JP6795083B2 - Steel plate and its manufacturing method - Google Patents

Description

The present invention relates to steel plates used for steel structures such as buildings, bridges, shipbuilding, offshore structures, construction machines, tanks, penstocks, etc., particularly thick steel plates having a thickness of 100 mm or more and methods for manufacturing the same.

When steel materials are used for structures such as buildings, bridges, shipbuilding, marine structures, construction machines, tanks, pen stocks, etc., it is desirable to join the steel materials by welding according to the shape of the structures. It is finished in the shape of. In recent years, the size of such steel structures has increased remarkably, and the strength and wall thickness of the steel materials used have been increased. For example, Non-Patent Document 1 reports an extremely thick steel plate having a thickness of 210 mm, which was developed for a rack of a jack-up rig. This non-patent document 1 describes a component composition and manufacturing conditions for ensuring the toughness of the central portion of the thick steel plate.

Kozaburo Otani, 4 others, "Development of extra-thick (210mm) 800N / mm 2nd grade steel plate for jack-up rig rack", Nippon Steel Technical Report, 1993, No. 348, p.10-16

A high-strength steel plate having a plate thickness of 100 mm or more is usually manufactured by imparting high toughness in addition to high strength by quenching and tempering after hot rolling. When a thick steel sheet is manufactured in this way, the cooling rate in the quenching process after hot rolling is lower inside the steel sheet inside the surface layer than the surface layer of the steel sheet, so that the inside of the steel sheet has a relatively low strength structure such as ferrite. Is more likely to be formed. In order to suppress the formation of such a low-strength structure inside the steel sheet, it is necessary to add a large amount of alloying elements.
Here, the surface layer of the steel sheet refers to each region on the front surface side and the back surface side, which is defined by the position of 1 / 4t (t represents the plate thickness) in the plate thickness direction from the front and back surfaces of the steel plate, respectively, and this surface layer. The inner side (including 1 / 4t) is the inside of the steel sheet.

In particular, in order to satisfy the strength and toughness inside the thick steel sheet, it is important to generate bainite or a mixed structure of bainite and martensite inside the steel sheet during quenching, and alloying elements such as Mn, Ni, Cr and Mo. Need to be added in large quantities.

On the other hand, when a large amount of alloying elements as described above is added, a martensite structure having inferior toughness is formed on the surface layer of the steel sheet, which has a faster cooling rate during quenching than the inside of the steel sheet, so that the steel sheet is formed even after being tempered. The toughness of the surface layer of the steel sheet is lower than that of the inside.

However, as mentioned in Non-Patent Document 1, the decrease in toughness of the surface layer of the steel sheet that cools quickly has not been studied so far.

The present invention has been made in view of the above circumstances, and an object of the present invention is to stably produce a high-strength steel sheet having excellent toughness not only inside the steel sheet but also on the surface layer of the steel sheet.

In order to solve the above problems, the present inventors have made a microstructure control factor for suppressing a decrease in toughness on the surface layer of a steel sheet and strength inside the steel sheet for a thick steel sheet having a yield strength of 620 MPa or more and a sheet thickness of 100 mm or more. As a result of diligent investigation, the following findings I to III were obtained.

I. In order to obtain high strength while maintaining good toughness inside the steel plate, where the cooling rate is significantly lower than that of the surface layer of the steel plate during quenching, the microstructure is martensite and / or bainite structure even when quenching at a low cooling rate. For that purpose, it is necessary to appropriately select the component composition and set the carbon equivalent to 0.57% or more.

II. When quenching a steel sheet having the component composition selected as described above, a martensite structure that is disadvantageous for ensuring toughness is formed in the surface layer of the steel sheet in which the cooling rate during quenching becomes high, and once formed even after quenching. Since the organizational unit of the martensite structure called a hardened block or packet does not change, it becomes difficult to secure stable toughness.

III. In order to suppress the formation of rewound martensite single-phase structure, which is disadvantageous to toughness, when the surface layer of the steel sheet and the inside of the steel sheet are in the temperature range of (Ar ₃ transformation point + 50) ° C to (Ar ₃ transformation point -20) ° C. It is important to form bainite in a predetermined ratio or more on the surface layer of the steel sheet by controlling the average cooling rate of the steel sheet in the range of 0.2 to 10 ° C./s.

