Low yield ratio extra thick steel plate and its manufacturing method

Abstract

Provided is a low-yield ratio type extra-thick steel plate having excellent toughness and a thickness of greater than 100 mm without the need for special facilities. The low-yield ratio type extra-thick steel plate having a predetermined component composition and a thickness of over 100 mm is characterized by having a component composition in which a Ceq is defined by equation (1) and a plate thickness t [mm] satisfies 0.22t0.1≤Ceq≤0.50, (1): Ceq= [C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15 (in equation (1), a symbol of each element represents the content (mass%) of said element, which is 0 if said element is not added), wherein the average grain size of parent-phase ferrite is at most 25 μm, the area ratio of island martensite is 3-20%, the tensile strength is at least 520 MPa, the yield ratio is at most 80%, and the Charpy absorbed energy at 0 °C is at least 47 J.

Description

[Technical Field]

[0001] The present invention relates to a low yield ratio extra heavy steel plate and a method for producing the same. [Background technology]

[0002] In recent years, structures have become larger in fields such as ships, buildings, bridges, marine structures, wind turbines, construction and industrial machinery, and pressure vessels. As a result, the thickness of the steel plates used has also increased, resulting in increased demand for extra-thick steel plates with a thickness of over 100 mm.

[0003] Additionally, to improve the earthquake resistance of structures, there is an increasing demand for steel materials with a low yield ratio (= yield strength / tensile strength x 100(%)), which is the ratio of yield strength to tensile strength obtained in a tensile test.

[0004] Furthermore, to improve the reliability of structures, good Charpy absorbed energy is required. In Japan, the Charpy absorbed energy required for structural steel is generally 0°C, but if the structure is to be constructed in a cold region, excellent low-temperature toughness of -40°C may be required.

[0005] Prior patents relating to low yield ratio steel sheets include, for example, Patent Documents 1 to 4. Patent Document 1 discloses a method for obtaining a low yield ratio steel sheet with excellent toughness by controlling the structure to a mixed structure consisting of soft and hard phases, i.e., fine ferrite, island martensite, and bainite.

[0006] Patent Document 2 discloses a method for obtaining a low yield ratio steel sheet with excellent toughness by forming a structure consisting of acicular ferrite and island martensite.

[0007] Patent Document 3 discloses a method for obtaining a low yield ratio steel sheet with excellent toughness by forming a multi-phase structure consisting of fine ferrite and a hard phase.

[0008] Patent Document 4 discloses a method for obtaining a low yield ratio steel sheet with excellent toughness by forming a multi-phase structure consisting of fine ferrite and a hard phase. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-266735 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-127065 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-90406 [Patent Document 4] Japanese Patent Application Laid-Open No. 2010-280932 Summary of the Invention [Problem to be solved by the invention]

[0010] The technology of Patent Document 1 is an invention intended for thin steel plate thicknesses of 12 mm or 30 mm, and requires a cooling rate of 10°C / sec or more after hot rolling. This high cooling rate is difficult to achieve with extra-thick steel plates with thicknesses exceeding 100 mm, and therefore cannot be applied to extra-thick steel plates.

[0011] The technology of Patent Document 2 is an invention intended for the thin range of plate thickness of 15 to 40 mm or less, and requires a cooling rate of 5°C / sec or more after hot rolling, which is a high cooling rate that is difficult to achieve in extra-thick steel plates with a plate thickness of over 100 mm, so it cannot be applied to extra-thick steel plates. In addition, the addition of Mo, an expensive alloying element, is essential, which causes a problem of significantly increasing production costs when manufacturing extra-thick steel plates.

[0012] The technology of Patent Document 3 is an invention that targets the thin range of plate thickness of 10 to 40 mm or less, and requires the addition of expensive alloy elements Cu, Ni, Cr, and Mo, which poses a problem of significantly increasing manufacturing costs when manufacturing extra-thick steel plates.

[0013] The technology of Patent Document 4 is an invention that targets a plate thickness range of up to 100 mm, and requires a cooling rate of 5 to 40°C / sec after hot rolling. However, with regard to the characteristics at a position 1 / 4 depth from the surface of the steel plate in the plate thickness direction (hereinafter sometimes referred to as "plate thickness 1 / 4 position") that is the target of the present invention, there is a problem that it is inapplicable because it is impossible to obtain a cooling rate of 5°C / sec or more in a plate thickness range exceeding 100 mm using a normal water cooling method.

[0014] The present invention has been developed in view of the above-mentioned current situation, and aims to provide a low-yield-ratio extra heavy steel plate having a thickness of over 100 mm and excellent toughness without requiring special equipment. Another aim of the present invention is to provide a method for manufacturing the low-yield-ratio extra heavy steel plate. [Means for solving the problem]

