JP5487683B2 - High-toughness high-tensile steel plate with excellent strength-elongation balance and method for producing the same - Google Patents

Description

  The present invention relates to a high-tensile steel plate suitable for use in steel structures such as bridges, storage tanks, pressure vessels, and line pipes, and a method for producing the same, and particularly provides a yield strength of 480 MPa or more and excellent low-temperature toughness. Thus, the strength-elongation balance is advantageously improved.

Steel sheets used for steel structures such as bridges, storage tanks, pressure vessels and line pipes are of course high in strength and excellent in toughness, but in addition to these, high ductility is required from the viewpoint of earthquake resistance. It is done. In general, steel materials for construction increase plastic deformability by reducing the yield ratio to ensure earthquake resistance.
However, these steel materials achieve a low yield ratio by introducing a soft ferrite structure into a martensite- or bainite-based structure by means such as two-phase quenching and making it a micro-homogeneous structure. Therefore, it is difficult to balance the yield strength required for structures with high loads, which leads to the early occurrence of steel yielding, and requires a complicated heat treatment process. Therefore, it was not necessarily suitable as a practical mass-produced product.

  On the other hand, it is known that the higher the uniform elongation, the better the earthquake resistance, and the line pipe or the like is required to have a large total elongation (uniform elongation + local elongation). This means that the amount of deformation from the start of deformation due to external stress to the time of destruction is large, which is an indicator of safety for steel materials.

  As a means for increasing the total elongation, it is conceivable to improve one or both of uniform elongation and local elongation. The longer the gauge distance of the tensile test piece, the larger the ratio of uniform elongation to the total elongation, but within the range of commonly used tensile test pieces, the percentage of local elongation is also about 40-50%. In many cases, in order to increase the total elongation, it is not preferable that either the uniform elongation or the local elongation is reduced as a result.

In general, it is considered that multiphase organization is effective for improving elongation (including uniform elongation). Examples thereof include Patent Document 1 and Patent Document 2.
Patent Document 1 discloses a method of forming a ferrite + martensitic structure by controlling cooling in a two-phase region after rolling is completed in the austenite recrystallization temperature region.
However, in this method, although the uniform elongation is improved, the ferrite grains are coarsened, so that the low temperature toughness is not good. Further, since the microstructure is non-uniform, there is a risk that the local elongation is significantly reduced.

  Patent Document 2 discloses a technique for improving elongation by generating retained austenite. For thin steel sheets, TRIP steel and the like that generate retained austenite have been put into practical use, but there has been no practical application in the field of thick steel sheets. The reason is that the alloy component cost is high and it is difficult to achieve compatibility with weldability.

On the other hand, it has been reported that elongation is improved by utilizing Cu precipitation. This is considered to be because the use of soft reinforcing particles suppresses microscopic non-uniform deformation because the plastic deformability of the reinforcing particles themselves is high. For example, Patent Document 3 discloses the technique.
However, in order to develop Cu precipitation strengthening, approximately 1% or more of Cu needs to be added. Therefore, the feasibility as a practical steel is low from the viewpoint of manufacturing cost and stability of characteristics.

Japanese Patent No. 3345501 JP 2006-131958 A Japanese Patent No. 3694383

As described above, the conventional techniques have left problems such as a decrease in productivity, an increase in manufacturing cost, and a decrease in weldability and toughness.
The present invention was developed in view of the above-mentioned present situation, and by individually controlling the structure of the center portion and the surface layer portion of the steel sheet, without causing a decrease in productivity and an increase in manufacturing cost, 480 MPa We propose a high-toughness, high-tensile steel sheet that has the above-mentioned yield strength and excellent low-temperature toughness, as well as excellent strength-elongation balance and uniform hardness distribution in the thickness direction, along with its advantageous manufacturing method. Objective.

Now, the inventors have conducted extensive research on a method for improving the total elongation of a full-thickness tensile specimen while ensuring excellent low-temperature toughness and yield strength of 480 MPa or more.
As a result, the factors governing the total elongation of the full-thickness tensile specimen include the microstructure of the center of the plate thickness and the front and back layers, the hardness of the front and back layers, and the uniformity of the material within the specimen. The knowledge that the intended purpose is achieved advantageously by controlling appropriately was acquired.

