JP2011127207A - High-strength thick steel plate having excellent brittle-crack arrestability, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength thick steel plate having excellent brittle-crack arrestability under high productivity. <P>SOLUTION: The high-strength thick steel plate having excellent brittle-crack arrestability has a chemical composition comprising, by mass, 0.01 to 0.12% C, &le;0.5% Si, 0.4 to 2% Mn, &le;0.05% P, &le;0.008% S, 0.002 to 0.05% Al, &le;0.01% N, and the balance Fe with impurities, in which carbon equivalent Ceq shown by formula (1): Ceq=C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5 (wherein the C, Mn, Cu, Ni, Cr, Mo and V in the formula denote the contents (mass%) of the respective elements in the steel plate) is 0.32 to 0.40. The X-ray intensity ratio of the (100) face in the rolling face in the central part at the plate thickness is &ge;2, and the X-ray intensity ratio of the (110) face in the rolling face in the (1/4)t part of the plate thickness is &lt;1.5. Further, the steel plate may comprise Cu, Cr, Mo, V, Nb, B, Ni, Ti, Ca, Mg and rare earth metals. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention is a thick steel plate with excellent toughness, especially high-strength thick wall with excellent brittle crack propagation stopping characteristics (arrest characteristics) in order to prevent collapse of the entire structure when a brittle crack occurs. It is related with a steel plate and its manufacturing method. In this case, the high-strength thick steel plate is for a class having a thickness exceeding 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. Therefore, a high-strength thick steel plate for a hull structure having a plate thickness exceeding 50 mm has been demanded. 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 at the center of the plate thickness is still insufficient.

  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 Documents 1 and 2 disclose a method of manufacturing a steel sheet that promotes shear lip formation during brittle crack propagation by making the surface layer structure finer.

  Patent Document 3 discloses a steel sheet that has improved brittle crack propagation stopping characteristics by defining the texture of the steel sheet, and a method for manufacturing the steel sheet.

  Further, Non-Patent Document 1 has a thickness of 16 mm and is not intended for the thick material aimed at in the present invention, but by performing controlled rolling in a low temperature region (austenite-ferrite region), Is disclosed, and a method for improving the arrest characteristics in the L direction is disclosed.

JP-A-3-2322 JP 7-126798 A JP 2008-214652 A

Junichi Katsuta, “Fundamental Study on Application of Steel Plates with Separation to Steel Structures” (Doctoral Dissertation), Nagasaki University, September 1991

  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 and 2, it is possible to improve arrest characteristics without adding expensive elements 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.

  Non-Patent Document 1 states that steel subjected to controlled rolling in a low temperature range can significantly improve the arrest characteristics in the L direction as compared with steel subjected to controlled rolling in a high temperature range (austenite range). However, the arrest characteristics in the 45 ° direction are low and remain at the same level as steel rolled in the high temperature range. Furthermore, the result of the Charpy impact test is excellent in the L direction and C direction, but not in the 45 ° direction.

  Depending on the structure, the principal stress direction is clear, and there is a case where it is not necessary to consider characteristics other than the fixed direction, but in an emergency situation where brittle fracture occurs and propagates, the stress acting direction assumed in the design is It is unclear whether it will be maintained, and there is no measured data. In other words, if a strong force acts in the 45 ° direction and a crack propagates in the same direction, it would be meaningless to have excellent characteristics in the L direction.

  It is considered that the difference in brittle crack propagation stopping characteristics due to such a difference in direction is related to the texture of the steel sheet. Patent Document 3 discloses a technique for improving the brittle crack propagation stop property by defining the texture of the steel sheet, but only considers the brittle crack propagation stop property in the thickness direction. .

  In view of such a situation, the present invention provides a high strength thick steel plate excellent in arrest properties, particularly a high strength thick steel plate excellent in arrest properties with small in-plane anisotropy, without performing expensive element addition. It aims to provide under productivity.

  First, since the stress state at the fracture stage of the structure is unpredictable, the inventors started research on a technology that uniformly improves the characteristics in all directions in the plane, and conducted various studies and experiments. As a result, the following findings (a) to (f) were obtained.

