JP5625694B2 - High-strength, high-toughness thick-walled steel plate with excellent material uniformity in the steel plate and method for producing the same - Google Patents

Description

The present invention is suitable for use in the fields of architecture, offshore structures, shipbuilding, civil engineering, construction industry machines, line pipes, and the like, and is a high-strength, high-toughness thick-walled steel plate excellent in material uniformity in the steel plate and its production It is about the method.
In the present invention, the thick steel plate means a steel plate having a thickness of 40 mm or more.

From the viewpoint of increasing the size of steel structures and reducing costs, there is an increasing demand for steel sheets having higher strength and higher toughness. For the purpose of improving the properties of steel sheets, reducing alloy elements, and omitting heat treatment, the so-called TMCP technique, which combines controlled rolling and controlled cooling, is usually applied in the production of high-strength steel sheets. In order to increase the strength of steel using this TMCP technology, it is effective to increase the cooling rate during controlled cooling and to decrease the cooling stop temperature.
However, when controlled cooling is performed at a high cooling rate, the surface layer portion of the steel sheet is rapidly cooled, so that the hardness of the surface layer portion is higher than that inside the steel plate, and the hardness distribution in the thickness direction varies. Further, the variation in the hardness distribution in the thickness direction tends to increase as the cooling stop temperature is lowered and / or as the thickness increases. Therefore, it becomes a problem from the viewpoint of ensuring the material uniformity in the steel plate.

In order to solve the above problems, various solutions have been proposed in the past. For example, in Patent Document 1, the cooling rate is controlled to a relatively low cooling rate of 3 to 12 ° C./s during controlled cooling. Thus, a method of suppressing an increase in surface hardness with respect to the center portion of the plate thickness is disclosed.
Patent Document 2 discloses that the structure of the steel sheet is made into a two-phase structure of ferrite and bainite by waiting in the temperature range where ferrite precipitates during the cooling process, and the difference in hardness between the surface layer and the center of the sheet thickness is indicated. A method for producing a reduced steel sheet with a small material difference in the thickness direction is disclosed.
Furthermore, in Patent Documents 3 and 4, after rolling, by performing controlled cooling at a high cooling rate that reheats the surface before the surface layer portion completes the bainite transformation, the manufacture of a steel sheet with a small material difference in the thickness direction is produced. A method is disclosed.

On the other hand, if there is unevenness in the scale properties on the surface of the steel plate, the cooling rate of the steel plate underneath will vary depending on the thickness of the scale during cooling, and as a result, the cooling stop temperature will vary locally within the steel plate. Corresponding to the properties, the steel plate material varies in the plate width direction.
On the other hand, Patent Documents 5 and 6 disclose a method of reducing the cooling unevenness caused by the scale properties and improving the steel plate shape by performing descaling immediately before cooling.

Japanese Patent Publication No.7-116504 Japanese Patent No. 3911834 Japanese Patent No. 3951428 Japanese Patent No. 3951429 JP-A-9-57327 Japanese Patent No. 3796133

However, in the technique described in Patent Document 1, since the cooling rate is limited and the cooling stop temperature is relatively high at 400 ° C. or higher, the strength is increased by the high cooling rate, the number of alloy elements is reduced, the control rolling is simplified, etc. The effect of control cooling cannot be fully utilized. In addition, since the manufacturing method disclosed in Patent Document 2 deposits ferrite in a cooling standby at an Ar ₃ transformation point or lower, not only the strength is reduced, but a cooling standby time is required, so that the manufacturing efficiency is improved. Getting worse. Furthermore, in the manufacturing methods described in Patent Documents 3 and 4, if the transformation behavior differs depending on the components of the steel sheet, sufficient material homogenization effect due to recuperation may not be obtained. In addition, since highly accurate cooling control is required, the application range is limited, and the production efficiency is inevitably reduced.

  On the other hand, in the methods described in Patent Documents 5 and 6, the cooling unevenness of the steel plate is reduced by descaling to improve the steel plate shape, but no consideration is given to the hardness distribution in the plate thickness direction. Absent.

