JP5477578B2 - Thick high-strength steel sheet excellent in brittle crack propagation stopping characteristics and method for producing the same - Google Patents

Description

  The present invention is suitable for welded structures such as shipbuilding, architecture, bridges, tanks, offshore structures, etc., and has a high thickness and thickness of 50 mm or more, which is excellent in brittle crack propagation stopping characteristics (hereinafter also referred to as arrestability). It is related with a steel plate and its manufacturing method.

  With the recent increase in the size of steel structures, the demand for brittle crack propagation stopping characteristics has become stricter from the viewpoint of increasing the thickness and strength of steel materials used, and ensuring safety. However, in a high strength steel plate with a yield strength of 355 to 460 MPa class, it is extremely difficult to ensure arrestability when the plate thickness is 50 mm or more. Therefore, a thick high-strength steel sheet excellent in arrestability that can be applied to a large welded structure is required.

As means for improving arrestability, crystal grain refinement has been well known, but in recent years, studies have also been made from the viewpoint of texture control. That is, since a brittle crack propagates through the cleavage plane of steel, the texture can be controlled so that the cleavage plane of the steel sheet does not coincide with the crack propagation direction and the arrestability of the steel material can be improved. Specifically, a steel sheet has been proposed in which the degree of integration on the (100) plane is increased in a texture parallel to the rolled surface by controlling the temperature at the center of the plate thickness and rolling conditions (for example, Patent Document 1). 2). These are produced by increasing the rolling reduction in a specific temperature range in the vicinity of Ar ₃ that is the ferrite transformation start temperature during cooling.

Further, in order to stop the propagation of a brittle crack generated from the surface of a steel sheet, a technique of hot rolling at a temperature of Ar ₃ or lower, that is, a two-phase region in which austenite and ferrite coexist has been proposed (for example, Patent Document 3). Furthermore, a method has been proposed in which the surface temperature of the steel sheet is cooled to Ar ₃ or lower and hot rolling is performed in a double heat process (see, for example, Patent Document 4). This improves the arrestability by refining the structure of the surface layer of the steel sheet and controlling the texture.

  In addition, a technology has been proposed in which the steel sheet is hot-rolled from the austenite low-temperature region to the two-phase region, and the thickness at the center of the plate thickness is austenite, and the distribution is such that the texture is different in the plate thickness direction ( For example, see Patent Document 5). This is to improve the arrestability by branching the brittle crack generated in the surface layer portion.

JP 2009-221585 A JP 2009-235458 A JP 2008-156750 A JP 2002-020835 A JP 2008-169467 A

  However, the method proposed by Patent Document 4 focuses on the texture and texture of the surface layer portion, but does not control the texture and texture inside the plate thickness. However, since arrestability is a characteristic of the total thickness of the steel sheet, the arrestability may not be improved if the structure and texture inside the sheet thickness are inappropriate. Further, the method proposed by Patent Document 5 does not significantly change the distribution of texture in the thickness direction, and it is considered that there is a limit to improving the arrestability.

  Furthermore, although it is a problem common to the method of patent documents 1-5, even if it is a steel plate which has the X-ray surface intensity ratio of a specific surface, there are cases where arrestability is not enough. For example, even if a (100) plane strength ratio parallel to the rolling surface is defined, contributions to arrestability are different between (001) [010] and (001) [110]. That is, in order to improve arrestability, it is necessary to control a three-dimensional texture.

In addition to the arrestability, demands for shortening the delivery time of customers are increasing year by year, and improvement in productivity in the steel sheet manufacturing process is strongly desired. However, in order to recrystallize the ferrite in the surface layer portion as in the method proposed by Patent Document 4, it is necessary to secure a sufficient rolling reduction in the temperature range of Ac ₁ or higher, and the productivity is significantly reduced. End up. The method proposed by Patent Document 5 is an extension of normal control rolling technology, and improvement in productivity cannot be expected.

The present invention has been made in view of such a situation, and even when the plate thickness is 50 mm or more and the yield strength is 355 to 460 MPa class, the temperature T _{Kca = 6000 at} which _{Kca = 6000 N} / mm ^1.5 is obtained. A steel plate excellent in brittle crack propagation stopping characteristics (hereinafter also referred to as a high arrested steel plate) and a high arrested steel plate that can be applied to a large structure having a temperature of −10 ° C. or lower, and a high arrested steel plate is stably and efficiently manufactured. Provide technology. Hereinafter, the temperature T _{Kca = 6000 at} which _{Kca = 6000 N} / mm ^1.5 is sometimes referred to as an _{arrestability} index.

