JP2012162801A - Method for producing taper plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a taper plate having a tensile strength of 570 MPa or more, to which high heat input welding having a welding heat input exceeding 300 kJ/cm can be applied.SOLUTION: A steel slab contains, by mass, 0.03 to 0.12% C, 0.03 to 0.5% Si, 0.8 to 2.2% Mn, 0.015% or less P, 0.0005 to 0.0050% S, 0.005 to 0.1% Al, 0.003 to 0.014% Nb, 0.003 to 0.02% Ti, 0.0003 to 0.0025% B, 0.0030 to 0.0070% N, 0.0005 to 0.005% Ca, satisfies formula (1), with the balance consisting of Fe and unavoidable impurities. The steel slab is heated to 1000°C to 1200°C and then subjected to hot rolling in which the plate thickness varies in a tapered shape in the longitudinal direction at a rolling finish temperature of 900°C or less and an Arpoint or above, followed by accelerated cooling to 500°C or less. Formula (1): 0≤N-Ti/3.42≤0.0025, wherein N and Ti are the content (mass%) of each component.

Description

  The present invention relates to a method for manufacturing a tapered plate (also referred to as a tapered steel plate or LP steel plate) whose thickness changes continuously in the longitudinal direction, which is suitable for shipbuilding, construction, and the like. A method for producing a taper plate having a difference in thickness (thickness difference) between a thickness of 10 mm or more and a thickness of 10 mm or more in the longitudinal direction with a tensile strength of 570 MPa or more applicable to large heat input welding with a heat input exceeding 300 kJ / cm About.

  The shape of the thick steel plate is generally uniform in both the width direction and the longitudinal direction. However, when the plate thickness is continuously changed in the longitudinal direction, it may have a great effect in reducing the weight of the material and reducing the number of welding processes. Such a thick steel plate is called a tapered plate, a tapered steel plate, an LP steel plate, or the like, and there are many proposals such as Patent Document 1, Patent Document 2, and Patent Document 3 for its manufacturing method. These proposals are aimed at how to produce a taper plate with high dimensional accuracy. However, unless the material characteristics and material uniformity of the steel sheet are satisfied in addition to the dimensional accuracy, it cannot be put into practical use.

  Recently, the quality requirements for thick steel plates have become stricter, and in particular, the demand for higher tension and the demand for improved weldability have become stronger. In response to such demands, TMCP methods such as controlled rolling and controlled cooling are employed. In this method, ferrite grains are refined by strong processing in the austenite non-recrystallized region and (austenite + ferrite) two-phase region, followed by austenite → ferrite transformation, and further cooling is performed as necessary. It is intended to increase strength and toughness.

  However, when this method is applied to a taper plate, temperature management becomes extremely difficult and material fluctuations increase.

  In particular, when the control rolling is strong processing at a low temperature such as austenite non-recrystallization zone rolling or (austenite + ferrite) two-phase zone rolling, when the thickness is different in the plate thickness direction like a taper plate, And the temperature difference of the steel plate temperature of the thick part becomes too large, and there remains a problem that the difference in strength becomes large. Several proposals have been made to eliminate such material non-uniformity and to produce a uniform tapered plate.

  For example, in Patent Document 4, in order to obtain a uniform material, the temperature in the longitudinal direction before cooling is measured, and the optimum cooling condition at each point is calculated based on this measured value, and cooling is performed according to the plate thickness. A method of cooling the tapered plate to correct the plate passing speed is shown. In Patent Document 5, the cooling is started at the thin part and the thick part of the steel sheet at the same time, and the taper plate cooling method for changing the timing of exiting the cooling device, or the cooling is started sequentially in the longitudinal direction of the steel sheet, and the cooling is finished at the same time. A taper plate cooling method is shown. Both are proposals to reduce the variation of the material in the steel plate when accelerated cooling is performed.

