KR101728362B1 - Method for manufacturing thick steel plate having excellent long brittle crack arrestability and thick steel plate - Google Patents

Abstract

Brittle crack propagation suitable for use in large container ships or bulk carriers Long-brittle crack propagation of a thick steel plate (mainly steel plate with a plate thickness of 50 mm or more) A method for evaluating performance, and a testing apparatus. When the tensile load was applied to a large test piece having a width of 1.5 m or more in the direction perpendicular to the width of the specimen and the magnitude of brittle crack propagation stopping performance against the brittle brittle crack with a crack length of 1 m or more was evaluated, the tensile load Of the large test piece is greater than or equal to at least 2.8 times the width of the large test piece, more preferably at least equal to 4.1 times the width of the large test piece, , The tensile load from the tensile tester is loaded at a position between the centers of the large test pieces in the widthwise direction at the thickness increasing portion. Test apparatus with conveying part.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a steel sheet having excellent brittle brittle crack propagation stop performance,

The present invention relates to a steel plate (mainly a steel plate having a plate thickness of 50 mm or more) which is preferably used for manufacturing a large-sized container carrier (Mega-container carrier) or a bulk carrier, A method for evaluating long brittle crack arrestability of a long brittle crack equivalent to a solid 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. Therefore, in order to ensure the rigidity and longitudinal strength of the hull, it is necessary to thicken the outer plate of the vessel's body in particular.

In recent years, the container ships have become large-sized, and the plate thickness of the outer shell of the hull is 50 mm or more in a large-sized line of 6,000 to 20,000 TEU (Twenty-foot Equivalent Unit). When the plate thickness is 50 mm or more, the fracture toughness is lowered by the thickness effect, and the weld heat input becomes larger. Therefore, the fracture toughness of the welded part Is lowered. TEU 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 the steel plates used for ships and line pipes (mainly, relatively thin steel plates having a plate thickness of less than 50 mm), grain refinement is performed by a TMCP (Thermo-Mechanical-Control-Process) method, low-temperature toughness, and can provide excellent brittle crack arrestability.

As a means for improving the brittle crack propagation stopping performance without raising the alloy cost, there has been proposed a technique of microfabricating the microstructure of the surface part of the steel. For example, Patent Document 1 discloses a technique in which a shear-lips (plastic deformation area) generated in a surface layer of a steel material when a brittle crack propagates is effective in improving brittle crack propagation stopping performance A method of making the crystal grains of the shear rib portion finer and absorbing the propagation energy of the propagating brittle cracks has been disclosed.

Patent Document 1 discloses a method of cooling a surface portion of a steel sheet to an Ar ₃ transformation point or lower by controlled cooling after hot rolling the steel sheet and thereafter stopping the control cooling to reheat the surface layer portion to the transformation point or more at least once Repeatedly, and during this time, the steel material is pressed down. In Patent Document 1, by adopting such a method, the ferrite structure or the bainite structure is formed in the surface layer portion by repeatedly transforming or recrystallization due to deformation.

In Patent Document 2, in a steel material having a microstructure mainly composed of ferrite-pearlite, both surface portions have an average of equivalent circle diameter of 5 μm or less, an aspect ratio ratio of ferrite grains having a ferrite grain size of 2 or more. Patent Document 2 discloses that when a maximum rolling reduction per pass in finish rolling is set to 12% or less to suppress a local recrystallization phenomenon and suppress variation in ferrite grain size, And an excellent effect of improving the brittle crack propagation stopping performance can be obtained.

Patent Document 3 discloses a steel material excellent in resistance to brittle crack propagation after subjected to plastic deformation and which is produced by employing the conditions described in the following (a) to (d) there is disclosed a steel material having a micro-ferrite in which a sub-grain is formed as a main structure. Patent Document 3 improves the brittle crack propagation stop performance after plastic deformation without requiring complicated temperature control such as cooling and double heating of the steel sheet surface layer.

