KR101584235B1 - Thick steel plate having thickness of 50mm or more with excellent long brittle crack arrestability, method for manufacturing the same and method and testing apparatus for evaluating long brittle crack arresting performance - Google Patents

Abstract

The region of 20% of the plate thickness at the central portion of the plate thickness in the shape of the tip of the long brittle fracture stop portion is at least the length of the plate thickness portion in the region where the area is 1/4 to 1/10 of the plate thickness from the steel plate surface (211) plane or a (100) plane on the rolled surface at least in a region of 20% of the plate thickness in the central portion of the plate thickness is smaller than the X-ray intensity ratio of the 1.5 or more, the area of 1/4 to 1/10 of the plate thickness, or the X-ray intensity ratio of the (110) face at the rolling surface of 3/4 to 9/10 of the plate thickness is 1.3 or more, After rolling at a temperature of 900 to 1350 캜 and at a steel sheet surface temperature of 1000 to 850 캜 and a cumulative rolling reduction of 10% or more, a one-pass rolling reduction of 7% or more and a cumulative rolling reduction of 50% The method of manufacturing the steel sheet after hot rolling at the temperature of the surface of the steel sheet at the temperature of 800 to 550 占 폚 at the end of rolling, A method of evaluating the propagation stopping performance against brittle fracture and an evaluation apparatus are provided.

Description

TECHNICAL FIELD [0001] The present invention relates to a steel plate having a thickness of 50 mm or more, which is excellent in radial brittle crack propagation stopping property, a method of manufacturing the steel plate, and a method and a device for evaluating the stiffness crack propagation stopping performance. METHOD FOR MANUFACTURING THE SAME AND METHOD AND TESTING APPARATUS FOR EVALUATING LONG BRITTLE CRACK ARRESTING PERFORMANCE}

The present invention relates to a thick steel plate having a thickness of 50 mm or more and excellent brittle crack arrestability suitable for use in a large container line (Mega-container carrier or bulk carrier) And a method for producing the same. The present invention also relates to a method for evaluating a long brittle crack propagation stop performance equivalent to an actual ship, and to a test apparatus.

The container line and the bulk carrier are structured such that the upper aperture is largely taken in order to improve the carrying capacity and the cargo handling efficiency. For this reason, it is necessary to thicken the outer plate of the vessel's body, especially in the ship, in order to secure the rigidity and longitudinal strength of the hull.

In recent years, container ships have become larger, and in large ships with 6,000 to 20,000 TEU (twenty-foot equivalent unit), the plate thickness of the outer shell of the hull is 50 mm or more, and the fracture toughness is increased by the thickness effect. The weld heat input is also increased, so that the fracture toughness of the welded part tends to be further lowered. In addition, TEU (Twenty-foot Equivalent Unit) represents the number converted into a container having a length of 20 feet and indicates an index of the loading capacity of the container ship.

For relatively thin steels having a plate thickness of less than 50 mm in steel plates used for ships or line pipes, grain refinement is performed by the TMCP method (Thermomechanical controlled processing) (low-temperature toughness) can be improved, and excellent brittle crack propagation stopping properties can be imparted.

A technique of superfineizing the texture of the surface part of the steel material without raising the alloy cost has been proposed as a means for improving the brittle crack propagation stopping property. For example, in Patent Document 1, shear-lips (plastic deformation area) generated in a surface layer of a steel material is effective in improving brittle crack propagation stopping characteristics when a brittle crack propagates , A method of making crystal grains in the shear rib portion finer and absorbing the propagation energy of the propagated brittle crack is disclosed.

After the steel sheet is hot-rolled, the surface layer portion is cooled to a temperature below the Ar ₃ transformation point by controlled cooling, and then the control cooling is stopped to repeat the surface layer portion at least over the transformation point. And by repeatedly transforming or recrystallization due to deformation by applying a pressure to the steel material in the meantime to create a superfine ferrite structure or bainite structure in the surface layer portion will be.

Patent Document 2 discloses a steel material having a microstructure of ferrite-pearlite as its main body, wherein both surface portions have a circle-equivalent average particle diameter of 5 μm or less and an aspect ratio of 2 Or more of a ferrite structure having 50% or more of ferrite grains and further having a maximum rolling reduction per pass of 12% or less during finish rolling, thereby forming a local recrystallization phenomenon. It is disclosed that excellent brittle crack propagation stopping property can be improved by suppressing unevenness of ferrite grain size.

Patent Document 3 discloses a steel material excellent in anti-brittle crack propagation characteristics after plastic deformation, and has a structure in which sub-grains (sub) are formed in the grains produced by the methods described in the following (a) to (d) -grain) is formed on the surface of the steel sheet as a main structure.

(a) rolling conditions for securing fine ferrite grains; (b) rolling conditions for producing fine ferrite structures in at least 5% of the steel sheet thickness; (c) processing (rolling) And (d) suppressing the coarsening of the fine ferrite grains and the fine sub-grain grains formed thereon. In the present invention, Cooling conditions and the like of the surface layer of the steel sheet do not require complicated temperature control and improve brittle crack propagation stopping characteristics after plastic deformation due to cooling conditions.

