JP2011052244A - METHOD FOR MANUFACTURING THICK HIGH-STRENGTH STEEL SHEET HAVING SUPERIOR CHARACTERISTICS OF STOPPING PROPAGATION OF BRITTLE CRACK SHEET AND SHEET THICKNESS OF 50-125 mm - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method which can industrially stably and effectively manufacture a thick high-strength steel sheet that has sufficient arrestability as a large-scale structural steel, superior characteristics of stopping the propagation of a brittle crack and a sheet thickness of 50-125 mm. <P>SOLUTION: This manufacturing method includes: heating a steel slab having an appropriate chemical composition to 950-1,150°C; rough-rolling the steel slab with a cumulative rolling reduction of 30% or more at 900°C or higher; finish-rolling the rough-rolled material with a cumulative rolling reduction of 40% or more within a surface temperature range of Ar<SB>3</SB>-60°C to Ar<SB>3</SB>; subsequently acceleration-cooling the finish-rolled material to 200°C or lower with a cooling rate of 8°C/s or more by average in a sheet thickness; subsequently heating the steel sheet with a temperature-raising rate of 1°C/s or more on the surface while keeping the surface of the steel sheet at a higher temperature than that in the central part in the sheet thickness direction by 30°C or higher; stopping heating when the surface of the steel sheet has reached 400-650°C; and tempering the steel sheet on the condition of keeping the surface at 400-650°C. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a method for producing a thick high strength steel plate (hereinafter also referred to as a thick high strength high arrest steel plate or simply a high arrest steel plate) excellent in brittle crack propagation stopping characteristics (hereinafter also referred to as arrestability). A thick material with a thickness of 50 mm or more (hereinafter also simply referred to as a thick material), and a yield strength of 355 to 460 MPa class has a temperature at which _{Kca = 6000 N} / mm ^1.5 (hereinafter also referred to as arrest property index T _{Kca = 6000} ). The present invention relates to a method for producing a thick high-strength steel sheet having a thickness of 50 to 125 mm, which has excellent brittle crack propagation stopping characteristics at -10 ° C or lower. The steel plate to which the present invention is applied is particularly applied to welded structures such as shipbuilding, architecture, bridges, tanks, and offshore structures, and may be distributed as secondary processed products processed into steel pipes, columns, and the like.

  In recent years, with the increase in the size of steel structures, the demand for brittle crack propagation stopping characteristics has become stricter from the viewpoint of ensuring safety, along with the increase in thickness and strength of steel materials used. However, generally, as the strength and thickness of the steel material increase, securing arrestability increases rapidly, which is a factor that hinders the application of thick high-strength steel plates to steel structures. At the same time, demands for shortening the delivery time on the demand side are increasing year by year, and thus there is a strong demand for improving productivity in the steel sheet manufacturing process.

Known metallurgical factors for improving the arrestability of steel materials include (i) grain refinement, (ii) Ni addition, (iii) embrittlement second phase control, (iv) texture control, and the like. Yes.
Examples of the method (i) for refining crystal grains include the technique described in Patent Document 1. This is a method of performing 30 to 50% two-phase rolling in the range of 700 to 750 ° C. after rolling at a reduction rate of 50% or more in an unrecrystallized region of Ar ₃ points or more. In addition, as a special method for refining the crystal grains of the steel sheet, the steel slab surface is cooled before rolling or after completion of rough rolling, and rolled in the recuperation process while maintaining a temperature difference from the inside. For example, Patent Documents 2 and 3 describe methods for refining ferrite (α) by recrystallization.

Regarding Ni addition in (ii), it is reported that by promoting cross-slip in a low temperature region, the propagation of brittle cracks is suppressed (see, for example, Non-Patent Document 1) and the arrestability of the matrix is improved. (For example, see Non-Patent Document 2).
As a method for controlling the embrittled second phase of (iii), for example, the technique described in Patent Document 4 can be mentioned. This is a technique in which martensite, which is an embrittled phase, is finely dispersed in α of the matrix.
Regarding the texture control of (iv), for example, Patent Document 5 describes a method of performing low-temperature, large-pressure rolling with ultra-low carbon bainitic steel and developing the (211) plane in parallel with the rolled surface.