(a) a rolling condition for securing fine ferrite grains; (b) a rolling condition for producing a micro ferrite structure at a depth of 5% or more of the steel sheet thickness from the surface; (c) Rolling conditions in which dislocations introduced by machining (rolling) are rearranged by thermal energy to form sub-grains; (d) rolling conditions for suppressing coarsening of fine fine ferrite grains and fine sub- 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 as to cause separation on a fracture surface of a steel in a direction parallel to the thickness direction, Discloses a method for improving stress-crack propagation performance by mitigating the stress at the brittle crack tip. It is also known that the controlled rolling causes the (110) plane X-ray intensity ratio to be 2 or more and the coarse overlapping with the equivalent circle diameter of 20 μm or more to 10% or less 4 < / RTI >

Patent Document 5 discloses a steel for welded structure. By using the steel for welded structure, the brittle crack propagation stopping performance in a welded joint can be enhanced. Specifically, Patent Document 5 discloses a steel sheet characterized in that the X-ray intensity ratio of the (100) face on the rolled surface inside the plate thickness is 1.5 or more as the welded structure steel. Then, in the case of this steel sheet, the crack propagation direction is changed in a direction vertical to the stress loading direction by the texture development, and the brittle crack is led from the weld joint to the base material side , And the brittle crack propagation stopping performance as a joint is improved in Patent Document 5.

Patent Document 6 discloses that the X-ray intensity ratio of the (211) plane on the rolled surface at the center of the plate thickness is 1.3 or more, and the (100) plane X-ray intensity And a (100) plane X-ray intensity ratio at the rolled surface in the surface layer portion of the plate is 1.5 or more. In this steel sheet, cracks are generated in the vicinity of the brittle crack tip protruding from the steel sheet surface through a T-joint or the like due to development of aggregate structure, and the crack acts as a crack propagation resistance, The brittle crack propagation stopping performance against the brittle crack propagated in the plate thickness direction is improved is disclosed in Patent Document 6. [

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 fracture 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 It was confirmed that the brittle cracks were easily separated from the welded portion to the base material side. In addition, in Article 147, a plurality of brittle cracks propagated along the welded portion were confirmed.

This suggests that brittle fracture can not be assumed to be unlikely to propagate straight along the weld. However, there has been a great deal of evidence that a welded ship, which is equivalent to that used in the 147th Committee, is applied to a steel plate with a thickness of less than 50mm and the ship is actually operated without any problem. Due to the recognition that steel sheet base materials (shipbuilding E grade steel, etc.) with good toughness are capable of stopping brittle cracks, the brittle crack propagation stopping performance of steel welded joints for shipbuilding is compiled in Rules and Guidance for the survey and the construction of steel ships.

However, in recent large container ships exceeding 6,000 TEU, the plate thickness of steel plates exceeds 50 mm. If the plate thickness exceeds 50 mm, the fracture toughness is lowered due to the plate thickness effect, and further, the heat input to welding becomes larger, so that the fracture toughness of the welded portion tends to be lowered further.

In recent years, brittle cracks generated from a welded portion of a large heat input welded joint of a heavy gauge steel plate have been used as long as they are straight and long (stiffeners) (Also referred to as a stiffener) or the like (see Non-Patent Document 1). This poses a serious problem in terms of securing the safety of the hull structure using a steel plate having a thickness of 50 mm or more.

A large-scale duplex ESSO (ESSO) test is a test to assess the safety of such hulls. In this pole ESSO test, there is a problem in that the test results change depending on the evaluation method and the constraints of the test apparatus, and thus the pole brittle crack propagation stopping performance corresponding to the solid line is not necessarily evaluated.

As the test method for evaluating the brittle crack propagation stopping performance (brittle crack propagation stop toughness) of the steel sheet by the techniques described in the above Patent Documents 1 to 6, there are a test such as a double tensile test and an ESSO test using a test piece having a width of about 500 mm (Steel grade qualified method of the Japan Welding Engineering Society, Nippon Kaiji Kyokai's brittleness test method), and so on. Kca test method of guidelines on brittle crack arrest design.

Further, when these steel sheets are actually applied to a structure, it is required to demonstrate the performance against a brittle brittle crack having a length of 1 m or more by a very large test such as a pole ESSO test with a width of 1.5 m or more , The test method is not specified in detail.

Japanese Patent Application Laid-Open No. 4-141517 Japanese Patent Application Laid-Open No. 2002-256375 Japanese Patent Application Laid-Open No. 11-256228 Japanese Patent Application Laid-Open No. 10-88280 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.