The present invention is based on the above-mentioned novel findings, and the gist structure thereof is as follows.

1. 1. By mass%
C: 0.080% or more and 0.200% or less,
Si: 0.40% or less,
Mn: 0.50% or more and 5.00% or less,
P: 0.015% or less,
S: 0.0050% or less,
Cr: 3.00% or less,
Ni: 5.00% or less,
Al: 0.080% or less,
N: 0.0070% or less and B: 0.0030% or less are contained in a range satisfying the following formula (1), and the balance is a steel sheet having a component composition of Fe and unavoidable impurities.
A steel sheet having a bainite surface integral of 10% or more on the surface layer of the steel sheet and a yield strength of 620 MPa or more inside the steel sheet inside the surface layer.
Note [C] + [Mn] / 6 + [Ni] / 15 + [Cr] / 15 ≧ 0.57… (1)
here,
[] Is the content (mass%) of the element in [].

2. 2. The component composition further
By mass%
Cu: 0.50% or less,
Mo: 1.50% or less,
Nb: 0.100% or less,
V: 0.200% or less and
Ti: The steel sheet according to 1 above, which contains one or more selected from 0.005% or more and 0.020% or less in place of the above formula (1) within a range satisfying the following formula (2).
Note [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 15 ≧ 0.57… (2)
here,
[] Is the content (mass%) of the element in [].

3. 3. The component composition further
By mass%
Mg: 0.0005% or more and 0.0100% or less,
Ta: 0.010% or more and 0.200% or less,
Zr: 0.0050% or more and 0.1000% or less,
Y: 0.001% or more and 0.010% or less,
Ca: 0.0005% or more and 0.0050% or less and
REM: The steel sheet according to 1 or 2 above, which contains one or more selected from 0.0005% or more and 0.0200% or less.

4. By mass%
C: 0.080% or more and 0.200% or less,
Si: 0.40% or less,
Mn: 0.50% or more and 5.00% or less,
P: 0.015% or less,
S: 0.0050% or less,
Cr: 3.00% or less,
Ni: 5.00% or less,
Al: 0.080% or less,
N: 0.0070% or less and B: 0.0030% or less are contained in a range satisfying the following formula (1), and the balance is heat-rolled by hot rolling on a steel material having a component composition of Fe and unavoidable impurities. Rolled steel plate
After cooling the hot-rolled steel sheet, it is heated to a temperature range above the Ac ₃ transformation point and below 1050 ° C.
A method for manufacturing a steel sheet that is cooled to 350 ° C or lower by performing a cooling treatment in which the average cooling rate in the temperature range of (Ar ₃ transformation point +50) ° C to (Ar ₃ transformation point-20) ° C is 0.2 to 10 ° C / s.
Note [C] + [Mn] / 6 + [Ni] / 15 + [Cr] / 15 ≧ 0.57… (1)
here,
[] Is the content (mass%) of the element in [].

5. The component composition further
By mass%
Cu: 0.50% or less,
Mo: 1.50% or less,
Nb: 0.100% or less,
V: 0.200% or less and
Ti: Manufacture of the steel sheet according to 4 above, which contains one or more selected from 0.005% or more and 0.020% or less in place of the above formula (1) within a range satisfying the following formula (2). Method.
Note [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 15 ≧ 0.57… (2)
here,
[] Is the content (mass%) of the element in [].

6. The component composition further
By mass%
Mg: 0.0005% or more and 0.0100% or less,
Ta: 0.010% or more and 0.200% or less,
Zr: 0.0050% or more and 0.1000% or less,
Y: 0.001% or more and 0.010% or less,
Ca: 0.0005% or more and 0.0050% or less and
REM: The method for producing a steel sheet according to 4 or 5 above, which contains one or more selected from 0.0005% or more and 0.0200% or less.

According to the present invention, it is possible to stably produce a high-strength steel sheet having excellent toughness not only inside the steel sheet but also on the surface layer of the steel sheet.

[Ingredient composition]
Hereinafter, the manufacturing conditions of the steel sheet according to the embodiment of the present invention will be described. First, the reasons for limiting the composition of steel will be described. In the present specification, "%" representing the content of each component element means "mass%" unless otherwise specified.