[0015] The present inventors have conducted extensive research to solve the above problems and have come to the following findings. There are two types of stress-strain curve shapes obtained in tensile tests: the yield point type, where an upper yield point appears, and the roundhouse type, where no yield point appears. By suppressing the appearance of the upper yield point and creating a roundhouse type, it is possible to significantly reduce the yield strength and yield ratio. In order to suppress the occurrence of the upper yield point, it is necessary to increase the mobile dislocation density in the steel structure. To do this, it is necessary to increase the proportion of shear-type transformation structures, such as bainite and martensite structures, which introduce dislocations into the surrounding structure during structure formation. To generate bainite or martensite structures, large amounts of alloying elements must be added to improve hardenability, which significantly increases the manufacturing cost of extra-thick steel plates. Furthermore, a high cooling rate is required, making it unsuitable for the production of extra-thick steel plates. Therefore, the inventors came up with the idea of ​​utilizing island martensite. Island martensite is formed when alloying elements such as C are concentrated in the untransformed austenite during the transformation from austenite to a low-temperature transformation structure. Therefore, island martensite can be formed even at a relatively slow cooling rate with the addition of trace amounts of alloying elements, making it the most suitable structure for low-yield-ratio extra-thick steel plates. Based on the above findings, the inventors have further investigated and found that in order to achieve a high tensile strength of 520 MPa or more, a low yield ratio of 80% or less, and good toughness of 47 J or more at 0°C, it is necessary to use 0.22t 0.1 It was found that the composition must be such that Ceq≦Ceq≦0.50, the average grain size of the ferrite matrix must be 25 μm or less, and the area fraction of island martensite must be 3 to 20%. At the same time, to achieve a high tensile strength of 520 MPa or more, a low yield ratio of 80% or less, and good toughness of 47 J or more at -40°C, the 0.22t 0.1 It was also found that the composition must be such that Ceq≦0.48, the average grain size of the ferrite matrix must be 15 μm or less, the difference between the maximum grain size and the average grain size must be 15 μm or less, and the area fraction of island martensite must be 3 to 15%.

[0016] In addition, the inventors have discovered that in the process of heating a slab, hot rolling, and subsequent water cooling, in order to obtain a structure that combines a high tensile strength of 520 MPa or more, a low yield ratio of 80% or less, and good toughness of 47 J or more at 0°C, the slab is heated to 1000 to 1150°C, reduced by 30% or more in the range of 800 to 930°C, and further cooled at a rate of 2100 t / min between 700 and 400°C after hot rolling. -1.5 It was found that it is effective to cool the material to a temperature of 350°C or less. In addition, in order to obtain a structure that combines high strength (tensile strength of 520 MPa or more), low yield ratio (yield ratio of 80% or less), and good toughness (47 J or more at -40°C), the material is heated at 1000 to 1100°C, and then heated in the range of 800 to 930°C. d / h m The total reduction rate is 40% or more with a rolling rate of 0.3 / pass or more, and after hot rolling, the cooling rate between 700 and 400°C is 2100t. -1.5 It was found that it is effective to cool the material to a temperature of 350°C or less. The present invention was completed based on the above findings and further investigations.

[0017] That is, the gist and configuration of the present invention are as follows.

[0018] [1] In mass %, C: 0.05 to 0.16%, Si: 0.03 to 0.70% Mn: 0.50~2.50%, P: 0.030% or less, S: 0.0200% or less, Nb: 0.003 to 0.100%, Ti: 0.005 to 0.100%, Al: 0.001 to 0.100%, O: 0.0100% or less, and N: 0.0100% or less, The balance is Fe and unavoidable impurities, Ceq defined by the following formula (1) and plate thickness t [mm] are 0.22t 0.1 ≦Ceq≦0.50, Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15...(1) (Note that each element symbol in the above formula (1) represents the content (mass%) of the element, and if the element is not added, it is set to 0.) A low yield ratio type extra thick steel plate having a thickness of over 100 mm, characterized in that the average grain size of the parent ferrite is 25 μm or less, the area ratio of island martensite is 3 to 20%, the tensile strength is 520 MPa or more, the yield ratio is 80% or less, and the Charpy absorbed energy at 0°C is 47 J or more.

[0019] [2] In mass%, C: 0.05 to 0.12%, Si: 0.03 to 0.70% Mn: 0.50~2.50%, P: 0.030% or less, S: 0.0200% or less, Nb: 0.003 to 0.100%, Ti: 0.005 to 0.100%, Al: 0.001 to 0.100%, O: 0.0100% or less, and N: 0.0100% or less, The balance is Fe and unavoidable impurities, Ceq defined by the following formula (1) and plate thickness t [mm] are 0.22t 0.1 ≦Ceq≦0.48, Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15...(1) (Note that each element symbol in the above formula (1) represents the content (mass%) of the element, and if the element is not added, it is set to 0.) A low yield ratio type extra-thick steel plate having a thickness of over 100 mm, characterized in that the average grain size of the parent ferrite is 15 μm or less and the difference between the maximum grain size and the average grain size is 15 μm or less, the area ratio of island martensite is 3 to 15%, the tensile strength is 520 MPa or more, the yield ratio is 80% or less, and the Charpy absorbed energy at -40°C is 47 J or more.

[0020] [3] The component composition further comprises, in mass%, Cu: 1.00% or less, Ni: 5.00% or less, Cr: 1.00% or less, Mo: 1.00% or less V: 0.20% or less, B: 0.0050% or less, Ca: 0.0100% or less, Mg: 0.0200% or less, and REM: 0.0500% or less 3. The low yield ratio extra thick steel plate according to claim 1, comprising one or more selected from the group consisting of:

[0021] [4] A method for producing a low yield ratio extra thick steel plate according to the above [1] or the above [3] citing the above [1], comprising: a preparation step of preparing a slab having the above composition; a heating step of heating the slab; a hot rolling step of hot rolling the heated slab to form a hot rolled steel plate; and a cooling step of water-cooling the rolled steel plate, In the heating step, the slab is heated at 950 to 1150°C, In the rolling step, a reduction of 30% or more is performed in the range of 800 to 930°C, In the cooling step, the cooling rate of the rolled steel plate is set to 2100 t between 700 and 400 ° C. -1.5 ℃ / sec or more, and the cooling stop temperature is 350℃ or less.