  The present invention is a brittle crack in which productivity is significantly increased by performing accelerated cooling after rough rolling, which has been conventionally air-cooled, and the three-dimensional texture of the front and back surfaces of the steel sheet and the center of the sheet thickness is controlled. A thick high-strength steel sheet having excellent propagation stop characteristics and a method for producing the same. The gist of the present invention is as follows.

(1) In mass%,
C: 0.02 to 0.18%,
Si: 0.03 to 0.5%,
Mn: 0.3 to 2.0%,
P: 0.020% or less,
S: 0.010% or less,
Nb: 0.003 to 0.050%,
Ti: 0.003 to 0.050%,
Al: 0.002 to 0.10%,
N: 0.0010 to 0.0080%
The effective equivalent of the effective crystal grains having a composition having the balance Fe and inevitable impurities and having a crystal orientation difference of 15 ° or more as a grain boundary is 1/10 of the plate thickness from the surface. At the position of 25 μm or less, at the center of the sheet thickness, 35 μm or less, and when the texture strength ratio I _{{hkl} <uvw>} relative to the rolling surface and the rolling direction is 1/10 of the sheet thickness from the surface,
I _{{001} <110>} + I _{{112} <110>} + I _{{332} <113>} ≧ 5,
I _{{110} <001>} + I _{{110} <110>} + I _{{001} <010>} ≦ 3
In the center of the plate thickness,
I _{{001} <110>} + I _{{112} <110>} + I _{{332} <113>} ≧ 3.5
A thick, high-strength steel sheet with excellent brittle crack propagation stopping characteristics.

(2) By mass%
Cu: 0.05 to 1.5%,
Cr: 0.05 to 1.0%,
Mo: 0.05-1.0%,
Ni: 0.05-2.0%,
V: 0.005-0.10%,
B: 0.0002 to 0.0030%
A thick high-strength steel sheet having excellent brittle crack propagation stopping characteristics as described in (1) above, comprising one or more of the following.

(3) In mass%,
Mg: 0.0003 to 0.0050%,
Ca: 0.0005 to 0.0030%,
REM: 0.0005 to 0.010%
A thick high-strength steel sheet having excellent brittle crack propagation stopping characteristics as described in (1) or (2) above, comprising one or more of the following.

(4) Using the steel slab having the composition described in any one of (1) to (3) above, the steel slab is heated to 950 to 1150 ° C. and subjected to rough rolling with a cumulative reduction ratio of 30% or more at a temperature of 900 ° C. or more. After the cooling, the steel sheet was cooled at a cooling rate of 5 to 30 ° C./s until the front and back surfaces of the steel sheet were 300 ° C. or higher and Ar ₃ −100 ° C. or lower, and then the front and back surfaces of the steel sheet were Ar ₃ −50 ° C. or higher and Ar ₃ + 50 ° C. Thereafter, finish rolling is performed at a temperature at which the central portion of the plate thickness is Ar ₃ + 80 ° C. or more and 900 ° C. or less, and each pass reduction rate is 10% or less and the cumulative reduction rate is 40% or more. A method for producing a thick high-strength steel sheet excellent in brittle crack propagation stopping characteristics, wherein accelerated cooling is performed to a temperature of 500 ° C. or lower at the above cooling rate.

  (5) The method for producing a thick high-strength steel sheet having excellent brittle crack propagation stopping characteristics as described in (4) above, wherein tempering is performed at a temperature of 300 to 650 ° C. after completion of the accelerated cooling.

According to the present invention, a high arrested steel plate applicable to a large structure, in which the thickness is 50 mm or more and the yield strength is 355 to 460 MPa, and the _{arrestability} index T _{Kca = 6000} is −10 ° C. or less. It is possible to provide by an efficient manufacturing method, and the industrial effect is extremely large, such as providing a thick and high strength high arrested steel sheet at low cost.

It is a figure which shows the example of ODF showing the sheet thickness direction texture distribution of conventional steel. It is a figure which shows the relationship between the crystal orientation shown on ODF, and the material axis. It is a figure which shows the example of ODF showing the plate | board thickness direction texture distribution of this invention steel. It is a graph which shows the influence of the sum of {001} <110>, {112} <110>, {332} <113> texture intensity ratio which has on arrest property. It is a graph which shows the influence of the sum of {110} <001>, {110} <110>, {001} <010> texture strength ratio which has on arrest property. It is a graph which shows the influence of the EBSP particle size which has on arrest property. It is a graph which shows the influence of the surface layer part temperature of finish rolling which has on arrest property. It is a graph which shows the influence of the center part temperature of finish rolling which has on arrest property. It is a graph which shows the influence of each pass rolling reduction of finish rolling which has on arrestability.

