JP6536331B2 - High strength steel plate and method of manufacturing the same - Google Patents

Description

  The present invention relates to a high strength steel plate and a method of manufacturing the same.

  With the increase in height of buildings, etc., the enlargement of construction machines such as cranes and industrial machines is being promoted. In order to reduce the weight of structural members, steel plates used in construction machines and industrial machines are required to have high strength, and a method of manufacturing high strength steel plates with a tensile strength of 780 MPa or more, and further 950 MPa is proposed. (See, for example, Patent Document 1).

  In addition, when using high strength steel plate as a member of construction machine or industrial machine, high yield strength is required so that plastic deformation does not occur. The upper limit is limited. Therefore, a high yield ratio is required for high-strength steel plates used in construction machines, industrial machines, etc. (hereinafter sometimes referred to collectively as construction machines).

  In general, precipitation strengthening is advantageous to increase the yield ratio of steel. Ti is conventionally used as a precipitation strengthening element, and a high strength steel plate containing 0.12% or more of Ti has been proposed (see, for example, Patent Documents 2 and 3). These are hot-rolled steel sheets manufactured by winding up in a coil shape after hot rolling.

JP, 2009-287081, A Unexamined-Japanese-Patent No. 5-230529 JP-A-8-73985

  High strength steel plates used in construction machines are required to have a high yield ratio, but when the yield ratio is too high, there is a problem that the formability is reduced. In addition, ductility is also required to enhance the workability such as bending. Moreover, when manufacturing a member of a construction machine, since welding is performed on a high strength steel plate, it is necessary to suppress softening due to the influence of heat.

  In view of such circumstances, the present invention has an object of providing a high-strength steel sheet which can be suitably used particularly for a construction machine, and a method of manufacturing the same.

  The present inventors have found that the yield ratio can be increased by minimizing the amount of carbon in solid solution in steel. On the other hand, the addition of Ti is advantageous for the precipitation strengthening and the reduction of the amount of solid solution carbon, but when C is added excessively to enhance the strength, the yield ratio becomes too high and the ductility decreases. Furthermore, in consideration of the difference in the direction of the tensile properties of the steel sheet, the optimum hot rolling conditions were found.

  Furthermore, in high-strength steel plates manufactured without cooling by tempering such as quenching and tempering, as they are water-cooled, the strength of the part affected by heat during welding is lower than when tempering is applied. Cheap. In the welded joint, since the lowest point of the hardness controls the strength of the entire welded joint, the relationship between the hardness of the softest part of the welded joint and the alloy component is investigated, and the strength and JS value of the welded joint It turned out that there is a good correlation.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] mass%,
C: 0.03 to 0.09%,
Mn: 1.30 to 2.50%,
Nb: 0.005 to 0.03%,
Mo: 0.05 to 0.3%,
Al: 0.01 to 0.05%,
Ti: 0.10 to 0.20%
Contains
Si: 0.50% or less,
P: 0.02% or less,
S: 0.006% or less,
N: 0.005% or less,
B: limited to 0.003% or less, consisting of balance Fe and unavoidable impurities, JS value determined by the following formula (1) is 1.8 or more, plate thickness is 4.5 to 25 mm Features high strength steel plate.
JS = 5.5 [C] +1.2 [Si] +0.5 [Mn] +0.8 [Ni] +0.2 [Cr] +1.8 [Mo] +0.6 [Nb] + [B] 1)
Here, [X] is the mass% of each element X, and in the above formula (1), it is 0 if the element is not contained.
[2] Furthermore, in mass%,
Cu: 0.5% or less,
Ni: 0.5% or less,
Cr: One or two or more of 1.0% or less of Cr is contained, The high strength steel plate according to the above [1].
[3] Furthermore, in mass%,
Ca: 0.01% or less,
Mg: 0.01% or less,
REM: One or two or more of 0.01% or less of the high strength steel plate described in the above [1] or [2].
[4] The high strength steel plate according to any one of the above [1] to [3], which has a metal structure composed of a lath-like structure.