  (a) The difference in brittle crack propagation stopping characteristics due to the difference in direction depends on the texture of the steel sheet. In a thick steel plate manufactured by normal controlled rolling, the (100) plane, which is a cleavage plane of α-iron, is accumulated in the 45 ° direction. Therefore, when a fracture toughness test is performed on the controlled rolled material at various angles, 45 ° Directional characteristics are worst. This was because the (100) plane, which is the cleavage plane of α-iron, was accumulated in the 45 ° direction.

  (b) In order to avoid this texture, the (100) plane of the thick steel plate is controlled, and the cleavage plane is parallel to the plate thickness front and back surfaces, which are the direction most unrelated to the destruction of the structure (rolling). The cleaved surface should be accumulated on the surface. In particular, the X-ray intensity ratio of the (100) plane of the rolled surface at the central portion of the plate thickness (sometimes referred to as “(1/2) t portion of plate thickness)” is controlled to 2 or more, and the brittleness in the 45 ° direction. It is necessary to improve crack propagation stopping characteristics.

  (c) However, it is not sufficient that the X-ray intensity ratio of the (100) plane of the rolled surface at the center of the thickness of the steel sheet of thickness t is 2 or more. This is because the thick steel plate used in the present invention has a certain thickness, and the texture differs depending on the position in the thickness direction. In the plate thickness part 1/4 from the surface of the steel plate with thickness t (sometimes referred to as “(1/4) t part of plate thickness)”, the texture is different and the exact mechanism is unknown, It was found that the X-ray intensity ratio of the (110) plane of the rolled surface is greatly related to the toughness characteristics in the 45 ° direction. It is also important to suppress the accumulation of the (110) plane on the rolled surface at the position of the (1/4) t portion of the plate thickness. It was found that the surface X-ray intensity ratio should be less than 1.5.

  (d) The large texture that contributes to the improvement of the brittle crack propagation stopping property is the above-mentioned (100) plane and (110) plane. However, as the main cleavage plane of α iron, there are (211) plane and (332) plane in addition to the (100) plane, and these planes are also accumulated on the rolling surface in the same manner, thereby improving the fracture toughness in the plane. It was found that the in-plane anisotropy of the arrest characteristic can be reduced by achieving uniformity. In particular, it is preferable that the (211) plane X-ray intensity ratio of the rolled surface at the center portion of the plate thickness is 1.8 or more, and the X-ray intensity ratio of the (332) plane of the rolled surface at the center portion of the plate thickness is 1. .5 or more is preferable.

  (e) In addition to appropriate control of the texture described above, the carbon equivalent, which is an indicator of chemical composition and hardenability, should be appropriately controlled in order to provide the basic properties necessary for structural steel. It is also important. In particular, since a high-strength thick steel plate is used as a strength member, sufficient strength as a standard material is required, and thus it is necessary to have 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). The carbon equivalent Ceq is preferably 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.

  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. As described later, the preferable range of the carbon equivalent Ceq is 0.32 to 0.38.

  (f) And it is also important to examine the heating and processing conditions and the cooling conditions when the steel ingot is processed hot.

In particular, the control of the heating condition of the steel ingot, which is the material of the thick steel plate, that is, the control of the heating temperature and the heating time controls the initial γ grain size at the time of reheating the steel ingot. As will be described later, by lowering the heating time or shortening the time, it is possible to cause ferrite transformation at the transformation after rolling, so that the initial γ grain size can be made fine. In the ingot heating process, the steel ingot heating temperature Tr (° C) and heating time t (hr) satisfy the following formula (2). Was found to be preferable as a condition for economical production of
^{t × exp (Tr 3/270000000} ) ≦ 580 ····· (2)
Here, t represents the heating time (hr) of the steel ingot, and Tr represents the heating temperature (° C.).

  In addition, control of the adjustment plate thickness, rolling temperature, and finish rolling temperature in the subsequent rolling process is also effective. 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 Can be expected.

