JP6835054B2 - High-strength steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a high-strength steel plate, and more particularly to a high-strength steel plate having excellent mechanical properties at a plate thickness of 1/2 position, excellent joint toughness and joint workability over the entire plate thickness, and excellent weldability. Therefore, the high-strength steel plate of the present invention can be suitably used for steel structures such as marine structures, pressure vessels, penstocks, bridges, and tunnels. The present invention also relates to a method for manufacturing the high-strength steel sheet.

In recent years, there has been a demand for thicker and higher strength steel materials used for reasons such as larger steel structures. Above all, a steel sheet having high strength and excellent low temperature toughness is required, and various techniques have been proposed in order to provide a high-strength steel sheet satisfying such a requirement.

For example, Patent Document 1 proposes a method for producing a high-strength steel plate in which a preliminary heat treatment is performed under specific conditions when the slab is reheated prior to hot rolling. According to Patent Document 1, by performing the preliminary heat treatment, the expression of mixed grains is avoided, and as a result, the low temperature toughness of the steel sheet is improved.

Further, Patent Document 2 describes a method for producing a high-strength steel plate in which a slab having a specific composition is hot-rolled and then cooled and tempered under specific conditions. According to Patent Document 2, the manufacturing method is excellent in productivity and enables production of a high-strength steel plate having excellent low-temperature toughness.

Patent Document 3 describes a method for producing a high-strength steel sheet in which a steel material having a specific composition is hot-rolled and then rapidly cooled and tempered under specific conditions. According to the manufacturing method, it is possible to obtain a high-strength steel plate having excellent toughness of the weld heat-affected zone in addition to the strength and toughness of the base metal.

Japanese Unexamined Patent Publication No. 2003-129134 Japanese Patent No. 4842402 Japanese Unexamined Patent Publication No. 2012-172243

However, in the prior art as described in Patent Documents 1 to 3, although the low temperature toughness of the base steel sheet is improved to some extent, only the toughness at the plate thickness 1/4 position is focused on. Mechanical properties up to the thick center are not guaranteed. Generally, the thicker the plate, the more likely it is that internal defects and welding defects (zaku flaws) in the steel sheet due to central segregation of the cast slab and internal micropores called porosity occur, which adversely affects the material properties. Therefore, in order to fully satisfy the recent demands for thickening and increasing strength, it is necessary to control the mechanical characteristics at the position of 1/2 of the plate thickness, which is called "internal quality".

Further, from the viewpoint of fracture safety of the steel structure, it is required to have excellent weldability and high toughness and joint workability in the entire plate thickness direction of the welded portion. Sufficient consideration was not given to weldability, toughness of the welded portion in the plate thickness direction, and joint workability.

The present invention has been made in view of the above-mentioned actual conditions, and has excellent mechanical properties at a plate thickness of 1/2 position, excellent joint toughness and joint workability over the entire plate thickness, and excellent weldability. It is an object of the present invention to provide a tension steel plate. Here, "joint workability" refers to a characteristic that cracks do not occur in the joint side bending test.

The present inventors diligently studied ways to solve the above problems and obtained the following findings.

(1) It is necessary to control impurity elements, inclusions, and microstructure in order to ensure excellent mechanical properties at the 1/2 plate thickness position.

(2) Regarding the above-mentioned impurity elements, it is necessary to reduce P and S, which are impurity elements that cause central segregation, and further fix the solid solution S as a Ca-based inclusion.

(3) Regarding the inclusions, it is necessary to control the contents of Ca and S so that the size and distribution density of the Ca inclusions are within a specific range.

(4) Regarding the microstructure, it is necessary to make the microstructure at the position of 1/2 of the plate thickness a mixed structure of bainite and martensite, and to control the particle size of the old austenite so as to be within a predetermined range.

(5) In order to satisfy the above conditions, it is necessary to suppress central segregation and reduce porosity by high-temperature-high-pressure rolling in the hot rolling process.

(6) Further, in order to ensure weldability, it is necessary to reduce the C content and keep the Pcm below a certain value.

(7) In addition, in order to ensure joint workability, in addition to (i) solid solution S and (ii) internal defects and welding defects of the steel sheet, (iii) is generated in the segregated portion at the plate thickness 1/2 position. It is necessary to reduce three of MnS, which are easy to make, at the same time.

The present invention has been made based on the above findings, and its gist structure is as follows.

1. 1. By mass%
C: 0.02 to 0.12%,
Si: 0.01-0.50%,
Mn: 0.5-2.0%,
P: 0.015% or less,
S: 0.0020% or less,
Al: 0.005 to 0.10%,
Ni: 0.01-5.00%,
Mo: 0.01-1.00%,
Ti: 0.005 to 0.030%,
B: 0.0003 to 0.0030%,
N: 0.0015 to 0.0065%, and Ca: 0.0001 to 0.0050% or less.
The balance consists of Fe and unavoidable impurities, and
The Pcm defined by the following equation (1) is 0.30 or less,
It has a component composition in which PS _{and Ca} defined by the following equation (2) are 0.0010 or less.
It has a microstructure at the plate thickness 1/2 position where the total area ratio of bainite and martensite is 95% or more and the average particle size of old austenite is 40 μm or less.
It contains Ca-based inclusions defined as inclusions containing S and having a mass fraction of Ca of 5% or more in the element excluding Fe.
The distribution density of the Ca-based inclusions at the plate thickness 1/2 position is ^{more than 0 pieces / mm 2} and less than 100 pieces / mm ² , and the average circle equivalent diameter of the Ca-based inclusions at the plate thickness 1/2 position. High-strength steel plate with a diameter of less than 2.0 μm.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ... (1)
PS _{, Ca} = S-Ca × 32/40 ... (2)
However, the element symbol in the above formulas (1) and (2) is the content of each element in the high-strength steel plate expressed in mass% units, and is set to zero when the element is not contained.

