High strength thick steel plate and its manufacturing method

Abstract

To provide a steel plate having yield strength of 500 MPa or more, excellent host strength and host toughness before and after SR treatment, and when performed multilayer welding, having excellent joint CTOD characteristics, and a manufacturing method thereof.SOLUTION: A thick steel plate has a plate thickness of 60 to 100 mm, the yield strength of 500 MPa or more, the tensile strength of 570 MPa or more, and HAZ maximum hardness of a welding joint of 250 HV or less, comprises components consisting of C, Si, Mn, Ti, Cu, Ni, Mo, Nb, N, O, P, S, Al, Mg, Ca, B in a predetermined range, and the balance being Fe and impurities, the plate thickness t / 4 part being a predetermined multiphase texture, an average circle equivalent diameter of crystal grains of a plate thickness center section being 50 μm or less, the maximum hardness HVmax of the plate thickness center section satisfying a formula (2), and furthermore, regarding particles of Ti-containing inclusions contained in a steel texture, the number density (EIGFD) of particles of Ti-containing inclusion effective for intragranular transformation shown in equation (4) being 20 pieces / mm2 or more.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention is primarily directed to steel plates for marine structures, which require low-temperature toughness in welded joints. However, the present invention is not limited to this application and relates to high-strength thick steel plates that can be used in a wide range of welded structures, such as ships, buildings, bridges, and tanks. [Background technology]

[0002] Welding is used to assemble various structures, including not only marine structures but also ships, buildings, bridges, tanks, etc., into a specified shape. When the thickness of the steel plates used increases, multi-pass welding is generally used. The HAZ (Heat Affected Zone) refers to the area on the steel plate side of a welded joint where the metal structure of the base material, which was created by thermomechanical treatment, is thermally affected by reheating during welding.

[0003] The most embrittled part of the weld HAZ is known to be the coarse grain heat affected zone (CGHAZ), which is the base metal part in contact with the weld metal and has a coarse metallurgical structure. The CGHAZ is reheated to near the fusion line of the steel, causing the austenite grains formed during reheating to grow. During subsequent cooling, the coarse austenite grains retransform, resulting in a coarse metallurgical structure.

[0004] In the CGHAZ, coarse austenite grains are cooled first, making it easier to harden and increasing its hardness compared to the base material. In addition, when the CGHAZ is reheated to the two-phase region of ferrite and austenite by a subsequent welding pass, a martensite-austenite constituent (MA) is formed, which is hard and acts as the initiation point for brittle fracture.

[0005] When strength is required for steel plates, alloying elements such as carbon are added. As the amount of alloying elements added increases with increasing strength and thickness, the hardness of the HAZ increases and the amount of brittle phases such as MA increases. Factors known to affect HAZ toughness include the grain size of the structure, the size of the brittle phases, and hardness. The CGHAZ has a coarse metal structure, is rich in brittle phases, and is hard, making it the most brittle part of the weld HAZ. Controlling the CGHAZ structure in multi-layer welded joints is important to ensure the safety of structures.

[0006] Patent Document 1 describes a microstructure refinement technology for improving the toughness of the CGHAZ, in which TiN particles are dispersed in the steel to suppress austenite grain growth in the CGHAZ that is reheated to the vicinity of the fusion line, refine the transformed microstructure, and achieve both high strength and HAZ toughness in the steel plate. However, depending on the welding conditions, if the CGHAZ is reheated to 1350°C or higher, TiN may dissolve, preventing sufficient grain refinement due to grain growth suppression and reducing toughness.

[0007] Another method for refining the CGHAZ structure is to finely disperse Ti oxides in the steel and use them as transformation nuclei to generate intragranular ferrite (IGF).For example, as shown in Patent Document 2, a steel plate with excellent HAZ toughness at low temperatures has been developed.

[0008] Furthermore, as offshore structures continue to grow in size, steel plates for these structures are required to be even stronger and thicker. High-strength steel plates are often subjected to stress relief (SR) treatment to remove residual stresses generated during welding. To maintain the stability of the steel's mechanical properties, minimal change in the steel's mechanical properties is required before and after SR treatment. To ensure the base material's toughness after SR treatment, it is necessary to reduce impurities such as P and S. However, the central segregation region has high impurity element concentrations, and the concentrated alloying elements cause localized high hardness. Therefore, it has been difficult to maintain the same base material toughness after SR treatment for high-strength steel plates containing a large amount of alloying elements. One method to maintain the base material's toughness after SR is to hold the slab at high temperatures for a long period of time (soaking process (SP)), which reduces impurities and suppresses the concentrated alloying elements in the central segregation region. However, this method increases production costs. Furthermore, although homogenization treatment (SP treatment) is effective in maintaining the toughness of the base material, it is difficult to suppress the decrease in strength due to SR. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-207237 [Patent Document 2] Japanese Patent Application Publication No. 7-278653 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention aims to provide a steel plate having a yield strength of 500 MPa or more, good base metal strength and toughness before and after SR treatment, and good joint CTOD characteristics when multi-layer welded, and a manufacturing method thereof. [Means for solving the problem]

[0011] The inventors of the present invention have conducted extensive research into steel plates that satisfy the objective of achieving base material strength of 500 MPa or more, as well as good base material toughness and joint toughness. 1) Ti-containing inclusions, including Ti oxides, promote IGF transformation and refine the CGHAZ structure. 2) Suppression of CGHAZ hardness by optimizing steel composition, 3) Ensuring base material strength through optimization of steel composition, refining the metal structure at the center of the plate thickness, and ensuring toughness at the center of the plate thickness by reducing hardness and stress concentration sources at the center of the plate thickness. 4) The addition of Mo prevents the reduction in strength after SR treatment by utilizing the temper softening resistance and maintains the toughness of the base material. It was found that this is effective in solving the above problems.

[0012] 1) In the past, inclusions were often considered as a single entity, and their size, shape, and / or number were treated as factors that affect the properties of steel sheets. However, the inventors' experiments have revealed that the composition of the inclusions themselves plays a significant role in the formation of intragranular transformed ferrite in the HAZ. Specifically, Ti-containing inclusions containing Ti oxides (TiO, Ti2O3) also contain other oxides such as Al oxide, Mg oxide, and Ca oxide. The inventors have found that as the Ti oxide content in Ti-containing inclusions containing these oxides increases, (i-1) the formation of intragranular transformed ferrite (IGF) is promoted and (i-2) the formation of grain boundary ferrite (GBF) and ferrite side plates (FSP) (both of which are embrittled structures) is suppressed, resulting in a significant improvement in low-temperature toughness in the HAZ.

[0013] 2) The toughness of steel plates is said to be in a trade-off relationship with their strength, and it is necessary to adjust the strength to an appropriate level to ensure CGHAZ toughness. The structure of the CGHAZ is determined by the heating and cooling conditions based on the welding heat input, so optimizing the chemical composition is necessary to optimize strength. It was discovered that this chemical composition optimization can ensure joint toughness by suppressing the maximum hardness of the CGHAZ.

[0014] 3) To ensure the strength of the base material, alloys are added to ensure hardenability, but toughness decreases. It was discovered that the target strength and toughness can be achieved by reducing the average circular equivalent diameter of crystal grains surrounded by high-angle grain boundaries with a misorientation of 15° or more at the center of the plate thickness to 50 μm or less, reducing stress concentration sources such as MnS that can cause fractures, and further reducing the hardness at the center of the plate thickness. Specifically, it was discovered that toughness can be ensured by satisfying -726[S]+315≧HVmax.

[0015] 4) To maintain the base metal strength after SR, Mo, which has high resistance to temper softening, was added to suppress the decrease in strength. Mo hardens the HAZ, reducing the toughness of the CGHAZ, but by optimizing the steel composition, it was possible to achieve both joint toughness and maintaining strength after SR. Furthermore, it was found that adding Mo can also suppress the decrease in base metal toughness after SR.

[0016] The present invention was completed based on the above findings and further investigations. The gist of the present invention is as follows.