[0022] [5] A method for producing a low yield ratio extra thick steel plate according to the above [2] or the above [3] citing the above [2], comprising: a preparation step of preparing a slab having the above component composition; a heating step of heating the slab; a hot rolling step of hot rolling the heated slab to form a hot rolled steel plate; and a cooling step of water-cooling the rolled steel plate, In the heating step, the slab is heated at 950 to 1100°C, In the rolling process, the temperature is in the range of 800 to 930°C. d / h m The total reduction rate of the reduction is 40% or more, and the reduction rate is 0.3 / pass or more. In the cooling step, the cooling rate of the rolled steel plate is set to 2100 t between 700 and 400 ° C. -1.5and a cooling stop temperature of 350°C or less. [Effects of the Invention]

[0023] According to the present invention, it is possible to obtain low yield ratio extra thick steel plates having excellent toughness and a plate thickness of over 100 mm without requiring any special equipment. The low yield ratio extra heavy steel plate of the present invention is not particularly limited in its use, and can be applied to a wide range of fields in which heavy steel plates are generally used, such as ships, line pipes, buildings, bridges, marine structures, wind power generators, construction and industrial machinery, and pressure vessels. DETAILED DESCRIPTION OF THE INVENTION

[0024] The low yield ratio extra thick steel plate of the present invention will be described based on the following embodiments.

[0025] First, the chemical composition of the low yield ratio extra thick steel plate according to the present invention will be described. The unit of the content of elements in the chemical composition is "mass %", and hereinafter, unless otherwise specified, it will be simply expressed as "%". Furthermore, a numerical range expressed as "A to B" using an upper limit value A and a lower limit value B includes the upper limit value A and the lower limit value B.

[0026] C: 0.05 to 0.16% C is the most important element in the formation of island martensite. If the C content is less than 0.05%, the amount of island martensite becomes too small, resulting in a high yield ratio. On the other hand, if the C content exceeds 0.16%, the amount of island martensite becomes too large, resulting in a decrease in toughness. Therefore, the C content is set to 0.05 to 0.16%. The C content is preferably 0.06% or more. The C content is preferably 0.15% or less. From the viewpoint of ensuring good toughness at -40°C, the C content is preferably set to 0.05 to 0.12%, and more preferably to 0.11% or less.

[0027] Si: 0.03 to 0.70% Si is an element effective for deoxidation. If the Si content is less than 0.03%, a sufficient effect cannot be obtained. However, if the Si content exceeds 0.70%, weldability decreases. Therefore, the Si content is set to 0.03 to 0.70%. The Si content is preferably 0.04% or more. Furthermore, the Si content is preferably 0.60% or less.

[0028] Mn: 0.50 to 2.50% Mn is an element that improves the hardenability and strength of steel at low cost. From the viewpoint of obtaining such effects, the Mn content is set to 0.50% or more. On the other hand, if the Mn content exceeds 2.50%, the weldability decreases. Therefore, the Mn content is set to 0.50 to 2.50%. The Mn content is preferably 0.60% or more. Furthermore, the Mn content is preferably 2.20% or less.

[0029] P:0.030% or less P is an element that has a strong effect of embrittling grain boundaries. Therefore, if a large amount of P is contained, the toughness of the steel decreases. Therefore, the P content is set to 0.030% or less. The P content is preferably 0.025% or less. On the other hand, the lower the P content, the better, so the lower limit of the P content is not particularly limited and may be 0%. However, P is an element that is inevitably contained in steel as an impurity, and excessively low P content leads to increased refining time and costs. Therefore, the P content is preferably 0.001% or more.

[0030] S: 0.0200% or less S reduces the toughness of steel. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0100% or less. On the other hand, the lower the S content, the better, so the lower limit of the S content is not particularly limited and may be 0%. However, S is an element that is inevitably contained in steel as an impurity, and excessively low S content leads to increased refining time and costs. Therefore, the S content is preferably 0.0001% or more.

[0031] Nb: 0.003 to 0.100% Nb is an element that suppresses recrystallization when strain is applied to the austenite structure through the presence of solute Nb and finely precipitated NbC. Nb is also an element that has the effect of raising the non-recrystallization temperature range. To obtain this effect, the Nb content must be 0.003% or more. However, if the Nb content exceeds 0.100%, the weldability decreases. Therefore, the Nb content is set to 0.003 to 0.100%. The Nb content is preferably 0.005% or more. The Nb content is preferably 0.075% or less.

[0032] Ti: 0.005 to 0.100% Ti is an element that has the effect of pinning the movement of grain boundaries and suppressing grain growth by precipitating as TiN. To obtain this effect, the Ti content must be 0.005% or more. However, if the Ti content exceeds 0.100%, the cleanliness of the steel decreases and the ductility decreases. Therefore, the Ti content is set to 0.005 to 0.100%. The Ti content is preferably 0.006% or more. Furthermore, the Ti content is preferably 0.080% or less.

[0033] Al: 0.001 to 0.100% Al is an element effective for deoxidation. To obtain this effect, 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, and 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. Furthermore, the Al content is preferably 0.080% or less.

