JP7006154B2 - Manufacturing method of thick steel plate and thick steel plate - Google Patents

Description

The present invention relates to a thick steel plate and a method for manufacturing a thick steel plate.

UO steel pipes obtained by processing thick steel plates are often used as line pipes used for transporting crude oil and natural gas. Petroleum resources such as crude oil and natural gas may contain hydrogen sulfide, in which case UO steel pipes will be used in sour environments.

In such a sour environment, hydrogen embrittlement fracture (Sulfide Stress Cracking: SSC), which is generated by hydrogen embrittlement generated by hydrogen generated on the steel surface due to a corrosion reaction in an acidic environment and entering the steel, may occur. .. Further, hydrogen is accumulated around inclusions in the steel, and internal cracking called hydrogen-induced cracking (HIC) may occur. Therefore, the thick steel sheet used for such a line pipe has properties such as strength, toughness, and weldability, as well as resistance in a sour environment containing hydrogen sulfide, that is, sour resistance, which is one of them. SSC property is required.

The SSC resistance has a correlation with the hardness of the surface layer of the thick steel sheet, and at present, it is required to limit the hardness in the vicinity of the surface layer of the thick steel sheet in order to improve the SSC resistance. For example, in the X65 grade steel pipe surface layer (range of less than about 1 mm from the surface in the thickness direction), the Vickers hardness (Hv) is required to be about 250 or less.

However, since the thick steel sheet, which is a steel pipe material, has a thickness of, for example, about 10 to 40 mm, the vicinity of the surface layer of the thick steel sheet is cooled preferentially over the vicinity of the center in the thickness direction during accelerated cooling after hot rolling. Will be done. As a result, the hardness near the surface layer of the thick steel sheet tends to be higher than that near the center.

On the other hand, in order to obtain a steel sheet having excellent sour resistance, Patent Documents 1 to 3 propose a method for manufacturing a steel sheet in which a predetermined heat treatment is performed after hot rolling and accelerated cooling.

In Patent Document 1, a steel having a predetermined chemical component is heated, hot-rolled, accelerated and cooled under predetermined conditions, and then induced to heat to a steel sheet surface temperature of 550 to 700 ° C. and a steel sheet cross-sectional average temperature of 400 to 580 ° C. A method for manufacturing a steel sheet for a high-strength sour-resistant pipe having a plate thickness of 30 mm or more, which is characterized by heating to ° C., is disclosed.

Patent Document 2 describes that steel containing a predetermined chemical component is hot-rolled under the conditions of heating temperature: 1000 to 1300 ° C. and rolling end temperature: Ar3 temperature or higher, and then cooled at a cooling rate of 5 ° C./sec or higher to 400. For line pipes with excellent HIC resistance, characterized by accelerated cooling to ~ 600 ° C. and reheating to a temperature of 600 to 700 ° C. at a heating rate of 0.5 ° C./sec or higher immediately after cooling. A method for manufacturing a high-strength steel plate is disclosed.

In Patent Document 3, a steel having a predetermined chemical component is reheated, hot-rolled, and accelerated and cooled under predetermined conditions, and immediately reheated at a surface temperature of 525 ° C. or higher and a plate thickness center temperature of 400 to 500 ° C. Disclosed is a method for manufacturing a thick-walled high-strength sour-resistant pipe, which is bent into a pipe shape by cold working and welded at the butt portions at both ends to form a welded steel pipe.

Japanese Unexamined Patent Publication No. 2009-52137 Japanese Unexamined Patent Publication No. 2008-101242 International Publication No. 2013/190750

However, the methods described in Patent Documents 1 to 3 cannot sufficiently reduce the hardness of the surface of the steel sheet, and it is difficult to sufficiently improve the SSC resistance. On the other hand, when the steel sheet is heated by the method described in Patent Documents 1 to 3, the strength of the steel sheet may decrease due to the transformation of the structure inside the steel sheet depending on the heating conditions.

Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to have excellent sour resistance, particularly SSC resistance, while maintaining excellent strength characteristics. It is an object of the present invention to provide a thick steel plate and a method for manufacturing a thick steel plate.

The present inventor has made diligent studies to improve the SSC resistance of the thick steel sheet, and found that the thick steel sheet is heated to a higher temperature range than conventionally known, that is, a temperature range exceeding the Ac1 transformation point. It has been found that the hardness of the surface layer of the steel sheet can be sufficiently lowered and the SSC resistance can be improved. On the other hand, when heating a thick steel sheet, we focused on the possibility that the effect of heating on the strength reduction of the thick steel sheet can be suppressed by selectively heating the vicinity of the surface layer. Then, focusing on the surface temperature of the thick steel sheet, the selective heating conditions for the surface of the thick steel sheet were examined, and the present invention was conceived.

Furthermore, the present inventors can improve sour resistance, particularly SSC resistance, while maintaining excellent strength characteristics by including a relatively large amount of a specific ferrite structure in the vicinity of the surface layer of the thick steel sheet. I found.
The gist of the present invention completed based on the above findings is as follows.

(1) The first step of hot rolling the steel to obtain a rolled material, and
A second step of accelerating and cooling the rolled material from a temperature above the Ar3 transformation point, and
Regarding the surface of the rolled material, the first temperature range of (Ac1 transformation point −50 ° C.) or more and below the Ac1 transformation point is the second temperature range exceeding the Ac1 transformation point at an average temperature rise rate of 2.0 ° C./sec or more. The third step of heating to the above and holding for 1 second or more and 600 seconds or less in the second temperature range, and
A method for manufacturing a thick steel plate having.
(2) The method for manufacturing a thick steel sheet according to (1), wherein the second temperature range is equal to or lower than the Ac3 transformation point.
(3) The method for producing a thick steel sheet according to (1) or (2), wherein the heating in the third step is performed by induction heating.
(4) The method for manufacturing a thick steel sheet according to (3), wherein the applied frequency in the induction heating is 2 kHz or more and 200 kHz or less.
(5) The thick steel sheet according to any one of (1) to (4), further comprising a fourth step of heating the rolled material to a temperature lower than the Ac1 transformation point after the third step. Production method.
(6) The thick steel sheet according to any one of (1) to (5), wherein in the second step, the rolled material is cooled at an average cooling rate of 5 ° C./sec or more and 60 ° C./sec or less. Manufacturing method.
(7) Central region ferrite surface integral in the central region of -2.5 mm to 2.5 mm in the thickness direction from the center of the plate thickness with respect to the surface integral surface integral in the surface integral region of 0.1 mm or more and 1.0 mm or less from the surface. The ratio (central region ferrite surface integral / surface integral surface integral) is 0.10 or less.
When observing the cross section in the rolling direction and the plate thickness direction, the average value of the ratio of the length in the rolling direction (the length in the rolling direction / the length in the plate thickness direction) to the length in the plate thickness direction of the ferrite structure in the surface layer region is , 5.0 or less, and the Vickers hardness in the surface layer region is 250 Hv or less.

