KR20130114239A - Thick steel plate of at least 50mm in thickness with superior long brittle fracture propagation stopping properties, manufacturing method for same, and method for evaluating long brittle fracture propagation stopping performance and test apparatus for same - Google Patents

Abstract

In the shape of the tip of the long brittle crack stop, at least the length of the plate thickness of the region of 20% of the plate thickness at the center of the plate thickness becomes 1/4 to 1/10 of the plate thickness from the steel plate surface. The recessed part is formed in the traveling direction of the long brittle crack, and the X-ray intensity ratio of the (211) plane or the (100) plane in the rolled surface of at least 20% of the sheet thickness in the sheet thickness center portion A thick steel sheet having a X-ray intensity ratio of 1.3 or more in the region of 1/4 to 1/10 of the plate thickness or the rolled surface of 3/4 to 9/10 of the plate thickness of 1.3 or more, the steel of the specific component After heating to 900-1350 degreeC, after rolling by steel plate surface temperature of 1000-850 degreeC and 10% or more of cumulative reduction rates, in a state of specific surface temperature and internal temperature, 1 pass reduction rate of 7% or more and 50% of cumulative reduction rates The manufacturing method of the thick steel plate hot-rolled at the steel plate surface temperature 800-550 degreeC at the time of rolling completion above, and An evaluation method and apparatus for evaluating propagation stop performance against large brittle cracks are provided.

Description

THICK STEEL PLATE OF AT LEAST 50MM IN THICKNESS WITH SUPERIOR LONG BRITTLE FRACTURE PROPAGATION STOPPING PROPERTIES, MANUFACTURING METHOD FOR SAME, AND METHOD FOR EVALUATING LONG BRITTLE FRACTURE PROPAGATION STOPPING PERFORMANCE AND TEST APPARATUS FOR SAME}

The present invention provides a thick steel plate having a plate thickness of 50 mm or more having excellent brittle crack arrestability suitable for use in a mega-container carrier or a bulk carrier. It relates to a manufacturing method thereof. The present invention also relates to a method for evaluating long brittle crack propagation stopping performance of an actual ship and 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, in order to ensure the rigidity and longitudinal strength of the hulls, it is necessary to thicken the outer plate of vessel's body in particular in their ships.

In recent years, container ships have become large, and in large ships of 6,000 to 20,000 TEU (twenty-foot equivalent unit), the plate thickness of the hull shell plate is 50 mm or more, and the fracture toughness is caused by the thickness effect. In addition to this decrease, the welding heat input also increases, so that the fracture toughness of the welded part tends to be further lowered. In addition, TEU (Twenty-foot Equivalent Unit) shows the number converted into the container of 20 feet in length, and has shown the index of the loading capacity of a container ship.

Relatively thin steel plates with a sheet thickness of less than 50 mm used for ships or linepipes are refined by TMCP (Thermomechanical controlled processing), and the low temperature toughness is achieved. (low-temperature toughness) can be improved and excellent brittle crack propagation stopping characteristic can be provided.

The technique of making the microstructure of the surface part of steel materials extremely fine without raising an alloy cost is proposed as a means of improving brittle crack propagation stop characteristic. For example, in Patent Literature 1, shear-lips (plastic deformation areas) generated in the steel surface layer portion when brittle cracks propagate are effective in improving the brittle crack propagation stop characteristics. In view of this, a method of miniaturizing the crystal grains of the shear grain portion and absorbing propagation energy of the brittle cracks propagated is disclosed.

After the hot rolling of the steel sheet, the process of cooling the surface layer portion below the Ar ₃ transformation point by controlled cooling, and then repeating the step of reheating the surface layer portion above the transformation point by stopping the control cooling. By repetitive transformation or recrystallization due to deformation to produce a very fine ferrite structure or bainite structure in the surface portion. will be.

Patent Literature 2 discloses a circle equivalent average particle diameter of 5 μm or less and an aspect ratio of 2 in a steel material having ferrite-pearlite as a main microstructure. A local recrystallization phenomenon is made up of a layer having 50% or more of the ferrite structure having the above-described ferrite grains, and further having a maximum rolling reduction per pass during the finish rolling of 12% or less. Is suppressed, and it is disclosed that an excellent brittle crack propagation stop characteristic improvement can be obtained by suppressing the nonuniformity of the ferrite particle size.

Patent Literature 3 describes a steel material having excellent brittle crack propagation characteristics after plastic deformation, and having subgrains in crystal grains produced by the method described in (a) to (d) below. Disclosed is a steel whose main structure is a fine ferrite having a grain formed therein.

(a) rolling conditions for securing fine ferrite grains, (b) rolling conditions for generating fine ferrite structures in portions of 5% or more of the steel plate thickness, and (c) processing (rolling) together with developing aggregated structures in fine ferrites. (D) rolling conditions for rearrangement of dislocations introduced by thermal energy and forming subgrains, and (d) suppressing coarsening of the formed fine ferrite grains and fine subgrain grains. Cooling conditions improve the brittle crack propagation stop characteristics after plastic deformation without the need for complicated temperature control such as cooling and reheating of the steel sheet surface layer.