[0034] O: 0.0100% or less O is an element that reduces ductility. Therefore, the O content is set to 0.0100% or less. On the other hand, the lower the O content, the better, so the lower limit of the O content is not particularly limited and may be 0%. However, O is an element that is inevitably contained in steel as an impurity, and excessive reduction in O content leads to increased refining time and costs. Therefore, the O content is preferably 0.0005% or more.

[0035] N: 0.0100% or less N is an element that reduces ductility. Therefore, the N content is set to 0.0100% or less. On the other hand, since the lower the N content, the better, the lower limit of the N content is not particularly limited and may be 0%. However, since N is an element that is inevitably contained in steel as an impurity, industrially it may be greater than 0%. However, excessively low N content increases refining time and increases costs. Therefore, the N content is preferably 0.0005% or more.

[0036] The basic component composition of the low yield ratio extra thick steel plate of the present invention has been described above. However, from the viewpoint of further improving strength and weldability (toughness of welded parts, welding workability, etc.), one or more of the following optional additional elements may be appropriately contained. Cu: 1.00% or less, Ni: 5.00% or less, Cr: 1.00% or less, Mo: 1.00% or less V: 0.20% or less, B: 0.0050% or less, Ca: 0.0100% or less, Mg: 0.0200% or less and REM: 0.0500% or less

[0037] Cu: 1.00% or less Cu is an element that improves the strength of steel without significantly deteriorating toughness. However, if the Cu content exceeds 1.00%, hot cracking due to the Cu-enriched layer formed directly below the scale becomes a problem. Therefore, when Cu is contained, the Cu content is preferably 1.00% or less. The Cu content is preferably 0.01% or more. Furthermore, the Cu content is more preferably 0.70% or less.

[0038] Ni: 5.00% or less Ni is an element that improves the hardenability of steel. Ni is also an element that has the effect of improving toughness. However, if the Ni content exceeds 5.00%, an increase in manufacturing costs becomes a problem. Therefore, when Ni is contained, the Ni content is preferably 5.00% or less. The Ni content is preferably 0.01% or more. The Ni content is more preferably 1.50% or less, and even more preferably 1.30% or less.

[0039] Cr:1.00% or less Cr is an element that improves the hardenability of steel, thereby improving its strength. However, if the Cr content exceeds 1.00%, weldability decreases. Therefore, when Cr is contained, the Cr content is preferably 1.00% or less. The Cr content is preferably 0.01% or more. Furthermore, the Cr content is more preferably 0.80% or less.

[0040] Mo: 1.00% or less Mo is an element that improves the hardenability of steel, thereby improving its strength. However, if the Mo content exceeds 1.00%, weldability decreases. Therefore, when Mo is added, the Mo content is preferably 1.00% or less. The Mo content is preferably 0.01% or more. Furthermore, the Mo content is more preferably 0.80% or less.

[0041] V: 0.20% or less V is an element that improves the hardenability of steel and increases the strength of steel by forming carbonitrides. However, if the V content exceeds 0.20%, weldability decreases. Therefore, when V is contained, the V content is preferably 0.20% or less. The V content is preferably 0.01% or more. Furthermore, the V content is more preferably 0.15% or less.

[0042] B: 0.0050% or less B is an element that improves the hardenability of steel, thereby improving its strength. However, if the B content exceeds 0.0050%, weldability decreases. Therefore, when B is contained, the B content is preferably 0.0050% or less. The B content is preferably 0.0001% or more. Furthermore, the B content is more preferably 0.0040% or less.

[0043] Ca:0.0100% 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 0.0100%, the effect of adding Ca becomes saturated and no effect commensurate with the content can be expected, which is economically disadvantageous. Therefore, when Ca is added, the Ca content is preferably 0.0100% or less. The Ca content is preferably 0.0001% or more. Furthermore, the Ca content is more preferably 0.0080% or less.

[0044] 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 no effect commensurate with the content can be expected, which is economically disadvantageous. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less. The Mg content is preferably 0.0001% or more. Furthermore, the Mg content is more preferably 0.0180% or less.

[0045] 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 saturates and no effect commensurate with the content can be expected, which is economically disadvantageous. Therefore, if REM is added, the REM content is preferably 0.0500% or less. The REM content is preferably 0.0001% or more. Furthermore, the REM content is more preferably 0.0450% or less.

[0046] The balance other than the above elements in the chemical composition of the low yield ratio extra heavy steel plate according to one embodiment of the present invention is Fe and unavoidable impurities. Note that, with respect to the elements related to the above-mentioned optional added components, when the content thereof is less than the respective preferable lower limit values, the element is treated as an unavoidable impurity.

[0047] In addition, in the low yield ratio type extra heavy steel plate according to one embodiment of the present invention, Ceq defined by the following formula (1) and the plate thickness t (mm) of the extra heavy steel plate are 0.22t 0.1 It is important to satisfy the relationship ≦Ceq≦0.50. Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15...(1) Here, the [element symbol] in formula (1) represents the content (mass%) of each element in the composition, and if the element is not contained, it is calculated as 0.