2. 2. When the component composition is mass%,
Cu: 1.50% or less,
Cr: 2.00% or less,
The high-strength steel plate according to 1 above, further containing 1 or 2 or more selected from the group consisting of Nb: 0.10% or less and V: 0.20% or less.

3. 3. When the component composition is mass%,
The high-strength steel sheet according to 1 or 2 above, further containing one or both of REM: 0.02% or less and Mg: 0.005% or less.

4. By mass%
C: 0.02 to 0.12%,
Si: 0.01-0.50%,
Mn: 0.5-2.0%,
P: 0.015% or less,
S: 0.0020% or less,
Al: 0.005 to 0.10%,
Ni: 0.01-5.00%,
Mo: 0.01-1.00%,
Ti: 0.005 to 0.030%,
B: 0.0003 to 0.0030%,
N: 0.0015 to 0.0065%, and Ca: 0.0001 to 0.0050% or less.
The balance consists of Fe and unavoidable impurities, and
The Pcm defined by the following equation (1) is 0.30 or less,
_{A steel material having a component composition in which PS and Ca} defined by the following equation (2) are 0.0010 or less is heated to a heating temperature of 900 to 1250 ° C.
The heated steel material is hot-rolled to form a hot-rolled steel sheet.
The hot-rolled steel sheet was reheated to an Ac3 transformation point to a reheating temperature of 1000 ° C.
Hold at the reheating temperature for 10 minutes or more,
The hot-rolled steel sheet after holding is accelerated and cooled at an average cooling rate of 1 to 200 ° C./s.
The accelerated cooled hot-rolled steel sheet is tempered at a tempering temperature of 450 ° C. to less than the Ac1 transformation point.
The hot rolling includes one or more rolling under the conditions that the rolling temperature is 1000 ° C. or higher, the rolling shape ratio is 1.0 or higher, and the rolling reduction is 15% or higher, and the rolling end temperature is 900 ° C. or higher. , A method for manufacturing high-strength steel sheets.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ... (1)
PS _{, Ca} = S-Ca × 32/40 ... (2)
However, the element symbol in the above formulas (1) and (2) is the content of each element in the high-strength steel plate expressed in mass% units, and is set to zero when the element is not contained.

5. When the component composition is mass%,
Cu: 1.50% or less,
Cr: 2.00% or less,
The method for producing a high-strength steel sheet according to 4 above, further containing 1 or 2 or more selected from the group consisting of Nb: 0.10% or less and V: 0.20% or less.

6. When the component composition is mass%,
The method for producing a high-strength steel sheet according to 4 or 5 above, further containing one or both of REM: 0.02% or less and Mg: 0.005% or less.

According to the present invention, it is possible to provide a high-strength steel plate which is excellent in mechanical properties at a plate thickness of 1/2 position, is excellent in joint toughness and joint workability over the entire plate thickness, and is also excellent in weldability. The high-strength steel plate of the present invention can be suitably used for steel structures such as marine structures, pressure vessels, penstocks, bridges, and tunnels.

Hereinafter, embodiments of the present invention will be specifically described. The following description shows a preferred embodiment of the present invention, and the present invention is not limited thereto.

[Ingredient composition]
The high-strength steel plate of the present invention and the steel material used for producing the high-strength steel plate need to have the above-mentioned composition. Hereinafter, each component contained in the component composition will be described. Unless otherwise specified, "%" representing the content of each component means "mass%".

C: 0.02 to 0.12%
C is an element having the effect of increasing the strength of steel and ensuring the strength required as a structural steel material. In order to obtain the above effect, the C content is set to 0.02% or more. The C content is preferably 0.05% or more. On the other hand, if the C content exceeds 0.12%, the weldability is lowered. Therefore, the C content is set to 0.12% or less.

Si: 0.01-0.50%
Si is an element that functions as a deoxidizing material and has an effect of increasing the strength of the base material. In order to obtain the above effect, the Si content is set to 0.01% or more. On the other hand, when the Si content exceeds 0.50%, the formation of island-shaped martensite is promoted, and the toughness and weldability are lowered. Therefore, the Si content is set to 0.50% or less. The Si content is preferably 0.35% or less.

Mn: 0.5-2.0%
Mn is an element having the effect of increasing hardenability and increasing the strength and toughness of steel. In order to secure the tensile strength of the base metal, the Mn content needs to be 0.5% or more. The Mn content is preferably 0.8% or more. On the other hand, if the Mn content exceeds 2.0%, the toughness of the base metal is significantly deteriorated. Therefore, the Mn content is set to 2.0% or less. The Mn content is preferably 1.8% or less.