As a technological thought that is different from the patent documents 1 to 3, Patent Document 4 discloses a technique in which a texture is developed so that a separation is formed on a fracture surface of a steel in a direction parallel to the thickness direction (110) plane X-ray intensity ratio is controlled to be not less than 2 by controlled rolling so as to improve crack propagation characteristics of the brittle crack by mitigating the stress of the brittle crack tip, And a large grain having a circle-equivalent diameter of 20 μm or more is set to 10% or less.

Patent Document 5 discloses a welded structure steel excellent in brittle crack propagation stopping performance of a welded joint and characterized in that the X-ray plane intensity ratio of the (100) face in the rolled surface inside the plate thickness is 1.5 or more And crack propagation direction is changed with respect to a direction perpendicular to the stress loading direction due to the development of texture so that the brittle crack is guided from the welded joint to the base material side And improve brittle crack propagation stopping performance as a joint.

Patent Document 6 discloses that the X-ray intensity ratio of the (211) plane at the rolled surface in the center of the plate thickness is 1.3 or more, and the (100) plane X-ray intensity at the rolled surface at 1/4 of the plate thickness And a (100) plane X-ray intensity ratio of 1.5 or more at the rolled surface of the plate surface layer portion is 1.5 or more. The steel sheet is characterized in that the steel sheet enters from the steel sheet surface through T- Cracks are generated in the vicinity of the brittle crack tip and the brittle crack propagation stopping performance against the brittle crack propagating in the plate thickness direction is improved because the crack acts as crack propagation resistance.

On the other hand, in the hull structure, it is considered necessary to prevent the separation of the hull by stopping the propagation of the brittle crack even if brittle failure occurs from the welded portion. The brittle crack propagation behavior of the welded steel plate for shipbuilding with a plate thickness of less than 50 mm has been experimentally examined at the 147th Committee of the Shipbuilding Research Association of Japan.

In the 147th Committee, if the fracture toughness of the weld is secured to some extent as a result of an experimental investigation of the propagation path and the propagation behavior of the brittle crack forced by the weld, the influence of the welding residual stress The brittle cracks often deviate from the welded portion to the base material side, but a plurality of brittle cracks propagated along the welded portion were also confirmed. This suggests that brittle fracture can not be assured that there is no possibility of propagating straight along the weld.

However, in addition to the fact that ships constructed by applying the same welding as that applied by the 147th Committee on steel plates less than 50mm in thickness have been performing actual service without any problems, From the recognition that steel base materials (such as shipbuilding grade steel) have sufficient ability to stop brittle cracks, the brittle crack propagation stopping properties of steel welded joints for shipbuilding are subject to the Rules and Guidance for the Survey and Construction of Steel Ships It was not required.

Japanese Patent Application Laid-Open No. 4-141517 Japanese Patent Application Laid-Open No. 2002-256375 Japanese Patent Publication No. 3467767 Japanese Patent Publication No. 3548349 Japanese Unexamined Patent Publication No. 6-207241 Japanese Patent Application Laid-Open No. 2008-214652

 Yamaguchi et al., "Development of Mega-Container Carrier - Practical Application of a New High Strength Heavy Gauge Steel Plate," Journal of the Japan Ship and Ocean Engineering Society, 3, (2005), p.

However, in a recent large container ship exceeding 6,000 TEU, the plate thickness of the steel plate exceeds 50 mm, the fracture toughness is lowered by the plate thickness effect, and the heat input of welding becomes larger, It is in a tendency to deteriorate.

In recent years, brittle cracks generated from the welded portion of a large heat input welded joint of a heavy gauge steel plate have been made long and large without going out to the base material side, and the aggregate (also referred to as a stiffener) stiffeners, etc. (Non-Patent Document 1), it has been a great problem in securing the safety of the hull structure using a steel sheet having a thickness of 50 mm or more. In addition, there is a pole ESSO test as a test for evaluating securing of the safety of the hull, but the test results vary depending on the difference in the evaluation method and the constraints of the test apparatus, and the pole brittle crack propagation stop performance There has been a problem in that it is not necessarily evaluated.

The steel sheets described in the above-mentioned Patent Documents 1 to 6 do not have a description of the long-brittle brittle crack propagation stopping property, so that the problem that is clear in the non-patent document 1 can not be solved. In addition, the method described in Patent Documents 1 to 6 is not described in the method and the test apparatus for evaluating the long-brittle fracture propagation stopping property equivalent to the solid line, and the problem of safety evaluation equivalent to the solid line can not be solved.

Therefore, the present invention provides a steel sheet having a plate thickness of 50 mm or more and a welded portion thereof, wherein a brittle fracture occurs, and a brittle crack is stopped before reaching a catastrophic fracture, and a manufacturing method thereof The purpose is to provide. In addition, it is an object of the present invention to provide a method for evaluating the brittle fracture crack propagation stop performance equivalent to the solid line and a test apparatus. The term "brittle brittle crack" as used herein refers to a brittle crack having a length of 1 m or more protruding from another adjacent steel sheet.

The inventors of the present invention investigated the relationship between aggregate structure and brittle crack propagation stopping characteristics (sometimes referred to as "arrestability") for many steel sheets whose chemical composition and rolling conditions were changed, The influence of the distribution in the thickness direction of the arresting performance (affected by toughness or texture) on the development was examined. In addition, by dynamic FEM analysis of the distance between tips and distance between loading points, it is possible to simulate the long-brittle crack propagation characteristics of the solid line by using the pole ESSO Evaluation method of test, and test apparatus were examined.