  On the other hand, Patent Document 6 is an example of trying to solve such a problem by devising the component composition of the steel sheet. This technique discloses that the intensity variation can be reduced by increasing the Nb addition amount to 0.015% to 0.06%.

Further, corresponding to the air-cooling rate in the 800 to 500 ° C. of the steel sheet of Patent In Document _{7, Hv 20-50 = -110 + 460C} + 44Si + 39Mn-31Cu-9Ni + 11Cr + 22Mo + 180V + 9600B-23000Mo × Hv 20-50 value represented by B (plate thickness 20mm and 50mm It is disclosed that variation in strength can be reduced by setting the difference in Hv hardness when cooled to room temperature at a cooling rate to 15 or less.

  However, steel materials that can be applied to high heat input welding, whose needs have been increasing in recent years, have various component design restrictions to ensure the toughness of the welded part, so that a taper plate can reduce strength variation. In particular, in the case of a steel material for high heat input welding containing B, there is a problem that the tendency of variation in strength becomes conspicuous due to changes in plate thickness and finishing temperature.

Japanese Patent Publication No. 50-36826 Japanese Patent Publication No. 60-124 JP-A-5-49361 JP-A-62-166013 JP 7-68309 A Japanese Patent No. 3180944 Japanese Patent No. 3972553

  The present invention advantageously solves the above-mentioned problems, has a tensile strength of 570 MPa or more with small variation in strength, and has a high heat input weld toughness exceeding 300 kJ / cm, and is excellent in toughness in the longitudinal direction. It is an object of the present invention to provide a method of manufacturing a tapered plate having a difference in thickness (taper amount) of 10 mm or more.

  In order to solve the above-mentioned problems, the present inventors investigated the influence of Ti and N contents on the strength difference between the thick part and thin part of B-containing taper plates having different Ti and N contents, and Ti and N contents. However, when 0 ≦ N—Ti / 3.42 ≦ 0.0025 is satisfied, an appropriate amount of the solid solution B can be stably secured, and the knowledge that the strength difference between the thick part and the thin part becomes small is obtained. It was.

The present invention has been made by further study based on the above knowledge, that is, the present invention,
1.
% By mass
C: 0.03-0.12%,
Si: 0.03 to 0.5%,
Mn: 0.8 to 2.2%
P: 0.015% or less,
S: 0.0005 to 0.0050%,
Al: 0.005 to 0.1%,
Nb: 0.003 to 0.014%,
Ti: 0.003 to 0.02%,
B: 0.0003 to 0.0025%,
N: 0.0030 to 0.0070%,
Ca: 0.0005 to 0.0050%,
And satisfies the formula (1),
After heating the steel slab composed of the remaining Fe and inevitable impurities to 1000 ° C to 1200 ° C, hot rolling in which the plate thickness changes in a taper shape in the longitudinal direction is performed at a rolling finishing temperature of 900 ° C or less at Ar ₃ points or more, Thereafter, accelerated cooling to 500 ° C. or lower, and a method for producing a tapered plate having a tensile strength of 570 MPa or more and a difference between the thickness of the thick part and the thickness of the thin part of 10 mm or more.
0 ≦ N-Ti / 3.42 ≦ 0.0025 (1)
However, N and Ti are content (mass%) of each component.
2. The component composition of the steel slab is further mass%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
Cr: 0.05 to 0.5%,
Mo: 0.05-0.5%
V: 0.02-0.1%,
2. A method for producing a taper plate having a tensile strength of 570 MPa or more according to 1, wherein the difference between the thickness of the thick part and the thickness of the thin part is 10 mm or more.
3. The component composition of the steel slab is further mass%, Mg: 0.0005 to 0.005%,
Zr: 0.003 to 0.02%,
REM: 0.003-0.02%,
A method for producing a taper plate having a tensile strength of 570 MPa or more according to 1 or 2, wherein the difference between the thickness of the thick part and the thickness of the thin part is 10 mm or more .
4). The component composition of the steel slab is further mass%.
O: 0.0030% or less, and further, each content of Ca, O, and S satisfies the following formula (2): Tensile strength according to any one of 1 to 3 A method of manufacturing a taper plate having a thickness of 570 MPa or more and a difference in thickness between the thick part and the thin part of 10 mm or more.
0.3 ≦ ACR ≦ 0.8 (2)
Here, ACR = (Ca− (0.18 + 130 × Ca) × O) /1.25/S
Moreover, Ca, O, and S represent content (mass%) of each component.