That is,
(1) Elongation improves by adopting ferrite + bainite structure.
(2) Processed ferrite introduced into the front and back layers by rolling is disadvantageous for elongation.
(3) By controlling the cooling start temperature and cooling stop temperature in the TMCP process, it is clear that the center of the plate thickness can be made into a single-phase structure, which can suppress the generation of microvoids that cause ductility reduction. did.
Also,
(4) It was found that a relatively high yield strength can be obtained by controlling the microstructure.
further,
(5) Elongation is improved by applying a tempering treatment that preferentially heats only the surface layer.
(6) Moreover, it discovered that the surface hardness in a steel plate reduced and elongation improved by heating a surface layer.
And such steel is
(7) It has been found that high efficiency can be obtained by building in a series of processes using accelerated cooling and heating equipment arranged on the line.
The present invention has been completed based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. In mass%, C: 0.03-0.18%, Si: 0.01-0.55%, Mn: 0.5-2.0%, Al: 0.005-0.1% and N: 0.0005-0.005%, the balance being Fe and inevitable impurities The composition is a mixed structure of ferrite and bainite, and the microstructure of the region including 1 mm above and below the center of the plate thickness does not contain processed ferrite, and polygonal ferrite is mainly composed of bainite with an area ratio of 5% or less. On the other hand, the microstructure of 1.5mm in the thickness direction from the front and back surfaces is a mixed structure of ferrite and bainite with an area ratio of processed ferrite of 5% or less and a polygonal ferrite area ratio of 10% or more. The difference between the maximum and minimum hardness distribution in the thickness direction from 0.5 mm below the front and back surfaces of the steel sheet is less than 30 HV in Vickers hardness, the thickness is 20 mm or more, and the yield strength is 480 MPa or more. Strength-elongation balun characterized by High toughness high tensile strength steel sheet excellent in.
Here, for processed ferrite and polygonal ferrite, the aspect ratio (major axis / minor axis) of the ferrite is measured with an optical microscope observation photograph of a sample corroded with 3% nital, and the one with an aspect ratio of 3.0 or more is processed. Ferrite, less than 3.0 is defined as polygonal ferrite.

2. The steel sheet is further in mass%, Cu: 0.8% or less, Ni: 2% or less, Cr: 1% or less, Mo: 0.8% or less, Nb: 0.05% or less, V: 0.1% or less, Ti: 0.025% or less B: 0.002% or less and Ca: 0.005% or less, the composition containing one or more kinds selected from the above, high toughness and high strength steel plate excellent in strength-elongation balance as described in 1 above .

3. A method for producing the high toughness high-tensile steel sheet according to 1 or 2,
After heating the slab having the component composition described in 1 or 2 above to a temperature of 1050 to 1250 ° C., it is hot under the conditions of cumulative rolling reduction: 50% or more, steel sheet surface temperature: Ar ₃ or more, Ar ₃ + 15 ° C. or less. After rolling, accelerated cooling is started from the temperature where the thickness center is Ar ₃ or higher, and the steel sheet is cooled to a temperature range of 350 ° C or higher and 550 ° C or lower, and then air-cooled. , In the temperature range where the steel sheet surface temperature is 300 ° C or higher, provide a non-cooling period of 0.3 seconds or more, a temporary non-cooling period of 0.3 seconds or more, and a total time of 1.5 seconds or more and 15 seconds or less. A method for producing a high-toughness, high-strength steel sheet having an excellent strength-elongation balance.

4). After the accelerated cooling including the non-cooling period, a tempering process is performed by rapid heating to a plate thickness center temperature of the steel sheet of 650 ° C. or lower and a maximum surface temperature of 580 ° C. or higher and 730 ° C. or lower by induction heating. 4. A method for producing a high-toughness, high-tensile steel sheet excellent in strength-elongation balance as described in 3 above.

  According to the present invention, it has high yield strength and excellent low temperature toughness without causing a decrease in productivity and an increase in manufacturing cost, and thus has an excellent strength-elongation balance, and has a hardness distribution in the thickness direction. A uniform high toughness and high strength steel sheet can be obtained stably.

It is the photograph which compared and showed processed ferrite (a) and polygonal ferrite (b).

