JP5598485B2 - Thick steel plate having a thickness of 50 mm or more excellent in long and brittle crack propagation stopping characteristics and method for producing the same - Google Patents

Description

  The present invention relates to a thick steel plate having a thickness of 50 mm or more, which is excellent in brittle crack propagation stopping characteristics suitable for use in large container ships, bulk carriers, and the like, and a method for producing the same. The present invention also relates to a method and a test apparatus for evaluating the long brittle crack propagation stopping performance equivalent to an actual ship.

  Container ships and bulk carriers have a structure with a large upper opening in order to improve loading capacity and cargo handling efficiency. For this reason, in these ships, it is necessary to increase the thickness of the hull skin.

  In recent years, the size of container ships has increased, and in the case of large ships of 6,000 to 20,000 TEU, the plate thickness of the hull outer plate becomes 50 mm or more, and in addition to the fracture toughness being reduced due to the plate thickness effect, the welding heat input is also increased. Therefore, the fracture toughness of the welded portion tends to be further reduced. Note that TEU (Twenty Fee Equivalent Unit) represents the number converted into a 20-foot container and indicates an index of the loading capacity of the container ship.

  For relatively thin steel materials with a thickness of less than 50 mm, which is used for ships and line pipes, fine graining is achieved by the TMCP method to improve low temperature toughness and excellent brittle crack propagation stopping properties. Can be granted.

  A technique for making the structure of the surface layer portion of a steel material ultrafine without increasing the alloy cost has been proposed as a means for improving brittle crack propagation stopping characteristics. For example, in Patent Document 1, when a brittle crack propagates, attention is paid to the fact that a shear lip (plastic deformation region) generated in the steel surface layer portion is effective in improving the brittle crack propagation stopping property. A method is disclosed in which the grains are refined to absorb the propagating energy of the propagating brittle crack.

After the steel sheet is hot-rolled, the process of cooling the surface layer part below the Ar ₃ transformation point by controlled cooling, and then repeating the process of stopping the controlled cooling and reheating the surface layer part above the transformation point, By applying a reduction to the steel material, it is repeatedly transformed or processed and recrystallized to produce an ultrafine ferrite structure or bainite structure in the surface layer portion.

  Patent Document 2 discloses a steel material mainly composed of ferrite-pearlite having a ferrite structure having a ferrite structure having a circle equivalent average particle size of 5 μm or less and an aspect ratio of 2 or more on a surface portion of both surfaces. In addition, the maximum reduction rate per pass during finish rolling is 12% or less to suppress local recrystallization phenomenon, and to suppress the variation in ferrite grain size, it improves the excellent brittle crack propagation stopping characteristics. Is obtained.

  In Patent Document 3, as a steel material having excellent brittle crack propagation characteristics after being subjected to plastic deformation, subgrains are formed in crystal grains produced by the methods described in the following (a) to (d). Further, a steel material mainly composed of fine ferrite is disclosed.

  (A) Rolling conditions for securing fine ferrite crystal grains, (b) Rolling conditions for generating a fine ferrite structure in a portion of 5% or more of the steel plate thickness, (c) Developing and forming a texture in fine ferrite ( The rolling condition in which dislocations introduced by rolling are rearranged by thermal energy to form subgrains, and (d) cooling conditions to suppress coarsening of the formed fine ferrite crystal grains and fine subgrain grains are used. Without requiring complicated temperature control such as cooling and recuperation of the surface layer, the brittle crack propagation stopping characteristics after being subjected to plastic deformation are improved.

  Further, as a technical idea different from Patent Documents 1 to 3, Patent Document 4 describes that by developing a texture, separation is generated on the fracture surface of the steel material in a direction parallel to the plate thickness direction, and a brittle crack tip is formed. In a method for improving the brittle crack propagation characteristics by relaxing stress, the (110) plane X-ray intensity ratio is set to 2 or more and coarse grains having an equivalent circle diameter of 20 μm or more are controlled to 10% or less by controlled rolling. Is described.

  In Patent Document 5, as a steel for welded structures having excellent brittle crack propagation stopping performance in a welded joint, the X-ray surface strength ratio of the (100) plane at the rolled surface within the plate thickness is 1.5 or more. A steel sheet characterized by the following is disclosed, and by the texture development, the crack propagation direction is changed with respect to the direction perpendicular to the stress load direction, and a brittle crack is induced from the welded joint to the base metal side, and the joint is It is described that the brittle crack propagation stopping performance is improved.

  In Patent Document 6, the X-ray intensity ratio of the (211) plane at the rolled surface at the center of the plate thickness is 1.3 or more, and the (100) plane X-ray at the rolled surface at the 1/4 thickness portion. A steel sheet characterized by having a strength ratio of 1.5 or more and a (100) plane X-ray intensity ratio at the rolled surface in the plate surface layer portion of 1.5 or more is disclosed. It is described that a crack occurs near the tip of a brittle crack that penetrates from the surface of the steel plate, and that the crack acts as a crack propagation resistance, improving the brittle crack propagation stopping performance against the brittle crack propagating in the plate thickness direction. Yes.