[0048] 0.22t 0.1 ≦Ceq≦0.50 Ceq is a parameter that correlates with the strength of the steel structure. The ratio of this Ceq to the plate thickness t (mm) of the extra-thick steel plate is appropriately controlled. Specifically, Ceq is set to 0.22t 0.1 By setting Ceq to 0.22 or more, the desired tensile strength can be obtained. On the other hand, if Ceq exceeds 0.50, the amount of island martensite formed becomes too large, and the desired base material toughness at 0°C cannot be satisfied. Therefore, the range of Ceq is set to 0.22 or more. 0.1 ≦Ceq≦0.50. Note that Ceq is 0.23t 0.1 It is preferable that ≦Ceq≦0.49 is satisfied. In addition, from the viewpoint of ensuring the toughness of the base material at -40°C, the range of Ceq is set to 0.22t 0.1 It is preferable that ≦Ceq≦0.48, and 0.23t 0.1 It is more preferable that ≦Ceq≦0.47 is satisfied.

[0049] The steel structure of the low yield ratio extra heavy steel plate according to the present invention will be described.

[0050] Average grain size of ferrite matrix: 25 μm or less The structure of the extra-thick steel plate of the present invention is mainly composed of ferrite with a ferrite phase ratio of more than 51%, and the smaller the ferrite grain size, the better the toughness. In order to obtain the desired toughness at 0°C, the average grain size of ferrite must be 25 μm or less. Therefore, the average grain size of the ferrite matrix is ​​set to 25 μm or less. The average grain size is preferably 24 μm or less. Furthermore, brittle fractures tend to originate from low points in the structure, i.e., points with coarse grain sizes. Therefore, to ensure good toughness at a low temperature of -40°C, it is necessary to reduce not only the average grain size but also the maximum grain size of the structure. Therefore, to obtain the desired toughness at -40°C, it is preferable that the average grain size of the ferrite matrix is ​​15 μm or less and the difference between the maximum and average grain sizes is 15 μm or less. It is even more preferable that the average grain size is 14 μm or less and the difference between the maximum and average grain sizes is 14 μm or less.

[0051] Area ratio of island martensite: 3 to 20% Island martensite is a structure necessary for suppressing the upper yield point and reducing the yield ratio. To achieve this effect, the area fraction of island martensite must be 3% or more. On the other hand, if the area fraction of island martensite exceeds 20%, the toughness of the base material deteriorates, and the desired toughness at 0°C cannot be obtained. Therefore, the area fraction of island martensite is set to 3 to 20%. The area fraction of island martensite is preferably 4 to 19%. Furthermore, from the viewpoint of checking toughness at −40° C., the area fraction of island martensite is preferably 3 to 15%, and more preferably 4 to 14%. In addition to the parent phase ferrite and island martensite, the alloy may contain pearlite, bainite, and cementite.

[0052] Next, a method for manufacturing a low yield ratio extra heavy steel plate according to the present invention will be described. The method for manufacturing a low yield ratio extra heavy steel plate according to the present invention includes a preparation step of preparing a slab (steel material) having the above-described chemical composition, a slab heating step of heating the slab, a hot rolling step of hot rolling the heated slab to form a hot-rolled steel plate, and a cooling step of cooling the hot-rolled steel plate. Each step will be described below.

[0053] The temperatures in the heating process, hot rolling process, and cooling process described below are measured at the 1 / 4 position in the sheet thickness direction. The temperature at the 1 / 4 position in the sheet thickness direction can be measured using a thermocouple or the like.

[0054] [Preparation process] First, in the preparation step, a slab having the above-mentioned composition is prepared. The preparation method is not limited. For example, molten steel is produced using a converter, an electric furnace, a vacuum melting furnace, or the like. Optionally, secondary refining such as ladle refining may be performed. Next, the produced molten steel is formed into a slab by, for example, a continuous casting method or an ingot casting method, to prepare a slab having the above-mentioned composition. Note that the respective conditions may be in accordance with conventional methods.

[0055] [Heating process] Heating temperature: 950~1150℃ Next, in the heating step, the slab having the above-mentioned component composition is heated. Here, it is important that the slab heating temperature is 950 to 1150°C. If the slab heating temperature is less than 950°C, portions that are not transformed into austenite remain, and the desired structure cannot be obtained. On the other hand, if the slab heating temperature exceeds 1150°C, the austenite grain size during heating becomes coarse, and the grain size of the finally obtained ferrite also becomes coarse, making it impossible to obtain the desired toughness. For this reason, the slab heating temperature is set to 950 to 1150°C. Note that the slab heating temperature is preferably set to 970 to 1140°C. From the viewpoint of ensuring toughness at -40°C, the heating temperature of the slab is preferably 950 to 1100°C, and more preferably 970 to 1100°C.

[0056] [Hot rolling process] Reduction rate in the range of 800 to 930°C: 30% or more Subsequently, in the hot rolling process, the slab heated in the heating process is hot rolled to obtain a rolled steel sheet. Here, it is important that the hot rolling of the slab is carried out in the range of 800 to 930°C. By carrying out the reduction at 800 to 930°C, which is the non-recrystallization temperature range of austenite, shear bands are introduced into the austenite, and these shear bands act as nuclei for the ferrite transformation from austenite, thereby obtaining fine ferrite. Due to this effect, a reduction rate of 30% or more is required to obtain the desired toughness. The reduction rate is preferably 32% or more. In order to ensure toughness at -40°C, it is also preferable to satisfy the following conditions: d / h m is a parameter called the rolling shape ratio, which is expressed by the following formula. The larger this value, the more easily shear strain is introduced from the steel plate to the center of the plate thickness during rolling. In order to introduce sufficient shear strain at the 1 / 4 position of the plate thickness, which is the evaluation position for the Charpy impact test of the steel of the present invention, l d / h m It is necessary to reduce the rolling under the condition that l is 0.3 or more. d and h m is defined by the following equations (2) and (3).

