JP2011068952A - High-strength thick steel plate superior in arrest properties - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength thick steel plate superior in arrest properties at a low cost. <P>SOLUTION: The high-strength thick steel plate superior in the arrest properties has a chemical composition comprising, by mass%, 0.01-0.12% C, 0.5% or less Si, 0.4-2.0% Mn, 0.05% or less P, 0.008% or less S, 0.002-0.05% Al, 0.01% or less N, 0.003-0.1% Nb, and the balance Fe with unavoidable impurities. The carbon equivalent Ceq expressed by the formula (1) of Ceq=C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5 is 0.32 to 0.40. The fraction of a ferrite structure in the (1/2)t part of the plate thickness is 80% or more, and the effective crystal grain size in the (1/2)t part of the plate thickness is 25 &mu;m or less. The average value of the sum of the X-ray intensity ratios of the (321), (211) and (110) faces in the direction of an angle of 45 degrees in the (1/2)t part and the (1/4)t part of the plate thickness is 3.3 or less. Furthermore, the high-strength thick steel plate may include Cu, Ni and Cr. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a thick steel plate having excellent toughness. In particular, the present invention relates to a high-strength thick steel plate having excellent properties (arrest properties) for stopping crack propagation in order to prevent collapse of the entire structure when a brittle crack occurs. Here, the high-strength thick steel plate is for a class exceeding a thickness of 50 mm, and the strength class is for a tensile strength of 490 MPa or more.

  In recent years, as the scale of various steel structures has increased, the thickness and strength required for various steel sheets used as the materials have been increasing. In particular, in recent years, international commercial transactions have become active, and the demand for maritime transportation has increased, so commercial ships have become larger. Among them, container ships of 10,000 TEU class (TEU: Twenty-foot equivalent units) are in operation, and higher strength and thicker steel plates have been required for high strength thick steel plates for containers. In such a high-strength thick steel plate, since the mechanical restraint force at the time of use also becomes large, there exists a tendency for the further improvement of the characteristic of a plate | board thickness center part to be requested | required. However, the improvement in the characteristics of the central portion of the plate thickness has not yet been fully satisfied.

  In all structures, brittle fracture instantly causes collapse of the entire structure and enormous damage is assumed. Therefore, although the building is designed to avoid the occurrence of brittle fracture, brittle fracture has occurred due to abnormal situations unexpected by the designer, such as the effects of external force exceeding the design and defects caused by construction. It is necessary to consider the case. In general, when a brittle fracture occurs, the brittle fracture spreads over the entire structure due to extremely high-speed crack propagation, and the entire structure is destroyed.

  However, a steel material that has remarkably enhanced resistance to crack propagation has the property of stopping cracks that have propagated due to propagation. This characteristic is generally called “arrest characteristic”. A structure with arrested members in place not only avoids the occurrence of brittle cracks, but even if a brittle crack occurs, it can stop the crack that has propagated due to propagation. It has double safety (double integrity) at the stage of initiation and propagation of brittle cracks. This is extremely important as the design concept of the structure.

  For example, in the shipbuilding field, there is a direction in which ships are built under a design philosophy based on this double integrity. However, as mentioned above, as the structure of commercial vessels and other structures grows in size, the thickness of the steel used is increasing, so the characteristics of the thick steel are improved both in terms of material characteristics and mechanical characteristics. The demand for is becoming more severe.

  The simplest method for imparting arrest properties to a steel material is to add Ni, which is an element that significantly improves toughness. It has been found that the effect of improving the arrest characteristics by the addition of Ni is large, and the arrest characteristics can be improved. For example, as a steel material that guarantees double integrity in a cryogenic environment of -165 ° C., a so-called 9% Ni steel added with 9% Ni is generally used, and is also defined in Japanese Industrial Standards (JIS).

Patent Document 1 promotes the refinement of crystal grains and the occurrence of separation on the fracture surface by defining the amount of reduction (two-phase rolling) between the Ar ₃ point and Ar ₁ point during rolling. Thus, a method for improving the arrest characteristic is disclosed. Patent Documents 2 and 3 disclose a method of manufacturing a steel sheet that promotes shear lip formation when a brittle crack propagates by making the surface layer structure very fine. Patent Document 4 discloses a thick steel plate and a manufacturing method thereof in which the structure fraction and the particle size of the center portion of the plate thickness are defined. According to these methods, it is possible to improve arrest characteristics without relying on expensive elements such as Ni.

JP 55-148746 A JP-A-3-2322 JP 7-126798 A JP 2009-41083 A

  As described above, the effect of improving the arrest characteristics by Ni is large, and the arrest characteristics can be improved. However, Ni is a very expensive element, and if Ni is added as much as 9%, the steel material cost will rise. Therefore, improvement of arrest characteristics by adding Ni has many problems in terms of cost.

  On the other hand, according to the invention disclosed in Patent Documents 1 to 4, it is possible to improve the arrest characteristics without adding an expensive element such as Ni. However, although the arrest characteristics in the direction parallel to the rolling direction (L direction) and the arrest characteristics in the direction perpendicular to the rolling direction (C direction) can be improved, an angle of 45 ° with respect to the rolling direction. It is difficult to improve the arrest characteristics in the direction. In the structure, since the direction in which stress is applied is not always constant, even if the arrestability is improved by these methods, the safety is not sufficient.

