electron beam or laser welding in a vacuum. If stacked slabs are produced without polishing, poor bonding due to oxides will occur at the joint interface after rolling, leading to a decrease in internal quality and toughness, so polishing is essential.

[0055] If the heating temperature of one stacked slab (this temperature is referred to as the product heating temperature) is less than 1000°C, mixed grains will form during austenite recrystallization, resulting in a deterioration in toughness. For this reason, the heating temperature of stacked slabs is set to 1000°C or higher. On the other hand, if the product heating temperature exceeds 1150°C, the austenite grains will coarsen, resulting in a decrease in toughness. Therefore, the product heating temperature of stacked slabs is set to 1150°C or lower. From the perspective of improving the toughness of steel plate, the preferred product heating temperature range is 1100°C or lower. Note that the temperature of stacked slabs means the temperature at the center of the plate thickness of the stacked slabs.

[0056] In hot rolling, the stacked slabs are first rolled with a cumulative reduction of 10% or more when the temperature at the center of the thickness of the stacked slabs is in the austenite recrystallization temperature range. By achieving a cumulative reduction of 10% or more in this temperature range, the Charpy fracture transition temperature (vTrs) at the center of the thickness can be reduced to −50°C or less and the Charpy fracture transition temperature (vTrs) at the quarter of the thickness of the stacked slabs can be reduced to −70°C or less. If the cumulative reduction is less than 10%, the austenite grain refinement is insufficient, resulting in poor toughness and failure to achieve these Charpy fracture transition temperatures. While there are no particular limitations on the upper limit of the cumulative reduction, a cumulative reduction of 45% or less is preferable because the effect of improving grain refinement is diminished when the cumulative reduction exceeds 45%. With the composition of the present invention, the cumulative reduction is set to 10% or more when the austenite recrystallization temperature range is 1100 to 950°C and the temperature at the center of the thickness is 1100 to 950°C.

[0057] Furthermore, rolling is performed with a cumulative reduction of 60% or more when the temperature at the center of the thickness of the stacked slab is in the austenite non-recrystallization temperature range. By setting the cumulative reduction at 60% or more in this temperature range, a texture is obtained in which the (211) plane density on the rolled surface at the center of the thickness of the stacked rolled steel sheet is 1.2 or more, and the Charpy fracture transition temperature (vTrs) at the center of the thickness of the stacked rolled steel sheet is −50°C or less, and further, the Charpy fracture transition temperature (vTrs) at the quarter-thickness position of the stacked rolled steel sheet is −70°C or less. Furthermore, if the cumulative reduction in this temperature range is less than 60%, a texture in which the (200) plane density on the rolled surface at the center of the thickness of the stacked rolled steel sheet is 1.7 or more cannot be obtained, and therefore the desired Charpy fracture transition temperature (vTrs) at each thickness position cannot be achieved. While there are no particular limitations on the upper limit of the cumulative reduction, it is preferably set to 75% or less so as not to impair rolling efficiency. In the composition of the present invention, the austenite non-recrystallization temperature range is 950 to 700°C, and the cumulative reduction when the temperature at the center of the thickness of the stacked slab is 950 to 700°C is 60% or more.

[0058] Furthermore, in the present invention, the cumulative reduction ratio during hot rolling is set to more than 20% when the surface temperature of the stacked slabs is below the Ar3 transformation point and the temperature at the center of the sheet thickness is above the Ar3 transformation point. This is an important requirement in the present invention, and by performing hot rolling under these conditions, the (200) plane integration degree at the rolled surface of the stacked rolled steel sheet can be developed to 1.7 or more, and the Charpy fracture transition temperature (vTrs) at the surface of the stacked rolled steel sheet can be set to -80°C or less. If the cumulative reduction ratio is 20% or less when the thickness surface of the stacked slabs is below the Ar3 transformation point, the desired texture and vTrs cannot be obtained. Here, the Ar3 transformation point is expressed by the following formula (2). Ar3(℃)=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo...(2) The element symbols in the above formula (2) represent the content (mass%) of each element, and those that are not included are represented as 0. The temperature range below the Ar3 transformation point suitable for rolling is the temperature range where the temperature of the surface of the stacked slabs is between the Ar3 transformation point and (Ar3 transformation point - 80)°C. Also, the temperature range above the Ar3 transformation point suitable for rolling is the temperature range where the temperature of the center of the thickness of the stacked slabs is between (Ar3 transformation point + 80)°C and the Ar3 transformation point.

[0059] Furthermore, in the hot rolling of the present invention, rolling outside the temperature range specified above is not limited, and it is sufficient that rolling is performed at least in the temperature range specified above with the specified cumulative reduction rate.