  The present invention has been developed in view of the above-mentioned present situation, effectively reducing the variation in hardness in the plate thickness direction and the plate width direction of the steel plate, and improving the material uniformity in the steel plate. The object is to propose a tough thick steel plate together with its advantageous production method.

  In order to reduce the variation in hardness in the plate thickness direction and the plate width direction of the high-strength steel sheet and to improve the material uniformity in the steel sheet, the present invention provides a large number of chemical components, microstructures and manufacturing conditions of the steel material. It was developed after repeated experiments and examinations.

That is, the gist configuration of the present invention is as follows.
1. In mass%, C: 0.02 to 0.15%, Si: 0.01 to 1.0% and Mn: 0.5 to 2.0%, with the balance from the composition of Fe and inevitable impurities The steel structure is a ferrite and bainite structure, and the hardness variation in the plate thickness direction is 50 or less in terms of Vickers hardness ΔHV, and the hardness variation in the plate width direction is 50 or less in terms of Vickers hardness ΔHV. In addition , the material in the steel sheet is characterized in that the tensile strength is 490 MPa or more, the Charpy impact test value vE (−40 ° C.) at −40 ° C. is 27 J or more, and the plate thickness is 40 mm or more. High strength, high toughness thick steel plate with excellent uniformity.

2. Further, the steel is one or two selected from the following by mass: Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 0.5% or less. 2. The high-strength, high-toughness thick steel plate according to 1 above, which contains seeds or more.

3. Further, the steel may be one selected from Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1% by mass%. 2. The high-strength, high-toughness thick steel plate according to 1 or 2 above, which contains two or more kinds.

4). 4. The high-strength and high-toughness thick steel plate according to any one of 1 to 3, wherein the steel further contains B: 0.0003 to 0.002% by mass%.

5. A method for producing the high-strength, high-toughness thick steel plate according to any one of 1 to 4,
After heating the steel slab comprising the component composition according to any one of 1 to 4 to a temperature of 900 ° C. to 1300 ° C. and hot rolling at a rolling end temperature of 700 ° C. to 900 ° C. at the steel sheet surface temperature. From the temperature range where the steel sheet surface temperature is 700 ° C. or higher to the temperature range where the steel sheet surface temperature is 400 ° C. or higher and 600 ° C. or lower at a cooling rate of the steel sheet surface of 20 ° C./s or higher and 100 ° C./s or lower (1) The first stage cooling is performed under the condition satisfying the equation, and then the second stage cooling is performed such that the average cooling rate of the steel sheet is 4 ° C./s or more and the recuperated temperature after cooling is 600 ° C. or less. A method for producing a high-strength, high-toughness thick steel plate with excellent material uniformity in the steel plate.
Record
3 ≦ (700−T) / V (1)
Here, T: cooling end temperature (° C.) of the steel sheet surface in the first stage cooling
V: Cooling rate of steel sheet surface in 1st stage cooling (° C / s)

6). A method for producing the high-strength, high-toughness thick steel plate according to any one of 1 to 4,
The steel slab comprising the component composition according to any one of 1 to 4 is heated to a temperature of 900 ° C. or higher and 1300 ° C. or lower, and the rolling end temperature is hot rolled at a steel sheet surface temperature of 700 ° C. or higher and 900 ° C. or lower, Immediately before the subsequent controlled cooling, descaling is performed under the condition that the impinging pressure of the jet flow on the steel sheet surface is 1 MPa or more, and within 5 seconds, the steel sheet surface cooling rate is within the temperature range of 700 ° C. or more. The first stage of cooling is performed at a rate of 20 ° C./s or more and 100 ° C./s or less to a temperature range where the steel sheet surface temperature is 400 ° C. or more and 600 ° C. or less, and then the average cooling rate of the steel plate. Of high strength and high toughness thick steel plate excellent in material uniformity in the steel plate, characterized in that the second stage cooling is performed at a cooling rate of 4 ° C./s or higher and the recuperated temperature after cooling is 600 ° C. or lower. Method.
Record
3 ≦ (700−T) / V (1)
Here, T: cooling end temperature (° C.) of the steel sheet surface in the first stage cooling
V: Cooling rate of steel sheet surface in 1st stage cooling (° C / s)

  According to the present invention, by utilizing controlled cooling technology and further descaling technology, cooling is performed at a high cooling rate using a low-cost component system, which has been difficult to achieve with conventional controlled cooling technology. However, it is possible to obtain a thick steel plate having excellent material uniformity and high strength and high toughness.