  On the other hand, in the hull structure, it is considered necessary to prevent the hull separation by stopping the propagation of a brittle crack even if a brittle fracture occurs from a weld. The brittle crack propagation behavior of welded steel plates for shipbuilding with a thickness of less than 50 mm has been experimentally studied by the Japan Shipbuilding Research Association No. 147 Committee.

  In the 147th Committee, as a result of experimental investigation of the propagation path and propagation behavior of the brittle crack that was forcibly generated in the weld, if the fracture toughness of the weld was secured to some extent, Due to the influence, brittle cracks often deviate from the weld to the base metal side, but multiple examples of brittle cracks propagating along the weld were also confirmed. This suggests that there is no possibility that brittle fracture will propagate straight along the weld.

  However, in addition to the fact that ships constructed by applying welding equivalent to the welding applied in the 147th Committee to steel sheets with a thickness of less than 50 mm have been put into service without any problems, toughness is good. Since steel plate base materials (shipbuilding class E steel, etc.) have sufficient ability to stop brittle cracks, the brittle crack propagation stop characteristics of steel welds for shipbuilding are not required by the classification rules. It was.

JP-A-4-141517 JP 2002-256375 A Japanese Patent No. 3467767 Japanese Patent No. 3548349 JP-A-6-207241 JP 2008-214652 A

Yamaguchi et al. “Development of ultra-large container ship-practical use of new high-strength extra-thick steel sheet”, Journal of Japan Society of Marine Science and Technology, 3, (2005), P70.

  However, in recent large container ships exceeding 6,000 TEU, the plate thickness of the steel plate exceeds 50 mm, and the fracture toughness is lowered due to the plate thickness effect. In addition, the weld heat input becomes larger, so the fracture toughness of the welded portion is reduced. There is a tendency to further decrease.

  Recently, in such thick-walled large heat input welded joints, it is experimental that brittle cracks generated from the welds do not escape to the base metal side and go straight, and do not stop even in the steel base material parts such as aggregates. (Non-Patent Document 1), which is a big problem in ensuring the safety of the hull structure to which a steel plate having a thickness of 50 mm or more is applied. In addition, there is a long-sized ESSO test as a test for evaluating the safety of such a hull, but the test results change due to differences in the evaluation method and restrictions on the test equipment, and a long brittle crack corresponding to an actual ship. There was a problem that propagation stop performance was not evaluated.

  The steel sheets described in Patent Documents 1 to 6 described above have no description of the lengthened brittle crack propagation stopping characteristics, and cannot solve the problems clarified in Non-Patent Document 1. Moreover, there is no description in the technique described in Patent Documents 1 to 6 for a method and a test apparatus for evaluating the long brittle crack propagation stop characteristic equivalent to an actual ship, and to solve the problem of safety evaluation equivalent to an actual ship. I can't.

  Accordingly, the present invention provides a steel plate having a thickness of 50 mm or more and a welded portion thereof, and a method of manufacturing the steel plate that stops a brittle crack that has become long before a large-scale fracture occurs even when a brittle fracture occurs. The purpose is to provide. In addition, an object of the present invention is to provide a method and a test apparatus for evaluating the long brittle crack propagation stopping performance equivalent to an actual ship. Here, the long and brittle crack is a brittle crack having a length of 1 m or more that enters from another adjacent steel plate.

  The present inventors investigated the relationship between texture morphology and brittle crack propagation stopping characteristics (sometimes referred to as arrest performance) for many steel plates with different chemical compositions and rolling conditions, and stopped long brittle crack propagation. The effect of distribution in the thickness direction of arrest performance (affected by toughness and texture) on the phenomenon was investigated. In addition, a long-scale ESSO test evaluation method and test apparatus capable of simulating long-brittle crack propagation characteristics equivalent to actual ships were studied by dynamic FEM analysis with varying distance between tab plate tips or load-loading points.

  As a result, when the chemical composition and rolling conditions are controlled and the toughness that affects arrest performance and the distribution of texture in the thickness direction are specified, the long brittle crack propagation stoppage performance has been dramatically improved and has been stopped so far. It has been found that a thick brittle crack or a long brittle crack propagating through its weld can be stopped with a steel plate under conditions equivalent to an actual ship without stress reflection. Furthermore, as a result of dynamic FEM analysis, the evaluation method and test apparatus for a long-sized ESSO test corresponding to an actual ship without stress reflection were found by setting the distance between the tab plate tips and the distance between the load points to predetermined values. . In addition, since the steel plate with a thickness of less than 50 mm can stop a long and brittle crack with a current steel plate (for example, E grade steel for shipbuilding), the present invention is intended for a steel plate with a thickness of 50 mm or more.