C: 0.080% or more and 0.200% or less C is an element useful for obtaining the strength required for structural steel at low cost, and it is necessary to add 0.080% or more in order to obtain the effect. On the other hand, if the content exceeds 0.200%, the toughness of the base metal and the welded portion is significantly deteriorated, so the upper limit is set to 0.200%. It is preferably 0.080% or more and 0.140% or less.

Si: 0.40% or less
Si is preferably added at 0.05% or more for deoxidation, but if it is added in excess of 0.40%, the toughness of the base metal and the heat-affected zone of the weld is significantly reduced, so the Si amount should be 0.40% or less. It is preferably 0.05% or more and 0.30% or less. More preferably, it is 0.05% or more and 0.25% or less.

Mn: 0.50% or more and 5.00% or less
Mn is added from the viewpoint of ensuring the strength of the base metal, but the effect is not sufficient if the addition is less than 0.50%. On the other hand, if it is added in excess of 5.00%, not only the toughness of the base metal deteriorates but also the central segregation is promoted, so the upper limit is set to 5.00%. It is preferably 0.60% or more and 2.00% or less. More preferably, it is 0.60% or more and 1.60% or less.

P: 0.015% or less When P is contained in excess of 0.015%, the toughness of the base metal and the heat-affected zone of the weld is significantly reduced. Therefore, limit it to 0.015% or less. Preferably, it is 0.010% or less. Since it is difficult to make it less than 0.001% in industrial scale manufacturing, a content of 0.001% or more is allowed.

S: 0.0050% or less When S is contained in excess of 0.0050%, the toughness of the base metal and the heat-affected zone of the weld is significantly reduced. Therefore, S should be 0.0050% or less. Preferably, it is 0.0010% or less. Since it is difficult to make it less than 0.0001% in industrial scale manufacturing, a content of 0.0001% or more is allowed.

Cr: 3.00% or less
Cr is an element effective for increasing the strength of the base material, and is preferably added at 0.10% or more, but when added in a large amount, the weldability is lowered. Therefore, Cr should be 3.00% or less. Preferably, it is 0.10% or more and 2.00% or less.

Ni: 5.00% or less
Ni is a beneficial element that improves the strength of steel and the toughness of the heat-affected zone of welding, and is preferably added at 0.50% or more, but if it is added in excess of 5.00%, the economic efficiency is significantly reduced. Therefore, Ni should be 5.00% or less. Preferably, it is 0.50% or more and 4.00% or less.

Al: 0.080% or less
Al is added to sufficiently deoxidize the molten steel, but if it is added in excess of 0.080%, the amount of Al that dissolves in the base metal increases and the toughness of the base metal decreases. Therefore, Al should be 0.080% or less. Preferably, it is 0.030% or more and 0.080% or less. More preferably, it is 0.030% or more and 0.060% or less.

N: 0.0070% or less N has the effect of refining the structure by forming a nitride with Al or the like and improving the toughness of the base metal and the heat-affected zone of welding. Therefore, 0.0020% or more of N is preferably added. You may. However, if it is added in excess of 0.0070%, the amount of nitrides precipitated in the base metal increases, the toughness of the base metal is remarkably lowered, and coarse carbonitrides are formed even in the weld heat affected zone to lower the toughness. Therefore, N is set to 0.0070% or less. It is preferably 0.0050% or less, and more preferably 0.0040% or less. In addition, N may be 0%.

B: 0.0030% or less B is preferably added at 0.0003% or more because it has the effect of suppressing ferrite transformation from the grain boundaries by segregating at the austenite grain boundaries and enhancing hardenability. On the other hand, if it is added in excess of 0.0030%, it precipitates as a carbonitride, which lowers the hardenability and causes a decrease in toughness. Therefore, B is set to 0.0030% or less. Preferably, it is 0.0003% or more and 0.0030% or less. More preferably, it is 0.0005% or more and 0.0020% or less.

Carbon equivalent CeqIIW
In the present invention, it is necessary to design an appropriate component composition in order to secure a yield strength of 620 MPa or more and good toughness, particularly inside a steel sheet having a plate thickness of 100 mm or more, and the following equation (1) regarding carbon equivalent CeqIIW ) Satisfying the component composition. This is because when the carbon equivalent does not satisfy the following formula (1), ferrite or the like having inferior strength is likely to be formed, and it becomes difficult to stably secure the desired strength.
Note [C] + [Mn] / 6 + [Ni] / 15 + [Cr] / 15 ≧ 0.57… (1)
here,
[] Is the content of the element in [] in terms of content (mass%).