P: 0.015% or less P is an element that deteriorates the low temperature toughness of the base metal. Since P tends to segregate at the center and deteriorates the toughness of the base steel sheet at the center of the plate thickness, it is desirable to reduce it as much as possible. Therefore, the P content is set to 0.015% or less. On the other hand, the lower limit of the P content is not limited and may be 0%. However, excessively low P causes an increase in refining time and an increase in cost, so it is preferably 0.001% or more.

S: 0.0020% or less S is an element that deteriorates the low temperature toughness and internal quality of the base material, and it is desirable to reduce it as much as possible. If the S content exceeds 0.0020%, the low temperature toughness and the internal quality deteriorate, so the S content is set to 0.0020% or less. On the other hand, the lower limit of the S content is not limited and may be 0%. However, excessively low S causes an increase in refining time and an increase in cost, so it is preferably 0.0001% or more.

Al: 0.005-0.10%
Al is an element that acts as an antacid and is most commonly used in the molten steel deoxidation process for high-strength steel. In order to obtain the above effect, the Al content is 0.005% or more. The Al content is preferably 0.010% or more. On the other hand, when the Al content exceeds 0.10%, the toughness of the base metal decreases. Therefore, the Al content is set to 0.10% or less. The Al content is preferably 0.07% or less.

Ni: 0.01-5.00%
Ni is an element capable of increasing strength while maintaining high toughness. In order to obtain the above effect, the Ni content is set to 0.01% or more. The Ni content is preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, if the Ni content exceeds 5.00%, the addition effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, the Ni content is set to 5.00% or less.

Mo: 0.01-1.00%
Mo is an element that contributes to improving the strength of steel. In order to obtain the above effect, the Mo content is set to 0.01% or more. The Mo content is preferably 0.05% or more. On the other hand, if the Mo content exceeds 1.00%, the toughness deteriorates. Therefore, the Mo content is set to 1.00% or less.

Ti: 0.005 to 0.030%
Ti has a strong affinity for N and precipitates as TiN during solidification. The toughness of the weld heat-affected zone can be improved by suppressing the coarsening of austenite crystal grains in the weld heat-affected zone due to the TiN pinning effect that is stable even at high temperatures. In order to obtain the above effect, the Ti content needs to be 0.005% or more. The Ti content is preferably 0.006% or more. On the other hand, when the Ti content exceeds 0.030%, the TiN particles are coarsened, and the effect of suppressing the coarsening of the austenite particles is saturated. Therefore, the Ti content is set to 0.030% or less. The Ti content is preferably 0.025% or less.

B: 0.0003 to 0.0030%
B is an element having an action of improving the strength of steel by improving hardenability. In order to obtain the above effect, the B content is set to 0.0003% or more. On the other hand, when the B content exceeds 0.0030%, the hardenability becomes excessively high, and the toughness and weldability of the base metal decrease. Therefore, the B content is set to 0.0030% or less. The B content is preferably 0.0020% or less.

N: 0.0015 to 0.0065%
N combines with Ti to precipitate and form a carbonitride, suppresses coarsening of austenite grains, and improves the toughness of the base metal and the toughness of the joint portion. In order to obtain the above effect, the N content is set to 0.0015% or more. The N content is preferably 0.0030% or more. On the other hand, when the N content exceeds 0.0065%, the toughness of the base metal and the welded portion is remarkably lowered due to the increase in the solid solution N content. Therefore, the N content is set to 0.0065% or less. The N content is preferably 0.0060% or less.

Ca: 0.0001 to 0.0050%
Ca is an element that not only improves the toughness of the base metal, but also fixes S, which causes deterioration of the internal quality, and is effective for obtaining the desired internal quality. In order to obtain the above effect, the Ca content is set to 0.0001% or more. The Ca content is preferably 0.0010% or more. On the other hand, if the Ca content exceeds 0.0050%, Ca-based inclusions become excessive, which in turn causes deterioration of low temperature toughness and the like. Therefore, the Ca content is set to 0.0050% or less.

In one embodiment of the present invention, the high-strength steel sheet has a component composition consisting of each of the above elements, the balance Fe, and unavoidable impurities.

Further, in another embodiment of the present invention, the above-mentioned component composition further contains 1 or 2 or more selected from the group consisting of Cu, Cr, Nb, and V in the amounts described below. Can be done.

Cu: 1.50% or less Cu is an element capable of increasing the strength while maintaining high toughness, like Ni, and can be arbitrarily contained depending on the desired strength. However, if the Cu content exceeds 1.50%, hot brittleness occurs and the surface properties of the steel sheet deteriorate. Therefore, when Cu is contained, the Cu content is 1.50% or less. The Cu content is preferably 0.70% or less. On the other hand, the lower limit of the Cu content is not limited, but from the viewpoint of obtaining a higher effect of adding Cu, the Cu content is preferably 0.01% or more, and more preferably 0.10% or more. It is preferably 0.20% or more, and more preferably 0.20% or more.

Cr: 2.00% or less Cr is an element that contributes to improving the strength of steel, and can be arbitrarily contained depending on the desired strength. However, if the Cr content exceeds 2.00%, the toughness deteriorates. Therefore, when Cr is contained, the Cr content is set to 2.00% or less. On the other hand, although the lower limit of the Cr content is not limited, the Cr content is preferably 0.05% or more from the viewpoint of obtaining a higher strength improving effect by Cr.