As a result, when the distribution of the toughness and the texture in the plate thickness direction, which affects the performance of the alloy, is defined by controlling the chemical composition and the rolling conditions, the pole brittle crack propagation stopping performance is dramatically improved, It has been recognized that a large brittle crack propagated on a thick steel plate or its welded portion which has been considered to be a steel plate can be stopped in a steel plate under a condition equivalent to a solid line without stress reflection. As a result of the dynamic FEM analysis, the evaluation method and the testing apparatus for the pole ESSO test corresponding to the solid line without stress reflection were recognized by setting the distances between the tab plate front ends and the load load points to predetermined values. Further, since the post-steel sheet having a thickness of less than 50 mm can stop the brittle brittle crack with the current steel sheet (for example, E grade steel for shipbuilding), the present invention is intended for a post-steel sheet having a thickness of 50 mm or more.

The present invention has been accomplished on the basis of the above recognition, and the present invention has been accomplished in the following manner. That is, the present invention provides: (1) a steel sheet having a plate thickness t of 50 mm or more, The length of the stopping crack in the region of the width of 20% of the plate thickness t at the central portion of the plate thickness is 1/4 to 1/10 of the plate thickness t from the surface of the steel plate or Is shorter than the maximum crack length of the region where the plate thickness (t) is 3/4 to 9/10 of the plate thickness (t) by at least the length of the plate thickness (t) and a concave portion is formed on the surface of the steel sheet after the brittle crack propagation stopping property.

(2) the X-ray intensity ratio of the (211) plane or the (100) plane at the rolled surface of the region in the region of at least 20% of the plate thickness t at the center of the plate thickness is 1.5 or more, Ray intensity ratio of the (110) face of the rolled surface of the region which is 1/4 to 1/10 of the thickness of the sheet or the region of 3/4 to 9/10 of the sheet thickness t of 1.3 or more. , And a plate thickness (t) excellent in the brittle fracture propagation stopping property described in (1) is 50 mm or more.

(3) The (211) plane X-ray intensity ratio X ₍₂₁₁₎ and (100) plane X-ray intensity ratio X at the rolled surface of the region in the region of at least 20% ₁₀₀ and to cool the wave front transition temperature vTrs (℃) obtained by 2㎜ V-notch Charpy impact test in the same _{area: vTrs-12X (100) -22X} (211) ≤ (T-75) /0.64 [T is grated (T) of the plate thickness (t), and the area of the area which is 1/4 to 1/10 of the plate thickness (t) or the range of 3/4 to 9/10 of the plate thickness (t) (1) or (2), wherein the X-ray intensity ratio of the (110) plane at the (110) plane of the steel sheet is at least 1.3.

(4) A steel composition comprising, by mass%, 0.15% or less of C, 0.60% or less of Si, 0.80 to 1.80% of Mn, 0.001 to 0.05% of S, 0.005 to 0.050% Cu: at most 2.0%, V: at most 0.2%, Ni: at most 2.0%, Cr: at most 0.6%, Mo: at most 0.6%, W: at most 0.5% (1) to (3), wherein the brittle brittle crack propagation stop according to any one of (1) to (3), which comprises at least one selected from the group consisting of B, at most 0.0050% and Z at most 0.5% A steel plate with excellent properties and having a plate thickness (t) of 50 mm or more.

(5) A steel material having the composition described in (4) is heated to a temperature of 900 to 1350 占 폚, followed by rolling at a cumulative rolling reduction of 10% or more at a temperature range of 1000 to 850 占 폚, A temperature of 900 to 600 deg. C, and a temperature within the steel sheet becomes 50 to 150 deg. C higher than the surface temperature of the steel sheet. Thereafter, the steel sheet surface temperature Wherein the hot-rolled steel sheet is hot-rolled at 800 to 550 占 폚.

(6) The steel sheet according to the above item (5), wherein the steel sheet is cooled to 400 DEG C at a cooling rate of 5 DEG C / s or more after completion of the hot rolling. Mm or more.

(7) Test specimens In the test for evaluating and confirming the stopping performance against the brittle brittle cracks with a crack propagation length of 1 m or more using a large specimen of 2 m or more in width, the test piece length or the distance between the tip ends of the test apparatus Wherein the test piece is at least 2.8 times the width of the test piece.

(8) The evaluation method described in (7), wherein the distance between the load points of the test apparatus is 4.1 times or more of the width of the test piece, and the method further comprises the step of evaluating the magneto-resistive crack propagation stopping performance of the steel material or structure.

(9) Specimen The test apparatus for evaluating and confirming the stopping performance of a brittle brittle crack having a crack propagation length of 1 m or more by using a large test piece having a width of 2 m or more, Of the maximum brittle crack propagation stopping performance.

(10) The test apparatus as described in (9), wherein the distance between the load points of the test apparatus is at least 4.1 times the width of the test piece.