In the techniques described in the above-mentioned Patent Documents 1 to 6, there is no description of a method and a test apparatus for evaluating the brittle brittle crack propagation stop performance equivalent to the solid line. Therefore, the problem of safety evaluation equivalent to the solid line can not be solved by using the technique described in Patent Documents 1 to 6. Further, the steel sheets described in the above-mentioned Patent Documents 1 to 6 do not relate to the brittle crack propagation stop performance. Therefore, even if the techniques described in Patent Documents 1 to 6 are used, the problem clarified in Non-Patent Document 1 can not be solved.

Therefore, it is an object of the present invention to provide a method, a testing apparatus, and a method of manufacturing a steel plate for evaluating the brittle fracture crack propagation stopping performance equivalent to a solid line.

The present inventors have found that a more thicker part of a transfer part of a tensile testing machine (consisting of a tab plate of the tester and a pin-chuck of the tester) Evaluation method and test equipment of pole ESSO test that can simulate full-scale long-brittle crack propagation performance by dynamic finite element method analysis with varying thickness and spacing did. As a result, it was recognized that a situation corresponding to a solid line without stress reflection (reflection of a stress wave) could be reproduced by setting the thickness of the thickness increasing portion and the interval therebetween to a predetermined value.

In the present invention, the brittle fracture is a brittle crack having a length of 1 m or more protruding from another adjacent steel plate.

Further, with respect to most of steel sheets whose chemical composition and rolling conditions were changed by using the evaluation method and the test apparatus of the obtained pole ESSO test, the production conditions and brittle crack propagation stopping performance (called arrestability) of the pole- ), And obtained the recognition.

The present invention has been made based on the above recognition, and the present invention is as follows.

(1) A large test piece having a width of 1.5 m or more was loaded with a tensile load in a direction perpendicular to the width of the large test piece, and the propagation stopping performance against the brittle brittle crack with a crack length of 1 m or more was obtained. A method of evaluating crack propagation stopping performance, comprising the steps of: providing a large-sized test piece with a transfer portion for transferring a tensile load from a tensile tester to a thickness increasing portion of 2.5 times or more of the thickness of the large- And a tensile load from the tensile tester is loaded at a position between the center of the large-size test piece at the widthwise direction of the large-size test piece at a distance therebetween Wherein said steel sheet is a steel sheet.

(2) A method for evaluating the magneto-brittle crack propagation stopping performance of the steel sheet according to (1), wherein the thickness increasing portion is at an interval of at least 4.1 times the width of the large test piece.

(3) Applying a tensile load in the direction perpendicular to the width of a large test piece having a width of 1.5 m or more, the propagation stopping performance against a brittle brittle crack having a crack length of 1 m or more is obtained, Wherein a transfer portion for transferring a tensile load to the large test piece is provided with a thickness increasing portion which is at least 2.5 times the thickness of the large test piece at an interval of 2.8 times or more the width of the large test piece, And the tensile load is applied to the thickened portion at a position between the center of the large test piece in the direction of the width of the large test piece so that the load capacity becomes 50 MN ( mega-newton) or more of the brittle brittle crack propagation stopping performance of the steel sheet.

(4) An apparatus for evaluating the stopping brittle crack propagation stopping performance of the steel sheet according to (3), wherein the thickness increasing portion is provided at an interval of at least 4.1 times the width of the large test piece.

(5) The apparatus according to (3) or (4), wherein the load capacity is 80MN or more.

(6) A method of evaluating a brittle brittle crack propagation stopping performance of a post-steel plate using the apparatus described in any one of (3) to (5) ≪ / RTI >

(7) The method of producing a steel sheet according to (6), wherein the steel composition contains 0.15% or less of C, 0.6% or less of Si, 0.8 to 2.4% of Mn and 0.001 to 0.05% , At least one selected from the group consisting of 0.005 to 0.05% of Ti, and 0.001 to 0.1% of Nb, and further comprising at least one selected from the group consisting of Cu: at most 2.0%, V: at most 0.2%, Ni: at most 2.0% Or less and at least one selected from the group consisting of Fe and Mo, at most 0.6% of Mo, at most 0.5% of W, at most 0.5% of W, at most 0.0050% of B and at most 0.5% of Zr, The steel sheet is rolled at a temperature of 1350 DEG C and then subjected to a cumulative rolling reduction of 10% or more in a temperature range of 1000 to 850 DEG C on the surface of the steel sheet, Wherein the hot rolled steel sheet is produced by hot rolling at a temperature of the steel sheet surface at the end of rolling of 800 to 550 占 폚 After the process for producing a high crack propagation performance stop plate.