  Thus, the conventional steel sheet has not taken into consideration the arrest characteristics in the direction of an angle of 45 ° with respect to the rolling direction.

In view of such circumstances, an object of the present invention is to provide a high-strength thick steel plate excellent in arrest characteristics in a direction at an angle of 45 ° with respect to the rolling direction at a low cost. In particular, an object is to provide a high-strength thick steel plate having an arrest characteristic of 6000 N / mm ^1.5 or more at −10 ° C. and a tensile strength TS of 490 MPa or more at low cost.

  First, the present inventors performed an operation for extracting problems from a structural design of a container manufactured by a shipbuilding manufacturer, particularly for a steel plate for a container of a container ship.

  FIG. 1 schematically shows a cross section of a container mounted on a container ship. This container has a large opening (hatch opening) for loading and unloading cargo on the upper deck (upper deck). The hatch mouth is provided with hatch side combing. Hatch side combing is a part erected so as to surround the hatch mouth, and prevents the inflow of seawater due to waves and supports the hatch cover.

  Since this container is made of a structure having a hatch opening, it is difficult to ensure strength. Since the hull receives a large bending moment, especially a longitudinal bending moment, due to waves and the like, the container is easily damaged. In particular, hatch side combing close to the hatch mouth is subject to the largest moment. For this reason, conventionally, damage is prevented by increasing the strength and thickness of the steel sheet used for hatch side combing. However, there are certain limits to increasing the strength and thickness of the steel sheet.

  On the other hand, it is also considered to prevent damage from the viewpoint of the structural design of the container. That is, in a container, particularly hatch side combing, a crack is likely to occur in the weld line, and the crack is likely to propagate along the weld line. For this reason, a structural design is made in which the propagation of the generated crack is stopped at the end of the weld line by shifting the position of the weld line and the steel plate is welded (see FIG. 2). ).

  As described above, a steel plate having excellent arrest characteristics in a direction parallel to the rolling direction (L direction) and a direction perpendicular to the rolling direction (C direction) is usually in the L direction and the C direction, respectively. It is used as a steel plate for hatch side combing or the container itself so as to coincide with the horizontal direction. However, even if the propagation of the crack is temporarily stopped at the end of the weld line, the crack may further develop from the end. Thus, when a crack further propagates starting from the end of the weld line, the crack does not necessarily propagate from the end of the weld line in the vertical direction or only in the horizontal direction with respect to the weld line. Therefore, when a crack further propagates starting from the end of the weld line, the crack hardly propagates in the vertical direction or the horizontal direction with respect to the weld line, but 45 in the L direction and the C direction. Cracks tend to propagate in the direction of the angle of °. Therefore, it has been found that arrestability in the direction of an angle of 45 ° is also required.

  Therefore, the present inventors have conducted various studies and experiments in order to solve the above problems, and as a result, have obtained the knowledge shown in the following (a) to (h).

  (a) In order to provide a thick steel plate having arrest properties at a low cost, the addition of expensive alloy elements should be avoided.

  (b) In order to improve the arrest characteristic in a direction at an angle of 45 ° with respect to the rolling direction, the orientation of the crystal plane of the steel sheet is preferably within a certain range. Specifically, the plate thickness center part of the steel plate (hereinafter also referred to as “(1/2) t part”) and the thickness part of the steel plate that is 1/4 (hereinafter referred to as “(1/4) t part”). The average value of the sum of the X intensity ratios of the (321), (211), and (110) planes in the direction of an angle of 45 ° is preferably 3.3 or less. In (alpha) iron, (321), (211), (110) plane is a typical cleavage plane, and when the X-ray intensity ratio of these planes is low, the arrestability in an angle direction of 45 ° is excellent.

(c) Applying the TMCP method (Thermo-Mechanical Controlled Process), which utilizes the effect of grain refinement due to the reduction of the unrecrystallized region, in order to give the thick steel plate an arrest property in the direction of an angle of 45 °. Can be considered. However, in the thick steel plate to which the normal TMCP method is applied, the grain size reduction effect due to the reduction in the non-recrystallized region does not penetrate to the center of the plate thickness, so the structure size of the (1/2) t part of the thick steel plate is There is a tendency to be coarser, and the Charpy impact characteristics at the center of the plate thickness tend to be worse than the Charpy impact characteristics at the (1/4) t portion or the surface layer portion of the thick steel plate. Further, the central part of the plate thickness may have a low processing penetration, and the upper bainite structure is mainly used. In general, the upper bainite structure has lower toughness due to the influence of the hard structure (MA) between the laths than the fine-grained ferrite structure. As described above, within the range of TMCP that has been widely used so far, even if the TMCP conditions are variously adjusted, the arrest characteristic is about 4000 N / mm ^1.5 due to insufficient toughness of the structure at the center of the plate thickness. It stays and does not reach the target. Therefore, it is necessary to conduct experiments not only in general-purpose TMCP conditions but also in a wider range of TMCP conditions.

