JP5082475B2 - Manufacturing method of high toughness and high strength steel sheet with excellent strength-elongation balance - Google Patents

Description

  The present invention relates to a method for producing a high-strength steel sheet suitable for welded steel structures such as bridges, storage tanks, pressure vessels, and line pipes, and in particular, has a yield strength of 480 MPa or more and excellent low-temperature toughness, and strength-elongation. The present invention relates to a method for producing a high-strength steel sheet with excellent balance.

In recent years, steel sheets used for welded steel structures such as bridges, storage tanks, pressure vessels, and line pipes have high strength and low temperature toughness, as well as high ductility from the viewpoint of earthquake resistance ( It is often required to have (elongation).
From the viewpoint of improving seismic resistance, it has been conventionally recommended to increase the plastic deformability by reducing the yield ratio and increasing the uniform elongation. Further, line pipes and the like are required to have a large total elongation (uniform elongation + local elongation). This means that the amount of deformation from the start of deformation to the failure due to external stress is large, and is an index of safety for steel materials. The ratio of the uniform elongation to the total elongation becomes larger as the gauge distance of the tensile test piece is longer, but within the range of the tensile test piece that is generally used even if it is a long gauge tensile test piece, Since the ratio of local elongation is often about 40 to 50%, it is necessary to increase both uniform elongation and local elongation in order to increase the total elongation.

  The low yield ratio is, for example, for steel for construction, and the like by introducing a soft ferrite phase into the structure mainly composed of martensite or bainite by means of two-phase quenching, etc. It is realized by. However, with such means, the yield phenomenon of the steel material has occurred at an early stage, and it is difficult to balance the yield strength required as a structure, and a complicated heat treatment process is required. As a practical mass-produced product, it remains a problem.

  In addition, it is considered that multiphase organization is effective for improving elongation (including uniform elongation). For example, Patent Document 1 includes C: 0.02 to 0.20% by weight, Si, Mn, P, S, Al, and N are adjusted to an appropriate range, and the carbon equivalent Ceq is 0.33 to 0.5%, Ni equivalent. After heating, the steel with a steel content of 0.5% or more is hot-rolled in the austenite recrystallization temperature range, cooled at a constant temperature or controlled at the cooling rate in the ferrite-austenite two-phase region, and ferrite with an area ratio of 70% or more. Is produced at a cooling rate higher than a predetermined temperature from a specific temperature in a two-phase region, and the remaining structure other than the ferrite phase is a second phase mainly including a martensite phase, which is disclosed in a method for producing a high-strength steel sheet Has been. According to the technique described in Patent Document 1, the uniform elongation in the tensile properties of the steel sheet is improved. However, in the technique described in Patent Document 1, the uniform elongation is improved, but the ferrite grains are coarsened, so the low temperature toughness is not good. In addition, there is a problem that the microstructure is non-uniform and the local elongation may be significantly reduced.

  Patent Document 2 proposes a method for producing a thick steel plate that improves uniform elongation (uniform elongation) and is excellent in strength-ductility balance and weldability. The technique described in Patent Document 2 includes a steel material containing, by mass%, C: 0.01 to 0.10%, Mn: 1.5 to 7.0%, and further adjusting Si, Al, Ti, and N to an appropriate range. Heating and forced cooling after hot rolling, followed by heat treatment to maintain the temperature in a specific range of the two-phase region, the structure fraction of residual austenite (residual γ) in the range of 1.0-30% It is the manufacturing method of a thick steel plate which makes it the quantity which satisfies specific conditions in the inside. According to the technique described in Patent Document 2, the residual γ is stabilized and the residual γ of a desired amount or more can be secured, the uniform elongation can be improved, and an excellent strength-ductility balance can be secured. However, in the technique described in Patent Document 2, it is necessary to contain a large amount of alloy elements, which leads to an increase in material cost and weldability.