Nb: 0.10% or less Nb, like Cr and Mo, is an element that contributes to improving the strength of steel, and can be arbitrarily contained depending on the desired strength. However, if the Nb content exceeds 0.10%, the toughness of the base metal deteriorates. Therefore, when Nb is contained, the Nb content is set to 0.10% or less. On the other hand, although the lower limit of the Nb content is not limited, the Nb content is preferably 0.005% or more from the viewpoint of obtaining a higher strength improving effect by Nb.

V: 0.20% or less V is an element that contributes to improving the strength of steel, like Cr, Mo, and Nb, and can be arbitrarily contained depending on the desired strength. However, if the V content exceeds 0.20%, the toughness deteriorates. Therefore, when V is contained, the V content is set to 0.20% or less. On the other hand, the lower limit of the V content is not limited, but the V content is preferably 0.01% or more from the viewpoint of obtaining the strength improving effect by higher V.

In another embodiment of the present invention, the component composition can optionally further contain one or both of REM (rare earth metal) and Mg.

REM: 0.02% or less REM has a toughness improving effect like Ca, and can be arbitrarily contained depending on desired properties. However, if the REM content exceeds 0.02%, the addition effect is saturated. Therefore, when the REM is contained, the REM content is set to 0.02% or less. On the other hand, the lower limit of the REM content is not limited, but the REM content is preferably 0.002% or more from the viewpoint of obtaining a toughness improving effect by a higher REM.

Mg: 0.005% or less Mg is an element having an effect of improving toughness by refining crystal grains like Ca, and can be arbitrarily contained according to desired properties. However, if the Mg content exceeds 0.005%, the addition effect is saturated. Therefore, when Mg is contained, the Mg content is set to 0.005% or less. On the other hand, the lower limit of the Mg content is not limited, but from the viewpoint of obtaining a higher toughness improving effect by Mg, the Mg content is preferably 0.001% or more.

Pcm ≤ 0.30
In the present invention, the component composition further needs to satisfy the condition of Pcm ≦ 0.30. Here, Pcm is an index of weldability and is defined by the following equation (1). If the Pcm exceeds 0.30, weldability cannot be ensured.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ... (1)
(However, the element symbol in the above formula (1) is the content of each element in the high-strength steel plate expressed in mass% units, and if the element is not contained, it is set to zero).

_{PS, Ca} ≤ 0.0010
In the present invention, the above-mentioned component composition further needs to satisfy the condition of PS _{, Ca ≤ 0.0010.} Here _{, PS and Ca} are defined by the following equation (2). In order to fix S with Ca and further obtain the effect of reducing MnS, the Ca content and the S content are adjusted so as to satisfy the above conditions. If _{PS and Ca} exceed 0.0010, the solid solution S becomes excessive and the internal quality deteriorates.
PS _{, Ca} = S-Ca × 32/40 ... (2)
(However, the element symbol in the above formula (2) is the content of each element in the high-strength steel plate expressed in mass% units, and if the element is not contained, it is set to zero).

[Micro tissue]
The high-strength steel plate of the present invention has a microstructure at a plate thickness 1/2 position where the total area ratio of bainite and martensite is 95% or more and the average particle size of the former austenite is 40 μm or less. The reasons for limiting the microstructure will be described below.

Total area ratio of bainite and martensite: 95% or more In the present invention, in order to obtain the desired base material strength and toughness, the total area ratio of bainite and martensite with respect to the entire microstructure at the plate thickness 1/2 position is used. , 95% or more. If the total area ratio is less than 95%, it becomes difficult to secure strength and toughness. The upper limit of the total area ratio is not particularly limited and may be 100%. If the total area ratio satisfies the above conditions, it is permissible for the balance of the microstructure to contain another structure such as ferrite. Here, the area ratio does not include the area of island-shaped martensite and cementite in bainite and the area of cementite in martensite. In the microstructure, as long as the total area ratio of bainite and martensite is 95% or more, the individual area ratios of bainite and martensite are not particularly limited. However, the microstructure preferably contains bainite as an essential structure.

Further, the high-strength steel plate of the present invention has a microstructure in which the total area ratio of bainite and martensite is 95% or more and the average particle size of the former austenite is 40 μm or less over the entire thickness direction. Is preferable. However, normally, if the microstructure at the plate thickness 1/2 position satisfies the above condition, the entire microstructure in the plate thickness direction satisfies the above condition. This is because, in the manufacturing process of the high-strength steel plate, the cooling rate is the slowest at the position where the plate thickness is 1/2, and therefore, it is the most difficult to harden. Here, it is assumed that the "whole in the plate thickness direction" does not include the polar surface layer (layer from the surface of the steel plate to a depth of 1 mm). Since the surface of the steel sheet is affected by decarburization, the microstructure of the polar surface may be different from the other parts. However, the influence of the microstructure of the polar surface layer on the characteristics of the steel sheet can be ignored.

Average particle size of the former austenite: 40 μm or less When the average particle size of the former austenite exceeds 40 μm, the structure of the base metal becomes coarse and the desired low temperature toughness cannot be obtained. Therefore, in the present invention, in order to secure the low temperature toughness of the base material, the average particle size of the old austenite at the plate thickness 1/2 position is set to 40 μm or less. On the other hand, the lower limit of the average particle size is not particularly limited, but if it is less than 10 μm, the manufacturing load at the time of rolling increases. Therefore, the average particle size is preferably 10 μm or more.