Hereinafter, the present invention will be specifically described.
[Chemical composition]
First, chemical components of the high-strength steel plate of the present invention will be described. In the following description, all units represented by% are mass%.
C: 0.02-0.15%
C contributes effectively to the improvement of strength. However, if the content is less than 0.02%, sufficient strength cannot be secured. On the other hand, if it exceeds 0.15%, the toughness is deteriorated. It is limited to the range of 02 to 0.15%.

Si: 0.01 to 1.0%
Si is added for deoxidation, but if the content is less than 0.01%, the deoxidation effect is not sufficient. On the other hand, if it exceeds 1.0%, the toughness and weldability are deteriorated. It is limited to the range of 01 to 1.0%.

Mn: 0.5 to 2.0%
Mn contributes effectively to the improvement of strength and toughness. However, if the content is less than 0.5%, the effect of addition is poor. On the other hand, if it exceeds 2.0%, the weldability deteriorates. It is limited to a range of 5 to 2.0%.

The basic components of the present invention have been described above. In the present invention, one or more selected from Cu, Ni, Cr, and Mo are selected in order to further improve the strength and toughness of the steel sheet. It can contain suitably in the range.
Cu: 1.0% or less Cu is an element effective for improving toughness and increasing strength. To obtain this effect, it is preferable to contain 0.03% or more, but if the content is too large, the surface Therefore, when Cu is added, the upper limit is 1.0%.

Ni: 1.0% or less Ni is an element effective for improving toughness and increasing strength. To obtain this effect, Ni is preferably contained in an amount of 0.03% or more. Therefore, when Ni is added, the upper limit is 1.0%.

Cr: 1.0% or less Cr, like Mn, is an element effective for obtaining sufficient strength even at low C. To obtain this effect, it is preferable to contain 0.02% or more. If the amount is too large, weldability deteriorates, so when adding Cr, the upper limit is 1.0%.

Mo: 0.5% or less Mo is an element effective for improving toughness and increasing strength. To obtain this effect, it is preferable to contain 0.02% or more, but if the content is too large, welding is performed. When the Mo is added, the upper limit is 0.5%.

In the present invention, one or more selected from Nb, V and Ti can be further contained in the following range.
Nb: 0.005 to 0.1%, V: 0.005 to 0.1% and Ti: One or more selected from 0.005 to 0.1% Nb, V and Ti are either Is an optional element that can be added to increase the strength and toughness of the steel sheet, and one or more elements can be added depending on the required strength. When the content of each element is less than 0.005%, the effect of addition is poor. On the other hand, when the content exceeds 0.1%, the toughness of the welded portion deteriorates. A range of 0.1% is preferable.

Furthermore, in this invention, B can also be contained in the following ranges.
B: 0.0003 to 0.002%
B is an element effective for increasing the strength. However, if the content is less than 0.0003%, the effect is not sufficient. On the other hand, if it exceeds 0.002%, the toughness of the welded portion is significantly deteriorated. Is added in the range of 0.0003 to 0.002%.

  In addition, there are P and S as elements inevitably mixed in the steel as impurities, but these elements deteriorate the toughness of the steel base material and the weld heat affected zone. It is preferable to reduce as much as possible, and the P content and S content are preferably 0.05% or less and 0.01% or less, respectively.

The balance other than the above elements is made of Fe and inevitable impurities.
However, the content of other trace elements is not hindered unless the effects of the present invention are impaired. For example, from the viewpoint of improving toughness, one or more of Ca: 0.003% or less, Mg: 0.02% or less, and REM (rare earth metal): 0.02% or less can be contained.