The basic components of the present invention have been described above. The rest other than the above components are Fe and unavoidable impurities, but in the present invention, other elements can be appropriately contained as needed.

Specifically, for the purpose of further increasing strength and toughness, Cu: 0.50% or less, Mo: 1.50% or less, Nb: 0.100% or less, V: 0.200% or less, and Ti: 0.005% or more and 0.020% or less. It can contain one or more selected species.
In this case, for the carbon equivalent CeqIIW, the component composition is adjusted within a range that satisfies the following formula (2) instead of the above formula (1).
[C] + [Mn] / 6+ ([Cu] + [Ni]) / 15+ ([Cr] + [Mo] + [V]) / 15 ≧ 0.57… (2)
here,
[] Is the content of the element in [] in terms of content (mass%).

Cu: 0.50% or less
Cu can improve the strength of steel without impairing toughness, but if more than 0.50% is added, the surface layer of the steel sheet will crack during hot working. Therefore, when Cu is contained, it should be 0.50% or less. Preferably, it is 0.03% or more and 0.40% or less.

Mo: 1.50% or less
Mo is an element effective for increasing the strength of the base material, but if it is added in excess of 1.50%, the hardness increases due to the precipitation of alloy carbides and the toughness decreases. Therefore, when Mo is contained, it should be 1.50% or less. Preferably, it is 0.02% or more and 0.80% or less.

Nb: 0.100% or less
Nb is effective because it is effective in improving the strength of the base metal, but addition of more than 0.100% significantly reduces the toughness of the base metal. Therefore, when Nb is contained, the upper limit is set to 0.100%. Preferably, it is 0.025% or less. If it is less than 0.003%, the effect of improving the characteristics cannot be obtained. Therefore, when it is added, it should be 0.003% or more.

V: 0.200% or less V is effective in improving the strength and toughness of the base metal, and is effective in reducing the solid solution N by precipitating as VN, but when added in excess of 0.200%, it is a hard VC. The toughness decreases due to the precipitation of. Therefore, when V is contained, it should be 0.200% or less. Preferably, it is 0.010% or more and 0.100% or less.

Ti: 0.005% or more and 0.020% or less
Ti produces TiN during heating, effectively suppresses the coarsening of austenite, and improves the toughness of the base metal and weld heat-affected zone. However, if it is added in excess of 0.020%, the Ti nitride becomes coarse and the toughness of the base metal is reduced. Therefore, when Ti is contained, it should be 0.005% or more and 0.020% or less. Preferably, it is 0.008% or more and 0.015% or less.

Further, for the purpose of further improving the material, Mg: 0.0005% or more and 0.0100% or less, Ta: 0.010% or more and 0.200% or less, Zr: 0.0050% or more and 0.1000% or less, Y: 0.001% or more and 0.010% or less, Ca: 0.0005% It can contain one or more selected from 0.0050% or more and REM: 0.0005% or more and 0.0200% or less.

Mg: 0.0005% or more and 0.0100% or less
Mg is an element that forms a stable oxide at high temperature, effectively suppresses the coarsening of old γ grains in the weld heat affected zone, and improves the toughness of the weld. However, when the addition amount is less than 0.0005%, a clear effect cannot be obtained, and when the addition amount exceeds 0.0100%, the amount of inclusions increases and the toughness decreases. Therefore, when Mg is contained, it should be 0.0005% or more and 0.0100% or less. Preferably, it is 0.0005% or more and 0.0050% or less.

Ta: 0.010% or more and 0.200% or less
Ta is effective in improving strength. However, if the addition amount is less than 0.010%, a clear effect cannot be obtained, and if it exceeds 0.200%, the toughness is lowered due to the formation of precipitates. Therefore, when Ta is contained, it should be 0.010% or more and 0.200% or less.

Zr: 0.0050% or more and 0.1000% or less
Zr is an element effective for increasing strength, but when the amount added is less than 0.0050%, no remarkable effect is obtained, and when it exceeds 0.1000%, coarse precipitates are formed and the toughness is lowered. .. Therefore, when Zr is contained, it should be 0.0050% or more and 0.1000% or less.