  Generally, the arrestability of a steel sheet is evaluated by a temperature gradient type ESSO test or a double tensile test. When the fracture surface after the test is observed with a scanning electron microscope (SEM) and the unit of the cleavage plane surrounded by the ductile fracture portion called tear ridge is defined as “fracture surface unit”, the smaller the fracture surface unit, the smaller the fracture surface unit. It is known that arrestability is improved. The fracture surface unit has a correlation with effective crystal grains having a boundary whose crystal orientation difference is 15 ° or more, and refinement of the microstructure is effective for improving arrestability.

  Therefore, the present inventors conducted various experimental verifications on means for efficiently miniaturizing the fracture surface unit. First, we investigated controlled rolling (CR) and subsequent accelerated cooling (ACC) in the austenite (γ) non-recrystallization temperature range, which is an effective means for refining the microstructure. . Specifically, a steel slab is hot-rolled under various rolling conditions and accelerated cooling conditions to produce a steel sheet having a yield strength of 355 MPa class and a thickness of 50 mm or more, and the arrestability is evaluated by a temperature gradient type ESSO test. Furthermore, the microstructure was observed and the unit of fracture surface was measured.

  As a result, the lower the rolling temperature, the higher the cumulative rolling reduction, the higher the cooling rate, and the lower the cooling stop temperature, the smaller the fracture surface unit, the finer the microstructure, and the better the arrestability of the steel sheet. It was confirmed. However, with this method, it is difficult to reduce the microstructure, for example, the ferrite grain size, over the entire thickness of the thick material, and a long wait time until the start of finish rolling occurs, resulting in a significant reduction in productivity. End up.

Therefore, the present inventors have made a detailed study on the combination of finish rolling and water cooling before and after that in order to secure the arrestability by controlling the structure distribution in the sheet thickness direction while improving the productivity. Specifically, after cooling the surface to an appropriate temperature range of Ar ₃ or lower, wait until the surface and the interior reach the specified temperature range respectively, and secure the cumulative reduction rate while regulating the reduction rate per pass. It is a method of rolling so as to.

  Analysis using EBSP (Electron Back Scattering Pattern) was performed in order to clarify the influence of the texture of steel sheets on arrestability using steel sheets manufactured by various processes. The measurement surface was a surface perpendicular to the rolling direction (RD) (referred to as the RD surface), and the measurement positions were t / 10, t / 4, and t / 2 (t: plate thickness) from the surface. The measurement was performed on a 1.5 × 1 mm region at 10 μm intervals, and after creating a crystallite orientation distribution function (ODF), the ratio of the specific texture strength to the random strength was read. For the analysis of the texture, an analyzer manufactured by TSL Solutions Co., Ltd. was used.

FIG. 1 shows an example of an ODF having a cross section of φ ₂ = 45 ° according to the Bunge display method. The test steel is a conventional steel material and is manufactured by ordinary CR-ACC. When the crystal orientation is expressed using a crystal plane {hkl} and a crystal direction <uvw> parallel to the rolling plane (ND plane) and the rolling direction (RD direction), the ODF at t / 2 part is {001} <110. >, {112} <110>, {332} <113> orientation is seen. This is a typical texture when transformed from γ subjected to compression strain.

  FIG. 2 schematically shows the relationship between the crystal orientation shown on the ODF and the crystal plane of the steel material. In addition, the propagation direction of a brittle crack is a direction (TD) orthogonal to a rolling direction. As shown in FIG. 2, in the conventional steel plate, the {100} plane which is a cleavage plane, that is, {001} <010>, {110} <110>, {110} <001> has a crack propagation direction ( TD). For this reason, in conventional steel plates, brittle cracks are likely to go straight in the propagation direction and are considered to be inferior in arrestability.

