CN113737103A - Steel sheet and method for producing same - Google Patents

Steel sheet and method for producing same Download PDF

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Publication number
CN113737103A
CN113737103A CN202110924659.6A CN202110924659A CN113737103A CN 113737103 A CN113737103 A CN 113737103A CN 202110924659 A CN202110924659 A CN 202110924659A CN 113737103 A CN113737103 A CN 113737103A
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steel sheet
less
surface layer
toughness
steel
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木津谷茂树
一宫克行
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JFE Steel Corp
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JFE Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21D2211/00—Microstructure comprising significant phases
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    • C21D2211/00—Microstructure comprising significant phases
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  • Heat Treatment Of Steel (AREA)

Abstract

The present invention stably produces a high-strength steel sheet having excellent toughness not only in the interior of the steel sheet but also in the surface layer of the steel sheet. The steel plate has the following composition: contains, in mass%, C: 0.080-0.200%, Si: 0.40% or less, Mn: 0.50% -5.00%, P: 0.015% or less, S: 0.0050% or less, Cr: 3.00% or less, Ni: 5.00 percentThe following, Al: 0.080% or less, N: 0.0070% or less and B: less than 0.0030%, the balance being Fe and unavoidable impurities; and, carbon equivalent CeqIIW0.57% or more, a bainite surface integral ratio of a surface layer of 10% or more, and a yield strength of 620MPa or more.

Description

Steel sheet and method for producing same
The present application is a divisional application of an invention application having an application number of 201880057703.4, an international application date of 07/09 in 2018, and an invention name of "steel sheet and method for manufacturing the same".
Technical Field
The present invention relates to a steel sheet used for steel structures such as buildings, bridges, ships, marine structures, construction machines, tanks, and pressure pipes, and particularly relates to a thick steel sheet having a sheet thickness of 100mm or more and a method for producing the same.
Background
When steel materials are used for structures such as buildings, bridges, shipbuilding, marine structures, construction machines, tanks, and pressure pipes, the steel materials are welded and joined to each other in accordance with the shape of the structures, and are processed into a desired shape. In recent years, such steel structures have been significantly increased in size, and the steel materials used have been increased in strength and thickness. For example, non-patent document 1 reports an extremely thick steel sheet having a thickness of 210mm, which is developed for use in a stand of a jack-up drill. Non-patent document 1 describes a composition of components and production conditions for ensuring toughness in the thickness center of a thick steel plate.
Documents of the prior art
Non-patent document
Non-patent document 1, Haighur, Happy Sanlang, Ex 4, and "the frame of the jack-up drill is 800N/mm in thickness (210mm)2Development of grade Steel plate ", New Ri iron report, 1993, No. 348, p.10-16.
Disclosure of Invention
High-strength steel sheets having a thickness of 100mm or more are conventionally manufactured by applying high strength and high toughness by performing quenching and tempering after hot rolling. In the case of producing a thick steel sheet in this manner, the cooling rate in the quenching step after hot rolling is lower in the steel sheet interior more inward than the surface layer of the steel sheet, and therefore a relatively low-strength structure such as ferrite is likely to be formed in the steel sheet interior. In order to suppress the formation of such a low-strength structure in the steel sheet, a large amount of alloying elements must be added.
Here, the surface layer of the steel sheet means each of the front and back regions bounded by a position of 1/4t (t represents the sheet thickness) in the sheet thickness direction from the front and back surfaces of the steel sheet, and the portion located more inward (including 1/4t) than the surface layer is the inside of the steel sheet.
In particular, in order to satisfy the strength and toughness of the inside of a thick steel plate, it is important that bainite or a mixed structure of bainite and martensite is formed inside the steel plate during quenching, and a large amount of alloying elements such as Mn, Ni, Cr, and Mo must be added.
On the other hand, when a large amount of the above-mentioned alloying elements are added, the cooling rate during quenching is higher than that of the steel sheet surface layer in the steel sheet, and a martensite structure having poor toughness is formed, so that even after tempering, the toughness of the steel sheet surface layer is lowered as compared with that of the steel sheet interior.
