JP5171327B2 - Steel plate for skin plate excellent in thickness direction toughness of heat-affected zone with large heat input and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel plate applied to a skin plate of a box column (also referred to as a four-sided box column, a welded assembled box-shaped cross-section column, etc.) used as a building steel frame of a high-rise building structure, for example, electroslag welding or corner submerging. The present invention relates to a steel sheet excellent in sheet thickness direction toughness of a part (hereinafter sometimes referred to as “HAZ”) that is affected by heat when large heat input welding with a heat input of 400 kJ / cm or more, such as arc welding.

  Columns that are applied to high-rise building structures are welded to the long side corners of four long steel plates (skin plates) in a box column shape, and where the beams are to be welded. It is manufactured by welding a diaphragm in a column to ensure strength. When performing these weldings, welding in one pass is desired in order to improve construction efficiency and shorten construction time. Therefore, in the corner welding that is the welding between the skin plates, a large heat input submerged arc welding is performed with a heat input of about 400 to 600 kJ / cm, and in the welding between the skin plate and the diaphragm, the heat input is about 400 to 1200 kJ / cm. High heat input electroslag welding has been performed. However, if high heat input welding is performed, the toughness of the welded portion deteriorates due to the coarsening of the metal structure of the weld heat affected zone (HAZ). In order to improve the earthquake resistance, it is rather desirable to improve the toughness of the weld. In particular, high heat input welding with a heat input of 400 kJ / cm or more, unlike the electrogas welding performed in some building structures, has a high heat input and a low cooling rate, and thus the toughness is more likely to deteriorate.

  Conventional metallographic studies to improve toughness in heat-affected zone of high heat input welding, such as 1) research using TiN, 2) research using oxide, 3) research using sulfide, etc. It is made from.

1) In research using TiN, for example, Patent Document 1 proposes a HAZ toughness improvement technique using TiN and VN. In Patent Document 2, the amount ratio of Ti and N [Ti] / [
N] ratios of 3.5 to 5.0 have been proposed in which the Ti amount is increased compared to the N amount. However, near the interface (fusion line) between the weld metal and the weld heat affected zone (HAZ), the temperature exceeds 1400 ° C. In particular, since welding in architecture is performed with ultra-high heat input welding, the residence time at high temperatures becomes longer. The longer the residence time at high temperature, the more the TiN particles are dissolved, and the HAZ toughness improving effect is reduced. Further, in order to prevent such dissolution of TiN particles, if the Ti amount is increased and the TiN particles are enlarged, the toughness deteriorates due to the coarsening of the TiN particles.

  2) As research utilizing oxides, for example, Patent Documents 3, 4 and 5 propose techniques utilizing oxide particles added with Mg. However, it is difficult to uniformly disperse fine oxides, and further improvements are required for industrially stable production.

3) As a research utilizing sulfide, a technique for dispersing sulfide, oxide and sulfate particles by adding REM (for example, Patent Documents 6, 7, and 8); for utilizing CaS particles In addition, a technique for controlling the contents of Ca, S, and O within the range of a formulated parameter called ACR (for example, Patent Documents 9 and 10) has been proposed. However, these technologies that utilize sulfides require a certain amount of S, so that the sulfides become coarse and the toughness deteriorates, or the toughness in the plate thickness direction deteriorates due to MnS inclusions.
Japanese Patent Laid-Open No. 5-186848 JP 2002-266050 A JP 2000-80436 A JP 2000-80437 A JP 2003-293077 A JP 2004-176100 A JP 2004-10951 A JP 2003-286540 A JP 2005-220379 A JP 2005-68478 A

  By the way, during an earthquake, the building structure is deformed, and tensile stress acts on the welded joint in the thickness direction of the column material. Although the steel plate is strong against stress acting in the plate surface (direction parallel to the plate surface), the strength against the stress in the plate thickness direction, which is the direction perpendicular to the plate surface, decreases. Therefore, it is desirable to consider the toughness in the thickness direction in order to improve the earthquake resistance of the building structure. The above-described prior art does not always satisfy the high toughness stably, and does not consider the toughness in the thickness direction. In order to evaluate the toughness in the plate thickness direction, a Charpy test piece with the same direction as the longitudinal direction must be collected, but from a normal butt welded steel plate, the test piece with the plate thickness direction as the longitudinal direction Cannot be collected.

  This invention is made | formed in view of the above situations, The objective is to improve the earthquake resistance of the box column used as a steel frame of a building structure. More specifically, in the heat affected zone (HAZ) of the sub-merged arc welding of the box column corner that is welded between the skin plates and the electroslag welding of the skin plate and the diaphragm, the plate thickness direction (Z Direction) to improve toughness.

The steel plate for skin plates according to the present invention, which has solved the above-mentioned problems and is excellent in the toughness in the thickness direction of the high heat input welding heat-affected zone, is C: 0.02 to 0.10% (meaning mass%; hereinafter, The same), Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, P: 0.015% or less (excluding 0%), S: 0.0010% or less (0 %), Al: 0.01 to 0.05%, Cu: 0.05 to 1.5%, Ni: 0.05 to 1.5%, Ti: 0.003 to 0.02%, B: 0.0005-0.0030%, Ca: 0.0015-0.0030%, N: 0.0040-0.008%, O: 0.0005-0.0030%, the balance being iron And inevitable impurities, wherein the content (mass%) of Ti, B, N satisfies the following formulas (1) to (3), inclusions or equivalent circle diameter 5μm containing a is at 5 / mm ² or less, MnS based inclusions above length 50μm is at 2 / cm ² or less, the C concentration of the center segregation area of the steel sheet, The main point is that the average C concentration of the whole steel sheet is 1.2 times or less.
(1) 1.0 ≦ [Ti] / [N] ≦ 3.0
(2) 0.0003 ≦ [N] − [Ti] /3.4≦0.0035
(3) −0.0005 ≦ [B] − {([N] − [Ti] /3.4) × 11/14} ≦ 0.0015
(However, [Ti], [N], and [B] represent the contents (mass%) of Ti, N, and B, respectively.)
The steel plate for skin plate of the present invention may further contain Cr: 0.05 to 1.5% and / or V: 0.005 to 0.05%.

