JPWO2019050010A1 - Steel sheet and manufacturing method thereof - Google Patents

Abstract

A high-strength steel sheet having excellent toughness is stably produced not only inside the steel sheet but also on the steel sheet surface layer. In mass%, C: 0.080% to 0.200%, Si: 0.40% or less, Mn: 0.50% to 5.00%, P: 0.015% or less, S: 0.0050% or less, Cr: 3.00% or less, Ni: 5.00% Hereafter, Al: 0.080% or less, N: 0.0070% or less and B: 0.0030% or less, the balance is a steel plate having a component composition of Fe and inevitable impurities, the carbon equivalent CeqIIW is 0.57% or more, The surface layer has a bainite area fraction of 10% or more and a yield strength of 620 MPa or more.

Description

  The present invention relates to a steel plate used for steel structures such as buildings, bridges, shipbuilding, marine structures, construction machinery, tanks, penstocks, etc., and more particularly to a steel plate having a thickness of 100 mm or more and a method for manufacturing the same.

  When steel materials are used for structures such as buildings, bridges, shipbuilding, offshore structures, construction machinery, tanks, penstocks, etc., the steel materials are joined by welding according to the shape of the structure. Finished in the shape of In recent years, the size of such steel structures has been remarkably increased, and the strength and thickness of steel materials used have been increased. For example, Non-Patent Document 1 reports a very 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 plate thickness center portion of the thick steel plate.

Kosaburo Otani and four others, "Development of extra-thickness (210mm) 800N / mm2 grade steel plate for racks of jack-up rigs", Nippon Steel Technical Report, 1993, No. 348, p.10-16

A high-strength steel sheet having a 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. Thus, when producing a thick steel plate, the cooling rate in the quenching process after hot rolling is lower in the steel plate inside the surface layer than the steel plate surface layer, so a relatively low strength structure such as ferrite inside the steel plate. Is easily formed. In order to suppress the formation of such a low-strength structure inside the steel plate, it is necessary to add a large amount of alloy elements.
Here, the surface layer of the steel sheet refers to each region on the front surface side and the back surface side with a position of 1/4 t (t represents the plate thickness) from the front and back surfaces of the steel sheet in the thickness direction. The inner side (including 1 / 4t) is the inside of the steel plate.

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

  On the other hand, when a large amount of the above alloying elements is added, the steel sheet surface layer, which has a faster cooling rate than the inside of the steel sheet, forms a martensite structure that is inferior in toughness, so even after tempering. Compared to the inside, the toughness of the steel sheet surface layer decreases.

  However, as described in Non-Patent Document 1, no consideration has been made so far on the toughness reduction in the steel sheet surface layer that is rapidly cooled.

  This invention is made | formed in view of said situation, Comprising: It aims at manufacturing stably the high strength steel plate excellent in toughness not only about the inside of a steel plate but also about the steel plate surface layer.

  In order to solve the above-mentioned problems, the present inventors have targeted a thick steel plate with a yield strength of 620 MPa or more and a plate thickness of 100 mm or more, with respect to a microstructure control factor for suppressing toughness in the steel sheet surface layer and strength reduction in the steel plate. As a result of earnest investigation, the following findings I to III were obtained.

  I. In order to obtain high strength while maintaining good toughness inside the steel sheet where the cooling rate is significantly lower than the steel sheet surface layer during quenching, the microstructure is martensite and / or bainite structure even when quenching at a low cooling rate. For this purpose, it is necessary to appropriately select the component composition and to make the carbon equivalent 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 securing toughness is formed in the steel sheet surface layer where the cooling rate at the time of quenching is fast, and once formed even after tempering Since the organizational unit of the martensite structure called a block or packet is not changed, it is difficult to ensure stable toughness.

III. In order to suppress the formation of a tempered martensite single-phase structure that is disadvantageous to toughness, the steel sheet surface layer and the inside of the steel sheet are in a temperature range of (Ar3 transformation point +50) ° C or higher (Ar3 transformation point -20) ° C or lower. It is important to form bainite at a predetermined ratio or more on the steel sheet surface layer by controlling the average cooling rate in the range of 0.2 to 10 ° C./s.