In order to control the amount of reduction in the non-recrystallized region, in the rolling process, the rolling temperature (adjusting rolling temperature) B (° C.) at an arbitrary thickness (adjusted plate thickness) A (mm) during rolling The final rolling temperature C (° C) when finishing to the final thick steel sheet thickness G (mm) by final rolling is expressed by the following formulas (3), (4), (5) and (6) It was found that rolling should be performed so as to satisfy the above.
A-1.5G ≧ 0 (3)
C-670-G ≦ 0 (4)
BC-20-1400 / G ≦ 0 (5)
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.

In order to ensure sufficient strength, in the case of such a thick material, it is also effective to control the cooling rate and the cooling stop temperature in the cooling process after rolling, and the water cooling stop temperature E (° C.) and the plate It was also found that it is preferable to perform water cooling so that the average cooling rate F (° C./s) during water cooling in the thickness center portion satisfies the following formulas (6) and (7).
E-500 ≦ 0 (6)
F-2 ≧ 0 (7)
Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during water cooling at the center of the plate thickness.

In addition, when tempering 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 may be performed as necessary.

  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 (7) The gist of the present invention is a method for producing a high-strength thick steel plate having excellent arrest characteristics shown in (8) and (9) below. Hereinafter, the present invention (1) to the present invention (9), respectively. The present invention (1) to the present invention (9) 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%, P: 0.05% or less, S: 0.008% or less , Al: 0.002 to 0.05%, N: 0.01% or less, a thick steel plate having a chemical composition consisting of the remaining Fe and impurities, and represented by the following formula (1) The equivalent Ceq is 0.32 to 0.40, the X-ray intensity ratio of the (100) plane of the rolled surface at the center of the plate thickness is 2 or more, and the (110) of the rolled surface at the (1/4) t portion of the plate thickness. ) A high-strength thick steel plate excellent in arrest characteristics, characterized in that the X-ray intensity ratio of the surface is less than 1.5.
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 each element in the steel sheet.

  (2) The high-strength thick steel plate having excellent arrest characteristics according to (1) above, wherein the (211) plane X-ray intensity ratio of the rolled surface at the center of the plate thickness is 1.8 or more.

  (3) The (332) plane X-ray intensity ratio of the rolled surface at the center of the plate thickness is 1.5 or more, and the high strength and thick wall having excellent arrest characteristics as described in (1) or (2) above steel sheet.

  (4) A high-strength thick steel plate excellent in arrest properties according to any one of (1) to (3) above, further comprising Ni: 1.0% or less by mass%.

  (5) Further, by mass, Cu: 2.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, V: 0.1% or less, Nb: 0.1% or less, and B : A high-strength thick steel plate excellent in arrest properties according to any one of the above (1) to (4), comprising one or more elements of 0.005% or less.

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

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

(8) The steel ingot having the chemical composition described in any one of (1) and (4) to (7) is heated, rolled, and cooled by the following steps 1 to 3. A method for producing a high-strength thick steel plate having excellent arrest characteristics.
[Step 1] A step of heating the ingot so that the heating temperature Tr (° C.) and the heating time t (hr) of the ingot satisfy the following expression (2).

^{t × exp (Tr 3/270000000} ) ≦ 580 ·········· (2)
Here, t represents the heating time (hr) of the steel ingot, and Tr represents the heating temperature (° C.).
[Step 2] The rolling temperature B (° C.) at an arbitrary thickness A (mm) during rolling and the finish rolling temperature C (° C.) when finishing to the final thick steel sheet thickness G (mm) by final rolling are as follows. The step of rolling so as to satisfy the following expressions (3), (4) and (5).

A-1.5G ≧ 0 (3)
C-670-G ≦ 0 (4)
BC-20-1400 / G ≦ 0 (5)
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.
[Step 3] Water cooling is performed so that the water cooling stop temperature E (° C.) and the average cooling rate F (° C./s) during water cooling at the center of the plate thickness satisfy the following equations (6) and (7). Process.

E-500 ≦ 0 (6)
F-2 ≧ 0 (7)
Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during water cooling at the center of the plate thickness.