  According to the present invention, the tensile strength is 570 MPa or more, the difference in strength between the thick part and the thin part is small, and the thick part thickness is applicable to large heat input welding applications such as submerged arc welding, electrogas welding, and electroslag welding. A taper plate having a thickness difference (taper amount) of 10 mm or more can be manufactured, which is extremely useful industrially.

In this invention, a component composition and manufacturing conditions are prescribed | regulated. In the description,% is mass%.
[Ingredient composition]
C: 0.03-0.12%
C is added in an amount of 0.03% or more in order to obtain the strength required for structural steel. On the other hand, if added over 0.12%, the weld heat affected zone toughness is lowered, so the content is made 0.03% to 0.12%. Preferably it is 0.04 to 0.09%.

Si: 0.03-0.5%
Si is added in an amount of 0.03% or more to ensure deoxidation and strength. If added over 0.5%, in the case of high heat input welding, island martensite is generated in the weld heat affected zone and the toughness is deteriorated, so the content is made 0.5% or less. Preferably it is 0.4% or less.

Mn: 0.8 to 2.2%
Mn is added in an amount of 0.8% or more in order to ensure the strength of the base material. On the other hand, if it exceeds 2.2%, the toughness of the welded portion is remarkably deteriorated, so 0.8 to 2.2%, preferably 1.2 to 2.1%, more preferably 1.2 to 2.0%. And

P: 0.015% or less P is an unavoidable impurity in the present invention. When P is contained in an amount exceeding 0.015%, island-like martensite is generated in the heat-affected zone by high heat input welding, and in particular, CTOD. In order to reduce the characteristics, the content is made 0.015% or less. Preferably it is 0.012% or less.

S: 0.0005 to 0.0050%
S is made to be 0.0005% or more in order to generate CaS and MnS. On the other hand, 0.0050
If it exceeds 50%, the toughness of the base material is reduced, so 0.0005 to 0.0050%.

Al: 0.005 to 0.1%
Al is made 0.005% or more in order to deoxidize steel. On the other hand, if it exceeds 0.1%, the toughness of the base metal is lowered and the toughness of the weld metal is also lowered, so 0.005 to 0.1%, preferably 0.01 to 0.06%.

Nb: 0.003 to 0.014%
Nb is effective to ensure the strength, toughness and weld joint strength of the base metal, and is required to be 0.003% or more to obtain the effect, but when it exceeds 0.014%, large heat input welding was performed. At this time, the toughness of the weld heat affected zone is lowered, so 0.003 to 0.014%. Preferably it is 0.005 to 0.013%.

Ti: 0.003-0.02%
Ti forms and precipitates TiN during solidification, suppresses coarsening of austenite grains in the weld heat-affected zone, precipitates ferrite as a ferrite transformation nucleus, and improves toughness, so 0.003% or more Add. On the other hand, if it exceeds 0.02%, TiN particles become coarse and lower toughness, so 0.003 to 0.02%. Preferably it is 0.005 to 0.018%.

B: 0.0003 to 0.0025%
B contributes to hardenability as solid solution B during steel plate production and improves the strength of the base metal. When large heat input welding is performed, BN is generated in the heat affected zone to reduce solid solution N. Further, 0.0003% or more is added in order to become ferrite transformation nuclei and generate ferrite to improve toughness.

  On the other hand, if it exceeds 0.0025%, the hardenability increases and the toughness decreases, so the content is made 0.0003 to 0.0025%. Preferably it is 0.005 to 0.0022%.