[0017] <1> The plate thickness is 60 to 100 mm, the yield strength is 500 MPa or more, and the tensile strength is 570 MPa or more; In mass%, C: 0.020~0.120%, Si: 0.05 to 0.30%, Mn: 0.80 to 2.00% Ti: 0.005 to 0.018%, Cu: 0.05 to 1.50% Ni: 0.05 to 2.00% Mo: 0.05% to 0.50%, Nb: 0.005 to 0.025%, N: 0.0015~0.0060%, O: 0.0010~0.0045%, Contains P: 0.015% or less, S: 0.0050% or less, Al: 0 to 0.004%, Mg: 0 to 0.0010%, Ca: 0 to 0.0010%, B: 0~0.0015%, and The Ceq. value calculated by the following formula (1) satisfies 0.440≦Ceq., with the remainder being composed of Fe and impurities, The ferrite fraction in the t / 4 portion of the plate thickness is 0 to 15% by area, and the remainder is a multi-phase structure consisting of one or more of bainite and martensite, The average circle equivalent diameter of the top 10 crystal grains in terms of the largest area in the region surrounded by a high-angle grain boundary with a crystal orientation misorientation of 15° at the center of the plate thickness is 50 μm or less, The maximum hardness HVmax at the center of the plate thickness satisfies formula (2), Furthermore, for particles of Ti-containing inclusions contained in the steel structure, which contain Ti and one or more of Al, Mg, Si, Ca, and Mn and have a circle equivalent diameter of 0.5 μm or more and 5.0 μm or less, the Ti content ratio (TCP) of each Ti-containing inclusion particle was calculated using formula (3) based on the mass ratio of elements measured by EDS. When these particles were classified into Group A where the TCP was 40% or more and Group B where the TCP was less than 40% and 20% or more, the number density (EIGFD) of Ti-containing inclusions effective for intragranular transformation shown in formula (4) was 20 particles / mm 2 A high-strength thick steel plate characterized by the above. Ceq=[C]+[Mn] / 6+Mo / 5+[Cu] / 15+[Ni] / 15 … Formula (1) -726[S]+315≧HVmax … Formula (2) TCP=[Ti] / ([Ti]+[Al]+[Mg]+[Ca]) … Formula (3) EIGFD=(XA×0.8)+(XB×0.5) … Formula (4) In the formula (1), [C] is the mass % of C, [Mn] is the mass % of Mn, [Mo] is the mass % of Mo, [Cu] is the mass % of Cu, and [Ni] is the mass % of Ni. In the formula (2), [S] is the mass % of S. In the formula (3), [Ti], [Al], [Mg], and [Ca] are the Ti, Al, Mg, and Ca contents (mass%) obtained from EDS analysis of Ti-containing inclusions, and 0 is substituted if none are contained. In the formula (4), XA and XB are the inclusion density (number / mm) of inclusions classified into Group A and Group B, respectively. 2 ) is the measured value.

[0018] <2> Furthermore, in mass%, Cr: 0.50% or less, V: 0.03% or less, Contains one or two of the following: The high-strength thick steel plate according to <1>, wherein the Ceq value is calculated by the following formula (1)' instead of the formula (1): Ceq.=[C]+[Mn] / 6+[Cu] / 15+[Ni] / 15+[Cr] / 5+[Mo] / 5+[V] / 5 … Equation (1)' In the formula (1)', [C] is the mass% of C, [Mn] is the mass% of Mn, [Cu] is the mass% of Cu, [Ni] is the mass% of Ni, [Cr] is the mass% of Cr, [Mo] is the mass% of Mo, and [V] is the mass% of V.

[0019] <3> A steel slab having the chemical composition according to either <1> or <2> and produced by a continuous casting method is reheated at 950°C to 1200°C, subjected to rough rolling so as to achieve an average reduction of 7.5% or more per pass, and reduced to a thickness of 135 to 210 mm. Thereafter, the steel slab is finish-rolled from 670°C to 800°C so as to achieve an average reduction of 6.0% or more per pass, and then cooled at a cooling rate of 10°C / s or less at the center of the plate thickness. 、 The ferrite fraction in the t / 4 portion of the plate thickness is 0 to 15% by area, and the remainder is a multi-phase structure consisting of one or more of bainite and martensite, The average circle equivalent diameter of the top 10 crystal grains in terms of the largest area in the region surrounded by a high-angle grain boundary with a crystal orientation misorientation of 15° at the center of the plate thickness is 50 μm or less, The maximum hardness HVmax at the center of the plate thickness satisfies formula (2), Furthermore, for particles of Ti-containing inclusions contained in the steel structure, which contain Ti and one or more of Al, Mg, Si, Ca, and Mn and have a circle equivalent diameter of 0.5 μm or more and 5.0 μm or less, the Ti content ratio (TCP) of each Ti-containing inclusion particle was calculated using formula (3) based on the mass ratio of elements measured by EDS. When these particles were classified into Group A where the TCP was 40% or more and Group B where the TCP was less than 40% and 20% or more, the number density (EIGFD) of Ti-containing inclusions effective for intragranular transformation shown in formula (4) was 20 particles / mm 2 That's all A method for manufacturing high-strength thick steel plate. -726[S]+315≧HVmax … Formula (2) TCP=[Ti] / ([Ti]+[Al]+[Mg]+[Ca]) … Formula (3) EIGFD=(XA×0.8)+(XB×0.5) … Formula (4) In the formula (1), [C] is the mass % of C, [Mn] is the mass % of Mn, [Mo] is the mass % of Mo, [Cu] is the mass % of Cu, and [Ni] is the mass % of Ni. In the formula (2), [S] is the mass % of S. In the formula (3), [Ti], [Al], [Mg], and [Ca] are the Ti, Al, Mg, and Ca contents (mass%) obtained from EDS analysis of Ti-containing inclusions, and 0 is substituted if none are contained. In the formula (4), XA and XB are the inclusion density (number / mm) of inclusions classified into Group A and Group B, respectively. 2 ) is the measured value.

[0020] <4> The method for producing a high-strength thick steel plate according to <3>, wherein the plate is cooled at a cooling rate of 10°C / s or less at the plate thickness center, and then heat treated at 300°C or more and 670°C or less. [Effects of the Invention]

[0021] This invention makes it possible to manufacture high-strength thick steel plates with thicknesses of 60 mm or more that have excellent CTOD properties in the HAZ of multi-pass welds produced with low to medium heat input, a base metal yield strength of 500 MPa or more, a tensile strength of 570 MPa or more, and good base metal strength and toughness before and after SR treatment. This will enable the manufacture of steel structures used in extremely harsh environments, such as offshore structures, with larger sizes and lighter weights, and cost reductions due to the reduced amount of steel used. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a diagram showing the relationship between the toughness of the base material after SR (ΔvTrs, ° C.) and the amount of Mo added for steel plates according to the present embodiment with Ceq. of 0.48 to 0.50. [Figure 2] FIG. 1 is a diagram showing the relationship between the amount of Mo added and the joint HAZ toughness (joint CTOD, mm) after SR for steel plates according to the present embodiment with Ceq. of 0.48 to 0.50. [Figure 3] FIG. 2 is a schematic diagram of an inclusion in the steel plate according to the present embodiment. [Figure 4] FIG. 1 is a diagram showing a thermal cycle when investigating the ability to generate intragranular transformed ferrite. [Figure 5] This is an example of determining whether intragranular transformation originating from inclusions occurs in the steel plate according to the present embodiment. [Figure 6-1]1 is a graph illustrating an example of the composition analysis results of all inclusions present in a 1 mm×1 mm region in the plate thickness t / 4 portion after thermal cycling of the steel plate according to the present embodiment, and the IGF generation behavior. [Figure 6-2] 1 is a graph illustrating an example of the composition analysis results of all inclusions present in a 1 mm×1 mm region in the plate thickness t / 4 portion after thermal cycling of the steel plate according to the present embodiment, and the IGF generation behavior. [Figure 7] 1 is a graph illustrating an example of the TCP calculation results of Ti-containing inclusions present in a 1 mm×1 mm area in a plate thickness t / 4 portion after thermal cycling of a steel plate according to this embodiment. [Figure 8] FIG. 1 is a diagram showing the relationship between formula (4) and the HAZ toughness of a SR joint for a steel plate according to the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0023] An embodiment of the present invention will be described below.

[0024] Plate Thickness The present invention is directed to high-strength thick steel plates suitable for large welded structures such as ships, buildings, bridges, and tanks, and particularly relates to steel plates with a plate thickness of 60 mm to 100 mm.

[0025] [Yield strength, tensile strength] The yield strength must be 500 MPa or more. As marine structures become larger, steel plates for marine structures are also required to be stronger and thicker. Therefore, for steel plates with a thickness of 60 to 100 mm, the guideline is a yield strength of 500 MPa or more and a tensile strength of 570 MPa or more. In addition, the guideline is a maximum hardness of 250 HV or less in the HAZ of the welded joint.

[0026] [Chemical composition] The chemical composition of the steel sheet according to this embodiment will be described. The steel sheet according to this embodiment contains, in mass%, C: 0.020 to 0.120%, Si: 0.05 to 0.30%, Mn: 0.80 to 2.00%, Ti: 0.005 to 0.018%, Cu: 0.05 to 1.50%, Ni: 0.05 to 2.00%, Mo: 0.05% to 0.50%, Nb: 0.005 to 0.025%, N: The content of each of the elements is limited to 0.0015-0.0060%, O: 0.0010-0.0045%, P: 0.015% or less, S: 0.0050% or less, Al: 0.004% or less, Mg: 0.0010% or less, Ca: 0.0010% or less, and B: 0.0015% or less. The Ceq. value calculated by the following formula (1) satisfies 0.440≦Ceq., and the balance consists of Fe and unavoidable impurities. Ceq.=[C]+[Mn] / 6+Mo / 5+[Cu] / 15+[Ni] / 15 … Formula (1) In the formula (1), [C] is the mass % of C, [Mn] is the mass % of Mn, [Mo] is the mass % of Mo, [Cu] is the mass % of Cu, and [Ni] is the mass % of Ni.