As described above, according to the present invention, a thick steel sheet can be obtained as having excellent sour resistance, particularly SSC resistance, while maintaining excellent strength characteristics.

Hereinafter, preferred embodiments of the present invention will be described in detail.
Unless otherwise specified in the present specification, the "surface layer" is a depth (for example, 1.0 mm depth) in a predetermined plate thickness direction from the front surface (or back surface) of a steel material such as a thick steel plate or a rolled material. ) Up to. Unless otherwise specified, the "internal region" is a portion of a steel material such as a thick steel plate or a rolled material that is deeper in the plate thickness direction than the surface layer, for example, 1/4 t to 3/4 t in the plate thickness direction of the steel material (1 / 4t to 3/4t). t indicates the plate thickness).

[1. Manufacturing method of thick steel plate]
The method for producing a thick steel sheet of the present invention includes a first step of hot rolling a steel (steel ingot or a piece of steel) to obtain a rolled material, and a second step of accelerating and cooling the rolled material. Regarding the surface of the rolled material, the first temperature range of (Ac1 transformation point −50 ° C.) or higher and lower than the Ac1 transformation point is the second temperature range exceeding the Ac1 transformation point at an average temperature rise rate of 2.0 ° C./sec or higher. It has a third step of heating to and holding in a second temperature range for 1 second or more and 600 seconds or less.

Hereinafter, steel as a rolling material in the method for manufacturing a thick steel sheet of the present embodiment will be described first, and then each process will be described in detail.

<Steel>
The steel that can be used in this embodiment is, for example, a steel piece processed into a required shape and size from an ingot (steel ingot) cast in a mold or a slab obtained by continuous casting, that is, a known manufacturing method. Steel pieces manufactured by can be used. Further, the shape and dimensions of the steel pieces may be appropriately set within a range that can be applied to each process described later, and are not particularly limited.

Further, the chemical composition of the steel is not particularly limited, and can be appropriately selected depending on, for example, the application required for the obtained thick steel sheet. Steel is, for example, by mass%
C: 0.02 to 0.09%,
Si: 0.01-0.60%,
Mn: 0.1-2.0%,
sol. Al: 0.005 to 0.09%,
Containing, in addition
Cu: 0.90% or less,
Ni: 0.90% or less,
Cr: 0.90% or less,
Mo: 0.90% or less,
V: 0.09% or less,
Nb: 0.09% or less,
Ti: 0.09% or less,
B: 0.004% or less,
Ca: 0.01% or less,
REM: 0.1% or less,
It contains one or more selected from the group consisting of Zr: 0.1% or less and Mg: 0.01% or less, and the balance can consist of Fe and impurities. Such a chemical composition is advantageous for obtaining sufficient strength, toughness, and sour resistance in the obtained thick steel sheet.

Hereinafter, each element will be described. In addition, also in the following description, the description of "%" regarding the content of the component means "mass%".

(C: 0.02 to 0.09%)
C is a component that contributes to improving the strength of steel. In order to sufficiently obtain this effect, the content of C is preferably 0.02% or more, more preferably 0.04% or more. On the other hand, from the viewpoint of preventing deterioration of toughness and sour resistance, the content of C is preferably 0.09% or less, more preferably 0.07% or less, still more preferably 0.06% or less.

(Si: 0.01-0.60%)
Si is an effective deoxidizing element in steelmaking and is a component that contributes to the improvement of steel strength. In order to sufficiently obtain this effect, the Si content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, from the viewpoint of preventing deterioration of toughness, the Si content is preferably 0.60% or less, more preferably 0.40% or less, still more preferably 0.30% or less.

(Mn: 0.1-2.0%)
Mn is a component that contributes to improving the strength of steel. In order to sufficiently obtain this effect, the Mn content is preferably 0.1% or more, more preferably 1.0% or more. On the other hand, from the viewpoint of preventing deterioration of toughness and sour resistance, the Mn content is preferably 2.0% or less, more preferably 1.8% or less, still more preferably 1.6% or less.

(Sol.Al: 0.005 to 0.09%)
Al is an element effective for deoxidation like Si. Therefore, sol. The Al (acid-soluble Al) is preferably contained in an amount of 0.005% or more, more preferably 0.01% or more, still more preferably 0.02% or more. On the other hand, from the viewpoint of preventing deterioration of toughness, sol. The Al content is preferably 0.09% or less, more preferably 0.06% or less.

Further, in the above-mentioned example of the steel, the steel is one or more selected from the group consisting of Cu, Ni, Cr, Mo, V, Nb, Ti, B, Ca, REM, Zr, and Mg. Contains. The content of these elements does not need to be particularly limited to the lower limit and may be 0%.

(Cu: 0 to 0.90%)
Cu is a component that contributes to improving the strength of steel, and may be added as needed. In order to sufficiently obtain this effect, the content when Cu is contained is preferably 0.10% or more. On the other hand, from the viewpoint of preventing deterioration of toughness, the Cu content is preferably 0.90% or less, more preferably 0.50% or less, still more preferably 0.30% or less.

(Ni: 0 to 0.90%)
Ni is an element that contributes to the improvement of the strength of steel and the improvement of toughness, and may be added as necessary. In order to sufficiently obtain this effect, the content when Ni is contained is preferably 0.10% or more. On the other hand, since Ni is an expensive element, the content of Ni is preferably 0.90% or less, more preferably 0.50% or less, still more preferably 0.30% or less.

(Cr: 0 to 0.90%)
Cr is a component that contributes to improving the strength of steel, and may be added as necessary. In order to sufficiently obtain this effect, the content when Cr is contained is preferably 0.10% or more. On the other hand, from the viewpoint of preventing deterioration of toughness, the Cr content is preferably 0.90% or less, more preferably 0.50% or less, still more preferably 0.30% or less.

(Mo: 0 to 0.90%)
Mo is a component that contributes to improving the strength of steel, and may be added as necessary. In order to sufficiently obtain this effect, the content when Mo is contained is preferably 0.05% or more. On the other hand, from the viewpoint of preventing deterioration of toughness, the Mo content is preferably 0.90% or less, more preferably 0.50% or less.

(V: 0 to 0.09%)
V is a component that contributes to the improvement of the strength of the steel, and may be added as needed. In order to sufficiently obtain this effect, the content when V is contained is preferably 0.01% or more. On the other hand, from the viewpoint of preventing deterioration of toughness, the V content is preferably 0.09% or less, more preferably 0.05% or less.

(Nb: 0 to 0.09%)
Nb is an element that contributes to the improvement of the strength of steel and the improvement of toughness, and may be added as necessary. In order to sufficiently obtain this effect, the content when Nb is contained is preferably 0.01% or more. On the other hand, from the viewpoint of preventing deterioration of sour resistance, the content of Nb is preferably 0.09% or less, more preferably 0.06% or less, still more preferably 0.04% or less.

(Ti: 0 to 0.09%)
Ti has the effect of improving the toughness of the weld heat-affected zone (HAZ) by combining with N to form TiN, and Ti may be added as necessary. In order to sufficiently obtain this effect, the content when Ti is contained is preferably 0.01% or more. On the other hand, from the viewpoint of preventing deterioration of toughness and sour resistance, the Ti content is preferably 0.09% or less, more preferably 0.03% or less.