Next, the steel structure (microstructure) of the steel of the present invention will be described.
Tensile strength targeted in the present invention: In order to achieve a high strength of 490 MPa or more, the steel structure is substantially made of a ferrite and bainite structure. In particular, in the surface layer portion, when a hard phase such as martensite or island martensite (MA) is generated, the surface layer hardness increases, the hardness variation in the steel sheet increases, and the material uniformity deteriorates. In order to suppress the increase in the surface hardness, the steel sheet structure, particularly the surface layer part, has a ferrite and bainite structure. When heterogeneous structures such as martensite, pearlite, island martensite, and retained austenite are mixed in ferrite and bainite structures, strength decreases, toughness deteriorates, and surface hardness increases. The smaller the tissue fraction, the better. However, when the volume fraction of the structure other than the ferrite and the bainite phase is low, the influence thereof can be ignored, so that a certain amount is allowed. Specifically, if the total volume fraction is 5% or less, mixing of other steel structures, that is, martensite, pearlite, island martensite, retained austenite, and the like is allowed.

[Hardness variation]
Hardness variation in the plate thickness direction: Vickers hardness ΔHV of 50 or less and hardness variation in the plate width direction: Vickers hardness ΔHV of 50 or less Steel strength and elongation, formability, HIC resistance, SSCC resistance From the viewpoint of performance and the like, it is required to suppress variation in hardness in the steel sheet. When the variation in hardness in the plate thickness direction exceeds 50 in terms of Vickers hardness ΔHV, or if the variation in hardness in the plate width direction exceeds 50 in terms of Vickers hardness ΔHV, the above characteristics are adversely affected. For example, when the hardness of the steel plate surface layer portion exceeds ΔHV50 compared to the inside of the steel plate, springback is likely to occur after forming, and cracking susceptibility to hydrogen sulfide is increased. In addition, if the hardness distribution in the plate width direction exceeds ΔHV50, a difference occurs in the way of deformation between the hard part and the soft part during molding, and the desired shape cannot be obtained, or when it is cut into small plates Each plate has different strength and elongation.
Therefore, from the viewpoint of material uniformity in the steel plate, the hardness variation in the plate thickness direction and the plate width direction is set to 50 or less in terms of Vickers hardness ΔHV. Preferably, ΔHV is 40 or less. More preferably, ΔHV is 30 or less.

Next, manufacturing conditions for the high-strength steel sheet according to the present invention will be described.
The process feature of the present invention is that the cooling process is divided into two stages. In other words, the first stage of cooling is intended to increase the strength of the entire steel sheet, while building a microstructure that suppresses hardening in the surface layer of the steel sheet, and in the second stage of cooling, the steel sheet is made to have higher strength and higher toughness. Strive to

[Slab heating temperature]
Slab heating temperature: 900-1300 ° C
If the heating temperature is less than 900 ° C., the microstructure is not sufficiently homogenized, and the necessary strength and toughness cannot be obtained. On the other hand, if the heating temperature exceeds 1300 ° C., the toughness deteriorates, so the slab heating temperature is 900 to 1300 ° C. . This temperature is the furnace temperature of the heating furnace, and the slab is heated to this temperature up to the center.

[Rolling end temperature]
Rolling end temperature: 700 ° C. or more and 900 ° C. or less at the surface temperature of the steel sheet When the rolling end temperature falls below 700 ° C. at the surface temperature of the steel sheet, the start of cooling is delayed and sufficient strength cannot be obtained, whereas it exceeds 900 ° C. Since the microstructure becomes coarse and the toughness deteriorates, the rolling end temperature is set to 700 ° C. or more and 900 ° C. or less at the surface temperature of the steel sheet. In addition, the surface temperature of a steel plate can be measured with a radiation thermometer or the like.

[Cooling start temperature of the first cooling stage]
Cooling start temperature shall be performed from the temperature range of 700 degreeC or more with the surface temperature of a steel plate. If the cooling start temperature is less than 700 ° C., sufficient strength cannot be obtained.