N: 0.0030 to 0.0070%
N generates 0.0030% or more in order to generate TiN effective for improving toughness. On the other hand, if it exceeds 0.0070%, solid solution B that contributes to hardenability may not be ensured at the time of manufacturing the steel sheet, and TiN in the vicinity of the bond portion melts when high heat input welding is performed, and the weld metal In order to increase the solid solution N in the steel and deteriorate its toughness, the content is made 0.0030 to 0.0070%.

0 ≦ N-Ti / 3.42 ≦ 0.0025
In the present invention, it is required that the tensile strength is 570 MPa or more, the strength variation is small, and the high heat input weld toughness exceeds 300 kJ / cm, and the parameter composition is defined in the above component composition. To do. When the content of Ti and N is N-Ti / 3.42> 0.0025, an appropriate amount of solid solution B cannot be stably secured, and the strength against the change in the plate thickness and rolling conditions The variation becomes large. On the other hand, in the case of N-Ti / 3.42 <0, the toughness of the weld heat-affected zone significantly deteriorates when high heat input welding is performed. Therefore, 0 ≦ N−Ti / 3.42 ≦ 0.0025.

Ca: 0.0005 to 0.0050%
Ca makes the toughness of the weld heat-affected zone good when subjected to high heat input welding. MnS, TiN, BN precipitates on CaS, and the toughness of the weld heat-affected zone is increased by increasing the nucleation frequency of ferrite. To improve. In order to obtain the effect, the content is made 0.0005% or more. On the other hand, if it exceeds 0.0050%, the effect is saturated, so 0.0005 to 0.0050%. Preferably it is 0.0005 to 0.0030%, More preferably, it is 0.0007 to 0.0030%.

  The above is the basic component of the present invention, and sufficient effects can be obtained. However, when further improving the characteristics, one or more of Cu, Ni, Cr, Mo, V, Mg, Zr, and REM should be included. Is possible.

Cu: 0.05 to 1.0%
Cu is effective for increasing the strength of the base material, and it is preferable to contain 0.05% or more in order to obtain the effect. However, if it exceeds 1.0%, hot brittleness occurs, and the steel sheet surface properties deteriorate. Therefore, when it contains, it is preferable to set it as 1.0% or less. More preferably, it is 0.1 to 0.8%.

Ni: 0.05-1.0%
Ni increases the strength while maintaining the base material with high toughness, so it is preferable to contain 0.05% or more in order to obtain the effect. On the other hand, since the effect will be saturated when it exceeds 1.0%, when it contains, it is preferable to set it as 0.05 to 1.0%. More preferably, the content is 0.1 to 0.9%.

Cr: 0.05-0.5%
Cr is effective for increasing the strength of the base material, and is preferably contained in an amount of 0.05% or more in order to obtain the effect. However, when added in a large amount, the toughness is deteriorated. It is preferable to make it 5% or less. More preferably, the content is 0.1 to 0.4%.

Mo: 0.05-0.5%
Mo is effective for increasing the strength of the base material and is preferably contained in an amount of 0.05% or more in order to obtain the effect. However, if added in a large amount, the toughness deteriorates. It is preferable to set it to 5% or less. More preferably, the content is 0.07 to 0.4%.

V: 0.02-0.1%
V is effective for increasing the strength of the base material, and is preferably contained in an amount of 0.02% or more in order to obtain the effect. However, if it exceeds 0.1%, the toughness will be lowered. Is preferably 0.1% or less. More preferably, the content is 0.04 to 0.08%.

Mg: 0.0005 to 0.005%
Mg is an element having an effect of improving toughness due to dispersion of oxides. In order to exert such an effect, it is preferable to contain at least 0.0005% or more, but even if it contains more than 0.005%, the effect is saturated. It is preferable to do.