Further, for example, Patent Document 3 describes that uniform elongation is improved by utilizing precipitation of Cu. The technique described in Patent Document 3 is weight%, C: 0.2% or less, Si and Mn are adjusted to appropriate amounts, and further rolling of steel containing Cu: 0.5 to 5.0% is completed at 750 ° C. or higher. After that, it cools to room temperature at a relatively slow cooling rate to room temperature, or finishes rolling at 750 ° C. or higher, and then rapidly cools and ages within a predetermined temperature range, so that 9R structure Cu is formed in the crystal grains. It is a high-strength steel in which particles and Cu particles with bcc structure or Cu particles with fcc structure are combined and dispersed, and it is said that uniform elongation is excellent.
Japanese Patent No. 3345501 JP 2006-131958 A Japanese Patent No. 3694383

However, in the technique described in Patent Document 3, in order to ensure the desired high strength and high uniform elongation, a large amount of Cu content exceeding 1% is required. There is a concern about a decrease in hot workability, which may lead to a significant decrease in productivity, and also has a problem in practical use because it deteriorates the weldability.
The present invention advantageously solves the above-mentioned problems of the prior art, has yield strength of 480 MPa or more and excellent low-temperature toughness without causing reduction in productivity and increase in manufacturing cost, and strength-elongation balance. An object of the present invention is to provide a method for producing a high-strength steel sheet that is capable of stably producing a high-strength steel sheet that is excellent in terms of economy and that is excellent in economy.

  In the present invention, “excellent low temperature toughness” and “high toughness” refer to the case where the fracture surface transition temperature vTrs of the Charpy impact test is −80 ° C. or lower, and “excellent in strength-elongation balance”. Means the product of tensile strength TS and elongation El obtained using JIS No. 5 tensile test specimen (total thickness), and TS × El is 30,000 MPa% or more.

In order to achieve the above-described problems, the present inventors have intensively studied various factors that affect strength, low temperature toughness, and elongation. As a result, yield strength: 480MPa and higher strength and excellent low temperature toughness can be maintained while improving the elongation (total elongation), the microstructure of the center of the steel sheet, the hardness of the front and back layers, and the strength of the front and back layers It came to mind that it is important to properly adjust the hardness uniformity. Further, (a) the elongation is improved by making the steel sheet structure a composite structure of ferrite and bainite, (b) the processed ferrite introduced into the surface layer by hot rolling inhibits the improvement of the elongation,
(C) Elongation is improved by applying a tempering treatment that preferentially heats only the surface layer,
(D) The above-described tempering treatment reduces the steel sheet surface hardness, reduces the surface hardness variation in the steel sheet, and the hardness variation in the plate thickness direction,
I found out. Also,
(F) In the tempering process in which only the surface layer is preferentially heated, it is effective to use an induction heating device and concentrate the induced current on the surface layer.
I came up with it.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, including C: 0.03-0.18%, Si: 0.01-0.55%, Mn: 0.5-2.0%, Al: 0.005-0.1%, N: 0.005% or less, from the remainder Fe and inevitable impurities the composition of the steel material made, after heating the steel material to a temperature in the range of 1000-1350 ° C., a temperature in the range rolling end temperature is the surface temperature Ar less than ₃ transformation point (Ar ₃ transformation point -80 ° C.) or more Accelerated cooling to cool the steel sheet to a temperature range of 620 ° C. or less at the average temperature of the steel sheet at the cooling rate exceeding air cooling after completion of the hot rolling, After completion of the accelerated cooling, using an induction heating device, a tempering treatment in which the plate thickness center temperature of the steel plate is 580 ° C. or lower and the maximum reached temperature of the steel plate surface is a temperature in the range of 580 to 700 ° C. Yield strength: High toughness high tensile strength steel sheet with high strength of 480 MPa or more and excellent strength-elongation balance Manufacturing method.

(2) In (1), in addition to the above composition, in terms of 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 A method for producing a high-toughness high-tensile steel sheet, comprising one or more selected from: 0.1% or less and B: 0.002% or less.
(3) In (1) or (2), in addition to the said composition, the manufacturing method of the toughness high-tensile steel plate characterized by containing Ti: 0.025% or less further by the mass%.

  (4) In any one of (1) to (3), in addition to the above-described composition, the method further comprises a mass% of Ca: 0.005% or less, and a method for producing a high toughness high-tensile steel sheet.