[Cooling speed of the first cooling stage]
Steel plate surface cooling rate: 20 ° C./s or more and 100 ° C./s or less In order to reduce the hardness variation in the steel plate and improve the material uniformity while increasing the strength, the cooling rate should be controlled. is important. If the cooling rate of the steel sheet surface is less than 20 ° C./s, sufficient strength cannot be obtained in the whole steel sheet. On the other hand, if it exceeds 100 ° C./s, hard surface such as martensite or island martensite (MA) is formed in the steel sheet surface layer. Since the phase is generated and the surface hardness is remarkably increased, the cooling rate is set in the range of 20 ° C./s to 100 ° C./s on the steel sheet surface.

[Cooling stop temperature at first stage of cooling]
Cooling stop temperature: 400 ° C or higher and 600 ° C or lower at the surface temperature of the steel plate. Cooling at a rate of 20 ° C / s or higher and 100 ° C / s or lower from a temperature range of 700 ° C or higher to generate ferrite and bainite phases on the surface layer of the steel plate. However, if the cooling stop temperature falls below 400 ° C., the start of the subsequent second-stage cooling is delayed and the cooling effect becomes insufficient, and high strength and toughness cannot be obtained. On the other hand, when the cooling stop temperature exceeds 600 ° C., ferrite and bainite are not sufficiently generated, and when the second stage cooling is started in that state, martensite and island martensite (MA) are generated in the surface layer portion. End up. Accordingly, the cooling stop temperature at the first stage is set to a range of 400 ° C. or more and 600 ° C. or less at the surface temperature of the steel plate.

[Restriction conditions for the first cooling stage]
In the first stage cooling described above, when T is the cooling end temperature (° C.) of the steel sheet surface in the first stage cooling and V is the cooling rate (° C./s) of the steel sheet surface in the first stage cooling, these are the following: It is important to perform the first stage cooling under the condition satisfying the formula (1).
3 ≦ (700−T) / V (1)
The right side of the above equation (1) means the cooling time of the first stage cooling. That is, the first stage cooling needs to be continued for 3 seconds or more. This is because a time of 3 seconds or more is required for the ferrite and bainite phases to be sufficiently generated so that the surface layer structure is not hard.
When the formula (1) is not satisfied, martensite and island-like martensite (MA) are generated in the steel sheet surface layer, the hardness of the surface layer increases significantly, and the variation in hardness increases. It is necessary to satisfy the expression (1) as the eye constraint.

[Cooling speed of the second stage of cooling]
Average cooling rate of steel sheet: 4 ° C./s or more The cooling rate of the second stage of cooling is “(the average temperature of the steel sheet at the start of second stage cooling” − “the second stage cooling is completed and the steel sheet surface is reheated. Steel plate average temperature ”) / (“ time when the second stage cooling is completed and the steel sheet surface is reheated ”−“ second stage cooling start time ”). When the cooling of the second stage is completed, the surface of the steel sheet is lower in temperature than the central part in the thickness direction of the steel sheet, but then the surface temperature rises because heat is transferred from the central part of the thick plate thickness to the surface. The surface temperature takes a maximum value. This phenomenon is called recuperation, and in the reheated state, that is, in the state where the surface temperature reaches the maximum value, the temperature difference in the plate thickness direction of the steel sheet becomes small. By subtracting the temperature difference obtained by subtracting the steel sheet average temperature when the steel sheet surface is reheated from the steel sheet average temperature at the start of the second stage cooling, by the required time from the start of cooling to the completion of reheating, the average steel sheet cooling rate Can be calculated.
If the average cooling rate of the steel sheet is less than 4 ° C./s, the effect of increasing the strength cannot be obtained sufficiently. As the thickness of the steel plate increases, the cooling rate at the central portion in the plate thickness direction becomes the rate of heat transfer in the steel, so the physical limit of the average cooling rate of the steel plate with a plate thickness of 100 mm is approximately 4 ° C./s. Moreover, when it is going to obtain this cooling condition with a thick steel plate, it is necessary to perform cooling exceeding 100 ° C./s as the cooling rate of the steel plate surface in a temperature range of 200 ° C. or more on the steel plate surface.