  In the steel plate for skin plate of the present invention, a steel slab satisfying the above component composition and satisfying the above formulas (1) to (3) is heated to 1) 950 to 1250 ° C., and 2) the rolling finishing temperature is 800. After rolling to ˜900 ° C., it can be produced by 3) air cooling for 30 seconds or more, and 4) cooling to a temperature of 300 ° C. or less at a cooling rate of 1 to 100 ° C./s.

  In the manufacturing method, 4) after cooling at 300 ° C. or lower, 5) reheated to 450 to 600 ° C. and air cooled. In addition, between 4) cooling of 300 ° C. or less and 5) reheating and air cooling of 450 to 600 ° C., 700 to 850 ° C. followed by 200 ° C. at a cooling rate of 1 ° C./s or more. The following cooling may be performed.

  In the steel plate for skin plate of the present invention, the amount of each element is controlled, and the interrelation between the contents of Ti, B, and N is appropriately controlled to finely disperse nitrides such as TiN and BN, and the coarseness Suppresses Ca-containing inclusions and expanded MnS inclusions harmful to sheet thickness direction toughness, and further suppresses center segregation of C, so super-high heat input welding such as submerged arc welding and electroslag welding is performed. Even in this case, the toughness in the plate thickness direction (Z direction) at the weld heat affected zone (HAZ) can be ensured.

  Usually, in butt electrogas welded joints between steel plates and steel plates and butt submerged arc welded joints made by shipbuilders, etc., HAZ toughness in the rolling direction (L direction) or perpendicular to the rolling direction (C direction, width direction) We are evaluating. However, in a butt-welded joint, a test piece having the plate thickness direction (Z direction) as the longitudinal direction cannot be collected, and the HAZ toughness in the plate thickness direction (Z direction) cannot be evaluated. In addition, even in a reproducible thermal cycle test that simulates the thermal history of large heat input welding that is normally performed, the HAZ toughness in the Z direction was evaluated only by taking the specimen in the L direction or the C direction. There was nothing.

  The rolled steel sheet exhibits different states in the rolling direction (L direction), the direction perpendicular to the rolling direction (C direction), and the sheet thickness direction (Z direction). For example, there are inclusions expanded by rolling. When there is a metal structure that has undergone rolling history, the HAZ toughness in the plate thickness direction (Z direction) deteriorates compared to the toughness in the L direction or C direction, so the toughness in the plate thickness direction (Z direction). In order to improve the quality, it is necessary to control inclusions and the structure.

  Therefore, the present inventor conducted research focusing on improving the high heat input HAZ toughness not only in the L direction and the C direction but also in the plate thickness direction (Z direction). From the viewpoint of microstructure control, we have found that it is possible to ensure the toughness in the plate thickness direction at the weld heat affected zone of high heat input welding by controlling the composition of the component at a high level and (ii) directly controlling the inclusions and the structure state. It was.

(I) Component control from the viewpoint of controlling inclusions and structures From the viewpoint of controlling inclusions and structures, (i-1) Ti is controlled to control the precipitation state of TiN and BN and the amount of dissolved B. (I-2) Inclusions containing Ca, MnS inclusions that reduce the toughness in the thickness direction by stretching (A inclusions) to control the amount of N and B It is important to control components such as controlling the amount of sulfur and S, and (i-3) controlling the amount of C in order to suppress MA (martensite austenite constituent). It is also important not to add Nb that is harmful to HAZ toughness and to ensure strength by adding Cu, Ni, and B.

  Since (i-1) control of Ti amount, N amount, and B amount is particularly important, it will be described in more detail below.

In the present invention, in order to control the precipitation state of TiN, BN, etc. and the amount of dissolved B, the amount of Ti, the amount of N, and the amount of B are controlled so as to satisfy all the relationships of the following formulas (1) to (3). .
(1) 1.0 ≦ [Ti] / [N] ≦ 3.0
(2) 0.0003 ≦ [N] − [Ti] /3.4≦0.0035
(3) −0.0005 ≦ [B] − {([N] − [Ti] /3.4) × 11/14} ≦ 0.0015
(However, [Ti], [N], and [B] represent the contents (mass%) of Ti, N, and B, respectively.)

  The reason for determining the relationships (1) to (3) will be described with reference to FIGS. 1 to 3, the basic components are 0.04% C-0.15% Si-1.35% Mn-0.008% P-0.001% S-0.030% Al-0.6% Cu As shown in Table 1, the HAZ toughness was evaluated for steel sheets with -0.6% Ni-0.6% Cr-0.0020% Ca-0.0015% O and varying amounts of Ti, N, and B as shown in Table 1. The relationship with Ti-N-B component control is organized. Below, the manufacturing method of the steel plate used for FIGS. 1-3 and the evaluation procedure of HAZ toughness are demonstrated.

  First, the manufacturing method of the steel plate used for FIGS. 1-3 is demonstrated. Steel whose components were adjusted to the amounts shown in Table 1 was continuously cast to obtain a slab having a thickness of 280 mm (deoxidation was performed by Al deoxidation and RH degassing in a converter). During continuous casting, the reduction roll gap (distance between the upper roll and the lower roll) was reduced near the solidification completion position. Next, after the slab was reheated to 1200 ° C., it was reduced to a breakdown slab having a thickness of 230 mm and cooled to room temperature. Further, the breakdown slab was rolled to a plate thickness of 60 mm at a heating temperature of 1100 ° C. and a finishing temperature of 850 ° C., and then accelerated and cooled at a cooling rate of 9 ° C./s.