Japanese Patent Laid-Open No. 02-129318 Japanese Patent Publication No. 06-004903 JP 2003-221619 A JP 59-047323 A JP 2002-241891 A

Tamura Imao, “Steel Material Strength Science”, published by Nikkan Kogyo Shimbun, July 5, 1969, p. 125 Hasebe, Kawaguchi, “About the brittle fracture propagation stop property of Ni-added steel sheet by taper type DCB test”, iron and steel, vol. 61 (1975), p. 875

  However, the method described in Patent Document 1 is intended for low-temperature steel having a microstructure that is mainly α, a relatively low strength, and a plate thickness of about 20 mm. For this reason, when applied to a thick material having a thickness of 50 mm or more as the object of the present invention, it is difficult in the first place to secure a reduction ratio from the viewpoint of the slab thickness, and the temperature waiting time becomes longer and the productivity is remarkably increased. There is a problem that it falls.

  In addition, when the inventions described in Patent Documents 2 and 3 are to be applied to thick materials as the subject of the present invention, it is necessary to ensure a sufficient cumulative rolling reduction ratio in the recuperation process. There is a problem that productivity is greatly hindered. Furthermore, there is a problem that the rolling reduction of rough rolling becomes insufficient, austenite (γ) is not sufficiently finely divided, coarse α is generated immediately below the surface layer region, and arrestability is lowered.

Moreover, when manufacturing the steel plate which has desired arrestability by Ni addition like said (ii), there exists a problem that an alloy cost takes too much.
Further, since there is a limit to the cooling rate that can be achieved at the center of the plate thickness of the thick material, it is difficult to finely disperse martensite as in the invention described in Patent Document 4.
Moreover, since the invention described in Patent Document 5 has a C content of 0.03% or less, the steelmaking cost increases, and when the rolling reduction in the non-recrystallized γ region is 75% or more, There is a problem that productivity is extremely lowered and it is not suitable for industrial production.

As described above, the present invention is applied to a large structure having a thickness of 50 mm or more and an _{arrestability} index T _{Kca = 6000} of −10 ° C. or less even when the yield strength is 355 to 460 MPa. A technology for stably and efficiently producing a possible high arrested steel sheet has not yet been established.

  The present invention has been made in view of the above circumstances, has sufficient arrestability as a large structural steel, and is excellent in brittle crack propagation stopping characteristics that can be industrially stable and efficient. An object of the present invention is to provide a method for producing a thick and high-strength steel sheet.

  The present invention is a method for producing a thick high-strength steel sheet having a thickness of 50 to 125 mm, which has excellent brittle crack propagation stopping characteristics that can solve the above-mentioned problems, and the gist thereof is as follows.

[1] By mass%, C: 0.05 to 0.15%, Si: 0.03 to 0.5%, Mn: 0.3 to 2.0%, P: 0.020% or less, S: 0.010% or less, Nb: 0.005-0.030%, Ti: 0.005-0.030%, Al: 0.002-0.10%, N: 0.0010-0.0080% The steel slab is contained at the balance, and the balance is composed of Fe and inevitable impurities. The steel slab is heated to 950 to 1150 ° C. and subjected to rough rolling at a temperature of 900 ° C. or higher and a cumulative rolling reduction of 30% or higher. after, in a range the surface temperature of _{Ar 3} -60 ℃ _~Ar 3, performs the finish rolling 40% or more cumulative rolling reduction, subsequently, the temperature of 200 ° C. or less in thickness on average 8 ° C. / s or more cooling rate Accelerating cooling is performed until the temperature rises, and then the heating rate on the steel sheet surface is set to 1 ° C./s or more, and The temperature is raised with the surface maintained at a temperature 30 ° C. higher than the center in the thickness direction, and the temperature is raised when the surface of the steel sheet reaches 400 to 650 ° C., and the temperature is kept at 400 to 650 ° C. A method for producing a thick high-strength steel having a thickness of 50 to 125 mm and having excellent brittle crack propagation stopping characteristics, characterized in that tempering is performed under the conditions of:
[2] The steel slab is further in mass%, Cu: 0.05 to 1.5%, Cr: 0.05 to 1.0%, Mo: 0.05 to 1.0%, Ni: 0 0.05% to 2.0%, V: 0.005% to 0.10%, B: 0.0002% to 0.0030%, or one or more of the above, [1 ]. The manufacturing method of the thick high-strength steel plate of 50-125 mm in thickness excellent in the brittle crack propagation stop characteristic of description.
[3] The steel slab further includes, in mass%, Mg: 0.0003 to 0.0050%, Ca: 0.0005 to 0.0030%, REM: 0.0005 to 0.010%. A method for producing a thick high-strength steel sheet having a thickness of 50 to 125 mm and having excellent brittle crack propagation stopping characteristics as described in [1] or [2] above, comprising seeds or two or more kinds.