Further, as a technical thought different from Patent Literatures 1 to 3, Patent Literature 4 develops a texture to separate the separation on the fracture surface of the steel in a direction parallel to the sheet thickness direction. In the method of increasing the brittle crack propagation characteristics by generating the stresses at the brittle crack tip, the (110) plane X-ray intensity ratio is 2 or more by controlled rolling. And it is described to make a large grain of 10 micrometers or more in circular equivalent diameter 20 micrometers or more.

Patent document 5 is a welded structural steel having excellent brittle crack propagation stop performance of a welded joint, wherein the X-ray surface strength ratio of the (100) plane in the rolled surface in the sheet thickness is 1.5 or more. A steel sheet is disclosed, and by the texture of the aggregate structure, the crack propagation direction is changed in a direction perpendicular to the stress loading direction to induce brittle cracks from the weld joint to the base metal side. In addition, it is described to improve the brittle crack propagation stop performance as a joint.

In addition, Patent Document 6 discloses that the X-ray intensity ratio of the (211) plane on the rolled surface in the plate thickness center portion is 1.3 or more, and the (100) plane X-ray intensity on the rolled surface in the plate thickness 1/4 portion. A steel sheet having a ratio of 1.5 or more and a (100) plane X-ray intensity ratio in the rolled surface at the plate surface layer portion is 1.5 or more, and the steel sheet is intruded from the surface of the steel sheet through a T-joint by the development of an aggregate structure. It has been described that a crack occurs near the brittle crack tip, and the crack acts as a crack propagation resistance to improve brittle crack propagation stop performance against brittle cracks propagating in the plate thickness direction.

On the other hand, in a hull structure, it is thought that it is necessary to stop propagation of brittle cracks and to prevent hull separation, even if brittle failure occurs from a welded part. The brittle crack propagation behavior of the welded steel plate welded portion having a sheet thickness of less than 50 mm is experimentally examined by the 147th Committee of The Shipbuilding Research Association of Japan.

According to the 147th Commission, the propagation paths and propagation behavior of the brittle cracks forced by the welds were experimentally determined, and if the fracture toughness of the welds was secured to some extent, the influence of the welding residual stress Although the brittle crack often escapes from the welded portion to the base material side, a plurality of examples in which the brittle crack propagated along the welded portion were also confirmed. This suggests that brittle fracture cannot be propagated straight along the weld.

However, in addition to having a good track record that ships constructed by applying welding equivalent to the welding applied by the 147th Committee to steel sheets less than 50 mm in thickness are in service without any problem, the toughness is good. From the recognition that steel plate base materials (such as shipbuilding grade E) have sufficient ability to stop brittle cracking, the brittle crack propagation stopping characteristic of ship's steel welds is not included in Rules and Guidance for the survey and construction of steel ships. It has not been 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 Carriers-Practical Use of New High Strength Heavy Gauge Steel Plates, '' Journal of the Korean Society of Ship Engineers and Engineers, 3, (2005), P 70.

However, in recent large container ships exceeding 6,000TEU, the plate thickness of the steel sheet exceeds 50 mm, and the fracture toughness decreases due to the sheet thickness effect. It tends to be lowered.

Recently, in such a large heat input welded joint of heavy gauge steel plate, brittle cracks generated from the welded portion are long and large by going straight without escaping to the base material side, and aggregates (or also called reinforcement materials). It is experimentally shown that it does not stop even in the steel plate base material parts, such as stiffeners, (nonpatent literature 1), and it becomes a big problem in ensuring the safety of the ship body structure to which the steel plate of 50 mm or more is applied. In addition, as a test for evaluating the safety of such a hull, there is a long ESSO test, but the test results vary depending on the differences in the evaluation method, the limitations of the test apparatus, and the like. There was a problem that it was not necessarily evaluated.

The steel sheets of Patent Documents 1 to 6 described above have no descriptions about the extended brittle crack propagation stop characteristics, and thus the problems clarified in Non-Patent Document 1 cannot be solved. Moreover, about the method and test apparatus for evaluating the long brittle crack propagation stop characteristic equivalent to a solid line, the technique of patent documents 1-6 does not have description, and the problem of safety evaluation of a solid line cannot be solved.

Therefore, the present invention relates to a thick steel sheet and a method of manufacturing the same, in which a steel plate having a plate thickness of 50 mm or more and a welded portion thereof stops the elongated brittle crack before the catastrophic fracture even when brittle fracture occurs. It aims to provide. In addition, it is an object to provide a method and a test apparatus for evaluating the solid brittle crack propagation stopping performance equivalent to a solid line. In addition, the long brittle crack here means a brittle crack of 1 m or more in length which penetrates from another adjacent steel plate.

The inventors investigated the relationship between the aggregate structure and the brittle crack propagation stop characteristics (sometimes called an arrestability) for many steel sheets which changed the chemical composition and the rolling conditions. The influence of the distribution in the plate thickness direction on the arrestor performance (affected by toughness and texture) on the phenomenon was investigated. In addition, the pole ESSO can simulate the long brittle crack propagation characteristics of the solid line by dynamic FEM analysis, which changes the distance between tips of tab plates or distance between loading points. The evaluation method of a test and the test apparatus were examined.