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of steel is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.03-0.18%
C is an element necessary to ensure the strength of the base material of the high-tensile steel sheet. However, if the content is less than 0.03%, it is necessary to add a large amount of a hardenability improving element such as Cu, Ni, Cr, and Mo. Not only is the cost high, but also weldability is deteriorated, and when high heat input welding is performed, the dilution of C into the weld metal is reduced, and it is difficult to ensure the joint strength. On the other hand, if the C content exceeds 0.18%, the base metal toughness and weldability deteriorate, and the weld joint toughness deteriorates. Therefore, the C content is limited to a range of 0.03 to 0.18%.

Si: 0.01-0.55%
Si is an element useful for ensuring the strength of the base metal and the welded joint, so it was included in an amount of 0.01% or more. However, if the Si content exceeds 0.55%, the weld crack sensitivity and weld joint toughness are deteriorated. Therefore, the Si content is limited to a range of 0.01 to 0.55%.

Mn: 0.5-2.0%
Mn is useful for ensuring the strength of the base metal and the welded joint, so it was added at 0.5% or more. However, if the amount of Mn exceeds 2.0%, not only the weld cracking sensitivity is deteriorated, but also hardenability more than necessary is brought about, and the base metal toughness and joint toughness are deteriorated. Therefore, the Mn content is limited to a range of 0.5 to 2.0%.

Al: 0.005-0.1%
Al is useful as a deoxidizer for steel, so 0.005% or more is contained. In addition, addition of 0.01% or more is suitable for securing the base material toughness by refining crystal grains. However, if the Al content exceeds 0.1%, the toughness of the base metal is impaired, so Al is included in the range of 0.005 to 0.1%.

N: 0.0005-0.005%
N reacts with Al, Nb, and the like to form precipitates and thereby has the effect of refining crystal grains and improving the base material toughness. However, if the content is less than 0.0005%, precipitates necessary for refining the crystal grains and securing the strength are not formed. On the other hand, if the content exceeds 0.005%, the toughness of the base metal and the high heat input welded joint is impaired. It was made to contain in 0.0005 to 0.005% of range.

  Although the basic components have been described above, in the present invention, one or more selected from Cu, Ni, Cr, Mo, Nb, V, Ti, B, and Ca are also included in the following ranges. It can be contained as appropriate.

Cu: 0.8% or less, Ni: 2% or less, Cr: 1% or less, Mo: 0.8% or less, Nb: 0.05% or less, V: 0.1% or less In the present invention steel, particularly high tensile strength of 600 MPa class or more When obtaining a steel plate or when weather resistance is required, it is advantageous to add at least one selected from Cu, Ni, Cr, Mo, Nb and V. In this case, for Cu, Ni, Cr, and Mo, the addition of a large amount is expensive, and in order to reduce weldability, the upper limit is 0.8% for Cu and the upper limit is 1% for Cr. For Ni, the upper limit was 2%, and for Mo, the upper limit was 0.8%. Nb is effective in securing the strength of the base metal, but adding a large amount does not contribute to strengthening, and conversely deteriorates the weld joint toughness, so the upper limit when added is 0.05%, preferably 0.03%. It is. Furthermore, V acts effectively in securing the base metal strength and weld joint strength, but addition exceeding 0.1% degrades the weld crack sensitivity, so the upper limit was made 0.1%.

Ti: 0.025% or less, B: 0.002% or less
Ti is added to refine the microstructure and to ensure B effective in hardenability in the case of B-added steel, but addition over 0.025% impairs the base metal toughness. 0.025% or less. In addition, B has an effect of improving hardenability by adding a very small amount, but if added excessively, BN is formed and conversely the hardenability is lowered, and the weld heat affected zone is remarkably hardened. The upper limit of B was 0.002%.

Ca: 0.005% or less
Ca changes the precipitation form of MnS, which degrades toughness, and has the effect of mitigating the adverse effects. However, excessive addition causes a decrease in hardenability, so the upper limit was made 0.005%.

The balance is Fe and inevitable impurities.
Here, P, S, etc. are conceivable as inevitable impurities, but in order to obtain a sound base metal and a welded joint, it is desirable to suppress both to 0.015% or less.
In addition, within the range not impairing the effects of the present invention, the content of components other than the above, for example, 0.0050% or less of Mg and / or 0.02% or less of REM (rare earth metal) for the purpose of improving toughness. It does not refuse to contain.