According to the present invention, it is possible to impart excellent brittle crack propagation stopping performance to a steel sheet having a plate thickness t of 50 mm or more, and it is possible to provide a brittle crack propagation stopping property in a post- It is industrially very useful because it is possible to stop the brittle brittle crack under conditions equivalent to a solid line without stress reflection.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view schematically showing the shape of a tip of a pole fracture crack propagation stop portion in a plate thickness direction section of a steel sheet having a plate thickness t of 50 mm or more according to the present invention. Fig.
2 is a view showing a dimensional shape of a large-scale duplex ESSO (ESSO) test piece.
3A shows a dynamic finite element method analysis model in the case of a parametric model for examining the influence of stress reflection on the evaluation of the brittle brittle crack propagation stopping characteristics.
Fig. 3B shows a dynamic FEM analysis model in the case where the distance between load points is 10 m for examining the influence of the stress reflection on the evaluation of the brittle brittle crack propagation stopping characteristics.
3C shows a dynamic FEM analysis model in the case where the distance between load points is 5 m for examining the influence of stress reflection on the evaluation of the brittle brittle crack propagation stopping characteristics.
Fig. 4 is a graph showing the influence of test conditions (distance from the end of the test piece) on the dynamic stress intensity factor as an analysis result by the dynamic analysis model of Fig.

(Mode for carrying out the invention)

In the present invention, the shape of the tip of the long-brittle fracture propagation stop portion in the plate thickness direction section is defined. The reason for limiting the present invention will be described below.

1 shows the shape of the leading end of the propagation stop portion (the long pole brittle fracture stop position 3) of the pole brittle crack 2 in the plate thickness direction section of the steel sheet 1 having the plate thickness t of 50 mm or more according to the present invention, Fig.

In the present invention, the shape of the tip of the long brittle fracture propagation stopper is set so that the long-brittle fracture stop position in the region of the width of 20% of the plate thickness t in the central portion of the plate thickness, (Hereinafter referred to as " depth a ") between the positions of the long brittle fracture stop positions in the width region of from 1/4 to 1/4 and 1/4 to 1/10 of the plate thickness t and from 3/4 to 9/10 of the plate thickness t, Of the long brittle fracture by a depth t which is the length of the long brittle fracture and has a concave substantially U-shaped concaved portion.

In order to improve the overall performance of the steel plate, it is preferable that a region having a width of at least 20% of the plate thickness t at the central portion of the plate thickness in the plate thickness direction section of the steel sheet, t _a (inclusive of 1/2 position of the plate thickness t) , The upper and lower 10% or more of the width region). In addition, it is preferable that the area t _a for improving the performance of the roll is set to 50% or less from the constraint of the rolling load.

(1/4) of the plate thickness (t) when the area width t _a near the center of the plate thickness, in which the arrester performance is improved, becomes less than 20% of the plate thickness (The area between the 1/4 position and the 1/10 position, including the 1/10 position and the 1/10 position) and the 3/4 to 9/10 part of the plate thickness t Position and the 9/10 position and the region between the 3/4 position and the 9/10 position) is not sufficiently lowered so that the vicinity of 1/4 to 1/10 of the plate thickness t and the plate thickness is at least 20% because the cracks do not stop and propagate in the vicinity of 3/4 to 9/10 parts of the thickness t.

In the region in the plate thickness direction cross section in which the arresting performance is superior to the other regions, the stopping length of the brittle brittle crack is short and the concave crucible is formed with respect to the traveling direction. Therefore, A region of at least 20% of the thickness t is formed into a substantially U-shaped recessed portion recessed with respect to the traveling direction of the large brittle fracture.

The shape of the substantially U-shaped concavo-convex portion is such that the fracture driving force in the vicinity of 1/4 to 1/10 of the plate thickness t and about 3/4 to 9/10 of the plate thickness t is reduced, It is preferable that the brittle crack stopping length in the region of 20% of the plate thickness of the central portion is within the range of 1/4 to 1/10 of the plate thickness t and 3/4 to 9/10 of the plate thickness t The depth a of the projected portion has to be concave at least equal to the length of the plate thickness t with respect to the traveling direction of the long brittle fracture because it is required to be shorter than the brittle crack stopping length by at least the length of the plate thickness t .

The depth a is defined as the maximum brittle crack stop position (also referred to as the maximum crack length) in the region where the plate thickness t is 1/4 to 1/10 of the plate thickness t and the plate thickness t is 3/4 to 9/10. Perpendicular to the plate thickness direction passing through the intersection of the line perpendicular to the plate thickness direction and the line parallel to the plate thickness direction indicating the area width of 20% of the plate thickness at the center of the plate thickness and the pole brittle crack propagation stop position It shall be defined as the shortest interval between lines.

In the brittle failure surface of the steel sheet having a thickness of 50 mm or more, the longest crack propagation portion ((t)) in the region of 1/4 to 1/10 of the plate thickness t or 3/4 to 9/10 of the plate thickness The vicinity of the point A and the point A 'in FIG. 1) is observed, the shape in which the position of the pole for brittle fracture propagation stops in the plate thickness direction is compared with the vicinity of the center of the plate thickness is defined in the present invention. Further, with respect to the half position of the plate thickness t, the area of 1/4 to 1/10 of the plate thickness t in the plate thickness direction from the vertically symmetrical steel plate surface and the area of 1/3 of the plate thickness t, In the range of 4 to 9/10, the arrest performance and the tip shape of the pole brittle fracture preventing stopper are substantially the same.