  The present invention is based on the above-described novel findings, and the gist of the present invention is as follows.

1. % By mass
C: 0.080% or more and 0.200% or less,
Si: 0.40% or less,
Mn: 0.50% to 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: 0.0030% or less in a range satisfying the following formula (1), the balance being a steel plate having a component composition of Fe and inevitable impurities,
A steel sheet having a structure having a bainite area fraction of 10% or more on a surface layer of the steel sheet, and a yield strength inside the steel sheet inside the surface layer of 620 MPa or more.
[C] + [Mn] / 6 + [Ni] / 15 + [Cr] /15≧0.57 (1)
here,
[] Is the content (% by mass) of the element in [].

2. The component composition further includes:
% 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 plate according to 1, wherein one or two or more selected from 0.005% or more and 0.020% or less are contained in a range satisfying the following formula (2) instead of the formula (1).
[C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 15 ≧ 0.57 (2)
here,
[] Is the content (% by mass) of the element in [].

3. The component composition further includes:
% 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 plate according to 1 or 2 above, containing one or more selected from 0.0005% to 0.0200%.

4). % By mass
C: 0.080% or more and 0.200% or less,
Si: 0.40% or less,
Mn: 0.50% to 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: 0.0030% or less in a range satisfying the following formula (1), with the balance being hot rolled on a steel material having a component composition of Fe and inevitable impurities A rolled steel sheet,
After cooling the hot-rolled steel sheet, after heating to a temperature range of Ac ₃ transformation point or more and 1050 ° C. or less,
(Ar ₃ transformation point +50) A method for producing a steel sheet that is cooled to 350 ° C. or less by performing a cooling treatment with an average cooling rate of 0.2 to 10 ° C./s in a temperature range of (Ar ₃ transformation point −20) ° C. or less. .
[C] + [Mn] / 6 + [Ni] / 15 + [Cr] /15≧0.57 (1)
here,
[] Is the content (% by mass) of the element in [].

5). The component composition further includes:
% 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 plate of said 4 containing 1 type or 2 types or more chosen from 0.005% or more and 0.020% or less in the range which satisfies the following formula (2) instead of the said formula (1) Method.
[C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 15 ≧ 0.57 (2)
here,
[] Is the content (% by mass) of the element in [].

6). The component composition further includes:
% 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 manufacturing method of the steel plate of said 4 or 5 containing 1 type, or 2 or more types chosen from 0.0005% or more and 0.0200% or less.

  ADVANTAGE OF THE INVENTION According to this invention, the high strength steel plate excellent in toughness not only about the inside of a steel plate but about the steel plate surface layer can be manufactured stably.

[Ingredient composition]
Hereinafter, the manufacturing conditions of the steel plate concerning one embodiment of the present invention are explained. First, the reasons for limiting the component composition of steel will be described. In the present specification, “%” representing the content of each component element means “% by 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 a low cost, and in order to obtain the effect, addition of 0.080% or more is necessary. On the other hand, if the content exceeds 0.200%, the toughness of the base metal and the welded portion is remarkably deteriorated, so the upper limit is made 0.200%. Preferably it is 0.080% or more and 0.140% or less.

Si: 0.40% or less
Si is preferably added in an amount of 0.05% or more for deoxidation, but if added over 0.40%, the toughness of the base metal and the weld heat-affected zone is remarkably reduced, so the Si content is 0.40% or less. Preferably it is 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% to 5.00%
Mn is added from the viewpoint of securing the strength of the base material, but if it is added less than 0.50%, the effect is not sufficient. On the other hand, if added over 5.00%, not only does the toughness of the base material deteriorate, but also promotes center segregation, so the upper limit is made 5.00%. Preferably it is 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 exceeds 0.015%, the toughness of the base metal and the weld heat-affected zone is remarkably lowered. Therefore, it is limited to 0.015% or less. Preferably, it is 0.010% or less. In addition, since it is difficult to make it less than 0.001% in industrial scale production, the content of 0.001% or more is allowed.