[5] mass%,
C: 0.03 to 0.09%,
Mn: 1.30 to 2.50%,
Nb: 0.005 to 0.03%,
Mo: 0.05 to 0.3%,
Al: 0.01 to 0.05%,
Ti: 0.10 to 0.20%
Contains
Si: 0.50% or less,
P: 0.02% or less,
S: 0.006% or less,
N: 0.005% or less,
B: A billet limited to 0.003% or less, consisting of the balance Fe and unavoidable impurities, and heating a steel piece having a component composition having a JS value of 1.8 or more determined by the following formula (1) to 1000 ° C. or more And rough rolling at 900 ° C. or higher, and then finish rolling with a cumulative rolling reduction of 40 to 60% within the range of the temperature T _CR that simultaneously satisfies the following formulas (2), (3) and (4) A plate thickness of 4.5 to 25 mm and water-cooling as it is from a temperature of 700 ° C. or more to a temperature of 100 ° C. or less.
JS = 5.5 [C] +1.2 [Si] +0.5 [Mn] +0.8 [Ni] +0.2 [Cr] +1.8 [Mo] +0.6 [Nb] + [B] 1)
T _CR ≦ 900 ° C. (2)
T _CR 800800 × [Ti] +720 (3)
T _CR ≦ 2250 × [Ti] + 590 (4)
Here, [X] is the content [mass%] of each element X, and in the above formula (1), when the element is not contained, it is determined as 0.
[6] Furthermore, in mass%,
Cu: 0.5% or less,
Ni: 0.5% or less,
Cr: 1.0% or less,
A method for producing a high strength steel sheet according to the above [5], characterized in that it contains one or two or more of them.
[7] Furthermore, in mass%,
Ca: 0.01% or less,
Mg: 0.01% or less,
REM: 0.01% or less,
The manufacturing method of the high strength steel plate according to any one of the above [5] or [6], characterized in that it contains one or two or more of them.

  According to the present invention, it is possible to provide a high-strength steel plate suitable for structural members of welded structures such as construction machines, industrial machines and the like which have strong needs for strengthening, by a non-refining process which does not require tempering treatment. . In particular, according to the present invention, preferably the tensile strength is 765 MPa or more, the yield ratio is 0.75 to 0.90, the elongation is 16% or more, and the strength of the welded joint is 85% or more of the strength of the base material, It is possible to provide a high strength steel plate having a thickness of 4.5 to 25 mm, preferably having a yield strength of 685 MPa or more and a tensile strength of 780 to 930 MPa, at low cost. Therefore, the present invention is extremely significant for industrial contribution.

It is a graph which shows the relationship between the amount of solid solution carbons of steel, and a yield ratio. It is a graph which shows the relationship between the yield strength in the rolling direction (YS-L), the tensile strength in the width direction (TS-C), and the amount of added Ti.

  A high strength steel plate having a thickness of 4.5 to 25 mm is used for a construction machine. From the viewpoint of the manufacturing cost, it is desirable to omit the manufacturing process such as the refining heat treatment, and the inventors of the present invention manufacture a steel plate having a tensile strength of 765 MPa or more as it is by water cooling after hot rolling. We examined ways to improve growth. As a result, it was first found that the yield ratio is increased by reducing the amount of C in the steel. As a result of further investigations, as shown in FIG. 1, it was found that the essential factor affecting the yield ratio is the amount of solid solution carbon.

  The vertical axis in FIG. 1 shows the yield strength measured by subjecting steels of various compositions in which the content of C is 0.05% and the addition amount of Ti is changed and which is subjected to hot rolling and water cooling and subjected to a tensile test. And the yield ratio determined from the tensile strength. The horizontal axis of FIG. 1 is the amount of solid solution carbon in an equilibrium state at 920 ° C. determined by thermal scientific calculation from the composition of the steel used to measure the yield ratio. As the amount of solid solution carbon decreases, the increase in yield strength becomes larger relative to the increase in tensile strength, and the yield ratio becomes higher.