According to the method for producing a thick high strength steel plate having a thickness of 50 to 125 mm, which has excellent brittle crack propagation stopping characteristics according to the present invention, the yield strength is a thick material having a thickness of 50 mm or more, particularly a thickness of 50 to 125 mm. Even in the case of the 355 to 460 MPa class, it is possible to provide a high arrest steel plate applicable to a large structure in which the _{arrestability} index T _{Kca = 6000} is −10 ° C. or less by an efficient manufacturing method. Therefore, the industrial effect is extremely large.

It is a schematic diagram for demonstrating an example of the manufacturing method of the thick high strength steel plate of 50-125 mm in thickness excellent in the brittle crack propagation stop characteristic based on this invention, and the temperature increase rate of the steel plate surface at the time of temper is arrest It is a graph which shows the influence which acts on sex. BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram for demonstrating an example of the manufacturing method of the thick high-strength steel plate of 50-125 mm in thickness excellent in the brittle crack propagation stop characteristic based on this invention, and the steel plate surface at the time of tempering and plate thickness direction center part It is a graph which shows the influence which the temperature difference between and has on the arrestability. It is a schematic diagram for demonstrating an example of the manufacturing method of the thick high strength steel plate of 50-125 mm in thickness excellent in the brittle crack propagation stop characteristic based on this invention, and the temperature of the steel plate surface at the time of tempering is arrestability It is a graph which shows the influence which acts.

  Hereinafter, an embodiment of a method for producing a thick high-strength steel sheet having a thickness of 50 to 125 mm and excellent in brittle crack propagation stopping characteristics of the present invention will be described. In addition, since this embodiment is described in detail for better understanding of the gist of the invention, the present invention is not limited unless otherwise specified.

The method for producing a thick high-strength steel sheet having a thickness of 50 to 125 mm and excellent in brittle crack propagation stopping characteristics of the present invention is mass%, C: 0.05 to 0.15%, Si: 0.03 to 0 0.5%, Mn: 0.3-2.0%, P: 0.020% or less, S: 0.010% or less, Nb: 0.005-0.030%, Ti: 0.005-0. 950%, Al: 0.002 to 0.10%, N: 0.0010 to 0.0080%, the balance is composed of Fe and unavoidable impurities, the steel slab is 950 was heated to to 1150 ° C., 900 after rough rolling of more than 30% cumulative rolling reduction at ° C. or higher, in the range surface temperature of _{Ar 3} -60 ℃ _~Ar 3, finishing more than 40% cumulative rolling reduction Rolling is performed, and subsequently the temperature within the plate thickness reaches 200 ° C. or lower at a cooling rate of 8 ° C./s or higher. Then, the steel plate surface is heated at a temperature rising rate of 1 ° C./s or more while maintaining a temperature higher by 30 ° C. than the central portion in the plate thickness direction. Is a method in which the temperature rise is stopped when the temperature reaches 400 to 650 ° C., and the tempering process is performed under the condition of maintaining the temperature at 400 to 650 ° C.

<Production conditions>
Below, the manufacturing conditions prescribed | regulated by this invention are demonstrated in detail.
Generally, the arrest property (brittle crack propagation stop property) 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 fracture surface unit is fine. It is known that the arrestability is improved. As a means for refining the fracture surface unit, it is effective to perform a controlled rolling (CR) by increasing the cumulative rolling reduction in the lowest possible temperature range. However, when CR is performed in the temperature range immediately above the Ar ₃ transformation point in order to refine the microstructure in the center part in the thickness direction of the thick material, the temperature of the steel sheet surface layer is inevitably lower than Ar ₃ and the processing α is It was found that since the hardness of the surface layer portion is remarkably increased, the arrestability is lowered despite the fine structure.