(B: 0 to 0.004%)
B is a component that improves the hardenability and contributes to the improvement of the strength of the steel, and may be added as necessary. In order to sufficiently obtain this effect, the content when B is contained is preferably 0.0003% or more. On the other hand, from the viewpoint of preventing a decrease in toughness, the content of B is preferably 0.004% or less, more preferably 0.002% or less.

(Ca: 0 to 0.01%)
Ca may be added as needed because it controls the morphology of sulfides (particularly MnS) and contributes to improved toughness and sour resistance. In order to sufficiently obtain this effect, the content when Ca is contained is not particularly limited, but is preferably 0.001% or more. On the other hand, from the viewpoint of preventing deterioration of toughness and sour resistance due to excessive addition, the Ca content is preferably 0.01% or less, more preferably 0.005% or less.

(REM: 0 to 0.1%)
REM may be added as needed to control the morphology of the sulfide and contribute to the improvement of toughness. In order to sufficiently obtain this effect, the content when REM is contained is preferably 0.001% or more. On the other hand, from the viewpoint of preventing deterioration of toughness due to excessive addition, the content of REM is preferably 0.1% or less, more preferably 0.05% or less. Here, "REM" refers to Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and any one of the above is used as REM. More than a seed can be contained in steel. The content of the above REM is the total content of the REM.

(Zr: 0 to 0.1%)
Zr may be added as needed because it forms oxides and nitrides, suppresses the coarsening of austenite grains in HAZ, and contributes to the improvement of toughness. In order to sufficiently obtain this effect, the content when Zr is contained is preferably 0.005% or more. On the other hand, from the viewpoint of preventing a decrease in toughness due to excessive addition, the Zr content is preferably 0.1% or less.

(Mg: 0 to 0.01%)
Mg forms finely dispersed oxides, and in particular, suppresses the coarsening of the austenite particle size in the weld heat-affected zone and contributes to the improvement of toughness. Therefore, Mg may be added as needed. In order to sufficiently obtain this effect, the content when Mg is contained is preferably 0.0005% or more. On the other hand, from the viewpoint of preventing adverse effects on toughness due to excessive addition, the Mg content is preferably 0.01% or less, more preferably 0.005% or less.

Further, the above-mentioned steel contains Fe and impurities as a balance.
Fe in steel is the main component of steel. Steel contains, for example, 95% or more Fe.

Impurities are components that are present in steel and do not need to be present in the resulting thick steel sheet, regardless of the intention of addition. The term "impurity" is a concept that includes unavoidable impurities that are mixed in from ore, scrap, or the manufacturing environment as a raw material when a steel material is industrially manufactured. Such impurities may be contained in an amount that does not adversely affect the effects of the present invention. Examples of impurities include P, S, N, O and the like. Hereinafter, typical impurity elements will be described in detail.

(P: 0 to 0.02%)
P is an impurity element, and if its content is excessively large, it may deteriorate the toughness. Therefore, the content of P is preferably as low as possible, for example, 0.02% or less, preferably 0.01% or less. The lower limit of the P content is preferably 0%, but may be, for example, 0.0001% or more.

(S: 0 to 0.01%)
S is an impurity element, and if its content is excessively large, it may deteriorate the toughness. Therefore, the content of S is preferably as small as possible, for example, 0.01% or less, preferably 0.005% or less, more preferably 0.002% or less, still more preferably 0.0010% or less. The lower limit of the S content is preferably 0%, but may be 0.0001% or more, for example.

(N: 0 to 0.009%)
N is an impurity element, and if its content is excessively large, it may deteriorate the toughness. Therefore, it is desirable that the content of N is as small as possible, for example, 0.009% or less, preferably 0.007% or less. Further, when the content of N is 0.004% or more, the toughness of HAZ may be improved by containing Ti or Zr. The lower limit of the N content is preferably 0%, but may be 0.0001% or more, for example.

(O: 0 to 0.003%)
O is an impurity element, and if its content is excessively large, it may deteriorate toughness. Therefore, it is desirable that the content of O is as small as possible, for example, 0.003% or less, preferably 0.002% or less. The lower limit of the O content is preferably 0%, but may be 0.0001% or more, for example.

(CeqL: 0.10 to 0.60%)
CeqL means carbon equivalent and is defined by the following equation (1). The Ceq is not particularly limited, but it is desirable that the CeqL is 0.10% or more because increasing the CeqL contributes to the improvement of the strength. On the other hand, if CeqL is excessive, welding cracks are likely to occur and the toughness may be deteriorated. Therefore, it is desirable that CeqL is 0.60% or less.
CeqL = C + Mn / 6 + (Cu + Ni) / 15+ (Cr + Mo + V) / 5 ... (1)
Here, each element symbol in the formula represents the content (mass%) of each element contained in the steel sheet, and when a certain element is not contained, the value of the element symbol corresponding to the element is set to zero.

The example of the steel applicable in this embodiment has been described above. However, the steel that can be used in the present invention is, of course, not limited to the above-mentioned example. For example, steel may contain elements other than the above-mentioned elements. Further, each element in the steel may be present in a content outside the above-mentioned range.

<First step (hot rolling step)>
In the first step, the steel is hot-rolled to obtain a rolled material. Hot rolling may be carried out as it is, for example, after the steel pieces are manufactured by a continuous casting method, or may be carried out after the steel pieces are once cooled and reheated.

The conditions for reheating as a pretreatment for hot rolling are not particularly limited and can be appropriately set.
For example, the heating temperature before rolling can be 900 ° C. or higher in order to easily perform hot rolling. By raising the heating temperature, carbides such as Nb, V, Ti, and Zr, nitrides, and the like can be solid-dissolved, and the effect of improving strength, toughness, and sour resistance can be obtained. The heating temperature is preferably 1000 ° C. or higher, more preferably 1100 ° C. or higher. Further, in order to more reliably suppress the coarsening of austenite crystal grains and the deterioration of toughness associated therewith, the heating temperature is preferably 1250 ° C. or lower.

Further, the conditions for hot rolling are not particularly limited. For example, hot rolling can be performed under the condition that the total rolling reduction in the temperature range of 900 ° C. or lower is 50% or more. This makes it easier to refine the structure of the steel sheet to ensure good toughness. Here, the "total reduction rate in the temperature range of 900 ° C. or lower" is expressed by the following formula.
Total reduction rate (%) = [(thickness when reaching 900 ° C)-(rolled finish thickness)] / (thickness when reaching 900 ° C) x 100%

Further, the finishing temperature of hot rolling can be set to exceed the Ar3 transformation point, for example. As a result, the start temperature of accelerated cooling can be set to exceed the Ar3 transformation point, and the sour resistance of the obtained thick steel sheet can be further improved. The Ar3 transformation point referred to here means the transformation start temperature in the case of air cooling, and can be obtained by thermal expansion measurement.

