JP6736959B2 - Steel plate manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing a steel plate, in particular, tensile strength at 490 N / mm ² grade and 570N / mm ² class, for example, a method for producing a thick steel plate acoustic anisotropy and excellent small ductility.

In recent years, due to the increase in size of structures, the opportunities for using high-strength steel having a tensile strength of 490 N/mm ² or higher are increasing. On the other hand, from the viewpoint of safety, high toughness of the base material and the weld is required. As described above, as a method for increasing the strength and improving the toughness, a method has been proposed in which accelerated cooling is performed using an online water cooling device to control the metallographic structure and make the crystal grains fine (for example, Patent Document 1). 1 and 2).

The methods proposed in Patent Documents 1 and 2 utilize controlled elements such as Nb and Ti to perform controlled rolling and accelerated cooling. However, when the steel sheet is water-cooled after hot rolling, the hardness of the steel sheet may excessively increase and the ductility may decrease depending on the manufacturing conditions. In particular, this tendency becomes stronger when high temperature rolling is performed for the purpose of improving the productivity of rolling or improving the acoustic anisotropy of the steel sheet.

Therefore, application of mild cooling has been attempted and proposed for the purpose of alleviating the tendency that the ductility decreases as described above (see, for example, Patent Document 3). Further, as a method for improving uniform elongation and improving ductility, after performing first forced cooling to a temperature range of 500 to 600° C., cooling is performed at a cooling rate equal to or lower than cooling, and then second forced cooling. Has been proposed to control the metallographic structure (see, for example, Patent Document 4).

Japanese Patent Publication No. 57-21007 Japanese Patent Publication No. 59-14535 JP-A-5-214440 JP-A-7-233414

However, conventionally, it has not been possible to optimally control various characteristics such as strength, ductility, toughness, and acoustic anisotropy while maintaining high productivity. Therefore, various properties, for example, the strength is improved without impairing the productivity, but other properties, for example, ductility and toughness, and the production of thick steel sheet without degrading acoustic anisotropy. A method was needed.

The present invention has been made in view of such circumstances, for example, capable of acoustic anisotropy is small, an excellent ductility, tensile strength efficiently produce 490 N / mm ² grade and 570N / mm ² class steel plate It is an object to provide a method for manufacturing a thick steel plate.

In order to solve the above problems, the present inventors have made detailed studies on the composition of steel and manufacturing conditions, particularly the relationship between the transformation temperature of steel and the temperature history of the surface. As a result, after hot rolling, start controlled cooling, until the temperature of the surface of the steel is lowered to a temperature at which bainite transformation or martensitic transformation occurs, in order to sufficiently generate ferrite, in particular, We have found that it is necessary to control the temperature history of the surface. Here, the controlled cooling is a process including accelerated cooling such as water cooling, which has a faster cooling rate than air cooling, and slow cooling, in which cooling is slower than air cooling, and heating may be performed during the process. Controlled cooling also includes multi-stage cooling in which the cooling rate of accelerated cooling is changed midway, and intermittent cooling in which accelerated cooling is stopped midway, air cooling is performed, and accelerated cooling is restarted.

In the controlled cooling of a steel sheet, after hot rolling, accelerated cooling reduces the temperature of the surface of the steel sheet to a temperature range lower than Ar ₃ (the temperature at which transformation from austenite to ferrite starts during cooling) to generate ferrite. The present inventors have found that the amount of ferrite generated in a steel sheet during controlled cooling is affected by the difference between the surface temperature of the steel sheet and Ar ₃ and the time during which the surface temperature of the steel sheet stays in a temperature range of Ar ₃ or lower. I thought, and examined. As a result, if the amount of ferrite generated on the surface of the steel sheet is controlled before the temperature of the surface decreases below 600° C. at which bainite transformation may occur, the production of bainite is suppressed, and the hardness of the surface of the steel sheet decreases. It was found that the rise in

Then, the cooling condition is set such that the parameter (ferrite formation index) SI indicating the degree of ferrite formation is 0.55 or more from the start of the controlled cooling to the time when the surface temperature of the steel sheet falls below 600°C. It has been found that the control can suppress the increase in hardness of the surface of the steel sheet. The ferrite formation index SI is calculated by the following equations (1) to (3). The larger the value, the larger the amount of ferrite produced, the lower the surface hardness, and the more the ductility and toughness are improved.

The SI is obtained by adding SI ₂ calculated by the following equation (3) in the range of 600°C or more and 650°C or less to SI ₁ calculated by the following equation (2) in the range of the surface temperature of 600°C or more from the start of the controlled cooling. Ask for. Here, SI ₂ calculated by the following equation (3) is a term that takes into account an increase in nucleation frequency due to an increase in ferrite transformation driving force at 650° C. or lower. When the controlled cooling is finished when the surface temperature exceeds 650° C., SI ₁ obtained by the above equation (2) becomes SI as it is.
SI=SI ₁ +SI ₂ (1)
SI ₁ =∫(1/2037)×(Ar ₃ −T)dt (where T≧600° C.) (2)
SI ₂ =∫(2/2037)×(650-T)dt (however, 600° C.≦T≦650° C.) (3)
Here, T in the above equations (2) and (3) is the surface temperature (°C) of the steel.

The present invention has been made on the basis of the above findings, by a higher degree of controlled cooling than conventional, to refine the structure of the steel, and prevent excessive surface hardening, for example, acoustic anisotropy is small, This is a method for efficiently manufacturing a thick steel sheet having excellent ductility.
The gist of the present invention is as follows.