S: 0.0050% or less When S exceeds 0.0050%, the toughness of the base metal and the weld heat-affected zone is significantly reduced. Therefore, S is 0.0050% or less. Preferably, it is 0.0010% or less. In addition, since it is difficult to manufacture less than 0.0001% in industrial scale production, the 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 metal, and is preferably added at 0.10% or more, but if added in a large amount, the weldability is lowered. Therefore, Cr is 3.00% or less. Preferably, the content 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 the steel and the toughness of the heat affected zone, and is preferably added in an amount of 0.50% or more, but if added over 5.00%, the economic efficiency is significantly reduced. Therefore, Ni is 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 added over 0.080%, the amount of Al dissolved in the base metal increases and the base metal toughness is reduced. Therefore, Al is made 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 an effect of refining the structure by forming a nitride with Al or the like and improving the toughness of the base material and the weld heat affected zone, so 0.0020% or more of N is preferably added. May be. However, if added over 0.0070%, the amount of nitride precipitated in the base material increases, the base material toughness is remarkably reduced, and coarse carbonitrides are also formed in the weld heat affected zone to reduce the toughness. Therefore, N is made 0.0070% or less. Preferably, it is 0.0050% or less, more preferably 0.0040% or less. N may be 0%.

B: 0.0030% or less B is preferably added in an amount of 0.0003% or more because B has the effect of suppressing the ferrite transformation from the grain boundary by segregating at the austenite grain boundary and improving the hardenability. On the other hand, if added over 0.0030%, it precipitates as carbonitride, lowers the hardenability and causes a decrease in toughness. Therefore, B is 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, in order to ensure a yield strength of 620 MPa or more and good toughness particularly in a steel plate having a thickness of 100 mm or more, it is necessary to design an appropriate component composition, and the following equation (1 for carbon equivalent CeqIIW) It is necessary to adjust the component composition within a range satisfying This is because, if the carbon equivalent does not satisfy the following formula (1), ferrite having inferior strength is likely to be formed, and it becomes difficult to ensure a desired strength stably.
[C] + [Mn] / 6 + [Ni] / 15 + [Cr] /15≧0.57 (1)
here,
[] Is the content (% by mass) of the element in [].

  The basic components of the present invention have been described above. The balance other than the above components is Fe and inevitable impurities, but in the present invention, other elements can be appropriately contained as necessary.

Specifically, in order to further increase the 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 One kind or two or more kinds selected can be contained.
In this case, regarding the carbon equivalent CeqIIW, the component composition is adjusted in a range satisfying 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 (% by mass) of the element in [].

Cu: 0.50% or less
Cu can improve the strength of the steel without impairing the toughness, but if added over 0.50%, it will cause cracks in the steel sheet surface layer during hot working. Therefore, when it contains Cu, it is 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 added over 1.50%, it increases hardness and decreases toughness due to precipitation of alloy carbides. Therefore, when Mo is contained, the content is made 1.50% or less. Preferably, the content 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 material, but addition over 0.100% significantly reduces the toughness of the base material. Therefore, when Nb is contained, the upper limit is made 0.100%. Preferably, it is 0.025% or less. If the content is less than 0.003%, the effect of improving the characteristics cannot be obtained. Therefore, when added, the content 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 solid solution N by precipitating as VN, but if added over 0.200%, hard VC The toughness is reduced by the precipitation of. Therefore, when V is contained, the content is made 0.200% or less. Preferably, the content is 0.010% or more and 0.100% or less.

Ti: 0.005% to 0.020%
Ti generates TiN during heating, effectively suppresses coarsening of austenite and improves the toughness of the base material and the weld heat affected zone. However, if added over 0.020%, the Ti nitride becomes coarse and the toughness of the base material is lowered. Therefore, when Ti is contained, the content is made 0.005% or more and 0.020% or less. Preferably, the content is 0.008% or more and 0.015% or less.

  In order to further improve the material, Mg: 0.0005% to 0.0100%, Ta: 0.010% to 0.200%, Zr: 0.0050% to 0.1000%, Y: 0.001% to 0.010%, Ca: 0.0005% One or more selected from the group consisting of 0.0050% or less and REM: 0.0005% or more and 0.0200% or less can be contained.