(9) A steel ingot having the chemical composition described in any one of (1) and (4) to (7) above is heated, rolled, cooled, according to the following steps 1 to 4; A method for producing a high-strength thick steel plate excellent in arrest properties, characterized by tempering.
[Step 1] A step of heating the ingot so that the heating temperature Tr (° C.) and the heating time t (hr) of the ingot satisfy the following expression (2).

^{t × exp (Tr 3/270000000} ) ≦ 580 ·········· (2)
Here, t represents the heating time (hr) of the steel ingot, and Tr represents the heating temperature (° C.).
[Step 2] The rolling temperature B (° C.) at an arbitrary thickness A (mm) during rolling and the finish rolling temperature C (° C.) when finishing to the final thick steel sheet thickness G (mm) by final rolling are as follows. The step of rolling so as to satisfy the following expressions (3), (4) and (5).

A-1.5G ≧ 0 (3)
C-670-G ≦ 0 (4)
BC-20-1400 / G ≦ 0 (5)
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.
[Step 3] Water cooling is performed so that the water cooling stop temperature E (° C.) and the average cooling rate F (° C./s) during water cooling at the center of the plate thickness satisfy the following equations (6) and (7). Process.

E-500 ≦ 0 (6)
F-2 ≧ 0 (7)
Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during water cooling at the center of the plate thickness.
[Step 4] Ac Tempering step at a temperature of ₁ point or less.

  According to the present invention, a high-strength thick steel plate excellent in arrest properties, particularly a high-strength thick steel plate excellent in arrest properties with small in-plane anisotropy, without high-value element addition, under high productivity. Can be provided.

  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 standpoint of balance between strength and arrest properties, a preferred range for the C content is 0.03 to 0.10%. A more preferable range is 0.03 to 0.07%.

Si: 0.5% or less Si is an element necessary for deoxidation in the refining stage and 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.

  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 contains at least one of Ni, Cu, Cr, Mo, V, Nb, B, Ti, Ca, Mg and REM in addition to the above elements as follows. May be.

Ni: 1.0% or less Ni can be contained if necessary. 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 develop the improvement effect of the arrest property 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%. 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 1.0%, 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 1.0%. And 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.

Nb: 0.1% or less Nb is an element effective for refining the structure, improving the hardenability and increasing the strength by precipitation hardening, and applies the TMCP method because it has a particularly large effect of expanding the non-recrystallized region. It is preferable to contain in steel materials. However, when the content exceeds 0.1%, the increase in precipitates causes toughness deterioration. Therefore, the upper limit of the Nb content is 0.1%. A preferable upper limit of Nb is 0.04%. It should be noted that it is preferable to contain Nb in an amount of 0.003% or more in order to stably speak out the effect of refinement of the structure by Nb, improvement of hardenability, and strength increase by precipitation hardening.

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, it is necessary to ensure the ferrite content at the center of the plate thickness. Therefore, 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 preferable range of the carbon equivalent Ceq is 0.32 to 0.38.

(C) Texture The difference in brittle crack propagation stopping characteristics due to the difference in direction depends on the texture of the steel sheet. In a thick steel plate manufactured by normal controlled rolling, the (100) plane, which is a cleavage plane of α-iron, is accumulated in the 45 ° direction. Therefore, when the fracture toughness test is performed, the characteristics in the 45 ° direction become the worst. Therefore, it is necessary to control the (100) plane of the thick steel plate and accumulate the cleavage planes in the direction most unrelated to the destruction of the structure. More specifically, it is necessary to control the X-ray intensity ratio of the (100) plane of the rolled surface at the center of the plate thickness to 2 or more to improve the brittle crack propagation stopping characteristics in the 45 ° direction.

  However, it is not sufficient that the X-ray intensity ratio of the (100) plane of the rolled surface at the center of the plate thickness is 2 or more. Since the thick steel plate used in the present invention has a certain thickness, the texture differs depending on the position in the thickness direction. Specifically, the texture is different between the center portion of the plate thickness and the (1/4) t portion of the plate thickness. Although the exact mechanism is unknown, it has been found that in the (1/4) t part of the plate thickness, the X-ray intensity ratio of the (110) plane of the rolled surface is greatly related to the toughness characteristics in the 45 ° direction. . It is also necessary that the (110) plane X-ray intensity ratio of the rolled surface at the (1/4) t portion of the plate thickness is less than 1.5.