Zr: 0.003 to 0.02%
Zr is an element having an effect of improving toughness due to dispersion of oxides. In order to exert such an effect, it is preferable to contain at least 0.003% or more, but even if it exceeds 0.02%, the effect is saturated. It is preferable to do. More preferably, it is 0.004 to 0.018%.

REM: 0.003-0.02%
REM is an element having an effect of improving toughness due to dispersion of oxides. In order to exert such an effect, it is preferable to contain at least 0.003% or more, but even if it exceeds 0.02%, the effect is saturated. It is preferable to do. More preferably, it is 0.004 to 0.018%.

O: 0.0030% or less O is contained as an unavoidable impurity, and exists as an oxide in the steel, which reduces cleanliness. For this reason, it is preferable to reduce as much as possible in the present invention. If the O content exceeds 0.0030%, CaO inclusions are coarsened, which adversely affects toughness. Further, in the present invention, in order to crystallize Ca as CaS, O having a strong binding force with Ca strengthens degassing or introduces a deoxidizer before adding Ca, It is preferable to reduce O to 0.0030% or less.

0.3 ≦ ACR ≦ 0.8
Here, ACR = (Ca− (0.18 + 130 × Ca) × O) /1.25/S
Ca, O, and S represent the content (% by mass) of each component.
In order to achieve high toughness by making it possible to finely disperse the ferrite transformation nuclei that do not dissolve even at high temperatures during high heat input welding, and to make the structure of the heat affected zone of the weld a fine ferrite + pearlite structure. And S must be contained so as to satisfy the relationship of 0.3 ≦ ACR ≦ 0.8.

  By setting the value of ACR to 0.3 or more and 0.8 or less, MnS that acts as ferrite nuclei precipitates on CaS and finely disperses, so the structure of the heat affected zone during high heat input welding is fine. High toughness can be achieved as a simple ferrite + pearlite structure.

  If the value of ACR is less than 0.3, since CaS does not crystallize, S precipitates in the form of MnS alone. This MnS is elongated by rolling at the time of manufacturing the steel sheet to cause a decrease in the toughness of the base material, and MnS melts in the weld heat affected zone which is the main point of the present invention, so that fine dispersion is not achieved.

  On the other hand, when the value of ACR exceeds 0.8, most of S is fixed by Ca, and MnS acting as a ferrite formation nucleus does not precipitate on CaS, so that a sufficient function is not exhibited.

[Production conditions]
In the present invention, by using the above-described component composition, an appropriate amount of the solid solution B can be stably ensured, so that variations in strength can be reduced with respect to changes in sheet thickness and rolling conditions. For this reason, conventionally, when accelerated cooling is applied in order to increase the strength of the taper plate, fluctuations in the steel plate strength are inevitable as the plate thickness changes from the thick part to the thin part. Even if is applied, a high-strength taper plate with a small strength difference between the thick part and the thin part can be obtained.

  The steel slab used as the material of the taper plate of the present invention is obtained by melting a steel having the above-described composition by a normal smelting process such as a converter, an electric furnace, a vacuum melting furnace, etc. It can be produced using a conventional method such as a lump-slab rolling method, and is not particularly limited.

  In the present invention, the slab heating temperature, hot rolling conditions, and cooling conditions are defined as follows.

Slab heating temperature: 1000-1200 ° C
When the slab heating temperature is less than 1000 ° C., the additive components are not sufficiently dissolved. On the other hand, when the temperature exceeds 1200 ° C., the austenite grains become coarse, and even after rolling, the fineness does not advance and the toughness deteriorates. For this reason, slab heating temperature shall be 1000-1200 degreeC. Preferably it is set as the range of 1030-1180 degreeC.

Hot rolling conditions After the steel slab is heated, hot rolling is performed. In hot rolling, tapers with different plate thicknesses are provided in the longitudinal direction. The change in the plate thickness in the longitudinal direction of the taper plate can be achieved by biting the steel plate and then hot rolling by changing the roll opening degree in a preset pass.