The thickness of the rolled material after hot rolling corresponds to the thickness desired for the obtained thick steel sheet.

<Second process (accelerated cooling process)>
Next, in the second step, the rolled material obtained in the first step is accelerated and cooled from the temperature above the Ar3 transformation point.
As described above, the starting temperature of accelerated cooling is above the Ar3 transformation point. In the central part of the plate thickness of the rolled material, the concentration unevenness of the band-shaped component due to solidification segregation tends to occur as the transformation progresses. The formation of band tissues such as pearlide bands is suppressed. As a result, the hard structure in the band structure can be reduced and the sour resistance can be improved. The starting temperature of accelerated cooling is preferably (Ar3 transformation point + 20 ° C.) or higher.

The average cooling rate of accelerated cooling is not particularly limited, but can be, for example, 5 ° C./sec or higher. As a result, the formation of the band structure described above can be suppressed more reliably, and the sour resistance can be further improved. The average cooling rate is preferably 10 ° C./sec or higher. On the other hand, when the average cooling rate is high, it becomes difficult to cool the rolled material uniformly, and the hardness near the surface layer of the rolled material tends to be higher than the hardness at the center in the thickness direction. Therefore, the average cooling rate is preferably 60 ° C./sec or less, more preferably 40 ° C./sec or less. Even when the hardness near the surface layer increases, the hardness of the surface layer can be sufficiently reduced in the third step described later, and the obtained thick steel sheet naturally has excellent sour resistance. Become.

The cooling stop temperature for accelerated cooling is not particularly limited, but may be, for example, 600 ° C. or lower, preferably 500 ° C. or lower, from the viewpoint of improving the strength of the thick steel sheet. Further, the cooling shutdown temperature of the accelerated cooling can be, for example, 300 ° C. or higher, preferably 400 ° C. or higher, from the viewpoint of ensuring good sour resistance.

Further, the specific method of accelerated cooling is not particularly limited, but for example, oil cooling, water cooling and the like generally used for quenching steel can be adopted.
The temperature of the rolled material in this step can be measured by observing the surface temperature of the rolled material. That is, the start temperature of accelerated cooling means the surface temperature. Further, the cooling shutdown temperature of accelerated cooling means the surface temperature after the completion of reheating.

<Third step (surface heating step)>
Next, in the third step, with respect to the surface of the rolled material, the first temperature range of (Ac1 transformation point −50 ° C.) or more and below the Ac1 transformation point is Ac1 at an average temperature rise rate of 2.0 ° C./sec or more. It is heated to the second temperature range above the transformation point and held in the second temperature range above the Ac1 transformation point for 1 second or more and 600 seconds or less.

As described above, the present inventor has found that by heating the surface of the rolled material to a temperature range exceeding the Ac1 transformation point, the surface layer can be softened and the hardness of the surface layer can be sufficiently reduced. In the surface layer of the rolled material having a depth of less than 1 mm, a composite structure of martensite and bainite tends to be formed. By heating in a temperature range above the Ac1 transformation point, in the microstructure near the surface of the rolled material, at least partly, reverse transformation of the structure occurs, austenite is generated, and finally austenite (γ phase). ) Produces a soft ferrite structure. Further, even in the portion not reverse-transformed, the hardness is lowered by the effect of heating exceeding the Ac1 transformation point. As a result, the surface layer of the obtained thick steel sheet is softened. Therefore, the sour resistance of the obtained thick steel sheet is improved.

Such heat treatment is not only for the purpose of decomposing island-shaped martensite as described in Patent Documents 1 to 3, but is for modifying the entire structure including martensite and bainite existing near the surface layer. With the goal. Then, in the above heat treatment, heating is performed in a temperature range higher than the heating temperature range described in these, and it is generated in the vicinity of the surface layer, at least partially, by reverse transformation at the time of temperature rise and transformation at the time of temperature decrease. Contains a relatively soft ferrite structure. Further, the ferrite structure generated in this step has a lower degree of anisotropy than the ferrite structure that can be generated during hot rolling and subsequent cooling, and it is difficult to propagate cracks and the like once formed. Therefore, the heat treatment performed in this step can more reliably soften the surface layer of the rolled material and improve the sour resistance of the obtained thick steel sheet as compared with the methods described in Patent Documents 1 to 3. can.

On the other hand, if the rolled material is simply heated, the entire rolled material exceeds the Ac1 transformation point, and as a result, the strength of the obtained thick steel sheet is significantly reduced. In view of this point, in the present invention, attention is paid to the surface temperature of the rolled material, and by rapidly heating under the above conditions, the temperature rise in the internal region of the rolled material is suppressed, and the vicinity of the surface layer is selectively selected. We devised to heat it. Hereinafter, the heating conditions in this step will be described in detail. The temperature described in this step refers to the surface temperature of the rolled material unless otherwise specified.

First, in this step, the surface of the rolled material is heated from a temperature of (Ac1 transformation point −50 ° C.) or lower. By rapidly heating from a temperature sufficiently lower than the Ac1 transformation point in this way, it is possible to prevent the temperature of the internal region of the rolled material from rising and to improve the strength of the obtained thick steel sheet. Preferably, heating is performed from a temperature of (Ac1 transformation point −150 ° C.) or lower. On the other hand, when the heating start temperature exceeds the upper limit value, the temperature rise in the internal region of the rolled material cannot be prevented, and the strength of the obtained thick steel sheet is inferior. In carrying out this third step, the cooling stop temperature in the accelerated cooling step of the second step, which is the previous step, may be set to a temperature of (Ac1 transformation point -50 ° C) or less, or after the accelerated cooling step, this step may be performed. It is important to surely heat from a temperature below (Ac1 transformation point −50 ° C.) by performing additional cooling such as air cooling before.

The Ac1 transformation point can be obtained by measuring the amount of expansion of the sample due to heating using a thermal expansion measuring device.

Further, in this step, the average heating rate of the surface of the rolled material during heating is 2.0 ° C./sec or more. As a result, the heat applied to the surface is reduced from diffusing into the internal region of the rolled material, and the temperature rise in the internal region of the rolled material can be suppressed. On the other hand, when the average temperature rise rate is less than the lower limit, the heat applied to the surface diffuses into the internal region of the rolled material, and the temperature rise in the internal region of the rolled material cannot be prevented, so that the obtained thick steel sheet cannot be prevented. The strength of is inferior. The average heating rate is not limited as long as it is within the above range, but is preferably 10 ° C./sec or more, more preferably 20 ° C./sec or more, from the viewpoint of preventing the diffusion of heat into the internal region of the rolled material. .. On the other hand, the upper limit of the average temperature rise rate is not particularly limited, but is, for example, 400 ° C./sec or less, preferably 200 ° C./sec or less due to restrictions on the temperature control performance of the heating device.