Y: 0.001% or more and 0.010% or less Y is effective in forming a stable oxide at high temperature, effectively suppressing the coarsening of old γ grains in the weld heat affected zone, and improving the toughness of the weld. It is an element. However, if less than 0.001% is added, no effect is obtained, and if more than 0.010% is added, the amount of inclusions increases and the toughness decreases. Therefore, when Y is contained, it should be 0.001% or more and 0.010% or less.

Ca: 0.0005% or more and 0.0050% or less
Ca is an element useful for morphological control of sulfide-based inclusions, and it is necessary to add 0.0005% or more in order to exert its effect. However, if it is added in excess of 0.0050%, the cleanliness will be deteriorated and the toughness will be deteriorated. Therefore, when Ca is contained, it should be 0.0005% or more and 0.0050% or less. It is preferably 0.0005% or more and 0.0025% or less.

REM: 0.0005% or more and 0.0200% or less
Like Ca, REM (rare earth metal) has the effect of forming oxides and sulfides in steel to improve the material, and it is necessary to add 0.0005% or more to obtain this effect. However, even if it is added in excess of 0.0200%, the effect is saturated. Therefore, when REM is contained, it should be 0.0005% or more and 0.0200% or less. It is preferably 0.0005% or more and 0.0050% or less.

[Organization]
In the present invention, it is important that the bainite surface integral in the surface layer of the steel sheet is 10% or more. When the steel sheet surface layer has such a structure, excellent toughness can be obtained for the steel sheet surface layer as well. The bainite surface integral of the surface layer of the steel sheet is preferably 20% or more. The rest is reheated martensite, ferrite, etc.

Further, it is preferable that the bainite surface integral inside the steel sheet as well as the surface layer of the steel sheet is 10% or more. By having such a structure inside the steel sheet, it is possible to obtain a steel sheet having a small difference in characteristics between the surface layer of the steel sheet and the inside of the steel sheet. The bainite surface integral inside the steel sheet is more preferably 20% or more.

To evaluate the area fraction of the structure on the surface of the steel sheet and inside the steel sheet, take a sample of the cross section of the hardened steel in the rolling direction, reveal the structure with a nital corrosive solution, and use a 200x optical microscope for 5 fields. This can be done by observing the above and obtaining the area fraction of each structure such as bainite by image analysis. For the surface layer of the steel sheet, a sample of a cross section in the rolling direction having a thickness of 15 mm is taken centering on a position of 1 / 8t in thickness. For the inside of the steel sheet, a sample of a cross section in the rolling direction having a thickness of 15 mm is taken around the position of the sheet thickness of 3 / 8t.

In order to obtain a structure in which the bainite area fraction of the surface layer of the steel sheet is at least 10% or more, the hot-rolled steel sheet obtained by hot-rolling a steel material whose composition is adjusted to the above range is cooled and then cooled. After heating to a temperature range of Ac3 transformation point or higher and 1050 ° C or lower, cooling with an average cooling rate of 0.2 to 10 ° C / s in the temperature range of (Ar ₃ transformation point + 50) ° C to (Ar ₃ transformation point -20) ° C. It needs to be treated and cooled to 350 ° C or below. It is important that the temperature conditions specified here satisfy both the surface layer of the hot-rolled steel sheet and the inside of the steel sheet. Details will be described later.

[Toughness]
The toughness of the surface layer of a steel sheet has not been paid attention so far, but due to the increasing demand for improving the safety of structures, the same level as the inside of a steel sheet is being required. In the steel sheet of the present invention, when the toughness difference between the surface layer surface of the steel sheet and the inside of the steel sheet is evaluated by the ductility-brittle fracture surface transition temperature (vTrs), the difference in vTrs is preferably within 20 ° C. This is because it can be evaluated that substantially the same toughness is obtained on the surface of the steel sheet and the inside of the steel sheet. Here, vTrs was evaluated by the method described in JIS Z 2242. The reason why the difference in vTrs is within 20 ° C is that even if the toughness is evaluated by vTrs at the same toughness level, the value may occur up to about 20 ° C due to the measurement error of the brittle fracture surface ratio. The temperature was set within 20 ° C, which is considered to be substantially equivalent.