However, as is not mentioned in non-patent document 1, no study has been made so far on the reduction in toughness of the steel sheet surface layer that is rapidly cooled as described above.
The present invention has been made in view of the above circumstances, and an object thereof is to stably produce a high-strength steel sheet having excellent toughness both in the inside and in the surface layer of the steel sheet.
In order to solve the above problems, the present inventors have intensively studied microstructure controlling factors for suppressing the reduction in toughness of the steel sheet surface layer and strength inside the steel sheet, aiming at a thick steel sheet having a yield strength of 620MPa or more and a sheet thickness of 100mm or more, and have obtained the following findings I to III.
I. In order to obtain high strength while maintaining good toughness in the steel sheet interior where the cooling rate is significantly reduced as compared to the steel sheet surface layer during quenching, it is important that the microstructure be martensite and/or bainite even in quenching where the cooling rate is low, and therefore it is necessary to appropriately select the composition and set the carbon equivalent to 0.57% or more.
When a steel sheet having a composition selected as described above is quenched, a martensite structure which is not favorable for ensuring toughness is formed in the surface layer of the steel sheet having a high cooling rate during quenching, and even after tempering, the structure unit of the martensite structure called a lump or a packet which is temporarily formed does not change, so that it is difficult to ensure stable toughness.
To suppress toughnessFormation of a tempered martensite single-phase structure which is unfavorable in properties is important when the surface layer and the inside of the steel sheet are made to be in the form of (Ar)3Transition point +50) DEG C or higher to (Ar)3The average cooling rate in a temperature range of-20) DEG C or less is controlled to be in the range of 0.2 to 10 ℃/s, and bainite is formed in a predetermined ratio or more in the surface layer of the steel sheet.
The present invention is based on the above-described novel findings, and the gist thereof is as follows.
1. A steel sheet having a composition containing C in mass% within a range satisfying the following formula (1): 0.080-0.200%, Si: 0.40% or less, Mn: 0.50% -5.00%, 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: less than 0.0030%, the balance being Fe and inevitable impurities,
the surface layer of the steel sheet has a structure in which the bainite area fraction is 10% or more, and the yield strength of the steel sheet inside the surface layer is 620MPa or more.
[C]+[Mn]/6+[Ni]/15+[Cr]/5≥0.57…(1)
Here, [ ] is the content (mass%) of the element in the [ ].
2. The steel sheet according to 1, wherein the composition further contains, in mass%, Cu selected from the group consisting of Cu in a range satisfying the following formula (2) in place of the formula (1): 0.50% or less, Mo: 1.50% or less, Nb: 0.100% or less, V: 0.200% or less and Ti: 0.005-0.020% of 1 or more than 2.
[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5≥0.57…(2)
Here, [ ] is the content (mass%) of the element in the [ ].
3. The steel sheet according to claim 1 or 2, wherein the composition further contains, in mass%, a metal selected from the group consisting of Mg: 0.0005 to 0.0100%, Ta: 0.010-0.200%, Zr: 0.0050 to 0.1000%, Y: 0.001% -0.010%, Ca: 0.0005% -0.0050% and REM: 1 or more than 2 of 0.0005 to 0.0200%.
4. A method for manufacturing a steel sheet, comprising the steps of:
a hot-rolled steel sheet is produced by hot-rolling a steel slab having a composition,
cooling the hot-rolled steel sheet to Ac3After heating in a temperature region of the transformation point to 1050 ℃,
carry out the following (Ar)3Transition point +50) DEG C or higher to (Ar)3Cooling at an average cooling rate of 0.2 to 10 ℃/s in a temperature region of-20) DEG C or lower, cooling to 350 ℃ or lower,
the steel blank material has a composition containing C in mass% within a range satisfying the following formula (1): 0.080-0.200%, Si: 0.40% or less, Mn: 0.50% -5.00%, 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: less than 0.0030%, the balance being Fe and inevitable impurities,
[C]+[Mn]/6+[Ni]/15+[Cr]/5≥0.57…(1)
here, [ ] is the content (mass%) of the element in the [ ].