  Therefore, the present inventors examined in detail the method of utilizing tempering after accelerated cooling in order to improve arrestability while taking advantage of the microstructure refinement advantage of this surface layer two-phase region rolling. As a result, we found that the strength distribution in the plate thickness direction can be made uniform and the arrestability can be improved by tempering so that the heating rate is increased and the surface layer portion is hotter than the center portion in the plate thickness direction. did. The strength equalizing action is caused by the fact that dislocations introduced by rolling are reduced in the surface layer portion and the strength is lowered, while carbide is finely precipitated in the center portion due to the effect of rapid heating and the strength is increased. To do. In addition, fine carbides contribute to the improvement of arrestability by generating microcracks during the propagation of brittle cracks and relaxing the stress state at the crack tip when the main crack propagates.

Furthermore, the present inventors have studied the optimal combination of CR and tempering conditions for ensuring stable arrest characteristics, and obtained the following knowledge.
First, finish rolling (CR) is in the temperature range of the steel sheet surface _{Ar 3} -60 ℃ _~Ar 3, it is necessary to perform as a cumulative rolling reduction of 40% or more. That is, the aim is to refine the structure over the entire thickness of the plate so that the surface layer is light two-phase rolling and the central portion in the thickness direction is rolled in a temperature range immediately above Ar ₃ . If the surface temperature of the steel sheet is lower than Ar ₃ -60 ° C., the toughness and arrestability are deteriorated because the processing α is generated not only from the surface layer portion but also to the center portion in the plate thickness direction and the hardness is remarkably increased. When the surface temperature of the steel plate is higher than Ar ₃ , the CR effect becomes small, and in particular, the refinement of the structure cannot be achieved at the central portion in the plate thickness direction. Further, even when the cumulative rolling reduction is less than 40%, the CR effect is insufficient and a fine structure cannot be obtained, and the arrestability is not improved. Although there is no need to set an upper limit on the cumulative rolling reduction, it is usually limited by the relationship between the product thickness, the slab thickness, and the cumulative rolling reduction in rough rolling.

  Moreover, after holding A steel slab with a plate thickness of 180 mm as shown in Table 1 to be described later at 1120 to 1140 ° C., rough rolling with a cumulative reduction ratio of 33% between 1010 and 980 ° C., and between 780 and 730 ° C. By performing finish rolling with a cumulative rolling reduction of 50%, the sheet thickness is 60 mm, and after accelerated cooling from 770 to 720 ° C. to 160 to 150 ° C. at a cooling rate of 14 ° C./s, by the upper and lower burner flame injection control of the heating furnace, (A) The heating rate of the steel sheet surface portion is 1.5 ° C./s, and (b) the surface is heated by 30 to 40 ° C. higher than the central portion in the plate thickness direction during the heating, and (c) the steel plate After the surface temperature reached 480 to 500 ° C., the temperature increase was stopped and tempering treatment was performed at that temperature for 5 minutes. Thereafter, the following test was performed while performing a temperature gradient type ESSO test based on the method described in WES 3003 as a basic test.

  (I) Of the basic tests, only the heating rate during tempering in (a) was varied from 1.5 ° C./s by the burner flame injection control of the heating furnace, as shown in the graph of FIG. Results were obtained. In this heating, it was confirmed by a heat transfer computer simulation for a plate thickness of 60 mm that a temperature difference between the surface near the burner flame and the central portion in the plate thickness direction was 30 to 50 ° C. It was found by observation with an optical microscope and an electron microscope of the test piece after the test that when the heating rate was less than 1 ° C./s, the precipitates were coarsened and the propagation of the brittle crack was promoted. As a result, as shown in FIG. 1, it was confirmed that the arrestability deteriorates.

  (Ii) Among the basic tests, the temperature difference between the surface of the steel sheet and the central part in the thickness direction during the tempering of (b) was variously changed by the burner flame injection control of the heating furnace, and is shown in the graph of FIG. The result was obtained. When the temperature difference between the surface of the steel plate and the central portion in the plate thickness direction is less than 30 ° C., it was confirmed that the strength distribution in the plate thickness direction could not be made uniform and the arrestability was not improved as shown in FIG.