  According to the present invention, a high-toughness high-tensile steel sheet having high yield strength: 480 MPa or more and excellent low-temperature toughness and excellent strength-elongation balance can be produced at low cost and with a decrease in productivity. It can be manufactured stably without any significant industrial effects.

The present invention, after hot rolling the steel material to a steel plate with a desired thickness, after the hot rolling is finished, subjected to accelerated cooling and tempering using an induction device, yield strength This is a method for producing a high-tensile steel sheet having a high strength of 480 MPa or more and a high toughness high-tensile steel sheet having an excellent strength-elongation balance.
First, the reasons for limiting the composition of the steel material used will be described. The mass% in the composition is simply expressed as%.

C: 0.03-0.18%
C is an element that increases the strength of the base material of the steel sheet, and needs to be contained by 0.03% or more in order to ensure a desired high strength. When the content is less than 0.03%, a large amount of hardenability-improving elements such as Cu, Ni, Cr, and Mo is required, which causes a rise in manufacturing cost, a decrease in weldability, and high heat input welding. In this case, the dilution of C into the weld metal is reduced, and it becomes difficult to ensure the desired weld joint strength. On the other hand, an excessive content exceeding 0.18% leads to a decrease in the toughness of the steel plate base metal and the resistance to weld cracking, and also a decrease in the weld joint toughness. For this reason, C was limited to the range of 0.03-0.18%.

Si: 0.01-0.55%
Si is an element effective in securing the base metal strength and weld joint strength of the steel sheet, and in the present invention, it needs to be contained in an amount of 0.01% or more. However, a large content exceeding 0.55% causes a decrease in resistance to weld cracking and a decrease in weld joint toughness. For this reason, Si was limited to the range of 0.01 to 0.55%.

Mn: 0.5-2.0%
Mn is an element effective in securing the base metal strength and weld joint strength of the steel sheet, and in the present invention, it needs to be contained at 0.5% or more. However, a large content exceeding 2.0% lowers the resistance to weld cracking and leads to an increase in hardenability more than necessary, thereby lowering the base metal toughness and weld joint toughness. For this reason, Mn was limited to the range of 0.5 to 2.0%. In addition, Preferably, it is 1.6% or less.

Al: 0.005-0.1%
Al is an element that acts as a deoxidizer for steel and combines with N to refine crystal grains and contribute to the improvement of the toughness of the base metal. To ensure the effect as a deoxidizer, 0.005% or more In addition, the content of about 0.01% or more is required for crystal grain refinement. On the other hand, the content exceeding 0.1% lowers the base metal toughness. For this reason, Al was limited to the range of 0.005 to 0.1%.

N: 0.005% or less N reacts with Al, Nb and the like to form precipitates, refines crystal grains, improves the base material toughness, and contributes to the improvement of the base material strength of the steel sheet. Such an effect becomes remarkable when the content of N is 0.0005% or more. However, when the content exceeds 0.005%, the base metal toughness and the high heat input welded joint toughness are deteriorated. For this reason, N was limited to 0.005% or less, preferably 0.0005% or more.

The above components are basic components. In addition to the basic components, 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, B: One or more selected from 0.002% or less, and / or Ti: 0.025% or less, and / or Ca: 0.005% or less, optionally selected and contained it can.
One selected from 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, B: 0.002% or less 2 or more types
Cu, Ni, Cr, Mo, Nb, V, and B are all elements that have an effect of contributing to an increase in the strength of the steel sheet. Two or more kinds can be contained.

Cu has the effect of improving the weather resistance in addition to the above-described effects. In order to ensure such an effect, it is desirable to contain 0.05% or more. However, if it exceeds 0.8%, hot workability and weldability are deteriorated. For this reason, when it contains, it is preferable to limit Cu to 0.8% or less.
Ni contributes to the increase in steel plate strength as described above through improvement in hardenability, and has the effect of improving weather resistance and toughness. In order to ensure such an effect, it is desirable to contain 0.05% or more, but inclusion exceeding 2% causes a rise in material cost. For this reason, when it contains, it is preferable to limit Ni to 2% or less.