The present invention has been made by further study based on the above knowledge, that is, the present invention,
(1) A steel plate having a thickness (t) of 50 mm or more, and a width of 20% of the thickness (t) at the center of the thickness in the tip shape of the long brittle crack propagation stop portion in the cross section in the thickness direction With respect to the maximum crack length in the region where the stop crack length in the region is from 1/4 to 1/10 of the plate thickness (t) or 3/4 to 9/10 of the plate thickness (t) from the steel plate surface, A plate thickness (t) excellent in a long brittle crack propagation stop characteristic, characterized in that it is formed in a concave recess portion that is shorter than the length of the long brittle crack by at least the length of the plate thickness (t). Is a thick steel plate of 50 mm or more.
(2) The X-ray intensity ratio of the (211) plane or the (100) plane at the rolled surface of the portion in the region of at least 20% of the plate thickness (t) at the plate thickness central portion is 1.5 or more, and the plate thickness The X-ray intensity ratio of the (110) plane at the rolled surface in the region of 1/4 to 1/10 of (t) or 3/4 to 9/10 of the plate thickness (t) is 1.3. A thick steel plate having a thickness (t) of 50 mm or more, which is excellent in the long brittle crack propagation stopping property as described in (1), which is as described above.
(3) The (211) plane X-ray intensity ratio X ₍₂₁₁₎ and the (100) plane X-ray intensity ratio X ₍₂ ) at the rolling surface of the portion in the region of at least 20% of the sheet thickness (t) at the center of the sheet thickness ₁₀₀₎ and the fracture surface transition temperature vTrs (° C.) obtained by the 2 mm V notch Charpy impact test of the same part is represented by the formula: vTrs-12X ₍₁₀₀₎ -22X ₍₂₁₁₎ ≦ (T-75) /0.64 [T is steel plate In the range of 1/4 to 1/10 of the plate thickness (t) or 3/4 to 9/10 of the plate thickness (t). The thickness (t) having excellent long-brittle crack propagation stopping property as described in (1) or (2) is 50 mm, wherein the X-ray intensity ratio of (110) plane at 1.3 is 1.3 or more The above thick steel plate.
(4) Steel composition includes mass%, C: 0.15% or less, Si: 0.60% or less, Mn: 0.80 to 1.80%, S: 0.001 to 0.05% Ti: 0.005 to 0.050% or Nb: At least one selected from 0.001 to 0.1%, Cu: 2.0% or less, V: 0.2% or less Ni: 2.0% or less, Cr: 0.6% or less, Mo: 0.6% or less, W: 0.5% or less, B: 0.0050% or less, Zr: 0.5% or less (1) to (3) characterized in that it comprises at least one selected from the group consisting of the remaining Fe and unavoidable impurities (1) to (3). A steel plate having a t) of 50 mm or more.
(5) A steel material having the component composition described in (4) is heated to a temperature of 900 to 1350 ° C., and then rolled at a cumulative rolling reduction of 10% or more in a temperature range of a steel plate surface temperature of 1000 to 850 ° C. The surface temperature is 900 to 600 ° C. and the internal temperature of the steel sheet is 50 to 150 ° C. higher than the steel sheet surface temperature, and then the steel sheet surface temperature at the end of rolling at a one-pass reduction ratio of 7% or more and a cumulative reduction ratio of 50% or more. A method for producing a thick steel plate having a thickness (t) of 50 mm or more, which is excellent in long brittle crack propagation stopping characteristics, characterized by hot rolling at 800 to 550 ° C.
(6) Further, after the hot rolling is finished, the sheet is cooled to 400 ° C. at a cooling rate of 5 ° C./s or more, and the plate thickness (t ) Is a manufacturing method of a thick steel plate of 50 mm or more.
(7) In a test that evaluates and confirms the propagation stop performance for a large brittle crack with a crack propagation length of 1 m or more using a large test piece with a width of 2 m or more of the test piece, A method for evaluating the long brittle crack propagation stopping performance of a steel material or structure, wherein the distance between the tab plate tips is at least 2.8 times the width of the test piece.
(8) In the evaluation method according to (7), the distance between the load points of the test apparatus is 4.1 times or more the width of the test piece, and the long brittle crack propagation of the steel material or structure Stop performance evaluation method.
(9) The tip of the tab plate of the test device to which the test piece is attached in a test device that evaluates and confirms the propagation stop performance of a large brittle crack with a crack propagation length of 1 m or more using a large test piece with a width of 2 m or more. A test apparatus for evaluating long brittle crack propagation stopping performance, characterized in that the distance between them is 2.8 times or more the width of the test piece.
(10) In the test apparatus described in (9), the long brittle crack propagation stop performance is further characterized in that the distance between the load points of the test apparatus is 4.1 times or more of the test piece width. Test equipment.

  According to the present invention, it is possible to provide excellent brittle crack propagation stopping performance in a thick steel plate having a thickness (t) of 50 mm or more, and it is long in a thick material having a thickness of 50 mm or more, which has been difficult until now. Brittle cracks can be stopped under conditions equivalent to actual ships without stress reflection, which is extremely useful in industry.

The figure which shows typically the front-end | tip shape of the long brittle crack propagation stop part in the plate | board thickness direction cross section of the steel plate whose thickness (t) which concerns on this invention is 50 mm or more. The figure which shows the dimension shape of a long ESSO test piece. The dynamic FEM analysis model for investigating the influence of the stress reflection on the evaluation of the long brittle crack propagation stop characteristic is shown, (a) is a parametric model, (b) is a case where the distance between load points is 10 m, (c ) Shows a dynamic FEM analysis model when the distance between load points is 5 m. The figure which shows the influence of the test conditions (distance from a test piece edge part) which acts on a dynamic stress intensity factor as an analysis result by the dynamic analysis model of FIG.