  In addition, (i) illustrated for the above (a) and (ii) illustrated for (b), the heating rate of 1.0 ° C./s or higher, the steel plate surface temperature, and the center in the plate thickness direction. Means for achieving a temperature difference of 30 ° C. or more as well as not only the burner flame injection control of the heating furnace, but also the introduction of steel sheet into the heating furnace maintained at a temperature higher than the target temperature, induction heating, electric heating, etc. Although it is conceivable to heat the surface of the steel sheet by means of, it is not necessary to limit the means.

  (Iii) Of the basic tests, the graph of FIG. 3 shows the results of various changes in the tempering treatment temperature of the steel sheet surface after the temperature rise was stopped in (c). The temperatures shown in the graph of FIG. 3 are all temperatures maintained for 5 to 6 minutes for tempering treatment. As shown in FIG. 3, when the temperature in the central part in the thickness direction is less than 400 ° C., precipitates effective for arrestability are not sufficiently generated, and when it exceeds 650 ° C., precipitates and structures become coarse. It is clear that arrestability is also reduced. In addition, after the steel sheet reaches the tempering temperature, it may be maintained at that temperature in order to make the temperature distribution in the plate uniform, but in order to prevent the coarsening of precipitates, it should be 10 minutes or less. desirable.

  Then, the reason for limitation of the other manufacturing conditions in this invention is demonstrated.

"Steel reheating temperature" 950-1150 ° C
In the present invention, first, the heating temperature of the steel slab having the above composition is set to 950 to 1150 ° C.
If the reheating temperature of the steel slab after steel making is less than 950 ° C., the solution of the alloy elements is insufficient and causes material non-uniformity, and if it exceeds 1150 ° C., the heated γ grain size becomes coarse, There is a possibility that it is difficult to refine the structure.

“Rough rolling conditions” Temperature: 900 ° C. or higher, cumulative rolling reduction: 30% or higher In the present invention, rough rolling needs to be performed at a temperature of 900 ° C. or higher and a cumulative rolling reduction of 30% or higher. If these conditions are not satisfied, recrystallization of γ does not proceed sufficiently, resulting in a mixed grain structure, which may cause non-uniform material.

“Conditions for CR (Controlled Rolling)”
CR performed subsequent to the rough rolling; in (controlled rolling finish rolling), as described above, the range of the surface temperature Ar _{3 -60} ℃ ~Ar 3 of the steel sheet, performing a cumulative reduction of 40% or more conditions . Thereby, it becomes possible to refine the structure over the entire plate thickness of the steel plate.

"Accelerated cooling after rolling"
In the present invention, after finish rolling (CR) under the above conditions is completed, accelerated cooling is continuously performed at a cooling rate of 8 ° C./s or more on the average in the plate thickness until the temperature reaches 200 ° C. or less. If the cooling rate is less than 8 ° C./s or the cooling stop temperature is higher than 200 ° C., not only the strength is insufficient and the structure is not sufficiently refined, but also necessary in the tempering process. There is a risk that the amount of solid solution C will be insufficient, and improvement in arrestability may not be achieved.

"Tempering treatment"
In this invention, after performing accelerated cooling on the said conditions, as mentioned above, the temperature increase rate in a steel plate surface shall be 1 degree-C / s or more, and the steel plate surface is 30 degreeC or more higher than plate thickness direction center part. When the steel sheet surface reaches 400 to 650 ° C., the temperature increase is stopped and tempering is performed under the condition of maintaining the temperature at 400 to 650 ° C.
In the production method of the present invention, by performing the tempering treatment under the above conditions, even if the plate thickness is 50 mm or more, particularly a thick material having a thickness of 50 to 125 mm and the yield strength is 355 to 460 MPa class, the _{arrestability} index T _{Kca = 6000} becomes −10 ° C. or lower, and a thick high-strength steel sheet having excellent arrestability can be produced.