  Among the ODFs shown in FIG. 1, in the t / 2 part ODF, no accumulation is observed in {001} <010>, {110} <110>, {110} <001>, and the crack goes straight. It is considered difficult. On the other hand, although the ODF of the t / 4 part in FIG. 1 is close to random, some accumulation is observed in {001} <010> and {110} <001>, and cracks are observed compared to the t / 2 part. It is assumed that it is easy to propagate. The t / 4 ODF is similar to a texture transformed from γ subjected to shear strain. Further, the ODF of the t / 10 part in FIG. 1 is close to that of the t / 4 part.

  On the other hand, FIG. 3 shows the ODF at each plate thickness position of the steel plate having excellent arrestability. Of the ODFs in FIG. 3, the ODFs at t / 2 and t / 4 are substantially the same as those of the conventional steel shown in FIG. However, it was confirmed that the ODF of the t / 10 part in FIG. 3 is greatly different from the ODF of the t / 10 part in FIG. 1 and is similar to the ODF of the t / 2 part. This suggests that the crack is difficult to advance straight in the surface layer portion as in the case of t / 2 portion, which is considered to be the cause of improvement in arrestability.

The ODF shown in FIG. 3 is a hot rolling in a state where the surface layer region is a light two-phase region rolling structure, the inside is a low temperature CR structure, the surface is Ar ₃ or less, the inside is Ar ₃ or more, and a temperature difference is given. It is the texture of the steel plate. Further, by hot rolling the inside at a non-recrystallization temperature of Ar ₃ or higher, the CR effect at the center portion is enhanced and fine graining is promoted. On the other hand, in the surface layer portion, a specific texture is developed, and the crack propagation resistance can be increased.

Next, the relationship between arrestability and texture was examined in detail. Note that the ratio of {hkl} <uvw> orientation to random intensity is expressed as I _{{hkl} <uvw>} . FIG. 4 shows the sum of I _{{001} <110>} , I _{{112} <110>} , I _{{332} <113>} of the center portion (t / 2) and the surface layer portion (t / 10) This shows the relationship with the temperature T _{Kca = 6000 at} which _{Kca = 6000 N} / mm ^1.5 , which is the arrestability index of the invention. FIG. 5 shows the sum of I _{{110} <001>} , I _{{110} < 110>} and I _{{001} <010> in} the surface layer portion (t / 10), and Kca = This shows the relationship with the temperature T _{Kca = 6000 at 6000 N} / mm ^1.5 .

As shown in FIGS. 4 and 5, it was found that the following conditions are necessary to ensure arrestability.
(A) The sum (ΣIa) of I _{{001} <110>} , I _{{112} <110>} and I _{{332} <113>} is 5 or more at t / 10 part (b) I _{{110 at} t / 10 part _{} <001>} , I _{{110} < 110>} , I _{{001} <010>} sum (ΣIp) is 3 or less (c) When t / 2 part, I _{{001} <110>} , I _{{112} <110 >} , I _{{332} <113>} sum (ΣIa) is 3.5 or more Note that the texture of the t / 4 part does not vary greatly depending on the manufacturing process. In addition, although EBSP is used for the measurement of the texture in the present invention, it may be measured by other methods such as X-ray diffraction or ECP (Electronic Channeling Pattern).

  Furthermore, the present inventors also investigated the microstructure of the steel sheet using EBSP. In the EBSP measurement, a region of 400 × 300 μm was performed at 1 μm intervals, and the equivalent circle diameter of crystal grains (effective crystal grains) was calculated when an interface having a crystal orientation difference of 15 ° or more was defined as a grain boundary. Note that the equivalent-circle diameter of effective crystal grains is sometimes referred to as effective crystal grain diameter. FIG. 6 shows the result of detailed analysis of the relationship between the effective crystal grain size and the arrestability index at each plate thickness position. As shown in FIG. 6, in order to ensure arrestability, it is necessary that the effective crystal grain size is 25 μm or less at t / 10 parts and 35 μm or less at t / 2 parts.

  In order to obtain a steel sheet having a microstructure and texture excellent in arrestability, it is necessary to carry out controlled rolling (CR) as finish rolling with the surface layer as a two-phase region and the thickness center as a γ non-recrystallized region. It is. Therefore, in the steel sheet manufacturing method of the present invention, the most important steps are water cooling before finish rolling and finish rolling. The present inventors examined water cooling and finish rolling conditions for obtaining good arrest properties, and obtained the following knowledge. In addition, although water cooling is performed from the front and back of a steel plate, it may only be called the surface below.