(8) The method according to (7), 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.

(9) A post-steel plate excellent in the brittle brittle crack propagation stopping performance, which is produced by the manufacturing method according to any one of (6) to (8).

According to the present invention, the evaluation of the brittle brittle fracture stopping performance, which has been difficult to accurately evaluate up to now, can be carried out under a condition corresponding to a solid line with no reflection of stress waves. In addition, it is possible to impart excellent brittle crack propagation stopping performance to the steel sheet having a plate thickness (t) of 50 mm or more, which has been difficult to date, and can stop the brittle brittle crack under conditions equivalent to a solid line without stress reflection It is very useful in industry.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view showing a dimensional shape of a rod-type ESSO test piece having a test piece width of 2400 mm. Fig.
Fig. 2 is a view showing a dimensional shape of a long ESSO test piece having a test piece width of 1500 mm. Fig.
3 is a diagram showing a dynamic FEM analysis model (Model 1) for examining the influence of stress reflection on the evaluation of the magneto-brittle crack propagation stopping performance.
Fig. 4 is a diagram showing a dynamic FEM analysis model (model 2) for examining the influence of stress reflection on the evaluation of the brittle brittle crack propagation stopping performance. Fig.
5 is a diagram showing a dynamic FEM analysis model (model 3) for examining the influence of stress reflection on the evaluation of the brittle fracture propagation stopping performance.
Fig. 6 shows the results of the analysis by the dynamic analysis model of Figs. 3 to 5, showing the test conditions (the thickness of the thickened portion and the thickness of the thickened portion) on the dynamic stress intensity factor (Point C in Figs. 3 to 5) Spacing). Here, Kd is the dynamic stress intensity factor (dynamic stress intensity factor at the point C in Figs. 3 to 5) when the brittle brittle crack _penetrates into the _{test plate} , Kd _{Aeff / W} = in the condition of _eff /W=8.3 (for 8.3 times the a _eff is the test piece width W), the dynamic stress intensity factor at the time of the pole brittle crack is entered the trial. The condition of A _{eff /} W = 8.3 is a condition under which a decrease in load (reduction in reaction force of pin) does not occur until the pole brittle fracture rushes into the test plate (that is, Until the unloading stress elastic wave due to crack initiation and crack opening does not reach the pin hole position.
Fig. 7 is a view showing the shape of a test piece, a tapping plate of a testing machine, and a pin chuck of a testing machine applied to the test of the pole ESSO test.

(Mode for carrying out the invention)

The present invention is for evaluating the magneto-brittle crack propagation stopping performance under a condition equivalent to a solid line without stress reflection. The present invention relates to a transducer for transmitting a tensile load from a tensile tester to a long ESSO test piece (also referred to as a large test piece), wherein a thickness increase portion of 2.5 times or more of the plate thickness t of the large test piece is 2.8 And the width of the large test piece is set at a distance from the center of the large test piece. In addition, large specimens refer to large test specimens described in Annex B (2009) of the Japan Marine Industry Association's "Guide to the Design of Brittle-Cracked Arrests".

Hereinafter, the present invention will be described with reference to Figs. Except for Fig. 6, these figures show a large tensile test jig. 6, crack-running plate 1 of a pole ESSO test specimen 1, test pole 1 of a pole ESSO test specimen 1, numeral 12 a crack-running plate of a pole ESSO test specimen 1, machined notch), 14 are electro gas arc weld (welded part of electrogas arc welding), 15 is a CO ₂ arc welding part (welded part of CO ₂ arc welding), 2 is taeppan of the testing machine, 3 pincheok, 31 of the tester is testing machine 32 denotes a nut-shaped portion (thickness increasing portion to reinforce the circumference of the fin), and W denotes the width of the pole ESSO test piece. The present invention is intended for large test specimens having a width W of at least 1.5 m. Further, the width W of the large test piece is usually 3 m or less.

First, the influence of the stress reflection was evaluated by dynamic FEM analysis, and test conditions that can neglect the influence of the stress reflection were obtained.

In Figs. 1 and 2, the shape of the rod-like ESSO test piece 1 used in the analysis and the dimensions of each part are shown. 1, the test strip 11 and the coarse abrasive plate 12 are welded to each other by a CO ₂ arc welded joint 15 (FIG. 1) along the rolling direction RD of the test plate 11 Hereinafter referred to as a CO ₂ arc welded portion 15).