Mg: 0.0005% to 0.0100%
Mg is an effective element for forming a stable oxide at a high temperature, effectively suppressing the coarsening of old γ grains in the weld heat affected zone, and improving the toughness of the weld zone. However, when the addition amount is less than 0.0005%, a clear effect cannot be obtained. When the addition amount exceeds 0.0100%, the amount of inclusions increases and the toughness decreases. Therefore, when Mg is contained, the content is made 0.0005% or more and 0.0100% or less. Preferably, the content is 0.0005% or more and 0.0050% or less.

Ta: 0.010% to 0.200%
Ta is effective for improving the strength. However, when the addition amount is less than 0.010%, a clear effect cannot be obtained, and when it exceeds 0.200%, the toughness decreases due to precipitate formation. Therefore, when Ta is contained, the content is made 0.010% or more and 0.200% or less.

Zr: 0.0050% or more and 0.1000% or less
Zr is an element effective in increasing the strength, but if the added amount is less than 0.0050%, a remarkable effect cannot be obtained, and if it exceeds 0.1000%, coarse precipitates are formed and the toughness decreases. . Therefore, when Zr is contained, the content is made 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 heat affected zone of the weld and improving the toughness of the weld. It is an element. However, if the addition is less than 0.001%, the effect cannot be obtained. If the addition exceeds 0.010%, the amount of inclusions increases and the toughness decreases. Therefore, when Y is contained, the content is made 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 controlling the morphology of sulfide inclusions, and 0.0005% or more must be added to exert its effect. However, if added over 0.0050%, the cleanliness is lowered and the toughness is deteriorated. Therefore, when Ca is contained, the content is made 0.0005% or more and 0.0050% or less. Preferably it is 0.0005% or more and 0.0025% or less.

REM: 0.0005% or more and 0.0200% or less
REM (rare earth metal) also has the effect of improving the quality of the material by forming oxides and sulfides in the steel, like Ca, and 0.0005% or more must be added to obtain this effect. However, even if added over 0.0200%, the effect is saturated. Therefore, when it contains REM, it is 0.0005% or more and 0.0200% or less. Preferably it is 0.0005% or more and 0.0050% or less.

[Organization]
In the present invention, it is important that the bainite area fraction 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. The bainite area fraction of the steel sheet surface layer is preferably 20% or more. The balance is tempered martensite, ferrite and the like.

  In addition to the steel sheet surface layer, the bainite area fraction in the steel sheet is preferably 10% or more. By having such a structure in the steel plate, a steel plate having a small difference in characteristics between the steel plate surface layer and the steel plate can be obtained. The bainite area fraction inside the steel plate is more preferably 20% or more.

  In addition, the evaluation of the area fraction of the steel sheet surface layer and the structure inside the steel sheet was made by taking a sample of the cross-section in the rolling direction of the as-quenched steel material, revealing the structure with a nital corrosive solution, and viewing it with a 200 × optical microscope for 5 fields of view. It can carry out by observing above and calculating | requiring the area fraction of each structure | tissue, such as a bainite, by image analysis. As for the steel sheet surface layer, a sample of a cross section in the rolling direction having a thickness of 15 mm is collected centering on the position of the plate thickness 1 / 8t. About the inside of a steel plate, the sample of a 15-mm-thickness rolling direction cross section is extract | collected centering on the position of plate thickness 3 / 8t.

In order to obtain a structure in which the bainite area fraction of the steel sheet surface layer is at least 10%, after cooling the hot-rolled steel sheet obtained by hot rolling the steel material having the component composition adjusted to the above range, After heating to a temperature range of Ac ₃ transformation point to 1050 ° C, 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. It needs to be cooled to 350 ° C or below by applying some cooling treatment. 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 steel sheet surface layer has not been noticed so far, but in response to the increasing demand for improving the safety of structures, a level equivalent to that inside the steel sheet is being demanded. In the steel sheet of the present invention, when the difference in toughness between the steel sheet surface and the inside of the steel sheet is evaluated by the ductile-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 steel sheet surface and the steel sheet interior. Here, vTrs was evaluated by the method described in JIS Z2242. The difference in vTrs should be within 20 ° C because even if the toughness evaluation by vTrs is the same toughness level, the value may occur up to about 20 ° C due to measurement error of the brittle fracture surface ratio. The temperature was 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 an increase in the size of the structure.