[Yield strength]
In the present invention, the yield strength inside the steel sheet is 620 MPa or more. The reason is that a yield strength of 620 MPa or more is required to contribute to the increase in size of the structure.

Next, the method for manufacturing the steel sheet of the present invention will be described. Unless otherwise specified, the temperature in the following description means the temperature at the center of the plate thickness (1 / 2t).
[Steel material]
The molten steel having the above composition is melted by a usual method such as a converter, an electric furnace, or a vacuum melting furnace, and used as a steel material such as a slab or a billet by a usual casting method such as a continuous casting method or an ingot forming method. Further, if there are restrictions such as the load of the rolling mill, the steel material may be further forged or lump-rolled to reduce the plate thickness of the steel material.

[Hot rolling]
Hot rolling is applied to the steel material. In order to achieve both the toughness on the surface layer of the steel sheet and the strength and toughness inside the steel sheet, it is effective to promote recrystallization in the γ region during hot rolling and to reduce the old γ grain size. Therefore, in hot rolling, it is preferable that the rolling end temperature is Ar ₃ points or more.
As the Ar ₃ transformation point, a value calculated by the equation (4) described later can be used.

[Cooling after hot rolling]
The steel sheet after the hot rolling is air-cooled or accelerated cooled. In particular, accelerated cooling is effective for improving toughness. This is because accelerated cooling shortens the residence time in a high temperature region as compared with air cooling, and can suppress finer crystal grain size and coarser precipitates. Therefore, when accelerating cooling, limit the number to less than ₃ Ar points. Cooling during accelerated cooling is performed by water cooling or impulse, and in either case, it is preferable that the cooling rate is 0.1 ° C./s or higher on the surface of the steel sheet.

[Heating temperature after hot rolling: Ac ₃ transformation point or more and 1050 ° C or less]
The hot-rolled steel sheet after cooling is heated to 1050 ° C. or higher at the Ac ₃ transformation point or higher. The reason for heating above the Ac ₃ transformation point is to homogenize the steel into an austenite single phase. The reason why the reheating temperature is set to 1050 ° C. or lower is that the decrease in the toughness of the base metal due to the coarsening of the austenite grains is significantly reduced by the reheating at a high temperature exceeding 1050 ° C. Preferably, the temperature is not less than the Ac ₃ transformation point and not more than 1000 ° C. Further, it is more preferable that the Ac ₃ transformation point or more and 950 ° C. or less.

For the Ac ₃ transformation point, the value calculated by the following equation (3) is used.
Ac ₃ = 937.2-476.5 [C] +56 [Si] -19.7 [Mn] -16.3 [Cu] -26.6 [Ni] -4.9 [Cr] +38.1 [Mo] +124.8 [V] +136.3 [Ti] ] + 198.4 [Al] + 3315 [B] ... (3)
Here, each element symbol in the formula (3) indicates the content (mass%) of each component composition in the steel material, and those not contained are calculated as 0.

[Cooling process: Average cooling rate in the range of (Ar ₃ transformation point + 50) ° C to (Ar ₃ transformation point -20) ° C is 0.2 to 10 ° C / s]
After the above heating, a cooling treatment is performed. This cooling treatment cools the surface layer of the steel sheet and the inside of the steel sheet to 350 ° C or lower in the temperature range of (Ar ₃ transformation point + 50) ° C to (Ar ₃ transformation point -20) ° C in each of the surface layer of the steel sheet and the inside of the steel sheet. It is important to perform the cooling treatment so that the average cooling rate of the steel sheet is 0.2 to 10 ° C / s. By performing such cooling, a structure having a bainite surface integral of 10% or more can be formed on the surface layer of the steel sheet, and the toughness of the surface layer of the steel sheet can be remarkably improved. Similarly, inside the steel sheet, bainite can form a structure of 10% or more.

The cooling rate can be controlled by adjusting the flow rate of water, performing cooling intermittently, cooling by an impulse, or the like.
Specifically, the control of the average cooling rate on the surface layer of the steel sheet and the inside of the steel sheet is performed by deriving the cooling method, the amount of water adjustment, and the intermittent conditions by simulation or the like so as to obtain the desired cooling rate.