The shape of the tip of the above-described large-brittle fracture propagation stopper can be confirmed from the failure surface of the rod-like ESSO test piece 4 shown in Fig. The test piece 6 and the crack-running plate 5 of the long ESSO test piece 4 are bonded to each other at the CO ₂ welding portion 8 and the auxiliary plate 5 is welded to the CO ₂ welding portion 8 at a right angle A brittle crack (not shown) generated from a mechanical notch 9 is propagated along the electrogas weld 7 to form an electrogas arc weld 7 on the test plate 6, Into the test plate 6 at a right angle to the loading direction of the test plate 6. The direction of the load load is the rolling direction of the arrow RD in the drawing. In the present invention, the long brittle crack propagation stopping property is defined as a value obtained by using the long ESSO test piece 4 having a propagation length of the brittle crack until it rushes into the test plate 6, The distance between the tip end of the tapping plate and the distance between the load points is evaluated by a sufficiently long tester. The stress reflection referred to here is a reflection of a compressive stress wave generated by the generation and propagation of brittle cracks on a tab plate of a testing machine or the like. When this stress reflection occurs, the stress wave of compression returns to the brittle crack propagation portion, so that the brittle crack becomes easy to stop. In actual ships and other structures, stress reflection does not occur (or is difficult to do) because the size of the structure is sufficiently large for brittle cracks. For this reason, it is necessary to evaluate the propagation stopping characteristics of the brittle brittle crack by a tester in which the distance between the tip end of the tab plate and the load-point distance is sufficiently long.

The steel sheet according to the present invention preferably has the following texture.

The X-ray intensity ratio of the (211) plane or the (100) plane on 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 is 1/4 to 1/10 t, The X-ray intensity ratio of the (110) face at the rolled face of 4 t to 9/10 t is 1.3 or more.

When the X-ray intensity ratio of the (211) plane or the (100) plane on the rolled surface in the vicinity of the center of the plate thickness is 1.5 or more, minute subcracks are generated and brittle crack propagating surfaces are formed. The crack propagation resistance is increased, and the brittle crack propagation stop toughness is greatly improved. If the X-ray intensity ratio is less than 1.5, this effect is not recognized. From the above, the X-ray intensity ratio of the (211) plane or the (100) plane on the rolled surface in the region where the plate thickness is 20% or more 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 of the plate thickness of 1/4 to 1/10 t is less than 1.3, the brittle crack stopping length in the region of 20% The brittle crack stopping length in the region of plate thickness 1 / 4t to 1 / 10t does not become shorter than the brittle crack stopping length in the region of plate thickness 1 / 4t to 1/10t portion (around point A and point A ' The cracking propagation force of the crack propagation portion of the longest crack propagating in the crack propagation region does not decrease. Therefore, the X-ray intensity ratio of the (110) face on the rolled surface of the plate thickness of 1/4 to 1/10 t was limited to 1.3 or more. The above requirements shall be the same for plating thickness of 3 / 4t to 9 / 10t.

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

vTrs-12X ₍₁₀₀₎ -22X ₍₂₁₁₎ ? (T-75) /0.64

(Where X ₍₂₁₁₎ is the (211) plane X-ray intensity ratio at the rolled surface of the region in the region of at least 20% of the plate thickness t at the center of the plate thickness, and X ₍₁₀₀₎ (100) plane X-ray intensity ratio, vTrs (占 폚) is a wave front transition temperature obtained by a 2 mm V notch Charpy impact test at the same site, and T is a common temperature (占 폚) .

This parameter formula defines the toughness of the steel sheet as vTrs in accordance with the aggregate structure in order to secure the brittle crack propagation stop toughness of the target site in the aggregated structure at a common temperature and is characterized in that the Charpy wavefront transition temperature To make vTrs cold, specify vTrs to satisfy the above equation. In order to improve brittle crack propagation stop toughness as described above, the X-ray intensity ratio of the (211) plane or the (100) plane is required to be not less than 1.5, The coefficient of X ₍₂₁₁₎ is increased with respect to X _{( 100)} in the equation because the contribution to toughness improvement is large.

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

[Composition of ingredients]

C: 0.15% or less

C is necessary to secure strength. From the viewpoint of ensuring the strength, the lower limit is preferably set at 0.02%. However, when the amount of C exceeds 0.15%, the toughness of the weld heat affected zone (HAZ) is lowered, so that the content is limited to 0.15% or less. In addition, the preferable range for developing the texture of the (211) and (100) planes is 0.03% or less.

Si: not more than 0.60%

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

Mn: 0.80 to 1.80%

Mn is an effective element for increasing the strength, and the lower limit is set to 0.80% from the viewpoint of securing strength. However, if the Mn content exceeds 1.80%, the deterioration of the toughness of the base material is a concern. For this reason, Mn was set in the range of 0.80 to 1.80%. The 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 a crack (crack parallel to the surface of the steel sheet) in the brittle leading edge of brittle crack, it is necessary to contain 0.001% or more of S. However, since S forms a non-metallic inclusion and deteriorates ductility and toughness, it is limited to 0.05% or less.