As a result, when the chemical composition and the rolling conditions are controlled to define the distribution in the plate thickness direction of the toughness and texture that affect the arrest performance, the pole brittle crack propagation stop performance is dramatically improved, and stopping is difficult until now. It was recognized that the thick steel plate or the long brittle crack which propagated its welded portion can be stopped in the steel sheet under the condition of solid line without stress reflection. In addition, as a result of the dynamic FEM analysis, the distance between the tip ends of the tabs and the distance between the load load points were set to predetermined values, thereby recognizing the evaluation method and the test apparatus for the long ESSO test corresponding to the solid line without stress reflection. Moreover, since the thick steel plate less than 50 mm in thickness can stop a large brittle crack with a current steel plate (for example, ship grade E steel etc.), this invention made the thick steel plate 50 mm or more thick.

The present invention has been made by further examining on the basis of the above-mentioned recognition, that is, the present invention is (1) a thick steel plate having a plate thickness (t) of 50 mm or more, and the brittle brittleness in the plate thickness direction cross section. In the tip shape of a crack propagation stop part, the stop crack length in 20% of the width | variety of the plate | board thickness t of a sheet thickness center part is 1/4-1/10 of the plate | board thickness t from the steel plate surface, or A concave recess, which is short and concave with respect to the advancing direction of the long brittle crack at least by the length of the plate thickness t with respect to the maximum crack length in the region of 3/4 to 9/10 of the plate thickness t; A thick steel sheet having a plate thickness (t) of 50 mm or more, which is excellent in pole brittle crack propagation stop characteristics, characterized by forming a concave portion.

(2) X-ray intensity ratio of (211) surface or (100) surface in the rolling surface of the site | part in the area | region at least 20% of plate | board thickness t in the said plate | board thickness center part is 1.5 or more, and said plate | board thickness (t) X-ray intensity ratio of (110) plane in the rolling surface of the area | region used as 1/4-1/10 of the area | region) or the area | region used as 3/4-9/10 of the said plate | board thickness t is 1.3 or more, It is characterized by the above-mentioned. And a thick steel sheet having a sheet thickness (t) of 50 mm or more, which is excellent in the pole brittle crack propagation stop characteristic according to (1).

(3) (211) plane X-ray intensity ratio X ₍₂₁₁₎ and (100) plane X-ray intensity ratio X in the rolling surface of the site | part in the area | region at least 20% of plate | board thickness t in the said plate | board thickness center part. ₍₁₀₀₎ and the wavefront transition temperature vTrs (° C.) obtained by the 2 mm V notch Charpy impact test of the same site is represented by the formula: vTrs-12X ₍₁₀₀₎ -22X ₍₂₁₁₎ ≤ (T-75) /0.64 [T is a steel sheet Of the common temperature (° C.), and the rolling surface of the region of 1/4 to 1/10 of the sheet thickness t or of the region of 3/4 to 9/10 of the sheet thickness t. A thick steel sheet having a thickness of 50 mm or more, which is excellent in the long brittle crack propagation stop characteristic according to (1) or (2), wherein the X-ray intensity ratio of the (110) plane in (1) is 1.3 or more.

(4) The steel composition contains, in mass%, C: 0.15% or less, Si: 0.60% or less, Mn: 0.80 to 1.80%, S: 0.001 to 0.05%, and Ti: 0.005 to 0.050% or Nb: 0.001 At least 1 sort (s) chosen from -0.1% is contained, Furthermore, 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 member selected from 0.5% or less, and the long brittle crack propagation stop according to any one of (1) to (3), which is composed of residual Fe and unavoidable impurities. A thick steel sheet having a sheet thickness (t) of 50 mm or more with excellent characteristics.

(5) The steel sheet surface after heating the steel raw material which has a component composition as described in (4) at the temperature of 900-1350 degreeC, and then rolling it by 10% or more of cumulative reduction rates in the temperature range of 1000-850 degreeC of steel plate surface temperature. The steel plate surface temperature at the end of rolling at a temperature of 900 to 600 ° C and a steel sheet internal temperature of 50 to 150 ° C higher than the steel plate surface temperature, and then at a pass reduction rate of 7% or more and a cumulative reduction rate of 50% or more. Hot-rolling at 800-550 degreeC The manufacturing method of the thick steel plate of 50 mm or more whose plate | board thickness (t) excellent in the pole brittle crack propagation stop characteristic.

(6) Furthermore, after finishing hot rolling, it cools to 400 degreeC by the cooling rate of 5 degree-C / s or more, The plate | board thickness (t) excellent in the pole brittle crack propagation stop characteristic as described in (5) is 50. The manufacturing method of the thick steel plate of mm or more.

(7) In the test for evaluating and confirming the propagation stop performance against a long brittle crack of 1 m or more in crack propagation length by using a large test specimen of 2 m or more in test piece width, the distance between the test piece length or the tab plate tip of the test apparatus to which the test piece is attached A method for evaluating the long brittle crack propagation stopping performance of a steel or a structure, characterized by being at least 2.8 times the width of the test piece.

(8) The evaluation method according to (7), wherein the distance between the load points of the test apparatus is 4.1 times or more the width of the test piece, wherein the evaluation method for the long brittle crack propagation stopping performance of the steel or the structure is performed.