Cr is an element that has the effect of improving the weather resistance and contributes to the increase in strength of the steel sheet through the improvement of hardenability, and can be added as necessary. Reduce. For this reason, when contained, Cr is preferably limited to 1% or less.
Mo is an element that contributes to the increase in steel sheet strength through the improvement of hardenability and the formation of precipitates, and can be added as necessary. On the other hand, the content exceeding 0.8% is harder than necessary. Increase the weldability. For this reason, when it contains, it is preferable to limit Mo to 0.8% or less.

  Nb is an element that contributes to the increase in the steel sheet strength described above through the formation of precipitates. In order to ensure such an effect, Nb is preferably contained in an amount of 0.005% or more, but a large amount exceeding 0.05%. Containing does not contribute to an increase in steel sheet strength, and lowers the toughness of the heat affected zone. For this reason, when it contains, it is preferable to limit Nb to 0.05% or less. More preferably, it is 0.03% or less.

V is an element that contributes effectively to the increase of the steel sheet strength and the securing of the weld joint strength through the formation of precipitates. To obtain such an effect, V is contained in an amount of 0.01% or more. However, the content exceeding 0.1% lowers the weld cracking resistance. For this reason, when it contains, it is preferable to limit V to 0.1% or less.
B is an element that enhances hardenability by addition of a very small amount and contributes effectively to the increase in steel sheet strength as described above through improvement of hardenability. To obtain such an effect, B is contained in an amount of 0.0005% or more. It is preferable. On the other hand, if the content exceeds 0.002%, the formation of BN becomes remarkable, the hardenability decreases, and the weld heat affected zone becomes hardened. For this reason, when it contains, it is preferable to limit B to 0.002% or less.

Ti: 0.025% or less
Ti has the effect of forming precipitates and refining the structure, forming TiN, preventing B from binding with N, and contributing effectively to securing B content effective for hardenability. It can be contained as needed. In order to acquire such an effect, it is preferable to contain 0.005% or more, but inclusion exceeding 0.025% reduces the toughness of the steel sheet. For this reason, when it contains, it is preferable to limit Ti to 0.025% or less.

Ca: 0.005% or less
Ca is an element having an action of controlling the form of sulfide that adversely affects toughness, such as MnS, to a nearly spherical form that is advantageous for improving toughness, and can be contained as necessary. In order to acquire such an effect, it is preferable to contain 0.001% or more, but inclusion exceeding 0.005% reduces the cleanliness of steel. For this reason, when it contains, it is preferable to limit Ca to 0.005% or less.

The balance other than the above components is Fe and inevitable impurities. As unavoidable impurities, P: 0.015% or less and S: 0.015% or less are acceptable.
The manufacturing method of the steel material having the above-described composition is not particularly limited in the present invention, and any generally known method can be applied. It is preferable that the molten steel having the above composition is melted by a known melting method such as a converter and used as a steel material such as a slab by a known method such as a continuous casting method or an ingot-bundling rolling method.

In the present invention, the steel material having the above composition is used as a starting material, the steel material is heated to form a steel plate having a desired thickness, and accelerated cooling is performed immediately after the hot rolling to cool the steel plate. After the accelerated cooling, a tempering process using an induction heating device is performed.
Hot rolling heating temperature: 1000-1350 ° C
The heating temperature of the steel material is 1000-1350 ° C. When the heating temperature is less than 1000 ° C., the alloying elements in the steel material are made uniform and the precipitate strengthening elements such as Mo, Nb, and V are not sufficiently dissolved, and the desired strength and toughness cannot be secured. On the other hand, when the heating temperature exceeds 1350 ° C., the crystal grains are coarsened and the toughness of the base material may be lowered. For this reason, the heating temperature of the steel material was limited to a temperature in the range of 1000 to 1350 ° C. In addition, Preferably it is 1250 degrees C or less.