Further, the target heating temperature reached in this step is a second temperature range exceeding the Ac1 transformation point, as described above. As a result, the vicinity of the surface layer of the rolled material can be sufficiently softened, and the sour resistance of the obtained thick steel sheet can be made excellent. On the other hand, when the heat reached temperature is equal to or lower than the Ac1 transformation point, the reverse transformation of the microstructure of the surface layer cannot be partially generated, and the vicinity of the surface layer of the rolled material cannot be sufficiently softened. As a result, the sour resistance of the obtained thick steel sheet becomes inferior.

The second temperature range (temperature range of the ultimate heating temperature) is not particularly limited as long as it exceeds the Ac1 transformation point, but is preferably 720 ° C. or higher, more preferably 740 ° C. or higher. This makes it possible to more reliably soften the vicinity of the surface layer of the rolled material. The upper limit of the second temperature range is not particularly limited, but is preferably Ac3 transformation point or less, more preferably 840 ° C or lower, and further preferably 820 ° C or lower. This makes it possible to more reliably prevent the excessive heat existing in the vicinity of the surface layer from diffusing into the internal region of the rolled material.

The Ac3 transformation point can be obtained by measuring the amount of expansion of the sample due to heating using a thermal expansion measuring device.

The time (maintenance time) for maintaining the second temperature range is 1 second or more and 600 seconds or less. As a result, the vicinity of the surface layer of the rolled material can be sufficiently softened, and the temperature of the internal region of the rolled material can be prevented from rising. On the other hand, when the maintenance time is less than the lower limit, the microstructure near the surface layer of the rolled material cannot be sufficiently transformed, and the surface layer of the obtained thick steel sheet cannot be softened. As a result, the obtained thick steel sheet is inferior in sour resistance. Further, when the maintenance time exceeds the upper limit value, the heat existing in the vicinity of the surface layer diffuses into the inner region of the rolled material, and as a result, the strength of the obtained thick steel sheet decreases.

The maintenance time may be 1 second or longer, preferably 3 seconds or longer, and more preferably 10 seconds or longer. As a result, the vicinity of the surface layer of the rolled material can be further softened, and the sour resistance of the thick steel sheet can be further improved. The maintenance time may be 600 seconds or less, but is preferably 120 seconds or less, and more preferably 30 seconds or less. As a result, it is possible to more reliably prevent the temperature from rising in the inner region of the rolled material, and the strength of the obtained thick steel sheet can be made excellent.

Further, in this step, heating may be performed by any method. The heating can be performed by, for example, induction heating, laser heating, or a heating method using electromagnetic wave radiation. These means are suitable for rapid heating. Induction heating is particularly preferable because it can selectively heat the vicinity of the surface layer of the rolled material with high efficiency. In this case, the applied frequency in the induction heating is not particularly limited, but for example, in order to selectively heat the vicinity of the surface layer with high efficiency, the frequency is 2 kHz or more and 200 kHz or less, preferably 6 kHz or more and 120 kHz or less, and more preferably 20 kHz or more and 80 kHz or less. be.

The cooling rate after heating is not particularly limited, and can be performed by slow cooling such as air cooling or mist spraying. Further, the cooling shutdown temperature after heating is not particularly limited.

The above-mentioned third step may be performed immediately after the completion of the second step, or may be performed after a time after the second step.
Further, in the latter case, preheating may be performed prior to the implementation of the third step. The method of preheating is not particularly limited, and can be performed by a heat treatment furnace or the like by gas combustion. Further, the preheating can be performed up to a temperature equal to or lower than the above-mentioned (Ac1 transformation point −50 ° C.). The rate of temperature rise in this case is also not particularly limited.

By going through the above steps, a thick steel plate can be obtained. However, if necessary, other treatments, for example, the following fourth step may be performed on the thick steel sheet.

<Fourth step (temper processing)>
The obtained thick steel sheet may be heat-treated (tempered) if necessary. As a result, even if island-like martensite (Martensite-Austenite Constituent: MA), which is a hard structure, is generated as a result of heating and cooling to the two-phase region in the third step, this can be achieved in this step. Can be disassembled. By decomposing the island-shaped martensite that can adversely affect the sour resistance in this way, the sour resistance of the obtained thick steel sheet can be further improved.

The temperature of such heat treatment can be, for example, a temperature below the Ac1 transformation point, preferably 650 ° C. or lower, and more preferably 600 ° C. or lower. This makes it possible to decompose the island-like martensite while preventing the reverse transformation to austenite again. The heat treatment temperature is not particularly limited as long as the island-shaped martensite can be decomposed, and can be, for example, 400 ° C. or higher, preferably 500 ° C. or higher.

Further, in the heat treatment, the time for maintaining the temperature (maintenance time) is not particularly limited, but may be, for example, 600 seconds or longer, preferably 1200 seconds or longer. The maintenance time can be 3600 seconds or less, preferably 2400 seconds or less, for the reason of heat treatment cost, particularly energy saving.

Further, the heating rate and the cooling rate of the thick steel sheet in the heat treatment are not particularly limited, and can be appropriately selected depending on, for example, the configuration of the heat treatment apparatus used in a general thick steel sheet factory.

In the thick steel sheet obtained by the above steps, the surface layer portion of the rolled material is selectively heated to the Ac1 transformation point or higher in the third step described above, whereby a specific ferrite structure is sufficiently formed on the surface layer, and the surface layer is formed. The hardness is sufficiently reduced. Therefore, the obtained thick steel sheet is excellent in sour resistance, particularly SSC resistance. On the other hand, by selectively heating the surface layer of the rolled material in the third step, the temperature rise in the internal region of the rolled material is suppressed. As a result, the strength of the thick steel sheet obtained by the third step is not adversely affected. Therefore, the strength of the obtained thick steel sheet can be maintained as excellent.

[2. Thick steel plate]
Next, the thick steel plate according to this embodiment will be described.
The thick steel sheet according to the present embodiment contains abundant a predetermined ferrite structure in the surface layer as compared with the internal region in the thickness direction.

The microstructure of the internal region of the thick steel sheet, for example, the portion of 1/4 t to 3/4 t in the plate thickness direction (t indicates the plate thickness) has a great influence on the mechanical properties such as the tensile strength of the thick steel plate.

The thick steel sheet can mainly contain, for example, bainite as a microstructure in the internal region. Thereby, the thick steel plate can have sufficient strength and toughness. Further, as the microstructure of the internal region, ferrite, pearlite, fresh martensite, martensite such as tempered martensite, island-like martensite (MA) and the like may be contained.

In addition, the thick steel sheet contains, for example, tempered bainite and ferrite as microstructures on the surface layer. Further, the thick steel sheet may contain other pearlite, tempered martensite, island-shaped martensite (MA) and the like in the surface layer.

In the present embodiment, as the surface layer of the thick steel sheet, the structure and hardness are evaluated for a portion of 0.1 mm or more and 1.0 mm or less (also referred to as “surface layer region”) from the surface in the plate thickness direction. It should be noted that changes in the chemical composition may occur in the portion less than 0.1 mm from the surface, which is not suitable for the evaluation of the surface structure and hardness of the thick steel sheet of the present embodiment. Exclude from the target site.