  In this invention, the front-end | tip shape of the long brittle crack propagation stop part in a plate | board thickness direction cross section is prescribed | regulated. The reason for limitation of the present invention will be described below.

  FIG. 1 schematically shows a tip shape (long brittle crack stop position 3) of a propagation stop portion of a long brittle crack 2 in a cross section in the thickness direction of a steel plate 1 having a thickness (t) of 50 mm or more according to the present invention. .

  In the present invention, the shape of the tip of the long brittle crack propagation stop portion is determined from the position of the long brittle crack stop in the region having a width of 20% of the plate thickness (t) at the plate thickness central portion, and the plate thickness (t) from the steel plate surface. Is the shortest interval (hereinafter referred to as depth a) among the positions of the large brittle crack stop positions in the width region of 1/4 to 1/10 of the thickness and 3/4 to 9/10 of the plate thickness (t). At least the length of the plate thickness (t) is set to a shape having a recessed portion having a substantially U-shape that is short by a depth a in the advancing direction of the long brittle crack.

In order to improve the arrest performance of the entire steel plate, _a region having a width of at least 20% of the plate thickness (t) in the plate thickness central portion of the cross section in the plate thickness direction of the steel plate, including _a half position of the plate thickness (t) However, the arrest performance in the upper and lower width regions of 10% or more) is improved. The area of improving the arrest performance, t _a is preferably 50% or less from the constraints of the rolling load.

Arrest performance area width around the mid-thickness with improved and _{t a} is the thickness of less than 20%, of the 1 / 4-1 / 10 parts near the plate thickness (t) (sheet thickness (t) 1 / 4 position and 1/10 position, the region between 1/4 position and 1/10 position) and the vicinity of 3/4 to 9/10 part of sheet thickness (t) (3/4 of sheet thickness (t)) And the 9/10 position, the area between the 3/4 position and the 9/10 position) is not sufficiently reduced, and the vicinity of 1/4 to 1/10 part of the plate thickness (t) and the plate In the vicinity of 3/4 to 9/10 part of the thickness (t), the crack does not stop and propagates, so at least 20%.

  In the cross section in the plate thickness direction, the area where the arrest performance is superior to other areas has a short stop length for the large brittle crack, and a concave recess is formed in the direction of travel, so the tip shape of the long brittle crack propagation stop section Is a substantially U-shaped recessed portion in which a region of at least 20% of the plate thickness (t) at the central portion of the plate thickness is recessed with respect to the traveling direction of the long brittle crack.

  Moreover, the shape of the substantially U-shaped recess is to reduce the breaking driving force in the vicinity of 1/4 to 1/10 of the plate thickness (t) and in the vicinity of 3/4 to 9/10 of the plate thickness (t). The brittle crack stop length in the region of 20% of the plate thickness at the center of the plate thickness is 1/4 to 1/10 of the plate thickness (t) and 3/4 to 9/10 of the plate thickness (t). Since it is necessary to shorten at least the length of the plate thickness (t) from the brittle crack stop length in the region, the depth a of the recessed portion is at least the plate thickness (t) with respect to the traveling direction of the large brittle crack. A concave shape equal to the length of.

  Depth a is a long brittle crack stop position (maximum crack length) in the region of 1/4 to 1/10 of the plate thickness (t) and 3/4 to 9/10 of the plate thickness (t) in FIG. Also passes through the intersection of a line perpendicular to the plate thickness direction and a line parallel to the plate thickness direction showing a region width of 20% of the plate thickness at the center of the plate thickness and the long brittle crack propagation stop position. It is defined as the length of the shortest interval among the intervals with the line perpendicular to the thickness direction.

  In the brittle fracture fracture surface of a steel plate having a thickness of 50 mm or more, the longest crack propagation portion (in the region of 1/4 to 1/10 of the plate thickness (t) or 3/4 to 9/10 of the plate thickness (t) ( 1 is observed), the shape of the long brittle crack propagation stop position drawn in the thickness direction is defined in the present invention in the comparison between the vicinity of the thickness center portion and these regions. In addition, with respect to the 1/2 position of the plate thickness (t), the region of ¼ to 1/10 of the plate thickness (t) in the plate thickness direction from the steel plate surface that is vertically symmetric, and 3 / of the plate thickness (t). In the region of 4-9 / 10, the arrest performance and the tip shape of the long brittle crack propagation stop portion are substantially equal.

The tip shape of the long brittle crack propagation stop portion described above can be confirmed on the fracture surface of the long ESSO test piece 4 shown in FIG. In the long ESSO test piece 4, the test plate 6 and the run-up plate 5 are joined by a CO ₂ weld 8. The run-up plate 5 is provided with an electrogas weld 7 in a direction perpendicular to the CO ₂ weld 8. The generated brittle crack (not shown) propagates along the electrogas weld 7 and enters the test plate 6 at a right angle to the load direction of the test plate 6. The load direction is indicated by the arrow R. D. The rolling direction. In the present invention, the long brittle crack propagation stop characteristic is a tab having no influence of stress reflection as in an actual ship, using a long large ESSO test piece 4 having a long brittle crack propagation distance before entering the test plate 6. Refers to those evaluated by a testing machine with a sufficiently long distance between the plate tips and the distance between load points.