Rolling end temperature of hot rolling: less than Ar ₃ transformation point (Ar ₃ transformation point−80 ° C.) or more In the present invention, the rolling end temperature of hot rolling is defined as the surface temperature in order to refine austenite (γ) grains. Thus, it is limited to a range below the Ar ₃ transformation point (Ar ₃ transformation point−80 ° C.) or more. When the rolling end temperature is not less than the Ar ₃ transformation point at the surface temperature, the γ grains become coarse and the toughness decreases. On the other hand, when the rolling end temperature is as low as less than (Ar ₃ transformation point −80 ° C.), ferrite processed to the vicinity of the center of the plate thickness is formed, and the elongation is lowered. For this reason, the rolling finishing temperature of hot rolling is limited to more than Ar less than ₃ transformation point (Ar ₃ transformation point -80 ° C.).

The Ar ₃ transformation point is, for example, the following formula (1)
Ar ₃ (° C.) = 910−310C−80Mn−20Cu−15Cr−55Ni−80Mo (1)
(Here, C, Mn, Cu, Cr, Ni, Mo: content of each element (mass%))
Thus, it can be calculated from the amount of alloying elements contained. In addition, in the above-described equation (1), for elements not included, the content of the element in the equation is assumed to be zero.

  In addition, from the viewpoint of improving the base material toughness and maintaining it stably, the cumulative rolling reduction in hot rolling is preferably 50% or more, and in order to obtain further effects, a temperature of 1050 ° C. or less It is desirable that the cumulative rolling reduction in the region is 20% or more. Thereby, recrystallization of austenite grains is promoted, the resulting structure is refined, and the base material toughness is stably improved. Further, from the viewpoint of improving the base material toughness and maintaining it stably, it is preferable that the rolling reduction of each rolling pass is 5% or more. More preferably, it is 10% or more.

Cooling stop temperature of accelerated cooling: 620 ° C or less Accelerated cooling starts immediately after hot rolling is finished (preferably within 180 s) and starts cooling at a cooling rate exceeding air cooling, and the average temperature of the steel sheet is 620 ° C or less. It is set as the process which cools to an area. By performing such accelerated cooling, the growth of the produced ferrite grains is suppressed and the ferrite grains are refined, and the transformation of untransformed γ to bainite (bainite transformation) is promoted. Thereby, desired steel plate strength and excellent base material toughness can be ensured.

  If the cooling stop temperature of accelerated cooling exceeds 620 ° C., which is the average temperature of the steel sheet, the bainite transformation does not proceed sufficiently, so that it is difficult to ensure a desired high strength. Further, when the stop temperature is low, for example, less than 400 ° C., the recuperation of the surface after cooling is small, the surface is cured, and the effect of softening by subsequent tempering is small. For this reason, the cooling stop temperature for accelerated cooling is limited to 620 ° C. or lower, preferably 400 to 620 ° C. The accelerated cooling is performed at a cooling rate exceeding air cooling. Preferably, it is 5 ° C./s or more on average from the start of cooling to the stop of cooling. Note that when the cooling rate is equal to or lower than air cooling, a desired steel plate strength cannot be secured.

  Moreover, the cooling stop temperature and cooling rate of accelerated cooling were defined by the average temperature in the steel sheet thickness direction. The average temperature is the temperature most closely related to the overall material of the steel sheet, and in the present invention, the average temperature is used as a standard for defining accelerated cooling conditions. The average temperature in the steel plate thickness direction can be obtained by simulation calculation or the like when the plate thickness, surface temperature, cooling conditions, and the like are given. For example, the temperature obtained by averaging the temperature distribution in the thickness direction using the difference method can be set as the average temperature.