Next, the HAZ toughness evaluation procedure will be described. First, a heat cycle test piece was produced from the steel sheet produced as described above. Next, a reproducible welding heat cycle test was carried out in order to reproduce the structure of the most embrittled region in the vicinity of the weld line corresponding to the heat input 800 kJ / cm of electroslag welding. In this reproduction welding heat cycle test, the heat cycle test piece was held at 1400 ° C. for 30 seconds and then cooled at a cooling rate such that the cooling time from 800 to 500 ° C. was 730 seconds. A V-notch standard test piece (JIS Z 2242) whose longitudinal direction is the thickness direction (Z direction) is taken from the thermal cycle test piece, and a Charpy impact test (JIS Z 2242; impact blade radius 2 mm) at a test temperature of 0 ° C. And the absorbed energy vE ₀ was determined.

FIG. 1 is a graph showing the relationship between “[Ti] / [N]” and vE ₀ , and FIG. 2 is a graph showing the relationship between “[N] − [Ti] /3.4” and vE ₀ . FIG. 3 is a graph showing the relationship between “[B] − {([N] − [Ti] /3.4) × 11/14}” and vE ₀ .

(1) 1.0 ≦ [Ti] / [N] ≦ 3.0
Based on FIG. 1, “[Ti] / [N]” was set to 1.0 or more and 3.0 or less. “[Ti] / [N]” is related to the dispersion state of TiN particles. TiN particles have an effect of preventing coarsening of austenite particles and an effect of promoting transformation from within the austenite grains during the cooling process, and these actions improve the HAZ toughness. As the TiN particles are finely dispersed, the HAZ toughness is improved. By setting [Ti] / [N] to 1.0 or more, fine TiN particles can be increased, and HAZ toughness is improved. On the other hand, if “[Ti] / [N]” exceeds 3.0, the TiN particles become coarse, the number of TiN particles decreases, and the HAZ toughness rapidly decreases. The preferable lower limit of “[Ti] / [N]” is 1.5 (particularly 2.0), and the preferable upper limit is 2.9 (particularly 2.8).

(2) 0.0003 ≦ [N] − [Ti] /3.4≦0.0035
Based on FIG. 2, “[N] − [Ti] /3.4” was determined to be 0.0003 or more and 0.0035 or less.

FIG. 2 shows the relationship between “[N] − [Ti] /3.4” and vE ₀ . “[Ti] /3.4” indicates the amount of N used for TiN when Ti and N are combined in a stoichiometric ratio. Therefore, “[N] − [Ti] /3.4” means a value obtained by subtracting the amount of N used for TiN from the total amount of N, that is, the amount of free N remaining after TiN is generated. N has a strong bonding force with Ti, and TiN is preferentially generated during casting. In the cooling process after being heated to a high temperature during welding, free N (N remaining after TiN is generated) is combined with B and precipitated as BN. In other words, “[N] − [Ti] /3.4” can be said to mean the amount of residual N that can effectively act on the precipitation of BN. BN, like TiN, promotes transformation from within the austenite grains and improves HAZ toughness, if appropriately dispersed.

  When the value of “[N] − [Ti] /3.4” is small, the amount of free N is small, so BN is insufficient and the HAZ toughness is low. When the value of “[N] − [Ti] /3.4” is 0.0003 or more, BN is appropriately dispersed, and the HAZ toughness is rapidly improved. On the other hand, when “[N] − [Ti] /3.4” exceeds 0.0035, the free N becomes excessive, the free N that cannot be fixed as BN increases, and the HAZ toughness deteriorates. The preferable lower limit of “[N]-[Ti] /3.4” is 0.0005 (particularly 0.0008), and the preferable upper limit is 0.0025 (particularly 0.0020).

(3) −0.0005 ≦ [B] − {([N] − [Ti] /3.4) × 11/14} ≦ 0.0015
Based on FIG. 3, “[B] − {([N] − [Ti] /3.4) × 11/14}” (hereinafter sometimes referred to as BK value) is −0.0005 or more, 0. 0015 or less. The BK value is obtained by subtracting the amount of B precipitated as BN from the total amount of B, and means the amount of dissolved B remaining after BN is precipitated. However, the BK value does not represent the absolute value of the solid solution B amount, but relatively represents the magnitude of the solid solution B amount. As the amount of solute B (BK value) increases, the HAZ toughness improves and the strength of the base material also improves. Therefore, the BK value is set to -0.0005 or more. On the other hand, if the amount of solute B (BK value) is too large, transformation promotion from within the austenite grains is inhibited. When the BK value exceeds 0.0015, the toughness deteriorates rapidly, so the BK value was set to 0.0015 or less. The preferable lower limit of the BK value is -0.0003 (particularly 0.0000), and the preferable upper limit is 0.0012 (particularly 0.0010).

  (I-2) Ca amount, S amount control, (i-3) C amount control, and addition of Cu, Ni, B, etc. will be described later.

(Ii) Direct control of inclusions and microstructures From the viewpoint of improving the HAZ toughness in the thickness direction, (ii-1) reducing coarse Ca-containing inclusions and (ii-2) expanded MnS It is important to reduce system inclusions and (ii-3) reduce center segregation.

(Ii-1) Reduction of coarse Ca-containing inclusions Ca-containing inclusions having a large particle diameter, particularly Ca-containing inclusions having a particle diameter (equivalent circle diameter) of 5 μm or more adversely affect HAZ toughness. Therefore, in the present invention, inclusions having an equivalent circle diameter of 5 μm or more are 5 pieces / mm ² or less, preferably 4 pieces / mm ² or less, more preferably 3 pieces / mm ² or less. The lower limit of the number of inclusions is not particularly limited, but may be, for example, about 0.1 / mm ² (particularly about 0.5 / mm ² ).