  As described above, when Ti is added to precipitate TiC and carbon is fixed as a carbide, the amount of solid solution carbon is reduced, and the yield ratio can be increased. Furthermore, the strength of the steel plate changes in the rolling direction (L direction) and in the width direction (C direction) perpendicular to the rolling direction, and in particular, when the yield strength (YS-L) in the rolling direction is increased, the width direction The knowledge that control of tensile strength (TS-C) was difficult was acquired.

Then, the inventors examined the conditions for setting the yield strength in the rolling direction (YS-L) in the rolling direction to 685 MPa or more and the tensile strength in the width direction (TS-C) to 930 MPa or less. It has been found that it is preferable to satisfy 2) to (4). Formulas (2) to (4), as shown in FIG. 2, the content of Ti [% Ti] satisfying (YS−L) ≧ 685 MPa and (TS−C) ≦ 930 MPa obtained by experiment is an expression that represents the relationship between the finishing temperature T _CR hot rolling. Among the line segments shown in FIG. 2, YS-L is a condition that the yield strength in the rolling direction is 685 MPa, and TS-C is a condition that the tensile strength in the width direction is 930 MPa. It has high strength below the line segment.

T _CR ≦ 900 ° C. (2)
T _CR 800800 × [% Ti] +720 (3)
T _CR ≦ 2250 × [% Ti] + 590 (4)
Further, by performing cumulative rolling reduction finish such that 40% to 60% rolling within these relations from sought T _CR, yield strength above 685MPa, a tensile strength of hereinafter referred to 930 MPa, It has been found that favorable tensile properties are obtained.

  By the way, the high strength steel plate manufactured without cooling and heat treatment such as quenching and tempering, as it is with water cooling, is lower in strength of the part affected by heat at the time of welding compared with the case where the refining treatment is performed It's easy to do. In the welded joint, since the place with the lowest hardness controls the strength of the entire welded joint, the relationship between the hardness of the softest part of the welded joint and the alloy composition was investigated.

  As a result, it was found that the hardness of the most softened portion is greatly affected by the alloy components and has a good correlation with the JS value calculated by the following formula (1). The alloy composition of the weld heat affected zone is not affected by dilution by the weld metal and the like, and is the same as the alloy composition of the high strength thick steel plate as the base material.

JS = 5.5 [C] + 1.2 [Si] + 0.5 [Mn] + 0.8 [Ni] + 0.2 [Cr] + 1.8 [Mo] + 0.6 [Nb] + [B] ... ( 1)
Here, [X] is the mass% of each element X, respectively.

Furthermore, when the relationship between the JS value and the strength (joint strength) of a welded joint was statistically examined, it was found that it can be approximated by the following formula (5).
Joint strength = 705 × JS-491 (MPa) · · · (5)

  That is, if the alloy component of the high strength thick steel plate is designed such that the JS value becomes appropriate, the joint strength can be adjusted, and a sufficient joint strength as a welded structure can be secured. However, when a tensile test is actually performed on a welded joint, breakage occurs at the weakest portion. Accordingly, when the estimated value of the above equation (5) exceeds the tensile strength of the base material, fracture occurs in the base material, so the joint strength becomes equal to the tensile strength of the base material.

  Below, the reason for limitation of each component and manufacturing method in this invention is demonstrated. First, components will be described, but unless otherwise noted, “%” means “mass%”.

[C: 0.03 to 0.09%]
C is an element that affects not only the strength and the yield ratio but also the manufacturability in the steel making process. If the C content is less than 0.03%, the process load in steelmaking becomes too large, so the content is made 0.03% or more. On the other hand, when the amount of C exceeds 0.09%, the yield ratio becomes large and the tensile strength becomes too high with respect to the yield strength, so the content is made 0.09% or less. Preferably, it is 0.06% or less.