The steel sheet subjected to accelerated cooling is then tempered using an induction heating device.
In the tempering process in the present invention, only the surface layer of the steel sheet is preferentially heated. By preferentially heating only the surface layer, dislocations in the processed ferrite introduced in a large amount in the vicinity of the surface layer by hot rolling disappear, the processed ferrite recovers, and the elongation of the steel sheet is improved. Further, the processed ferrite and bainite of the surface layer are tempered and softened, whereby the hardness of the steel plate surface layer is lowered and the hardness distribution in the steel plate thickness direction is made uniform. Moreover, due to the cooling rate variation at the time of accelerated cooling due to differences in scale properties etc. of the steel sheet surface, there is a variation in surface hardness even within the same steel sheet, but by heating only the surface layer of the steel sheet, Variations in surface hardness within the steel sheet are reduced. In the present invention, such a tempering process in which only the surface layer of the steel sheet is preferentially heated can be performed using, for example, an induction heating apparatus 10 as shown in FIG. By using an induction heating device, the induction current can be concentrated on the surface layer and heated rapidly, giving a temperature distribution in which the temperature of the surface layer is higher than the center of the steel sheet, and preferentially heating only the surface layer it can.

In addition, the location of the induction heating device may be on-line or off-line, and is not particularly limited. However, from the viewpoint of energy cost and production efficiency, on-line so that heating can be performed immediately after completion of accelerated cooling. It is preferable to be on.
In the tempering treatment of the present invention, the heating temperature is set so that the center thickness of the steel sheet is 580 ° C. or less and the maximum temperature reached on the steel sheet surface is in the range of 580 to 700 ° C. according to the target characteristics. Set and heat. Such heating control can be performed by controlling input power, frequency, and the like of the induction heating apparatus.

  In the tempering process, if the maximum temperature reached on the surface of the steel sheet is less than 580 ° C, the surface layer processed ferrite is not sufficiently softened, the degree of elongation is insufficient, and the desired strength-elongation balance cannot be secured. . On the other hand, when the temperature exceeds 700 ° C., the temperature rise inside the steel sheet also increases, and the strength of the steel sheet as a whole may decrease significantly, and the carbides become coarse and the toughness decreases. For this reason, the maximum temperature reached on the surface of the steel sheet in the tempering process is set to a temperature in the range of 580 to 700 ° C. In addition, Preferably it is 620 degreeC or more.

  Moreover, when the plate thickness center temperature of the steel sheet exceeds 580 ° C. by heating in the tempering treatment, the strength of the steel sheet is significantly reduced, and the desired steel sheet strength cannot be ensured. For this reason, in the tempering process, the plate thickness center temperature of the steel plate was limited to 580 ° C. or less. In addition, Preferably it is 560 degrees C or less. Here, the plate thickness center temperature of the steel plate refers to the highest temperature reached at the plate thickness center when the temperature distribution inside the steel plate becomes substantially uniform after heating by the induction device.

  Hereinafter, the present invention will be described in detail based on examples.

The steel material having the composition shown in Table 1 was subjected to hot rolling, accelerated cooling, and tempering treatment under the conditions shown in Table 2 to obtain a thick steel plate having the thickness shown in Table 2. In the accelerated cooling, water cooling was started from the cooling start temperature (surface temperature) shown in Table 2 immediately after the end of hot rolling, the cooling was stopped at the temperature (average temperature) shown in Table 2, and air cooling was performed. Moreover, the tempering process was heated using the induction heating apparatus so that it might become the highest ultimate temperature of a steel plate surface shown in Table 2, and plate | board thickness center temperature. In some cases, the tempering process was omitted after the accelerated cooling was stopped. The Ar ₃ transformation point shown in the table was calculated using equation (1).

Test pieces were collected from the obtained thick steel plates and subjected to a tensile test, a Charpy impact test, and a hardness test. The test method was as follows.
(1) Tensile test Tensile test pieces (JIS No. 5 full-thickness test pieces) were taken from 1 / 2t part of the thickness of each steel plate obtained and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to yield. Strength YS, tensile strength TS, and elongation El were determined. Furthermore, TS × El was calculated from the obtained tensile strength TS and elongation El, and the strength-elongation balance was evaluated.