  The stress reflection here refers to reflection of a compressive stress wave generated by the occurrence and propagation of a brittle crack at a testing machine tab plate portion or the like. When this stress reflection occurs, the compressive stress wave returns to the brittle crack propagation portion, so that the brittle crack tends to stop. In a structure such as an actual ship, stress reflection does not occur (or is difficult) because the size of the structure is sufficiently large for a brittle crack. For this reason, it is necessary to evaluate the propagation stop characteristic of a long and brittle crack with a test machine having a sufficiently long distance between the tab plate tips and the distance between the load points.

The steel sheet according to the present invention preferably has a texture described below.
The X-ray intensity ratio of the (211) plane or the (100) plane at the rolled surface in the region of at least 20% of the plate thickness at the center of the plate thickness is 1.5 or more, the plate thickness from 1/4 t to 1/10 t, or the plate The X-ray intensity ratio of the (110) plane at the rolling surface of thickness 3 / 4t to 9 / 10t is 1.3 or more. The X-ray intensity of the (211) surface or (100) surface at the rolling surface near the center of the plate thickness. When the ratio is 1.5 or more, fine subcracks are generated, the unevenness of the brittle crack propagation surface is increased, crack propagation resistance is increased, and brittle crack propagation stop toughness is greatly improved. This effect is not observed when the X-ray intensity ratio is less than 1.5. From the above, the X-ray intensity ratio of the (211) plane or the (100) plane on the rolled surface in the region of 20% or more of the plate thickness at the central portion of the plate thickness is limited to 1.5 or more.

  On the other hand, when the X-ray intensity ratio of the (110) plane on the rolled surface with a thickness of 1/4 t to 1/10 t is less than 1.3, the brittleness in a region of 20% or more of the thickness of the central portion of the thickness is obtained. The crack stop length is not shorter than the brittle crack stop length in the region of the plate thickness of ¼t to 1 / 10t, and is not shorter than the plate thickness by about the plate thickness of ¼t to 1 / 10t (point A in FIG. 1). And the failure driving force of the longest crack propagation part near the point A ′ does not decrease. Therefore, the X-ray intensity ratio of the (110) plane on the rolled surface having a thickness of 1/4 t to 1/10 t was limited to 1.3 or more. The same applies to the thickness 3 / 4t to 9 / 10t part.

  Furthermore, in order to improve the brittle crack propagation stop toughness at the service temperature of the steel sheet, it is preferable to satisfy the following formula.

vTrs-12X ₍₁₀₀₎ -22X ₍₂₁₁₎ ≤ (T-75) /0.64
(However, in the formula, X ₍₂₁₁₎ is the (211) plane X-ray intensity ratio at the rolling surface in the region at least 20% of the plate thickness (t) at the center of the plate thickness, and X ₍₁₀₀₎ is the same site (100) Plane X-ray intensity ratio, vTrs (° C.) is the fracture surface transition temperature obtained by the 2 mm V notch Charpy impact test at the same site, and T is the service temperature (° C.) of the steel sheet)
This parameter formula defines the toughness of the steel sheet by vTrs according to the texture in order to ensure the brittle crack propagation stop toughness of the target area in the texture. In order to make the fracture surface transition temperature vTrs low, vTrs is defined so as to satisfy the above equation. As described above, in order to improve brittle crack propagation stop toughness, the X-ray intensity ratio of the (211) plane or the (100) plane needs to be 1.5 or more. Since the texture greatly contributes to the improvement of brittle crack propagation stop toughness, the coefficient of X ₍₂₁₁₎ is made larger than X ₍₁₀₀₎ in the equation.

  The preferable component composition and manufacturing conditions of the steel sheet having the above-described characteristics are as follows. In the description,% is mass%.

[Ingredient composition]
C: 0.15% or less C is necessary to ensure strength. From the viewpoint of securing strength, the lower limit is preferably 0.02%. However, if the C content exceeds 0.15%, the weld heat affected zone (HAZ) toughness decreases, so the content was limited to 0.15% or less. In order to further develop the texture of the (211) plane and the (100) plane, the preferable range is 0.03% or less.

Si: 0.60% or less Si is an element effective for increasing the strength. In order to acquire the effect, it is preferable to contain 0.01% or more. If the Si content exceeds 0.60%, the weld heat affected zone (HAZ) toughness is remarkably deteriorated, so it is limited to 0.60% or less.

Mn: 0.80 to 1.80%
Mn is an element effective for increasing the strength, and the lower limit is set to 0.80% from the viewpoint of securing the strength. However, if the amount of Mn exceeds 1.80%, there is a concern about deterioration of the base material toughness. For this reason, Mn was taken as 0.80 to 1.80% of range. A preferable range is 1.00 to 1.70%.

S: 0.001 to 0.05% or less In the present invention, since it is necessary to generate cracks (cracks parallel to the steel plate surface) at the leading edge of the brittle crack, it is necessary to contain 0.001% or more of S. It is. However, since S forms nonmetallic inclusions and deteriorates ductility and toughness, it is limited to 0.05% or less.