(9) In the test apparatus for evaluating and confirming the propagation stop performance against long brittle cracks having a crack propagation length of 1 m or more using a large test specimen having a test piece width of 2 m or more, the distance between the tab plate ends of the test apparatus to which the test piece is attached is the test piece width. It is 2.8 times or more of the test apparatus for evaluating the pole brittle crack propagation stop performance.

(10) The test apparatus according to (9), wherein the distance between load points of the test apparatus is 4.1 times or more of the width of the test piece, wherein the test apparatus for evaluating pole brittle crack propagation stop performance.

ADVANTAGE OF THE INVENTION According to this invention, it is possible to give the brittle crack propagation stop performance excellent in the thick steel plate whose plate | board thickness t is 50 mm or more, and in the thick-material material of plate thickness 50 mm or more which was difficult until now. The large brittle cracks can be stopped under solid line equivalent conditions without stress reflection, which is very useful industrially.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows roughly the tip shape of the pole brittle crack propagation stop part in the plate thickness direction cross section of the steel plate whose sheet thickness t is 50 mm or more which concerns on this invention.
2 is a diagram showing the dimensional shape of a large-scale duplex ESSO (ESSO) test piece.
FIG. 3A shows a dynamic finite element method analysis model in the case of a parametric model for investigating the effect of stress reflection on the evaluation of pole brittle crack propagation stop characteristics.
FIG. 3B shows a dynamic FEM analysis model when the distance between load points is 10 m to investigate the effect of stress reflection on the evaluation of pole brittle crack propagation stop characteristics. FIG.
FIG. 3C shows a dynamic FEM analysis model when the distance between load load points is 5 m to investigate the effect of stress reflection on the evaluation of pole brittle crack propagation stop characteristics. FIG.
FIG. 4 is a diagram showing the influence of test conditions (distance from the end of a test piece) on a dynamic stress intensity factor as a result of analysis by the dynamic analysis model of FIG. 3.

(Mode for carrying out the invention)

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

The tip shape (pole brittle crack stop position 3) of the propagation stop part of the pole brittle crack 2 in the plate thickness direction cross section of the steel plate 1 whose plate thickness t which concerns on this invention is 50 mm or more in FIG. Schematically.

In the present invention, the tip shape of the pole brittle crack propagation stop part is defined by the pole brittle crack stop position in the 20% width region of the plate thickness t of the plate thickness center portion, and the plate thickness t from the steel plate surface. Among the positions of the long brittle crack stop position of the width | variety area which becomes / 4-1/10 and 3/4-9/10 of the plate | board thickness t, the shortest space (hereinafter, depth a) is at least plate | board thickness t It is set as the shape which is short with respect to the advancing direction of a long brittle crack by the depth t of the length of), and has a recessed substantially U-shaped recessed part.

In order to improve the restoring performance of the whole steel plate, the area | region of the width | variety of at least 20% of the plate thickness t in the sheet thickness center part of the plate thickness direction cross section of a steel plate, including t _a (1/2 position of the plate thickness t) , The arrestor performance of the region (up to 10% or more in width) is improved. In addition, the region, _a t eoreseuteu improve the performance, it is preferable that from the constraints of the rolling load (rolling load) to 50% or less.

When the area width near the center of plate thickness and t _a which improved the arrestor performance is less than 20% of the plate thickness, 1/4 to 1/10 part of the plate thickness t (a quarter of the plate thickness t) 3/4 to 9/10 parts of the plate thickness t, including the position and the 1/10 position, and the area between the 1/4 position and the 1/10 position (3/4 of the plate thickness t) Position and the 9/10 position, the breakage driving force of the region (between the 3/4 position and the 9/10 position) is not sufficiently reduced, so that 1/4 to 1/10 part of the sheet thickness t is near and the sheet thickness In the vicinity of 3 / 4-9 / 10 part of (t), since a crack does not stop but propagates, it is made into at least 20%.

In the sheet thickness direction cross section, the region having better arrest performance than the other regions has a short stop length of the long brittle crack and forms a concave recess in the advancing direction. An area of at least 20% of the thickness t is regarded as a substantially U-shaped recessed portion concave with respect to the advancing direction of the long brittle crack.

In addition, the shape of the substantially U-shaped recessed portion is such that the plate thickness is reduced in the reduction of the fracture 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 in the center portion is 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. Since it is necessary to be shorter at least by the length of the plate | board thickness t than a brittle crack stop length, the depth a of a recessed part is made into a concave shape at least equal to the length of plate | board thickness t with respect to the advancing direction of a pole brittle crack. .

Depth a is the pole brittle crack stop position (also called the maximum crack length) in the area which becomes 1/4 to 1/10 of the plate thickness t and 3/4 to 9/10 of the plate thickness t in FIG. Perpendicular to the plate thickness direction passing through the intersection of the line parallel to the plate thickness direction showing the area width of 20% of the plate thickness and the pole brittle crack propagation stop position at the plate thickness center part. It is defined as the length of the shortest interval among the intervals with the line.