Next, the reason why the steel structure is limited as described above in the present invention will be described.
Although the microstructure of the present invention is a mixed structure of ferrite and bainite, in the present invention, the structure is individually controlled at the center portion and the surface layer portion of the steel sheet.
Microstructure at the center of the plate thickness: Main structure of bainite In the full thickness tensile test of the steel material, after reaching the maximum load, voids are generated from the center of the steel plate, and they grow and connect to break. Therefore, in order to suppress the occurrence of voids at the center of the plate thickness, it is necessary to reduce the interface of the heterogeneous structure that is the source of the voids. It is important that the structure is a bainite-based structure that does not include processed ferrite and that polygonal ferrite has an area ratio of 5% or less. In addition, an area ratio shows the average area fraction in the area | region measured from the microstructure of a steel plate cross section.

Here, the region to be subjected to the structure control at the center of the plate thickness is the region including 1 mm above and below the center of the plate thickness. If the region within 1 mm above and below the center of the plate thickness is controlled as described above, This is because the generation of voids is effectively suppressed.
In addition, since the processed ferrite has a structure which is disadvantageous to elongation, the processed ferrite is not allowed to exist in this region.
Furthermore, if the amount of polygonal ferrite in this region exceeds 5% in terms of area ratio, voids are likely to occur at the interface due to the difference in strength between polygonal ferrite and bainite. The amount was limited to 5% or less in terms of area ratio.

Microstructure of surface layer: Mixed structure of ferrite and bainite In order to ensure high uniform elongation, it is effective to suppress the processed ferrite and introduce polygonal ferrite with excellent ductility, and at the same time reduce the hardness of the front and back layers. The ductility is also improved by lowering.
However, when polygonal ferrite is less than 10% in area ratio, the effect of improving uniform elongation and ductility is small, so the microstructure of the steel sheet surface layer 1.5mm from the front and back surfaces to the sheet thickness direction has an area ratio of processed ferrite. The mixed structure of ferrite and bainite is 5% or less, and the area ratio of polygonal ferrite is 10% or more, preferably 20% or more, more preferably 30% or more.

FIGS. 1 (a) and 1 (b) show photographs of processed ferrite and polygonal ferrite, respectively.
The processed ferrite has a flat shape because strain is applied by rolling after ferrite transformation. Polygonal ferrite, on the other hand, has a relatively equiaxed shape because no strain is applied by rolling after ferrite transformation. Therefore, although the processed ferrite and the polygonal ferrite have a difference such as a difference in internal dislocation density, both can be discriminated from the difference in shape.
In the present invention, the aspect ratio (major axis / minor axis) of the ferrite is measured with an optical microscopic photograph of a sample corroded with 3% nital. It is defined as null ferrite.

Difference between maximum and minimum hardness distribution in the plate thickness direction: Vickers hardness of less than 30HV If the hardness distribution in the plate thickness direction is large, the elongation of the portion with the greater hardness is the elongation of the portion with the smaller hardness. Therefore, it is desirable to make the hardness distribution as small as possible because problems such as low elongation occur when compared at the same strength.
Therefore, in the present invention, the difference between the maximum value and the minimum value of the hardness distribution in the thickness direction is suppressed to less than 30 HV in terms of Vickers hardness.
Here, in order to obtain such a uniform hardness distribution, a tempering process using induction heating may be performed after accelerated cooling according to the present invention in the manufacturing process described later.
Note that the difference between the maximum value and the minimum value of the hardness distribution in the thickness direction in the past could only be reduced to about 50 HV at most by the Vickers hardness.

In the present invention, the thickness of the target steel sheet is set to 20 mm or more, for the following reason.
That is, in order to realize the steel sheet having the microstructure of the present invention by the manufacturing method of the present invention, as described below, it is necessary to define the temperature of the surface layer and the center of the sheet thickness at the start of cooling, but the sheet thickness is 20 mm. If it is less than this, it is difficult to control practically, so the plate thickness is set to 20 mm or more.