[1]% by mass,
C: 0.03 to 0.25%,
Si: 0.001 to 0.50%,
Mn: 0.5-3.0%
Containing
P: 0.050% or less,
S: 0.050% or less,
Al: 0.05% or less,
N: 0.010% or less,
O: limited to 0.010% or less, after casting the steel consisting of the balance Fe and impurities, and as it is, or once cooled and heated to Ac _{3 to} 1300° C., the end temperature is within the range of Ar _{3 to} 1100° C. After performing hot rolling to
From the temperature where the surface temperature is Ar ₃ or higher, the controlled cooling that makes the average cooling rate in the plate thickness direction 3 to 50° C./s is started,
The controlled cooling is performed so that the ferrite formation index SI calculated from the temperature history of the surface temperature T (° C.) by the following equations (1) to (3) becomes 0.55 to 2.00,
The average temperature in the plate thickness direction is 450 to 675° C., and the surface temperature is 350 to 675° C.
A method for manufacturing a thick steel plate, which is characterized by allowing to cool to room temperature as it is.
SI=SI ₁ +SI ₂ (1)
SI ₁ =∫(1/2037)×(Ar ₃ −T)dt (where T≧600° C.) (2)
SI ₂ =∫(2/2037)×(650-T)dt (however, 600° C.≦T≦650° C.) (3)
[2] The method for producing a thick steel sheet according to [1], wherein the controlled cooling is terminated in an average temperature in the plate thickness direction of 550 to 675°C and a surface temperature of 450 to 675°C.
[3] The method for producing a thick steel sheet according to the above [1], wherein the controlled cooling is terminated in an average temperature in the plate thickness direction of 450 to 600°C and a surface temperature of 350 to 600°C.

[4] Further, the steel is in a mass%,
Cu: 2.0% or less,
Ni: 3.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
W: 2.0% or less,
V: 0.30% or less,
Nb: 0.05% or less,
Ti: 0.05% or less,
B: 0.002% or less,
1 or 2 or more are contained, The manufacturing method of the thick steel plate as described in any one of said [1]-[3] characterized by the above-mentioned.
[5] Further, the steel is in a mass%,
REM: 0.10% or less,
Mg: 0.02% or less,
Ca: 0.02% or less,
Zr: 0.30% or less,
Hf: 0.30% or less,
Ta: 0.30% or less of 1 type or 2 types or more are contained, The manufacturing method of the thick steel plate as described in any one of said [1]-[4] characterized by the above-mentioned.

According to steel plate production method of the present invention, for example, the acoustic anisotropy is small, an excellent ductility, tensile strength is produced with high efficiency 490 N / mm ² grade and 570N / mm ² class steel plate It becomes possible and the industrial contribution is extremely remarkable.

Hereinafter, an embodiment of a method for manufacturing a thick steel plate according to the present invention will be described. It should be noted that this embodiment will be described in detail for better understanding of the gist of the invention, and therefore does not limit the invention unless otherwise specified.

In the method for manufacturing a thick steel plate of the present invention, in particular, the amount of ferrite generated on the surface of the steel plate is controlled by a parameter (ferrite formation index) SI that is calculated by the following equations (1) to (3) and represents the degree of ferrite formation. However, the greatest feature is that the formation of bainite is suppressed and the increase in surface hardness is suppressed.
SI=SI ₁ +SI ₂ (1)
SI ₁ =∫(1/2037)×(Ar ₃ −T)dt (where T≧600° C.) (2)
SI ₂ =∫(2/2037)×(650-T)dt (however, 600° C.≦T≦650° C.) (3)
Here, T in the above equations (2) and (3) is the surface temperature (°C) of the steel.

The conventional controlled rolling and accelerated cooling control heating conditions and rolling conditions of hot rolling and further cooling conditions, and have an effect of refining crystal grains to some extent. Particularly, by combining controlled rolling in which hot rolling is performed at a low temperature with accelerated cooling, it is possible to significantly reduce the size of crystal grains. On the other hand, when performing controlled rolling at a low temperature, it is necessary to wait for the temperature of the steel sheet to drop during hot rolling, which causes a problem of reduced productivity. Further, when hot rolling is performed at a low temperature, the crystal grains are elongated in the rolling direction, causing anisotropy in the properties of the steel sheet, and particularly, the acoustic anisotropy deteriorates.

In order to reduce the anisotropy without impairing the productivity, a method of accelerating and cooling the steel sheet after finishing hot rolling at high temperature can be considered. However, when water cooling is started at a high temperature, the hardness of the steel sheet, especially the hardness of the surface increases, so that the ductility and toughness decrease. The causes are 1) the temperature difference between the surface and the center of the plate thickness, and 2) coarsening of austenite.

Here, the temperature difference between the central portion of the plate thickness and the surface, which is the first cause, is enlarged by starting accelerated cooling from a high temperature. In particular, on the surface of the steel sheet, since the cooling rate is high, the amount of bainite and martensite increases and the hardness rises significantly. Further, the second cause of the coarsening of austenite is that the rolling temperature becomes high when hot rolling is finished at a high temperature. When the rolling temperature is high, the crystal grains of austenite are large and the hardenability is high. Therefore, after accelerated cooling, the hardness of both the central portion and the surface of the steel sheet is likely to increase.

Therefore, in order to improve the ductility and toughness of the steel sheet, a method in which hot rolling is completed at a high temperature and the amount of water cooling is reduced to gradually cool (slow cooling) can be considered. However, if the cooling rate is simply slowed down, the initially aimed effect of refining crystal grains by water cooling cannot be obtained as expected. Therefore, the present inventors have investigated a method of refining the structure of steel and preventing excessive hardening by a higher degree of controlled cooling. As a result, they have found that a method of performing hot rolling at a high temperature and precisely performing highly controlled cooling, that is, gentle cooling or intermittent cooling is optimal.