5. The method for producing a steel sheet according to 4, wherein the composition further contains, in mass%, Cu selected from the group consisting of: 0.50% or less, Mo: 1.50% or less, Nb: 0.100% or less, V: 0.200% or less and Ti: 0.005-0.020% of 1 or more than 2.
[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5≥0.57…(2)
Here, [ ] is the content (mass%) of the element in the [ ].
6. The method for producing a steel sheet according to the above 4 or 5, wherein the composition further contains, in mass%, a component selected from the group consisting of Mg: 0.0005 to 0.0100%, Ta: 0.010-0.200%, Zr: 0.0050 to 0.1000%, Y: 0.001% -0.010%, Ca: 0.0005% -0.0050% and REM: 1 or more than 2 of 0.0005 to 0.0200%.
According to the present invention, a high-strength steel sheet having excellent toughness not only in the inside of the steel sheet but also in the surface layer of the steel sheet can be stably produced.
Detailed Description
[ composition of ingredients ]
Hereinafter, the production conditions of the steel sheet according to one embodiment of the present invention will be described. First, the reasons for the limitation of the composition of the steel will be explained. In the present specification, "%" indicating the content of each component element represents "% by mass" unless otherwise specified.
C：0.080％～0.200％
C is an element useful for obtaining the strength required for structural steel at low cost, and in order to obtain this effect, it is necessary to add 0.080% or more. On the other hand, if it is contained in an amount exceeding 0.200%, the toughness of the base material and the weld portion is significantly deteriorated, so the upper limit is set to 0.200%. Preferably 0.080 to 0.140%.
Si: less than 0.40%
It is preferable to add 0.05% or more of Si for deoxidation, but if it exceeds 0.40%, the toughness of the base material and the weld heat-affected zone is significantly reduced, so the Si content is 0.40% or less. Preferably 0.05% to 0.30%. More preferably 0.05% to 0.25%.
Mn：0.50％～5.00％
Mn is added from the viewpoint of ensuring the strength of the base material, but the effect is insufficient in the case of adding less than 0.50%. On the other hand, if the amount exceeds 5.00%, the toughness of the base material is not only deteriorated but also the center segregation is promoted, so the upper limit is 5.00%. Preferably 0.60% to 2.00%. More preferably 0.60% to 1.60%.
P: less than 0.015%
If P is contained in an amount of more than 0.015%, the toughness of the base material and the weld heat-affected zone is significantly reduced. Therefore, the content is limited to 0.015% or less. Preferably 0.010% or less. It is difficult to make the content of the compound less than 0.001% in industrial scale production, and the content of the compound is allowed to be 0.001% or more.
S: 0.0050% or less
If S is contained in an amount of more than 0.0050%, the toughness of the base metal and the welding heat-affected zone is significantly reduced. Therefore, S is 0.0050% or less. Preferably 0.0010% or less. It is difficult to make the content of the compound less than 0.0001% in industrial scale production, and the content of the compound is allowed to be 0.0001% or more.
Cr: 3.00% or less
Cr is an element effective for increasing the strength of the base material, and is preferably added in an amount of 0.10% or more, and when added in a large amount, the weldability is lowered. Therefore, Cr is 3.00% or less. Preferably 0.10% to 2.00%.
Ni: 5.00% or less
Ni is an element which is advantageous for improving the strength of steel and the toughness of the weld heat-affected zone, and is preferably added in an amount of 0.50% or more, but if it exceeds 5.00%, the economical efficiency is remarkably lowered. Therefore, Ni is 5.00% or less. Preferably 0.50% to 4.00%.