First, water cooling before finish rolling needs to be performed at a cooling rate of 5 to 30 ° C./s until the front and back surfaces are 300 ° C. or higher and Ar ₃ -100 ° C. or lower. This is because when the water is cooled to less than 300 ° C., the temperature does not increase due to the subsequent double heat, the rolling temperature of the surface layer portion decreases, and the arrestability cannot be ensured. On the other hand, if the surface temperature exceeds Ar ₃ −100 ° C. immediately after water cooling before finish rolling, it takes time until the temperature of the central portion decreases, and the ferrite of the surface layer portion becomes coarse or the productivity decreases. Resulting in. On the other hand, if the cooling rate of the surface layer is less than 5 ° C./s, coarse ferrite is generated during cooling, and the structure after finish rolling becomes non-uniformly processed ferrite, resulting in reduced toughness. On the other hand, when the cooling rate exceeds 30 ° C./s, it is difficult to cool uniformly, and the final structure and material may vary.

  There is no need to specify the starting temperature for cooling before finish rolling. From the viewpoint of productivity, it is desirable to immediately cool without waiting for the temperature. However, the temperature may be waited to meet the conditions calculated in advance. Under the rough rolling conditions of the present invention, there is almost no possibility that austenite grows significantly during the temperature waiting and adversely affects the material.

The finish rolling after water cooling needs to be performed at a surface temperature of Ar ₃ −50 ° C. or higher and Ar ₃ + 50 ° C. or lower. As shown in FIG. 7, when the surface temperature of the finish rolling is less than Ar ₃ -50 ° C., excessive two-phase region rolling is applied to the surface layer portion, the processed ferrite in the surface layer portion increases, and the toughness and arrestability are increased. descend. On the other hand, if the surface temperature of finish rolling exceeds Ar ₃ + 50 ° C., ferrite in the surface layer portion generated by cooling before finish rolling may become coarse and the arrestability may be reduced.

Moreover, the finish rolling after water cooling needs to be performed at a temperature at the center of the plate thickness of Ar ₃ + 80 ° C. or higher and 900 ° C. or lower. As shown in FIG. 8, when the temperature at the central portion of the plate thickness exceeds 900 ° C., the structure is not sufficiently refined and the arrestability is lowered. Further, when the temperature at the center of the plate thickness is high, a texture that is advantageous for arrestability may not develop. On the other hand, if the center temperature is less than Ar ₃ + 80 ° C., it takes time to lower the temperature, and the ferrite on the surface layer may become coarse or the productivity may be reduced.

  The rolling reduction per pass of finish rolling must be suppressed to 10% or less. As shown in FIG. 9, when it exceeds 10%, the structure of the central portion of the plate thickness is coarsened by processing heat generation, and the texture of the surface layer portion is also affected, and the arrest characteristics may be deteriorated. The lower limit is not particularly limited, but if the number of passes is too large, rolling efficiency is lowered, so usually it is often about 3%. It is necessary to secure 40% or more of the cumulative rolling reduction of finish rolling. If it is less than 40%, the effect of CR becomes insufficient, a fine structure cannot be obtained, and the arrest characteristics deteriorate. In addition, when the cumulative rolling reduction of finish rolling is low, a texture that is advantageous for arrestability may not develop in the surface layer portion and the plate thickness center portion. The cumulative rolling reduction may be appropriately determined from the thickness of the steel slab and the product and the cumulative rolling reduction in rough rolling, and does not define an upper limit.

  In the present invention, by controlling the water cooling conditions before finish rolling, the temperature of finish rolling, and the rolling reduction, it becomes possible to ensure arrestability and the temperature waiting time of finish rolling is about 1/3 or less of the conventional one. Therefore, productivity is greatly improved.

  Next, the reasons for limiting other manufacturing conditions in the present invention will be described.

  In the present invention, the heating temperature of the steel slab was 950 to 1150 ° C. If the reheating temperature is less than 950 ° C., the alloy elements are not sufficiently solutioned, which causes unevenness of the material. On the other hand, if the reheating temperature exceeds 1150 ° C., the heated γ grain size becomes coarse, and it may be difficult to refine the final structure.

  Rolling performed at a temperature of 900 ° C. or higher after reheating is called rough rolling, and is performed at a cumulative reduction of 30% or more. If the cumulative rolling reduction at 900 ° C. or higher is less than 30%, the recrystallization of austenite (γ) does not proceed sufficiently to form a mixed grain structure, and the material may become non-uniform. The upper limit of the rough rolling start temperature and the cumulative rolling reduction is not particularly specified. Rough rolling may be started immediately after the steel slab is extracted from the heating furnace. Further, the cumulative rolling reduction ratio of the rough rolling may be appropriately determined according to the thickness of the steel slab, the thickness of the product, and the rolling reduction ratio of the finish rolling so as not to be less than 30%.