(d) Therefore, the present inventors deviated from general-purpose TMCP conditions and conducted various experiments under a wider range of TMCP conditions. As a result, the tensile strength TS of 490 MPa or more, which is the target strength of a high-strength thick steel plate. In order to realize the above, it is necessary to control to a chemical component having an appropriate hardenability, and the carbon equivalent Ceq represented by the following formula (1) is used as an index of the hardenability. And found that the carbon equivalent Ceq needs to be 0.32 to 0.40.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
Here, C, Mn, Cu, Ni, Cr, Mo and V in the formula mean the content (% by mass) of each element.

  (e) And the control of the heating conditions of the steel ingot, which is the material of the thick steel plate, is extremely effective. In particular, if the heating temperature of the steel ingot is not made lower than a certain temperature, the crystal grain size is increased. Furthermore, by setting the heating temperature and the heating time to values within a certain range, the temperature is lowered or shortened, and ferrite transformation is caused at the transformation after rolling, thereby making the initial γ grain size fine. I found out that I can.

Specifically, in the heating process of the steel ingot that is the material of the thick steel plate, the heating temperature Tr (° C.) and the heating time t (hr) of the steel ingot are both the following formulas (2) and (3). It is preferable to lower the temperature and shorten the time by heating the steel ingot so as to satisfy the above.
Tr <1050 (2)
^{400 ≦ t × exp (Tr 3} /270000000) ≦ 550 ····· (3)
Here, t represents the heating time (hr) of the steel ingot, and Tr represents the heating temperature (° C.).

  The “steel ingot” is a material that is supplied to each process such as rolling, forging, and extrusion, and is a slab that is manufactured by continuous casting and supplied to the next process by omitting the ingot process. (Continuous cast steel pieces) are also included.

  (f) Next, the ferrite structure fraction in the central part of the thickness of the thick steel sheet obtained after rolling is 80% or more, and the effective crystal grain size in the central part of the thickness is 25 μm or less. It was found to be effective for improving the toughness of the steel sheet. Here, the “effective crystal grain size” means a crystal grain size of a portion surrounded by a tissue boundary having an orientation difference of 15 ° or more when evaluated by an EBSP (Electron Backscatter Diffraction Pattern) method. .

  Thus, in order to make the thick steel plate obtained after rolling have a ferrite structure fraction of 80% or more and an effective crystal grain size of 25 μm or less at the center of the plate thickness, it is effective to lower the rolling temperature. Yes, it is effective to control the adjustment plate thickness, rolling temperature, and finish rolling temperature. This is because the dislocation density in γ before transformation can be increased by increasing the adjustment plate thickness and lowering the temperature.

For example, in the rolling step, the rolling temperature B (° C.) at an arbitrary thickness A (mm) during rolling and the finish rolling temperature C (when finishing to the final thick steel plate thickness G (mm) by final rolling ( Is rolled so that the following equation (3), (4), (5) and (6) are satisfied.
A-3.5G ≦ 0 (4)
A-1.5G ≧ 0 (5)
C-670-G ≦ 0 (6)
BC-20-1400 / G ≦ 0 (7)
Here, A is an arbitrary thickness (mm) during rolling, B is a rolling temperature (° C.) in A, C is a finish rolling temperature (° C.), and G is a thickness of the final thick steel ( mm) respectively.

(h) If tempering is performed at a temperature of Ac ₁ point or less after cooling, the hardened structure in the bainite structure may be partially detoxified, and therefore, it can be carried out as necessary.

  By such a manufacturing method, the crystal orientation as described above can be obtained. The reason why such a crystal orientation can be obtained is not necessarily clear, but in the steel of the present invention, since there are many nucleation sites at the time of austenite and ferrite transformation at the time of manufacture, the structure is mainly composed of fine ferrite as described above. As a result, the austenite orientation is inherited, and the ratio of the bainite structure, which is easy to form a texture at the time of transformation, is low, so that the crystal orientation is randomized. There seems to be little degradation of arrest characteristics in the direction.

  (i) As described above, the steel sheet in which the ferrite structure is refined at the center of the plate thickness ((1/2) t part) is extremely thick in the direction of an angle of 45 ° with respect to the rolling direction, despite being thick. It shows good arrest characteristics and fully reaches the target characteristics. However, when the heating and rolling conditions are inappropriate and the structure size at the center of the plate thickness is coarse, the toughness may deteriorate even when the ferrite fraction is high.

  The high-strength thick steel plate excellent in arrest properties according to the present invention is completed based on such knowledge, and the high-strength thick steel plate excellent in arrest properties shown in the following (1) to (6) Is the gist. Hereinafter, the present invention (1) to the present invention (6), respectively. The present invention (1) to the present invention (6) may be collectively referred to as the present invention.