Al: 0.080% or less
Al is added to sufficiently deoxidize molten steel, but if it is added in excess of 0.080%, the amount of Al dissolved in the base material increases, and the toughness of the base material decreases. Therefore, Al is 0.080% or less. Preferably 0.030% to 0.080%. More preferably 0.030% to 0.060%.
N: 0.0070% or less
N has an effect of forming nitrides with Al or the like to refine the structure and improve the toughness of the base material and the welding heat-affected zone, and therefore, it is preferable to add 0.0020% or more of N. However, if the amount of the carbon nitride is more than 0.0070%, the amount of the nitride precipitated in the base material increases, the toughness of the base material is remarkably reduced, and coarse carbon nitride is formed also in the welding heat-affected zone, and the toughness is reduced. Therefore, N is 0.0070% or less. Preferably 0.0050% or less, more preferably 0.0040% or less. Note that N may be 0%.
B: less than 0.0030%
B is preferably added in an amount of 0.0003% or more because it has the effect of suppressing ferrite transformation from grain boundaries by segregating in austenite grain boundaries and improving hardenability. On the other hand, if the amount exceeds 0.0030%, carbonitrides precipitate to lower hardenability and cause a reduction in toughness. Therefore, B is 0.0030% or less. Preferably 0.0003% to 0.0030%. More preferably 0.0005% to 0.0020%.
Carbon equivalent CeqIIW
In the present invention, in order to ensure strength of 620MPa or more in yield strength and good toughness particularly in the interior of a steel sheet having a thickness of 100mm or more, it is necessary to design an appropriate composition, and it is necessary to satisfy the carbon equivalent CeqIIWThe composition of the components is adjusted in the following formula (1). This is because if carbon does not satisfy the following formula (1), ferrite or the like having poor strength is easily formed, and it is difficult to stably secure desired strength.
[C]+[Mn]/6+[Ni]/15+[Cr]/5≥0.57…(1)
Here, [ ] is the content (mass%) of the element in the [ ].
The essential components of the present invention are explained above. The balance other than the above components is Fe and inevitable impurities, but in the present invention, other elements may be appropriately contained as necessary.
Specifically, for the purpose of further improving the strength and toughness, a metal selected from Cu: 0.50% or less, Mo: 1.50% or less, Nb: 0.100% or less, V: 0.200% or less and Ti: 0.005-0.020% of 1 or more than 2.
At this time, the carbon equivalent CeqIIWIn place of the above formula (1), the composition of the components is adjusted within a range satisfying the following formula (2).
[C]+[Mn]/6+([Cu]+[Ni])/5+([Cr]+[Mo]+[V])/5≥0.57…(2)
Here, [ ] is the content of the element in the [ ] in terms of content (mass%).
Cu: less than 0.50%
Cu improves the strength of the steel without impairing toughness, but if added in an amount exceeding 0.50%, cracks occur in the surface layer of the steel sheet during hot working. Therefore, when Cu is contained, the content is 0.50% or less. Preferably 0.03 to 0.40%.
Mo: 1.50% or less
Mo is an element effective for increasing the strength of the base material, and if Mo is added in an amount exceeding 1.50%, the hardness is increased by precipitation of alloy carbides, and the toughness is lowered. Therefore, when Mo is contained, it is 1.50% or less. Preferably 0.02 to 0.80 percent.
Nb: less than 0.100%
Nb is effective for improving the strength of the base material, but addition exceeding 0.100% significantly reduces the toughness of the base material. Therefore, when Nb is contained, the upper limit is set to 0.100%. Preferably 0.025% or less. When the content is less than 0.003%, the effect of improving the characteristics is not obtained, and therefore, the content is 0.003% or more.
V: less than 0.200%
V is effective for improving the strength and toughness of the base material, and is effective for reducing the amount of solid solution N by precipitation as VN, but when V is added in an amount exceeding 0.200%, the toughness is reduced by precipitation of hard VC. Therefore, when V is contained, it is 0.200% or less. Preferably 0.010 to 0.100%.