[0060] After rolling, the lap-rolled steel sheet is cooled to a cooling stop temperature of 500°C or less at an average cooling rate of 0.5°C / s or more. If the average cooling rate is less than 0.5°C / s, it is not possible to ensure a (211) plane density of 1.2 or more on the rolled surface at the center of the thickness of the lap-rolled steel sheet. Furthermore, if the cooling stop temperature exceeds 500°C, it is not possible to obtain the desired steel sheet strength and texture. The average cooling rate refers to the average cooling rate between 700°C and 550°C.

[0061] Furthermore, after cooling to a cooling stop temperature of 500°C or less, tempering can be performed for the purpose of reducing residual stress in the lap-rolled steel sheet. Tempering must be performed when the temperature at the center of the thickness of the lap-rolled steel sheet is below the Ac1 transformation point. This is because if the temperature at the center of the thickness of the lap-rolled steel sheet during tempering is above the Ac1 transformation point, the texture developed during rolling will be lost. Here, the Ac1 transformation point is expressed by the following formula (3). Ac1(℃)=751-26.6C+17.6Si-11.6Mn-169Al-23Cu-23Ni+24.1Cr+22.5Mo+233Nb-39.7V-5.7Ti-895B...(3) The element symbols in formula (3) represent the content (mass%) of each element, and those that are not included are represented as 0.

[0062] In the above description, the temperature at the center of thickness is determined by heat transfer calculation from the steel sheet surface temperature measured with a radiation thermometer. The temperature condition in the cooling conditions after rolling is the temperature at the center of thickness of the lap-rolled steel sheet, and the cooling rate also means an average cooling rate of 700°C to 550°C calculated based on the temperature at the center of thickness of the lap-rolled steel sheet.

[0063] In the lap-rolled steel plate of the present invention, the thicknesses of the two slabs used as raw materials are adjusted and controlled so that the position of the joining interface is 30 to 70% of the total thickness of the two integrated steel plates in the plate thickness direction. [Example]

[0064] Next, an embodiment of the present invention will be described.

[0065] Molten steels with the chemical compositions shown in Table 1 were melted in a converter and continuously cast into slabs. These slabs were reheated, hot-rolled, and then cooled to room temperature. Two slabs with the same chemical composition were polished on one side, and the polished surfaces were overlapped and fillet-welded to form a single stacked slab. After reheating, the slabs were hot-rolled to a thickness of 70–120 mm and further cooled to obtain the test steels shown in Table 2. Table 2 shows the heating, hot-rolling, and cooling conditions. For samples that were tempered after cooling, the tempering temperature is also shown. The location of the joint interface was varied between 15–80% of the total thickness. Product No. 33 was produced from a single slab. The slab was reheated, hot-rolled, cooled to room temperature, reheated, hot-rolled, and further cooled.

[0066] [Table 1]

[0067] [Table 2]

[0068] From the obtained lap-rolled steel sheets (hereinafter, only Product No. 33 is referred to as a rolled steel sheet), 14 mm diameter JIS No. 14A test pieces were taken from the 1 / 4 position of the sheet thickness so that the longitudinal axis of the test piece was parallel to the rolling direction, and a tensile test was performed to measure the yield strength (YS) and tensile strength (TS). Steel sheets with a YS of 390 MPa or more and a TS of 510 MPa or more were evaluated as having high strength.

[0069] JIS No. 4 impact test specimens were taken from the center and quarter positions of the thickness of the lap-rolled steel plate and from the steel plate surface, with the longitudinal axis of the test specimen parallel to the rolling direction, and Charpy impact tests were performed to determine the Charpy fracture transition temperature (vTrs). Lap-rolled steel plates with vTrs of -50°C or less at the center position, -70°C or less at the quarter position, and -80°C or less at the surface were evaluated as having good toughness.

[0070] In addition, to evaluate the texture of the lap-rolled steel sheets, the (211) plane integration degree on the rolled surface at the center of the sheet thickness of the lap-rolled steel sheets and the (200) plane integration degree on the rolled surface at the surface of the lap-rolled steel sheets (the steel sheet surface refers to the range from the extreme surface to 1 mm below the surface) were measured.

[0071] The planar integration degree was measured using an X-ray diffractometer (manufactured by Rigaku Corporation) at 40 kV using a Mo radiation source, and the texture orientation integration degree was measured from the inverted pole figure intensity, and was calculated by calculating the ratio of the X-ray diffraction intensity to that of annealed pure iron.

[0072] Next, to evaluate the brittle crack propagation arrestability, a temperature gradient ESSO test was conducted in accordance with WES2815, and the Kca (hereinafter referred to as Kca (-10°C) N / mm 1.5 (also written as Kca(-10℃)N / mm 1.5 is 8000N / mm 1.5 The above was considered to be good.