[0027] In the following explanation of chemical components, mass% will be expressed as %. Furthermore, in the following explanation, when an upper limit and a lower limit of an element content are connected by "~" to indicate a range, this means a range that includes both the upper and lower limits, unless otherwise noted. Therefore, when expressed as 0.01 to 0.20% by mass, this range means a range of 0.01 to 0.20% by mass.

[0028] C: 0.020 to 0.120% Carbon (C) is an element that increases the strength of the base metal. Because the effect of improving base metal strength is small when the C content is less than 0.020%, the lower limit is set to 0.020%. A more preferable lower limit for the C content is 0.030%. On the other hand, when the C content exceeds 0.120%, cementite and martensite-austenite composites (referred to as martensite-austenite constituents (MA)), which serve as initiation sites for brittle fracture, increase, resulting in reduced HAZ toughness. Therefore, the upper limit for the C content is set to 0.120%. In particular, for the HAZ toughness and low-temperature toughness of high-heat-input welding, even relatively small amounts of cementite and MA can easily serve as initiation sites for brittle fracture and reduce HAZ toughness, so it is preferable to strictly restrict the upper limit for the C content. The upper limit for the C content is preferably 0.110%, more preferably 0.100%, even more preferably 0.090%, and even more preferably 0.080%.

[0029] Si: 0.05 to 0.30% Si functions as a deoxidizer and contributes to increasing strength. In particular, in the case of Ti-deoxidized steel, Si is often added to enhance deoxidation ability, and the lower limit is set at 0.05%. A more preferable lower limit for the Si content is 0.06%. On the other hand, excessive Si content can easily lead to the formation of MA, a hard, embrittled structure, in the HAZ microstructure. Since MA deteriorates the toughness of the HAZ, it is desirable to limit the Si content, with the upper limit set at 0.30%. The upper limit for Si content is preferably 0.23%, more preferably 0.15%.

[0030] Mn: 0.80 to 2.00% Mn is an element that improves hardenability, and a content of 0.80% or more is required to ensure the strength of steel sheet. It also combines with S to form MnS on Ti oxides, promoting intragranular transformation. However, a large amount of Mn leads to segregation and the formation of hard phases, reducing HAZ toughness. Especially after SR, it promotes grain boundary embrittlement, degrading the toughness of the base material and HAZ. The upper limit of this limit is set at 2.00%, within the acceptable range. A more preferable upper limit of the Mn content is 1.80%.

[0031] Ti: 0.005 to 0.018% Ti is an important element in the present invention, as it forms Ti-containing inclusions, including Ti oxides, and promotes the formation of intragranular transformed ferrite in the HAZ. It also forms nitrides, which refine the microstructure through the pinning effect of γ grain boundaries, contributing to improved toughness. Ti content less than 0.005% may result in insufficient Ti-containing inclusions and nitride particles, so the lower limit is set to 0.005%. To generate a larger number of particles, the lower limit of the Ti content is preferably set to 0.008%, more preferably 0.009%, and even more preferably 0.010%. On the other hand, excessive addition of Ti leads to coarsely grown nitrides, which act as initiation sites for brittle fracture, degrading toughness. It also promotes intergranular embrittlement after SR, degrading the toughness of the base material and HAZ. To suppress the formation of coarse nitrides, the upper limit of Ti is preferably set to 0.018%. The upper limit of Ti is more preferably set to 0.016%.

[0032] Cu: 0.05% to 1.50% If Cu is not added in excess, it improves the strength and toughness of the base material without adversely affecting the toughness of the heat-affected zone (HAZ). To achieve these effects, a content of 0.05% or more is required, but adding too much Cu impairs HAZ toughness and weldability, so the upper limit is set at 1.50%.

[0033] Ni: 0.05% to 2.00% Like Cu, Ni improves the strength and toughness of the base material without adversely affecting the toughness of the heat-affected zone if not added in excess. To achieve these effects, at least 0.05% must be included. In addition to being an expensive element, excessive content impairs HAZ toughness and weldability, so the upper limit for industrial production is set at 2.00%.

[0034] Mo: 0.05% to 0.50% Mo is an element that improves hardenability and increases the strength of the base material. In particular, in the present invention, it is an essential element from the viewpoint of ensuring the toughness of the base material and HAZ after SR. Generally, grain boundary segregation of P is promoted during SR treatment and the subsequent slow cooling. However, the presence of a certain amount of Mo in steel can suppress the diffusion and segregation of P. To achieve this effect, the addition of 0.05% or more is necessary. More preferably, the lower limit of the Mo content is 0.07%. However, if the Mo content exceeds 0.50%, a hard structure may form in the HAZ, reducing HAZ toughness. Therefore, the upper limit of the Mo content is set to 0.50%. The upper limit of the Mo content is preferably 0.45%, more preferably 0.40%.

[0035] Nb: 0.005% to 0.025% Nb is effective in improving the strength of the base material. To obtain this effect, the addition of 0.005% or more is necessary. However, excessive addition has a negative effect on HAZ toughness. It also promotes grain boundary embrittlement after SR, degrading the base material and HAZ toughness. Therefore, the upper limit is set to 0.025%. The upper limit is more preferably set to 0.020%. The upper limit is even more preferably set to 0.018%.

[0036] N: 0.0015~0.0060% N is an element that forms nitrides and is essential for obtaining the γ-grain pinning effect of nitrides, so the lower limit is set to 0.0015%. The lower limit is preferably 0.0018%, more preferably 0.0020%. On the other hand, a high N content makes it easier to form coarse nitrides such as AlN and TiN. These coarse particles can become the initiation point for brittle fracture and may result in a decrease in HAZ toughness. Therefore, the upper limit of the N content is set to 0.0060%. The preferred upper limit of the N content is 0.0055%, more preferably 0.0050%.

[0037] O: 0.0010 to 0.0045% O is an oxide-forming element and is one of the important elements for generating Ti-containing inclusions that become nuclei for intragranular transformation ferrite. In order to obtain dispersion of Ti-containing inclusions, the lower limit is set to 0.0010%. On the other hand, if the content is too high, coarse oxides are more likely to be generated. Since coarse oxides become the starting point for fracture and reduce HAZ toughness, the upper limit of the O content is set to 0.0045%. The upper limit of the O content is preferably 0.0040%, and more preferably 0.0035%.

[0038] P:0.015% or less P is an element that causes grain boundary embrittlement and is detrimental to toughness. In particular, in the present invention, segregation to grain boundaries is promoted during SR treatment and the subsequent slow cooling, making it a very detrimental element from the perspective of ensuring the toughness of the base material and HAZ after SR. Therefore, a low P content is desirable. In the present invention, the P content in the center segregation and microsegregation is reduced by homogenization heat treatment, making it possible to moderate the amount of P added to some extent. Since a P content exceeding 0.015% significantly reduces the toughness of the base material and HAZ after SR, the upper limit of the P content is set to 0.015%. The upper limit is preferably 0.013%, and more preferably 0.011%. While there is no particular need to set a lower limit for the P content, it is technically difficult to achieve a P content of 0%, so it may be set to more than 0%. The lower limit of the P content may be 0.001%.

[0039] S: 0.0050% or less S is an element that forms inclusions such as MnS. If coarse, elongated MnS forms in the center of the sheet thickness, it reduces toughness (in the HAZ and base material) and elongation in the sheet thickness direction. Therefore, the upper limit of the S content is set to 0.0050%. A preferred upper limit of the S content is 0.0040%. To improve HAZ toughness, the upper limit of the S content may be set to 0.0030% or 0.0025%. There is no need to particularly limit the lower limit of the S content, but since it is technically difficult to set the S content to 0%, it may be set to more than 0%. On the other hand, if the aim is to complexly precipitate MnS in the intragranular transformation nuclei and achieve more stable intragranular transformation, the lower limit of the S content is preferably 0.0005%. To generate a larger amount of MnS, the lower limit of the S content may be set to 0.0010%.

[0040] Al: 0 to 0.004% Al functions as a deoxidizer and reduces the amount of dissolved oxygen in molten steel. However, if Ti-containing inclusions contain a large amount of Al, the Ti oxides lose their function as ferrite nuclei for intragranular transformation, resulting in a deterioration in HAZ toughness. Therefore, the upper limit of the Al content is set to 0.004%. A preferred upper limit of the Al content is 0.003%. There is no particular need to set a lower limit for the Al content, and the lower limit may be set to 0%.