  Inclusions containing Ca to be prevented from coarsening include all inclusions containing Ca, and inclusions that are particularly likely to be coarsened (and therefore should be controlled intensively) include sulfides such as CaS. In addition to physical inclusions, composite inclusions with oxides, composite inclusions with nitrides, and the like can be given.

  In order to reduce coarse Ca-containing inclusions, it is effective to control the degassing time as well as the steel material components (particularly Ca content and S content) and the oxygen content.

  In addition, the number of coarse Ca containing inclusions is based on the observation result of the longitudinal cross-section of a steel plate. That is, the number of coarse Ca-containing inclusions can be obtained by observing the cross section with EPMA or FE-SEM, extracting inclusions containing Ca, and measuring the particle diameter (equivalent circle diameter).

(Ii-2) Reduction of expanded MnS inclusions When MnS inclusions (A inclusions) expanded by rolling, especially MnS inclusions with a length of 50 μm or more, are present, The interface with the base iron peels off, and that part becomes the starting point of fracture occurrence, which deteriorates the toughness in the thickness direction. In order to prevent this, the number of MnS inclusions having a length of 50 μm or more is 2 pieces / cm ² or less, preferably 1 piece / cm ² or less, more preferably 0.5 pieces / cm ² or less.

  It is well known that lamellar tearing occurs during welding if expanded MnS inclusions (A inclusions) are present (for example, “New Steel for Building Structures”, Steel Structure Publishing, p. 88- 92). However, from the viewpoint of improving the HAZ toughness in the sheet thickness direction, the influence of MnS inclusions has not been clarified quantitatively, and has been clarified for the first time in the present invention.

  In order to reduce the expanded MnS inclusions, it is effective to control the steel material components (particularly the Ca content and the S content).

  The number of expanded MnS-based inclusions is determined by measuring the longitudinal section of the steel sheet with an optical microscope (magnification is about 100 times, for example).

(Ii-3) Reduction of center segregation If the chemical composition of the steel sheet is center segregated, when the influence of welding heat reaches the vicinity of the center of the plate thickness, A structure | tissue changes greatly and many hard phases, such as an island-like martensite (MA), a martensite, and a bainite, produce | generate on a surface parallel to a plate surface. When a stress is applied in the plate thickness direction, fracture occurs from this MA, so that the HAZ toughness in the plate thickness direction is greatly deteriorated. In order to prevent the deterioration of the HAZ toughness, it is necessary to reduce the center segregation. The degree of segregation can be evaluated by the degree of segregation of C. When the ratio of (C concentration in the central segregation part) / (average C concentration in the entire steel sheet) exceeds 1.2, the production of MA increases and the HAZ toughness Deteriorates. Therefore, the upper limit of the ratio of (C concentration of central segregation part) / (average C concentration of the entire steel sheet) was set to 1.2. A preferable range of the ratio of (C concentration of central segregation part) / (average C concentration of the entire steel sheet) is 1.1 or less.

  As with the expanded MnS inclusions (A inclusions), it is well known that lamellar tearing occurs during welding even when center segregation occurs (the above-mentioned “new steels for building structures”, steel structures). Publishing, p. 88-92). However, from the viewpoint of improving the HAZ toughness in the plate thickness direction, the influence of center segregation has not been clarified quantitatively, and has been clarified for the first time in the present invention.

  The degree of center segregation can be changed by the casting temperature of continuous casting, casting roll gap control, reheating breakdown of the casting slab, and the like. More specifically, the smaller the difference between the casting temperature and the solidification temperature of the mold, the smaller the center segregation. As for the gap between the casting rolls, the reduction roll gap (the distance between the upper roll and the lower roll) near the solidification completion position of the molten steel. ) Can be reduced and the center segregation can be reduced. Moreover, the center segregation component is diffused by reheating the casting slab and aging at a high temperature. Further, by performing breakdown rolling, the solidification gap at the center can be crimped and the center segregation can be reduced.

  The C concentration in the center segregation part varies depending on the measurement method. Therefore, in order to perform quantitative control, it is necessary to unify measurement methods. For example, according to EPMA, since an extremely small area is analyzed, the analysis value tends to vary. Therefore, in the present invention, the C concentration in the central segregation part is measured by the following method.

  A longitudinal section of the steel sheet is cut out, macro-etched to reveal the center segregation part, the steel material is shaved at a specific thickness from the concentrated part, and the C concentration of the center segregation part is determined by chemical analysis of the chips. . The machined thickness is set to plate thickness × 0.02. For example, if the plate thickness is 100 mm, the machined thickness is 100 × 0.02 = 2 mm. For machining, it is convenient to use a drill having the same diameter as the machined thickness. However, when the cutting thickness is determined in relation to the plate thickness, a tool with an appropriate drill diameter may not be available depending on the thickness, and if the drill diameter is changed for each plate thickness, the work efficiency decreases, It may be difficult to cut out. Therefore, in the case of a steel plate having a plate thickness of 50 to 100 mm, the machined thickness (drill diameter) may be set to 1 to 1.2 mm. Alternatively, the center segregation portion may be cut so as to come out on the surface, and the surface may be subjected to emission analysis (cantback) to determine the C concentration of the center segregation portion.

  As described above, the present invention has a great feature in that (i-1) the amount of components between Ti and B-N is controlled, or (ii) the inclusions and the tissue state are directly controlled. It is also important to control (i-2) Ca content, S content, and (i-3) C content, Cu content, Ni content, B content. It is also important to design. The component composition of the steel sheet of the present invention and the reasons for limitation are as follows.