  After finish rolling, accelerated cooling is performed to a temperature of 500 ° C. or less at a cooling rate of 8 ° C./s or more on the average of the sheet thickness. When the cooling rate is less than 8 ° C./s or the cooling stop temperature is higher than 500 ° C., not only the strength is insufficient, but the structure is not sufficiently refined, and the arrestability is deteriorated. Although the upper limit of the cooling rate of accelerated cooling is not particularly defined, it is difficult to exceed 30 ° C./s because the present invention targets thick materials having a plate thickness of 50 mm or more. The lower limit of the accelerated cooling stop temperature is not particularly limited, and may be cooled to room temperature. However, since the present invention is intended for thick materials with a plate thickness of 50 mm or more, it takes time to accelerate cooling to below 100 ° C. Therefore, considering productivity, the accelerated cooling is stopped at 100 ° C. or more. Is preferred.

  The start temperature of the accelerated cooling does not need to be specified in particular, but it is desirable to start the cooling as quickly as possible in order to fully enjoy the effects of CR.

  After accelerated cooling, a tempering treatment may be performed to adjust strength and toughness. In order to improve ductility and toughness, it is preferable to perform tempering at 300 ° C. or higher. On the other hand, the upper limit of the tempering temperature is preferably set to 650 ° C. or less in order to suppress the coarsening of cementite and crystal grains and ensure arrestability.

  Next, the reasons for limiting the components of the present invention will be described.

  C is an element essential for increasing strength at a low cost, so 0.02% or more is added. On the other hand, if the amount of C increases, it becomes difficult to ensure high heat input HAZ toughness, so the upper limit is made 0.18%. The C content is preferably 0.03% or more and 0.13% or less.

  Si is an inexpensive deoxidizing element and is added in an amount of 0.03% or more in order to strengthen the matrix by solid solution. On the other hand, if the Si content exceeds 0.5%, the weldability and the HAZ toughness are deteriorated, so the upper limit is made 0.5%. The amount of Si is preferably 0.1% or more and 0.3% or less.

  Mn is effective as an element for improving the strength and toughness of the base material, so 0.3% or more is added. On the other hand, if Mn is added excessively, the HAZ toughness and weld cracking properties are deteriorated, so 2.0% is made the upper limit. The amount of Mn is preferably 0.7% or more and 1.7% or less, and more preferably 0.9% or more and 1.5% or less.

  P and S are impurities, and in order to ensure toughness and arrestability, the upper limit of P is 0.02% and S is 0.01%. The smaller the P and S contents, the better.

  Nb and Ti are elements that contribute to refinement of structure, transformation strengthening, and precipitation strengthening when added in a small amount and are effective in securing the strength of the base material, and are each added in an amount of 0.003% or more. On the other hand, excessive addition of Ti and Nb hardens the HAZ and significantly deteriorates the toughness. Therefore, the upper limit is 0.050%. The upper limit of the addition amount of Nb and Ti is preferably 0.03% or less, and more preferably 0.02% or less.

  Since Al is an important deoxidizing element, 0.002% or more is added. On the other hand, if Al is added excessively, the surface quality of the steel slab is impaired and inclusions harmful to toughness are formed, so the upper limit is made 0.10%. The upper limit with preferable Al amount is 0.05% or less, More preferably, it is 0.03% or less.

  N forms a nitride with Ti and improves the HAZ toughness, so the lower limit of the content is 0.0010% or more. On the other hand, if the content of N is excessive, embrittlement due to solute N occurs, or coarse nitrides are generated and the toughness is lowered, so the upper limit is made 0.0080% or less. The upper limit with preferable N amount is 0.0050% or less, More preferably, it is 0.0004% or less.

  Furthermore, you may add the 1 type (s) or 2 or more types in the group of Cu, Cr, Mo, Ni, V, and B, and Mg, Ca, and REM as a selection component.

  Since Cu, Cr, and Mo all improve the hardenability and are effective for increasing the strength, it is preferable to add 0.05% or more. On the other hand, if Cu, Cr and Mo are added excessively, the HAZ toughness may be lowered. Therefore, it is preferable that the upper limit of Cu is 1.5% or less and the upper limit of Cr and Mo is 1.0% or less. More preferable upper limits of Cu, Cr, and Mo are 0.5% or less, 0.8% or less, and 0.6% or less, respectively.