Further, as for the internal region of a steel material such as a thick steel plate, it is complicated to evaluate the structure or the like in the entire region. Therefore, in the present embodiment, on behalf of the internal region of the steel material, the structure and the like are evaluated in the region of −2.5 mm to 2.5 mm (also referred to as “center region”) in the plate thickness direction from the center of the plate thickness. ..

As described above, in the present embodiment, the thick steel sheet contains abundant ferrite structure in the surface layer as compared with the internal region in the plate thickness direction. Specifically, in the thick steel plate according to the present embodiment, the ratio of the central region ferrite area fraction to the surface layer ferrite area fraction in the surface layer region (central region ferrite area fraction / surface layer ferrite area fraction, "ferrite fraction". (Also referred to as "ratio") is 0.10 or less.

As described above, the presence of many soft ferrite structures in the surface layer region of the thick steel sheet makes the hardness in the surface layer region of the thick steel sheet relatively small, and the ferrite structure in the internal region of the thick steel sheet is reduced, resulting in the thick steel sheet. The strength of the is high enough. Therefore, it is possible to improve the SSC resistance of the thick steel sheet while sufficiently increasing the strength of the thick steel sheet. On the other hand, if the ferrite fraction ratio exceeds 0.10, the ferrite structure in the surface layer region becomes too small, and as a result, sufficient sour resistance cannot be obtained. Alternatively, as a result of having too many ferrite structures in the internal region, the strength of the thick steel sheet becomes low.

The above-mentioned ferrite surface integral ratio (central region ferrite surface integral / surface integral surface integral) is preferably 0.07 or less from the viewpoint of further improving the SSC resistance while sufficiently increasing the strength of the thick steel sheet. More preferably, it is 0.05 or less. Further, the lower limit of the ferrite fraction ratio is not particularly limited and may be 0. When the surface integral surface integral ratio is completely 0, the above ferrite surface integral ratio cannot be calculated, but such a thick steel sheet is out of the scope of the present invention.

Further, in the thick steel plate according to the present embodiment, when the cross sections in the rolling direction and the plate thickness direction are observed, the ratio of the length in the rolling direction to the length in the plate thickness direction of the continuous ferrite structure in the surface layer region (length in the rolling direction). The average value (hereinafter, also referred to as “anisotropic index”) of rolling stock / length in the plate thickness direction) is 5.0 or less. This improves the SSC resistance of the thick steel sheet. It is considered that this is because the anisotropy of the shape and orientation of ferrite, bainite, etc. in the plane direction of the surface layer region becomes small, so that the crack once generated in the surface layer region becomes difficult to propagate. On the other hand, if the anisotropy index exceeds the upper limit, the SSC resistance cannot be improved. In this case as well, it is considered that the presence of highly anisotropy ferrite, bainite, etc. in the surface layer region facilitates the propagation of cracks once generated in the surface layer region.

The anisotropy index as described above is preferably 3.0 or less, more preferably 2.0 or less, from the viewpoint of further improving the SSC resistance. The lower limit of the anisotropy index is not particularly limited, but is usually 1.0 or more.

Further, the anisotropy index of 5.0 or less as described above is obtained by a specific heat treatment after hot rolling, for example, as performed in the third step. On the other hand, the ferrite structure generated in the hot rolling and the subsequent cooling process inherits the shape formed during the hot rolling and generally has a long shape in the rolling direction, which is not necessarily the above-mentioned 5.0 or less difference. It cannot have an anisotropy index.

The surface integral surface integral means the area ratio of the ferrite structure in the surface layer region in the cross section in the plate thickness direction of the thick steel plate, and the central region ferrite surface integral means the ferrite in the central region in the cross section in the plate thickness direction of the thick steel plate. It means the surface integral of the organization. These surface integral surface integrals and central region ferrite surface integrals can be measured, for example, as follows.

A test piece for microstructure investigation is collected from a position near the center of the thick steel sheet in the rolling direction and near the center in the width direction. The collected sample is mirror-polished with a surface parallel to the plate thickness direction and the rolling direction as an observation surface, corroded with nital, and photographed in the surface layer region and the central region using an optical microscope. By obtaining the ferrite fractions (surface integrals) of the surface layer region and the central region from the images thus obtained, the surface integral ferrite area fractions and the central region ferrite surface integrals can be obtained.

Further, the anisotropy index of the ferrite structure in the surface layer region can be measured as follows. First, as described above, a surface parallel to the plate thickness direction and the rolling direction is mirror-polished to obtain an observation sample corroded by nital.

Next, the observation sample is observed with an optical microscope or the like, a cross-sectional image of the observation sample is obtained, and each ferrite structure is specified from the cross-sectional image. In the present embodiment, one ferrite structure is composed of one ferrite crystal grain or an aggregate of a plurality of ferrite crystal grains. Then, for each ferrite structure in the cross-sectional image, the edge portion where the ferrite structure is adjacent to another structure (for example, bainite structure) can be used as the outer edge portion of the ferrite structure.

Then, among the specified individual ferrite structures, the length in the rolling direction and the length in the plate thickness direction are measured at random for 10 or more, preferably 20 or more, in the rolling direction with respect to the length in the plate thickness direction. Get the length ratio. Then, the ratio of the length in the rolling direction to the length in the obtained plate thickness direction is averaged to obtain the above anisotropy index. When the ferrite area ratio in the cross-sectional image is 50% or more, the above-mentioned length is measured for another structure (for example, bainite structure) surrounded by the ferrite structure to obtain the above-mentioned anisotropy index. You can also.

The Vickers hardness in the surface layer region of the thick steel sheet is 250 Hv or less. As a result, the SSC resistance of the thick steel sheet can be made sufficiently high. The Vickers hardness in the surface layer region is more preferably 230 Hv or less, still more preferably 220 Hv or less. The Vickers hardness in the surface layer region of the thick steel sheet is not particularly limited, but is usually 170 Hv or more, preferably 180 Hv or more, from the viewpoint of ensuring the tensile strength.

The Vickers hardness in the surface layer region of the thick steel sheet can be measured, for example, as follows. First, a test piece for measuring cross-sectional hardness is collected from a position near the center of the thick steel sheet in the rolling direction and near the center in the width direction. Then, using a Micro Vickers hardness tester, with a measured load of 100 g, a predetermined pitch, for example 0.1 mm, from a portion of the upper surface of the thick steel plate according to each example at a depth of 0.1 mm to a depth of 1.0 mm in the plate thickness direction. The pitch is measured, and the hardness distribution on the surface including the plate thickness direction and the rolling direction is measured. The maximum value among the obtained measured values can be evaluated as the maximum Vickers hardness.

The chemical composition of the thick steel sheet is not particularly limited. However, for example, it can have the same chemical composition as the above-mentioned steel as a raw material for a thick steel sheet.