number

[0057] [Cooling process] Cooling rate between 700 and 400°C: 2100t -1.5 End Finally, in the cooling process, the rolled steel plate is cooled. Here, the rolled steel plate is cooled at a cooling rate of 2100 t between 700 and 400°C. -1.5 It is important to cool the material to a temperature of 2100°C or higher. -1.5 If the cooling rate is less than 2100°C, the desired tensile strength and island martensite content cannot be obtained because the cooling rate is too small. -1.5 Although there is no specific upper limit for the cooling rate, a special cooling facility is required to significantly increase the cooling rate, so the upper limit is set at 6000t. -1.5 It is preferable to have the following:

[0058] Cooling stop temperature: 350℃ or less In addition, it is important to set the cooling stop temperature to 350°C or less during the cooling process. If the cooling stop temperature is higher than 350°C, the amount of island martensite formed will decrease, resulting in a high yield ratio. Therefore, the cooling stop temperature was set to 350°C or less.

[0059] The other conditions are not limited and may be conventional. For example, the cooling method may be water cooling or gas cooling. The cooling rate in the temperature ranges other than those described above is not particularly limited, and any cooling method may be used to cool to room temperature. [Example]

[0060] Example 1 Molten steel having the chemical composition shown in Table 1 was melted, and slabs with a thickness of 260 to 600 mm were prepared by continuous casting, ingot casting, etc. Note that blanks in the element columns of Table 1 indicate that the element was not intentionally added, and include not only cases where the element is not contained (0%), but also cases where the element is unavoidably contained.

[0061] Next, the prepared slab was hot rolled and cooled under the conditions shown in Table 2 to obtain low yield ratio extra heavy steel plates with thicknesses t (mm) shown in Table 2. The temperature at the 1 / 4 position in the plate thickness direction was measured using a thermocouple. The rolling reduction r between 800 and 930°C was calculated using the following formula (5). r=(t0-t1) / t0×100(%) (5) Here, t0 indicates the plate thickness when the temperature at the 1 / 4 position of the plate thickness reaches 930°C, and t1 indicates the plate thickness when the temperature at the 1 / 4 position of the plate thickness reaches 800°C.

[0062] [Organizational Evaluation] From each of the obtained extra-heavy steel plates, a sample for microstructure observation was taken at the center position in the longitudinal direction (rolling direction) of the extra-heavy steel plate so that the microstructure observation surface was parallel to the rolling direction of the extra-heavy steel plate, and microstructure observation was performed at a position 1 / 4 of the plate thickness. The average grain size of the parent ferrite phase was determined by mirror-polishing the sample for microstructure observation, etching it with 3% nital, taking an optical microscope photograph at a magnification of ×200, and calculating the circle equivalent diameter by image analysis. The area ratio of island martensite was determined by mirror-polishing a sample for microstructural observation, etching it with 3% nital, and then removing the cementite by electrolytic polishing with an aqueous solution of 200 g of sodium hydroxide and 40 g of picric acid dissolved in 800 ml of water. Five scanning electron microscope photographs at a magnification of 2000 times were then taken, and the average value obtained by image analysis was used.

[0063] Furthermore, tensile tests and Charpy tests were carried out on each of the obtained extra-thick steel plates in the following manner to evaluate the yield strength, tensile strength, yield ratio, and Charpy absorbed energy at 0° C. The evaluation results are also shown in Table 2.

[0064] [Tensile test] A tensile test specimen was taken from each of the obtained extra-heavy steel plates at the center position in the longitudinal direction (rolling direction) of the extra-heavy steel plate, so that the longitudinal direction of the tensile test specimen was parallel to the plate width direction (direction perpendicular to rolling) of the extra-heavy steel plate. Here, the tensile test specimen was taken so that the center position in the thickness direction of the tensile test specimen was at a position 1 / 4 of the plate thickness of the extra-heavy steel plate. The shape of the tensile test specimen was JIS No. 4 shape. Next, a tensile test was performed in accordance with JIS Z2241 (2011) using each of the taken tensile test specimens, and the yield strength and tensile strength were measured and the yield ratio was calculated.

[0065] [Charpy impact test] Charpy test specimens were taken from the longitudinal and transverse center positions of each extra-thick steel plate. The Charpy test specimens were taken so that the thickness-direction center of the specimen was 1 / 4 the thickness of the extra-thick steel plate and the longitudinal direction of the specimen was parallel to the transverse direction of the steel plate. Next, a Charpy impact test was performed in accordance with JIS Z2242 (2018) using each Charpy test specimen to evaluate the absorbed energy. Standard 2mm V-notch Charpy test specimens were used, and the average value of the results of three tests at 0°C was used for evaluation.

[0066] [Table 1]

[0067] [Table 2]

[0068] As shown in Table 2, inventive examples Nos. 1 to 15, 29, and 30, all of which are extra-thick steel plates with thicknesses exceeding 100 mm, achieve excellent tensile strength, toughness, and a low yield ratio.