A machining notch 13 is machined along a bond in the welded portion of the electro-arc arc welded portion 14 of the auxiliary plate 12 in order to generate a brittle crack. In the long ESSO test piece 1 shown in Fig. 2, the machining notch 13 is formed in the bond portion of the end portion of the electro-arc arc welded portion 14 of the auxiliary plate 12, have.

Figures 3 to 5 show the dynamic FEM analysis model. Figs. 3 and 5 show an analysis model using the pole ESSO test piece of Fig. 1, and Fig. 4 an analysis model using the pole ESSO test piece of Fig. The dynamic FEM analysis model shown is a parametrical model for identifying conditions without stress reflection, and is a model for analyzing the influence of the shape and dimensions of the transfer part.

The transmitting portion is a portion for transmitting the tensile load from the tensile tester to the large test piece (1). More specifically, the transferring portion is formed in a symmetrical manner with respect to the center of the large test piece 1 in the widthwise direction (the position of the notch tip of the machining notch 13) at the end portion to which the tensile load is applied by the large- (2) of the testing machine and a finch (3) of the testing machine. The portion where the thickness of the specimen in the thickness direction t in the transmitting portion is 2.5 times or more of the thickness t (thickness) of the specimen is defined as the thickness increasing portion. The width direction is a direction perpendicular to the rolling direction (R.D.) when viewed from the plate thickness direction, and the direction perpendicular to the width is the rolling direction (R.D.).

The thickness increase part of the transfer part is a finch 3 of the test machine which is three times the thickness t of the test piece in the case of the analytical model of Fig. 3, and 2.5 32 and the pinch roll 3 of the testing machine in the case of the analytical model of Fig. 5, the nut-shaped portion 32 (the thickness increasing portion reinforcing the circumference of the pinch hole) of the pinch roll 3 of the testing machine, to be. The upper limit of the thickness of the thickness increasing portion is not particularly limited, but usually the thickness of the thickness increasing portion is 20 times or less of the thickness t (thickness of the test piece) of the test piece.

The tensile load from the tensile tester (not shown) is loaded symmetrically in the direction perpendicular to the width of the large test piece 1 by a pin (not shown) of a tensile tester loaded in the fin hole 31 of the thickened portion .

In the present invention, the interval of the thickness increasing portion (sometimes referred to as A _eff ) is the shortest interval among the intervals of the thickness increasing portions. The interval between the thickness increasing portions is the interval (A _{eff in the} figure) between the finches 3 of the tester in which the thickness of the left and right test pieces is three times as large as the thickness t of the analytical model in Fig. 3, (A _{eff in the} figure) between the tabs 2 of the tester in which the thickness of the test piece is 2.5 times the thickness t of the test piece on the right and left sides. In the case of the analytical model of Fig. 5, (A _{eff in the} figure) of the opposed surfaces in the part 32. [

6, the test conditions (the thickness of the thickened portion and the gap ( _Aeff )) on the dynamic stress intensity factor at the time of entering the test plate 11 of the long crack (Point C in Figs. 3 to 5) .

As shown in Fig. 6, when A _eff is shortened, the dynamic stress intensity factor is determined by the shape of the thickness increasing portion around the pin hole 31 of the tester (the tap plate 2 of the tester, the pinch roller 3 or the nut- 32). And, if A _eff is shorter than 2.8 times of the test piece width W (A _eff / W is less than 2.8), the deterioration becomes even more remarkable.

6, when A _eff is 2.8 times or more of the width W of the test piece at a crack propagation speed in the range of 500 to 800 m / s (general cracking speed in which the weld propagates straightly) A dynamic stress intensity factor nearly equal to the test conditions of A _{eff /} W = 8.3 (95% or more of the test conditions in which no load drop occurs) is obtained. That is, in the pole ESSO test, if the interval A _eff of the thickness increasing portion in the transmitting portion is 2.8 or more of the width W of the test piece, the influence of the reflection of the wave of unloading characteristics is small, This is feasible.

In addition, when A _eff / W is 4.1 times or more, 97% or more of the test conditions in which the load drop does not occur make a more perfect test possible. When the A _eff / W is 6 times or more, the condition is completely the same as the test condition in which the load drop does not occur, and a more ideal test becomes possible.