(1) By mass%, C: 0.01 to 0.12%, Si: 0.5% or less, Mn: 0.4 to 2.0%, P: 0.05% or less, S: 0.008 % Or less, Al: 0.002 to 0.05%, N: 0.01% or less, Nb: 0.003 to 0.1%, and a steel plate having a chemical composition composed of the remaining Fe and inevitable impurities. The carbon equivalent Ceq represented by the following formula (1) is 0.32 to 0.40%, the ferrite structure fraction of the (1/2) t part of the plate thickness is 80% or more, and the plate The effective crystal grain size of the (1/2) t part of the thickness is 25 μm or less, and the thickness of the sum of the X-ray intensity ratios of the (321), (211), (110) planes in the direction of 45 ° A high-strength thick steel plate excellent in arrest characteristics, characterized in that an average value in a (1/2) t part and a (1/4) t part is 3.3 or less.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
Here, C, Mn, Cu, Ni, Cr, Mo and V in the formula mean the content (% by mass) of each element.

  (2) The high-strength thick steel plate having excellent arrest characteristics as described in (1) above, further containing Ni: 1.0% or less by mass%.

  (3) The above (1) or (2), characterized in that it further contains one or two elements out of elements of Cu: 2.0% or less and Cr: 1.0% or less by mass%. High strength thick steel plate with excellent arrest properties.

  (4) It is characterized by containing one or more elements out of elements of Mo: 0.5% or less, V: 0.1% or less, and B: 0.005% or less. A high-strength thick steel plate excellent in the arrest characteristics of any one of (1) to (3) above.

  (5) A high-strength thick steel plate excellent in arrest properties according to any one of (1) to (4), characterized by containing, by mass%, Ti: 0.1% or less.

  (6) It is characterized by further containing one or more elements out of elements of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less. A high-strength thick steel plate excellent in the arrest characteristics of any one of (1) to (5) above.

ADVANTAGE OF THE INVENTION According to this invention, the high intensity | strength thick steel plate excellent in the arrest characteristic of the direction of an angle of 45 degrees with respect to a rolling direction can be provided at low cost. In particular, a high-strength thick steel plate having an arrest characteristic of 6000 N / mm ^1.5 or more at −10 ° C. and a tensile strength TS of 490 MPa or more can be provided at low cost.

A cross section of a container mounted on a container ship is schematically shown. A structural design that stops the propagation of the generated crack at the end of the weld line and prevents further damage is shown.

  Below, each requirement of this invention is demonstrated in detail. Here, “%” representing the chemical composition means “mass%” unless otherwise specified.

(A) Chemical composition C: 0.01 to 0.12%
C is an element necessary for ensuring strength. And in order to make steel with practical strength, it is necessary to contain 0.01% or more. However, if its content exceeds 0.12%, the toughness deterioration of the bainite transformation region becomes remarkable and the toughness of the weld heat affected zone is also impaired. Therefore, the C content is set to 0.01 to 0.12%. From the point of balance between strength and arrest properties, the preferred range of C content is 0.03 to 0.10% Si: 0.5% or less Si is an element necessary for deoxidation in the refining stage At the same time, it is an element that contributes to an increase in strength. However, if the Si content exceeds 0.5%, the formation of island martensite in the weld heat affected zone is promoted, which adversely affects toughness. Therefore, the Si content needs to be 0.5% or less. The Si content is preferably 0.3% or less. In order to stably express the effect of Si, it is preferable to contain 0.03% or more of Si.

Mn: 0.4 to 2.0%
Mn is an element necessary for ensuring strength. And in order to make steel with practical strength, it is necessary to contain 0.4% or more. However, if it exceeds 2.0%, the toughness of the weld heat affected zone is greatly deteriorated. Therefore, the upper limit of the Mn content is 2.0%. A preferable upper limit of the Mn content is 1.6%. In order to stably obtain the strength by Mn, it is preferable to contain 0.4% or more. A more preferable content is 0.6% or more.

P: 0.05% or less P is present as an impurity and causes grain boundary cracking in the weld heat affected zone. If the P content exceeds 0.05%, the occurrence of intergranular cracks in the weld heat affected zone becomes significant, so the upper limit of the P content needs to be 0.05%. In addition, it is preferable to make the mixing amount as low as possible, and in order to obtain the arrest characteristics stably, the P content is preferably 0.03% or less.

S: 0.008% or less S is an element which is present as an impurity and forms MnS which becomes a base point of brittle fracture and impairs arrest properties. If the S content exceeds 0.008%, the arrest characteristics are remarkably deteriorated, so the upper limit of the S content as an impurity element needs to be 0.008%. In addition, it is preferable to make the mixing amount as low as possible, and in order to obtain the arrest characteristics stably, the S content is preferably 0.003% or less.

Al: 0.002 to 0.05%
Al is an element necessary for deoxidation of steel. In the case of the steel material according to the present invention, Al content of 0.002% or more is necessary for deoxidation. However, when the content exceeds 0.05%, the deterioration of arrest properties becomes remarkable through the increase of precipitates. Therefore, the Al content is 0.002 to 0.05%. Preferably it is 0.002 to 0.04%.

N: 0.01% or less N is an element which exists as an impurity and causes toughness deterioration by forming precipitates. When the content of N exceeds 0.01%, the deterioration of arrest characteristics becomes remarkable, so the content of N needs to be 0.01% or less. In addition, in order to ensure low temperature toughness, the lower one is good, and preferably 0.006% or less.