The thickness of the thick steel sheet can be appropriately set according to the intended use, but is, for example, 6 mm or more, preferably 7.0 mm or more, and more preferably 10 mm or more. When the steel sheet is relatively thick in this way, a temperature difference is likely to occur between the internal region of the rolled material and the surface layer during accelerated cooling during manufacturing, and the hardness of the surface layer is likely to increase. However, in the present embodiment, the ferrite structure as described above is sufficiently formed in the surface layer region by the third step, and the hardness of the surface layer region is sufficiently lowered. The method according to the present embodiment exhibits a particularly excellent effect, that is, excellent SSC resistance, when the thickness of the thick steel sheet is within the above-mentioned range. On the other hand, the thickness of the thick steel sheet can be 100 mm or less, particularly 40 mm or less, depending on the application.

The thick steel plate according to the present embodiment described above can be manufactured by, for example, the above-mentioned manufacturing method for the thick steel plate according to the present embodiment.
Further, since the above-mentioned thick steel sheet according to the present embodiment has excellent sour resistance and can maintain excellent strength, it can be used for applications in an environment containing hydrogen sulfide, for example, line pipes, especially UO line pipes. Can be suitably used. That is, the thick steel plate according to the present embodiment can be a thick steel plate for sour line pipes.

Hereinafter, the method for manufacturing the thick steel plate and the thick steel plate according to the above-described embodiment will be specifically described with reference to Examples and Comparative Examples. It should be noted that the examples shown below are merely examples, and the method for manufacturing the thick steel plate and the thick steel plate according to the present embodiment is not limited to the following examples.

<1. Manufacture of thick steel sheets>
(First step)
A steel piece (test steel) having a chemical composition shown in Table 1 below and having a thickness of 250 mm is heated to 1180 ° C., kept at a soaking temperature for 3600 seconds, and then hot-rolled at a finishing temperature of 840 ° C. And said. Here, hot rolling was performed under the condition that the total rolling reduction in the temperature range of 900 ° C. or lower was 50% or more. In Table 1, the transformation start temperature at the time of temperature rise, the Ac1 transformation point which is the end temperature, the Ac3 transformation point, and the Ar3 transformation point which is the transformation start temperature at the time of temperature decrease (non-water cooling) have a diameter of 3 mm and a length of 10 mm. The test piece was taken from a steel piece and obtained by a thermal expansion measuring tester.

(Second step)
The test steels according to each example were subjected to accelerated cooling by water cooling from the start temperature shown in Table 2 to the stop temperature of 460 ° C. Here, the start temperature is the surface temperature, and the stop temperature is the maximum value of the surface temperature at the time of reheating. The cooling speeds of accelerated cooling are as shown in Table 2. In Example 18, the test steel after accelerated cooling was obtained as the thick steel plate according to Example 18 without performing the subsequent preheating and the third and fourth steps.

(Steps between the second step and the third step)
The test steels of Examples 10 and 16 were preheated in a gas combustion furnace to the heating start temperature (Table 2) of the third step after the second step and prior to the third step.

(Third step)
The test steels according to each example were heated by induction heating or laser light irradiation under the conditions shown in Table 2. Specifically, the test steels according to Examples 1 to 7 and 9 to 17 are heated by induction heating at the applied frequency shown in Table 2, and the test steels according to Example 8 are heated by irradiation with laser light. went. Then, the temperature was raised from the heating start temperature shown in Table 2, and after reaching a predetermined surface temperature, the surface temperature was maintained for a certain period of time (“Ac1 super-holding time” in the table). Further, in Example 13 under the condition that the surface temperature did not reach the Ac1 transformation point, the same temperature was maintained for 15 seconds after reaching 650 ° C. The temperatures of the temperature rising conditions shown in Table 2 are all the surface temperatures of the test steel.
Further, the cooling after the temperature rise was performed by air cooling (leaving in the atmosphere at room temperature).
As a result, except for Example 9, the thick steel sheets according to each example were obtained.

(4th step)
For the test steel according to Example 9, after the third step, additional heat treatment was performed in the gas combustion furnace under the conditions shown in Table 2. As a result, the thick steel plate according to Example 9 was obtained.

<2. Tissue observation>
In each of the obtained thick steel sheets, the ferrite fraction ratio and the anisotropy index were measured and calculated as follows.

(Ferrite fraction ratio)
Test pieces for microstructure investigation were collected from the positions near the center of the rolling direction and the center of the width direction of the thick steel sheets according to each example. The collected sample is mirror-polished with the surface parallel to the plate thickness direction and the rolling direction as the observation surface, corroded with nital, and the surface layer region (0.1 mm to 1.0 mm in the plate thickness direction from the surface) is used using an optical microscope. (Position) and the central region (position of −2.5 mm to 2.5 mm in the thickness direction from the center of the plate thickness), 10 visual fields were photographed at a time. From the photographs thus obtained, the ferrite fractions (surface integrals) of the surface layer region and the central region were obtained, respectively, and the surface integral ferrite area fraction and the central region ferrite surface integral were obtained, and the ferrite fraction ratio was calculated. ..
The ferrite fraction ratio calculated for each thick steel sheet was evaluated as "◯" when the ferrite fraction ratio was 0.10 or less and as "x" when it exceeded 0.10.

(Anisotropy index)
As described above, the surface of the sample piece collected from the thick steel plate according to each example was mirror-polished on a surface parallel to the plate thickness direction and the rolling direction to obtain an observation sample corroded by nital. The observation sample was observed with an optical microscope, a cross-sectional image of the observation sample was obtained, and individual ferrite structures were identified from the cross-sectional image. Then, the length in the rolling direction and the length in the plate thickness direction were measured at random for 20 of the specified individual ferrite structures, and the ratio of the length in the rolling direction to the length in the plate thickness direction was obtained. .. When the ferrite area ratio in the cross-sectional image was 50% or more, the above-mentioned length ratio was obtained for other structures (bainite structure or the like) surrounded by the ferrite structure. Then, the ratio of the length in the rolling direction to the length in the obtained plate thickness direction was averaged to obtain the above anisotropy index.
The anisotropy index obtained for each thick steel sheet was evaluated as "◯" when the anisotropy index was 5.0 or less and as "x" when it exceeded 5.0.

<3. Surface area hardness test>
A test piece for measuring the cross-sectional hardness was collected from a position near the center of the thick steel sheet in the rolling direction and near the center in the width direction of each example, and the hardness distribution on the surface including the plate thickness direction and the rolling direction was measured. Using a Micro Vickers hardness tester, a measurement was performed at a pitch of 0.1 mm from a depth of 0.1 mm in the plate thickness direction to a depth of 1 mm on the upper surface of the thick steel plate according to each example with a measurement load of 25 g, and the maximum value was evaluated. .. In order to obtain sufficient SSC resistance, the target value of Vickers hardness was set to 250 Hv or less.

<4. Evaluation>
Tensile tests and SSC resistance evaluation tests were carried out on each of the obtained thick steel sheets as follows.

(Tensile test)
From the thick steel plates according to each example, plate-shaped test pieces were collected so that the center of the test piece was at the position 1/2 in the plate thickness direction and the axis of the test piece was perpendicular to the rolling direction. The shape of the test piece was measured at room temperature using a No. 14A test piece (JIS Z 2201) having a parallel portion having a diameter of 5 mm, and YS (stress at 0.5% strain) and TS (tensile strength) were measured. In order to satisfy the strength of the X65 class of the line pipe, the target value of YS was 450 MPa or more, and the target value of TS was 535 MPa or more.