One or two types of Ti: 0.005 to 0.050%, Nb: 0.001 to 0.1% Ti forms a precipitate of carbide or nitride, thereby heating at the time of manufacturing the steel sheet. This suppresses the growth of the austenite grains and contributes to the refinement, and also suppresses the grain coarsening of the weld heat affected zone (HAZ) and improves the HAZ toughness. In order to obtain these effects, a content of 0.005% or more is necessary. On the other hand, excessive content deteriorates toughness, so 0.050% is made the upper limit.

  Nb is also effective in improving precipitation strengthening and toughness. Moreover, the recrystallization of austenite is suppressed, and the effect by the rolling conditions described later is promoted. In order to obtain these effects, addition of 0.001% or more is necessary, but if added over 0.1%, the quenched structure tends to become needle-like and the toughness tends to deteriorate. The upper limit is 1%.

Cu: 2.0% or less, V: 0.2% or less, Ni: 2.0% or less, Cr: 0.6% or less,
Mo: 0.6% or less, W: 0.5% or less, B: 0.0050% or less, Zr: at least one selected from 0.5% or less Cu: 2.0% or less Cu is mainly used It can be used for precipitation strengthening. In order to obtain the effect, 0.05% or more is preferably added. If the Cu content exceeds 2.0%, precipitation strengthening becomes excessive and toughness deteriorates, so it is preferable to set the Cu content in a range of 2.0% or less.

V: 0.2% or less V is a component that can use solid solution strengthening and precipitation strengthening. In order to acquire the effect, containing 0.001% or more is preferable. However, if the content exceeds 0.2%, the base material toughness and weldability are greatly impaired.

Ni: 2.0% or less Ni is effective in improving strength and toughness and preventing Cu cracking during rolling when Cu is added. In order to obtain the effect, addition of 0.05% or more is preferable. However, since it is expensive and its effect is saturated even if it is added excessively, the content is preferably set to 2.0% or less.

Cr: 0.6% or less Cr has an effect of increasing strength. In order to acquire the effect, containing 0.01% or more is preferable. However, if the content exceeds 0.6%, the weld zone toughness deteriorates, so the Cr content is preferably in the range of 0.6% or less.

Mo: 0.6% or less Mo has an effect of increasing the strength at normal temperature and high temperature. In order to obtain the effect, 0.01% or more is preferably added. However, if the content exceeds 0.6%, the weldability deteriorates, so the content is preferably in the range of 0.6% or less.

W: 0.5% or less W has an effect of increasing the high-temperature strength. In order to acquire the effect, containing 0.05% or more is preferable. However, if it exceeds 0.5%, not only is the toughness deteriorated, but it is expensive, so it is preferably contained in a range of 0.5% or less.

B: 0.0050% or less B precipitates as BN during rolling, and fines ferrite grains after rolling. In order to acquire the effect, containing 0.0010% or more is preferable. However, if it exceeds 0.0050%, the toughness deteriorates, so it is limited to 0.0050% or less.

Zr: 0.5% or less Zr is an element that increases the strength and improves the plating cracking resistance of the galvanized material. In order to acquire the effect, containing 0.03% or more is preferable. However, since the weld toughness deteriorates if the content exceeds 0.5%, the Zr content is preferably 0.5% as an upper limit.
The steel according to the present invention is the balance Fe and unavoidable impurities in addition to the above component composition. Inevitable impurities include P: 0.035% or less, Al: 0.08% or less, N: 0.012% or less, O: 0.05% or less, Mg: 0.01% or less, and the like. Acceptable.

  In the production conditions, it is preferable to define heating temperature, hot rolling conditions, and cooling conditions. Unless otherwise specified in the description, the temperature and cooling rate are average values in the thickness direction.

[Heating temperature]
The steel material is heated to a temperature of 900 to 1350 ° C. A heating temperature of 900 ° C. or higher is heating necessary for homogenizing the material and controlled rolling described later, and 1350 ° C. or lower is that surface oxidation becomes remarkable when the temperature is excessively high, This is because coarsening of crystal grains cannot be avoided. In order to improve toughness, the upper limit is preferably set to 1150 ° C.

[Hot rolling conditions]
When the steel sheet surface temperature is 1000 to 850 ° C., the rolling reduction is 10% or more. By rolling in the temperature range, the austenite grains are partially recrystallized, so that the structure becomes fine and uniform.

  Note that rolling at a temperature exceeding 1000 ° C. promotes the growth of austenite grains, and thus is not preferable for making fine grains. On the other hand, if it is lower than 850 ° C., it completely enters the austenite non-recrystallized region, which is not preferable for making the crystal grains uniform.

After the steel plate surface temperature is 900 to 600 ° C. and the steel plate internal temperature is 50 to 150 ° C. higher than the steel plate surface temperature, the steel plate at the end of rolling has a one-pass reduction ratio of 7% or more and a cumulative reduction ratio of 50% or more. Hot rolling under conditions of a surface temperature of 850 to 550 ° C. When the steel plate surface temperature is 900 to 600 ° C. and the steel plate internal temperature is 50 to 150 ° C. higher than the steel plate surface temperature, the surface vicinity is almost two phases. And the inside of the steel sheet is almost an austenite non-recrystallized region.