[C: 0.02-0.10%]
C is an element necessary for ensuring the strength of the steel sheet. If the C content is less than 0.02%, the strength cannot be secured. On the other hand, if the amount of C exceeds 0.10%, a hard structure called island martensite (MA) is likely to occur in the weld heat affected zone (HAZ) during high heat input welding, and the HAZ toughness deteriorates. Therefore, the C content is determined to be 0.02 to 0.10%. The preferable lower limit of the amount of C is 0.03%, and the preferable upper limit is 0.09%.

[Si: 0.05-0.5%]
Si is an element necessary for deoxidation. When the amount of Si is less than 0.05%, the effect of deoxidation cannot be exhibited effectively. On the other hand, if the Si content exceeds 0.5%, the weldability deteriorates. Therefore, the Si amount is set to 0.05 to 0.5%. The preferable lower limit of the amount of Si is 0.1%, and the preferable upper limit is 0.4%.

[Mn: 1.0 to 2.0%]
Mn is an element effective for ensuring the strength of the steel sheet and improving toughness. If the amount of Mn is less than 1.0%, the strength and toughness of the steel sheet cannot be ensured. On the other hand, when the amount of Mn exceeds 2.0%, weldability deteriorates. Therefore, the amount of Mn is set to 1.0 to 2.0%. A preferable lower limit of the amount of Mn is 1.2%, and a preferable upper limit is 1.8%.

[P: 0.015% or less (excluding 0%)]
P is an element that is inevitably mixed in as an impurity element, but if the amount of P exceeds 0.015%, the toughness deteriorates, so the upper limit was made 0.015%. The upper limit with preferable P amount is 0.013%.

[S: 0.0010% or less (excluding 0%)]
S is an element inevitably mixed as an impurity element. S becomes an MnS-based or CaS-based inclusion, which deteriorates the performance of the base material in the thickness direction, and these inclusions serve as a starting point for occurrence of fracture and deteriorate the HAZ toughness. MnS-based inclusions are present in a stretched state at various positions in the sheet thickness center and sheet thickness direction after rolling. When high heat input welding is performed, the interface between the expanded MnS inclusions and the steel plate of the steel sheet is peeled off due to the influence of the welding heat and becomes a starting point of occurrence of fracture, which degrades the HAZ toughness in the thickness direction. In addition, the expanded MnS-based inclusion is likely to coexist with the central segregation portion, and the HAZ toughness is further deteriorated when adjacent to the island-like martensite (MA). When the amount of S becomes excessive, the aforementioned MnS-based and CaS-based inclusions become coarse and the number of inclusions also increases, so that the number of occurrence points of fracture increases and the HAZ toughness deteriorates. Therefore, the upper limit of the amount of S is set to 0.0010%. A preferable upper limit of the amount of S is 0.009%.

[Al: 0.01 to 0.05%]
Al is an element that acts as a deoxidizer. If the Al content is less than 0.01%, the effect of deoxidation is not exhibited effectively. On the other hand, if the Al content exceeds 0.05%, the toughness is deteriorated. Therefore, the Al content is determined to be 0.01 to 0.05%. A preferable lower limit of the amount of Al is 0.02%, and a preferable upper limit is 0.04%.

[Cu: 0.05 to 1.5%]
Cu is an element effective in increasing the strength and has a small deterioration in HAZ toughness. If the amount of Cu is less than 0.05%, the strength cannot be ensured. On the other hand, if the amount of Cu exceeds 1.5%, weldability deteriorates. Therefore, the amount of Cu is set to 0.05 to 1.5%. The preferable lower limit of the amount of Cu is 0.2%, and the preferable upper limit is 1%.

[Ni: 0.05 to 1.5%]
Ni is an element that is effective in increasing the strength and has little deterioration in HAZ toughness. If the Ni content is less than 0.05%, the strength cannot be secured. On the other hand, if the Ni content exceeds 1.5%, weldability deteriorates. Therefore, the amount of Ni is set to 0.05 to 1.5%. The preferable lower limit of the amount of Ni is 0.2%, and the preferable upper limit is 1.3%.

[Ti: 0.003 to 0.02%]
Ti is an extremely effective element for improving the HAZ toughness. By dispersing a large amount of TiN finely, when welding heat is applied to near the melting point, the austenite grain size is prevented by the pinning effect, and during cooling, it acts as a ferrite / bainite nucleation site in the austenite grains. , To refine the HAZ structure. If the amount of Ti is less than 0.003%, it is not possible to sufficiently secure the number of TiNs for exhibiting such effects. On the other hand, if the Ti content exceeds 0.02%, TiN becomes coarse and the number of TiNs decreases. Therefore, the amount of Ti is set to 0.003 to 0.02%. The preferable lower limit of the amount of Ti is 0.005%, and the preferable upper limit is 0.018%.

[B: 0.0005 to 0.0030%]
B is an element that is effective for increasing the strength in a small amount and is effective for improving the HAZ toughness in the same manner as Ti. During cooling after rolling, solid solution segregates at the austenite grain boundaries to improve hardenability and improve strength. In the weld heat affected zone, TiN particles do not reprecipitate once they melt at a high temperature of 1400 ° C. or higher near the melting line and the number of particles decreases, whereas B precipitates as BN in the cooling process after welding heating. It works as a ferrite / bainite nucleation site in austenite grains and refines the HAZ structure. In order to exert such an effect, the lower limit of the B amount is set to 0.0005%. On the other hand, when the content exceeds 0.0030%, the solid solution amount of B becomes excessive, and the weldability and the HAZ toughness deteriorate, so the upper limit of the B amount is set to 0.0030%. A preferable lower limit of the amount of B is 0.001%, and a preferable upper limit is 0.0025%.