In the present invention, the reduction amount for each pass is not particularly limited. The rolling finishing temperature of the hot rolling is set to 900 ° C. or less and Ar ₃ points or more at the steel sheet surface temperature. If the finishing temperature is less than ₃ points of Ar, a predetermined strength cannot be obtained, and if it exceeds 900 ° C., the toughness deteriorates, so the finishing temperature is 900 ° C. or less and Ar ₃ points or more. Preferably, it is in the range of (Ar ₃ + 10 ° C.) to 880 ° C.

Cooling conditions After the hot rolling, accelerated cooling is performed. If the cooling stop temperature exceeds 500 ° C., a steel plate strength with a tensile strength of 570 MPa or more cannot be obtained, so accelerated cooling is performed to a temperature of 500 ° C. or less at the steel plate surface temperature. Preferably it is set as the range of 490 degrees C or less.

In addition, the steel plate surface temperature which prescribes | regulates hot rolling conditions and cooling conditions can be measured using a radiation thermometer, for example.
In the present invention, an appropriate amount of solid solution B can be stably secured by the combination of the above-described component composition and manufacturing conditions, and the effect of improving the hardenability and the effect of improving the toughness of the weld heat affected zone of high heat input welding are obtained. Therefore, even if the difference between the thick part thickness and the thin part thickness (taper amount) of the taper plate is 10 mm or more in the steel plate, the tensile strength is 570 MPa or more and the toughness of the welding heat affected zone of the high heat input welding is excellent. A taper plate is obtained.

  A steel slab having the chemical composition shown in Table 1 is hot-rolled under the conditions shown in Table 2, and a taper plate having a thickness of 60 mm, a thin part of 50 mm, and a taper amount (difference between the thickness of the thick part and the thin part) of 10 mm is obtained. Manufactured.

  Rolling a 2mm V notch Charpy test piece in a direction perpendicular to the rolling direction from a round bar tensile test piece having a parallel part of 14φ x 85mm and a distance of 70mm between the thick part and thin part of the taper plate. The sample was taken in a direction parallel to the direction, and the strength of the base material and the absorbed energy at −40 ° C. were evaluated. The absorbed energy at −40 ° C. was an average value of three values.

  Further, in order to evaluate the toughness of the weld heat affected zone (hereinafter also referred to as HAZ), a test piece having a width of 80 mm, a length of 80 mm and a thickness of 15 mm was taken from these steel sheets for heating and heat cycle and heated to 1450 ° C. 2 mm V notch Charpy for a test piece provided with a welding heat cycle after cooling at 800 to 500 ° C. at 270 s (corresponding to the heat cycle of the weld heat-affected zone with heat input of 400 kJ / cm in electrogas welding on a steel plate having a thickness of 55 mm) Tests were performed to evaluate reproducible HAZ toughness.

  Table 2 shows the mechanical properties of the thick and thin portions of the taper plate and the toughness after the welding heat cycle. No. of the example of the present invention. 1-No. No. 8 satisfies YS: 460 MPa or more, TS: 570 MPa or more, absorbed energy at −40 ° C .: 300 J or more (average of three), and the strength difference between the thick part and the thin part is less than 20 MPa for TS, YS Is less than 30 MPa, and the reproduced HAZ toughness is excellent at vTrs: −40 ° C. or lower.

  On the other hand, No. 2 satisfying N-Ti / 3.42> 0.0025. 11, no. No. 14 has a large strength difference between the thick part and the thin part. In addition, those that are outside the proper components or production conditions cannot satisfy any one or more of YS: 460 MPa or more, TS: 570 MPa or more, absorbed energy: 300 J or more, and reproduced HAZ toughness vTrs: −40 ° C. or less. It is the result.

  A steel slab having the chemical composition shown in Table 3 is hot-rolled under the conditions shown in Table 4, and a taper plate having a thickness of 60 mm, a thickness of 50 mm, and a taper amount (difference between the thickness of the thickness and the thickness of the thin part) of 10 mm is obtained. Manufactured.