[Si: 0.50% or less]
Si is a deoxidizing element and is an element also contributing to the increase in strength. Although the amount of Si may be 0%, 0.01% or more is preferable. On the other hand, since toughness will fall if Si is added excessively, the upper limit of the amount of Si is made into 0.50% or less. Preferably, it is 0.40% or less.

[Mn: 1.30 to 2.50%]
Mn is an element that enhances hardenability, and in order to secure strength, the amount of Mn is made 1.30% or more. Preferably, it is 1.50% or more. On the other hand, since toughness will fall when Mn is added excessively, the upper limit of the amount of Mn is made into 2.50% or less. Preferably, it is 2.00% or less.

[P: 0.02% or less]
[S: 0.006% or less]
P and S are impurities, and the amounts of P and S are limited to 0.02% or less and 0.006% or less, respectively, to reduce the low temperature toughness of the base material and the joint. Preferably, it is 0.01% or less and 0.004% or less, respectively. Although the lower limit of the amount of P and the amount of S may be 0, the amount of P can be made 0.001% or more and the amount of S can be made 0.0001% or more from the viewpoint of manufacturing cost.

[Nb: 0.005 to 0.03%]
Nb is effective for securing hardenability and toughening by refining the structure, and the Nb content is made 0.005% or more. Preferably, it is 0.010% or more. On the other hand, when a large amount of Nb is added, the toughness of the welded portion decreases, so the upper limit of the Nb amount is made 0.03% or less. Preferably, it is 0.02% or less.

[Mo: 0.05 to 0.3%]
Mo is an element effective for securing the hardenability, and the Mo amount is 0.05% or more, preferably 0.10% or more. On the other hand, when adding Mo excessively, there is a concern that the toughness will be reduced, so the upper limit of the Mo amount is made 0.3% or less. Preferably, the amount of Mo is 0.25% or less.

[Al: 0.01 to 0.05%]
Al is a deoxidizing element, and the amount of Al is made 0.01% or more in order to precipitate TiC by suppressing the formation of an oxide of Ti. On the other hand, if the amount of Al exceeds 0.05%, coarse alumina inclusions are formed to lower the toughness, so the upper limit of the amount of Al is made 0.05% or less.

[Ti: 0.10 to 0.20%]
Ti is the most important element in the present invention. In order to form TiC to fix carbon, to increase the yield ratio and to suppress an increase in tensile strength, the amount of Ti is made 0.10% or more. On the other hand, when Ti is added excessively, the amount of solid solution carbon decreases excessively and the strength decreases, so the upper limit of the amount of Ti is made 0.20% or less.

[B: 0.003% or less]
B is an element improving the hardenability, and preferably the B content is 0.0003% or more. However, since the hardenability is saturated even when the B content exceeds 0.003%, the upper limit is limited to 0.003% or less. Preferably, the B amount is 0.002% or less.

[N: 0.005% or less]
Since N forms TiN and inhibits the formation of TiC, the amount of N is limited to 0.005% or less. Preferably, the N amount is 0.004% or less. Although the amount of N may be 0%, it may be 0.001% or more from the viewpoint of manufacturing cost.

[Cu: 0.5% or less]
[Ni: 0.5% or less]
[Cr: 1.0% or less]
Cu, Ni, and Cr are elements that enhance the hardenability and contribute to the increase in strength, and can contain one or more kinds. In order to obtain an effect, the content of each element is preferably 0.10% or more. On the other hand, when any of Cu, Ni, and Cr is added excessively, the toughness of the base material and the joint may decrease, and the upper limit of the content is 0.5% or less, 0.5% or less, 1.0 or less, respectively. It is preferable to set it as% or less.

[Ca: 0.01% or less]
[Mg: 0.01% or less]
[REM: 0.01% or less]
In addition to the above elements, one or more of Ca, Mg and REM may be contained to control the form of the oxide or sulfide. In order to acquire an effect, it is preferable to make content of each element 0.0005% or more. On the other hand, when any of Ca, Mg and REM is added excessively, coarse inclusions may be generated to lower the toughness of the base material and the joint. The upper limit of the content is 0.01% or less, 0 It is preferable to set it as .01% or less and 0.01% or less.