[Ca: 0.0015 to 0.0030%]
Ca is an element that has the effect of reducing the length by changing the length of MnS-based inclusions into single inclusions of CaS or composite inclusions with CaS, shortening the length, and contributing to the improvement of the plate thickness direction characteristics. It is. Furthermore, Ca forms complex inclusions with TiN, becomes fine inclusions stable at high temperatures, and improves HAZ toughness. In order to obtain this effect, the lower limit of the Ca content is 0.0015%. On the other hand, if the Ca content exceeds 0.0030%, inclusions containing Ca are coarsened, the number increases, and the HAZ toughness deteriorates. Therefore, the upper limit of the Ca content is set to 0.0030%. A preferable lower limit of the Ca content is 0.0017%, and a preferable upper limit is 0.0029%.

[N: 0.0040 to 0.008%]
N is an element that becomes nitride particles of TiN or BN and is effective in improving the HAZ toughness. If the N content is less than 0.0040%, the effect of improving the HAZ toughness cannot be exhibited. On the other hand, if the N content exceeds 0.008%, the solid solution N becomes excessive and the HAZ toughness deteriorates. Therefore, the N amount is set to 0.0040 to 0.008%. The preferable lower limit of the N amount is 0.0045%, and the preferable upper limit is 0.007%.

[O: 0.0005 to 0.0030%]
When O is added excessively, the alumina inclusions increase, the oxide containing Ca becomes coarse, and the number increases, so that the HAZ toughness is deteriorated. Therefore, the upper limit of the O amount is set to 0.0030%. A preferable upper limit of the amount of O is 0.0025%. Further, O inevitably remains in the steel making process, so the lower limit was made 0.0005%.

  The composition of the steel sheet of the present invention is as described above, and the balance is iron and inevitable impurities.

  In the present invention, as other elements, Cr: 0.05 to 1.5% and / or V: 0.005 to 0.05% may be included. Hereinafter, these components will be described.

[Cr: 0.05-1.5%]
Cr is an element effective for increasing the strength. In order to exert such effects, it is recommended that the Cr content is preferably 0.05% or more, more preferably 0.5% or more. On the other hand, if the amount of Cr is excessive, the weldability deteriorates, so it is preferably 1.5% or less, more preferably 1% or less.

[V: 0.005 to 0.05%]
V is an element effective for improving strength and toughness. In order to exert such effects, it is recommended that the V amount is preferably 0.005% or more, more preferably 0.01% or more. On the other hand, if the amount of V is excessive, the HAZ toughness deteriorates, so it is preferably 0.05% or less, and more preferably 0.04% or less.

  In the steel sheet of the present invention, a steel slab that satisfies the requirements of the above formulas (1) to (3) for Ti, N, and B is heated to 950 to 1250 ° C., and the rolling finishing temperature is 800 to 900. It can manufacture by air-cooling for 30 seconds or more after rolling so that it may become 0 degreeC, and cooling to the temperature of 300 degrees C or less after that with a cooling rate of 1-100 degrees C / s.

[Heating temperature during rolling: 950 to 1250 ° C.]
The heating temperature at the time of rolling the steel slab needs to be 950 ° C. or higher because it is necessary to uniformly transform the steel slab into an austenite structure. On the other hand, if the heating temperature is too high, the austenite crystal grain size becomes coarse and the toughness deteriorates, so the upper limit was set to 1250 ° C. The minimum with a preferable heating temperature is 1000 degreeC, and a preferable upper limit is 1200 degreeC.

[Rolling finishing temperature: 800-900 ° C]
The rolling finishing temperature is desirably a temperature at which strain (deformation band) can be introduced into the austenite grains in the non-recrystallized region in order to ensure toughness, and the upper limit is set to 900 ° C. On the other hand, if the rolling finishing temperature becomes too low, the acoustic anisotropy in the ultrasonic flaw detection test becomes large and the inspection efficiency of the welded part is adversely affected. The minimum with a preferable rolling finishing temperature is 820 degreeC, and a preferable upper limit is 870 degreeC.

[Air cooling for 30 seconds after rolling]
It is necessary to air-cool for 30 seconds or more before the start of cooling (water cooling or the like) of accelerated cooling (direct quenching (DQ)) after completion of rolling. By this air cooling, the surface temperature of the steel sheet can be made uniform to reduce material variation in the steel sheet, and the cooling start temperature can be lowered to reduce the yield ratio (YR). A preferable air cooling time is 60 seconds or more.

[Cooling rate: 1 to 100 ° C./s]
Since the strength increases as the cooling rate after rolling increases, the cooling rate is set to 1 ° C./s or more. On the other hand, if the cooling rate is too fast, the structure becomes hard and the toughness deteriorates, so the upper limit is set to 100 ° C./s. The preferable lower limit of the cooling rate is 3 ° C./s, and the preferable upper limit is 30 ° C./s (particularly 15 ° C./s).

[Cooling stop temperature: 300 ℃ or less]
Since the lower the cooling stop temperature is, the lower the temperature is and the higher the strength, the lower the cooling stop temperature. A preferable cooling stop temperature is 200 ° C. or lower.

[Tempered (T)]
In the production method of the present invention, the steel plate accelerated and cooled (directly quenched) as described above may be tempered offline, for example. Tempering can soften the hard tissue and further improve toughness.

  The tempering conditions are preferably reheating temperature: 450 to 600 ° C. and cooling method: air cooling. If the reheating temperature is less than 450 ° C, the softening of the hard tissue is not sufficient. On the other hand, if it exceeds 600 ° C, the tensile strength (TS) may be lower than the standard strength. In comparison, the yield strength (YS) decreases little and the yield ratio (YR) increases.