[0041] Mg: 0 to 0.0010% Mg is an element that functions as a deoxidizer and desulfurizer, reducing the amount of dissolved oxygen and sulfur in molten steel. However, as the amount of Mg oxide increases, the amount of Ti oxide, which is most effective in generating intragranular transformed ferrite, decreases, so it is preferable to minimize the generation of Mg oxide. Therefore, the upper limit of the Mg content is set to 0.0010%. Preferably, the upper limit of the Mg content may be set to 0.0005%. There is no need to particularly set a lower limit for the Mg content, and the lower limit may be set to 0%.

[0042] Ca: 0 to 0.0010% Ca functions as a deoxidizer and desulfurizer, and is an element that reduces the amount of dissolved oxygen and S in molten steel. However, if the Ti-containing inclusions contain a large amount of Ca oxides or Ca sulfides, the Ti oxides lose their function as ferrite nuclei for intragranular transformation, and the HAZ toughness deteriorates. Therefore, the upper limit of the Ca content is set to 0.0010%. Preferably, the upper limit of the Ca content may be set to 0.0005%. There is no need to particularly set a lower limit for the Ca content, and the lower limit may be set to 0%.

[0043] B: 0 to 0.0015% B is an element that significantly improves hardenability and improves the strength and toughness of the base material and HAZ, and B may be contained. However, adding a large amount of B can result in greater variations in strength, which can lead to unstable toughness. Therefore, the upper limit of the B content is set to 0.0015%. A preferred upper limit of the B content is 0.0013%, and a more preferred upper limit is 0.0010%. The lower limit of the B content may be 0%, but since it is technically not easy to achieve a B content of 0%, it may be set to more than 0%. To obtain the effect of increasing strength, the lower limit of the B content is preferably 0.0003%. More preferably, the lower limit of the B content is set to 0.0005%.

[0044] Ceq.≧0.440 The Ceq value calculated by the following formula (1) is an index showing the hardenability of steel components, and the higher the Ceq, the stronger the steel plate. To obtain a thick steel plate with a thickness of 60 to 100 mm and a YP of 500 MPa or more, Ceq should be 0.440 or more. Ceq=[C]+[Mn] / 6+Mo / 5+[Cu] / 15+[Ni] / 15 … Formula (1) In formula (1), [C] is the mass% of C, [Mn] is the mass% of Mn, [Mo] is the mass% of Mo, [Cu] is the mass% of Cu, and [Ni] is the mass% of Ni.

[0045] Furthermore, the steel plate according to this embodiment basically contains the above chemical components, but in order to improve the mechanical properties and HAZ toughness of the steel plate (base material), one or two of the following optional components may be contained in place of a portion of Fe: Cr: 0.50% or less and V: 0.03% or less, if necessary.

[0046] Cr:0.50% or less Cr is an element that increases the strength of the base metal by improving hardenability and precipitation strengthening, and Cr may be contained. However, if the Cr content exceeds 0.50%, MA is more likely to form in the HAZ, reducing HAZ toughness. Therefore, the upper limit of the Cr content is set to 0.50%. Preferably, the upper limit of the Cr content is set to 0.40%, and more preferably 0.30%. Cr may be mixed in as an impurity from scrap or the like during the production of molten steel, but there is no need to specifically set the lower limit, and it may be 0%. To improve the strength of the base metal, the lower limit of the Cr content is preferably 0.02%. More preferably, the lower limit of the Cr content is set to 0.10%.

[0047] V:0.03% or less V may be contained because it is an element that improves hardenability and forms carbides and nitrides, effectively increasing the strength of the base metal. However, if the V content exceeds 0.03%, the precipitation of carbonitrides in the HAZ becomes significant, which may reduce the HAZ toughness. Therefore, the V content is limited to 0.03% or less. Preferably, the V content is 0.025% or less. V may be mixed in as an impurity from scrap or the like during the production of molten steel, but there is no need to particularly limit its lower limit, and it may be 0%. In order to improve the strength of the base metal, the V content is preferably 0.01% or more.

[0048] When the high-strength steel plate of the present invention contains one or more of these as optional components, the Ceq value is calculated by the following formula (1)'. Ceq.=[C]+[Mn] / 6+[Cu] / 15+[Ni] / 15+[Cr] / 5+[Mo] / 5+[V] / 5 … Equation (1)' In formula (1)', [C] is the mass% of C, [Mn] is the mass% of Mn, [Cu] is the mass% of Cu, [Ni] is the mass% of Ni, [Cr] is the mass% of Cr, [Mo] is the mass% of Mo, and [V] is the mass% of V.

[0049] The remainder of the chemical composition of the steel sheet according to this embodiment is iron (Fe) and impurities. The impurities refer to components that are mixed in due to raw materials such as ore and scrap or other factors during industrial production of steel, and are acceptable within a range that does not adversely affect the steel according to this embodiment.

[0050] [Metal structure] If the ferrite fraction exceeds 15%, the base material will not be able to achieve a yield strength of 500 MPa. Therefore, the ferrite fraction in the quarter-thickness portion of the steel sheet is set to 15% or less, with the remainder consisting of bainite and / or martensite. If the ferrite fraction exceeds 15%, or if the ferrite fraction is 15% or less but the remainder is not a metallographic structure consisting of bainite and / or martensite, the base material will not be able to achieve a yield strength of 500 MPa. Therefore, the ferrite fraction in the quarter-thickness portion of the steel sheet is set to 15% or less by area, with the remainder consisting of one or more of bainite and martensite. The ferrite fraction can be measured by machining a metallographic observation sample from a cross section of the quarter-thickness portion of the steel sheet parallel to the rolling direction, and observing it under a 500x optical microscope.

[0051] [Average circular equivalent diameter ≦50μm] The finer the metal structure, the better the toughness of the base material. Because good base material toughness at low temperatures can be achieved if the average equivalent circle diameter of grains surrounded by high-angle grain boundaries with a misorientation of 15° or more in the center of the plate thickness (the t / 2 portion of the plate thickness) is 50 μm or less, the upper limit of the average equivalent circle diameter was set at 50 μm. The average equivalent circle diameter is the average of the equivalent circle diameters (diameters) of the top 10 largest grains in terms of area. Because large grain size is the cause of brittle fracture, the equivalent circle diameter of the top 10 largest grains in terms of area was specified. The measurement of the equivalent circle diameter of the metal structure does not take into account the structural classification, such as ferrite or bainite, but covers the entire observation field. The grain size can be determined, for example, by processing a metallographic specimen from a thickness-direction cross section parallel to the rolling direction, centered at the t / 2 portion of the plate thickness, and measuring it using Electro Back Scavenging Difraction (EBSD). The grain size can be determined by, for example, averaging the equivalent circle diameters of the top 10 largest grains in terms of area, which are bounded by boundaries with a misorientation of 15° or more.

[0052] [Maximum hardness at the center of the plate thickness HVmax] The maximum hardness HVmax at the center of the plate thickness must satisfy the following equation: -726.178[S]+315.7624≧HVmax (Equation (2)). To ensure base material strength, alloying elements are added to ensure hardenability, but toughness decreases. It has been discovered that base material toughness can be ensured by reducing stress concentration sources such as MnS, which are the starting point of fracture, and further reducing the hardness at the center of the plate thickness. If this relationship (Equation (2)) is not satisfied, the base material toughness at the center of the plate thickness decreases. Here, [S] is the mass percent of S.

[0053] [Ti-containing inclusions] The steel sheet according to this embodiment is assumed to be manufactured by a manufacturing method including deoxidation with Ti. The inventors conducted detailed research and studies on the relationship between HAZ structure and toughness. As a result, they found that improving HAZ toughness requires promoting ferrite transformation that occurs within prior austenite grains. To promote the formation of intragranular ferrite, it is effective to disperse inclusion particles that act as intragranular transformation nuclei, and as mentioned above, Ti oxides (TiO, Ti2O3) are desirable as oxide species. However, in actual manufacturing, even with Ti deoxidation, there are cases where a large amount of complex oxides containing elements such as Al, Mg, and Ca is contained, and it has been found that depending on the ratio of these elements, sufficient intragranular transformation does not occur.

[0054] In view of the above circumstances, the inventors have investigated the particle composition and microstructure of Ti-containing inclusions that serve as nuclei for intragranular transformation and confirmed that the probability of intragranular transformation varies depending on the composition of each Ti-containing inclusion. Furthermore, they have investigated conditions for optimizing the manufacturing conditions in the steelmaking process to generate particles of Ti-containing inclusions with a predetermined Ti content in steel so that the number density falls within a predetermined range and contribute to improving HAZ toughness.