Nb: 0.003 to 0.1%
Nb is an element effective for refining the structure, improving hardenability, and increasing the strength by precipitation hardening. Particularly, Nb is a necessary element for steel materials to which the TMCP method is applied because it has a large effect of expanding the non-recrystallized region. is there. In order to exert this effect, it is necessary to contain 0.003% or more of Nb. However, when the content exceeds 0.1%, the increase in precipitates causes toughness deterioration. Therefore, the Nb content is set to 0.003 to 0.1%. Preferably it is 0.003 to 0.04%.

  The thick steel plate according to the present invention has the above-described chemical composition, with the balance being Fe and impurities. Here, an impurity is a component that is mixed due to various factors in the manufacturing process including raw materials such as ore and scrap when industrially manufacturing a thick steel plate, and does not adversely affect the present invention. It means what is allowed in the range.

  The thick steel plate according to the present invention may contain at least one of Ni, Cu, Cr, Mo, V, B, Ti, Ca, Mg and REM in addition to the above elements as follows. Good.

Ni: 1.0% or less Ni can be contained as required. When Ni is contained, the arrest characteristics of the steel sheet can be improved. However, since the Ni content causes a cost increase, the content is made 1.0% or less. Preferably it is 0.6% or less. In order to stably express the effect of improving arrest properties by Ni, it is preferable to contain Ni by 0.03% or more.

Cu: 2.0% or less Cu can be contained as necessary. When Cu is contained, the strength can be improved without deteriorating toughness. However, if the content exceeds 2.0%, the arrest properties are deteriorated due to an increase in precipitates, and further, micro cracks are generated on the surface during hot processing. The upper limit is 2.0%. A preferable upper limit of Cu is 1.0%. In order to stably develop the strength improvement effect by Cu, it is preferable to contain 0.03% or more of Cu.

Cr: 1.0% or less Cr can be contained as necessary. When Cr is contained, the strength can be increased. However, if the content exceeds 1%, the toughness is deteriorated, and further, a hardened structure is formed in the weld heat affected zone and the toughness is deteriorated. Therefore, the upper limit of the content is set to 1%. A preferable upper limit of Cr is 0.6%. In order to stably develop the strength improvement effect by Cr, it is preferable to contain 0.05% or more of Cr.

Mo: 0.5% or less Mo can be contained as necessary. When Mo is contained, the hardenability can be improved and the strength can be improved. However, the content of Mo becomes a cost increase factor, and if the content exceeds 0.5%, the toughness of the weld heat affected zone is deteriorated, so the upper limit of the content is 0.5%. A preferable upper limit of Mo is 0.3%. In order to stably develop the hardenability and strength improvement effect of Mo, it is preferable to contain 0.02% or more of Mo.

V: 0.1% or less V can be contained as necessary. Inclusion of V is effective for improving hardenability and improving strength by precipitation hardening. However, if the V content exceeds 0.1%, the toughness is significantly deteriorated. Therefore, the upper limit of the content is set to 0.1%. A preferable upper limit of V is 0.06%. In order to stably develop the hardenability and strength improvement effect by V, it is preferable to contain V by 0.003% or more.

B: 0.005% or less B can be contained if necessary. When B is contained, the ferrite transformation from the austenite grain boundary is suppressed, the hardenability is improved, and the strength can be increased. However, since the toughness deteriorates when the B content exceeds 0.005%, the upper limit of the content is set to 0.005% or less. A preferable upper limit of B is 0.0015%. In order to stably express the effect of improving hardenability and strength by B, it is preferable to contain B by 0.0003% or more. Furthermore, in the present invention, since it is necessary to ensure the ferrite content in the central portion of the plate thickness, when B is contained, it is important to fully consider the balance with the hardenability indicated by the carbon equivalent. .

Ti: 0.1% or less Ti can be contained as necessary. When Ti is contained, it is effective as a constituent element of the oxide particles, and it is effective for preventing cracking of a steel ingot manufactured by continuous casting by increasing high temperature ductility. However, if the Ti content exceeds 0.1%, TiC is generated and the toughness is deteriorated, so the upper limit of the content is 0.1%. A preferable upper limit of Ti is 0.04%. In order to stably express these effects due to Ti, it is preferable to contain 0.003% or more of Ti.

Ca: 0.004% or less Ca can be contained as necessary. When Ca is contained, it has an effect of controlling the shape of inclusions and contributes to improvement of arrest characteristics. However, if the content exceeds 0.004%, the cleanliness of the steel itself is greatly reduced, so the upper limit of the content is 0.004% or less. A preferable upper limit of Ca is 0.002%. In order to stably express these effects by Ca, it is preferable to contain 0.0003% or more of Ca.

Mg: 0.002% or less Mg can be contained as necessary. When Mg is contained, the dispersion density of the fine oxide can be increased. However, if the content exceeds 0.002%, fine oxides cannot be obtained, and the cleanliness of the steel is greatly reduced, so the upper limit of the content is made 0.002% or less. A preferable upper limit of Mg is 0.0015%. In order to stably exhibit the effect of improving the fine oxide dispersion density by Mg, it is preferable to contain 0.0002% or more of Mg. Here, the step of containing Mg in the molten steel is preferably performed before Al is contained in the molten steel.