"Final thickness of steel sheet" 50-125mm
In the present invention, the reason why the final plate thickness of the steel plate is limited to 50 to 125 mm is that the surface of the steel plate and the plate thickness direction are less than 50 mm during the temperature rise for tempering, which is a technical feature of the present invention. This is because it becomes difficult to set the temperature difference at the center to 30 ° C. or more. Conversely, when the final thickness of the steel sheet exceeds 125 mm, a sufficient reduction ratio cannot be obtained, which is a technical feature of the present invention, that is, a cumulative reduction ratio of 30% or more during rough rolling and a cumulative reduction ratio during finish rolling. It is because it becomes difficult to ensure 40% or more.

<Chemical component composition>
Below, the reason for limitation about the chemical component of steel in this invention is demonstrated. In the production method of the present invention, a thick high-strength steel plate is produced using a steel slab that contains the following elements and the balance is composed of Fe and inevitable impurities.
In addition, unless otherwise specified, the unit of content of each element in the following description shall be represented by mass%.

“C: carbon” 0.05 to 0.15%,
Since C is an element essential for inexpensively increasing the strength of the steel sheet, it is necessary to add 0.05% or more. On the other hand, if the amount of addition of C increases, it becomes difficult to ensure high heat input HAZ toughness, so 0.15% is made the upper limit.

"Si: Silicon" 0.03-0.5%
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. However, if it exceeds 0.5%, the weldability and the HAZ toughness are deteriorated. 0.5%.

"Mn: Manganese" 0.3-2.0%
Mn is effective as an element for improving the strength and toughness of the base metal, so 0.3% or more is added. However, excessive addition degrades the HAZ toughness and weld cracking property, so 2.0% is the upper limit. Further, the Mn content is more preferably 0.85 to 2.0% in order to ensure the strength and toughness of the base material.

“P: Phosphorus” 0.020% or less “S: Sulfur” 0.010% or less P and S are desirable as the content is small, but it takes a great deal of cost to reduce this industrially. , P is 0.020%, and S is 0.010%.

"Nb: Niobium" 0.005-0.030%
Nb contributes to microstructure refinement, transformation strengthening and precipitation strengthening when added in a very small amount, and is added in an amount of 0.005% or more because it is an effective element for securing the strength of the base material. In order to degrade toughness, 0.030% is made the upper limit.

"Ti: Titanium" 0.005-0.030%
Addition of 0.005% or more of Ti is effective because it contributes to refinement of the structure and strengthening of precipitation by adding a small amount, and is effective in improving the strength / toughness and HAZ toughness of the base material by forming fine TiN. If added to the content, the HAZ toughness is remarkably deteriorated, so 0.030% is made the upper limit.

"Al: Aluminum" 0.002-0.10%
Al is an important deoxidizing element, so 0.002% or more is added, but if added excessively, the surface quality of the steel slab is impaired and inclusions harmful to toughness are formed, so the upper limit is 0.10%. And

“N: Nitrogen” 0.0010 to 0.0080%
N forms a nitride with Ti and improves the HAZ toughness, so 0.0010% or more is added. However, if excessively added, embrittlement due to solute N occurs, so the content is limited to 0.0080% or less.

  In the manufacturing method of this invention, after making each said element essential, 1 type (s) or 2 or more types of each element demonstrated below can be added selectively. Hereinafter, the reason for limiting the content of each selectively added element will be described.

"Cu: Copper" 0.05-1.5%
"Cr: Chromium" 0.05-1.0%
"Mo: Molybdenum" 0.05-1.0%
Since Cu, Cr, and Mo all improve the hardenability and are effective for increasing the strength, 0.05% or more is added. On the other hand, excessive addition of these elements reduces the HAZ toughness, so Cu is limited to 1.5% or less, and Cr and Mo are limited to 1.0% or less.

"Ni: Nickel" 0.05-2.0%
Ni is effective for ensuring strength, improving arrestability, and improving HAZ toughness, so 0.05% or more is added. However, an increase in the amount of Ni is limited to 2.0% or less because it increases the billet cost.

“V: Vanadium” 0.005 to 0.10%
V contributes to an increase in strength by precipitation strengthening, so 0.005% or more is added. However, if over 0.10% is added, HAZ toughness is reduced, so this is the upper limit.