[0055] In the steel material according to this embodiment, the formation of intragranular transformed ferrite is essential to ensure HAZ toughness. Therefore, the dispersion state of each particle of Ti-containing inclusions, including Ti oxides, is defined. In addition to Ti oxides, Ti-containing inclusions may contain one or more of Al oxides, Mg oxides, Si oxides, Ca oxides, Ca sulfides, and Mn sulfides. Furthermore, Ti-containing inclusions may contain trace amounts of impurity elements, such as Zr, Y, Hf, REM, Sn, Sb, Te, Se, Bi, and Pb, contained in steel. Figure 3 shows a schematic diagram of a Ti-containing inclusion (a composite inclusion containing Ti, Al, Mg, Ca, and Mn). Mn sulfides (MnS) are partially precipitated around the Ti-containing inclusion particles.

[0056] The inventors have found that the mass or mass ratio of Al oxide, Mg oxide, Ca oxide, Ca sulfide, etc. relative to Ti-containing inclusions significantly affects HAZ toughness. Ti oxides, which contain cation vacancies, contribute to the promotion of intragranular transformation ferrite formation by forming a Mn-depleted zone around Ti-containing inclusion particles during solidification and cooling. Al oxide, Mg oxide, Ca oxide, Ca sulfide, etc., form at a higher temperature than Ti oxide, and their presence in the Ti-containing inclusion particles is thought to inhibit the formation of the Mn-depleted zone. Therefore, the promotion of intragranular transformation ferrite formation is more effective with a higher proportion of Ti oxide in the Ti-containing inclusions, while the promotion is less effective with a higher proportion of other oxides, such as Al oxide, Mg oxide, Ca oxide, Ca sulfide, etc. Note that oxides such as Mn oxide and Si oxide, which form at lower temperatures than Ti oxide, are thought not necessarily to inhibit the formation of the Mn-depleted zone.

[0057] The contents of Ti, Al, Mg, and Ca contained in the particles of Ti-containing inclusions can be determined as the average values ​​by measuring EDS mapping (element mapping using an energy dispersive X-ray spectrometer) for the entire particle of the Ti-containing inclusion during cross-sectional observation using a scanning electron microscope (SEM).

[0058] The particle size (equivalent circle diameter) of Ti-containing inclusions effective for intragranular transformed ferrite is 0.5 to 5.0 μm. The lower limit was set at 0.5 μm because small Ti-containing inclusion particle sizes make it difficult to form intragranular transformed ferrite. Furthermore, the upper limit was set at 5.0 μm because coarse Ti-containing inclusion particles themselves can become the starting point for brittle fracture and reduce toughness. The particle size can be measured by taking a photograph of the Ti-containing inclusion particles using an SEM and then using image analysis to determine the equivalent circle diameter from the cross-sectional area.

[0059] Ti-containing inclusions can be observed by preparing a micro-sample from a steel material that has been heated to 1350-1400°C, held there for 3-30 seconds, and then rapidly cooled. This is because, for example, the presence of alloy carbonitrides makes it difficult to count the number of Ti-containing inclusion particles of 0.5 μm to 5.0 μm in size, which are the target of observation. A sample with fewer carbonitrides can be prepared by heating to a high temperature to dissolve precipitates other than the target of observation, followed by rapid cooling, or by applying a thermal cycle that generates ferrite during rapid cooling. Ti-containing inclusions, including Ti oxides, are stable even when heated to high temperatures and their morphology remains almost unchanged during cooling. Therefore, applying such a thermal cycle does not significantly affect the measurement results for the number of Ti-containing inclusion particles. Furthermore, Ti-containing inclusion particles can be observed either by revealing the structure using nital etching or by mirror polishing.

[0060] To determine the composition of Ti-containing inclusions, first exclude particles with low Ti content. To do this, EDS analysis of oxysulfides is performed to calculate [Ti] / ([Ti] + [Al] + [Mg] + [Ca] + [Mn] + [S] + [Si]) using the mass percentages of the main constituent elements Ti, Al, Mg, Ca, Mn, S, and Si. Ti-containing inclusions with a [Ti] / ([Ti] + [Al] + [Mg] + [Ca] + [Mn] + [S] + [Si]) ratio of 10% or more are considered Ti-containing inclusions, and those with a ratio of less than 10% are excluded. [Ti], [Al], [Mg], [Ca], [Mn], [S], and [Si] are the Ti, Al, Mg, Ca, Mn, S, and Si contents (mass%) obtained from EDS analysis of the inclusions, respectively. If no inclusions are present, a value of 0 is substituted.

[0061] Next, the proportion of Ti oxides contained in the Ti-containing inclusions (TCP) is calculated based on formula (3). TCP=[Ti] / ([Ti]+[Al]+[Mg]+[Ca]) … Formula (3)

[0062] The intragranular transformation potential of each particle of Ti-containing inclusions is then determined based on the Ti content ratio within the Ti-containing inclusions. By calculating the Ti content ratio (TCP) of each particle of Ti-containing inclusions based on the analysis values ​​of each particle by oxysulfide EDS, it is possible to determine whether the particle is a Ti-containing inclusion with a high probability of intragranular transformation occurring.

[0063] Specifically, when TCP is 40% or more, intragranular transformation is most likely to occur (Group A), when TCP is less than 40% and more than 20%, the intragranular transformation ability is somewhat poor (Group B), and when TCP is less than 20%, intragranular transformation is almost impossible. Here, TCP can be calculated using the following formula (3). TCP=[Ti] / ([Ti]+[Al]+[Mg]+[Ca]) … Formula (3) In the formula (3), [Ti], [Al], [Mg], and [Ca] are the Ti, Al, Mg, and Ca contents (mass%) obtained from EDS analysis of Ti-containing inclusions, respectively, and 0 is substituted if no Ti is contained.

[0064] Note that particles of Ti-containing inclusions may precipitate as complexes of sulfides and nitrides such as MnS and TiN. If these compounds are included in the composition calculation of Ti-containing inclusions, it will be impossible to correctly evaluate the oxide composition. Therefore, when calculating the contents of Ti, Al, Mg, and Ca from the results of the EDS mapping analysis described above, areas with relatively high S and N contents and where it is determined that sulfides and nitrides such as MnS and TiN are present are excluded.

[0065] Using the above-mentioned TCP, it is possible to estimate the degree to which intragranular transformation occurs in the structure. According to the study by the inventors, it has been found that the probability of intragranular transformation is approximately 80% for Ti-containing inclusions in group A, approximately 50% for Ti-containing inclusions in group B, and that intragranular transformation hardly occurs for other Ti-containing inclusions. Furthermore, even within the same group, intragranular transformation may or may not occur. This is thought to be because the likelihood of intragranular transformation occurring differs for each particle of Ti-containing inclusion depending on the composite morphology of the Ti-containing inclusions and the microsegregation of the steel material. Taking these intragranular transformation probabilities into consideration, it is assumed that the particle number density (EIGFD) of Ti-containing inclusions effective for intragranular transformation is 20 particles / mm 2 When the temperature is above this level, the effect of improving HAZ toughness can be obtained. Here, EIGFD can be calculated using the following equation (4). EIGFD=(XA×0.8)+(XB×0.5) … Formula (4) In equation (4), XA and XB are the particle number densities (particles / mm) of Ti-containing inclusions classified into Group A and Group B, respectively. 2 ) is the measured value.

[0066] The particle density XA and XB of each type of Ti-containing inclusion is measured in a continuous 1mm 2 It is desirable to observe and map the area and calculate the density. However, when the number of Ti-containing inclusion particles is large, it can be 100 or more, so it is a difficult task to map analyze all of the Ti-containing inclusion particles one by one. Therefore, it is sufficient to measure the particle composition of at least 30 or more Ti-containing inclusion particles with a circle equivalent diameter of 0.5 μm to 5.0 μm in a continuous measurement field and calculate the number density from their abundance ratio.

[0067] [Manufacturing method] Next, a method for manufacturing a steel material according to this embodiment will be described. [Smelting process] In order to control Ti-containing inclusions in steel, it is effective to control the melting process. Specifically, it is necessary to increase the mass% of Ti oxide in the Ti-containing inclusions and decrease the mass% of Al oxide, Mg oxide, Ca oxide, and Ca sulfide. Normally, molten steel is deoxidized with an Al-based deoxidizer, but in the manufacturing method of this embodiment, it is necessary to decrease the mass% of Al oxide, so it is preferable not to use an Al-based deoxidizer.

[0068] Furthermore, in the refining process using RH, which is a secondary refining facility, in order to ensure the necessary molten steel temperature, the temperature of the molten steel may be increased by adding metallic Al to the molten steel and by a heat-raising reaction caused by blowing oxygen onto the molten steel. However, in the production method of this embodiment, since it is necessary to reduce the mass % of Al oxide, the heat-raising reaction is not carried out in the refining process.