Ti: 0.005 to 0.050%, Nb: 0.001 to 0.1%

Ti forms precipitates of carbides or nitrides and thereby inhibits the growth of austenite grains in the heating step during the production of steel sheets and contributes to grain refinement and also contributes to the improvement of the strength And also has the effect of improving the HAZ toughness by suppressing grain coarsening of the welded heat-affected zone (HAZ). In order to obtain these effects, a content of 0.005% or more is required. On the other hand, an excessive content deteriorates the toughness, so the upper limit is 0.050%.

Nb is also effective in improving precipitation strengthening and toughness. Further, the recrystallization of the austenite is suppressed, and the effect of the rolling condition, which will be described later, is promoted. In order to obtain these effects, it is necessary to add 0.001% or more, but when it is added in an amount exceeding 0.1%, the hardenend structure tends to become needle-like and the toughness tends to deteriorate. Upper limit.

Cu: at most 2.0%, V: at most 0.2%, at least 2.0% of Ni, at most 0.6% of Cr, at most 0.6% of Mo, at most 0.5% of W, at most 0.0050% of B and at most 0.5% 1 species

Cu: 2.0% or less

Cu can be used mainly for precipitation strengthening. In order to obtain the effect, it is preferably contained in an amount of 0.05% or more. If the amount of Cu exceeds 2.0%, the precipitation strengthening becomes excessive and the toughness deteriorates. Therefore, the Cu content is preferably set to 2.0% or less.

V: not more than 0.2%

V is a component that can utilize solid solution strengthening and precipitation strengthening. In order to obtain the effect, it is preferably contained in an amount of 0.001% or more. If the V content exceeds 0.2%, the base material toughness and weldability are greatly impaired, and therefore, the V content is preferably set to 0.2% or less.

Ni: not more than 2.0%

Ni improves the strength and toughness, and when Cu is added, it is effective in preventing Cu cracking during rolling. In order to obtain the effect, it is preferably contained in an amount of 0.05% or more. However, since it is expensive and its effect is saturated even when it is added excessively, it is preferable to add it in the range of 2.0% or less.

Cr: not more than 0.6%

Cr has an effect of increasing the strength. In order to obtain the effect, it is preferable to contain 0.01% or more. However, if the content exceeds 0.6%, the toughness of the welded portion deteriorates. Therefore, it is preferable that the Cr content is within a range of 0.6% or less.

Mo: 0.6% or less

Mo has an effect of raising the strength at normal temperature and high temperature. In order to obtain the effect, it is preferable to contain 0.01% or more. However, if it exceeds 0.6%, the weldability deteriorates, and therefore, the content is preferably set to 0.6% or less.

W: not more than 0.5%

W has the effect of raising the high-temperature strength. In order to obtain the effect, it is preferably contained in an amount of 0.05% or more. However, when it exceeds 0.5%, not only does it deteriorate toughness, but it is expensive, and therefore, it is preferably contained in a range of 0.5% or less.

B: not more than 0.0050%

B precipitates as BN during rolling to make the ferrite lips after rolling down finer. In order to obtain the effect, it is preferable that the content is 0.0010% or more. However, if it exceeds 0.0050%, the toughness deteriorates, so it is limited to 0.0050% or less.

Zr: not more than 0.5%

Zr is an element which improves the cracking resistance of the plating of the zinc plating material in addition to the strength. In order to obtain the effect, it is preferable to contain 0.03% or more. However, if it exceeds 0.5%, the toughness of the welded portion deteriorates. Therefore, it is preferable that the Zr content is 0.5% as the upper limit.

The steel according to the present invention is the balance Fe and inevitable impurities other than the above-mentioned composition. As the inevitable impurities, 0.035% or less of P, 0.08% or less of Al, 0.012% or less of N, 0% or less and 0.05% or less of Mg, and 0.01% or less of Mg are acceptable.

The manufacturing conditions include a heating temperature, a hot rolling condition, and a cooling condition. In the case of no description in the explanation, the temperature and the cooling rate are the average values in the plate thickness direction.

[Heating temperature]

The steel material is heated to a temperature of 900 to 1350 ° C. The reason why the heating temperature is 900 ° C or higher is the heating required to homogenize the material and control rolling which will be described later. When the temperature is set to 1350 ° C or lower, surface oxidation becomes remarkable at an excessively high temperature, The coarsening of the crystal grain can not be avoided. Further, in order to improve the toughness, it is preferable to set the upper limit to 1150 캜.

[Hot rolling condition]

Rolling of steel sheet with a cumulative rolling reduction of 10% or more at a surface temperature of 1000 to 850 ° C

By rolling at this temperature range, the austenite grain is partially recrystallized, so that the structure becomes fine and uniform.

In addition, rolling at a temperature exceeding 1000 占 폚 is not preferable for grain refinement because it promotes the growth of austenite grains. On the other hand, when the temperature is lower than 850 占 폚, the austenite non-recrystallization range is not completely achieved, which is not desirable for uniformizing the grain size.

The steel sheet surface temperature is 900 to 600 占 폚 and the internal temperature of the steel sheet is higher than the steel sheet surface temperature by 50 to 150 占 폚 so that the one-pass rolling reduction of 8% or more and the cumulative rolling reduction of 50% And then hot-rolled at a temperature of 850 to 550 占 폚.