“B: Boron” 0.0002 to 0.0030%
B is an element that improves hardenability, and is effective in increasing the strength of steel by adding an appropriate amount. However, excessive addition impairs weldability, so the addition amount is 0.0002 to 0.0030%. Limit to range.

“Mg: Magnesium” 0.0003 to 0.0050%,
“Ca: Calcium” 0.0005 to 0.0030%
"REM: rare earth element" 0.0005 to 0.010%
Mg, Ca, and REM form fine oxides and sulfides and contribute to improving HAZ toughness. However, excessive addition coarsens inclusions and lowers toughness. For this reason, Mg is limited to 0.0003 to 0.0050%, Ca is limited to 0.0005 to 0.0030%, and REM is limited to 0.0005 to 0.010%.
In addition, REM demonstrated in this invention means rare earth elements, such as La and Ce, for example.

As described above, according to the method for manufacturing a thick high-strength steel plate having a thickness of 50 to 125 mm, which has excellent brittle crack propagation stop characteristics according to the present invention, a steel slab whose chemical composition is in an appropriate range is used. , after reheating - rough rolling under predetermined conditions, the surface temperature is carried out in the finish rolling 40% or more cumulative rolling reduction in the range of Ar _{3 -60} ℃ ~Ar 3, after the accelerated cooling subsequent, steel When the temperature rise rate on the surface is 1 ° C./s or higher, and the steel plate surface is heated at 30 ° C. or higher than the central portion in the plate thickness direction, and the steel plate surface reaches 400 to 650 ° C. The arrestability index is obtained even when the yield strength is 355 to 460 MPa class with a thick material having a plate thickness of 50 mm or more by performing the tempering process under the condition of holding the temperature at 400 to 650 ° C. T _{Kca = 6000} is -1. Since it is possible to provide a highly arrested steel sheet that can be applied to a large structure at 0 ° C. or less by an efficient manufacturing method, the industrial effect is immeasurable.

  Hereinafter, examples of a method for producing a thick high-strength steel sheet having a thickness of 50 to 125 mm, which is excellent in brittle crack propagation stopping characteristics according to the present invention, will be described in more detail. Of course, the present invention is not limited to the following examples, and the present invention can be carried out with appropriate modifications within a range that can be adapted to the gist of the preceding and following descriptions, all of which are included in the technical scope of the present invention. Is.

[Prototype of thick high-strength steel sheet]
In the steel making process, deoxidation / desulfurization and chemical components of the molten steel were controlled, and a steel piece having a thickness of 180 to 300 mm having chemical components shown in Table 1 below was produced by continuous casting. And by the manufacturing conditions shown in the following Table 2 and Table 3, the steel slab is reheated and rolled into a thick plate to finish the plate to a thickness of 50 to 80 mm, accelerated cooling is performed, and further, offline as necessary. A tempering treatment was carried out to produce a thick high-strength steel plate.
In addition, the “surface-center temperature difference” in the conditions of the tempering treatment shown in Table 3 is a value calculated by obtaining the center temperature in the plate thickness direction at each plate thickness of the steel plate by a temperature model using a heat transfer computer simulation. is there.

[Evaluation test]
The following evaluation tests were performed on the thick steel plate samples produced by the above method, and the test results of the base metal strength and arrestability are shown in Table 4 below.
Here, the yield strength (YP) and the tensile strength (TS) were evaluated using a No. 4 tensile test piece based on JIS Z 2201, which was taken in the direction perpendicular to the rolling direction from the center of the plate thickness direction of the steel sheet. did.
As for the arrestability, provides temperature gradient type ESSO test based on the method described in WES 3003, was evaluated at a temperature showing a Kca = 6000N ^{/ mm 1.5.}

  A list of chemical composition of the thick high-strength steel sheet of this example is shown in Table 1, a list of manufacturing conditions of the steel sheet is shown in Table 2 and Table 3, and a list of mechanical properties of the thick high-strength steel sheet is shown. 4 shows.

[Evaluation results]
Steels A to J shown in Table 1 are steels of the present invention in which the chemical components of the steel are optimized, and steels K and L are comparative steels whose chemical components are outside the specified range of the present invention.
Moreover, No. shown in Tables 2-4. Nos. 1 to 16 are examples of the present invention having chemical components within a predetermined range and manufactured under predetermined conditions, all having sufficient strength as YP: 355 to 460 MPa grade steel, It can be seen that the _{arrestability} index T _{Kca = 6000} is also favorable at −10 ° C. or lower.