On the brittle failure surface of the thick steel plate having a thickness of 50 mm or more, the longest crack propagation portion (1/4 to 1/10 of the plate thickness t or 3/4 to 9/10 of the plate thickness t) Since points A and A 'in Fig. 1) are observed, the shape in which the long brittle crack propagation stop position is drawn in the sheet thickness direction in the comparison between the sheet thickness center portion and these regions is defined in the present invention. Moreover, with respect to the 1/2 position of the plate | board thickness t, 3 / of the area | region of plate | board thickness t of 1/4 to 1/10 of the plate | board thickness t from the steel plate surface which becomes up-down symmetry, and plate | board thickness t The areas of 4 to 9/10 have roughly the same tip shape as the arrestor performance and the long brittle crack propagation stop.

The tip shape of the long brittle crack propagation stop part mentioned above can be confirmed by the failure surface of the long ESSO test piece 4 shown in FIG. The pole ESSO test piece 4 is joined to the test plate 6 and the crack-running plate 5 by the CO ₂ welded part 8, and the columnar plate 5 is perpendicular to the CO ₂ welded part 8 by the CO ₂ welded part 8. To form an electrogas arc weld 7 so that brittle cracks (not shown) generated from the mechanical notch 9 propagate along the electrogas weld 7 and the test plate 6 The test plate 6 is inclined at right angles to the loading direction. The load load direction is the rolling direction of the arrow RD in the figure. In the present invention, the long brittle crack propagation stop characteristic is the same stress reflection as that of an actual ship by using the long ESSO test piece 4 having a long propagation length of brittle crack until it enters the test plate 6. Indicates that the distance between the tip of the tab plate or the distance between the load points without any influence is evaluated by a tester that is sufficiently long. The stress reflection here is a reflection in the tab plate of testing machine of a compressive stress wave generated by the generation and propagation of brittle cracks. When this stress reflection occurs, the compressive stress wave returns to the brittle crack propagation portion, so that the brittle crack easily stops. In an actual structure of a ship or the like, stress reflection does not occur (or is difficult to do) because the size of the structure is sufficiently large for brittle cracking. For this reason, it is necessary to evaluate the propagation stop characteristic of the long brittle crack by the tester with a sufficiently long distance between the tip end of a tab board, or the distance between load load points.

It is preferable that the steel plate concerning this invention is equipped with the aggregate structure described below.

X-ray intensity ratio of (211) surface or (100) surface in the rolling surface in the area | region at least 20% of the plate | board thickness center part is 1.5 or more, plate | board thickness 1 / 4t-1 / 10t part, or plate | board thickness 3 The X-ray intensity ratio of the (110) plane on the rolled surface of the / 4t to 9 / 10t part is 1.3 or more.

When the X-ray intensity ratio of the (211) plane or the (100) plane at the rolled surface near the center of the sheet thickness is 1.5 or more, fine subcracks occur and a brittle crack propagating surface Unevenness increases, crack propagation resistance increases, and 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 in the rolled surface in the region of 20% or more of the plate thickness center portion was 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 1 / 4t to 1 / 10t part is less than 1.3, the brittle crack stop length in the region of 20% or more of the plate thickness of the plate thickness center portion is It is not shorter than the plate | board thickness more than the brittle crack stop length in the area | region of plate | board thickness 1 / 4t-1 / 10t, and is near plate | board thickness 1 / 4t-1 / 10t part (A point and A 'point of FIG. Decrease in the driving force of the longest crack propagation part). Therefore, the X-ray intensity ratio of the (110) plane in the rolling surface of the plate thickness 1 / 4t-1 / 10t part was limited to 1.3 or more. The above provision also applies to the plate thickness of 3 / 4t to 9 / 10t.

In addition, 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 equation:

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

(Wherein, X ₍₂₁₁₎ is the (211) plane X-ray intensity ratio in the rolled surface of the site in the region of at least 20% of the plate thickness t in the plate thickness center portion, X ₍₁₀₀₎ is the same. (100) plane X-ray intensity ratio of the site, vTrs (° C) is the wavefront transition temperature obtained by the 2 mm V notch Charpy impact test of the same site, T represents the common temperature (° C) of the steel sheet) .

In order to ensure the brittle crack propagation stop toughness of the target site in the aggregate at this temperature, this parameter expression defines the toughness of the steel sheet as vTrs according to the aggregate, and the Charpy wavefront transition temperature of the target site is higher than the common temperature. To make vTrs low, vTrs is defined to satisfy the above equation. In addition, as described above, in order to improve the brittle crack propagation stop toughness, the X-ray intensity ratio of the (211) plane or the (100) plane is required to be 1.5 or more. Since the contribution to toughness improvement is large, the coefficient of X ₍₂₁₁₎ is enlarged with respect to X _{( 100)} in a formula.

The preferable component composition and manufacturing conditions of the steel plate which have the above-mentioned characteristic are as follows. In description,% is taken as the mass%.

[Composition of ingredients]

C: 0.15% or less

C is necessary to secure strength. From the viewpoint of securing the strength, the lower limit is preferably 0.02%. However, when the amount of C exceeds 0.15%, the weld heat affected zone (HAZ) toughness is lowered, so it is limited to 0.15% or less. Moreover, in order to develop further the aggregate structure of (211) plane and (100) plane, the preferable range is 0.03% or less.

Si: 0.60% or less

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

Mn: 0.80 to 1.80%

Mn is an element effective for high strength, and the lower limit was 0.80% from the viewpoint of securing the strength. However, when Mn amount exceeds 1.80%, deterioration of base material toughness is feared. For this reason, Mn was made into 0.80 to 1.80% of range. Moreover, a preferable range is 1.00-1.70%.