The surface temperature of the steel sheet is 900 to 600 DEG C and the temperature inside the steel sheet is 50 to 150 DEG C higher than the surface temperature of the steel sheet so that the vicinity of the surface is almost a duplex phase region and the inside of the steel sheet is almost an austenitic non- (non-recrystallization region).

Under this condition, a rolling strain is preferentially introduced into the steel sheet having relatively low strength when rolled at a rolling reduction of 8% or more under a condition of 1% . By this process, an aggregate structure is formed in the austenite lips.

That is, a foundation of a (211) facet texture, which is a type of transformation texture effective for crack generation at the brittle crack tip, is formed. In order to introduce the aggregate structure into the region of at least 20% of the plate thickness in the central portion of the plate thickness, it is more preferable to set the one-pass reduction ratio to 10% or more.

Thereafter, the steel sheet is rolled up to a surface temperature of 850 to 550 占 폚, whereby the steel sheet is rolled into a bimetallic zone to form a (100) surface texture.

A cumulative reduction ratio of 50% or more is required in order to make the degree of integration of the texture to be at a level (an integration degree of 1.55 or more) effective for crack generation at the brittle crack tip.

[Cooling condition]

After completion of the hot rolling, the steel sheet is cooled to 400 DEG C at a cooling rate of 5 DEG C / s or more.

When the temperature range up to 400 DEG C is cooled at a cooling rate of 5 DEG C / s or higher, the inheritance from the austenite texture, which is the texture structure predominated by the (211) plane, is promoted and the brittle crack propagation stop toughness is improved.

By cooling under the above conditions, the X-ray surface strength of the (211) face becomes stronger, and the occurrence of sub cracks is further promoted, so that the cracks are easily stopped. Further, in the cooling method, a more preferable cooling start temperature is 700 ° C or more.

Needless to say, when the steel sheet according to the present invention has a steel sheet thickness of less than 50 mm, it has excellent brittle crack propagation characteristics.

[Evaluation method, test apparatus]

In order to evaluate the brittle brittle crack propagation stopping characteristics under the condition of a solid line without stress reflection, the influence of the stress reflection was evaluated by the dynamic FEM analysis, and the distance between the tip ends of the tab plates and the distance between the load points was determined. The size of the pole ESSO specimen was as shown in Fig.

Figs. 3A, 3B and 3C show the dynamic FEM analysis model, and Fig. 4 shows the results. Fig. 3A is a parametric model for confirming conditions without stress reflection, and is a model for analyzing the influence of the distance (2A in Fig. 3A) between the tester tapes 11 (thickness 200 mm) affecting the stress reflection. Fig. 3B is a model when the distance of the load load point 10 of the tester used is set to 10 m, and Fig. 3C is a model when the distance of the load load point 10 of the tester used is set to 5 m.

Fig. 4 shows the FEM analysis result. Fig. 4 is a graph showing the change in the dynamic stress intensity factor (fracture driving force during brittle crack propagation) Kd during the propagation from the occurrence of fracture to the start of the test plate. × The results shown in the table are the results in the case of 2A = 10000 mm, in which the brittle cracks do not cause stress reflection until the test plate rushes. It can be confirmed that the dynamic stress intensity factor Kd at the time of entering the test plate is lower than that in the case of 2A = 10000 mm, which is the condition corresponding to the solid line, because stress reflection occurs under the condition of 2A = 1800 to 4300 mm. This means that under the condition of 2A = 1800 to 4300 mm, the brittle brittle crack becomes easier to stop than the solid line condition. On the other hand, under the condition of 2A = 6800 mm, although it is recognized that the dynamic stress intensity factor Kd is slightly lowered, it is confirmed that it is not so different from the condition corresponding to the solid line.

Therefore, if the size of 2A is secured to be 6800 mm or more, the condition corresponding to the solid line can be evaluated. For example, if the large-size tensile test jig has a load-point distance of 10 m as shown in Fig. Fig. 4 shows the results of the analysis obtained by the model when the distance between the load points of the tester used is set to 5 m and 10 m. In the large tensile test jig shape of the 10 m model, When the ESSO test is conducted, it is recognized that evaluation under the condition of a solid line without stress reflection is made.

According to the FEM analysis described above, the evaluation of the magnitude of brittle crack propagation stopping performance under the condition of a solid line without stress reflection is performed by using a test piece length or a distance between the tip end of the test plate to which the test piece is attached is 2.8 times or more 2400 mm), and the distance between the load points of the test apparatus was 4.1 times or more of the width of the test piece (? 10000 mm / 2400 mm).

Similarly, a test apparatus capable of evaluating the magneto-brittle crack propagation stopping performance under a condition corresponding to a solid line without stress reflection is provided. The distance between the tip ends of the tab plates of the test apparatus for attaching the test specimens is 2.8 times or more (≒ 6800 mm / 2400 mm) Also, the distance between the load points of the test apparatus was 4.1 times or more of the width of the test piece (≒ 10000 mm / 2400 mm).

[Example]

The steel slabs adjusted to various chemical compositions shown in Table 1 were used to produce a steel sheet according to the conditions shown in Table 2. [ The X-ray intensity ratio between the (211) plane and the (100) plane of the central portion (high height) of the plate thickness t was measured for each of the steel plates thus obtained, and the Charpy wavefront transition Temperature (Ductile-brittle transition temperature of Charpy impact test) vTrs were investigated. The X-ray intensity ratio of the (110) face of 1/8 part of the plate thickness t (the representative part of the area of 1/4 to 1/10 of the plate thickness t) was measured.