Next, the manufacturing method of this invention is demonstrated.
The molten steel having the above-described component composition is melted using a known furnace such as a converter or an electric furnace, and then formed into a slab by a continuous casting method or an ingot-bundling method.
Next, after the obtained slab was heated to a temperature of 1050 to 1250 ° C., the hot rolling was completed under the conditions of the cumulative rolling reduction: 50% or more, the steel sheet surface temperature: Ar ₃ or more, and Ar ₃ + 15 ° C. or less. The center of thickness is accelerated from a temperature of Ar ₃ or higher, and the average temperature of the steel plate is cooled to a temperature range of 350 ° C to 550 ° C, and then air-cooled, providing excellent strength-elongation balance in the present invention. Manufactured high toughness and high strength steel sheet.
Hereinafter, the reason why the manufacturing conditions are limited to the above range will be described.

Heating temperature: 1050-1250 ° C
In slab heating, it is necessary to secure at least 1050 ° C in order to homogenize the components in the steel and dissolve precipitation strengthening elements such as Mo, Nb, and V. However, if the heating temperature becomes too high, the crystal grains become coarse In the center of the thickness, in addition to promoting the generation of microvoids, the toughness of the base material is deteriorated, so the temperature is limited to the range of 1050 to 1250 ° C. Preferably it is 1200 degrees C or less.

Cumulative rolling reduction in hot rolling: 50% or more In order to reduce the austenite grain size by hot rolling and to accelerate bainite transformation and refine the ferrite grain size by accelerated cooling in the subsequent process, The cumulative rolling reduction in rolling needs to be 50% or more. From the viewpoint of improving the toughness of the base material and ensuring more stability, it is desirable to apply a cumulative reduction of 20% or more in a temperature range of 1050 ° C. or lower and 900 ° C. or higher. Thereby, a structure | tissue refines | miniaturizes with recrystallization of austenite ((gamma)) grain, and the toughness of a base material is improved and stabilized. From the aspect of the same effect, it is desirable that the reduction amount for each rolling pass is 5% or more, preferably 10% or more.

Steel plate surface temperature at the end of rolling: Ar ₃ or higher, Ar ₃ + 15 ° C. or lower This is the most important control item in suppressing processed ferrite. When rolling is completed at a temperature lower than the Ar ₃ transformation point, pro-eutectoid ferrite is processed, and processed ferrite containing dislocations is generated. Therefore, the steel sheet surface temperature at the end of rolling is set to Ar ₃ or higher. On the other hand, if the rolling end temperature is Ar ₃ or higher, the formation of processed ferrite can be suppressed, but if the temperature is too high, the crystal grains become coarse, leading to a decrease in toughness and a decrease in elongation. Therefore, the steel sheet surface temperature at the end of rolling is set to Ar ₃ + 15 ° C. or lower.

The Ar ₃ point can be calculated using, for example, the following relational expression.
Ar ₃ (° C.) = 910−310 [% C] −80 [% Mn] −20 [% Cu] −15 [% Cr] −55 [% Ni] −80 [% Mo] where [% M] is , Represents the content (mass%) of the M element.

Accelerated cooling start temperature: The center of the plate thickness is Ar ₃ or more. When the rolling end temperature is in the temperature range described above, ferrite transformation proceeds from the surface layer portion immediately after the end of rolling. Therefore, in the surface layer portion, it is possible to secure the target ferrite fraction even if accelerated cooling is performed immediately, but since it is necessary to have a bainite-based structure at the center of the plate thickness, accelerated cooling starts. In order to suppress the previous ferrite transformation, this accelerated cooling is performed from a temperature at which the thickness center is Ar ₃ or higher. A suitable starting temperature for accelerated cooling is in the range of Ar ₃ + 5-50 ° C.
Here, the plate thickness center temperature can be obtained by simulation calculation or the like when plate thickness, surface temperature, cooling conditions, and the like are given.

  Here, the specific cooling rate of the above-described accelerated cooling is preferably about 4 ° C./s or more. This is because if the cooling rate is less than 4 ° C./s, ferrite is partially formed during cooling and the strength is lowered.