  On the other hand, No. shown in Tables 2-4. Nos. 17 to 31 are comparative examples in which either chemical components or production conditions depart from the scope of the present invention, and it is understood that arrestability is reduced.

No. No. 26 has a high heating temperature. In No. 25, since the cumulative rolling reduction of rough rolling was small, the structure was not refined and the arrestability was lowered.
No. No. 24 has a high finish rolling temperature. In No. 21, since the cumulative rolling reduction of finish rolling was small, the fine structure was not obtained as described above, and the arrestability was lowered.
No. In Nos. 17 and 18, the finish rolling temperature was too low, so that the two-phase rolling was performed up to the inside of the plate thickness, and the arrestability was lowered.
No. No. 22 has a low cooling rate for accelerated cooling. Since No. 27 had a high cooling stop temperature, the structure and carbides became coarse, and sufficient arrestability could not be obtained.

No. In Nos. 23 and 28, the rate of temperature increase during tempering was small, so that the carbides became coarse and the arrestability decreased.
No. In No. 19, since the tempering temperature was too low, sufficient carbide was not deposited, and the arrestability was not improved.
No. In No. 20, since the tempering temperature was high, the carbide and the structure became coarse, and the arrestability decreased.
No. In No. 29, the temperature difference between the surface of the steel sheet and the central portion in the plate thickness direction during tempering was small, so the strength distribution was not uniform and the arrestability was lowered.

No. Since No. 30 had much C content, intensity | strength became excessive and arrestability fell.
No. Since No. 31 had a large amount of Nb, the arrestability decreased due to the influence of coarse undissolved Nb remaining during heating.

From the results of the examples described above, the method for producing a thick high-strength steel sheet having a thickness of 50 to 125 mm, which has excellent brittle crack propagation stop characteristics according to the present invention, is a thick material with a thickness of 50 mm or more, and has a yield strength. Even in the case of the 355 to 460 MPa class, it is possible to provide a high arrest steel plate applicable to a large structure in which the _{arrestability} index T _{Kca = 6000} is −10 ° C. or less by an efficient manufacturing method. Is clear.

Claims (3)

% By mass
C: 0.05 to 0.15%,
Si: 0.03 to 0.5%,
Mn: 0.3 to 2.0%,
P: 0.020% or less,
S: 0.010% or less,
Nb: 0.005 to 0.030%,
Ti: 0.005 to 0.030%,
Al: 0.002 to 0.10%,
N: 0.0010 to 0.0080%
A steel slab having a composition consisting of Fe and inevitable impurities in the balance,
Heating the steel strip to 950 to 1150 ° C., after rough rolling of more than 30% cumulative rolling reduction at 900 ° C. or higher, the surface temperature is in the range of _{Ar 3} -60 ℃ _~Ar 3, the cumulative rolling reduction Finish rolling over 40%,
Subsequently, accelerated cooling is performed at a cooling rate of 8 ° C./s or more on average in the plate thickness until a temperature of 200 ° C. or less is reached, then the temperature rising rate on the steel plate surface is set to 1 ° C./s or more, and the steel plate surface is The temperature is raised while maintaining a temperature 30 ° C. higher than the central portion in the plate thickness direction, the temperature rise is stopped when the steel plate surface reaches 400 to 650 ° C., and the temperature is kept at 400 to 650 ° C. A method for producing a thick high-strength steel having a thickness of 50 to 125 mm, which is excellent in brittle crack propagation stopping characteristics, characterized by performing a tempering process. The billet is further in 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 method for producing a thick high-strength steel sheet having a thickness of 50 to 125 mm and having excellent brittle crack propagation stopping characteristics according to claim 1, comprising one or more of them. The billet is further in mass%,
Mg: 0.0003 to 0.0050%,
Ca: 0.0005 to 0.0030%,
REM: 0.0005 to 0.010%
A high-strength steel sheet having a thickness of 50 to 125 mm and having excellent brittle crack propagation stopping characteristics according to claim 1, wherein the steel sheet contains one or more of them. Method.

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