(2) Charpy impact test Charpy impact test specimens (V-notch test specimens) were sampled around the position of the thickness 1 / 2t of each steel plate obtained, and Charpy impact tests were conducted in accordance with the provisions of JIS Z 2242. The fracture surface transition temperature vTrs was obtained and the low temperature toughness was evaluated.
(3) Hardness test At the central part in the width direction of the obtained thick steel plate, a hardness test piece (t x 15 mm x 20 mm) is taken from the central part in the length direction, and a cross section in the thickness direction perpendicular to the rolling direction is taken. After polishing, measure the Vickers hardness HV10 at a pitch of 1mm in the thickness direction with a Vickers hardness tester (test force: 98N), obtain the hardness distribution in the thickness direction, and calculate the difference between the maximum hardness and the minimum hardness, ΔHV . A case where ΔHV was 45 HV or higher was evaluated as nonuniform hardness distribution.

  The obtained results are shown in Table 3.

本発明例はいずれも、降伏強さ：480MPa以上の高強度と、破面遷移温度vＴrs：−80℃以下の優れた低温靭性と、TS×Elが30000MPa以上の優れた強度−伸びバランスを有し、かつΔＨＶが45HV未満の均一な板厚方向の硬さ分布を有する高靭性高張力鋼板となっている。一方、本発明の範囲を外れる条件で製造された厚鋼板は、強度が不足するか、低温靭性が低下しているか、あるいは強度−伸びバランスが低下しているか、さらには板厚方向の硬さ分布が不均一となっていた。

Each of the inventive examples has a high yield strength of 480 MPa or more, an excellent low temperature toughness of fracture surface transition temperature vTrs of −80 ° C. or less, and an excellent strength-elongation balance of TS × El of 30000 MPa or more. In addition, a high toughness and high strength steel sheet having a uniform thickness distribution in the thickness direction with ΔHV of less than 45 HV. On the other hand, a thick steel plate manufactured under conditions outside the scope of the present invention has insufficient strength, low-temperature toughness, low strength-elongation balance, and hardness in the thickness direction. Distribution was uneven.

  In the comparative example (steel plate No. 2) in which the rolling end temperature of the hot rolling deviates from the range of the present invention is high, the low temperature toughness is lowered. Further, in the comparative example (steel plate No. 3) not subjected to the tempering treatment, no improvement in elongation was observed, and the strength-elongation balance was lowered. Moreover, the low-temperature toughness of the comparative example (steel plate No. 4) in which the plate thickness center temperature in the tempering treatment is outside the range of the present invention is low. Further, in the comparative examples (steel plates No. 7 and No. 15) in which the C and Mn contents of the steel material are out of the scope of the present invention, the desired high strength is not obtained.

It is explanatory drawing which shows typically an example of the induction heating apparatus suitable for the tempering process of this invention. (A) is a top view, (b) is a side view, (c) is a front view.

Explanation of symbols

1 Steel plate 10 Induction heating device 30 Table roller

Claims (4)

% By mass
C: 0.03-0.18%, Si: 0.01-0.55%,
Mn: 0.5-2.0%, Al: 0.005-0.1%,
N: A steel material containing 0.005% or less, the balance being Fe and inevitable impurities, and heating the steel material to a temperature in the range of 1000 to 1350 ° C., the rolling end temperature is the surface temperature and the Ar ₃ transformation point Less than (Ar ₃ transformation point −80 ° C.) or higher, and hot rolling to obtain a steel sheet with a desired plate thickness. After the hot rolling is completed, the average of the steel sheets is cooled at a cooling rate exceeding air cooling. Accelerated cooling that cools to a temperature range of 620 ° C. or less at the temperature, and after completion of the accelerated cooling, using an induction heating device, the plate thickness center temperature of the steel plate is 580 ° C. or less, and the maximum reached temperature of the steel plate surface is 580 to 700 ° C. A high toughness, high-tensile steel sheet having a high strength of 480 MPa or more and excellent strength-elongation balance. Manufacturing method.   In addition to the above composition, 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, B: 0.002 The method for producing a high-toughness high-tensile steel sheet according to claim 1, comprising one or more selected from 1% or less.   The method for producing a high-toughness high-tensile steel sheet according to claim 1 or 2, further comprising Ti: 0.025% or less by mass% in addition to the composition.   The method for producing a high-toughness high-tensile steel sheet according to any one of claims 1 to 3, further comprising Ca: 0.005% or less by mass% in addition to the composition.

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