[0073] Table 3 shows the results of these tests.

[0074] [Table 3]

[0075] As shown in Table 3, the stack-rolled steel sheets of the present invention have a texture in which the (211) plane integration degree on the rolled surface at the center of thickness of the stack-rolled steel sheet is 1.2 or more and the (200) plane integration degree on the rolled surface at the surface of the stack-rolled steel sheet is 1.7 or more, the Charpy fracture appearance transition temperature (vTrs) at the center of thickness of the stack-rolled steel sheet is -50°C or less, the Charpy fracture appearance transition temperature (vTrs) at the 1 / 4 position of the thickness of the stack-rolled steel sheet is -70°C or less, and the Charpy fracture appearance transition temperature (vTrs) at the surface of the stack-rolled steel sheet is -80°C or less, and the Charpy fracture appearance transition temperature (vTrs) at the surface of the stack-rolled steel sheet is excellent in toughness, and the Kca (-10°C) is 8000N / mm 1.5 As a result, excellent brittle crack propagation arrest properties were obtained.

[0076] On the other hand, the comparative examples outside the scope of the present invention failed to satisfy any one or more of YS, TS, vTrs, and Kca (-10°C).

Claims

1. The steel sheet has two integrated steel plates and a joining interface between the two steel plates, The two steel plates are, by mass%, C: 0.03-0.20%, Si: 0.03 to 0.5%, Mn: 0.5-2.2%, P: 0.01% or less, S: 0.005% or less, Ti: 0.005-0.03%, Al: 0.005-0.080%, and N: 0.0050% or less The composition contains: a Ceq defined by the following formula (1) of 0.36 or more and 0.43 or less, with the balance being Fe and unavoidable impurities; The (211) plane concentration on the rolled surface at the center of the thickness of the two integrated steel plates is 1.2 or more, The (200) plane concentration on the rolled surface of the steel sheet surface is 1.7 or more, the Charpy fracture appearance transition temperature vTrs at the center of the plate thickness of the two integrated steel plates is −50°C or less, the Charpy fracture appearance transition temperature vTrs at a 1 / 4 position of the plate thickness of the two integrated steel plates is −70°C or less, and the Charpy fracture appearance transition temperature vTrs on the steel plate surface is −80°C or less, A lap-rolled steel plate having a plate thickness of 70 mm or more, wherein the position of the joining interface is included in a position of 30 to 70% of the total plate thickness of the two integrated steel plates in the plate thickness direction. Ceq=C+Mn / 6+Cu / 15+Ni / 15+Cr / 5+Mo / 5+V / 5...(1) Here, C, Mn, Cu, Ni, Cr, Mo, and V in formula (1) represent the content (mass%) of each element, and are set to 0 when no element is contained.

2. The component composition is further expressed in mass % as follows: Nb: 0.005-0.05%, Cu: 0.05-1.0%, Ni: 0.05-1.5%, Cr: 0.01-0.5%, Mo: 0.01-0.5%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, REM: 0.0100% or less The lap-rolled steel sheet according to claim 1, further comprising one or more of the following:

3. A method for producing a lap-rolled steel sheet according to claim 1 or 2, comprising: heating two slabs having the above-mentioned composition to a temperature range of 950 to 1100°C, hot-rolling them, and cooling them to room temperature; The surfaces of one side of the two slabs are polished, and the two slabs are stacked so that the polished surfaces of the two slabs are in contact with each other, and the joints of the two slabs are fillet welded to form a single stacked slab; A method for manufacturing a stack-rolled steel plate with a thickness of 70 mm or more, comprising heating the stacked slabs to a temperature range of 1000 to 1150°C, and then hot rolling the stacked slabs under the conditions that the cumulative reduction in the austenite recrystallization temperature range at the temperature at the center of the thickness of the stacked slabs is 10% or more, the cumulative reduction in the austenite non-recrystallization temperature range at the temperature at the center of the thickness of the stacked slabs is 60% or more, and the cumulative reduction in the temperature range where the temperature of the surface of the stacked slabs is equal to or lower than the Ar3 transformation point and the temperature at the center of the thickness of the stacked slabs is equal to or higher than the Ar3 transformation point is more than 20%, and then cooling the stacked slabs at an average cooling rate of 0.5°C / s or more to a cooling stop temperature of 500°C or less.

4. The method for producing lap-rolled steel sheet according to claim 3, wherein after cooling to the cooling stop temperature of 500°C or less, tempering is performed at a temperature at which the temperature at the center of the sheet thickness is lower than the Ac1 transformation point.

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