S: 0.001 to 0.05% or less

In the present invention, since cracks (cracks parallel to the steel sheet surface) need to be generated at the leading edge of brittle cracks, the S content of 0.001% or more is necessary. However, S was limited to 0.05% or less in order to form nonmetallic inclusions and deteriorate ductility and toughness.

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

Ti forms carbide and nitride precipitates, thereby inhibiting the growth of austenite grains in the heating step during steel sheet production, contributing to finer grains, and welding. The grain coarsening of the welded heat-affected zone (HAZ) is also suppressed, thereby improving the HAZ toughness. In order to acquire these effects, 0.005% or more of containing is required. On the other hand, since excessive content deteriorates toughness, it makes 0.050% an upper limit.

Nb is also effective in improving precipitation strengthening and toughness. Moreover, recrystallization of austenite is suppressed and the effect by the rolling conditions mentioned later is promoted. In order to obtain these effects, addition of 0.001% or more is required, but when it is added in excess of 0.1%, the hardenend structure tends to be needle-shaped and the toughness tends to deteriorate. It is the upper limit.

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 selected at least Type 1

Cu: 2.0% or less

Cu can be used mainly for precipitation strengthening. In order to acquire the effect, it is preferable to contain 0.05% or more. If the amount of Cu is added in excess of 2.0%, the precipitation strengthening becomes excessive and the toughness deteriorates. Therefore, the amount of Cu is preferably 2.0% or less.

V: not more than 0.2%

V is a component that can use solid solution strengthening and precipitation strengthening. In order to acquire the effect, it is preferable to contain 0.001% or more. When V amount exceeds 0.2%, the base metal toughness and weldability are largely impaired. Therefore, the amount of V is preferably 0.2% or less.

Ni: 2.0% or less

Ni is effective for improving the strength and toughness and for preventing Cu cracking during rolling when Cu is added. In order to acquire the effect, it is preferable to contain 0.05% or more. However, since it is expensive and the effect is saturated even if it adds excessively, it is preferable to add in 2.0% or less of range.

Cr: not more than 0.6%

Cr has an effect of increasing the strength. In order to acquire the effect, it is preferable to contain 0.01% or more. However, since the weld part toughness will deteriorate when it contains exceeding 0.6%, it is preferable to make Cr content into 0.6% or less of range.

Mo: 0.6% or less

Mo has an effect of raising the strength at normal temperature and high temperature. In order to acquire the effect, it is preferable to contain 0.01% or more. However, when it contains exceeding 0.6%, since weldability will deteriorate, it is preferable to make content into the range of 0.6% or less.

W: not more than 0.5%

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

B: 0.0050% or less

B precipitates as BN during rolling to refine the ferrite grains after rolling. In order to acquire the effect, it is preferable to contain 0.0010% or more. However, since the toughness deteriorates when it exceeds 0.0050%, it was limited to 0.0050% or less.

Zr: not more than 0.5%

Zr is an element which not only raises strength but also improves the plating crack resistance of the zinc plating material. In order to acquire the effect, it is preferable to contain 0.03% or more. However, since weld part toughness deteriorates when it contains exceeding 0.5%, it is preferable to make Zr content an upper limit to 0.5%.

The steel according to the present invention is the remainder Fe and unavoidable impurities other than the above component composition. As unavoidable impurities, P: 0.035% or less, Al: 0.08% or less, N: 0.012% or less, 0: 0.05% or less, Mg: 0.01% or less can be tolerated.

As manufacturing conditions, it is preferable to define a heating temperature, a hot rolling condition, and a cooling condition. When there is no regulation in description, temperature and a cooling rate shall be an average value of the plate | board 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 means heating required to homogenize the material and control rolling described later. A temperature of 1350 ° C or lower indicates that surface oxidation becomes remarkable when the temperature becomes excessively high. This is because coarsening of grains is inevitable. In addition, in order to improve toughness, it is preferable to make an upper limit into 1150 degreeC.

[Hot rolling condition]

10% or more cumulative rolling reduction in the temperature range of 1000-850 ℃

By rolling in this temperature range, because the austenite grains are partially recrystallized, the structure becomes fine and uniform.

Moreover, rolling at the temperature exceeding 1000 degreeC is unpreferable for fine graining because it promotes growth of austenite grains. On the other hand, below 850 degreeC, since it fully enters the austenite fine recrystallization range, it is unpreferable for uniformizing a grain.

At the steel plate surface temperature of 900-600 degreeC, and after making steel plate internal temperature become 50-150 degreeC higher temperature than the steel plate surface temperature, the steel plate surface at the time of completion | finish of rolling with a 7% or more pass reduction rate and 50% or more cumulative reduction rate. Hot rolling is carried out under the conditions of a temperature of 850 to 550 ° C.

By bringing the steel plate surface temperature to 900-600 ° C. and the steel plate internal temperature being 50-150 ° C. higher than the steel plate surface temperature, the vicinity of the surface is almost a duplex phase region and the interior of the steel sheet is almost an austenite uncrystallized region. (non-recrystallization region).