Accelerated cooling stop temperature: 350 ° C. or higher and 550 ° C. or lower at the steel plate average temperature When the cooling stop temperature falls below 350 ° C. at the steel plate average temperature, martensite is generated by accelerated cooling and the toughness deteriorates. On the other hand, when the cooling stop temperature is higher than 550 ° C at the average temperature of the steel sheet, the bainite transformation does not proceed sufficiently, so it is difficult not only to secure the strength as a high-tensile steel sheet, but also a coarse pearlite structure is generated, and ductility Decreases. Therefore, in order to make the sheet thickness center a bainite main structure, the cooling stop temperature is in the range of 350 ° C. or higher and 550 ° C. or lower in terms of the average temperature of the steel plate. After the accelerated cooling is finished, it is desirable to cool by air except when the induction heating described later is performed.

Here, the reason for prescribing the temperature during cooling by the average temperature in the plate thickness direction is that when the plate thickness of the steel plate is large or the cooling rate is fast, the temperature history is different in each part in the plate thickness direction, In order to prevent the standard from becoming unclear, the average temperature, which is the most relevant to the overall quality of the steel, was used as the standard.
As the average temperature, a value obtained by simulation calculation or the like when a plate thickness, a surface temperature, a cooling condition, or the like is given can be used. For example, the temperature obtained by averaging the temperature distribution in the plate thickness direction using the difference method can be used as the average temperature.

Non-cooling period during accelerated cooling: Total time is 1.5 seconds or more and 15 seconds or less. The plate thickness is higher than the front and back layers by providing a non-cooling period during continuous cooling in the accelerated cooling process after hot rolling. The heat from the inside causes the front and back layers to recuperate, thereby reducing the hardness of only the front and back layers. At that time, the closer to the center of the steel plate, the less the effect of recuperation due to the provision of the non-cooling period, and the change in cooling heat history is small at the center of the steel plate and its surroundings, and there is almost no decrease in the cooling rate. Or since it can suppress very slightly, hardness hardly falls. Therefore, the strength as a whole thickness is not greatly reduced, and the time required for cooling after hot rolling does not change, so that a high-tensile steel sheet having an excellent strength-elongation balance can be obtained without reducing productivity. be able to.

Here, when the non-cooling period per time is shorter than 0.3 seconds, the decrease in front and back layer hardness is not sufficient, and the expected effect cannot be obtained, so the non-cooling period per time is 0.3 seconds or more. . Preferably it is 0.8 second or more.
Further, the length and number of times of the non-cooling period can be set according to the product plate thickness, size, and strength level. However, if the total non-cooling time is too short, the hardness of the front and back layers will not be sufficiently lowered and the expected effect will not be obtained, while if it is too long, the cooling rate at the center of the plate thickness and its surroundings will be reduced. As a result, not only the strength is lowered but also productivity is lowered as compared with the normal direct quenching. Therefore, the total time of the non-cooling period is 1.5 seconds or more and 15 seconds or less. Preferably, it is 3 seconds or more and 13 seconds or less.
Furthermore, for the temperature range providing the non-cooling time as described above, if the temperature of the steel sheet is low, the recuperation of the front and back layers becomes small and the expected effect cannot be obtained sufficiently, so the surface temperature of the steel sheet is 300 ° C. The above temperature range.

In the present invention, after accelerated cooling including the above-described non-cooling period, the center thickness of the steel sheet is rapidly heated to 650 ° C. or less and the highest surface temperature reaches 580 ° C. or more and 730 ° C. or less by induction heating. It is advantageous to apply a tempering treatment.
Although the surface hardness is reduced and the elongation is improved by the above-described cooling method, the surface hardness is increased even within the same steel sheet due to variations in the cooling rate during accelerated cooling due to the scale properties of the steel sheet surface. There is variation in the size. The presence of such a variation in the parallel portion of the tensile test piece causes a decrease in elongation. Therefore, by heating the surface, variations in surface hardness within the same steel sheet are reduced.