From the above, when A _eff is 2.8 times or more of the test piece width W, it is possible to evaluate the condition corresponding to the solid line. For example, the interval A _eff between the screw nut like part 32 (the thickness of the large test piece 1 to a thickness of 60 to 100 mm and the thickness of 400 mm) shown in Fig. 7, When evaluating the magnitude of brittle fracture propagation stopping performance using this 8800 mm transmission part, it is possible to evaluate sufficiently the condition corresponding to the solid line.

According to the above FEM analysis, the interval A _eff of the thickness increasing portion (the tap of the testing machine or the pinch of the testing machine) for loading / conveying the load is set to be 2.8 times or more, more preferably, to be 4.1 times or more of the width of the test piece. And ideally more than 6 times.

Under the above conditions, the maximum allowable stress level (about 242 to 300 N / mm 2) of the ship was measured using specimens with a specimen width of 1.5 m or more, In order to evaluate the crack stopping performance, it is necessary to set the load capacity of the tester to 50MN or more.

Therefore, a test apparatus capable of evaluating the magneto-brittle crack propagation stopping performance without stress reflection and under the condition of a solid line is characterized in that the thickness of the thickness increasing portion (the tap of the testing machine or the pinch of the testing machine) (Shortest distance) between the thickness increase portions located at both ends of the test piece is not less than 2.8 times the width of the test piece, and the load capacity is not less than 50 MN.

It is necessary to set the load capacity of the tester to 80MN or more in the case of performing evaluation at a test piece width of 2m or more as described in Guidelines on Brittle Crack Arrest Design of the Japan Maritime Association. Therefore, it is more preferable that the load capacity is 80 MN or more. The upper limit value of the load capacity is not particularly limited, but the load capacity of the test apparatus is usually 100 MN or less.

The method for evaluating the magnitude of brittle fracture propagation stopping performance under the condition of the solid line described above makes it possible to stop the elongate brittle crack before reaching a large scale fracture (havoc) even when brittle fracture occurs in the steel sheet and its welded portion, A steel sheet having a thickness of 50 mm or more can be selected. The preferable composition of the steel sheet after this, and preferable production conditions are as follows. In the description, "%" means mass%. For reference, a steel sheet with a thickness of less than 50 mm can stop the brittle brittle crack with an existing steel sheet (for example, an E crass shipbuilding steel or the like).

[Composition of ingredients]

C: 0.15% or less

C is necessary to secure strength. From the viewpoint of ensuring the strength, it is preferable to set the lower limit of the C content to 0.02%. However, when the amount of C exceeds 0.15%, the toughness of the welded heat-affected zone (HAZ) decreases, so the upper limit of the C amount is limited to 0.15% or less. Further, in order to further develop the texture of the (211) plane and the (100) plane, the C content is preferably 0.03% or less.

Si: 0.6% or less

Si is an effective element for increasing the strength. To obtain the effect, the content of Si is preferably 0.01% or more. If the amount of Si exceeds 0.6%, the toughness of the weld heat affected zone (HAZ) is remarkably deteriorated. Therefore, the amount of Si was limited to 0.6% or less.

Mn: 0.8 to 2.4%

Mn is an effective element for increasing the strength. From the standpoint of securing strength, the Mn content was 0.8% or more. However, if the amount of Mn exceeds 2.4%, the deterioration of the toughness of the base material is concerned. Therefore, the Mn content was in the range of 0.8 to 2.4%. The preferable range of the Mn content is 1.0 to 1.7%.

S: 0.001 to 0.05% or less

Since it is necessary to generate cracks (cracks parallel to the surface of the steel sheet) at the brittle crack front edge, the amount of S should be 0.001% or more. However, S forms a non-metal inclusion and deteriorates ductility and toughness. Therefore, the amount of S was set 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 There is also an effect of suppressing crystal grain coarsening of the heat affected zone (HAZ) and improving the HAZ toughness. In order to obtain these effects, the amount of Ti should be 0.005% or more. On the other hand, if the amount of Ti is excessively large, the toughness deteriorates. Therefore, the amount of Ti should be 0.050% or less.

Nb is also effective in improving precipitation strengthening and toughness. Further, Nb suppresses recrystallization of austenite and promotes the effect of the rolling condition described later. In order to obtain these effects, the amount of Nb is 0.001% or more. If the amount of Nb exceeds 0.1%, the hardened microstructure tends to be needle-like and toughness deteriorates. Therefore, the amount of Nb is 0.1% or less.