By performing slow cooling or intermittent cooling after hot rolling at a high temperature, it is possible to avoid a decrease in productivity and the occurrence of acoustic anisotropy due to temperature waiting. Furthermore, it is possible to promote the generation of ferrite in the surface layer and avoid an excessive increase in hardness. At the same time, the cooling rate in the central portion in the plate thickness direction increases, and the ferrite in the plate central portion becomes fine. Such slow cooling and intermittent cooling, near the lower limit temperature of ferrite is formed, the components of the present invention, a tensile strength of 490 N / mm ² class steel to about 550 to 675 ° C. (surface temperature of the steel sheet), tensile strength With 570 N/mm ² grade steel, it is necessary to cool to about 450 to 600° C. (surface temperature of steel sheet). In addition, gentle cooling is a method of cooling at a relatively slow cooling rate from the start of cooling to the stop of cooling. On the other hand, the intermittent cooling is a method of interrupting the cooling once or more after the start of cooling to reheat the temperature of the steel sheet.

(Ferrite formation index SI: 0.55 to 2.00)
In controlled cooling, the amount of ferrite generated on the surface of the steel sheet during cooling is affected by the size of the difference between Ar ₃ and the temperature of the steel sheet and the residence time in the temperature range below Ar ₃ , and therefore the optimum conditions Was examined. As a result, it was found that "SI (ferrite formation index)", which is a parameter indicating the degree of ferrite formation, is important. The SI is dependent on the integral of the time variation of the difference _(Ar 3 -T) between the temperature T of the surface of the Ar ₃ and the steel sheet. At 650°C or lower, it is necessary to consider the increase in the nucleation frequency due to the increase in the ferrite transformation driving force, and the increment depends on the integral of the time change of the difference (650-T) between 650°C and the surface temperature T of the steel sheet. .. The ferrite formation index SI is set to 0.55 or more in order to increase the amount of ferrite generated on the surface of the steel sheet, suppress an increase in surface hardness, and improve ductility and toughness. It is preferable that SI be 1.00 or more so that the amount of ferrite increases as SI increases and the surface becomes almost ferrite. On the other hand, if the upper limit of the ferrite formation index SI is more than 2, ferrite may grow and the crystal grains may be coarsened to lower the toughness and ductility, so it is set to 2.00 or less.

Below, the technical significance and calculation method of the ferrite formation index SI will be described.

The amount of ferrite generated on the surface of the steel sheet by the controlled cooling after hot rolling is affected by the difference between Ar ₃ and the surface temperature of the steel sheet and the time during which the surface temperature stays in the temperature range of Ar ₃ or lower. However, if the temperature is lower than 600° C., bainite transformation may occur, so that controlled cooling in the temperature range of 600° C. or higher from the start of controlled cooling is important. That is, the ferrite formation index depends on the integrated value of the time change of (Ar ₃ −T), and in the case of the steel having the composition of the present invention, the surface temperature T of the steel sheet is in the range of 600° C. or higher from the start of controlled cooling. Then, the ferrite formation index SI ₁ is calculated by the following equation (2). Here, 1/2037 is a coefficient obtained by an experiment using steel having the composition of the present invention.
SI ₁ =∫(1/2037)×(Ar ₃ −T)dt (where T≧600° C.) (2)

Further, in the case of the steel having the composition of the present invention, when the temperature of the surface of the steel sheet becomes 650° C. or lower, the frequency of nucleation increases due to the increase in ferrite driving force, and the generation of ferrite is promoted. The increase in the amount of ferrite generated due to the increase in the frequency of nucleation of ferrite transformation is affected by the difference between 650°C and the surface temperature of the steel sheet and the time during which the surface temperature stays in the temperature range of 650°C or lower. This increase in the amount of ferrite produced depends on the integral value of the time change of (650° C.−T), and in the case of the steel having the composition of the present invention, the surface temperature T of the steel sheet is 600 to 650° C. from the start of controlled cooling. Within the range, the ferrite formation index SI ₂ that represents the decrease in the amount of ferrite formation is calculated by the following equation (3). Here, 2/2037 is a coefficient obtained by an experiment using the steel having the composition of the present invention.
SI ₂ =∫(2/2037)×(650-T)dt (however, 600° C.≦T≦650° C.) (3)

As described above, ferrite the sum of the SI ₁ and SI ₂ calculated by the above equation (3) calculated by the equation (2) is, when performing the controlled cooling, was obtained from temperature history of the surface of the steel plate during cooling It is an index SI (Equation (1) below). SI can be calculated from the time change of the temperature difference of Ar ₃ by measuring the temperature of the surface of the steel sheet. For example, the temperature difference of Ar ₃ may be plotted against time, and the numerical value may be used to obtain the SI as the area under the curve showing the temperature history. Therefore, SI is not limited to the case where accelerated cooling is performed at a constant cooling rate (slow cooling), but also when the cooling rate is changed midway (multistage cooling) or when accelerated cooling is stopped midway (intermittent cooling). Can be applied.
SI=SI ₁ +SI ₂ (1)