(SSC resistance evaluation test)
Bending test pieces (thickness: 2 mm, width: 10 mm, length: 75 mm) for evaluating sour resistance are placed so that the test piece is on the surface of the rolled material and the long side of the test piece is in the rolling direction. Three pieces were taken from the thick steel plates according to each example so as to be vertical. A bending load stress of 90% of the yield stress was applied to these test pieces to saturate hydrogen sulfide with NaCl of 5%, CH ₃ COOH of 0.5%, and pH of about 3 and 1 atm. Was soaked in the aqueous solution of No. 30 for 30 days, and the presence or absence of breakage was evaluated. When all three were not broken, it was judged that the SSC resistance was good (◯). On the other hand, if even one of the test pieces was broken, it was judged that the SSC resistance was not good (×).
The above results are shown in Table 3.

As shown in Table 3, all of the thick steel sheets manufactured by the methods according to Examples 1 to 11 of the present invention have a ferrite fraction ratio of 0.1 or less and an anisotropy index of 5.0. The Vickers hardness of the surface layer region was 250 HV or less.

Further, as shown in Table 3, the thick steel sheets according to Examples 1 to 11 of the present invention have a relatively low hardness in the surface layer region and are excellent in tensile properties. Further, the thick steel plates according to Examples 1 to 11 were also excellent in SSC resistance. However, the physical characteristics of the thick steel sheet according to each example were slightly different depending on the heating conditions in the third step. Specifically, in Example 3 in which the surface temperature was high, YS and TS were low. In Example 5, where the applied frequency was high, the surface hardness was high. In Example 7, where the applied frequency was low, YS and TS were low. Further, in Example 10 in which the heating start temperature was high, YS and TS were low.

On the other hand, the thick steel sheets manufactured by the methods according to Examples 12 to 18 which are comparative examples all have a ferrite fraction ratio of more than 0.1 or an anisotropy index of more than 5.0. Or, the Vickers hardness of the surface layer region was more than 250 HV. The thick steel sheets according to Examples 12 to 18, which are comparative examples, were inferior in at least one of SSC resistance and tensile properties.

In the thick steel sheet according to Example 12, since the heating rate was too slow, heat diffused to the internal region of the test steel and the temperature of the internal region of the test steel increased, resulting in a decrease in tensile strength. It was guessed.
In the thick steel sheet according to Example 13, since the surface temperature was set to less than the Ac1 transformation point, it was presumed that the surface layer region could not be sufficiently softened.

In the thick steel sheet according to Example 14, the holding time after the surface temperature was set to the Ac1 transformation point or higher was too long, so that the heat diffused to the internal region of the test steel and the temperature of the internal region of the test steel rose. As a result, it was speculated that the tensile strength decreased.
In the thick steel sheet according to Example 15, the holding time after the surface temperature was set to the Ac1 transformation point or higher was too short, so that the reverse transformation of the surface layer could not be sufficiently performed and the surface layer could not be sufficiently softened. It was guessed.

In the thick steel sheet according to Example 16, as a result of the heating start temperature being too high, heat has already diffused to the internal region of the test steel before the temperature rise, and the temperature of the internal region of the test steel rises, resulting in yield strength. Was speculated to have decreased.
In the thick steel sheet according to Example 17, the holding time after the surface temperature was set to the Ac1 transformation point or higher was too long, so that the heat diffused to the internal region of the test steel and the temperature of the internal region of the test steel rose. As a result, it was speculated that the tensile strength decreased.

In the thick steel sheet according to Example 18, since the cooling start temperature was lowered, the ferrite fraction ratio and the anisotropy index were increased, and it is presumed that not only the yield strength was lowered but also the SSC resistance was lowered. rice field.

Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to such examples. It is clear that a person having ordinary knowledge in the field of technology to which the present invention belongs can come up with various modifications or modifications within the scope of the technical ideas described in the claims. , These are also naturally understood to belong to the technical scope of the present invention.

Claims (7)

The first step of hot rolling steel to obtain rolled material,
The second step of accelerating and cooling the rolled material from the temperature above the Ar3 transformation point to the temperature below (Ac1 transformation point -50 ° C), and the second step.
Regarding the surface of the rolled material, the first temperature range of (Ac1 transformation point −50 ° C.) or more and below the Ac1 transformation point is the second temperature range exceeding the Ac1 transformation point at an average temperature rise rate of 2.0 ° C./sec or more. The third step of heating to, holding in the second temperature range for 1 second or more and 600 seconds or less, and cooling.
The thickness is 19 mm or more and 100 mm or less.
The ratio of the central region ferrite surface integral in the central region of -2.5 mm to 2.5 mm in the thickness direction from the center of the plate thickness to the surface integral surface integral in the surface integral region of 0.1 mm or more and 1.0 mm or less from the surface (center). Region ferrite surface integral / surface ferrite surface integral) is 0.10 or less.
When observing the cross sections in the rolling direction and the plate thickness direction, the average value of the ratio of the length in the rolling direction (length in the rolling direction / length in the plate thickness direction) to the length in the plate thickness direction of the ferrite structure in the surface layer region is , 5.0 or less, and a method for producing a thick steel sheet, which obtains a thick steel sheet having a Vickers hardness of 250 Hv or less in the surface layer region. The method for manufacturing a thick steel sheet according to claim 1, wherein the second temperature range is equal to or lower than the Ac3 transformation point. The method for manufacturing a thick steel sheet according to claim 1 or 2, wherein the heating in the third step is performed by induction heating. The method for manufacturing a thick steel sheet according to claim 3, wherein the applied frequency in the induction heating is 2 kHz or more and 200 kHz or less. Any one of claims 1 to 4, comprising a fourth step of heating the thick steel sheet obtained by the method for producing a thick steel sheet according to any one of claims 1 to 4 to a temperature lower than the Ac1 transformation point. The method for manufacturing a thick steel sheet according to the section. The method for producing a thick steel sheet according to any one of claims 1 to 5, wherein in the second step, the rolled material is cooled at an average cooling rate of 5 ° C./sec or more and 60 ° C./sec or less. The thickness is 19 mm or more and 100 mm or less.
The ratio of the central region ferrite surface integral in the central region of -2.5 mm to 2.5 mm in the thickness direction from the center of the plate thickness to the surface integral surface integral in the surface integral region of 0.1 mm or more and 1.0 mm or less from the surface (center). Region ferrite surface integral / surface ferrite surface integral) is 0.10 or less.
When observing the cross section in the rolling direction and the plate thickness direction, the average value of the ratio of the length in the rolling direction (the length in the rolling direction / the length in the plate thickness direction) to the length in the plate thickness direction of the ferrite structure in the surface layer region is , 5.0 or less, and the Vickers hardness in the surface layer region is 250 Hv or less.