  Rolling a 2mm V notch Charpy test piece in a direction perpendicular to the rolling direction from a round bar tensile test piece having a parallel part of 14φ x 85mm and a distance of 70mm between the thick part and thin part of the taper plate. The sample was taken in a direction parallel to the direction, and the strength of the base material and the absorbed energy at −40 ° C. were evaluated. The absorbed energy at −40 ° C. was an average value of three values.

  Further, in order to evaluate the toughness of the weld heat affected zone (hereinafter also referred to as HAZ), a test piece having a width of 80 mm, a length of 80 mm and a thickness of 15 mm was taken from these steel sheets for heating and heat cycle and heated to 1450 ° C. 2 mm V notch Charpy for a test piece provided with a welding heat cycle after cooling at 800 to 500 ° C. at 270 s (corresponding to the heat cycle of the weld heat-affected zone with heat input of 400 kJ / cm in electrogas welding on a steel plate having a thickness of 55 mm) Tests were performed to evaluate reproducible HAZ toughness.

  Table 4 shows the mechanical properties of the thick and thin portions of the taper plate and the toughness after the welding heat cycle. No. of the present invention example satisfying the ACR regulations 21 and no. No. 22 satisfies YS: 460 MPa or more, TS: 570 MPa or more, absorbed energy at −40 ° C .: 300 J or more (average of three), and the strength difference between the thick part and the thin part is less than 20 MPa for TS, YS Is less than 30 MPa, and the reproduced HAZ toughness is excellent at vTrs: −65 ° C. or less.

Claims (4)

% By mass
C: 0.03-0.12%,
Si: 0.03 to 0.5%,
Mn: 0.8 to 2.2%
P: 0.015% or less,
S: 0.0005 to 0.0050%,
Al: 0.005 to 0.1%,
Nb: 0.003 to 0.014%,
Ti: 0.003 to 0.02%,
B: 0.0003 to 0.0025%,
N: 0.0030 to 0.0070%,
Ca: 0.0005 to 0.0050%,
And satisfies the formula (1),
After heating the steel slab composed of the remaining Fe and inevitable impurities to 1000 ° C to 1200 ° C, hot rolling in which the plate thickness changes in a taper shape in the longitudinal direction is performed at a rolling finishing temperature of 900 ° C or less at Ar ₃ points or more, Thereafter, accelerated cooling to 500 ° C. or lower, and a method for producing a tapered plate having a tensile strength of 570 MPa or more and a difference between the thickness of the thick part and the thickness of the thin part of 10 mm or more.
0 ≦ N-Ti / 3.42 ≦ 0.0025 (1)
However, N and Ti are content (mass%) of each component. The component composition of the steel slab is further mass%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
Cr: 0.05 to 0.5%,
Mo: 0.05-0.5%
V: 0.02-0.1%,
The method for producing a taper plate having a tensile strength of 570 MPa or more and a difference between the thickness of the thick part and the thickness of the thin part of 10 mm or more, comprising at least one selected from among . The component composition of the steel slab is further mass%, Mg: 0.0005 to 0.005%,
Zr: 0.003 to 0.02%,
REM: 0.003-0.02%,
A taper plate having a tensile strength of 570 MPa or more according to claim 1 or 2, wherein the difference between the thickness of the thick part and the thickness of the thin part is 10 mm or more. Production method. The component composition of the steel slab is further mass% O: 0.0030% or less,
Furthermore, each content of Ca, O, and S satisfies the following formula (2), and the tensile strength is 570 MPa or more according to any one of claims 1 to 3, A method of manufacturing a tapered plate having a difference between the thickness of the part and the thickness of the thin part of 10 mm or more.
0.3 ≦ ACR ≦ 0.8 (2)
Here, ACR = (Ca− (0.18 + 130 × Ca) × O) /1.25/S
Moreover, Ca, O, and S represent content (mass%) of each component.