Further, approximately, when the cooling rate is constant, Tdt is (1/CR)dT. When the surface temperature T of the steel sheet is higher than 650° C., SI can be calculated by the equation (4) using the cooling rate CR (° C./s) in the temperature range in which the cooling rate is constant. On the other hand, when the surface temperature T of the steel sheet is 600 to 650° C., SI can be calculated by the equation (5). When performing complex control cooling such that the cooling rate changes a plurality of times, SI is obtained from the equations (4) and (5) according to the surface temperature of the steel plate within a constant temperature range of the cooling rate. , All can be summed up. Further, in the intermittent cooling, the SI while the accelerated cooling is stopped may be calculated on the assumption that the recuperation temperature is reached immediately after the accelerated cooling is stopped.
SI=[{(1/2037)×(Ar ₃ −T ₂ ) ² }/CR]/2−[{(1/2037)×(Ar ₃ −T ₁ ) ² }/CR]/2 (however, T>650°C) (4)
SI=[{(1/2037)×(Ar ₃ −T ₂ ) ² }/CR]/2−[{(1/2037)×(Ar ₃ −T ₁ ) ² }/CR]/2+{(1 ^{_{/ 2037) × (650-T}} 2) 2} / CR - {(1/2037) × (650-T 1) 2} / CR ( although, 600 ℃ ≦ T ≦ 650 ℃ ) ··· (5)
Here, T ₁ is the surface temperature (° C.) of the steel when the cooling rate starts to change, and T ₂ is the surface temperature (° C.) of the steel when the cooling rate is further changed from the cooling rate of T _1. Yes (T ₁ >T ₂ ). CR (° C./s) is a cooling rate that is considered to be approximately constant, and is calculated by dividing the temperature difference between T ₁ and T ₂ by the time required for cooling. If 650°C is included in the temperature range where the cooling rate is constant, the temperature range is divided into 650°C and 650°C or less, and SI of 650°C or more is calculated by the formula (4). It suffices to obtain it by the formula (5).

Next, the chemical composition of the steel of the present invention will be described. In the following description, mass% in the composition will be simply described as %.

(C: 0.03 to 0.25%)
C is an element that enhances the hardenability of steel materials and forms carbides. In the present invention, the lower limit of the amount of C is set to 0.03% or more in order to improve the strength of the steel material. However, when C is excessively contained, the amount of hard second phase structure such as pearlite, martensite, and cementite formed increases, and the ductility and toughness of the steel material deteriorate. Therefore, the upper limit of the C content is 0.25% or less. Further, if the amount of C is large, the weldability of the steel material and the toughness of the welded portion may deteriorate, so it is more preferable to set the upper limit of the C content to 0.15% or less.

(Si: 0.001 to 0.50%)
Si is a deoxidizing element of the steel material, and is added for the purpose of reducing the oxygen concentration of the steel material, and the Si content is set to 0.001% or more in order to obtain the effect. However, if the Si content exceeds 0.50%, a high carbon martensite-austenite (MA) mixture or the like is generated in the steel, which deteriorates the toughness. Therefore, the Si content is limited to 0.50% or less. Further, if the Si content is large, the scale generation behavior may change and the surface properties of the steel may deteriorate, so the upper limit of the Si content is more preferably 0.30% or less. On the other hand, since Si contributes to the increase in strength as a solid solution strengthening element, the lower limit of the Si content is preferably 0.01% or more.

(Mn: 0.5-3.0%)
Mn is an element that enhances the hardenability of steel and contributes to the strength improvement. In the present invention, the Mn content is 0.5% or more in order to increase the strength of the steel. Moreover, the more preferable lower limit of the Mn content is 1.0% or more. On the other hand, when the Mn content exceeds 3.0%, the microsegregation generated during solidification becomes remarkable. In the steel material, in a portion where Mn is concentrated more than the addition amount, the hardenability is high and the MA mixture that deteriorates the toughness of the welded portion is easily generated. Therefore, in the present invention, the upper limit of the Mn content is 3.0% or less from the viewpoint of avoiding the formation of the M-A mixture. Moreover, the more preferable upper limit of the Mn content is 2.5% or less.

(P: 0.050% or less)
P is an impurity and mainly segregates at grain boundaries to reduce the toughness of thick steel plates. Therefore, in the present invention, the upper limit of the P content is limited to 0.050% or less. In order to improve the toughness of steel, it is more preferable to limit the upper limit of the amount of P to 0.020% or less. The lower limit of the amount of P is not particularly limited, but 0.001% or more may be contained in order to avoid an increase in manufacturing cost.

(S: 0.050% or less)
S is an impurity and combines with Mn in steel to form MnS. If the S content exceeds 0.050%, MnS becomes coarse and the hot workability is impaired, so the upper limit is limited to 0.050% or less. The more preferable upper limit of the S content is 0.010% or less, and more preferably 0.005% or less. Since fine MnS dispersed in steel is effective in suppressing the growth of crystal grains and acts as a nuclei for ferrite transformation, it is preferable to contain 0.0005% or more.

(Al: 0.05% or less)
Al is a deoxidizing element, is added for the purpose of reducing the oxygen concentration, and the Al content is set to 0.001% or more in order to obtain the effect. However, if the Al content exceeds 0.05%, the toughness and surface properties of the thick steel plate deteriorate, so the upper limit is made 0.05% or less. In order to suppress the formation of coarse inclusions, the upper limit of the Al content is more preferably 0.03% or less. The lower limit of the Al content is not particularly limited, but 0.01% or more is preferably added to form a nitride that contributes to the refinement of crystal grains.

(N: 0.010% or less)
N is an impurity, and if contained in excess, the toughness decreases due to the solid solution N, so the upper limit of the N content is limited to 0.010% or less. In addition, N forms a nitride with Ti, Nb, V, Al, Zr, Ta, Hf, etc., and acts effectively for refining the crystal grains, so it is preferable to contain 0.0003% or more. Further, in order to suppress the coarsening of the nitride and ensure the toughness, it is preferable to limit the upper limit of the N content to 0.005% or less.