[JS value: 1.8 or more]
Furthermore, in order to make the strength of the welded joint manufactured using the high strength steel plate according to the present invention equal to or higher than the strength of the base material, a good correlation with the hardness of the softest part in the welding heat affected zone is shown. Let JS value represented by be 1.8 or more. The upper limit of the JS value is preferably as high as possible, and is not particularly limited, and may be 3.1 determined by the upper limit of the content of C, Si, Mn, Ni, Cr, Mo, Nb, and B. In order to increase the JS value, it is necessary to increase the content of the alloying elements, so the JS value is preferably 3.0 or less, more preferably 2.5 or less, still more preferably 2. from the viewpoint of production cost. It can be set to 0 or less.

JS = 5.5 [C] + 1.2 [Si] + 0.5 [Mn] + 0.8 [Ni] + 0.2 [Cr] + 1.8 [Mo] + 0.6 [Nb] + [B] ... ( 1)
Here, [C], [Si], [Mn], [Ni], [Cr], [Mo], [Nb], [B] are C, Si, Mn, Ni, Cr, Mo, Nb, respectively. , B mass%.

  The high strength steel plate of the present invention preferably has a metal structure composed of a lath structure. The lath-like structure is a metal structure which exhibits a needle-like morphology when observed with an optical microscope, and does not include polygonal ferrite, pearlite or island martensite which exhibits a granular morphology. Among the lath-like structures, metal structures consisting of one or both of martensite and lower bainite are more preferable.

  The two are different metallographic structures in that martensite contains no centite and lower bainite contains fine cementite. Although discrimination between martensite and lower bainite is difficult with an optical microscope, observation with a transmission electron microscope (TEM) can be conducted depending on the presence or absence of cementite formation.

  The lath-like structure is a metal structure formed by performing water cooling after hot rolling. In particular, martensite and lower bainite are formed by water cooling after hot rolling to a temperature at which the metal structure is austenite, that is, a temperature at which the ferrite transformation starts, Ar 3 point or more to 100 ° C. or less Also referred to as metamorphosis.

  Next, the method for producing the high strength steel sheet of the present invention will be described. The high strength steel plate of the present invention can be manufactured by hot rolling a steel piece obtained by melting and casting steel by a conventional method so that the thickness becomes 4.5 to 25 mm. . Below, a preferable manufacturing method is demonstrated.

  The heating temperature for hot rolling is preferably 1000 ° C. or higher in order to reduce deformation resistance. In the present invention, since the C content is small, the austenite grain growth suppressing effect by NbC is small, and the initial austenite grains at the time of heating are likely to be coarsened, so it is not preferable to heat too high temperature.

In hot rolling, after rough rolling in the austenite recrystallization temperature range, finish rolling in the austenite non-recrystallization temperature range of 900 ° C. or less is preferably performed. In addition, it is not necessary to restrict | limit in particular about rough rolling in the temperature over 900 degreeC. The rolling in the unrecrystallized temperature range has a cumulative rolling reduction at a temperature _TCR that satisfies the equations (2), (3) and (4) simultaneously depending on the amount of Ti [% Ti] (mass%) It is preferable to make it 60%.

T _CR ≦ 900 ° C. (2)
T _CR 800800 × [% Ti] +720 (3)
T _CR ≦ 2250 × [% Ti] + 590 (4)

  The above equation (2) defines the upper limit temperature for performing non-recrystallization rolling, and means that the non-recrystallization temperature range is 900 ° C. or less in the above-described component range. Equation (3) is a condition for suppressing an excessive increase in tensile strength, and defines the lower limit value of the finish rolling temperature. Due to the reduction of the rolling temperature, the strength rises, and when the finish rolling finish temperature becomes lower than the value on the right side of the equation (3), the tensile strength may excessively increase and may exceed 930 MPa. The equation (4) is a condition for suppressing the decrease in yield strength, and defines the upper limit of the start temperature of finish rolling. When the numerical value on the right side of the equation (4) exceeds 900 ° C., the upper limit of finish rolling is set to 900 ° C. due to the limitation by the equation (2). When the finishing temperature becomes high, the strength decreases, and when the starting temperature exceeds either the numerical value on the right side of Formula (4) or 900 ° C., whichever is lower, the yield strength may be reduced to less than 685 MPa.