Cu: not more than 2.0%, V: not more than 0.2%, Ni: not more than 2.0%, Cr: not more than 0.6%, Mo: not more than 0.6%, W: not more than 0.5%, B: not more than 0.0050% At least one species

Cu: 2.0% or less

Cu can be used mainly for precipitation strengthening. In order to obtain the effect, the amount of Cu is preferably 0.05% or more. When the amount of Cu exceeds 2.0%, precipitation strengthening becomes excessive and toughness deteriorates. Therefore, the amount of Cu is preferably 2.0%.

V: not more than 0.2%

V is a component that can utilize solute strengthening and precipitation strengthening. To obtain the effect, the amount of V is preferably 0.001% or more. If the V content exceeds 0.2%, the toughness and weldability of the base material are seriously impaired. Therefore, the V content is preferably 0.2% or less.

Ni: not more than 2.0%

Ni improves strength and toughness. Further, Ni is effective in preventing Cu cracking during rolling when Cu is added. In order to obtain the effect, the amount of Ni is preferably 0.05% or more. However, Ni is expensive, and the effect is saturated even if Ni is added excessively. Therefore, the amount of Ni is preferably 2.0% or less.

Cr: not more than 0.6%

Cr has an effect of increasing the strength. In order to obtain the effect, the amount of Cr is preferably 0.01% or more. However, if the Cr content exceeds 0.6%, the toughness of the welded portion deteriorates. Therefore, the amount of Cr is preferably set to 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, the amount of Mo is preferably 0.01% or more. However, if the amount of Mo exceeds 0.6%, the weldability deteriorates, so that the amount of Mo is preferably 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, the amount of W is preferably 0.05% or more. However, if the amount of W exceeds 0.5%, not only the toughness is deteriorated but also the cost is high. Therefore, the amount of W is preferably 0.5% or less.

B: not more than 0.0050%

B precipitates as BN during rolling to make the ferrite grains after rolling down finer. In order to obtain the effect, the amount of B is preferably 0.001% or more. However, if the B content exceeds 0.005%, the toughness deteriorates. Therefore, the amount of B is limited to 0.005% or less.

Zr: not more than 0.5%

In addition to increasing the strength, Zr is an element that improves the plating cracking resistance of the galvanized material. In order to obtain the effect, the amount of Zr is preferably 0.03% or more. However, if the amount of Zr exceeds 0.5%, the toughness of the welded portion deteriorates. Therefore, the amount of Zr is preferably 0.5% or less.

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, 0.05% or less of O, and 0.01% or less of Mg can be used as the inevitable impurities.

In the manufacturing conditions, it is desirable to specify heating temperature, hot rolling condition, and cooling condition. Unless otherwise specified in the description, the temperature and cooling rate shall be the average values in the thickness direction.

[Heating temperature]

The steel material is heated to a temperature of 900 to 1350 ° C. Setting the heating temperature to 900 DEG C or more is necessary to homogenize the material and to perform controlled rolling, which will be described later. In addition, the heating temperature is set to 1350 DEG C or less because, when the temperature is excessively high, surface oxidization becomes remarkable, and coarsening of crystal grains becomes inevitable. Further, in order to improve the toughness, it is preferable to set the upper limit of the heating temperature to 1150 캜.

[Hot rolling condition]

The steel sheet is rolled under the condition that the cumulative rolling reduction is 10% or more in the temperature range of 1000 to 850 캜. By this rolling, since the austenitic grains partially recrystallize, the structure becomes finer 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, rolling at a temperature of less than 850 占 폚 is undesirable for the uniformization of grains, since it enters a no-recrystallization temperature range in austenite completely below 850 占 폚. Also, when the cumulative rolling reduction is less than 10%, it is not preferable because the austenite lips are not sufficiently fine.

The hot rolling is performed under the condition that the cumulative rolling reduction is 50% or more in the temperature region of the steel sheet surface temperature of 900 to 600 占 폚, and the steel sheet surface temperature at the end of rolling is 800 to 550 占 폚. This process introduces a deformation to refine the crystal grains after transformation.

There is an effect that the crystal grains are made finer by rolling in the temperature range of 900 to 600 DEG C and the aggregate structure favorable for the arrestability develops.

Further, by setting the cumulative reduction ratio to 50% or more, there is an effect that the grain refinement and the development of the texture are further promoted.