[Intervention]
Further, the high-strength steel plate of the present invention contains Ca-based inclusions. Here, the Ca-based inclusions are defined as inclusions containing S and having a mass fraction of Ca of 5% or more in the elements excluding Fe.

Distribution Density: 0 / mm ² greater, less than 100 pieces / mm ² S is the center easily segregated elements. Since S is fixed by Ca in the present invention, Ca-based inclusions are more distributed in the central portion of the plate thickness. If the distribution density of Ca-based inclusions at the plate thickness 1/2 position is 100 pieces / mm ² or more, the low temperature toughness deteriorates. Therefore, the distribution density of the Ca-based inclusions at the position of 1/2 of the plate thickness is set to more ^{than 0 pieces / mm 2} and less than 100 pieces / mm ^2.

Average circle equivalent diameter: less than 2.0 μm When the average circle equivalent diameter of the Ca-based inclusions at the plate thickness 1/2 position is 2.0 μm or more, the low temperature toughness deteriorates. Therefore, the average circle-equivalent diameter of the Ca-based inclusions at the plate thickness 1/2 position is set to less than 2.0 μm. On the other hand, the lower limit of the average circle equivalent diameter is not particularly limited, but is preferably 0.2 μm or more.

[Plate thickness]
The thickness of the high-strength steel sheet of the present invention is not particularly limited and may be any thickness, but it is preferably 6 mm or more and 100 mm or less.

[Mechanical characteristics]
(Yield strength)
The yield strength (YP) of the high-strength steel sheet of the present invention is not particularly limited and can be any value, but the yield strength at the position where the plate thickness is 1/2 is preferably 700 MPa or more. The yield strength can be measured by the method described in Examples.

(Tensile strength)
The tensile strength (TS) of the high-tensile steel sheet of the present invention is not particularly limited and can be any value, but the tensile strength at the position where the plate thickness is 1/2 is preferably 780 MPa or more. The tensile strength can be measured by the method described in Examples.

(Toughness)
The toughness of the high-strength steel plate of the present invention is not particularly limited and can be any value, but the Charpy absorption energy vE- ₄₀ at −40 ° C. at the plate thickness 1/2 position may be 100 J or more. preferable. The Charpy absorbed energy can be measured by the method described in Examples.

(Joint toughness)
The joint toughness of the high-strength steel plate of the present invention is not particularly limited and can be any value, but it is welded with Charpy absorption energy vE- _{40 at -40 ° C at the position of 1/2 of the plate thickness of the welded portion.} It is preferable that both _{of the Charpy absorption energy vE-40} at −40 ° C. at the position of 1/4 of the plate thickness of the portion are 100 J or more. The Charpy absorption energy of the welded portion can be measured by the method described in the examples.

(Weldability)
Although not particularly limited, it is preferable that the minimum preheating temperature in the y-shaped weld crack test defined in JIS Z 3158 is 100 ° C. or lower. The minimum preheating temperature can be measured by the method described in Examples.

[Production method]
Next, a method for manufacturing a high-strength steel sheet according to an embodiment of the present invention will be described. In the following description, unless otherwise specified, the temperature refers to the temperature at the position where the plate thickness is 1/2. The temperature at the position where the plate thickness is 1/2 can be obtained by heat transfer calculation from the surface temperature of the steel plate measured by the radiation thermometer.

The high-strength steel sheet of the present invention can be produced by sequentially applying the following treatments (1) to (6) under specific conditions to a steel material having the above-mentioned composition. Hereinafter, each step will be described.
(1) Heating (2) Hot rolling (3) Reheating (4) Holding at reheating temperature (5) Accelerated cooling (6) Tempering

(Steel material)
A steel material having the above-mentioned composition is hot-rolled to obtain a hot-rolled steel sheet. The method for producing the steel material is not particularly limited, and for example, molten steel having the above composition can be melted and cast by a conventional method. The melting can be carried out by any method such as a converter, an electric furnace, and an induction furnace. Further, the casting is preferably performed by a continuous casting method from the viewpoint of productivity, but it can also be performed by an ingot-decomposition rolling method. As the steel material, for example, a steel slab can be used.

(Heating process)
The steel material is heated to a heating temperature prior to rolling. The heating may be performed after the steel material obtained by a method such as casting is once cooled, or the obtained steel material may be directly subjected to the heating without being cooled.

In the present invention, the heating temperature is 900 ° C. or higher and 1250 ° C. or lower. If the heating temperature is less than 900 ° C., the deformation resistance of the steel material is high, so that the load on the rolling mill in hot rolling increases, and hot rolling becomes difficult. Therefore, the heating temperature is set to 900 ° C. or higher. On the other hand, when the heating temperature is higher than 1250 ° C., the oxidation of the steel becomes remarkable, the loss due to the oxidation increases, and the yield decreases. Therefore, the heating temperature is set to 1250 ° C. or lower.

(Hot rolling process)
Next, the heated steel material is hot-rolled. The hot rolling includes one or more rolling under the conditions that the rolling temperature is 1000 ° C. or higher, the rolling shape ratio is 1.0 or higher, and the rolling reduction is 15% or higher, and the rolling end temperature is 900 ° C. or higher. ..