  If the start temperature of water cooling after rolling is less than 700 ° C, polygonal ferrite, island martensite and cementite may be locally generated, and the strength and toughness of the base material may decrease. It is preferable to do.

  The stopping temperature of water cooling is preferably 100 ° C. or less in order to stably obtain a lath-like structure, in particular, a metallographic structure consisting of one or both of martensite and lower bainite, and water cooling may be performed to room temperature. After water cooling, it is allowed to cool to room temperature as it is, and thereafter heat treatment is not performed, and a high strength steel plate having excellent tensile properties and toughness can be obtained.

  The steel piece obtained by melting the steel having the component composition shown in Table 1 was hot-rolled under the production conditions shown in Table 2 and water-cooled to produce a steel plate. The metallographic structure of the steel sheet is observed with an optical microscope, and the specimen is collected to obtain the tensile properties (yield strength, tensile strength, elongation) of the base material, toughness, and tensile strength of the welded joint (welded joint strength) The evaluation of The tensile properties of the base material were obtained by collecting test pieces No. 1A (plate-like) or No. 4 (rod-like) in accordance with JIS Z 2241 and measuring the yield strength, tensile strength, and elongation. As for the tensile test pieces, No. 1A full thickness tensile test pieces are collected if the thickness is 20 mm or less, and if the thickness is more than 20 mm, the No. 4 round bar tensile test pieces are 1/4 part (t / 4 parts) of the thickness and the thickness center It collected from part (t / 2 part).

  The toughness of the base material is evaluated by collecting V-notch test pieces in the direction perpendicular to the rolling direction from the center of thickness according to JIS Z 2242, and determining Charpy absorbed energy (vE-20) at -20 ° C. did. The welded joint strength was measured by using a butt welded joint and conforming to JIS Z 3121 using No. 1A test pieces (plate-like) or No. 5 test pieces (rod-like). As for tensile test pieces, No. 1A full thickness tensile test pieces are collected if the board thickness is 20 mm or less, and No. 5 round bar tensile test pieces if the board thickness is more than 20 mm, 1/4 part (t / 4 part) of the board thickness It collected from part (t / 2 part). The results are shown in Table 2.

  The target values for the tensile properties of the base material are: yield strength 650 MPa or more (preferably 685 MPa or more), tensile strength of the base material 765 to 930 MPa (preferably 780 MPa or more), yield ratio 0.75 to 0.90 , Growth is more than 16%. The target value of the toughness (vE-20) of the base material is 47 J or more (preferably 60 J or more), and the weld joint strength is 765 MPa or more (preferably 780 MPa or more).

  Invention Examples 1 to 15 all had tensile strength characteristics, toughness, and weld joint strength of the base material satisfying the target values, and had a metal structure consisting of a lath-like structure. Particularly, in the invention examples 1 to 10, the yield strength of the base material is 685 MPa or more, the tensile strength is 780 MPa or more, the toughness (vE-20) of the base material is 60 J or more, and the weld joint strength is 780 MPa or more. I am satisfied with the target value. In addition, they have a metal structure consisting of a lath-like structure in observation with a light microscope, and further, because cementite can not be seen, they have a metal structure consisting of one or both of martensite and lower bainite It was determined that

  On the other hand, Comparative Example 16 lacks the C content, Comparative Example 21 the Nb content, and Comparative Example 22 the Mn content, and the yield strength and the tensile strength of the base material are lowered. Comparative Example 17 has a large amount of C and an excessively high tensile strength. Comparative Example 18 has a small amount of Ti, and Comparative Example 19 has a large amount of Ti, so the yield ratio is lowered. Since the comparative example 20 has much Mo amount, the toughness of a base material is falling. Since the comparative example 23 has a low JS value, the tensile properties of the base material are good, but the weld joint strength is reduced. In addition, it was determined by observation with an optical microscope that these also have a metal structure consisting of a lath-like structure.