  Since Ni is effective for securing strength, arrestability, and HAZ toughness, 0.05% or more is preferably added. However, if Ni is added in excess of 2.0%, the effect is saturated, and if added excessively, the cost increases, so the upper limit is preferably made 2.0% or less.

  V contributes to an increase in strength by precipitation strengthening, so 0.005% or more is preferably added. On the other hand, if adding more than 0.10% V, the HAZ toughness may be impaired, so it is preferable to set the upper limit to 0.10% or less. A more preferable upper limit of the V amount is 0.05% or less.

  B is an element that improves hardenability and is effective in increasing the strength of steel by adding a suitable amount. In order to increase the hardenability of the steel and ensure the strength, it is preferable to add 0.0002% or more. On the other hand, since excessive addition of B may impair weldability, the upper limit of the B amount is preferably 0.0030% or less.

  Mg, Ca, and REM are elements that contribute to improving HAZ toughness by forming fine oxides and sulfides. Mg is 0.0003% or more, Ca is 0.0005% or more, and REM is 0.0005. It is preferable to add 1% or 2% or more. On the other hand, if Mg, Ca, and REM are added excessively, inclusions become coarse and the toughness may be impaired. Therefore, Mg is 0.0050% or less, Ca is 0.0030% or less, and REM is 0.010% or less. Is preferably the upper limit of each. More preferable upper limits of Mg, Ca, and REM are as follows. 0.0030% or less, 0.0020% or less, or 0.0050% or less. Note that REM is a rare earth element such as La or Ce.

  A steel plate having a thickness of 50 to 80 mm was manufactured on the basis of the manufacturing conditions shown in Tables 2 and 3 using steel pieces having chemical components shown in Table 1. In addition, the center temperature of Table 3 is a calculated value by a rolling temperature model. Samples were taken from the position of 1/10 of the plate thickness from the surface of the steel plate and from the center of the plate thickness, and the texture and effective crystal grain size were determined by EBSP.

Yield strength (YP) and tensile strength (TS) were evaluated using a JIS Z 2201 No. 4 tensile test specimen taken from the center of the plate thickness in a direction perpendicular to the rolling direction. The tensile test was conducted in accordance with JIS Z 2241. For arrestability performs the temperature gradient type ESSO test based on the method described in WES 3003 (low temperature rolled steel criterion), was evaluated at a temperature showing a Kca = 6000N / mm ^1.5. Productivity was calculated by dividing the steel slab weight by the time from the start of rough rolling to the end of finish rolling. Table 4 shows the base material strength, arrestability, and productivity.

No. of the example of the present invention. Nos. 1 to 15 have chemical components within a predetermined range and manufactured under predetermined conditions, and thus all have sufficient strength as YP: 355 to 460 MPa class steel, and the _{arrestability} index T _{Kca = 6000} is also − It was good at 10 ° C. or lower, and the productivity was high at 180 ton / h or higher.

  On the other hand, no. Nos. 16 to 30 have chemical constituents or production conditions that deviate from the scope of the present invention, so that the predetermined crystal grain size and texture strength are not satisfied, and arrestability is reduced or productivity is reduced. . No. No. 18 is a normal manufacturing method in which cooling after rough rolling is not performed, and since the predetermined texture does not develop, the arrestability is low, and the productivity is extremely reduced.

  No. No. 25 has a high reheating temperature of the steel slab. No. 24 is an example in which the cumulative rolling reduction of rough rolling is small and the surface temperature of finish rolling is high, the effective crystal grain size does not become fine, the texture does not develop, and the arrestability is lowered. No. 16 is an example in which the temperature of the surface has decreased too much during the cooling process after rough rolling, the surface temperature during finish rolling does not rise sufficiently, an unfavorable texture develops in the surface layer, and the arrestability decreases. ing. No. 17 is an example in which the cooling rate after rough rolling is small, coarse α is generated in the surface layer portion, an advantageous texture is not developed, and arrestability is reduced.

  No. No. 23 is an example in which effective crystal grains were not refined and a texture advantageous to arrestability did not develop because the finish rolling temperature at the center of the plate thickness was high, and the arrestability was lowered. No. No. 20 is an example in which the cumulative rolling reduction of finish rolling is small, a texture advantageous to arrestability does not develop in the surface layer portion and the center portion of the plate thickness, effective crystal grains are not refined, and arrestability is reduced. . No. No. 28 is an example in which the finish rolling temperature is low on the surface and high in the center of the plate thickness, the crystal grains in the center are not refined, and a texture that is advantageous for arrestability develops in the surface layer and the center of the plate thickness. Without arresting.