Ti：0.005％～0.020％
Ti generates TiN when heated, effectively suppresses coarsening of austenite, and improves toughness of the base material and the welding heat affected zone. However, if the amount exceeds 0.020%, Ti nitride coarsens and the toughness of the base material decreases. Therefore, when Ti is contained, it is 0.005% to 0.020%. Preferably 0.008% to 0.015%.
In addition, for the purpose of further improving the material quality, a material selected from Mg: 0.0005 to 0.0100%, Ta: 0.010-0.200%, Zr: 0.0050 to 0.1000%, Y: 0.001% -0.010%, Ca: 0.0005% -0.0050% and REM: 1 or more than 2 of 0.0005 to 0.0200%.
Mg：0.0005％～0.0100％
Mg is an element that forms a stable oxide at high temperature, effectively suppresses coarsening of the original γ crystal grains in the weld heat affected zone, and is effective for improving the toughness of the weld zone. However, when the amount is less than 0.0005%, no significant effect is obtained, and when it exceeds 0.0100%, the amount of inclusions increases, resulting in a decrease in toughness. Therefore, when Mg is contained, it is set to 0.0005% to 0.0100%. Preferably 0.0005% to 0.0050%.
Ta：0.010％～0.200％
Ta is effective for improving strength. However, when the amount is less than 0.010%, no significant effect is obtained, and when it exceeds 0.200%, precipitates are formed and the toughness is lowered. Therefore, when Ta is contained, it is 0.010% to 0.200%.
Zr：0.0050％～0.1000％
Zr is an element effective for improving strength, but when the amount of Zr added is less than 0.0050%, no significant effect is obtained, and when it exceeds 0.1000%, coarse precipitates are formed and toughness is lowered. Therefore, when Zr is contained, it is 0.0050% to 0.1000%.
Y：0.001％～0.010％
Y is an element which forms a stable oxide at high temperature, effectively suppresses coarsening of the original γ crystal grains in the welding heat-affected zone, and is effective for improving the toughness of the welded portion. However, when the amount is less than 0.001%, no effect is obtained, and when the amount exceeds 0.010%, the amount of inclusions increases, and the toughness decreases. Therefore, when Y is contained, it is 0.001% to 0.010%.
Ca：0.0005％～0.0050％
Ca is an element useful for controlling the morphology of sulfide-based inclusions, and in order to exert its effect, it is necessary to add 0.0005% or more. However, if the amount exceeds 0.0050%, the cleanliness is reduced and the toughness is deteriorated. Therefore, when Ca is contained, it is 0.0005% to 0.0050%. Preferably 0.0005% to 0.0025%.
REM：0.0005％～0.0200％
REM (rare earth metal) has an effect of improving the quality by forming oxides and sulfides in steel, as in Ca, and in order to obtain this effect, 0.0005% or more needs to be added. However, if the amount of the compound is more than 0.0200%, the effect is saturated. Therefore, when REM is contained, it is 0.0005% to 0.0200%. Preferably 0.0005% to 0.0050%.
[ tissue ]
In the present invention, it is essential that the bainite surface integral rate of the surface layer of the steel sheet is 10% or more. By providing the steel sheet surface layer with such a structure, excellent toughness can be obtained also in the steel sheet surface layer. The bainite surface integral ratio of the steel sheet surface layer is preferably 20% or more. The balance being tempered martensite, ferrite, etc.
It is preferable that not only the surface layer of the steel sheet but also the bainite surface integral ratio in the steel sheet be 10% or more. By providing the steel sheet with such a structure also in the interior, a steel sheet with a small difference in characteristics between the surface layer and the interior can be obtained. The bainite surface integral ratio in the steel sheet is more preferably 20% or more.
The evaluation of the area fraction of the structure in the surface layer and the interior of the steel sheet can be performed as follows: a sample of a section of a steel material in a quenched state in a rolling direction is sampled, a structure is exposed by a nital etching solution, 5 fields or more are observed by an optical microscope of 200 times, and an area fraction of each structure such as bainite is obtained by image analysis. A sample was taken of a section of the steel sheet surface layer in the rolling direction with a thickness of 15mm centered on the position of 1/8 t. A sample was taken of the inside of the steel sheet, with a thickness of 3/8t as the center, in a cross section in the rolling direction of 15 mm.