[Quenching from two-phase area (Q ')]
Quenching (Q ′) from the ferrite-austenite two-phase region may be performed between the accelerated cooling (direct quenching) and tempering. Quenching (Q ′) from the two-phase region is a particularly effective method for reducing YR.

The reheating temperature is Ac ₁ point or higher (for example, 700 ° C. or higher) and Ac ₃ point or lower (for example, 850 ° C. or lower). By reheating the steel sheet to a two-phase region (Ac ₁ point or more and Ac ₃ point or less), a part of the steel sheet structure becomes austenite, and the rest softens or ferrites. Next, by quenching, the austenite becomes a hard phase, the hard phase and the ferrite phase can be appropriately balanced, and the YR can be lowered. A preferable reheating temperature is 700 ° C. or higher and 850 ° C. or lower. Preferred quenching conditions are a cooling rate of 1 ° C./s or more and a cooling end temperature of 200 ° C. or less. A steel sheet that has been subjected to two-phase quenching is usually tempered in the same manner as described above, because the toughness deteriorates as it is quenched.

  In addition, in order to reduce the inclusion shape control and center segregation, the above manufacturing method may further include control of the RH degassing treatment time, control of the casting temperature, control of the casting roll gap, and reheating breakdown. is important.

  The control of the treatment time for RH degassing is effective for controlling the size of oxide inclusions, and the preferred treatment time is about 20 to 30 minutes.

  Control of casting temperature, control of casting roll gap, and reheating breakdown are effective in controlling center segregation. As for the casting temperature, the smaller the difference (ΔT) between the casting temperature into the mold and the solidification temperature, the more the center segregation can be reduced, and the preferable ΔT range is about 15 to 25 ° C. With respect to the casting roll gap, central segregation can be reduced by reducing the rolling roll gap (the distance between the upper roll and the lower roll) in the vicinity of the solidification completion position of the molten steel. The degree of narrowing of the rolling roll gap is “small” when the rolling roll is gradually narrowed over the entire length of the rolling roll (that is, when the rolling gradient is small), and narrowing down immediately when solidification is completed (that is, the rolling gradient is reduced). It is preferable to narrow down by “large” or “standard” where “large” is “large” and the middle of these is “standard”. In particular, in an extremely thick steel plate (plate thickness of 80 mm or more), center segregation can be reduced by reheating the cast slab, performing diffusion soaking, and performing breakdown rolling. The preferable heating temperature of breakdown rolling is about 1200 ° C., and the preferable rolling reduction is about 10 to 20%.

  The steel sheet of the present invention is excellent in HAZ toughness and tensile strength (TS) in the thickness direction when high heat input welding is performed, and has a low yield ratio (YR). Therefore, when the steel plate of the present invention is used for a skin plate of a box column used as a building steel frame, the earthquake resistance of the box column can be significantly improved.

  The plate | board thickness of the steel plate of this invention is about 30-100 mm, for example, Preferably it is about 40-80 mm. The tensile strength is, for example, about 490 to 740 MPa. The yield ratio (YR) is, for example, about 65 to 80%.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  After the steel having the composition shown in Table 2 was melted in a converter, it was deoxidized by the method shown in Table 3, continuously casted under the conditions shown in Table 3, and then the cast slab was reheated under the conditions shown in Table 3. Then, breakdown rolling was performed to an intermediate slab thickness, and then hot rolling and accelerated cooling were performed to a predetermined plate thickness under the conditions shown in Table 3. Some steel plates were tempered as they were or after two-phase quenching.

  About the steel plate manufactured as mentioned above, it measured in the following way.

[Measurement of Ca-containing inclusions]
FE-SEM was used for the measurement of Ca-containing inclusions. An arbitrary measurement region (about 300 μm × 300 μm) at a t / 4 position (t: plate thickness) in the longitudinal section of the steel plate is measured at a magnification of 1000 times, and inclusions containing Ca are extracted, and the equivalent grain size is 5 μm. The number of inclusions was counted. Measurement was performed for 10 fields of view, and the number of inclusions obtained (total for 10 fields of view) was converted to the number per 1 mm ² .

[Measurement of MnS inclusions]
An optical microscope was used to measure MnS inclusions. An arbitrary measurement region (about 15 mm × 15 mm) at the t / 4 position and the t / 2 position (t: plate thickness) of the longitudinal section of the steel sheet is observed at a magnification of 100 times, and MnS-based inclusions having a length of 50 μm or more are observed. The number was counted. The measurement was performed for each of the t / 4 position and t / 2 position (t: plate thickness) of the longitudinal cross section of the steel sheet for 5 fields, and a total of 10 fields, and the number of MnS-based inclusions obtained (total for 10 fields) ) Was converted to the number per cm ² .

[Measurement of C concentration in center segregation part]
The steel sheet was crossed, a sample was cut out from the center in the width direction, and macro-etched to reveal a segregated portion. Cutting was performed with a drill having a drill diameter of 1 to 1.2 mm, and the chips were chemically analyzed by wet analysis. And (C density | concentration of a center segregation part) / (average C density | concentration of the whole steel plate) was calculated | required by calculation, and it was set as the center segregation degree.

[Measurement of HAZ toughness]
In the toughness evaluation of the welded joint, the steel plate obtained in the above experimental example was regarded as a skin plate. As shown in FIG. 4, a diaphragm having a thickness of 50 mm and a skin plate were arranged in a T shape, and these were electroslag welded under the conditions shown below. A Charpy standard impact test piece (JIS Z 2242) having a longitudinal direction in the thickness direction of the skin plate was collected from the connection portion between the skin plate and the weld metal on an extension of ½ of the diaphragm plate thickness. A V-notch whose longitudinal direction is the welding direction was placed on the weld fusion line of the test piece, and a Charpy impact test was performed at a test temperature of 0 ° C. in accordance with JIS Z 2242. Absorbed energy (vE ₀ ) was measured for three test pieces, and the average value was obtained. A welded joint having an absorbed energy (vE ₀ ) of 70 J or more was regarded as acceptable.