[0069] On the other hand, in all of the extra thick steel plates of the comparative examples, any one of the tensile strength, toughness, and yield ratio was insufficient. That is, in No. 16, the C content was too low, resulting in a small amount of island martensite and a high yield ratio. No. 17 had a high C content, which resulted in a large amount of island martensite and low toughness. In Nos. 18 and 19, the Ceq was too high, resulting in a large amount of island martensite and low toughness. No. 20 had low tensile strength because the plate thickness was too large compared to Ceq. In No. 21, the heating temperature was too high, which resulted in large ferrite grain size and low toughness. In Comparative Example No. 22, the rolling reduction rate between 800 and 930°C was too low, so the ferrite grain size was large and the toughness was low. In No. 23, the cooling rate was too low to obtain sufficient tensile strength, and the amount of island martensite was too small to obtain a low yield ratio. In No. 24, the cooling temperature was too high, resulting in a small amount of island martensite and a high yield ratio. No. 25 had an excessively high P content, which resulted in an embrittlement of the structure and low toughness. No. 26 had an excessively high S content, which made the structure brittle and resulted in low toughness. In No. 27, the Nb content was too low, which resulted in a coarse structure (i.e., the ferrite grain size was large) and low toughness. In No. 28, the Ti content was too low, which resulted in a coarse structure (i.e., the ferrite grain size was large) and low toughness.

[0070] Example 2 Molten steel having the chemical composition shown in Table 3 was melted, and slabs with a thickness of 260 to 600 mm were prepared by continuous casting, ingot casting, etc. Note that blanks in the element columns in Table 3 indicate that the element was not intentionally added, and include not only cases where the element was not contained (0%), but also cases where the element was unavoidably contained.

[0071] Next, the prepared slab was hot rolled and cooled under the conditions shown in Table 4 to obtain extra heavy steel plates having thicknesses t (mm) shown in Table 4. The temperature at the 1 / 4 position in the plate thickness direction was measured using a thermocouple. The reduction ratio r between 800 and 930°C was calculated using the following formula (5). r=(t0-t1) / t0×100(%) (5) Here, t0 indicates the plate thickness when the temperature at the 1 / 4 position of the plate thickness reaches 930°C, and t1 indicates the plate thickness when the temperature at the 1 / 4 position of the plate thickness reaches 800°C.

[0072] [Organizational Evaluation] From each of the obtained low yield ratio extra heavy steel plates, a sample for microstructural observation was taken at the center position in the longitudinal direction (rolling direction) of the extra heavy steel plate so that the microstructural observation surface was parallel to the rolling direction of the extra heavy steel plate, and microstructural observation was performed at a position 1 / 4 of the plate thickness. The average grain size and maximum grain size of the parent ferrite were calculated as the circle equivalent diameter by image analysis after mirror polishing the sample for microstructural observation and etching it with 3% nital, and then taking five optical microscope photographs at a magnification of ×200. The area ratio of island martensite was determined by mirror-polishing a sample for microstructural observation, etching it with 3% nital, and then removing the cementite by electrolytic polishing with an aqueous solution of 200 g of sodium hydroxide and 40 g of picric acid dissolved in 800 ml of water. Five scanning electron microscope photographs at a magnification of 2000 times were then taken, and the average value obtained by image analysis was used.

[0073] Furthermore, tensile tests and Charpy tests were carried out on each of the obtained extra-thick steel plates in the following manner to evaluate the yield strength, tensile strength, yield ratio, and Charpy absorbed energy at -40°C. The evaluation results are also shown in Table 4.

[0074] [Tensile test] A tensile test specimen was taken from each of the obtained extra-heavy steel plates at the center position in the longitudinal direction (rolling direction) of the extra-heavy steel plate, so that the longitudinal direction of the tensile test specimen was parallel to the plate width direction (direction perpendicular to rolling) of the extra-heavy steel plate. Here, the tensile test specimen was taken so that the center position in the thickness direction of the tensile test specimen was at a position 1 / 4 of the plate thickness of the extra-heavy steel plate. The shape of the tensile test specimen was JIS No. 4 shape. Next, a tensile test was performed in accordance with JIS Z2241 (2011) using each of the taken tensile test specimens, and the yield strength and tensile strength were measured and the yield ratio was calculated.

[0075] [Charpy impact test] Charpy test specimens were taken from the longitudinal and transverse center positions of each extra-thick steel plate. The Charpy test specimens were taken so that the thickness-direction center of the specimen was 1 / 4 the thickness of the extra-thick steel plate and the longitudinal direction of the specimen was parallel to the transverse direction of the steel plate. Next, a Charpy impact test was performed in accordance with JIS Z2242 (2018) using each Charpy test specimen to evaluate the absorbed energy. Standard 2mm V-notch Charpy test specimens were used, and three specimens were tested at -40°C. The average value of the results was used for evaluation.

[0076] [Table 3]

[0077] [Table 4]

[0078] As shown in Table 4, inventive examples Nos. 1' to 14', 33', and 34', although they are all extra-thick steel plates with a plate thickness of over 100 mm, they combine excellent tensile strength, toughness, and a low yield ratio. Ta.

[0079] On the other hand, in all of the extra thick steel plates of the comparative examples, any one of the tensile strength, toughness, and yield ratio was insufficient.