In order to obtain a structure having a bainite surface integral ratio of at least 10% in at least the surface layer of the steel sheet, it is necessary to obtain a hot-rolled steel sheet by hot-rolling a steel slab having a composition adjusted to the above-mentioned range, cool the hot-rolled steel sheet, and then subject the cooled steel sheet to Ac3Heating in a temperature range of a transformation point to 1050 ℃, and then carrying out a secondary heating of (Ar)3Transition point +50) DEG C or higher to (Ar)3Cooling at an average cooling rate of 0.2 to 10 ℃/s in a temperature range of-20) DEG C or less to 350 ℃ or less. It is important that the temperature conditions specified here must be satisfied simultaneously in the surface layer and the inside of the hot-rolled steel sheet. Details will be described later.
[ toughness ]
Although the toughness of the surface layer of the steel sheet has not been paid attention to until now, the demand for improving the safety of the structure has been increasing, and the level equivalent to that of the inside of the steel sheet has been demanded. In the steel sheet of the present invention, when the difference in toughness between the surface layer of the steel sheet and the inside of the steel sheet is evaluated by the ductile-brittle fracture transition temperature (vTrs), the difference in vTrs is preferably within 20 ℃. From this, it was evaluated that substantially the same toughness was obtained on the surface of the steel sheet as in the interior of the steel sheet. Here, vTrs was evaluated according to the method described in JIS Z2242. The reason why the difference in vTrs is within 20 ℃ is that the evaluation of toughness by vTrs is within 20 ℃ which is considered to be substantially equivalent because the highest value is about 20 ℃ due to the measurement error of the brittle fracture ratio even at the same toughness level.
[ yield Strength ]
In the present invention, the yield strength of the inside of the steel sheet is set to 620MPa or more. The reason for this is that a yield strength of 620MPa or more is required to contribute to the enlargement of the structure.
Next, a method for manufacturing a steel sheet according to the present invention will be described. Unless otherwise specified, the temperature described below indicates the temperature of the plate thickness center portion (1/2 t).
[ Steel blank ]
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 is formed into a slab such as a slab or a billet by a usual casting method such as a continuous casting method or an ingot casting method. In addition, when there is a limitation in the load of the rolling mill or the like, the steel blank may be further forged or cogging-rolled to reduce the thickness of the steel blank.
[ Hot Rolling ]
The steel blank is hot rolled. In order to achieve both the toughness of the surface layer of the steel sheet and the strength and toughness of the inside of the steel sheet, it is effective to promote the recrystallization of the γ region during hot rolling and to refine the original γ crystal grain size. Therefore, in the hot rolling, it is preferable to set the rolling end temperature to Ar3The point is above.
In addition, Ar3The transition point may use a value calculated by equation (4) described later.
[ Cooling after Hot Rolling ]
The hot-rolled steel sheet is air-cooled or accelerated-cooled. Accelerated cooling is effective particularly in the case of attempting to improve toughness. This is because the residence time in the high temperature region is shortened as compared with air cooling by accelerated cooling, and the grain size reduction of the crystal grains and the coarsening of the precipitates can be suppressed. Therefore, the temperature is set to be lower than Ar in accelerated cooling3And (4) point. The cooling in the accelerated cooling is performed by water cooling or air blowing, and in any case, the cooling rate is preferably 0.1 ℃/s or more on the steel sheet surface.