In this condition, when rolling of one pass reduction rate of 7% or more is performed, rolling strain is preferentially introduced into the steel sheet having a relatively low strength, and the aggregate structure is formed in at least 20% of the sheet thickness at the center of the sheet thickness. This is introduced. By this step, an aggregate structure is formed in the austenite grain.

That is, the foundation of the (211) plane aggregate structure, which is a kind of transformation texture effective for crack generation at the brittle crack tip, is formed. Moreover, in order to introduce an aggregate into the area | region of 20% of plate | board thickness at least 20% of plate | board thickness center part, It is good to set it as 10% or more of 1 pass reduction ratio more preferably.

Thereafter, by rolling to the steel plate surface temperature of 850 to 550 ° C, the inside of the steel sheet is rolled in two phases to form a (100) plane texture.

In order to make the degree of integration of the aggregate structure at an effective level (integration rate of 1.55 or more) for crack generation at the brittle crack tip, a cumulative reduction ratio of 50% or more is required.

[Cooling condition]

After finishing hot rolling, it cools to 400 degreeC at the cooling rate of 5 degrees C / s or more.

When the temperature range up to 400 ° C. is cooled at a cooling rate of 5 ° C./s or more, the inheritance from the austenite texture, which is the (211) plane is superior, promotes brittle crack propagation stopping toughness.

When cooling under the above conditions, the X-ray surface strength of the (211) plane becomes stronger, the generation of sub cracks is further promoted, and the cracks are more likely to stop. Moreover, in the said cooling method, more preferable cooling start temperature is 700 degreeC or more.

It is needless to say that the thick steel sheet according to the present invention has excellent brittle crack propagation characteristics when the steel sheet thickness is less than 50 mm.

[Evaluation method, test apparatus]

In order to evaluate the pole brittle crack propagation stop characteristics under the solid line equivalent condition without stress reflection, the influence of the stress reflection was evaluated by dynamic FEM analysis, and the distance between the tab plate tip and the load load point of the tester was determined. The large ESSO test piece size was shown in FIG.

The results of the dynamic FEM analysis model in Figs. 3A, 3B, and 3C are shown in Fig. 4. 3A is a parametric model for confirming a condition without stress reflection, and is a model for analyzing the influence of the distance (2A in FIG. 3A) between the tester tab plate 11 (thickness 200 mm) influencing the stress reflection. 3B is a model when the distance of the load load point 10 of the tester to be used is set to 10 m, and FIG. 3C is a model when the distance of the load load point 10 of the tester to be used is set to 5 m.

4 shows the results of FEM analysis. Fig. 4 shows the change in the dynamic stress expansion coefficient (fracture driving force during brittle crack propagation) Kd during the propagation of the crack during the propagation from failure to the test plate. The result shown by the x table | surface is a case where 2A = 10000mm, and a brittle crack is a result in the solid line equivalence condition which stress reflection does not generate | occur | produce until a test plate inrush. In the conditions of 2A = 1800-4300mm, since a stress reflection generate | occur | produces, it can be confirmed that the dynamic stress expansion coefficient Kd at the time of a test board rush is lower compared with the case of 2A = 10000mm which is a solid line equivalent condition. This means that under the condition of 2A = 1800-4300 mm, large brittle cracking becomes easier to stop than solid line conditions. On the other hand, on the condition of 2A = 6800mm, although the fall of some dynamic stress expansion coefficient Kd is recognized, it turns out that it does not differ so much from a solid line equivalent condition.

Therefore, when 2A is secured to 6800 mm or more, evaluation of solid line equivalence conditions is possible, and if it is a large tensile test jig shape with a distance of 10 m between load load points shown, for example, in FIG. Although the analysis result obtained by the model when the distance between the load load points of the tester to be used is set to 5 m and 10 m is shown in FIG. 4, it is long in the shape of the large tensile test jig of the 10 m model of distance between the load load points shown in FIG. When ESSO test is performed, it is recognized that evaluation is carried out under the conditions corresponding to the solid line without stress reflection.

According to the above FEM analysis, the method for evaluating the long brittle crack propagation stop performance under a solid line equivalent condition without stress reflection has a test piece length or a distance between the tip ends of the tab plate of the test apparatus to which the test piece is attached (≒ 6800mm / 2400 mm), and 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 apparatus capable of evaluating the long brittle crack propagation stop performance under a solid line equivalent condition without stress reflection, the distance between the tip ends of the tab plates of the test apparatus to which the test specimen is attached is 2.8 times or more (≒ 6800 mm / 2400 mm), Moreover, the distance between the load load points of the test apparatus was 4.1 times or more (# 10000 mm / 2400 mm) of the test piece width.

[Example]

The thick steel plate was manufactured according to the conditions shown in Table 2 using the steel slab adjusted to the various chemical composition shown in Table 1. For each thick steel sheet thus obtained, the X-ray intensity ratio of the (211) plane and the (100) plane of the center portion (high arrest performance range) of the plate thickness t was measured, and the Charpy wavefront transition was performed. Ductile-brittle transition temperature of Charpy impact test was investigated. Moreover, the X-ray intensity ratio of the (110) plane of 1/8 part (representative site of the area | region of 1/4 to 1/10 of plate | board thickness t) of the plate | board thickness t was measured.