By the way, under the influence of the first stage cooling, there is a possibility that the steel plate surface temperature at the start of the second stage cooling is lower than the temperature of the steel plate thickness central part at that time. However, in the present invention, the steel sheet surface temperature is used as described above as the steel sheet temperature at the start of the second stage cooling used for calculation of the steel sheet average cooling rate, so the steel sheet average cooling rate specified in the present invention is ensured. Then, even in a region where the temperature at the start of the second stage cooling is higher than the surface of the steel sheet, the steel sheet cooling rate specified in the present invention can be secured, so that a sufficient strength increasing effect can be obtained, which becomes a problem. There is no.
Note that the average temperature and cooling rate of the steel sheet cannot be directly measured physically, but can be obtained in real time by simulation calculation based on the temperature change of the surface.

[Recuperated temperature after cooling the second stage of cooling]
Recuperation temperature after cooling: The average temperature of the steel sheet when the second stage cooling is completed and the steel sheet surface is reheated is 600 ° C or less. If the average temperature of the steel sheet when the surface of the steel sheet is reheated after the cooling of the eyes is over 600 ° C., sufficient strength cannot be obtained, so the reheat temperature after cooling, that is, the second stage cooling is When finished, the average temperature of the steel sheet when the steel sheet surface is reheated is 600 ° C. or less.

[Descaling]
In the present invention, it is preferable to perform the descaling by the high collision pressure injection flow immediately before the two-stage control cooling as described above.
In the steel sheet after rolling, unevenness in the thickness of the scale may occur in the width direction due to descaling or the like before and during rolling. Further, when the scale thickness is large, the scale may be partially peeled off. When the scale thickness varies during cooling after rolling, the cooling rate of the steel sheet surface also changes according to the thickness, and the hardness of the steel sheet surface also changes according to the cooling rate. As a countermeasure, the above problem is solved by performing descaling with the jet flow of high collision pressure immediately before the control cooling, thereby reducing the thickness of the scale to such a degree that a large difference in the cooling rate does not occur. .
Here, when the scale thickness of the steel sheet after controlled cooling is 15 μm or less, the variation in hardness in the plate width direction and the plate thickness direction is ΔHV of 30 or less. Although it is practically difficult to measure the scale thickness of the steel sheet immediately before the controlled cooling, the scale thickness before the controlled cooling can be estimated by the scale thickness after the controlled cooling, and the scale thickness of the steel sheet after the cooling is 15 μm or less. It has been clarified that the desired effect can be obtained by performing descaling immediately before cooling.

Descaling pressure (impact pressure of jet flow on steel plate surface): 1 MPa or more In the present invention, descaling is performed under the condition that the impingement pressure of jet flow on the steel plate surface is 1 MPa or more immediately before control cooling. If the collision pressure of the jet flow on the surface of the steel sheet is less than 1 MPa, the descaling may be insufficient and unevenness in scale may occur, resulting in variations in surface hardness. Therefore, the collision pressure of the jet flow is 1 MPa or more. Although descaling is performed using high-pressure water, there is no problem even if another jet flow is used as long as the collision pressure of the jet flow on the steel plate surface is 1 MPa or more. More preferably, it is 2 MPa or more.

  Moreover, it is desirable to perform control cooling within 5 seconds after descaling. If the time until the controlled cooling is performed after descaling exceeds 5 seconds, the scale grows, the cooling rate of the surface layer portion increases, and the variation in hardness increases. In particular, when the scale thickness exceeds 15 μm, the surface layer hardness variation becomes significant. In this regard, if controlled cooling is started within 5 seconds after descaling, the scale thickness can be reduced to 15 μm or less. Therefore, when descaling is performed immediately before control cooling, the time from descaling to control cooling must be within 5 seconds.