-Internal quality improvement rolling The hot rolling step is performed once or more under the conditions that the rolling temperature is 1000 ° C. or higher, the rolling shape ratio is 1.0 or higher, and the rolling reduction ratio is 15% or higher. By performing the rolling, the inside of the steel sheet is deformed, and as a result, low temperature toughness and internal quality can be ensured. Therefore, rolling under the above conditions is also referred to as "internal quality improvement rolling". If rolling that satisfies the above conditions is not performed even once, internal defects and welding defects of the steel sheet cannot be sufficiently crushed, and as a result, desired joint workability cannot be obtained. Here, the rolled shape ratio is a value defined as the roll contact arc length / average plate thickness.

-Rolling end temperature: 900 ° C. or higher In the hot rolling process, the rolling end temperature is set to 900 ° C. or higher in order to miniaturize the steel structure to the center of the plate thickness. When the rolling end temperature is less than 900 ° C., the rolling is performed in the austenite unrecrystallized temperature range, so that a mixed grain structure having various particle sizes is likely to occur, and the toughness is lowered.

The cumulative reduction rate in the hot rolling step is not particularly limited, but is preferably 50% or more. When the cumulative reduction rate is 50% or more, the austenite grains are made finer, and the toughness of the base metal can be further improved.

The thickness of the hot-rolled steel sheet (equal to the final thickness of the high-strength steel sheet) finally obtained in the hot rolling is not particularly limited, but may be 6 mm or more and 100 mm or less as described above. preferable.

(Reheating process)
Next, the hot-rolled steel sheet obtained in the hot rolling step is reheated to the reheating temperature. By this reheating treatment, the ferrite structure of the hot-rolled steel sheet is reverse-transformed into austenite.

Reheating temperature: Ac 3 points to 1000 ° C
By heating the hot-rolled steel sheet to a reheating temperature of Ac 3 points to 1000 ° C., the structure of the hot-rolled steel sheet can be made into an austenite structure, and can be made finer and sized. If the reheating temperature is less than Ac3, the ferrite structure of the hot-rolled steel sheet does not reverse-transform to austenite and remains, so that the desired strength cannot be obtained. Further, when the reheating temperature exceeds 1000 ° C., not only the operating load increases, but also the austenite grains become coarse, and as a result, the toughness of the base metal decreases. The Ac3 points can be obtained by the following equation (3).
Ac3 (° C) = 937.2 --436.5C + 56Si --19.7Mn --16.3Cu --26.6Ni --4.9Cr + 38.1Mo + 124.8V + 136.3Ti --19.1Nb + 198.4Al + 3315B… (3)
However, the element symbol in the above formula (3) represents the content (mass%) of each element, and is set to 0 when the element is not contained.

(Holding process)
Retention time: After reaching the reheating temperature for 10 minutes or more, the reheating temperature is maintained. If the holding time in the holding is less than 10 minutes, the variation in austenite particle size becomes large. Therefore, the holding time is set to 10 minutes or more.

For the reheating and holding, any heating method can be used as long as the reheating temperature and holding time can be controlled as described above. An example of the heating method is furnace heating. The furnace heating is not particularly limited, and any heating furnace such as a general heat treatment furnace can be used.

(Accelerated cooling process)
After the above holding is completed, accelerated cooling is performed. By performing the accelerated cooling, the austenite reversely transformed in the reheating step is transformed into martensite and bainite.

Average cooling rate: 1-200 ° C / s
If the average cooling rate in the accelerated cooling step is less than 1 ° C./s, the desired hardened structure (bainite and martensite) cannot be obtained, and the strength is lowered. Therefore, the average cooling rate is set to 1 ° C./s or more. On the other hand, if the average cooling rate is higher than 200 ° C./s, it becomes difficult to control the temperature at each position in the steel sheet, and the material tends to vary in the plate width direction and the rolling direction. As a result, the material such as tensile characteristics Will vary. Therefore, the average cooling rate is set to 200 ° C./s or less.

The cooling stop temperature of the accelerated cooling is not particularly limited, but is preferably 200 ° C. or lower.

(Tempering process)
Next, the hot-rolled steel sheet after the accelerated cooling is tempered at a tempering temperature of 450 ° C. to less than the Ac1 transformation point. If the tempering temperature is less than 450 ° C., the residual stress removal due to quenching is not sufficient, and it is necessary to further perform the residual stress removal treatment. On the other hand, when the tempering temperature exceeds the Ac1 transformation point, the strength is greatly reduced. The Ac1 point can be obtained by the following equation (4).
Ac1 (℃) ＝ 750.8 ―― 26.6C + 17.6Si ―― 11.6Mn ―― 22.9Cu ―― 23Ni + 24.1Cr + 22.5Mo- 39.7V ―― 5.7Ti + 232.4Nb ―― 169.4Al ―― 894.7B… (4)
However, the element symbol in the above formula (4) represents the content (mass%) of each element, and is set to 0 when the element is not contained.