  The heating temperature in this case needs to be an appropriate temperature at which the strength at the center of the plate thickness or at the total thickness becomes the target strength, but if the maximum surface temperature is less than 580 ° C, the hardness of the surface layer portion The reduction effect and recovery effect of processed ferrite are not sufficient.On the other hand, if the temperature exceeds 730 ° C, not only may the strength as a whole thickness decrease significantly due to the temperature rise inside the steel sheet, but also the toughness will increase due to the coarsening of carbides. descend. Further, if the temperature at the center of the plate thickness exceeds 650 ° C., the toughness may decrease. Therefore, the heating temperature in this tempering treatment was set to 650 ° C. or lower at the center temperature and 580 ° C. or higher, preferably 620 ° C. or higher and 730 ° C. or lower as the surface temperature of the steel sheet. Here, the plate thickness center temperature refers to the temperature when the temperature distribution inside the steel plate becomes substantially uniform after induction heating. In addition, the cooling conditions after induction heating are not particularly specified, and normal air cooling may be used, but active cooling may be performed.

As a method for preferentially heating the surface layer, the temperature of the surface layer portion becomes higher than that inside the steel plate by using induction heating instead of heating by normal gas combustion and concentrating the induction current on the steel plate surface layer portion. A temperature distribution can be given. This induction heating treatment may be online or offline, but from the viewpoint of energy cost, it is advantageous to make it online that can be heated immediately after quenching.
Further, by using induction heating, the tempering treatment can be performed in a shorter time than in the prior art, so that productivity can be improved and at the same time the hardness difference between the steel sheet surface layer and the plate thickness center can be further reduced. That is, the difference between the maximum value and the minimum value of the hardness distribution in the thickness direction on the inner side from 0.5 mm below the front and back surfaces of the steel sheet can be made less than 30 HV in terms of Vickers hardness.
Thus, it is possible to stably produce a high-toughness, high-tensile steel plate having a yield strength of 480 MPa or more and excellent low-temperature toughness and excellent strength-elongation balance.

The steel having the composition shown in Table 1 was melted to produce a steel ingot, and after hot rolling to a predetermined plate thickness under the manufacturing conditions shown in Table 2, the test was conducted under various conditions shown in Table 2 as well. Steel was produced. Steel symbol C and steel symbol K are comparative steels in which the C content and Mn content are outside the proper ranges of the present invention, but the other steel types are compatible steels whose component compositions satisfy the proper ranges of the present invention.
The area ratio of polygonal ferrite in the center of the steel sheet was measured by randomly analyzing five optical micrographs of 400 times corroded with 3% nital in the area of 1 mm above and below the center of the plate thickness, and analyzing the polygonal ferrite fraction. Was calculated. Further, the processed ferrite and polygonal ferrite fractions in the surface layer portion were obtained by taking five optical micrographs 400 times at 0.3 mm intervals from directly below the surface layer, and calculating the processed ferrite and polygonal ferrite fractions by image analysis.
In addition, as an evaluation of the mechanical properties of the base metal, a full-thickness tensile test using JIS No. 5 tensile test pieces, hardness distribution measurement in the thickness direction by Vickers hardness, and Charpy impact test at 1 / 2t position went. Since elongation has a correlation with TS, the value of TS × El (total elongation) was used as an evaluation of elongation, and the greater this value, the better the strength-elongation balance (TS × El). TS × El was set to 30000 MPa ·% or more when the plate thickness was 25 mm, 35000 MPa ·% or more when the plate thickness was 35 mm, and the target value of vTs was −70 ° C. or less.

Table 3 shows the results of examining the microstructure and mechanical properties of each test steel.
In Table 3, Nos. 1-3, 9-11, 13, 15-21 are invention examples, and Nos. 4-8, 12, 14, 22 are comparative examples. In addition, about No. 9-12, the tempering process using induction heating was performed after rolling.

All of the inventive examples obtained according to the present invention have excellent characteristics such as yield strength (YS) of 480 MPa or more, vTs of −70 ° C. or less, and TS × El of 30000 MPa ·% or more.
On the other hand, among the comparative examples, No. 1 and No. 2 did not have a non-cooling period during accelerated cooling, so the hardness distribution in the thickness direction did not become small.
In No. 9, the strength decreased because the non-cooling period during accelerated cooling was too long.
In No. 13, the temperature range in which the non-cooling period was provided at the time of accelerated cooling was too low, so the effect of reducing the surface hardness distribution was not obtained.
No. 17 has a low accelerated cooling start temperature and a high ferrite fraction at the center of the plate thickness, and therefore has low strength and low toughness.
In No.18, the rolling end temperature was high, so polygonal ferrite was not sufficiently secured in the surface layer, and No.19 was processed because the rolling end temperature was low and the cooling start temperature was not kept. Ferrite was introduced, the surface layer portion became hard, and the strength-elongation balance was low.
In No. 20, since the cooling stop temperature was low, the surface layer was cured, the hardness difference between the surface layer and the central portion was large, the elongation was low, and the toughness was also decreased.
In No. 21, because the cooling stop temperature was high, coarse pearlite was generated at the center of the plate thickness, and it became a structure mainly composed of ferrite and pearlite, but did not become a structure mainly composed of bainite, so the strength and toughness decreased. did.
In No. 25, since the temperature in the induction heat treatment was too high, the strength decreased and the toughness also decreased.
In No. 27 and No. 35, the target yield stress was not obtained because the component was outside the claimed range.