[0069] Therefore, in the manufacturing method of this embodiment, it is necessary to ensure in advance a molten steel temperature at which a heat-up treatment is not required. The required molten steel temperature varies depending on the type of steel, refining equipment and process, casting conditions, etc., but in the manufacturing method of this embodiment, it is preferable to maintain the molten steel temperature at 1590°C or higher, a general temperature at which a heat-up treatment is not required.

[0070] Furthermore, preventing the inclusion of Al, Mg, and Ca as impurities during the melting process can suppress the formation of Al oxides, Mg oxides, Ca oxides, and Ca sulfides in Ti-containing inclusions. While some contamination is unavoidable due to the presence of Al, Mg, and Ca in furnace materials, flux, and slag used in melting processes, it is necessary to minimize the presence of Al, Mg, and Ca as impurity elements in the additive materials used to adjust the molten steel composition. While the concentrations of the Al, Mg, and Ca impurities in the additive materials cannot be clearly defined because they depend on the target addition amounts, the upper limits should be adjusted so that the molten steel sample after composition adjustment at RH is 0.0040% Al, 0.0010% Mg, and 0.0010% Ca. More preferably, the upper limits should be 0.0035% Al, 0.0005% Mg, and 0.0005% Ca.

[0071] After the composition adjustment at RH, the Al oxides, Mg oxides, and Ca oxides present in the molten steel can also be removed by refluxing the molten steel. By ensuring a reflux time of 3 minutes or more after the composition adjustment at RH described above, the proportions of Al oxides, Mg oxides, Ca oxides, and Ca sulfides in the Ti-containing inclusions can be reduced. As described above, the molten steel sample after the composition adjustment at RH and reflux treatment should have Al: 0.0040% or less, Mg: 0.0010% or less, and Ca: 0.0010% or less. More preferably, the Al: 0.0035% or less, Mg: 0.0005% or less, and Ca: 0.0005% or less.

[0072] The heating, rolling, and heat treatment conditions after casting may be appropriately selected depending on the target mechanical properties of the steel material, such as controlled rolling and controlled cooling, quenching and tempering directly after rolling, or quenching and tempering after cooling once after rolling. For example, a billet of steel having the same composition as the steel plate of the present invention is reheated at 950°C to 1200°C, rough rolled to an average reduction of 7.5% or more per pass, and reduced to a thickness of 135 to 210 mm. After that, finish rolling is performed from 670°C to 800°C to an average reduction of 6.0% or more per pass, and the center of the plate thickness is cooled at a cooling rate of 10°C / s or less. A typical example of the manufacturing process is shown below.

[0073] [Silver heating temperature: 950℃ or higher and 1200℃ or lower] During hot rolling, if the billet heating temperature is below 950°C, coarse inclusions formed during solidification that adversely affect toughness may not dissolve in the matrix and remain. Furthermore, a high rolling load may result in insufficient reduction depending on the capacity of the rolling mill, resulting in center porosity and internal defects, or insufficient diffusion of C elements, increasing the hardness at the thickness center, thereby reducing the toughness at the thickness center. Therefore, the billet heating temperature is set to 950°C or higher, preferably 980°C or higher. On the other hand, if the billet heating temperature exceeds 1200°C, Ti nitrides coarsen, deteriorating the toughness of the steel plate and making it impossible to expect improvements in the toughness of the weld heat-affected zone. Furthermore, coarsening of the initial austenite grains increases the hardenability of the steel, increasing the hardness at the thickness center and deteriorating the toughness at the thickness center. Considering the suppression of grain coarsening, a temperature of 1150°C or lower is more preferable.

[0074] [Rough rolling: reduction rate per pass (average 7.5% or more), thickness 135-210mm] Rough rolling immediately after heating is typically performed at approximately 900°C to 1200°C. This process crushes center porosity, which is a defect within the slab, and recrystallizes and refines the heated γ through rolling. If the reduction rate per pass is less than 7.5% on average, the reduction in the t / 2 portion will be insufficient, resulting in residual center porosity and insufficient γ recrystallization, making it impossible to ensure toughness. Similarly, to fully obtain the effects of the rough rolling, the upper limit of the rough rolling end thickness is set to 210 mm. On the other hand, if the rough rolling end thickness is less than 135 mm, the reduction amount in the subsequent finish rolling process cannot be ensured, so the lower limit is set to 135 mm. Note that the upper limit of the rough rolling end thickness is, for example, approximately 20% from the viewpoint of equipment capacity, etc.

[0075] [Finishing rolling temperature range: 670°C to 800°C, reduction rate per pass (average 6.0% or more)] Finish rolling after rough rolling involves cooling the steel to the non-recrystallization temperature range before rolling. This process introduces processing strain into the gamma grains, refining the ferrite transformation structure, and ensuring toughness. It also serves to finish the steel to a final thickness of 60 to 100 mm. Final finish rolling at temperatures lower than 670°C generates a large amount of processed ferrite, degrading the toughness of the base material. Furthermore, the ferrite fraction increases, potentially resulting in unsatisfactory strength. On the other hand, rolling at temperatures higher than 800°C does not adequately refine the final structure, degrading the toughness of the base material. Furthermore, the increased hardenability of the steel increases the hardness of the central segregation region, potentially worsening the toughness of the center of the plate thickness. Furthermore, if the average reduction rate per pass in finish rolling is less than 6.0%, the processing strain cannot be effectively introduced into the t / 2 portion, resulting in insufficient refinement of the metal structure. Therefore, the average lower limit of the reduction rate per pass is set to 6.0% or more. The upper limit of the reduction rate per pass in finish rolling is, for example, about 15% from the viewpoint of equipment capacity and the like.

[0076] [Cooling rate after rolling: 10°C / s or less] Cooling after rolling is carried out at a rate of 10°C / s or less in the thickness t / 2 portion because if the cooling rate exceeds 10°C / s, it becomes difficult to control the temperature during cooling. Also, if the cooling rate is too fast, the hardness at the center of the thickness increases, which may deteriorate the toughness at the center of the thickness. However, to make it easier to ensure the strength of the base material, a cooling rate of 1°C / s or more is preferred.

[0077] In the manufacturing method of the present invention, the cooled steel sheet may be heated to a temperature range of 300° C. or higher and 670° C. or lower to be subjected to heat treatment (SR treatment) in order to improve the properties of the steel sheet.

[0078] [Tempering temperature range: 300℃ or higher and 670℃ or lower] After the cooling, the steel plate may be heat-treated (SR treatment) to improve the strength-toughness balance of the base material. To improve the strength balance by heat treatment, heat treatment must be performed at 300°C to 670°C. If the temperature is lower than 300°C, a sufficient tempering effect cannot be obtained. Furthermore, if the temperature is higher than 670°C, coarsening of carbonitrides occurs, resulting in a decrease in strength and toughness. [Example]

[0079] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0080] Example 1 [Test steel] Steel plates with various compositions were manufactured through the converter-continuous casting-plate rolling process, and their properties were investigated. Molten steel at 1590°C or higher with 53 different compositions shown in Tables 1-1 and 1-2 was continuously cast into 240mm or 300mm thick continuous cast pieces.

[0081] [Smelting process] The Al, Mg, and Ca contents of the component analysis samples taken after RH for each slab are shown in Tables 2-1 and 2-2.

[0082] [Hot rolling] Hot-rolled steel plates were produced from the continuous cast slabs under the conditions of heating temperature, rolling (average reduction in rough rolling, transfer thickness, average reduction in finish rolling, final temperature), cooling rate, and heat treatment temperature shown in Tables 2-1 and 2-2. Plate thicknesses were 60 to 100 mm. Some of the steel plates were subjected to heat treatment (SR treatment) after water cooling. The laboratory-rolled steel plates produced in this way were used as test steels. The rolling performance of each steel plate is shown in Table 2. The properties were investigated before and after SR (Stress Relief) treatment. The SR conditions (heat treatment temperature) are also shown in Tables 2-1 and 2-2.

[0083] [Steel plate strength evaluation] The strength of the steel sheets before SR treatment was evaluated by taking JIS No. 4 tensile test pieces from a quarter section of the steel sheet in the thickness direction, in the direction perpendicular to the rolling direction. Steel sheets with a yield strength of 500 MPa or more and a tensile strength of 570 MPa or more were considered to have excellent base material strength. After SR treatment, JIS No. 4 tensile test pieces were taken from a quarter section of the steel sheet in the thickness direction, in the direction perpendicular to the rolling direction, in the same way as before SR treatment, and tested. Steel sheets with a yield strength of 500 MPa or more and a tensile strength of 570 MPa or more, and a decrease in tensile strength due to SR treatment, ΔTS, of -20 MPa or less, were considered to have excellent base material strength after SR treatment.