  If rolling with a one-pass reduction ratio of 7% or more is performed under these conditions, rolling strain is preferentially introduced into the steel plate having relatively low strength, and at least 20% of the thickness of the plate thickness central portion. Will be introduced into the organization. Through this process, a texture is formed in the austenite grains.

  That is, the basis of the (211) plane texture, which is a kind of transformation texture effective for crack generation at the brittle crack tip, is formed. In order to introduce a texture to at least 20% of the plate thickness at the center of the plate thickness, it is more preferable to set the one-pass rolling reduction rate to 10% or more.

  Then, by rolling to a steel plate surface temperature of 850 to 550 ° C., the inside of the steel plate is rolled in a two-phase region to form a (100) plane texture.

  In order to set the accumulation degree of the texture to a level effective for crack generation at the brittle crack tip (an accumulation degree of 1.55 or more), a cumulative rolling reduction of 50% or more is required.

[Cooling conditions]
After the hot rolling is finished, it is cooled to 400 ° C. at a cooling rate of 5 ° C./s or more. When the temperature range up to 400 ° C. is cooled at a cooling rate of 5 ° C./s or more, inheritance from the austenite texture of the texture having a predominant (211) surface is promoted, and the brittle crack propagation stopping toughness is improved.

  When cooled under the above conditions, the X-ray surface strength of the (211) plane becomes stronger, the generation of subcracks is further promoted, and cracks are likely to stop. In the above cooling method, a more preferable cooling start temperature is 700 ° C. or higher.

Needless to say, when the steel plate according to the present invention has a thickness of less than 50 mm, it has excellent brittle crack propagation characteristics.
[Evaluation method, test equipment]
In order to evaluate the long-brittle crack propagation stopping characteristics under conditions equivalent to actual ships without stress reflection, the effect of stress reflection was evaluated by dynamic FEM analysis, and the distance between the tab plate tips of the testing machine and between load points The distance was determined. The size of the long ESSO test piece was as shown in FIG.

  3A, 3B and 3C show the dynamic FEM analysis model, and FIG. 4 shows the result. FIG. 3A is a parametric model for determining the condition without stress reflection. The influence of the distance (2A in FIG. 3A) between the tester tab plates 11 (thickness 200 mm) that affects the stress reflection is shown. This is a model for analysis. FIG. 3B is a model in the case where the distance of the load load point 10 of the test machine to be used is set to 10 m, and FIG. 3C is a case in which the distance of the load load point 10 of the test machine to be used is set to 5 m. It is a model.

  FIG. 4 shows the FEM analysis results. FIG. 4 shows the change in the dynamic stress intensity factor (fracture driving force during brittle crack propagation) Kd of the crack during propagation from the occurrence of fracture to the entry into the test plate. The result shown by x is a case of 2A = 10000 mm, and is a result under conditions equivalent to an actual ship where a brittle crack does not cause stress reflection until the test plate enters.

  Under the conditions of 2A = 1800-4300mm, stress reflection occurs, so that the dynamic stress intensity factor Kd when entering the test plate is lower than in the case of 2A = 10000mm, which is an actual ship equivalent condition. Can be confirmed. This means that under the condition of 2A = 1800-4300 mm, a long and brittle crack is more likely to stop than under actual ship conditions. On the other hand, under the condition of 2A = 6800 mm, although a slight decrease in the dynamic stress intensity factor Kd is recognized, it can be confirmed that it is not so different from the actual ship equivalent condition.

  Therefore, if 2A is secured 6800 mm or more, it is possible to evaluate the actual ship equivalent conditions. For example, a large tensile test jig shape with a distance of 10 m between the load points shown in FIG. Conditions can be evaluated. FIG. 4 shows the analysis result obtained by the model when the distance between the load points of the test machine to be used is set to 5 m and 10 m, but the 10 m model between the load points shown in FIG. It is recognized that if a long-sized ESSO test is performed with a large-sized tensile test jig shape, evaluation under conditions equivalent to actual ships without stress reflection is recognized.

  Based on the above FEM analysis, the evaluation method of the long-brittle crack propagation stopping performance under conditions equivalent to an actual ship without stress reflection is the test piece length or the distance between the tab plate tips of the test apparatus to which the test piece is attached is 2 .Times.8 times or more (≈6800 mm / 2400 mm), and further, the distance between the load points of the test apparatus was 4.1 times or more (≈10000 mm / 2400 mm) of the test piece width.

  Similarly, as a test device that can evaluate the long brittle crack propagation stopping performance under conditions equivalent to an actual ship without stress reflection, the distance between the tab plate tips of the test device to which the test piece is attached is 2.8 times or more of the test piece width. (≈6800 mm / 2400 mm), and further, the distance between the load points of the test apparatus was 4.1 times or more the width of the test piece (≈10000 mm / 2400 mm).