The temperature of the surface layer of the steel sheet and the inside of the steel sheet can be obtained by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions and the like. For example, the temperature from the surface layer of the steel sheet to the inside of the steel sheet can be obtained by calculating the temperature distribution in the plate thickness direction using the difference method.

For the Ar ₃ transformation point, the value calculated by the following equation (4) is used.
Ar ₃ = 910-310 [C] -80 [Mn] -20 [Cu] -15 [Cr] -55 [Ni] -80 [Mo] ... (4)
Here, each element symbol in the formula (4) indicates the content (mass%) of each component composition in the steel material, and those not contained are calculated as 0.

[Cooling stop temperature: 350 ° C or less]
The cooling stop temperature is 350 ° C or lower. This is because when cooled to 350 ° C. or lower, transformation is completed in the entire steel sheet and a uniform structure can be obtained.
The cooling method is generally water-cooled industrially, but the cooling method may be other than water-cooling, and there is also a method such as gas cooling, for example.

[Burning]
After quenching as described above, if necessary, rebake in a temperature range of 450 ° C. or higher and 700 ° C. or lower. This is because if the temperature is lower than 450 ° C, the effect of removing residual stress is small, while if the temperature exceeds 700 ° C, various carbides are precipitated, the structure of the base metal becomes coarse, and the strength and toughness are significantly reduced.

In addition, industrially, there are cases where the steel is repeatedly hardened for the purpose of toughening the steel, but in the present invention as well, it may be repeatedly hardened. However, at the time of final quenching, the average cooling rate of the surface layer of the steel sheet and the inside of the steel sheet in the temperature range of (Ar ₃ transformation point + 50) ° C to (Ar ₃ transformation point -20) ° C is 0.2 ° C / s or more and 10 ° C / s. It is preferable to perform the following cooling, then cool to 350 ° C. or lower, and quench at 450 ° C. or higher and 700 ° C. or lower.

Steels Nos. 1 to 31 shown in Table 1 are melted to form slabs, and then steel sheets having a thickness of 100 mm or more and 240 mm or less are obtained according to the manufacturing conditions shown in Table 2, followed by cooling treatment and tempering treatment. The thick steel plates of Sample Nos. 1 to 37 were produced and subjected to the following tests.

[Tensile test]
Φ12.5mm round bar tensile test pieces were sampled from the 1 / 8t and 1 / 4t thickness of each steel plate with a length of 50mm in the direction perpendicular to the rolling direction, and yield strength (YS) and tensile strength (TS). ) Was measured. Yield strength (YS) and tensile strength (TS) were measured in accordance with JIS Z 2241.

[Charpy impact test]
Fifteen 2 mm V-notched Charpy test pieces with the rolling direction in the longitudinal direction were collected from 2 mm below the surface layer of each steel sheet and 1 / 4t in thickness, and vTrs (ductility-brittle fracture surface transition temperature) were measured for each test piece. Evaluated in accordance with JIS Z 2242.

The above test results are shown in Table 2. From this result, the steel sheets (Sample Nos. 1 to 22) of the invention examples in which the composition and structure of the steel conform to the present invention are all 1 / 4t portion of YS of 620MPa or more, TS of 720MPa or more, the surface layer of the steel sheet and the steel sheet. The toughness (vTrs) of 1 / 4t part is lower than -30 ℃, the difference of vTrs is within 20 ℃, the strength of the base material and the difference in toughness between the surface layer of the steel sheet and the inside of the steel sheet are small, and the surface layer of the steel sheet to the steel sheet It can be seen that the toughness is excellent up to the inside in the plate thickness direction.

On the other hand, the steel sheet (Sample Nos. 23 to 32) of the comparative example that deviates from the composition or structure of the present invention has a YS of less than 620 MPa and a TS of less than 720 MPa inside the steel sheet, or the surface layer of the steel sheet and 1 / 4t portion. The toughness (vTrs) is -30 ° C or higher, or the vTrs difference is more than 20 ° C, and any of the above characteristics is inferior.

Further, as shown in Sample Nos. 33 to 37, even if the steel sheet has a composition of steel conforming to the present invention, if the manufacturing conditions do not conform to the present invention, YS, TS, toughness, and toughness difference It can be seen that one or more of the characteristics are inferior.