  The large texture that contributes to the improvement of the brittle crack propagation stopping characteristics is the above-mentioned (100) plane and (110) plane. However, as the main cleavage plane of α-iron, there are (211) plane and (332) plane in addition to (100) plane, and these planes are also accumulated on the rolling plane to further increase the fracture toughness in the plane. Can be made uniform. It is preferable that the (211) plane X-ray intensity ratio of the rolled surface at the center of the plate thickness is 1.8 or more, and the X-ray intensity ratio of the (332) plane of the rolled surface at the center of the plate thickness is 1.5. The above is preferable.

  In addition, said X-ray intensity ratio is X in the (100) plane, (110) plane, (211) plane, and (332) plane of a rolling surface in a sheet thickness center part or (1/4) t part of sheet thickness. It means the ratio between the line intensity and the X-ray intensity of a non-oriented sample having a random crystal orientation, which can be measured using an X-ray diffractometer.

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

  The control of the heating condition of the steel ingot, which 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, so that the number of ferrite nuclei during α transformation decreases, the ferrite structure fraction in the final structure decreases, and the waiting time during rolling is long. As a result, economic efficiency is impaired. 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 has been found that satisfying the equation is preferable as a condition for economically producing a high-strength thick steel plate having excellent arrest properties.
^{t × exp (Tr 3/270000000} ) ≦ 580 ····· (2)
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-1.5G ≧ 0 (3)
C-670-G ≦ 0 (4)
BC-20-1400 / G ≦ 0 (5)
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 (3) to (5) 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 will decrease. 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 (6) and (7). preferable.
E-500 ≦ 0 (6)
F-2 ≧ 0 (7)
Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during water cooling at the center of the plate thickness.

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.

  Using various test steels having chemical components described in Table 1, various high-strength thick steel plates were manufactured based on the manufacturing conditions shown in Table 2, and the characteristic values of the obtained steel plates were measured (Table 3). And Table 4). Of these steels, steel Nos. 38 to 43 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). Moreover, about test No.1-2, although not clearly shown in Table 2, tempering was performed at 520 degreeC after cooling.

  In addition, 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). Here, the yield point was determined as a test yield of 10 N / (mm · s), and the yield point was determined as 0.2% proof stress when no clear yield point appeared. The target value of strength was set to tensile strength TS ≧ 490 MPa.

As an evaluation method for the arrest characteristics, a plurality of temperature gradient type ESSO tests were performed in the L direction and 45 ° direction, and the obtained results were plotted on an Arrhenius graph to perform linear approximation, and at −10 ° C. The Kca value was used as an evaluation representative value as the arrest property (N / mm ^1.5 ) of the steel. The target value of the arrest characteristics 6000N ^{/ mm 1.5,} the target value of the arrest characteristic difference between L direction and 45 ° direction was 1000 N ^{/ mm 1.5.} Note that the magnitude of the in-plane anisotropy of the arrest characteristic can be determined by the magnitude of the difference in the arrest characteristic between the L direction and the 45 ° direction.

  Table 3 shows the X-ray intensity ratios of the (100), (211) and (332) planes at the rolling surface at the center of the thickness of each test steel, and (1/4) t part of the thickness ( 110) X-ray intensity ratio of the plane. The target value of the X-ray intensity ratio is that the (100) plane X-ray intensity ratio at the rolled surface at the center of the plate thickness is 2 or more, and (110) at the (1/4) t portion of the plate thickness. The X-ray intensity ratio of the surface is less than 1.5.

  Table 4 shows the evaluation results of the mechanical properties (yield strength YS [MPa] and tensile strength TS [MPa]) and arrest properties (L direction and 45 ° direction) of each high-strength thick steel plate.