  Using steel slabs adjusted to various chemical compositions shown in Table 1, thick steel plates were produced according to the conditions shown in Table 2. For each thick steel plate thus obtained, the X-ray intensity ratio between the (211) plane and the (100) plane of the central portion (high arrest performance region) of the thickness (t) is measured, and the Charpy fracture surface transition temperature vTrs. investigated. Further, the X-ray intensity ratio of the (110) plane of 1/8 part of the plate thickness (t) (representative part of the region of 1/4 to 1/10 of the plate thickness (t)) was measured.

Next, in order to evaluate the long brittle crack propagation stopping characteristics, a long ESSO test piece having the dimensions shown in FIG. 2 was prepared using the above-mentioned thick steel plate (the original thickness of the plate thickness (t)). It was used for. The test was performed under the conditions of a stress of 257 N / mm ² and a temperature of −10 ° C. Here, the stress 257 N / mm ² is the maximum allowable stress of the yield strength 40 kgf / mm ² grade steel plate frequently used in the hull, and the temperature −10 ° C. is the design temperature of the ship. The long-sized ESSO test was conducted using a large tensile test jig shown in FIG. 3B with a distance between the tab plate tips of 6800 mm and a load load point distance of 10000 mm.

  Table 3 shows the results of the long ESSO test. In the present invention examples (Nos. 2, 3, 6, 8, 9, 12, 14), the brittle cracks stopped at the fillet welds, and comparative examples (No. 1, 4, 5, 7, 10, 11, In 13, 15, 16), the brittle crack did not stop.

1 steel plate 2 long brittle crack 3 long brittle crack stop position 4 long ESSO test piece 5 run-up plate 6 test plate 7 electrogas weld 8 CO ₂ weld 9 machine notch 10 load point 11 tab plate

Claims (4)

The steel composition includes, in mass%, C: 0.15% or less, Si: 0.60% or less, Mn: 0.80 to 1.80%, S: 0.001 to 0.05%, Ti: Contains 0.005 to 0.050%, Nb: 0.001 to 0.1%, Ni: 2.0% or less, consists of the balance Fe and inevitable impurities, and has a thickness (t) of 50 mm or more. A steel plate , wherein the X-ray intensity ratio of the (211) plane or (100) plane at the rolled surface of the portion in the region of at least 20% of the plate thickness (t) at the central portion of the plate thickness is 1.5 or more, The X-ray intensity ratio of the (110) plane at the rolling surface of the region that is 1/4 to 1/10 of the plate thickness (t) or 3/4 to 9/10 of the plate thickness (t) is 1. and at .3 above, the tip shape of the long brittle crack propagation stop in the sheet thickness direction cross-section of the fracture surface of the case of performing long ESSO test, thickness The stop crack length in the region having a width of 20% of the central plate thickness (t) is 1/4 to 1/10 of the plate thickness (t) or 3/4 to 9 of the plate thickness (t) from the steel plate surface. The long crack is characterized in that it is shorter than the maximum crack length in the region of / 10 by at least the plate thickness (t) with respect to the traveling direction of the large brittle crack, and has a concave recess. A thick steel plate having a thickness (t) of 50 mm or more, which has excellent brittle crack propagation stopping characteristics. The (211) plane X-ray intensity ratio X ₍₂₁₁₎ and the (100) plane X-ray intensity ratio X ₍₁₀₀₎ at the rolling surface of the portion in the region of at least 20% of the sheet thickness (t) in the center portion of the sheet thickness and The fracture surface transition temperature vTrs (° C.) obtained by the 2 mmV notch Charpy impact test at the same site is expressed by the formula: vTrs-12X ₍₁₀₀₎ -22X ₍₂₁₁₎ ≦ (T-75) /0.64 [T is the service temperature of the steel sheet (° C.)] and in a rolling surface of a region that is 1/4 to 1/10 of the plate thickness (t) or a region that is 3/4 to 9/10 of the plate thickness (t). 110) The X-ray intensity ratio of the plane is 1.3 or more, and the plate thickness (t) excellent in long brittle crack propagation stopping characteristics according to claim 1, wherein the thickness (t) is 50 mm or more. Steel composition, by mass%, in a further, Cu: 2.0% or less, V: 0.2% or less, Cr: 0.6% or less, Mo: 0.6% or less, W: 0.5% or less , B: 0.0050% or less, Zr: excellent long brittle crack arrest properties according to claim 1 or 2, characterized in that you contain at least one kind selected from among 0.5% or less A thick steel plate having a thickness (t) of 50 mm or more. The steel material having the component composition according to claim 1 or 3 is heated to a temperature of 900 to 1350 ° C, and then rolled at a cumulative rolling reduction of 10% or more in a temperature range of a steel plate surface temperature of 1000 to 850 ° C, and then the steel plate surface The steel sheet surface temperature is 800 to 600 ° C. and the steel sheet internal temperature is 50 to 150 ° C. higher than the steel sheet surface temperature. Thereafter, the steel sheet surface temperature is 800 at the end of rolling at a one-pass rolling reduction of 8 % or more and a cumulative rolling reduction of 50% or more. Thickness with excellent long-brittle crack propagation stopping characteristics, characterized in that after hot rolling at ˜550 ° C. and further after completion of hot rolling, it is cooled to 400 ° C. at a cooling rate of 5 ° C./s or more. The manufacturing method of the thick steel plate whose t) is 50 mm or more.

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