According to the present invention, it is possible to obtain a thick steel sheet having a yield strength of 620 MPa or more as a base material and having excellent toughness of the surface layer of the steel sheet, strength and toughness inside the steel sheet, and manufacturing stability of 100 mm or more. It greatly contributes to increasing the size of steel structures and improving the safety of steel structures.

Claims (6)

By mass%
C: 0.080% or more and 0.200% or less,
Si: 0.40% or less,
Mn: 0.50% or more and 5.00% or less,
P: 0.015% or less,
S: 0.0050% or less,
Cr: 0.10% or more and 3.00% or less,
Ni: 5.00% or less,
Al: 0.030% or more and 0.080% or less,
N: 0.0070% or less and B: 0.0003% or more and 0.0030% or less are contained in a range satisfying the following formula (1), and the balance is a steel sheet having a component composition of Fe and unavoidable impurities.
The plate thickness is 100 mm or more,
A steel sheet having a bainite surface integral of 10% or more on the surface layer of the steel sheet and a yield strength of 620 MPa or more inside the steel sheet inside the surface layer.
Note [C] + [Mn] / 6 + [Ni] / 15 + [Cr] / 15 ≧ 0.57… (1)
here,
[] Is the content (mass%) of the element in []. The component composition further
By mass%
Cu: 0.50% or less,
Mo: 1.50% or less,
Nb: 0.100% or less,
V: 0.200% or less and
Ti: The steel sheet according to claim 1, which contains one or more selected from 0.005% or more and 0.020% or less in place of the above formula (1) within a range satisfying the following formula (2).
Note [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 15 ≧ 0.57… (2)
here,
[] Is the content (mass%) of the element in []. The component composition further
By mass%
Mg: 0.0005% or more and 0.0100% or less,
Ta: 0.010% or more and 0.200% or less,
Zr: 0.0050% or more and 0.1000% or less,
Y: 0.001% or more and 0.010% or less,
Ca: 0.0005% or more and 0.0050% or less and
REM: The steel sheet according to claim 1 or 2, which contains one or more selected from 0.0005% or more and 0.0200% or less. By mass%
C: 0.080% or more and 0.200% or less,
Si: 0.40% or less,
Mn: 0.50% or more and 5.00% or less,
P: 0.015% or less,
S: 0.0050% or less,
Cr: 0.10% or more and 3.00% or less,
Ni: 5.00% or less,
Al: 0.030% or more and 0.080% or less,
N: 0.0070% or less and B: 0.0003% or more and 0.0030% or less are contained in a range satisfying the following formula (1), and the balance is hot-rolled on a steel material having a component composition of Fe and unavoidable impurities. It is applied to make a hot-rolled steel sheet with a thickness of 100 mm or more.
After cooling the hot-rolled steel sheet, it is heated to a temperature range above the Ac ₃ transformation point and below 1050 ° C.
The surface layer of the steel sheet and the inside of the steel sheet are cooled so that the average cooling rate in the temperature range of (Ar ₃ transformation point + 50) ° C to (Ar ₃ transformation point -20) ° C is 0.2 to 10 ° C / s. A method for producing a steel sheet having a bainite area fraction of 10% or more on the surface layer and having a yield strength inside the steel sheet inside the surface layer of 620 MPa or more, which is cooled to 350 ° C. or less.
Note [C] + [Mn] / 6 + [Ni] / 15 + [Cr] / 15 ≧ 0.57… (1)
here,
[] Is the content (mass%) of the element in []. The component composition further
By mass%
Cu: 0.50% or less,
Mo: 1.50% or less,
Nb: 0.100% or less,
V: 0.200% or less and
Ti: The steel sheet according to claim 4, which contains one or more selected from 0.005% or more and 0.020% or less in place of the above formula (1) within a range satisfying the following formula (2). Production method.
Note [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 15 ≧ 0.57… (2)
here,
[] Is the content (mass%) of the element in []. The component composition further
By mass%
Mg: 0.0005% or more and 0.0100% or less,
Ta: 0.010% or more and 0.200% or less,
Zr: 0.0050% or more and 0.1000% or less,
Y: 0.001% or more and 0.010% or less,
Ca: 0.0005% or more and 0.0050% or less and
REM: The method for producing a steel sheet according to claim 4 or 5, which contains one or more selected from 0.0005% or more and 0.0200% or less.