[ after Hot RollingHeating temperature: ac of3The transformation point is 1050 DEG C]
The hot-rolled steel sheet after cooling is Ac3Heating at the temperature of between the transformation point and 1050 ℃. Heating to Ac3The transformation point is above to homogenize the steel into an austenite single phase. The reheating temperature is set to 1050 ℃ or lower because the reduction in toughness of the parent material due to coarsening of austenite grains becomes remarkable when the reheating is performed at a high temperature exceeding 1050 ℃. Preferably Ac3The transformation point is 1000 ℃. And, more preferably Ac3The transformation point is 950 ℃.
Note that Ac3The transition point uses a value calculated by the following equation (3).
Ac3＝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 of the formula (3) represents the content (% by mass) in the steel material of each component composition, and the element not contained is calculated as 0.
[ Cooling treatment: (from Ar)3Transition point +50) DEG C or higher to (Ar)3An average cooling rate of 0.2 to 10 ℃/s in the range of-20) DEG C or less]
The heating is followed by a cooling treatment. In this cooling treatment, it is important that the steel sheet surface layer and the steel sheet interior be cooled to 350 ℃ or lower by the respective starting point (Ar) of the steel sheet surface layer and the steel sheet interior3Transition point +50) DEG C or higher to (Ar)3The cooling treatment is performed so that the average cooling rate in a temperature region of-20) DEG C or less is 0.2 to 10 ℃/s. By performing such cooling, a structure having a bainite area fraction of 10% or more can be formed in the surface layer of the steel sheet, and the toughness of the surface layer of the steel sheet can be significantly improved. Similarly, a structure having 10% or more bainite can be formed inside the steel sheet.
The cooling rate can be controlled by adjusting the flow rate of water, intermittently cooling, cooling by air blowing, or the like.
Specifically, the average cooling rate of the surface layer and the inside of the steel sheet is controlled as follows: the cooling method, the water amount adjustment, and the intermittent condition are simulated to derive a desired cooling rate.
The temperatures of the surface layer and the inside of the steel sheet can be determined by simulation calculation or the like based on the sheet thickness, the surface temperature, the cooling conditions, and the like. For example, the temperature distribution in the thickness direction is calculated by using the difference method, and the temperature from the surface layer of the steel sheet to the inside of the steel sheet can be obtained.
In addition, Ar3The transition point uses a value calculated using the following equation (4).
Ar3＝910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo]…(4)
Here, the element symbols in the formula (4) indicate the content (% by mass) in the steel material of each component composition, and the case where the content is not included is calculated as 0.
[ cooling stop temperature: below 350 ℃)
The cooling stop temperature is 350 ℃ or lower. This is because, when the steel sheet is cooled to 350 ℃ or lower, the transformation of the whole steel sheet is completed and a uniform structure is obtained.
The cooling method is usually water cooling in industry, but the cooling method may be a method other than water cooling, for example, a method of gas cooling or the like.
[ tempering ]
After the rapid cooling as described above, tempering is performed at a temperature of 450 to 700 ℃ as required. This is because when the temperature is lower than 450 ℃, the effect of removing residual stress is small, while when the temperature is higher than 700 ℃, various carbides are precipitated, the structure of the base material is coarsened, and the strength and toughness are greatly reduced.
In the present invention, the quenching may be repeated for the purpose of strengthening and toughening the steel in an industrial scale. However, in the final quenching, it is preferable that the surface layer and the inside of the steel sheet are subjected to secondary quenching (Ar)3Transition point +50) DEG C or higher to (Ar)3Cooling at an average cooling rate of 0.2 ℃/s to 10 ℃/s in a temperature range of-20) DEG C or less, cooling to 350 ℃ or less, and tempering at 450 to 700 ℃.
Examples
Steels of steels Nos. 1 to 31 shown in Table 1 were melted to prepare slabs, and then steel sheets having a thickness of 100mm to 240mm were prepared under the production conditions shown in Table 2, and then subjected to cooling treatment and tempering treatment to prepare thick steel sheets of samples Nos. 1 to 37, which were subjected to the following tests.