Next, in order to evaluate a pole brittle crack propagation stop characteristic, the pole ESSO test piece of the dimension shape shown in FIG. 2 was produced using the said thick steel plate (with the original thickness of plate | board thickness t), and, Provided. The test was performed on conditions of stress 257 N / mm <2> and temperature-10 degreeC. Here, stress 257 N / mm <2> is the maximum allowable stress of the yield stress 40 kgf / mm <2> grade steel sheet used extensively in a ship body, and temperature-10 degreeC is a design temperature of a ship. The long ESSO test was carried out in the large tensile test jig shown in Fig. 3B under a distance of 6800 mm between the tip ends of the tab plates and a distance of 10000 mm between the load load points.

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

1: steel plate
2: pole brittle crack
3: pole brittle crack stop position
4: pole ESSO test piece
5: crack-running plate
6: trial version
7: electro gas welding part
8: CO ₂ weld
9: machine notch
10: load load point
11: tab plate

Claims (10)

A thick steel sheet having a sheet thickness t of 50 mm or more, which is 20% of the sheet thickness t at the center portion of the sheet thickness in the tip shape of the long brittle crack propagation stop in the sheet thickness direction cross section. From the surface of the steel sheet to the maximum crack length in the region where the stationary crack length becomes 1/4 to 1/10 of the sheet thickness t or 3/4 to 9/10 of the sheet thickness t. A thick steel sheet having a sheet thickness t of at least 50 mm in length that is at least as long as the sheet thickness t and is formed in a concave concave portion, which is short in the traveling direction of the pole brittle crack. The method of claim 1,
X-ray intensity ratio of (211) plane or (100) plane in the rolling surface of the site | part in the area | region at least 20% of the plate | board thickness t in the said plate | board thickness center part is 1.5 or more, and 1 of the said plate | board thickness t The plate | board thickness t whose X-ray intensity ratio of the (110) plane in the rolling surface of the area | region used as / 4-1 / 10 or the area | region used as 3 / 4-9 / 10 of the said plate | board thickness t is 1.3 or more is 50 Thick steel plate of not less than mm. 3. The method according to claim 1 or 2,
(211) plane X-ray intensity ratio X ₍₂₁₁₎ and (100) plane X-ray intensity ratio X ₍₁₀₀₎ in the rolling surface of the site | part in the area | region at least 20% of plate | board thickness t in the said plate | board thickness center part _. And the wavefront transition temperature vTrs (° C.) obtained by a 2 mm V notch Charpy impact test of the same site _is expressed by the formula: vTrs-12X ₍₁₀₀₎ -22X ₍₂₁₁₎ ≤ (T-75) /0.64 [T is the common temperature of the steel sheet. (° C)] and (110) at the rolling surface of the region of 1/4 to 1/10 of the plate thickness t or of the region of 3/4 to 9/10 of the plate thickness t. A thick steel sheet having a thickness of 50 mm or more with a sheet thickness (t) having an X-ray intensity ratio of 1.3) or more. 4. The method according to any one of claims 1 to 3,
The steel composition contains, in mass%, C: 0.15% or less, Si: 0.60% or less, Mn: 0.80 to 1.80%, S: 0.001 to 0.05%, and Ti: 0.005 to 0.050% or Nb: 0.001 At least 1 sort (s) chosen from -0.1% is contained, Furthermore, 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: thick steel sheet containing at least one selected from 0.5% or less, and having a plate thickness (t) of remaining Fe and unavoidable impurities of 50 mm or more. The steel raw material having a component composition according to claim 4 is heated to a temperature of 900 to 1350 ° C, and then rolled at a cumulative reduction rate 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 temperature of 900 to The steel plate internal temperature becomes 50-150 degreeC higher temperature than the steel plate surface temperature, and after that, the steel plate surface temperature 800-550 at the end of rolling is 7% or more of 1 pass reduction rate and 50% or more of cumulative reduction rates. The manufacturing method of the thick steel plate of 50 mm or more of plate | board thickness t which hot-rolls at ° C. The method of claim 5,
Furthermore, after finishing hot rolling, the plate | board thickness t cooled to 400 degreeC by the cooling rate of 5 degreeC / s or more is a manufacturing method of the thick steel plate of 50 mm or more. In the test for evaluating and confirming the propagation stop performance against a large brittle crack having a crack propagation length of 1 m or more using a large test specimen having a test piece width of 2 m or more, the test piece length or the distance between the tab plate ends of the test apparatus to which the test piece is attached. (4) A method of evaluating the long brittle crack propagation stopping performance of a steel or structure having a width of at least 2.8 times the specimen width. The method of claim 7, wherein
Further, a method for evaluating the long brittle crack propagation stopping performance of a steel or structure in which the distance between the load points of the test device is at least 4.1 times the width of the test piece. In a test apparatus for evaluating and confirming the propagation stop performance against a large brittle crack having a crack propagation length of 1 m or more using a large test specimen having a test piece width of 2 m or more, the distance between the tip ends of the tab plates of the test apparatus to which the test piece is attached is 2.8 times the width of the test piece. The test apparatus which evaluates the above-mentioned pole brittle crack propagation stop performance. 10. The method of claim 9,
In addition, a test apparatus for evaluating the long brittle crack propagation stop performance of the test apparatus, the distance between the load load point is at least 4.1 times the test piece width.

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