[0084] The toughness of the steel plates before SR treatment was evaluated by taking 10mm x 10mm full-size V-notch Charpy test specimens from the 1 / 2t section of the steel plate in the thickness direction, perpendicular to the rolling direction, and testing three specimens at -40°C. The average absorbed energy at -40°C was evaluated, and a value of 100J or more was considered to have good base metal toughness. After SR treatment, as before SR treatment, full-size V-notch Charpy test specimens, 10mm x 10mm, were taken from the 1 / 2t section of the steel plate in the thickness direction, perpendicular to the rolling direction, and testing three specimens was conducted at -40°C. Steel plates with an average absorbed energy of 100J or more at -40°C and a decrease in the ductile-brittle transition temperature (vTrs) due to SR treatment (ΔvTrs) of 15°C or less were considered to have excellent base metal toughness after SR treatment.

[0085] A CTOD test was conducted to measure joint toughness in accordance with the BS7448 standard. Welding was performed using multi-layer submerged arc welding with a heat input of 3.5 kJ / mm. The straight side of the square groove was used as the notch introduction position for the CTOD test, and a three-point bending test was conducted at -10°C. Steel plates with a CTOD value of 0.50 or more at -10°C were considered to have good joint toughness. After SR treatment, a three-point bending test was conducted at -10°C, with the straight side of the square groove used as the notch introduction position for the CTOD test, just as before SR treatment. Steel plates with a CTOD value of 0.50 or more at -10°C were considered to have good joint toughness after SR.

[0086] [Evaluation of steel sheet structure, etc.] The ferrite fraction was measured using a 500x optical microscope. A cross-section of the steel plate, parallel to the rolling direction, was machined for metallographic observation at a thickness of 4 / 4 of the plate thickness. Under optical microscope observation, ferrite appears as a massive structure, observed as a white contrast. The grains exhibit a generally uniform contrast, with little cementite, observed as black dots, or lath or processed structures, observed as black lines. However, even within ferrite grains, a few black dots or lines may be present. Furthermore, ferrite grain boundaries exhibit clear, smooth curves with a clear black contrast, obscuring prior austenite grain boundaries. However, depending on the misorientation between crystal grains, the ferrite grain boundaries may be unclear. The remainder of the specimen is a complex phase consisting of one or more of bainite and martensite. The microstructure fraction is generally calculated visually, with the ferrite portion marked on the micrograph and then binarized.

[0087] The grain size was determined by processing a metallographic observation sample from a cross section in the thickness direction parallel to the rolling direction, centered on the t / 2 portion of the steel plate, and measuring it by EBSD (Electro Back Scatering Difraction) over a 500 μm × 500 μm field of view at 1 μm intervals. The grain size was calculated as the average of the 10 largest circle-equivalent diameters in terms of area defined by boundaries with a crystal orientation misorientation of 15° or more.

[0088] To measure the hardness at the center of the plate thickness, a metallographic observation sample was cut from a cross section parallel to the rolling direction, centered at the t / 2 portion of the plate thickness. Five randomly selected fields were photographed at the t / 2 portion using an optical microscope at 500x magnification, and the hardness at each of two points was measured using a Vickers test with a load of 25g. The maximum hardness of the 10 measurement points was taken as the maximum hardness at the center of the plate thickness (HVmax).

[0089] The hardness of the HAZ was measured by processing a sample for metal structure observation from a cross section perpendicular to the welding direction, and measuring the maximum hardness using a Vickers test with a load of 10 kg at 1 mm intervals from the weld metal across the HAZ to the base material at the center of the plate thickness (plate thickness t / 2 part) and the plate thickness t / 4 part.The maximum hardness was taken as the maximum hardness of the HAZ.

[0090] [Analysis of composition, size and number of Ti-containing inclusions] Thermal cycle test specimens (12 mm × 12 mm × 120 mm) were taken from the quarter-thickness portion of the test steel and subjected to a simulated welding thermal cycle test (hold at 1400°C for 3 seconds, then cool to 200°C at 20°C / s. Hold at 200°C for 3 seconds, then heat to 720°C at 20°C / s. Hold at 720°C for 3 seconds, then cool to 200°C at 12°C / s. Hold at 200°C for 3 seconds, then heat to 500°C at 20°C / s. Hold at 500°C for 3 seconds, then cool to 20°C at 12°C / s). These specimens were used for inclusion investigation. Elemental analysis of Ti-containing inclusions 0.5 μm or larger, which were suitable for particle analysis, was performed within a 1.0 mm × 1.0 mm observation field in the soaking zone of the simulated thermal cycle test specimens using SEM-EDS. Table 4 shows the EIGFD calculated by classifying the Ti-containing inclusions in the observed field of view according to the composition classification described above.

[0091] Tables 1-1 and 1-2 show the steel compositions of the developed and comparative steels, Tables 2-1 and 2-2 show the manufacturing conditions for the steel plates, Tables 3-1 and 3-2 show the mechanical properties of the steel plates, and Table 4 shows the analysis results for Ti-containing inclusions in the steel plates. All of the steel plates Nos. 1 to 20, which are examples of the present invention, have base material strengths of 500 MPa or more in yield strength and 570 MPa or more in tensile strength, vE-40 of 100 J or more, the change in vTRs before and after SR, ΔvTrs, within 15°C, and a CTOD test value at -10°C of 0.50 or more.

[0092] [Table 1-1]

[0093] [Table 1-2]

[0094] [Table 2-1]

[0095] [Table 2-2]

[0096] [Table 3-1]

[0097] [Table 3-2]

[0098] [Table 4]

[0099] On the other hand, the 33 steel plates of the comparative examples using slabs Nos. 21 to 53 had either or both of the chemical composition and the manufacturing method outside the scope of the present invention, and one or more of the base material strength, base material toughness, and joint CTOD before or after SR were not achieved.

[0100] The composition ranges of slabs Nos. 21 to 40 are outside the range of the present invention.

[0101] Slab No. 21 has a high C content, which increases the hardness of the quenched structure. As a result, the maximum hardness HVmax at the center of the plate thickness does not satisfy formula (2), and the toughness of the joint is inferior. The slab No. 22 has a high Si content, which increases the amount of MA produced and refines the grain boundary carbides, resulting in inferior joint toughness. The high Mn content of slab No. 23 results in a high Mn concentration in the central segregation area, and the maximum hardness HVmax at the center of the plate thickness does not satisfy formula (2), so the toughness of the base material and joint after SR is not ensured. Slab No. 24 has a low Mn content, which reduces hardenability and results in poor strength. In addition, upper bainite structures with poor toughness are formed, resulting in poor joint toughness.

[0102] Slab No. 25 has a low Ti content, and therefore has a low proportion of Ti2O3, which acts as a nucleus for the formation of intragranular transformed ferrite, and does not satisfy the required EIGFD. Slab No. 26 has a high Ti content, which may have resulted in the formation of coarse inclusions that cause brittle cracks, and the toughness of the base material and HAZ after SR is poor.

[0103] Since the slab No. 27 has a high Cu and Ni content, the Ni and Cu concentrations in the central segregation area are high, and the maximum hardness HVmax in the center of the plate thickness does not satisfy formula (2), so the toughness of the joint after SR is not ensured.

[0104] Slab No. 28 has a high Mo content, which results in excessive hardenability, particularly in excessive hardness at the center segregation area, and does not satisfy the base material and joint toughness before and after SR. In slab No. 29, the Mo content is low, so the effect of Mo in suppressing P segregation is not obtained, and the base material and joint toughness before and after SR are not satisfied.

[0105] Since the slab No. 30 has a high Nb content, the Nb concentration in the central segregation area is high, and the maximum hardness HVmax at the center of the plate thickness does not satisfy formula (2), and the toughness of the joint after SR is not ensured. The slab No. 31 has a high N content, which causes many coarse nitrides to form in the microstructure, especially near the grain boundaries, and the toughness of the joint after SR cannot be ensured. Slab No. 32 has a high O content, which causes the oxide inclusions in the steel to become coarse, resulting in poor toughness of the base material and joint.

[0106] The slab No. 33 has a high P content, which increases the amount of segregated P at the prior γ grain boundaries and ferrite grain boundaries, making it more susceptible to grain boundary embrittlement, and therefore the toughness of the base material and joint after SR cannot be ensured. Slab No. 34 has a high S content, which causes the sulfide inclusions in the steel to become coarse, resulting in poor joint toughness.

[0107] Slabs Nos. 35 to 37 have high Al, Mg, and Ca contents, and therefore have low proportions of Ti2O3, which acts as nuclei for the formation of intragranular transformed ferrite, and do not satisfy the required EIGFD. Slab No. 38 has a high B content, which results in excessive hardenability and the formation of a large amount of hard structure. As a result, the maximum hardness HVmax at the center of the plate thickness does not satisfy formula (2), and the toughness of the joint is inferior.