Figure BDA0003208708200000121
[ tensile test ]
From the 1/8t and 1/4t steel sheets, a round bar tensile test piece of 12.5mm in diameter was sampled at a length of 50mm in a direction perpendicular to the rolling direction, and the Yield Strength (YS) and the Tensile Strength (TS) were measured. The Yield Strength (YS) and Tensile Strength (TS) were measured according to JIS Z2241.
[ Charpy impact test ]
15 pieces of 2mmV notched Charpy test pieces each having a rolling direction as a longitudinal direction were sampled from a portion 2mm below the surface layer and a sheet thickness of 1/4t of each steel sheet, and each test piece was evaluated for vTrs (ductile-brittle fracture transition temperature) according to JIS Z2242.
The test results are shown in table 2. From these results, it is found that the composition and structure of the steel sheets (sample Nos. 1 to 22) according to the invention examples of the present invention are such that YS at the 1/4t portion is 620MPa or more, TS is 720MPa or more, toughness (vTrs) at the steel sheet surface layer and 1/4t portion is less than-30 ℃ and the difference between vTrs is within 20 ℃, the strength of the base material and the difference between toughness at the steel sheet surface layer and toughness at the steel sheet interior are small, and toughness in the sheet thickness direction from the steel sheet surface layer to the steel sheet interior is excellent.
Figure BDA0003208708200000141
On the other hand, in the steel sheets (sample Nos. 23 to 32) of comparative examples other than the composition and structure of the present invention, YS in the steel sheet was less than 620MPa and TS was less than 720MPa, or toughness (vTrs) of the steel sheet surface layer and 1/4t portion was-30 ℃ or higher or difference in vTrs was more than 20 ℃, and the above characteristics were inferior.
Further, as shown in samples nos. 33 to 37, even when the steel sheet has a composition according to the present invention, one or more of the characteristics of YS, TS, toughness, and toughness are inferior when the production conditions do not comply with the present invention.
Industrial applicability of the invention
According to the present invention, a thick steel plate of 100mm or more having a base metal yield strength of 620MPa or more, excellent toughness of the steel plate surface layer, strength and toughness of the steel plate interior, and excellent production stability can be obtained, and this contributes greatly to the enlargement of the steel structure and the improvement of the safety of the steel structure.

Claims (3)

1. A steel sheet having the following composition:
contains C in a mass% within a range satisfying the following formula (1): 0.080-0.200%, Si: 0.40% or less, Mn: 0.50% -5.00%, 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: less than 0.0030%, the balance being Fe and inevitable impurities,
the steel sheet has a structure in which the bainite area fraction is 10% or more in the surface layer, the yield strength of the steel sheet inside the steel sheet is 620MPa or more,
wherein the surface layer of the steel sheet is a region on the front side and the back side of the steel sheet in the sheet thickness direction and is bounded by 1/4 of the sheet thickness, and the inside of the steel sheet is a region more inside than the surface layer and includes 1/4 of the sheet thickness, and when the difference between the ductile-brittle fracture transition temperature vTrs at a position 2mm below the surface in the surface layer of the steel sheet and a position 1/4 of the sheet thickness in the inside of the steel sheet is within 20 ℃,
[C]+[Mn]/6+[Ni]/15+[Cr]/5≥0.57…(1)
here, [ ] is the content of the element in the [ ] in mass%.
2. The steel sheet according to claim 1, wherein the composition further contains, in mass%, Cu in a range satisfying the following formula (2) in place of the formula (1): 0.50% or less, Mo: 1.50% or less, Nb: 0.100% or less, V: 0.200% or less and Ti: 0.005-0.020% of 1 or more than 2,
[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5≥0.57…(2)
here, [ ] is the content of the element in mass%.
3. The steel sheet according to claim 1 or 2, wherein the composition further contains, in mass%, a metal selected from the group consisting of Mg: 0.0005 to 0.0100%, Ta: 0.010-0.200%, Zr: 0.0050 to 0.1000%, Y: 0.001% -0.010%, Ca: 0.0005% -0.0050% and REM: 1 or more than 2 of 0.0005 to 0.0200%.
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