  No. No. 27 is an example in which the temperature of the surface is lowered, the processed ferrite is formed in the surface layer portion, the texture advantageous to arrestability is not developed, the arrestability is lowered, and the waiting time before finish rolling However, productivity is also decreasing. No. No. 22 had a large rolling reduction in each pass of the finish rolling, so the crystal grain size at the center of the plate thickness increased due to processing heat generation, and the surface layer did not develop a texture favorable to arrestability, and the arrestability decreased. Yes.

  No. No. 21 has a low cooling rate of accelerated cooling after rolling. No. 26 is an example in which the cooling stop temperature is high, the refinement of effective crystal grains is insufficient, and the arrestability is lowered. No. No. 19 is an example in which the heat treatment temperature is high, cementite and the structure are coarsened, and the arrestability is lowered. No. No. 29 is an example in which the C content is large and the strength is excessive, and the arrestability is reduced. No. No. 30 is an example in which the amount of Nb is large. Coarse undissolved Nb remaining during heating becomes the starting point of brittle fracture, and the arrestability is lowered.

Claims (5)

% By mass
C: 0.02 to 0.18%,
Si: 0.03 to 0.5%,
Mn: 0.3 to 2.0%,
P: 0.020% or less,
S: 0.010% or less,
Nb: 0.003 to 0.050%,
Ti: 0.003 to 0.050%,
Al: 0.002 to 0.10%,
N: 0.0010 to 0.0080%
The effective equivalent of the effective crystal grains having a composition having the balance Fe and inevitable impurities and having a crystal orientation difference of 15 ° or more as a grain boundary is 1/10 of the plate thickness from the surface. The position is 25 μm or less, the center of the plate thickness is 35 μm or less, and the texture strength ratio I _{{hkl} <uvw>} relative to the rolling surface and the rolling direction is 1/10 of the plate thickness from the surface,
I _{{001} <110>} + I _{{112} <110>} + I _{{332} <113>} ≧ 5,
I _{{110} <001>} + I _{{110} <110>} + I _{{001} <010>} ≦ 3
In the center of the plate thickness,
I _{{001} <110>} + I _{{112} <110>} + I _{{332} <113>} ≧ 3.5
A thick, high-strength steel sheet with excellent brittle crack propagation stopping characteristics. % By mass
Cu: 0.05 to 1.5%,
Cr: 0.05 to 1.0%,
Mo: 0.05-1.0%,
Ni: 0.05-2.0%,
V: 0.005-0.10%,
B: 0.0002 to 0.0030%
The thick high-strength steel sheet having excellent brittle crack propagation stopping characteristics according to claim 1, comprising one or more of the following. % By mass
Mg: 0.0003 to 0.0050%,
Ca: 0.0005 to 0.0030%,
REM: 0.0005 to 0.010%
The thick high-strength steel sheet having excellent brittle crack propagation stopping characteristics according to claim 1 or 2, characterized by containing one or more of the following. Using the steel slab having the composition according to any one of claims 1 to 3, the steel sheet is heated to 950 to 1150 ° C, subjected to rough rolling at a temperature of 900 ° C or higher and a cumulative rolling reduction of 30% or higher. After cooling at a cooling rate of 5 to 30 ° C./s until the front and back surfaces are 300 ° C. or higher and Ar ₃ −100 ° C. or lower, the front and back surfaces of the steel sheet are Ar ₃ −50 ° C. or higher and Ar ₃ + 50 ° C. or lower, the center of the plate thickness Finish rolling at a pass reduction ratio of 10% or less and a cumulative reduction ratio of 40% or more at a temperature at which Ar ₃ + 80 ° C. or more and 900 ° C. or less, and subsequently, with a cooling rate of 8 ° C./s or more on average sheet thickness. A method for producing a thick high-strength steel sheet having excellent brittle crack propagation stopping characteristics, characterized by performing accelerated cooling to a temperature of ℃ or less.   The method for producing a thick high-strength steel sheet having excellent brittle crack propagation stopping characteristics according to claim 4, wherein tempering is performed at a temperature of 300 to 650 ° C after completion of the accelerated cooling.