  The high-tensile steel plate obtained according to the present invention has a high yield strength of YS: 480 MPa or more and an excellent low-temperature toughness of vTs: −70 ° C. or less, and TS × El: 30000 MPa ·% or more (when the plate thickness is 25 mm). Excellent strength-elongation balance, as well as the difference between the maximum and minimum hardness distributions in the plate thickness direction, Vickers hardness is uniform less than 30HV, so bridges, storage tanks, pressure It is extremely useful for use in steel structures such as containers and line pipes.

Claims (4)

In mass%, C: 0.03-0.18%, Si: 0.01-0.55%, Mn: 0.5-2.0%, Al: 0.005-0.1% and N: 0.0005-0.005%, the balance being Fe and inevitable impurities The composition is a mixed structure of ferrite and bainite, and the microstructure of the region including 1 mm above and below the center of the plate thickness does not contain processed ferrite, and polygonal ferrite is mainly composed of bainite with an area ratio of 5% or less. On the other hand, the microstructure of 1.5mm in the thickness direction from the front and back surfaces is a mixed structure of ferrite and bainite with an area ratio of processed ferrite of 5% or less and a polygonal ferrite area ratio of 10% or more. The difference between the maximum and minimum hardness distribution in the thickness direction from 0.5 mm below the front and back surfaces of the steel sheet is less than 30 HV in Vickers hardness, the thickness is 20 mm or more, and the yield strength is 480 MPa or more. Strength-elongation balun characterized by High toughness high tensile strength steel sheet excellent in.
Here, for processed ferrite and polygonal ferrite, the aspect ratio (major axis / minor axis) of the ferrite is measured with an optical microscope observation photograph of a sample corroded with 3% nital, and the one with an aspect ratio of 3.0 or more is processed. Ferrite, less than 3.0 is defined as polygonal ferrite.   The steel sheet is further in mass%, Cu: 0.8% or less, Ni: 2% or less, Cr: 1% or less, Mo: 0.8% or less, Nb: 0.05% or less, V: 0.1% or less, Ti: 0.025% or less B: 0.002% or less and Ca: 0.005% or less, the composition containing one or more selected from the group consisting of two or more, high toughness and high tension excellent in strength-elongation balance according to claim 1 steel sheet. It is a manufacturing method of the high toughness high tension steel plate according to claim 1 or 2,
The slab to be component composition according to claim 1 or 2, after heating to a temperature of 1050-1250 ° C., the cumulative rolling reduction: 50% or more, the steel sheet surface temperature: Ar ₃ or more, heat Ar ₃ + 15 ° C. the following conditions After the hot rolling is completed, accelerated cooling is started from a temperature where the center of the sheet thickness is Ar ₃ or higher, the steel sheet is cooled to a temperature range of 350 ° C. or higher and 550 ° C. or lower, and then air-cooled. In this case, in the temperature range where the steel sheet surface temperature is 300 ° C or higher, provide a non-cooling period of 0.3 second or more, which is temporarily not cooled once or twice, and a total time of 1.5 seconds or more and 15 seconds or less. A method for producing a high-toughness, high-tensile steel sheet with excellent strength-elongation balance.   After the accelerated cooling including the non-cooling period, a tempering process is performed by rapid heating to a plate thickness center temperature of the steel sheet of 650 ° C. or lower and a maximum surface temperature of 580 ° C. or higher and 730 ° C. or lower by induction heating. The manufacturing method of the high toughness high tensile steel plate excellent in the strength-elongation balance of Claim 3.

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