Thereafter, rolling is performed in a temperature region where the steel sheet surface temperature is in the range of 850 to 550 占 폚, whereby the crystal grains are made finer, and a good arrester performance (stiffness crack propagation stopping performance) is obtained.

[Cooling condition]

After completion of hot rolling, it is preferable to cool to 400 캜 at a cooling rate of 5 캜 / s or higher. When the temperature range up to 400 DEG C is cooled at a cooling rate of 5 DEG C / s or higher, a bainite lath develops and crack propagation resistance becomes, and a good arrester performance (a large brittle crack propagation stop Performance) is obtained.

Example

A steel sheet was prepared in accordance with the conditions shown in Table 2 by using a steel slab adjusted to various chemical compositions shown in Table 1. For each of the steel sheets thus obtained, a rod-shaped ESSO test piece having the dimensional shape shown in Fig. 1 was prepared and used for the test by the above-mentioned method of the present invention. The test was conducted under the conditions of a stress of 257 N / mm 2 and a test temperature of -10 캜. Here, the stress 257 N / mm 2 is the maximum permissible stress of a steel sheet having a yield strength of 40 kgf / mm 2, which is commonly used for the hull, and the temperature -10 ° C is the design temperature of the ship. The pole ESSO test was conducted using the large tensile test jig shown in Fig.

The results of the pole ESSO test are shown in Table 2. Nos. 2, 3, 5, 6, 8, 9, 12 and 14 are the inventions of the manufacturing method of the present invention, and the brittle cracks are stopped in the test plate. Therefore, these can be evaluated as " good " by the evaluation method of the present invention. No. 1, 4, 7, 10, 11, 13, 15 and 16 are comparative examples of the production method of the present invention, and the brittle cracks did not stop. Therefore, these can be evaluated as " defective " by the evaluation method of the present invention.

1: pole ESSO specimen
11: Pre-release
12: The abacus plate
13: Machined notch
14: Electro gas arc welding
15: CO ₂ arc welding portion
2: Tab of test machine
3: Pinch of the tester
31: Finer hole of testing machine
32: Nut portion

Claims (9)

delete delete delete delete delete A steel composition comprising, by mass%, C: 0.15% or less, Si: 0.6% or less, Mn: 0.8 to 2.4%, S: 0.001 to 0.05%, Ti: 0.005 to 0.050%, or Nb: 0.001 to 0.1% 2.0% or less of V, 0.2% or less of Ni, at most 2.0% of Cr, at most 0.6% of Cr, at most 0.6% of Mo, at most 0.5% of W, at most 0.5% of W, : 0.0050% or less, and Zr: 0.5% or less, and the remainder Fe and inevitable impurities are heated to a temperature of 900 to 1350 캜, and then a steel sheet surface temperature of 1000 to 850 캜 , A cumulative rolling reduction of 50% or more in a temperature region of the steel sheet surface temperature of 900 to 600 占 폚 and a hot rolling at a steel sheet surface temperature of 800 to 550 占 폚 at the end of rolling As a method,
Further, in order to evaluate the propagation stop performance against the large brittle fracture with a crack length of 1 m or more in the large test piece by applying a tensile load in the widthwise direction of the large test piece having a width (W) of 1.5 m or more, A distance Aeff between the thickness-increasing portions of the transmitting portion, a tensile load from the tensile tester is transmitted to the test piece, and the thickness of the thickness increasing portion and the thickness of the large test piece And an evaluation step that enables evaluation of a condition corresponding to a solid line by considering a correlation between a width W and a dynamic stress magnification coefficient,
Wherein the thickness increasing portion has a thickness at least 2.5 times the thickness t of the large test piece and the spacing Aeff between the thickness increasing portions is less than the permissible dynamic stress intensity factor from the test conditions in which no load drop occurs (W) of the large test piece is adjusted from a range of 2.8 times or more of the width (W)
The evaluation step is carried out at a crack propagation speed in the range of 500 to 800 m / s,
Wherein the tensile load from the tensile tester is loaded at a position between the center of the large test piece in the widthwise direction and the center in the width direction of the large test piece. The method according to claim 6,
Further, after the hot rolling is finished, the steel sheet is cooled to 400 DEG C at a cooling rate of 5 DEG C / s or higher. 7. A post-steel plate excellent in the brittle brittle crack propagation stopping performance, which is produced by the manufacturing method according to claim 6 or 7. delete

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