As a result, the test Nos. 1-2, 1-3, 2-5, 3-1 and the test Nos. 4 to 37 according to the examples of the present invention are necessary strengths despite being thick steel plates. While ensuring the characteristics, high arrest characteristics were secured. Further, the difference between the L direction and 45 ° direction of the arrest characteristics is within 1000 N / mm ^1.5 , and it was found that the arrest characteristics ensure a small in-plane anisotropy.

  On the other hand, the thick steel plates according to Test Nos. 1-1, 1-4, 2-1, 2-2, 2-3, 2-4, 3-2 and 38 to 43 shown as comparative examples are As shown below, at least one of the strength characteristics, arrest characteristics and in-plane anisotropy was insufficient.

  That is, Test No. 1-1 and No. 1-4 do not satisfy the heating condition related to the equation (2). Therefore, in test No. 1-1, the (110) plane X-ray intensity ratio at the (1/4) t part rolled surface of the plate thickness exceeds 1.5, and in test No. 1-4, the plate thickness ( In addition to the (110) plane X-ray intensity ratio at the rolling surface of the 1/4) t portion exceeding 1.5, the (100) plane X-ray intensity ratio at the rolling surface at the center of the plate thickness was reduced. For this reason, the arrest characteristic is insufficient. In addition, the in-plane anisotropy is large.

  Test No. 2-1 does not satisfy the rolling conditions related to equation (4). Therefore, in test No. 2-1, the (110) plane strength at the (1/4) t part rolled surface of the plate thickness exceeded 1.5, and (100) at the rolled surface at the center of the plate thickness. The surface X-ray intensity ratio was reduced. For this reason, the arrest characteristic is insufficient. In addition, the in-plane anisotropy is large.

  Test No. 2-2 does not satisfy the rolling conditions related to the expression (3). Therefore, in Test No. 2-2, the (100) plane X-ray intensity ratio on the rolled surface at the center of the plate thickness was less than 2.0. For this reason, the arrest characteristic is insufficient. In addition, the in-plane anisotropy is large.

  Test No. 2-3 does not satisfy the cooling rate condition regarding the expression (7). Therefore, in Test No. 2-3, the (100) plane X-ray intensity ratio on the rolled surface at the center of the plate thickness was less than 2.0. For this reason, the arrest characteristic in the 45 ° direction is insufficient. In addition, the in-plane anisotropy is large.

  Test No. 2-4 does not satisfy the cooling rate condition related to Equation (6). Therefore, in Test No. 2-4, the (100) plane X-ray intensity ratio on the rolled surface at the center of the plate thickness was less than 2.0. For this reason, although there is sufficient arrest characteristics in the L direction, the arrest characteristics in the 45 ° direction are insufficient. In addition, the in-plane anisotropy is large.

  Test No. 3-2 does not satisfy the heating conditions related to the equations (2) and (5). Therefore, in Test No. 3-2, the (110) plane X-ray intensity ratio at the (1/4) t part rolled surface of the sheet thickness exceeds 1.5, and the (211) surface at the rolled surface at the center of the sheet thickness. The X-ray intensity ratio was reduced. For this reason, the arrest characteristic is insufficient. In addition, the in-plane anisotropy is large.

  And Test No.38-43 does not satisfy the component range prescribed | regulated by this invention, or (1) Formula. Therefore, all have a low arrest characteristic. In addition, the in-plane anisotropy is large.

  As described above, according to the present invention, a high-strength thick steel plate excellent in arrest properties, particularly a high-strength thick steel plate excellent in arrest properties with small in-plane anisotropy, without adding an expensive element. Can be offered under high productivity.