(O: 0.010% or less)
O (oxygen) is an impurity, and the upper limit of the amount of O is limited to 0.010% or less in order to prevent coarsening of inclusions. Further, the more preferable upper limit of the O content is 0.002% or less. Further, when oxides of Ti, Al, Mn, etc. are finely dispersed in the steel, the growth of crystal grains is suppressed during reheating in hot rolling, and ferrite transformation within the grains occurs during cooling after hot rolling. Facilitate. In order to refine the crystal grains of the thick steel plate and improve the strength and toughness of the steel, the lower limit of the O content is preferably 0.0001% or more.

The steel used in the present invention is further selected from the group consisting of Cu, Ni, Cr, Mo, W, Ti, Nb, V and B, which improve the strength and strength of the thick steel plate, in addition to the above respective components. One or more elements can be added.

(Cu: 2.0% or less)
Cu is effective for improving the hardenability and is also an element for improving the strength of thick steel plates by solid solution strengthening and precipitation strengthening. When Cu is added and heat treatment is applied, fine metallic Cu is deposited, which also contributes to miniaturization of crystal grains and suppression of deterioration of ductility. In order to increase the strength of the thick steel plate, the lower limit of the Cu content is preferably 0.001% or more. On the other hand, when the Cu content is excessive, precipitation strengthening becomes remarkable, and therefore, in order to secure the toughness of the thick steel sheet, it is preferable to set the upper limit of the Cu content to 2.0% or less. Further, if Cu precipitates on the grain boundaries during casting, internal cracking is likely to occur, and surface defects of the steel slab and the steel plate are likely to occur. Therefore, the upper limit of the Cu content is more preferably 0.70% or less. Further, in order to suppress deterioration of hot workability and the like, it is more preferable to set the upper limit of the Cu amount to 0.50% or less.

(Ni: 3.0% or less)
Ni is a useful element that enhances hardenability and improves strength without particularly lowering toughness. In order to increase the strength and toughness of the thick steel plate, the lower limit of the Ni content is preferably 0.001% or more. On the other hand, even if Ni is contained in an amount exceeding 3.0%, the effect is saturated, so the upper limit of the Ni amount is preferably 3.0% or less. From the viewpoint of alloy cost, the upper limit of the amount of Ni is more preferably 1.50% or less, further preferably 0.50% or less.

(Cr: 1.0% or less)
Cr is an element that improves hardenability, and also produces carbides and nitrides, which also contributes to precipitation strengthening. In order to increase the strength of the thick steel plate, the lower limit of the Cr content is preferably 0.001% or more. On the other hand, if the Cr content exceeds 1.0%, the toughness may decrease, so the upper limit is preferably made 1.0% or less. Moreover, the more preferable upper limit of the amount of Cr is 0.50% or less.

(Mo: 1.0% or less)
Mo is an element that improves hardenability. Further, Mo is a useful solid solution strengthening element and forms carbide to contribute to precipitation strengthening. In order to increase the strength of the thick steel plate, the Mo content is preferably 0.001% or more. On the other hand, if the Mo content exceeds 1.0%, the alloy cost increases, so the upper limit is preferably made 1.0% or less. The more preferable upper limit of the amount of Mo is 0.30% or less.

(W: 2.0% or less)
W, like Mo, is an element that contributes to the improvement of hardenability, solid solution strengthening, and precipitation strengthening. In order to improve the strength of the thick steel plate, the W content is preferably 0.001% or more. If it is less than this content, it cannot contribute to precipitation strengthening and sufficient strength cannot be secured. On the other hand, if the W content exceeds 2.0%, the alloy cost increases, so the upper limit is preferably made 2.0% or less. The more preferable upper limit of the amount of W is 0.30% or less.

(Ti: 0.05% or less)
Ti is an element that forms carbides and nitrides. In order to make the crystal grains finer by utilizing these precipitates, it is preferable to add 0.0001% or more of Ti. On the other hand, if the Ti content exceeds 0.05%, the toughness of the thick steel plate may decrease, so the upper limit is preferably 0.05% or less. The Ti content is more preferably 0.03% or less, still more preferably 0.02% or less.

(Nb: 0.05% or less)
Nb, like Ti, is an element that forms carbides and nitrides. In order to make the crystal grains finer by utilizing these precipitates, it is preferable to add 0.0001% or more of Nb. On the other hand, if the Nb content exceeds 0.05%, the toughness of the thick steel plate may decrease, so the upper limit is preferably 0.05% or less. The Nb content is more preferably 0.03% or less, still more preferably 0.02% or less.

(V: 0.30% or less)
V is an element that forms carbides and nitrides like Ti and Nb, and it is preferable to add 0.0001% or more of V in order to refine the crystal grains. On the other hand, if the V content exceeds 0.30%, the toughness of the thick steel plate may decrease, so the upper limit is preferably 0.30% or less. The V content is more preferably 0.10% or less, still more preferably 0.05% or less.

(B: 0.002% or less)
B is an element that remarkably improves the hardenability of thick steel plates by adding a trace amount. In order to increase the strength of the thick steel plate, addition of 0.0001% or more is preferable. The B content is more preferably 0.0003% or less. On the other hand, if the B content exceeds 0.002%, the hardenability may be excessively improved and the toughness may be impaired. Therefore, the upper limit of the B content is preferably 0.002% or less.

Further, in the steel used in the present invention, one or more elements selected from the group consisting of Zr, Hf, Ta, REM, Mg and Ca may be added to the above-mentioned components. ..