Next, the manufacturing method of the steel plate of this invention is demonstrated. Unless otherwise specified, the temperature in the following description means the temperature at the thickness center portion (1/2 t).
[Steel material]
Molten steel having the above component composition is melted by an ordinary method such as a converter, an electric furnace, a vacuum melting furnace or the like, and is made into a steel material such as a slab or billet by an ordinary casting method such as a continuous casting method or an ingot forming method. Moreover, when there are restrictions such as the load of the rolling mill, the steel material may be further forged or split rolled to reduce the plate thickness of the steel material.

[Hot rolling]
Hot rolling is performed on the steel material. In order to achieve both the toughness in the steel sheet surface layer and the strength and toughness in the steel sheet, it is effective to promote recrystallization in the γ region and to refine the old γ grain size during hot rolling. For this reason, in hot rolling, it is preferable that the rolling end temperature is Ar ₃ point or higher.
As the Ar ₃ transformation point, a value calculated by 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 in improving toughness. This is because accelerated cooling shortens the residence time in a high temperature range as compared with air cooling, and can suppress refinement of crystal grain size and coarsening of precipitates. Therefore, when accelerating cooling, it should be less than Ar ₃ point. Cooling at the time of accelerated cooling is performed by water cooling or blast, and in either case, it is preferable to set a cooling rate of 0.1 ° C./s or more on the steel sheet surface.

[Heating temperature after hot rolling: Ac ₃ transformation point or higher and 1050 ° C or lower]
The cooled hot-rolled steel sheet is heated to an Ac ₃ transformation point or higher and 1050 ° C. or lower. 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, when reheating at a high temperature exceeding 1050 ° C., the reduction in the base material toughness due to coarsening of austenite grains is significantly reduced. Preferably, it is set to Ac ₃ transformation point or higher and 1000 ° C. or lower. Furthermore, the Ac ₃ transformation point or higher and 950 ° C. or lower is more preferable.

The Ac ₃ transformation point uses a value calculated by the following formula (3).
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%) in the steel material having the respective component composition, and those not contained are calculated as zero.

[Cooling treatment: (Ar ₃ transformation point +50) ° C to (Ar ₃ -20) ° C, the average cooling rate is 0.2 to 10 ° C / s]
A cooling process is performed after the said heating. In this cooling treatment, when the steel sheet surface layer and the inside of the steel sheet are cooled to 350 ° C. or less, the temperature range of (Ar ₃ transformation point +50) ° C. or more (Ar ₃ transformation point −20) ° C. or less in each of the steel sheet surface layer and the inside of the steel plate It is important to perform the cooling treatment so that the average cooling rate at 0.2 to 10 ° C./s. By performing such cooling, a structure having a bainite area fraction of 10% or more can be formed on the steel sheet surface layer, and the toughness of the steel sheet surface layer can be remarkably improved. Similarly, a structure in which bainite is 10% or more can be formed inside the steel plate.

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

  The steel plate surface layer and the temperature inside the steel plate 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 steel sheet surface layer to the inside of the steel sheet can be obtained by calculating the temperature distribution in the sheet thickness direction using the difference method.

The Ar ₃ transformation point uses a value calculated by the following formula (4).
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%) in the steel material having the respective component composition, and those not contained are calculated as zero.

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

[Tempering]
After the rapid cooling as described above, tempering is performed in a temperature range of 450 ° C. to 700 ° C. as necessary. When the temperature is lower than 450 ° C., the residual stress removal effect is small. On the other hand, when the temperature exceeds 700 ° C., various carbides are precipitated, the microstructure of the base material is coarsened, and the strength and toughness are greatly reduced.

In addition, industrially, there is a case where the steel is repeatedly hardened for the purpose of strengthening the steel, but it may be repeatedly hardened in the present invention. However, at the time of final quenching, 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 ° C / s to 10 ° C / It is preferable to perform cooling that is s or less, then cool to 350 ° C. or less, and temper at 450 ° C. or more and 700 ° C. or less.