[0080] That is, Nos. 15', 17' and 19' had a high C content and a large amount of island martensite, and therefore had low Charpy absorbed energy at -40°C. Nos. 16', 20' and 21' had high Ceq and a large amount of island martensite, and therefore had low Charpy absorbed energy at -40°C. No. 18' had a low C content, so the island martensite ratio was low and the yield ratio was high. For No. 22', the Charpy absorbed energy at -40°C was low because the slab heating temperature was high and the average grain size of ferrite was large. For No. 26', the slab heating temperature was high, which increased the average grain size of ferrite and the difference between the average grain size and the maximum grain size, resulting in low Charpy absorbed energy at -40°C. No.23' is between 800 and 930℃ d / h m However, the total reduction rate of 0.3 / pass or more was low, and the difference between the average grain size and the maximum grain size of ferrite was large, so the Charpy absorbed energy at -40°C was low. No.27' is between 800 and 930℃ d / h m The total reduction rate of 0.3 / pass or more was low, and the difference between the average grain size and the maximum grain size of ferrite was large, so the Charpy absorbed energy at -40°C was low. No. 24' had a low tensile strength due to the low cooling rate between 700 and 400°C, and a high yield ratio due to the low proportion of island martensite. In No. 25', the cooling stop temperature of the rolled steel plate was high, so the island martensite fraction was low and the yield ratio was high. No. 28' had low tensile strength due to its low Ceq relative to the plate thickness. No. 29' had an excessively high P content, which resulted in an embrittlement of the structure and low toughness. No. 30' had too high a S content, which made the structure brittle and resulted in low toughness. In No. 31', the Nb content was too low, resulting in coarse average grain size of ferrite and low toughness. In No. 32', the Ti content was too low, resulting in coarse average grain size of ferrite and low toughness. [Industrial Applicability]

[0081] According to the present invention, it is possible to obtain extra-thick steel plates with a low yield ratio and a thickness of over 100 mm, which have excellent low-temperature toughness, without requiring any special equipment.

Claims

1. In mass%, C: 0.05-0.16%, Si: 0.03-0.70%, Mn: 0.50 to 2.50%, P: 0.030% or less, S: 0.0200% or less, Nb: 0.003 to 0.100%, Ti: 0.005-0.100%, Al: 0.001-0.100%, O: 0.0100% or less, and N: 0.0100% or less, The balance is Fe and unavoidable impurities, Ceq defined by the following formula (1) and plate thickness t [mm] are 0.22t ０．１ ≦Ceq≦0.50, Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15...(1) (Note that each element symbol in the above formula (1) represents the content (mass%) of the element, and if the element is not added, it is set to 0.) A low yield ratio type extra thick steel plate having a thickness of more than 100 mm, characterized in that the average grain size of parent phase ferrite is 25 μm or less, the area ratio of island martensite is 3 to 20%, the tensile strength is 520 MPa or more, the yield ratio is 80% or less, and the Charpy absorbed energy at 0°C is 47 J or more.

2. In mass%, C: 0.05-0.12%, Si: 0.03-0.70%, Mn: 0.50 to 2.50%, P: 0.030% or less, S: 0.0200% or less, Nb: 0.003 to 0.100%, Ti: 0.005-0.100%, Al: 0.001-0.100%, O: 0.0100% or less, and N: 0.0100% or less, The balance is Fe and unavoidable impurities, Ceq defined by the following formula (1) and plate thickness t [mm] are 0.22t ０．１ ≦Ceq≦0.48, Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15...(1) (Note that each element symbol in the above formula (1) represents the content (mass%) of the element, and if the element is not added, it is set to 0.) A low yield ratio type extra thick steel plate having a thickness of more than 100 mm, characterized in that the average grain size of parent phase ferrite is 15 μm or less and the difference between the maximum grain size and the average grain size is 15 μm or less, the area ratio of island martensite is 3 to 15%, the tensile strength is 520 MPa or more, the yield ratio is 80% or less, and the Charpy absorbed energy at −40° C. is 47 J or more.

3. The component composition further comprises, in mass%, Cu: 1.00% or less, Ni: 5.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, V: 0.20% or less, Ca: 0.0100% or less, Mg: 0.0200% or less, and REM: 0.0500% or less The low yield ratio type extra thick steel plate according to claim 1 or 2, comprising one or more selected from the group consisting of:

4. A method for producing a low yield ratio extra heavy steel plate according to claim 1 or claim 3 which cites claim 1, comprising: a preparation step of preparing a slab having the component composition; a heating step of heating the slab; a hot rolling step of hot rolling the heated slab to form a hot-rolled steel plate; and a cooling step of water-cooling the rolled steel plate, In the heating step, the slab is heated at 950 to 1150°C, In the rolling step, a reduction of 30% or more is performed in the range of 800 to 930°C, In the cooling step, the rolling steel plate is cooled at a rate of 2100 t / min between 700 and 400°C. －１．５ ℃ / sec or more, and the cooling stop temperature is 350 ° C or less.

5. A method for producing a low yield ratio extra heavy steel plate according to claim 2 or claim 3 which cites claim 2, comprising: a preparation step of preparing a slab having the chemical composition; a heating step of heating the slab; a hot rolling step of hot rolling the heated slab to form a hot-rolled steel plate; and a cooling step of water-cooling the rolled steel plate, In the heating step, the slab is heated at 950 to 1100°C, In the rolling process, the temperature is in the range of 800 to 930°C. ｄ / h ｍ The total reduction rate of the rolling is 40% or more, and the rolling is 0.3 / pass or more. In the cooling step, the rolling steel plate is cooled at a rate of 2100 t / min between 700 and 400°C. －１．５ and a cooling stop temperature of 350°C or less.

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