(Zr: 0.30% or less)
Zr is an element that forms carbides and nitrides, and is also effective as a deoxidizing agent. In order to refine the crystal grains of the thick steel plate, it is preferable to add 0.0001% or more of Zr. On the other hand, if the Zr content exceeds 0.30%, the toughness and surface properties of the thick steel plate may deteriorate. Therefore, the upper limit of the Zr content is preferably 0.30% or less. The Zr content is more preferably 0.10 or less, still more preferably 0.05% or less.

(Hf: 0.30% or less)
Hf, like Zr, is an element that forms carbides and nitrides, and is also effective as a deoxidizing agent. In order to refine the crystal grains of the thick steel plate, it is preferable to add 0.0001% or more of Hf. On the other hand, if the Hf content exceeds 0.30%, the toughness and surface properties of the thick steel plate may deteriorate, so the upper limit is preferably made 0.30% or less. The Hf content is more preferably 0.10% or less, still more preferably 0.05% or less.

(Ta: 0.30% or less)
Ta, like Zr and Hf, is an element that forms carbides and nitrides, and is also effective as a deoxidizing agent. In order to make the crystal grains of the thick steel plate finer, it is preferable to add 0.0001% or more of Ta. On the other hand, if the Ta content exceeds 0.30%, the toughness and surface properties of the thick steel sheet may deteriorate, so the upper limit is preferably 0.30% or less. The Ta content is more preferably 0.10% or less, still more preferably 0.05% or less.

(REM: 0.10% or less)
REM is an element effective in controlling the form of inclusions such as sulfides, and contributes to the improvement of hot workability. In addition, fine oxides are generated, which is effective in preventing the coarsening of the crystal grains and the grain size of HAZ. In order to improve the toughness of the thick steel plate, it is preferable to add 0.0001% or more of REM. On the other hand, if REM is excessively added, inclusions may be coarsened and toughness may be impaired. Therefore, the content of REM is preferably 0.10% or less. The REM content is more preferably 0.05% or less, still more preferably 0.02% or less.

(Mg: 0.02% or less)
Like REM, Mg is an element effective in controlling the form of inclusions such as sulfides. In particular, Mg produces a fine oxide and, in addition to the pinning effect, promotes ferrite transformation within the grains, and is therefore extremely effective in preventing the coarsening of the grain size of crystal grains and HAZ. In order to improve the toughness of the thick steel plate, it is preferable to add 0.0001% or more of Mg. On the other hand, if Mg is added excessively, inclusions may be coarsened and the toughness may be impaired, so the upper limit of the content is preferably made 0.02% or less. The Mg content is more preferably 0.01% or less.

(Ca: 0.02% or less)
Like Mg and REM, Ca is an element effective in controlling the form of inclusions such as sulfides. In particular, Ca suppresses the formation of coarse MnS and is effective in controlling the form of sulfide. On the other hand, if Ca is added excessively, inclusions may be coarsened and the toughness may be impaired, so the upper limit of the content is preferably made 0.02% or less. The Ca content is more preferably 0.01% or less.

The components other than the above components of the steel used in the present invention are Fe and impurities. Here, impurities are components that are mixed by various factors of the manufacturing process, including raw materials such as ores and scraps when industrially manufacturing thick steel plates, and adversely affect the present invention. It means something that is acceptable within the range. However, in the present invention, it is necessary to define the upper limit of P, S, N, and O among the impurities as described above.

Next, the manufacturing process and manufacturing conditions of the thick steel plate of the present invention will be described.

The thick steel plate of the present invention is manufactured by melting steel, adjusting the composition, casting, hot rolling, and controlled cooling. Steel may be melted, and the billet obtained by casting may be hot-rolled as it is without cooling to room temperature. After casting, the billet is once cooled to Ar ₃ or less, and further to room temperature, It may be hot-rolled by reheating to a temperature higher than Ac ₃ (the temperature at which the transformation of ferrite to austenite starts during heating). In order to make the structure of the steel slab fine, it is preferable to cool to room temperature and reheat after casting. Ac ₃ and Ar ₃ can be determined by measuring the temperature at which the expansion and contraction of the volume accompanying the transformation by heating and cooling occur by a thermal expansion test.

After hot rolling, controlled cooling is started from a surface temperature of Ar ₃ or higher. Controlled cooling is preferable as the cooling rate is faster, and water cooling is desirable. Generally, the metal structure of steel changes to relatively coarse-grained ferrite, fine-grained ferrite, bainite, and martensite as the cooling rate increases due to cooling from fan cooling, fan cooling, mist cooling, water cooling, and the like. In the control cooling, in order to adjust the SI, the cooling speed may be controlled to perform one intermediate speed cooling, or intermittent cooling may be adopted in which the cooling is temporarily stopped and the surface temperature is reheated. .. In the intermittent cooling, cooling may be stopped twice or more.

(Heating temperature for hot rolling: Ac _{3 to} 1300° C.)
When the temperature of the steel slab is cooled to Ar ₃ or lower, the heating temperature needs to be Ac ₃ or higher in order to make the metal structure of the steel slab into an austenite single phase. In order to improve the acoustic anisotropy, it is preferable to finish the hot rolling at a high temperature, and it is preferable to set the heating temperature of the hot rolling to 900° C. or higher. Furthermore, in order to raise the end temperature of hot rolling, it is more preferable to set the heating temperature of hot rolling to 950° C. or higher. On the other hand, when the heating temperature of hot rolling exceeds 1300°C, the structure of the steel slab becomes remarkably coarse and it becomes difficult to refine the crystal grains of the steel sheet. Therefore, the upper limit of the heating temperature of hot rolling is set to 1300°C or less. To do. The heating temperature is the temperature in the heating furnace, and the center of the steel slab is heated to this temperature.