REM: 0.002% or less REM (rare earth element) can be contained as required. When REM is contained, the dispersion density of the fine oxide can be increased as in the case of Mg. Furthermore, the effect of fixing excess S as sulfides can also be obtained. However, if the content exceeds 0.002%, fine oxides cannot be obtained, and the cleanliness of the steel is greatly reduced, so the upper limit of the content is made 0.002% or less. A preferable upper limit of REM is 0.0015%. In addition, in order to stably express these effects by REM, it is preferable to contain REM 0.0002% or more.

  Here, the step of incorporating REM in the molten steel is preferably performed before Al is contained in the molten steel. REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanide, and one or more of these elements can be contained. Each REM element may be separated and contained in steel, or may be contained in steel in a mixed state called misch metal. Note that the content of REM means the total content of these elements.

(B) Hardenability Since the high-strength thick steel plate specified in the present invention is used as a strength member, it needs to have sufficient strength as a standard material. Therefore, it is necessary that the chemical composition of the high-strength thick steel plate not only satisfies each specified range but also has an appropriate hardenability. Carbon equivalent can be used as a parameter representing the hardenability of the high strength thick steel plate. In particular, in the case of a high-strength thick steel plate with a tensile strength of 490 MPa or more, a carbon equivalent formula defined by IIW (International Institute of Welding) can be used. That is, it can be arranged using the carbon equivalent Ceq represented by the following formula (1).
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
Here, C, Mn, Cu, Ni, Cr, Mo and V in the formula mean the content (% by mass) of each element.

  When the carbon equivalent Ceq is less than 0.32%, sufficient strength is not ensured. Conversely, when the carbon equivalent Ceq is more than 0.40%, the ferrite structure fraction at the center of the plate thickness cannot be ensured. Therefore, the carbon equivalent Ceq is defined as 0.32 to 0.40%. A preferred range for the carbon equivalent Ceq is 0.32 to 0.38%.

(C) Ferrite structure fraction The central part of the plate thickness may have a low processing penetration, and the upper bainite structure is the main component. In general, the upper bainite structure has lower toughness due to the influence of the hard structure (MA) between the laths than the fine-grained ferrite structure.

  Therefore, since the toughness tends to deteriorate when the upper bainite structure increases, it is necessary to increase the ferrite structure. Examples of methods for increasing the ferrite structure include an increase in γ grain boundaries due to low-temperature heating and an expansion of the ferrite generation range due to processing induction.

  As a result of testing on steels having various ferrite ratios, the present inventors have found that steels having a ferrite structure fraction of 80% or more possess excellent arrest properties. Therefore, the ferrite structure fraction in the steel is defined as 80% or more. Preferably it is 85% or more.

  In addition to the optical microscope, the ferrite structure fraction can be evaluated based on observation using a scanning electron microscope and a transmission electron microscope having an acceleration voltage of 100 to 200 kV. Here, the ferrite structure fraction is evaluated by the area ratio of ferrite. Specifically, for 100 visual fields observed by these observation methods, after calculating the area ratio of ferrite to the total visual field area in each visual field, the average value of the area ratio of ferrite of 100 visual fields is obtained.

(D) About effective crystal grain size The toughness of high-strength thick steel sheet defines the ferrite structure fraction of the thickness center part ((1/2) t part) of the thick steel sheet obtained after rolling as 80% or more. In addition, it has been found that the effective crystal grain size at the center of the plate thickness is improved by setting it to 25 μm or less.

  Note that the effective crystal grain size is measured based on the grain boundary observed with an optical microscope or a scanning electron microscope, and when the difference between the orientations of adjacent crystal grains is small, there is a correspondence with the fracture surface unit. It cannot be a numerical value representing the tissue size. Therefore, in the present invention, the “effective crystal grain size” means a crystal grain size of a portion surrounded by a structure boundary having an orientation difference of 15 ° or more when evaluated by EBSP. That is, by using EBSP (Electron Backscatter Diffraction Pattern) method, observation of 5 fields or more is performed at a magnification of 300 times, a structure boundary having an orientation difference of 15 ° or more is regarded as a grain boundary, The area inside the crystal was determined, and the area converted to the equivalent circle diameter was evaluated as the effective crystal grain size.

  (E) X-ray intensity ratio Regarding the arrest characteristics in the direction having a 45 ° angle with respect to the rolling direction, the X-ray intensity ratio of the (321), (211), (110) planes in the direction of the 45 ° angle. When the average value of the (1/2) t portion and (1/4) t portion of the sum of the thicknesses is 3.3 or less, the thickness is remarkably improved.

  When the amount of reduction at a low temperature exceeds a certain level, the accumulation of cleavage planes in the direction of an angle of 45 ° is relaxed, and the accumulation of cleavage planes in the Z direction becomes significant. For this reason, it is estimated that the arrest characteristic in the direction of an angle of 45 ° with respect to the rolling direction is improved.

  Note that the X-ray intensity ratio mentioned here is the ratio of X-ray intensity with a sample having a random crystal orientation, and this can be measured by using an X-ray diffractometer. Moreover, since the temperature history at the time of rolling is significantly different from the inside of the steel sheet, the X-ray intensity ratio varies greatly immediately below the surface of the steel sheet. For this reason, it is desirable to collect samples while avoiding the area directly below the surface. In the present invention, samples at an angle of 45 ° from the rolling direction are used in the (1/2) t and (1/4) t parts of the plate thickness. The sample was collected and the X-ray intensity ratio was measured.