(Electroslag welding conditions)
Heat input: 850 kJ / cm
Welding current: 380A
Welding voltage: 52V
Welding speed: 14mm / min

[Measurement of tensile strength]
Sample No. 4 of JIS Z 2201 was taken from t (plate thickness) / 4 part of the steel sheet, and the tensile strength (TS) and yield strength (YS) were measured according to JIS Z 2241 to obtain the yield ratio (YR). It was. The test speed during the tensile test was 10 N / mm ² · sec. A tensile strength (TS) of 490 MPa or more is acceptable, and a yield ratio (YR) of 80% or less is acceptable.

  These results are shown in Table 4.

  Steel plate No. in Table 4 1, 7 to 10 and 12 to 14 are produced by the production method of the present invention using steels A to E satisfying the composition of the present invention and the relational expressions (1) to (3) of Ti, B, and N. In order to satisfy the requirements of the present invention in terms of the number of Ca-containing inclusions, the number of MnS inclusions, and the C concentration of the central segregation part, it exhibits good sheet thickness direction HAZ toughness, and the base material properties (tensile strength , Yield ratio) is also good.

  Steel plate No. in Table 4 No. 2 has a low tensile temperature of the base metal because the heating temperature during rolling is low, and the steel plate No. 2 3 is an example in which the tensile strength of the base material is low because the rolling finishing temperature is low. Steel plate No. In Nos. 4 and 11, since the air cooling time from the end of rolling to the cooling is short, the cooling start temperature is high and the YR of the base material is high. Steel plate No. No. 5 is air cooled, not water cooled after rolling, so the cooling rate is slow and the tensile strength of the base material is low. Steel plate No. Since No. 6 has a high cooling stop temperature, the tensile strength of the base material is low.

  Steel plate No. 15 and 16 are examples in which the Z-direction welded joint toughness is low because the degree of center segregation (C concentration in the center segregation portion / average C concentration in the entire steel sheet) is high. Steel plate No. Since 17-19 have many MnS type inclusions with a long length, the weld joint toughness of a Z direction is low. Steel plate No. 19 and 24 are low in weld joint toughness in the Z direction because there are many coarse Ca-containing inclusions. Steel plate No. In Nos. 20 to 23 and 25 to 27, the balance of Ti, N, and B (the above formulas (1) to (3)) is not within the range defined by the present invention, and therefore the weld joint toughness in the Z direction is low.

It is a graph which shows the relationship between "[Ti] / [N]" and HAZ toughness. It is a graph which shows the relationship between "[N]-[Ti] /3.4" and HAZ toughness. It is a graph which shows the relationship between "[B]-{([N]-[Ti] /3.4) * 11/14}" and HAZ toughness. It is the schematic sectional drawing which showed typically the collection method of the welding method and a Charpy test piece.

Claims (5)

C: 0.02 to 0.10% (meaning mass%, hereinafter the same),
Si: 0.05 to 0.5%,
Mn: 1.0-2.0%,
P: 0.015% or less (excluding 0%),
S: 0.0010% or less (excluding 0%),
Al: 0.01 to 0.05%,
Cu: 0.05 to 1.5%,
Ni: 0.05 to 1.5%,
Ti: 0.003 to 0.02%,
B: 0.0005 to 0.0030%,
Ca: 0.0015 to 0.0030%,
N: 0.0040 to 0.008%,
O: 0.0005 to 0.0030%
The balance is iron and inevitable impurities, and the content (mass%) of Ti, B, and N satisfies the following formulas (1) to (3),
In the longitudinal section of the steel sheet, the number of inclusions with an equivalent circle diameter of 5 μm or more containing Ca is 5 / mm ² or less, and the number of MnS inclusions with a length of 50 μm or more is 2 / cm ² or less,
A steel plate for skin plate excellent in plate thickness direction toughness of a high heat input welding heat-affected zone, wherein the C concentration in the central segregation portion of the steel plate is 1.2 times or less of the average C concentration of the whole steel plate.
(1) 1.0 ≦ [Ti] / [N] ≦ 3.0
(2) 0.0003 ≦ [N] − [Ti] /3.4≦0.0035
(3) −0.0005 ≦ [B] − {([N] − [Ti] /3.4) × 11/14} ≦ 0.0015
(However, [Ti], [N], and [B] represent the contents (mass%) of Ti, N, and B, respectively.)   Furthermore, the steel plate of Claim 1 containing Cr: 0.05-1.5% and / or V: 0.005-0.05%. A method for producing the steel sheet according to claim 1 or 2,
The steel slab is heated to 950 to 1250 ° C. and rolled so that the rolling finishing temperature is 800 to 900 ° C., then air-cooled for 30 seconds or more, and then 300 ° C. or less at a cooling rate of 1 to 100 ° C./s. The manufacturing method of the steel plate for skin plates excellent in the plate | board thickness direction toughness of a high heat input welding heat affected zone characterized by cooling to the temperature of.   After cooling at 300 ° C. or lower according to claim 3, the skin plate is further reheated to 450 to 600 ° C. and air-cooled, and is excellent in plate thickness direction toughness of a large heat input welding heat affected zone. A method of manufacturing a steel sheet. After cooling below 300 ° C according to claim 3,
(1) 700-850 ° C. reheating, followed by cooling to 200 ° C. or lower at a cooling rate of 1 ° C./s or higher,
(2) A method for producing a steel plate for skin plate excellent in plate thickness direction toughness of a high heat input welding heat-affected zone, wherein reheating at 450 to 600 ° C. and subsequent air cooling are sequentially performed.

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