As an example of a rolling line suitable for use in the production of the steel sheet of the present invention, a hot rolling mill, a high collision pressure descaling device, a control cooling device for first stage cooling, from the upstream to the downstream side, The structure which arrange | positions the control cooling apparatus for a stage cooling in this order is mentioned. A hot straightening machine can also be installed in front of the descaling device. By improving the shape of the steel sheet with this hot straightening machine, it is possible to increase the collision pressure of the jet flow, so that more efficient descaling can be performed at low cost.
A hot straightening machine can also be installed in front of the descaling device. By improving the shape of the steel sheet with this hot straightening machine, it is possible to increase the collision pressure of the jet flow, so that more efficient descaling can be performed at low cost.

Steel (steel types A to G) having chemical components shown in Table 1 was made into a slab by a continuous casting method, and using this, a thick steel plate (No. 1 to 15 ) having a thickness of 60 mm to 100 mm was manufactured.
Next, after heating the slab to a predetermined thickness by hot rolling, or after descaling with high impact pressure, two-stage controlled cooling was performed using a water-cooled control cooling device. Table 2 shows the production conditions of each steel plate (No. 1 to 15 ).

The microstructure and scale properties of the obtained steel sheet were observed with an optical microscope and a scanning electron microscope. Ten cross-sectional structure photographs were obtained, and the phase fraction was measured using an image analyzer. Moreover, the scale thickness was measured and evaluated by the average value of 10 fields of view. The tensile strength was measured by using a round bar test piece in a direction perpendicular to the rolling direction and at a t / 4 position (plate thickness 1/4 position) as a tensile test piece. Moreover, the Vickers hardness was measured about the cross section orthogonal to a rolling direction based on JISZ2244, and the hardness distribution of the board thickness direction and the hardness distribution of the board width direction were calculated | required. For the plate thickness direction, the hardness of the entire thickness was measured at a pitch of 1 mm, and for the plate width direction, the hardness of the full width was measured at a pitch of 20 mm. The hardness in the plate width direction was measured at a position 1 mm below the surface, t / 4 position (plate thickness 1/4 position), and t / 2 position (plate thickness center). Since the variation in hardness showed the maximum at the position, the variation in hardness in the plate width direction was evaluated at a position 1 mm below the surface.
The target range of the present invention is a high strength steel plate with a tensile strength of 490 MPa or more, a Charpy impact test value vE (−40 ° C.) at −40 ° C. of 27 J or more, a microstructure of a ferrite and bainite multiphase structure, a thickness direction The variation in hardness in the plate width direction is ΔHV50 or less.
The obtained results are shown in Table 3.

No. 1-5 are invention examples in which chemical components and production conditions satisfy the appropriate range of the present invention. In either case, tensile strength: 490 MPa or more, Charpy impact test value at −40 ° C. vE (−40 ° C.): 27 J or more, variation in hardness in sheet thickness direction and sheet width direction ΔHV: 50 or less, The structure was substantially ferrite and bainite structure. In particular, no. In No. 5, the scale thickness after controlled cooling was 15 μm or less, and the hardness variation between the plate thickness direction and the plate width direction was ΔHV: 30 or less.
On the other hand, no. Nos. 6 and 7 are comparative examples in which chemical components are outside the scope of the present invention. 6 is C. In No. 7, since Si exceeded the appropriate range of the present invention, sufficient toughness could not be obtained. No. 8 to 15 are comparative examples in which the chemical components are within the proper range of the present invention, but the production conditions deviate from the proper range of the present invention. No. In No. 8, since the slab heating temperature was low, homogenization of the microstructure was insufficient, and sufficient strength and toughness could not be obtained. No. In No. 9, the rolling end temperature was lower than the appropriate range of the present invention, and at the same time, the control cooling start temperature was also lower than the appropriate range of the present invention. No. No. 10 had only a low strength because the cooling rate of the first stage cooling was less than the lower limit of the present invention. No. 11, 12 and 14, the cooling conditions of the first stage cooling deviated from the appropriate range of the present invention, and a ferrite and bainite multiphase structure was not obtained as a microstructure. The hardness variation exceeded ΔHV50. No. In No. 13, the first stage cooling stop temperature exceeded the upper limit of the present invention, so that the cooling effect in the second stage cooling could not be sufficiently obtained, and a satisfactory strength could not be obtained. No. For No. 15 , only a low strength was obtained because the cooling conditions of the second stage cooling deviated from the appropriate range of the present invention.