  The high-strength steel plate according to the present invention is suitable for structural members of welded structures such as construction machines, industrial machines and the like which have strong needs for high strength, and can be manufactured by a non-refining process which does not require tempering. It is. Therefore, the present invention can provide a high strength steel plate for construction machine with a thickness of 4.5 to 25 mm at low cost.

Claims (7)

In mass%,
C: 0.03 to 0.09%,
Mn: 1.30 to 2.50%,
Nb: 0.005 to 0.03%,
Mo: 0.05 to 0.3%,
Al: 0.01 to 0.05%,
Ti: 0.10 to 0.20%
Contains
Si: 0.50% or less,
P: 0.02% or less,
S: 0.006% or less,
N: 0.005% or less,
B: limited to 0.003% or less, consisting of balance Fe and unavoidable impurities, JS value determined by the following formula (1) is 1.8 or more, plate thickness is 4.5 to 25 mm Features high strength steel plate.
JS = 5.5 [C] + 1.2 [Si] + 0.5 [Mn] + 0.8 [Ni] + 0.2 [Cr] + 1.8 [Mo] + 0.6 [Nb] + [B] ... ( 1)
Here, [X] is the mass% of each element X, and in the above formula (1), it is 0 if the element is not contained. Furthermore, in mass%,
Cu: 0.5% or less,
Ni: 0.5% or less,
Cr: 1.0% or less of 1 type or 2 or more types is contained, The high strength steel plate according to claim 1 characterized by the above-mentioned. Furthermore, in mass%,
Ca: 0.01% or less,
Mg: 0.01% or less,
REM: One or more of 0.01% or less is contained, The high strength steel plate according to claim 1 or 2, characterized in that   The high strength steel plate according to any one of claims 1 to 3, characterized by having a metal structure consisting of a lath-like structure. In mass%,
C: 0.03 to 0.09%,
Mn: 1.30 to 2.50%,
Nb: 0.005 to 0.03%,
Mo: 0.05 to 0.3%,
Al: 0.01 to 0.05%,
Ti: 0.10 to 0.20%
Contains
Si: 0.50% or less,
P: 0.02% or less,
S: 0.006% or less,
N: 0.005% or less,
B: A billet limited to 0.003% or less, consisting of the balance Fe and unavoidable impurities, and heating a steel piece having a component composition having a JS value of 1.8 or more determined by the following formula (1) to 1000 ° C. or more And rough rolling at 900 ° C. or higher, and then finish rolling with a cumulative rolling reduction of 40 to 60% within the range of the temperature T _CR that simultaneously satisfies the following formulas (2), (3) and (4) A plate thickness of 4.5 to 25 mm and water-cooling as it is from a temperature of 700 ° C. or more to a temperature of 100 ° C. or less.
JS = 5.5 [C] + 1.2 [Si] + 0.5 [Mn] + 0.8 [Ni] + 0.2 [Cr] + 1.8 [Mo] + 0.6 [Nb] + [B] ... ( 1)
T _CR ≦ 900 ° C. (2)
T _CR 800800 × [Ti] +720 (3)
T _CR ≦ 2250 × [Ti] + 590 (4)
Here, [X] is the content [mass%] of each element X, and in the above formula (1), when the element is not contained, it is determined as 0. Furthermore, in mass%,
Cu: 0.5% or less,
Ni: 0.5% or less,
Cr: 1.0% or less,
A method for producing a high strength steel sheet according to claim 5, characterized in that it contains one or more of the following. Furthermore, in mass%,
Ca: 0.01% or less,
Mg: 0.01% or less,
REM: 0.01% or less,
The manufacturing method of the high strength steel plate according to any one of claims 5 or 6, characterized in that one or two or more of them are contained.

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