(Temperature of hot rolling (surface temperature): Ar _{3 to} 1100° C.)
The lower limit of the hot rolling finish temperature is set to be Ar ₃ or higher in order to finish the hot rolling at a temperature at which the metal structure is an austenite single phase. The lower limit of the hot rolling finish temperature is more preferably 900°C or higher. This is to avoid the controlled rolling at low temperature that lowers the productivity of rolling and suppress the development of rolling anisotropy that impairs the acoustic anisotropy of the steel sheet. Further, from the viewpoint of anisotropy, it is preferable to finish the rolling at the unrecrystallized temperature of austenite or higher. In order to avoid rolling in an unrecrystallized state, it is more preferable to finish hot rolling at 950°C or higher. On the other hand, the lower the rolling temperature is, the better from the viewpoint of grain refinement. Therefore, the upper limit of the hot rolling finish temperature is set to 1100° C. or lower. A more preferable upper limit of the hot rolling finish temperature is 1000°C or lower.

(Control cooling start temperature (surface temperature): Ar ₃ or more)
If controlled cooling is started when the surface temperature is less than Ar ₃ , coarse ferrite is generated before the start of cooling, the strength of the steel sheet is reduced, and the toughness is also deteriorated. Therefore, the controlled cooling start temperature is set to Ar ₃ or higher at the surface temperature. To do. The upper limit of the controlled cooling start temperature is not particularly limited, and the controlled cooling may be started immediately after the end of hot rolling. However, if the controlled cooling is started at a high temperature, the temperature difference between the central portion of the plate thickness and the surface of the steel plate becomes large when the controlled cooling is stopped. Therefore, in order to maintain the surface temperature at 600° C. or higher to generate ferrite and lower the end temperature at the center of the plate thickness to secure the strength, it is preferable to start the controlled cooling at 1000° C. or lower. ..

(Average cooling rate in plate thickness direction: 3 to 50°C/s)
In the present invention, controlled cooling is performed in order to refine the crystal grains of the steel sheet after hot rolling. Among the controlled cooling, the accelerated cooling is usually performed by water cooling, but the lower limit is 3° C./s or more as the average cooling rate in the plate thickness direction that can obtain a sufficient grain refining effect. Further, the lower limit of the average cooling rate of the controlled cooling in the plate thickness direction is more preferably 5° C./s or more in order to stably generate finer ferrite. More preferably, it is set to 15° C./s or more. In the present invention, since the ferrite transformation occurs even after the controlled cooling is finished and during the cooling, the upper limit of the average cooling rate in the plate thickness direction of the controlled cooling is 50° C./s or less as a level that can be realized on an industrial scale. And The upper limit of the average cooling rate of the controlled cooling in the plate thickness direction may be 30°C/s or less. The average cooling rate in the plate thickness direction is calculated from the change in average temperature in the plate thickness direction and the required time. The average temperature in the plate thickness direction (the average temperature from the surface of the steel plate to the center of the plate thickness) is obtained by simulation calculation based on the temperature of the surface of the steel plate. That is, the average temperature is obtained from the temperature distribution in the plate thickness direction at the start of controlled cooling, the average temperature is obtained from the temperature distribution in the plate thickness direction at the end of controlled cooling, and the temperature difference is divided by the required time. Ask for.

In order to suppress hardening of the surface of the steel sheet due to controlled cooling, the cooling rate of the surface of the steel sheet may be controlled. In particular, the accelerated cooling rate (however, approximately constant cooling rate) of the surface immediately before the termination of the controlled cooling is set to 3° C./s or more from the viewpoint of controlling ferrite generation and grain refinement. preferable. More preferably, it is 5° C./s or more. Moreover, the cooling rate of the surface of the accelerated cooling immediately before the termination of the controlled cooling does not exceed 200° C./s even at the highest level that can be realized on an industrial scale. Here, accelerated cooling is cooling that has a higher cooling rate than air cooling, and is cooling such as fan cooling, mist cooling, and water cooling described above.

(End temperature of controlled cooling (average temperature in the plate thickness direction): 450 to 675°C)
If the average cooling end temperature in the plate thickness direction exceeds 675° C., coarse ferrite increases and the strength and toughness of the steel plate decrease. Preferably, the average cooling end temperature in the plate thickness direction is 600° C. or lower. In order to increase the strength, it is preferable that the average cooling end temperature in the plate thickness direction is low. On the other hand, when the average cooling end temperature in the plate thickness direction is less than 450° C., the ductility and toughness decrease due to an increase in the amount of bainite and the formation of martensite. In order to improve ductility and toughness, it is preferable that the average cooling end temperature in the plate thickness direction is high.

The appropriate range of the average cooling end temperature in the plate thickness direction differs depending on the level of tensile strength. In the case of a steel sheet having a tensile strength of 490 N/mm ² , it is preferable that the average cooling end temperature in the sheet thickness direction be 550° C. or higher so that the strength does not become excessively high. In the case of a steel sheet having a tensile strength of 570 N/mm ² , the average cooling end temperature in the sheet thickness direction is preferably 600° C. or less, more preferably less than 550° C. in order to secure ductility and toughness.

(Control cooling end temperature (surface temperature): 350 to 675°C)
The surface cooling end temperature is set to 675° C. or lower in order to generate fine ferrite. In order to increase the strength, it is preferable to lower the cooling end temperature of the surface so that proper bainite is generated. On the other hand, when the cooling end temperature of the surface is less than 350° C., the amount of bainite is increased and the surface of the steel sheet is hardened due to the formation of martensite, and the ductility and toughness are reduced.