Next, in order to evaluate the brittle brittle crack propagation stopping property, a long ESSO test piece having the dimensional shape shown in Fig. 2 was prepared by using the above-mentioned post-steel plate (the original thickness of the plate thickness t) Provided. The test was carried out under the conditions of a stress of 257 N / mm 2 and a temperature of -10 캜. Here, the stress 257N / mm 2 is the maximum allowable stress of the steel sheet with a yield stress of 40 kgf / mm 2 commonly used for the hull, and the temperature -10 ° C is the design temperature of the ship. The large-scale ESSO test was conducted under the large tensile test jig shown in Fig. 3B with a distance between the tip ends of tabs of 6800 mm and a load-point distance of 10000 mm.

Table 3 shows the results of the pole ESSO test. In the examples (Nos. 2, 3, 6, 8, 9, 12 and 14), the brittle cracks were stopped in the fillet welded part, , 13, 15 and 16), the brittle crack did not stop.

1: steel plate
2: Strong brittle crack
3: Strong brittle crack stop position
4: Pole ESSO test specimen
5: Crack-running plate
6: Pre-release
7: electrogas welding
8: CO ₂ welded part
9: Machine notch
10: load load point
11: Tab plate

Claims (13)

C: more than 0% to 0.15%, Si: more than 0% to 0.60%, Mn: 0.80 to 1.80%, S: 0.001 to 0.05%, Ti: 0.005 to 0.050% The steel sheet having a thickness (t) of 50 mm or more, the balance being Fe and unavoidable impurities, the Nb being 0.001 to 0.1%, Ni: more than 0 to 2.0% the ratio of the X-ray intensity of the (211) plane or the (100) plane on the rolled surface of the region in the region of at least 20% of the thickness t is 1.5 or more and 1/4 to 1/10 of the plate thickness t (110) surface of the rolled surface of the region having a thickness of 3/4 to 9/10 of the plate thickness (t) is 1.3 or more, and the X-ray intensity ratio of the The tip end of the long brittle fracture propagation stop portion in the cross section has a shape in which the stopping crack length in the region of 20% of the plate thickness t in the central portion of the plate thickness is smaller than the plate thickness t) (T) of the maximum crack length of the region where the maximum bending stress is 1/4 to 1/10 or 3/4 to 9/10 of the plate thickness t, , And a concave concave portion (50) having a plate thickness (t) of 50 mm or more. delete The method according to claim 1,
(211) plane X-ray intensity ratio X ₍₂₁₁₎ and (100) plane X-ray intensity ratio X _{(100) on} the rolled surface of the region in the region of at least 20% And a wave front transition temperature vTrs (占 폚) obtained by a 2 mm V notch Charpy impact test at the same site satisfy the following formula: vTrs-12X ₍₁₀₀₎ -22X ₍₂₁₁₎ ? (° C)], and the area of the area where the area is 1/4 to 1/10 of the plate thickness t or the region where the area is 3/4 to 9/10 of the plate thickness t is 110 ) Surface of X-ray intensity ratio of 1.3 or more is 50 mm or more. The method according to claim 1,
The steel according to any one of claims 1 to 3, wherein the steel composition contains, by mass%, 2.0% or less of Cu, 0.2% or less of V, 0.6% or less of Cr, 0.6% or less of Mo, 0.5% or less of W, And Zr: 0.5% or less, and having a thickness (t) of 50 mm or more. A steel material having the composition described in any one of claims 1 to 4 is heated to a temperature of 900 to 1350 占 폚 and then rolled at a cumulative rolling reduction of 10% or more at a temperature range of 1000 to 850 占 폚, The surface temperature is 900 to 600 deg. C and the steel sheet internal temperature is higher than the steel sheet surface temperature by 50 to 150 deg. C, and then the steel sheet surface A method for producing a steel plate having a thickness (t) of 50 mm or more to be hot-rolled at a temperature of 800 to 550 占 폚. 6. The method of claim 5,
Further, a method for producing a post-hot-rolled steel sheet having a sheet thickness t of 50 mm or more cooled to 400 캜 at a cooling rate of 5 캜 / s or more after completion of hot rolling. In the test for evaluating and confirming the suspension performance against the brittle brittle cracks with a crack propagation length of 1 m or more using a large test piece having a width of 2 m or more, a test piece length or a distance between the tips of the tap plates Of the test piece is 2.8 times the width of the test piece. 8. The method of claim 7,
In addition, a method for evaluating the stopping performance of a brittle brittle crack propagation of a steel material or structure having a load-point distance between the test equipment of at least 4.1 times the width of the test piece. A test apparatus for evaluating and confirming the stopping performance of a brittle brittle crack having a crack propagation length of 1 m or more using a test piece having a width of 2 m or more with a test piece width of 2 m or more is characterized in that the distance between the tip ends of the test pieces is 2.8 times Or more of the elongation brittle crack propagation stopping performance. 10. The method of claim 9,
In addition, a test apparatus for evaluating the magneto-brittle crack propagation stopping performance in which the distance between the load points of the test apparatus is at least 4.1 times the width of the specimen.
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