The molten steel having the composition shown in Table 1 was melted in a converter to obtain a steel slab (thickness: 250 mm or 200 mm) as a steel material by a continuous casting method. Table 1 also shows the Ac3 transformation point (° C.) obtained by the above equation (3), the Ac1 transformation point (° C.) determined by the equation (4), and _{the values of PS and Ca.}

The steel slab was hot-rolled to obtain a hot-rolled steel sheet. The hot rolling was performed under the conditions shown in Tables 2 and 3. In addition, "the number of passes of internal quality improvement rolling" in Table 2 means the number of times of rolling which satisfies all the conditions (a) to (c) below.
(A) Rolling temperature is 1000 ° C or higher (b) Rolling shape ratio is 1.0 or higher (c) Reduction rate is 15% or higher

Next, the hot-rolled steel sheet was air-cooled to 200 ° C.

Next, the steel sheet was reheated and tempered under the conditions shown in Tables 2 and 3. A heat treatment furnace was used for the reheating.

The microstructure, Ca-based inclusions, mechanical properties, joint toughness, joint workability, and weldability were evaluated for each of the obtained high-strength steel sheets. The evaluation results are shown in Table 4. The evaluation was carried out by the method described below.

(Micro tissue)
From the obtained high-strength steel plate, test pieces for microstructure observation were collected so that the plate thickness 1/2 position was the observation position. The test piece was embedded in resin so that the cross section perpendicular to the rolling direction was the observation surface, and mirror-polished. Next, after performing nital corrosion, observation was performed with a scanning electron microscope (SEM) at a magnification of 200 times, and an SEM image of the structure at a plate thickness of 1/2 was photographed in five fields. The captured SEM image was analyzed by an image analyzer to determine the total area ratio of bainite and martensite tissues.

Further, from the high-strength steel plate, test pieces for microstructure observation were collected so that the plate thickness 1/2 position was the observation position. The test piece was embedded in resin so that the cross section perpendicular to the rolling direction was the observation surface, and mirror-polished. Next, after carrying out picric acid corrosion, observation was performed with an SEM at a magnification of 200 times, and an SEM image of the structure at the position of 1/2 the plate thickness was taken. The images of the five visual fields taken were analyzed by an image analyzer, and the particle size of the old austenite was determined as the diameter corresponding to the average circle.

(Ca-based inclusions)
A test piece for tissue observation was collected so that the plate thickness 1/2 position was the observation position. The test piece was embedded in resin so that the cross section perpendicular to the rolling direction was the observation surface, and mirror-polished. After taking a secondary electron image of inclusions existing in the range of 1 mm × 1 mm with an electron probe microanalyzer (EPMA), surface analysis of Ca, S, O, Si, Al, Mn and Fe is performed under the following conditions. The average circle-equivalent diameter and distribution density of Ca-based inclusions were determined.
Acceleration voltage: 15kV,
Irradiation current: 1 × ^10-8 A,
Magnification: 1000 times

(Mechanical characteristics)
A JIS No. 4 tensile test piece was taken from the plate thickness 1/2 position (1 / 2t) of the high-strength steel plate. Using the above tensile test piece, a tensile test was carried out in accordance with the provisions of JIS Z 2241 to evaluate the yield strength (YP) and tensile strength (TS) of the high-strength steel sheet.

Further, a V-notch test piece was taken from the position of 1/2 of the thickness of the high-strength steel plate in accordance with the provisions of JIS Z 2202. Using the V-notch test piece, a Charpy impact test was carried out in accordance with JIS Z 2242 _{to determine the Charpy absorbed energy (vE-40} ) at −40 ° C., and the toughness of the base metal was evaluated.

The tensile strength (TS) was 780 MPa or more, the yield strength (YP) was 700 MPa or more, and the absorbed energy (vE- ₄₀ ) at −40 ° C. was 100 J or more.

(Joint toughness)
In order to evaluate the joint toughness, the Charpy absorbed energy of the weld was measured by the following procedure. First, two welding test pieces collected from a high-strength steel plate are subjected to X groove processing (groove angle 60 °), and then gas shield metal arc welding (GMAW) with a heat input of 25 kJ / cm is performed. A multi-layer welded joint was manufactured. Then, from each of the plate thickness 1/4 position and 1/2 position of the welding portion of the joint, taken Charpy test pieces bond portion with V notch position by the three, it absorbed energy vE at -40 ° C. _{- 40} was measured and the average value was calculated. The Charpy absorption energy of 100 J or more was regarded as acceptable.

(Joint workability)
Using the joint manufactured by the above method, a joint side bending test was carried out in accordance with JIS Z3122 to evaluate the joint workability. After the test, those without cracks were marked with ○ (pass), and those with cracks were marked with × (fail).

(Weldability)
In order to evaluate the weldability, a y-shaped weld crack test was carried out according to JIS Z 3158. The welding method was GMAW with a heat input of 25 kJ / cm, and a minimum preheating temperature of 100 ° C. or less was accepted.

As can be seen from the results shown in Table 4, all of the high-strength steel sheets satisfying the conditions of the present invention have a tensile strength of 780 MPa or more, a yield strength of 700 MPa or more, and −40 ° C. at a plate thickness 1/2 position. Absorption energy vE- ₄₀ : 100J or more, and excellent mechanical properties at the plate thickness 1/2 position. Further, the high-strength steel plate satisfying the condition of the present invention has a Charpy absorption energy vE- ₄₀ of 100 J or more at both the plate thickness 1/2 position and the plate thickness 1/4 position of the welded portion, and reaches the center of the plate thickness. It also had excellent joint toughness. In addition, no cracks occurred in the joint side bending test. The high-strength steel plate satisfying the conditions of the present invention had a minimum preheating temperature of 100 ° C. or less in the y-shaped weld crack test and was excellent in weldability. On the other hand, the steel sheet that does not satisfy the conditions of the present invention is inferior in at least one of the mechanical properties, the joint toughness, the joint workability, and the weldability at the plate thickness 1/2 position.