Claims (9)

In mass%, C: 0.01 to 0.12%, Si: 0.5% or less, Mn: 0.4 to 2%, P: 0.05% or less, S: 0.008% or less, Al: A thick steel plate containing 0.002 to 0.05%, N: 0.01% or less, and having a chemical composition consisting of the balance Fe and impurities, and having a carbon equivalent Ceq represented by the following formula (1): 0.32 to 0.40, the X-ray intensity ratio of the (100) plane of the rolled surface at the center of the plate thickness is 2 or more, and the (110) plane of the rolled surface at the (1/4) t portion of the plate thickness A high-strength thick steel plate excellent in arrest characteristics, characterized in that the X-ray intensity ratio is less than 1.5.
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 each element in the steel sheet.   The high-strength thick steel plate with excellent arrest characteristics according to claim 1, wherein the (211) plane X-ray intensity ratio of the rolled surface at the center of the plate thickness is 1.8 or more.   The high-strength thick steel plate having excellent arrest characteristics according to claim 1 or 2, wherein the (332) plane X-ray intensity ratio of the rolled surface at the center of the plate thickness is 1.5 or more.   Furthermore, Ni: 1.0% or less is contained in the mass%, The high-strength thick steel plate excellent in the rest characteristic in any one of Claim 1 to 3 characterized by the above-mentioned.   Further, in terms of mass%, Cu: 2.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, V: 0.1% or less, Nb: 0.1% or less, and B: 0. The high-strength thick steel plate excellent in arrest properties according to any one of claims 1 to 4, characterized by containing one or more elements of 005% or less.   Furthermore, Ti: 0.1% or less is contained in the mass%, The high strength thick steel plate excellent in the arrest property in any one of Claim 1-5 characterized by the above-mentioned.   Furthermore, by mass%, it contains one or more 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 arrest properties according to any one of 1 to (6). A steel ingot having the chemical composition according to any one of claims 1 and 4 to 7 is heated, rolled and cooled by the following steps 1 to 3, A manufacturing method for high-strength thick steel plates with excellent arrest properties.
[Step 1] A step of heating the ingot so that the heating temperature Tr (° C.) and the heating time t (hr) of the ingot satisfy the following expression (2).
^{t × exp (Tr 3/270000000} ) ≦ 580 ·········· (2)
Here, t represents the heating time (hr) of the steel ingot, and Tr represents the heating temperature (° C.).
[Step 2] A rolling temperature B (° C.) at an arbitrary thickness A (mm) during rolling and a finish rolling temperature C (° C.) when finishing to a final thick steel plate thickness G (mm) by final rolling. The step of rolling so as to satisfy the following expressions (3), (4) and (5).
A-1.5G ≧ 0 (3)
C-670-G ≦ 0 (4)
BC-20-1400 / G ≦ 0 (5)
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.
[Step 3] Water cooling is performed so that the water cooling stop temperature E (° C.) and the average cooling rate F (° C./s) during water cooling at the center of the plate thickness satisfy the following formulas (6) and (7). Process.
E-500 ≦ 0 (6)
F-2 ≧ 0 (7)
Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during water cooling at the center of the plate thickness. A steel ingot having the chemical composition according to any one of claims 1 and 4 to 7 is heated, rolled, cooled and tempered by the following steps 1 to 4. The manufacturing method of the high-strength thick-walled steel plate excellent in the arrest characteristic.
[Step 1] A step of heating the ingot so that the heating temperature Tr (° C.) and the heating time t (hr) of the ingot satisfy the following expression (2).
^{t × exp (Tr 3/270000000} ) ≦ 580 ·········· (2)
Here, t represents the heating time (hr) of the steel ingot, and Tr represents the heating temperature (° C.).
[Step 2] A rolling temperature B (° C.) at an arbitrary thickness A (mm) during rolling and a finish rolling temperature C (° C.) when finishing to a final thick steel plate thickness G (mm) by final rolling. The step of rolling so as to satisfy the following expressions (3), (4) and (5).
A-1.5G ≧ 0 (3)
C-670-G ≦ 0 (4)
BC-20-1400 / G ≦ 0 (5)
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.
[Step 3] Water cooling is performed so that the water cooling stop temperature E (° C.) and the average cooling rate F (° C./s) during water cooling at the center of the plate thickness satisfy the following formulas (6) and (7). Process.
E-500 ≦ 0 (6)
F-2 ≧ 0 (7)
Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during water cooling at the center of the plate thickness.
[Step 4] Ac Tempering step at a temperature of ₁ point or less.