Claims (6)

In mass%, C: 0.02 to 0.15%, Si: 0.01 to 1.0% and Mn: 0.5 to 2.0%, with the balance from the composition of Fe and inevitable impurities The steel structure is a ferrite and bainite structure, and the hardness variation in the plate thickness direction is 50 or less in terms of Vickers hardness ΔHV, and the hardness variation in the plate width direction is 50 or less in terms of Vickers hardness ΔHV. In addition , the material in the steel sheet is characterized in that the tensile strength is 490 MPa or more, the Charpy impact test value vE (−40 ° C.) at −40 ° C. is 27 J or more, and the plate thickness is 40 mm or more. High strength, high toughness thick steel plate with excellent uniformity.   Further, the steel is one or two selected from the following by mass: Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 0.5% or less. The high-strength, high-toughness thick steel plate according to claim 1, comprising at least a seed.   Further, the steel may be one selected from Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1% by mass%. The high-strength, high-toughness thick-walled steel sheet according to claim 1, comprising two or more kinds.   The high-strength, high-toughness thick steel plate according to any one of claims 1 to 3, wherein the steel further contains B: 0.0003 to 0.002% by mass%. A method for producing the high-strength, high-toughness thick steel plate according to any one of claims 1 to 4,
A steel slab comprising the component composition according to any one of claims 1 to 4 is heated to a temperature of 900 ° C or higher and 1300 ° C or lower and hot rolled at a rolling end temperature of 700 ° C or higher and 900 ° C or lower at a steel sheet surface temperature. Then, from the temperature range where the steel sheet surface temperature is 700 ° C. or higher, to the temperature range where the steel sheet surface temperature is 400 ° C. or higher and 600 ° C. or lower at a cooling rate of the steel sheet surface of 20 ° C./s or higher and 100 ° C./s or lower (1 ), The first stage cooling is performed under the condition satisfying the equation, and then the second stage cooling is performed such that the average cooling rate of the steel sheet is 4 ° C./s or more and the recuperated temperature after cooling is 600 ° C. or less. The manufacturing method of the high strength high toughness thick-walled steel plate excellent in the material uniformity in the steel plate.
Record
3 ≦ (700−T) / V (1)
Here, T: cooling end temperature (° C.) of the steel sheet surface in the first stage cooling
V: Cooling rate of steel sheet surface in 1st stage cooling (° C / s) A method for producing the high-strength, high-toughness thick steel plate according to any one of claims 1 to 4,
A steel slab comprising the composition according to any one of claims 1 to 4 is heated to a temperature of 900 ° C or higher and 1300 ° C or lower and hot rolled at a rolling end temperature of 700 ° C or higher and 900 ° C or lower at a steel sheet surface temperature. The descaling is performed under the condition that the impinging pressure of the jet flow on the steel sheet surface is 1 MPa or more immediately before the subsequent controlled cooling, and within 5 seconds, the steel sheet surface temperature is cooled from the temperature range of 700 ° C. or more. Is cooled at a rate of 20 ° C./s or higher and 100 ° C./s or lower to a temperature range where the steel sheet surface temperature is 400 ° C. or higher and 600 ° C. or lower, satisfying the following formula (1), and then the average cooling of the steel plate. A high-strength, high-toughness thick steel plate excellent in material uniformity in a steel plate, characterized by performing second-stage cooling at a rate of 4 ° C./s or more and a reheat temperature after cooling of 600 ° C. or less. Production method.
Record
3 ≦ (700−T) / V (1)
Here, T: cooling end temperature (° C.) of the steel sheet surface in the first stage cooling
V: Cooling rate of steel sheet surface in 1st stage cooling (° C / s)