  After the steel No. 1-31 shown in Table 1 was melted and made into slabs, the steel plate with a thickness of 100 mm or more and 240 mm or less was manufactured 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 manufactured and subjected to the following tests.

[Tensile test]
Φ12.5mm round bar tensile test specimens were taken at a length of 50mm in the direction perpendicular to the rolling direction from the 1 / 8t and 1 / 4t thicknesses of each steel plate, yield strength (YS), tensile strength (TS ) Was measured. Yield strength (YS) and tensile strength (TS) were measured according to JIS Z2241.

[Charpy impact test]
Take 15 mm 2mm V-notch Charpy specimens with the rolling direction as the longitudinal direction from 2mm below the steel sheet surface layer and 1 / 4t part of each steel sheet. Evaluation was performed in accordance with JIS Z 2242.

  The test results are shown in Table 2. From these results, the steel composition (sample Nos. 1 to 22) of the invention examples in which the component composition and structure of the steel are suitable for the present invention are all 1/4 t part YS is 620 MPa or more, TS is 720 MPa or more, steel sheet surface layer and 1 / 4t part toughness (vTrs) is lower than -30 ° C, vTrs difference is within 20 ° C, strength of base metal and difference in toughness between steel plate surface and steel plate is small. It can be seen that the toughness is excellent in the thickness direction up to the inside.

  On the other hand, in the steel plate of the comparative example (sample No. 23 to 32) that deviates from the component composition or structure of the present invention, YS inside the steel plate is less than 620 MPa, TS is less than 720 MPa, or the steel sheet surface layer and 1/4 t part Has a toughness (vTrs) of −30 ° C. or higher, or a vTrs difference of more than 20 ° C., and any of the above properties is inferior.

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

  According to the present invention, the yield strength of the base material is a strength of 620 MPa or more, and a steel plate surface layer toughness, strength and toughness inside the steel plate, and a thick steel plate of 100 mm or more excellent in production stability can be obtained, 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% to 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: 0.0030% or less in a range satisfying the following formula (1), the balance being a steel plate having a component composition of Fe and inevitable impurities,
A steel sheet having a structure having a bainite area fraction of 10% or more on a surface layer of the steel sheet, and a yield strength inside the steel sheet inside the surface layer of 620 MPa or more.
[C] + [Mn] / 6 + [Ni] / 15 + [Cr] /15≧0.57 (1)
here,
[] Is the content (% by mass) of the element in []. The component composition further includes:
% 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 plate of Claim 1 which contains 1 type, or 2 or more types chosen from 0.005% or more and 0.020% or less in the range which replaces the said Formula (1) and satisfies following formula (2).
[C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 15 ≧ 0.57 (2)
here,
[] Is the content (% by mass) of the element in []. The component composition further includes:
% 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 plate of Claim 1 or 2 containing 1 type, or 2 or more types chosen 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% to 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: 0.0030% or less in a range satisfying the following formula (1), with the balance being hot rolled on a steel material having a component composition of Fe and inevitable impurities A rolled steel sheet,
After cooling the hot-rolled steel sheet, after heating to a temperature range of Ac ₃ transformation point or more and 1050 ° C. or less,
(Ar ₃ transformation point +50) A method for producing a steel sheet that is cooled to 350 ° C. or less by performing a cooling treatment with an average cooling rate of 0.2 to 10 ° C./s in a temperature range of (Ar ₃ transformation point −20) ° C. or less. .
[C] + [Mn] / 6 + [Ni] / 15 + [Cr] /15≧0.57 (1)
here,
[] Is the content (% by mass) of the element in []. The component composition further includes:
% 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 plate of Claim 4 which contains 1 type or 2 types or more chosen from 0.005% or more and 0.020% or less in the range which replaces the said Formula (1) and satisfies following formula (2). Production method.
[C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 15 ≧ 0.57 (2)
here,
[] Is the content (% by mass) of the element in []. The component composition further includes:
% 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 manufacturing method of the steel plate of Claim 4 or 5 containing 1 type, or 2 or more types chosen from 0.0005% or more and 0.0200% or less.