[0108] The slab No. 39 has a high Cr content, which results in excessive hardenability and the formation of a large amount of hard structure, which makes it difficult to ensure the toughness of the joint after SR. The slab No. 40 has a high V content, which results in excessive hardenability and the formation of a large amount of hard structure, which makes it difficult to ensure the toughness of the joint after SR.

[0109] Slab No. 41 had Al heated during the melting process, and the proportion of Ti2O3, which acts as a nucleus for the formation of intragranular transformed ferrite, was low, so the required EIGFD was not met.

[0110] Slab No. 42 is insufficient in strength because the heating temperature is low and undissolved elements such as Nb and B remain. The heating temperature of slab No. 43 was too high and was outside the range of the present invention.

[0111] In the case of slab No. 44, the average reduction rate per pass in rough rolling was low, and the effect of forming a fine grain structure by introducing transformation nuclei in the unrecrystallized region rolling was not fully achieved, resulting in inferior toughness of the base material after SR.

[0112] The transfer thickness of slab No. 45 was too large, and the effect of forming a fine grain structure by refining the pre-transformation austenite grains during recrystallization region rolling was not fully achieved, resulting in inferior toughness of the base material after SR. In the case of slab No. 46, the transfer thickness was too small, and the effect of forming a fine grain structure by introducing transformation nuclei during rolling in the unrecrystallized region was not fully achieved. As a result, the maximum hardness HVmax at the center of the plate thickness did not satisfy formula (2), and the toughness of the base material after SR was inferior.

[0113] In slab No. 47, the average reduction rate per pass in finish rolling was low, and the effect of forming a fine grain structure by introducing transformation nuclei during rolling in the unrecrystallized region was not fully achieved. As a result, the maximum hardness HVmax at the center of the plate thickness did not satisfy equation (2), and the toughness of the base material after SR was inferior.

[0114] The final finish rolling temperature of slab No. 48 was too high and was outside the range of the present invention. In slab No. 49, the cooling rate was too high, resulting in the formation of an excessive amount of hard structure. As a result, the maximum hardness HVmax at the center of the plate thickness did not satisfy formula (2), and the toughness of the base material after SR was inferior. In the case of slab No. 50, the heat treatment temperature after cooling was high, which resulted in excessive aggregation and coarsening of fine carbides due to tempering, and the toughness of the base material after SR was poor. [Industrial Applicability]

[0115] As described above, the present invention makes it possible to manufacture high-strength thick steel plates with thicknesses of 60 mm or more that have excellent CTOD characteristics in the HAZ of multi-pass welds produced with low to medium heat input welding, a base metal yield strength of 500 MPa or more, and good base metal toughness before and after SR treatment. This makes it possible to increase the size and weight of steel structures used in extremely harsh environments, such as offshore structures, and to reduce costs by reducing the amount of steel used. Therefore, the present invention has a high degree of industrial applicability.

Claims

1. The plate thickness is 60 to 100 mm, the yield strength is 500 MPa or more, and the tensile strength is 570 MPa or more; In mass%, C: 0.020-0.120%, Si: 0.05-0.30%, Mn: 0.80-2.00%, Ti: 0.005-0.018%, Cu: 0.05-1.50%, Ni: 0.05-2.00%, Mo: 0.05% to 0.50%, Nb: 0.005-0.025%, N: 0.0015-0.0060%, O: 0.0010 to 0.0045%, Contains P: 0.015% or less, S: 0.0050% or less, Al: 0 to 0.004%, Mg: 0 to 0.0010%, Ca: 0-0.0010%, B: 0 to 0.0015%, and The Ceq. value calculated by the following formula (1) is 0.440 ≦Ceq., with the balance being Fe and impurities, The ferrite fraction in the quarter-thickness portion of the plate is 0 to 15% by area, and the remainder is a multi-phase structure consisting of one or more of bainite and martensite, The average circle equivalent diameter of the top 10 crystal grains in terms of the largest area in the region surrounded by a high-angle grain boundary with a crystal orientation difference of 15° in the center of the plate thickness is 50 μm or less, The maximum hardness HVmax at the center of the plate thickness satisfies formula (2), Furthermore, for particles of Ti-containing inclusions contained in the steel structure, which contain Ti and one or more of Al, Mg, Si, Ca, and Mn and have an equivalent circle diameter of 0.5 μm or more and 5.0 μm or less, the Ti content ratio (TCP) of each Ti-containing inclusion particle is calculated using formula (3) based on the mass ratio of elements measured by EDS, and the particles are classified into group A where the TCP is 40% or more and group B where the TCP is less than 40% and 20% or more. When the number density (EIGFD) of the particles of Ti-containing inclusions effective for intragranular transformation shown in formula (4) is 20 particles / mm ２ A high-strength thick steel plate characterized by the above. Ceq=[C]+[Mn] / 6+Mo / 5+[Cu] / 15+[Ni] / 15... Formula (1) -726[S]+315≧HVmax... Formula (2) TCP=[Ti] / ([Ti]+[Al]+[Mg]+[Ca])...Equation (3) EIGFD=(XA×0.8)+(XB×0.5) … Formula (4) In the formula (1), [C] is the mass % of C, [Mn] is the mass % of Mn, [Mo] is the mass % of Mo, [Cu] is the mass % of Cu, and [Ni] is the mass % of Ni. In the formula (2), [S] is the mass % of S. In the formula (3), [Ti], [Al], [Mg], and [Ca] are the contents (mass%) of Ti, Al, Mg, and Ca obtained from EDS analysis of Ti-containing inclusions, and 0 is substituted when no Ti-containing inclusions are contained. In the formula (4), XA and XB are the density of inclusions (numbers / mm) classified into Group A and Group B, respectively. ２ ) is the measured value.

2. Furthermore, in mass%, Cr: 0.50% or less, V: 0.03% or less, Contains one or two of the following: The high-strength thick steel plate according to claim 1, wherein the Ceq value is calculated by the following formula (1)' instead of the formula (1): Ceq. =[C]+[Mn] / 6+[Cu] / 15+[Ni] / 15+[Cr] / 5+[Mo] / 5+[V] / 5... Formula (1)' In the formula (1)', [C] is the mass% of C, [Mn] is the mass% of Mn, [Cu] is the mass% of Cu, [Ni] is the mass% of Ni, [Cr] is the mass% of Cr, [Mo] is the mass% of Mo, and [V] is the mass% of V.

3. A steel slab having the component composition according to claim 1 or 2 and produced by a continuous casting method is reheated at 950°C to 1200°C, subjected to rough rolling so that the average reduction rate per pass is 7.5% or more, and the thickness is reduced to 135 to 210 mm. After that, the steel slab is finish rolled from 670°C to 800°C so that the average reduction rate per pass is 6.0% or more, and then cooled at a cooling rate of 10°C / s or less at the center of the plate thickness. The ferrite fraction in the quarter-thickness portion of the plate is 0 to 15% by area, and the remainder is a multi-phase structure consisting of one or more of bainite and martensite, The average circle equivalent diameter of the top 10 crystal grains in terms of the largest area in the region surrounded by a high-angle grain boundary with a crystal orientation difference of 15° in the center of the plate thickness is 50 μm or less, The maximum hardness HVmax at the center of the plate thickness satisfies formula (2), Furthermore, for particles of Ti-containing inclusions contained in the steel structure, which contain Ti and one or more of Al, Mg, Si, Ca, and Mn and have an equivalent circle diameter of 0.5 μm or more and 5.0 μm or less, the Ti content ratio (TCP) of each Ti-containing inclusion particle is calculated using formula (3) based on the mass ratio of elements measured by EDS, and the particles are classified into group A having a TCP of 40% or more and group B having a TCP of less than 40% and 20% or more, whereby the number density of particles of Ti-containing inclusions effective for intragranular transformation (EIGFD) shown in formula (4) is 20 particles / mm2 or more. -726[S]+315≧HVmax... Formula (2) TCP=[Ti] / ([Ti]+[Al]+[Mg]+[Ca])...Equation (3) EIGFD=(XA×0.8)+(XB×0.5) … Formula (4) In the formula (1), [C] is the mass % of C, [Mn] is the mass % of Mn, [Mo] is the mass % of Mo, [Cu] is the mass % of Cu, and [Ni] is the mass % of Ni. In the formula (2), [S] is the mass % of S. In the formula (3), [Ti], [Al], [Mg], and [Ca] are the contents (mass%) of Ti, Al, Mg, and Ca obtained from EDS analysis of Ti-containing inclusions, and 0 is substituted when no Ti-containing inclusions are contained. In the formula (4), XA and XB are measured values ​​of the number density (numbers / mm 2 ) of inclusions classified into Group A and Group B, respectively.

4. 4. The method for manufacturing a high-strength thick steel plate according to claim 3, wherein the plate is cooled at a cooling rate of 10°C / s or less at the center of the plate thickness, and then heat-treated at a temperature of 300°C or more and 670°C or less.

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