(F) Manufacturing conditions The manufacturing conditions described in detail below are one of the methods for realizing the above-mentioned thick steel plate economically and in a reasonable manner. The technical scope of the thick steel plate itself depends on the manufacturing conditions. It is not specified.

  Control of the heating condition of the steel ingot that is the material of the thick steel plate, that is, the control of the heating temperature and the heating time is the main production condition for controlling the initial γ grain size at the time of reheating the steel ingot. Very important.

High-temperature heating or long-time heating promotes the growth of γ grains, which reduces the number of ferrite formation nuclei during α transformation, which reduces the ferrite structure fraction in the final structure and increases the waiting time during rolling. Since it is timed, it results in a loss of economic efficiency. Therefore, it is necessary to control the heating temperature low and the heating time short. However, since temperature and time are equivalent, it is sufficient to satisfy one of the conditions. That is, by reducing the heating temperature or shortening the time, ferrite transformation can occur during transformation after rolling, and the initial γ grain size can be made fine. When this equivalence was experimentally clarified, the heating temperature Tr (° C.) and the heating time t (hr) of the steel ingot in the heating process of the steel ingot, which is the material of the thick steel plate, are the following (2) It was found that satisfying both the formula and the formula (3) is preferable as a condition for economically producing a high-strength thick steel plate having excellent arrest characteristics.
Tr <1050 (2)
^{400 ≦ t × exp (Tr 3} /270000000) ≦ 550 ····· (3)
Here, t represents the heating time (hr) of the steel ingot, and Tr represents the heating temperature (° C.).

  In addition, when the heating temperature is extremely low, it becomes difficult to realize rolling due to an increase in deformation resistance or the like. Therefore, the heating temperature is preferably 800 ° C. or higher. However, the heating temperature is preferably 1050 ° C. or lower.

  Next, as a method for economically obtaining a high-strength thick steel plate having excellent arrest characteristics, it is also effective to control the adjustment plate thickness, rolling temperature, and finish rolling temperature in the subsequent rolling process. By applying TMCP technology that promotes ferrite transformation by increasing the amount of rolling in the non-recrystallized region and increasing the dislocation density in γ before α transformation, it is sufficient even at the center of thick plate thickness This is because a good ferrite transformation can be expected.

  As manufacturing parameters for controlling the amount of reduction in the non-recrystallized region, it has been found that the adjustment plate thickness, the rolling temperature during adjustment, and the finish rolling temperature are important.

The conditions obtained by many experiments by the present inventors are, in the rolling process, rolling temperature (adjusting rolling temperature) B (° C.) at an arbitrary thickness (adjusted plate thickness) A (mm) during rolling, The finishing rolling temperature C (° C) when finishing to the final thick steel sheet thickness G (mm) by final rolling is the following (3), (4), (5) and (6). Roll to satisfy.
A-3.5G ≦ 0 (4)
A-1.5G ≧ 0 (5)
C-670-G ≦ 0 (6)
BC-20-1400 / G ≦ 0 (7)
Here, A is an arbitrary thickness (mm) during rolling, B is the rolling temperature (° C.) in A, C is the finish rolling temperature (° C.), and G is the thickness of the final thick steel ( mm) respectively.

  If any one of the above formulas (4) to (7) is not satisfied, the dislocation density before α transformation is insufficient, and the ferrite structure fraction in the structure at the center of the plate thickness is reduced. In addition, it is not possible to efficiently obtain a high-strength thick steel plate having excellent arrest characteristics.

In order to ensure sufficient strength, in the case of such a thick material, it is also effective to control the cooling rate and cooling stop temperature in the cooling process after rolling, and the cooling rate during water cooling is 2 ° C / The water cooling stop temperature is preferably 500 ° C. or lower. That is, the water cooling is performed so that the water cooling stop temperature E (° C.) and the average cooling rate F (° C./s) during the water cooling at the center of the plate thickness satisfy the following equations (8) and (9). preferable.
E-500 ≦ 0 (8)
F-5 ≧ 0 (9)
Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during the water cooling at the thickness center portion ((1/2) t portion).

In addition, when tempering at a temperature of Ac ₁ point or less after cooling, the hardened structure in the bainite structure may have an effect of detoxifying partly.

  Table 1 shows the chemical components of the steels tested this time. Of these steels, steel Nos. 30 to 35 are comparative steels, and do not satisfy the component range defined in the present invention or the carbon equivalent Ceq represented by the formula (1).

  Using these various steels, various high-strength thick steel plates were manufactured based on the manufacturing conditions shown in Table 2. In addition, although test No. 1-2 is not clearly shown in Table 2, tempering is performed at 520 ° C. after cooling.

  Table 2 shows the sum of the manufacturing parameters, the obtained structure and crystal grain size, and the X-ray intensity ratios of the (321), (211), and (110) planes in the direction of an angle of 45 °.