Claims (6)

By mass%
C: 0.02 to 0.12%,
Si: 0.01-0.50%,
Mn: 0.5-2.0%,
P: 0.015% or less,
S: 0.0020% or less,
Al: 0.005 to 0.10%,
Ni: 0.01-5.00%,
Mo: 0.01-1.00%,
Ti: 0.005 to 0.030%,
B: 0.0003 to 0.0030%,
N: 0.0015 to 0.0065%, and Ca: 0.0001 to 0.0050% or less.
The balance consists of Fe and unavoidable impurities, and
The Pcm defined by the following equation (1) is 0.30 or less,
It has a component composition in which PS _{and Ca} defined by the following equation (2) are 0.0010 or less.
It has a microstructure at the plate thickness 1/2 position where the total area ratio of bainite and martensite is 95% or more and the average particle size of old austenite is 40 μm or less.
It contains Ca-based inclusions defined as inclusions containing S and having a mass fraction of Ca of 5% or more in the element excluding Fe.
The distribution density of the Ca-based inclusions at the plate thickness 1/2 position is ^{more than 0 pieces / mm 2} and less than 100 pieces / mm ² , and the average circle equivalent diameter of the Ca-based inclusions at the plate thickness 1/2 position. High-strength steel plate with a diameter of less than 2.0 μm.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ... (1)
PS _{, Ca} = S-Ca × 32/40 ... (2)
However, the element symbol in the above formulas (1) and (2) is the content of each element in the high-strength steel plate expressed in mass% units, and is set to zero when the element is not contained. When the component composition is mass%,
Cu: 1.50% or less,
Cr: 2.00% or less,
The high-strength steel plate according to claim 1, further containing 1 or 2 or more selected from the group consisting of Nb: 0.10% or less and V: 0.20% or less. When the component composition is mass%,
The high-strength steel plate according to claim 1 or 2, further containing one or both of REM: 0.02% or less and Mg: 0.005% or less. A method for manufacturing high-strength steel sheets
By mass%
C: 0.02 to 0.12%,
Si: 0.01-0.50%,
Mn: 0.5-2.0%,
P: 0.015% or less,
S: 0.0020% or less,
Al: 0.005 to 0.10%,
Ni: 0.01-5.00%,
Mo: 0.01-1.00%,
Ti: 0.005 to 0.030%,
B: 0.0003 to 0.0030%,
N: 0.0015 to 0.0065%, and Ca: 0.0001 to 0.0050% or less.
The balance consists of Fe and unavoidable impurities, and
The Pcm defined by the following equation (1) is 0.30 or less,
_{A steel material having a component composition in which PS and Ca} defined by the following equation (2) are 0.0010 or less is heated to a heating temperature of 900 to 1250 ° C.
The heated steel material is hot-rolled to form a hot-rolled steel sheet.
The hot-rolled steel sheet was reheated to an Ac3 transformation point to a reheating temperature of 1000 ° C.
Hold at the reheating temperature for 10 minutes or more,
The hot-rolled steel sheet after holding is accelerated and cooled at an average cooling rate of 1 to 200 ° C./s.
The accelerated cooled hot-rolled steel sheet is tempered at a tempering temperature of 450 ° C. to less than the Ac1 transformation point.
The hot rolling includes one or more rolling under the conditions that the rolling temperature is 1000 ° C. or higher, the rolling shape ratio is 1.0 or higher, and the rolling reduction is 15% or higher, and the rolling end temperature is 900 ° C. or higher. ,
The high-strength steel plate
It has a microstructure at the plate thickness 1/2 position where the total area ratio of bainite and martensite is 95% or more and the average particle size of old austenite is 40 μm or less.
It contains Ca-based inclusions defined as inclusions containing S and having a mass fraction of Ca of 5% or more in the element excluding Fe.
The distribution density of the Ca-based inclusions at the plate thickness 1/2 position is ^{more than 0 pieces / mm 2} and less than 100 pieces / mm ^2.
A method for producing a high-strength steel sheet, wherein the average circle-equivalent diameter of the Ca-based inclusions at the plate thickness 1/2 position is less than 2.0 μm.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ... (1)
PS _{, Ca} = S-Ca × 32/40 ... (2)
However, the element symbol in the above formulas (1) and (2) is the content of each element in the high-strength steel plate expressed in mass% units, and is set to zero when the element is not contained. When the component composition is mass%,
Cu: 1.50% or less,
Cr: 2.00% or less,
The method for producing a high-strength steel sheet according to claim 4, further containing 1 or 2 or more selected from the group consisting of Nb: 0.10% or less and V: 0.20% or less. When the component composition is mass%,
The method for producing a high-strength steel sheet according to claim 4 or 5, further containing one or both of REM: 0.02% or less and Mg: 0.005% or less.