The appropriate range of the surface cooling end temperature also differs depending on the level of tensile strength. In the case of a steel sheet having a tensile strength of 490 N/mm ² , it is preferable that the surface cooling end temperature be 450° C. or higher so that the strength does not become excessively high. In the case of a steel sheet having a tensile strength of 570 N/mm ² , the surface cooling end temperature is preferably 600° C. or lower, more preferably 575° C. or lower in order to secure ductility and toughness.

Hereinafter, the present invention will be described more specifically by way of examples of the method for producing a thick steel sheet having low acoustic anisotropy and excellent ductility according to the present invention, but the present invention is not limited to the following examples. However, the present invention can be implemented with appropriate modifications within a range that is compatible with the gist of the above and below, and all of them are included in the technical scope of the present invention.

Steel having the composition shown in Table 1 was melted and cast by a conventional method, the obtained steel slab was reheated, hot-rolled and controlled-cooled, and then air-cooled to room temperature to produce a steel sheet. Hot rolling and controlled cooling were performed under the conditions shown in Table 2, Table 4, Table 5, Table 8, Table 9, Table 11 and Table 12. Table 2 is an example in which the controlled cooling is terminated without stopping the accelerated cooling (water cooling), and the other is an example in which the accelerated cooling (water cooling) is stopped once or more in the middle.

The tensile properties (yield stress YS, tensile strength TS, uniform elongation uEL, total elongation EL) of the obtained steel sheets were evaluated according to JIS Z 2241 using No. 5 test pieces taken from each steel sheet. In addition, a V-notch test piece was sampled from each steel sheet, a Charpy test was performed according to JIS Z 2242, and toughness (ductile-brittle fracture surface transition temperature vTrs and absorbed energy vE _{0 at} 0° C.) was evaluated. Further, the toughness of the weld heat affected zone (absorption energy vE- ₄₀ at -40°C of Charpy test) when submerged arc welding was performed with the welding heat input equivalent to 7 KJ/mm was evaluated according to JIS Z3128.

As for the hardness distribution of the base material in the plate thickness direction, the hardness of 1 mm below the surface and the hardness difference ΔHv at the center part in the plate thickness direction were measured with a 10 kg Vickers hardness meter. Furthermore, acoustic anisotropy (sound velocity ratio in the rolling direction and the width direction) was also measured using an ultrasonic measuring device.

The results are shown in Table 3, Table 6, Table 7, Table 10 and Table 13.

As is clear from Tables 1 to 13, the thick steel plate of the present invention produced within the scope of the present invention is excellent in both the strength and toughness of the base metal and the toughness of the weld heat affected zone. On the other hand, the thick steel plate of the comparative example manufactured outside the range of each component and conditions specified in the present invention resulted in a decrease in strength, ductility, toughness, or acoustic anisotropy. From these results, the above-mentioned findings can be confirmed, and the above-mentioned grounds for the limitation of each steel component can be supported. Therefore, it is clear that the method for producing a thick steel sheet according to the present invention enables highly efficient production of a thick steel sheet having a small acoustic anisotropy, excellent ductility, and a tensile strength of 490 N/mm ² grade or higher. Is.

Claims (5)

In mass %,
C: 0.03 to 0.25%,
Si: 0.001 to 0.50%,
Mn: 0.5-3.0%
Containing
P: 0.050% or less,
S: 0.050% or less,
Al: 0.05% or less,
N: 0.010% or less,
O: limited to 0.010% or less, after casting the steel consisting of the balance Fe and impurities, and as it is, or once cooled and heated to Ac _{3 to} 1300° C., the end temperature is within the range of Ar _{3 to} 1100° C. After performing hot rolling to
From the temperature where the surface temperature is Ar ₃ or higher, the controlled cooling that makes the average cooling rate in the plate thickness direction 3 to 50° C./s is started,
The controlled cooling is performed so that the ferrite formation index SI calculated from the temperature history of the surface temperature T (° C.) by the following equations (1) to (3) becomes 0.55 to 2.00,
The average temperature in the plate thickness direction is 450 to 675° C., and the surface temperature is 350 to 675° C.
A method for manufacturing a thick steel plate, which is characterized by allowing to cool to room temperature as it is.
SI=SI ₁ +SI ₂ (1)
SI ₁ =∫(1/2037)×(Ar ₃ −T)dt (where T≧600° C.) (2)
SI ₂ =∫(2/2037)×(650-T)dt (however, 600° C.≦T≦650° C.) (3) The method for manufacturing a thick steel sheet according to claim 1, wherein the controlled cooling is terminated in an average temperature in the plate thickness direction of 550 to 675°C and a surface temperature of 450 to 675°C. The method for producing a thick steel sheet according to claim 1, wherein the controlled cooling is finished in an average temperature in the plate thickness direction of 450 to 600°C and a surface temperature of 350 to 600°C. Further, the steel, in% by mass,
Cu: 2.0% or less,
Ni: 3.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
W: 2.0% or less,
V: 0.30% or less,
Nb: 0.05% or less,
Ti: 0.05% or less,
B: 0.002% or less,
1 type or 2 types or more of these are contained, The manufacturing method of the thick steel plate in any one of Claims 1-3 characterized by the above-mentioned. Further, the steel, in% by mass,
REM: 0.10% or less,
Mg: 0.02% or less,
Ca: 0.02% or less,
Zr: 0.30% or less,
Hf: 0.30% or less,
Ta: 0.30% or less of 1 type or 2 or more types are contained, The manufacturing method of the thick steel plate in any one of Claims 1-4 characterized by the above-mentioned.