  It should be noted that the thickness of the sum of the X-ray intensity ratios of the (321), (211), and (110) planes in the direction of an angle of 45 ° at the (1/2) t and (1/4) t parts The value was measured in an X-ray diffraction test by collecting a test piece of 20 mm × 20 mm from a direction at an angle of 45 ° with respect to the rolling direction.

  About the characteristic of the obtained steel plate, the specimen was extract | collected according to the test method as described in JIS-Z-2201 in the tension test. The sampling position was the (1/4) t portion of the plate thickness and the C direction (direction perpendicular to the rolling direction). The yield point was determined as a test speed of 10 N / (mm · s), and the yield point was 0.2% proof stress when no clear yield point appeared. The target value of strength was set to tensile strength TS ≧ 490 MPa.

As a method for evaluating the arrest properties, a plurality of temperature gradient type ESSO tests were conducted, and the obtained results were plotted on an Arrhenius graph to perform a linear approximation, and the Kca value at −10 ° C. was calculated for the arrest of the steel. The evaluation representative value as the characteristic (N / mm ^1.5 ) was used. The target value of the arrest characteristic was set to 6000 N / mm ^1.5 .

  The arrest characteristics and vTrs have a certain degree of correlation, and it is possible to roughly know whether the arrest characteristics are good or bad. Therefore, a Charpy impact test was performed in order to confirm that the arrest characteristics in the 45 ° direction are not inferior to those in the L and C directions. First, after determining the fracture surface transition temperature (vTrs) in the rolling direction and in the direction of an angle of 45 ° from the position of the (1/4) t portion of the sheet thickness in accordance with the test method described in JIS-Z-2242. According to the following formula (9), the difference in toughness between the rolling direction and the 45 ° angle is evaluated by obtaining ΔvTrs of the difference in fracture surface transition temperature (vTrs) between the rolling direction and the 45 ° angle direction. did.

ΔvTrs = vTrsL × (-1) -vTrs45 × (-1) ... (8)
Here, vTrsL: vTrs in the rolling direction and vTrs45: vTrs in the 45 ° direction, respectively. The target value of ΔvTrs was set to ≦ 10 ° C.

  Table 3 shows the evaluation results of mechanical properties (yield strength YS [MPa] and tensile strength TS [MPa]), arrest properties, and ΔvTrs of each high-strength thick steel plate.

  From Table 3, Test Nos. 1-2, 1-3, 2-6, 3-1 and Test Nos. 4 to 23 according to the present invention are necessary even though they are all thick steel plates. High arrest characteristics are secured while maintaining strength characteristics. In addition, no deterioration was observed with respect to the toughness in the direction of an angle of 45 °, and it is suggested that excellent arrest characteristics are exhibited regardless of the stress load direction.

As described above, according to the present invention, it is possible to provide a high-strength thick steel plate excellent in arrest characteristics in a direction at an angle of 45 ° with respect to the rolling direction at a low cost. In particular, a high-strength thick steel plate having an arrest characteristic of 6000 N / mm ^1.5 or more at −10 ° C. and a tensile strength TS of 490 MPa or more can be provided at low cost. Therefore, high brittle crack propagation stopping characteristics can be stably imparted to a high-strength thick steel plate applied to a steel structure that needs to prevent large-scale fracture due to brittle fracture at a low cost. It can contribute to the improvement of the destruction resistance safety of the structure, and the social effect is extremely large.

Claims (6)

In mass%, C: 0.01 to 0.12%, Si: 0.5% or less, Mn: 0.4 to 2.0%, P: 0.05% or less, S: 0.008% or less, A steel plate containing Al: 0.002-0.05%, N: 0.01% or less, Nb: 0.003-0.1%, and having a chemical composition consisting of the balance Fe and inevitable impurities, The carbon equivalent Ceq represented by the formula (1) is 0.32 to 0.40, the ferrite structure fraction of the (1/2) t part of the plate thickness is 80% or more, and the (1 / 2) The effective crystal grain size of the t part is 25 μm or less, and the thickness (1/2) of the sum of the X-ray intensity ratios of the (321), (211), (110) planes in the direction of an angle of 45 ° ) A high-strength thick steel plate excellent in arrest properties, characterized in that the average value at t part and (1/4) t part is 3.3 or less.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
Here, C, Mn, Cu, Ni, Cr, Mo and V in the formula mean the content (mass%) of the element in each steel plate.   The high-strength thick steel plate having excellent arrest characteristics according to claim 1, further comprising Ni: 1.0% or less in mass%.   The arrest characteristic according to claim 1 or 2, further comprising one or two elements out of elements of Cu: 2.0% or less and Cr: 1.0% or less in mass%. Excellent high strength thick steel plate.   The composition further comprises one or more elements selected from the group consisting of Mo: 0.5% or less, V: 0.1% or less, and B: 0.005% or less. A high-strength thick steel plate having excellent arrest characteristics according to any one of 1 to 3.   The high-strength thick steel plate having excellent arrest properties according to any one of claims 1 to 4, further comprising Ti: 0.1% or less in terms of mass%.   The composition further comprises one or more elements selected from the group consisting of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less. A high-strength thick steel plate excellent in arrest characteristics according to any one of 1 to 5.

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