JP2015193917A - Refining high tension thick steel and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for stably producing a refining high tension thick steel in which intensity and work ability are achieved, even when new equipment is not used, in a method for making only a surface layer of steel soft to secure intensity of inside of the steel for achieving both of intensity and work ability.SOLUTION: A thick steel is taken out from an atmosphere furnace in a midway of temperature rising, in normalizing processing, for acquiring temperature difference between a surface layer and inside of the thick steel. With the temperature difference, a rough ferrite is formed on the surface layer, and fine ferrite is formed in inside.

Description

  The present invention relates to a tempered high-tensile steel plate excellent in bending workability and toughness, and a method for producing the same.

  Steel sheets used for the manufacture of steel structures such as buildings, bridges, storage tanks, pressure vessels, etc. are required to have excellent strength and toughness as well as excellent workability during forming. Generally, bending workability worsens as the strength of a steel plate increases.

  Therefore, in order to achieve both strength and workability, a method of softening only the surface layer portion of the steel sheet has been proposed.

  For example, Patent Documents 1 and 2 propose a method in which only the surface layer is softened by accelerated cooling after rolling and then induction heating. However, the techniques of Patent Document 1 and Patent Document 2 require new heating equipment in addition to the existing equipment, which increases the manufacturing cost of the steel sheet.

  In addition, the technique described in Patent Document 3, which is a method of softening only the surface layer portion of the steel sheet and ensuring the strength inside the steel sheet, performs accelerated cooling during rolling, in which case the accelerated cooling is divided into a plurality of parts, and a predetermined It includes a step of rolling in a temperature range. However, in the technique described in Patent Document 3, a new cooling device is required to perform rolling even during or immediately after cooling. Moreover, since temperature control is difficult for accelerated cooling, it is difficult to stably produce a steel sheet that satisfies the rolling and cooling conditions proposed in Patent Document 3, and the yield is poor.

JP2011-195961A JP 2005-298963 A Japanese Patent Laid-Open No. 9-165552

  The present invention has been made in order to solve the above-mentioned problems. The purpose of the present invention is to provide a new facility in a method of softening only the surface portion of a steel sheet and ensuring the strength inside the steel sheet in order to achieve both strength and workability. It is to provide a technology capable of stably producing a tempered high-tension thick steel plate that achieves both strength and workability even when not used.

  Usually, when a thick steel plate formed by hot rolling is heated to a predetermined temperature T ° C. or higher and normalized, the thick steel plate is taken out of the heating furnace after the whole thick steel plate reaches T ° C. or higher. However, when trying to soften the surface layer of the thick steel plate by this method, coarse ferrite is generated up to the inside of the thick steel plate, and the strength of the entire steel plate is lowered.

  Therefore, as a result of intensive studies to solve the above problems, the present inventors have obtained the following knowledge.

  In the normalizing process, by removing the thick steel plate from the heating furnace in the middle of the temperature rise, a temperature difference can be made between the surface layer and the inside of the thick steel plate. By utilizing this, coarse ferrite can be formed on the surface layer, and fine ferrite can be formed inside. Since fine ferrite has higher strength than coarse ferrite due to the effect of grain refinement, the strength of the surface layer can be lowered and the bending workability can be improved while maintaining the strength of the entire steel sheet.

  Moreover, the higher the temperature of the heating furnace, the larger the temperature difference between the steel sheet surface layer and the steel sheet during the temperature raising process.

  The present invention has been completed based on the above findings, and specifically, the present invention provides the following.

(1) By mass%, C: 0.04 to 0.30%, Si: 0.50% or less, Mn: 2.0% or less, P: 0.020% or less, S: 0.006% or less, Al: 0.05% or less, N: 0.0060% or less, Ti: 0.005 to 0.30%, having a component composition consisting of the balance Fe and inevitable impurities, and the surface of the thick steel plate in the thickness direction To ₁ mm (t ₁ = (thick steel plate thickness t) × 0.1), the average ferrite grain size of the surface layer of the thick steel plate is 30 μm or more, and ±± in the thickness direction from the center of the plate thickness The average ferrite grain size in the central portion of the thick steel plate, which is 2 mm, is 15 μm or less, and the average ferrite grain size in the surface layer portion of the thick steel plate is 2.00-5. Tempered high-tensile steel plate, characterized by being 00 times.

  (2) Further, by mass%, one or two selected from Ca: 0.0005-0.0050%, REM: 0.0010-0.0050%, Mg: 0.0010-0.0050% The tempered high-tensile thick steel plate according to (1), which contains seeds or more.

(3) A hot rolling process in which a steel slab having the composition described in (1) or (2) is heated to a temperature of 1000 to 1250 ° C. and subjected to hot rolling at a rolling finishing temperature of 800 ° C. or higher, And a normalizing step in which a normalizing treatment is performed on the thick steel plate after the hot rolling step. In the normalizing step, the thick steel plate was calculated by heat transfer calculation from the heating conditions when the thick steel plate was taken out from the heating furnace. Thick steel plate surface temperature (T _s ) and thick steel plate center temperature (T _c ) in the thickness direction satisfy the following (Equation 1) to (Equation 4): The average ferrite grain size of the thick steel plate surface layer portion, which is an area up to ₁ mm (t ₁ = (thick steel plate thickness t) × 0.1), is 30 μm or more, and ± 2 mm in the plate thickness direction from the plate thickness center position. The average ferrite grain size in the central portion of the thick steel plate, which is the region, is 15 μm or less, and A method for producing a tempered high-tensile thick steel plate having an average ferrite grain size in the surface layer of the thick steel plate that is 2.00 to 5.00 times the average ferrite grain size in the central portion of the thick steel plate.
A _c3 + 30 ≦ T _s (° C.) ≦ A _c3 +120 (Formula 1)
V _s (° C./min)≧−0.0036×t+0.54 (Formula 2)
A _c3 ≦ T _c (° C.) ≦ A _c3 +20 (Formula 3)
T _s −T _c ≧ 30 (° C.) (Formula 4)
_However, an A c3 = 937-476.5C + 56Si-19.7Mn + 136.3Ti + 198.4Al ( element symbol means the content of each element (mass%).),
V _s is the 5 minutes before the surface temperature of the steel plate is _{T s,} the steel plate surface temperature calculated by the heat transfer calculations from the heating condition when the _{T s-5, V s =} (T s -T _s-5 ) / 5,
t is the thickness of the thick steel plate to be subjected to the normalizing process.

  In addition, since the manufacturing method of the steel plate of this invention provides a temperature difference in the surface layer and plate | board thickness center of a steel plate in a normalization process, application is difficult unless it is a steel plate to some extent or more. Therefore, the production method of the present invention can be suitably applied to a steel plate having a thickness of 10 mm to 80 mm.

  The tempered high tensile steel plate of the present invention is excellent in strength and bending workability and can be manufactured stably.

  Moreover, when manufacturing the tempered high tension thick steel plate of this invention, the existing heating furnace can be used and it is not necessary to use a new installation like the prior art. The steel sheet of the present invention has a tensile strength of 550 MPa or more.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

Component composition The tempered high-tensile steel plate of the present invention is in mass%, C: 0.04 to 0.30%, Si: 0.50% or less, Mn: 2.0% or less, P: 0.020%. Hereinafter, S: 0.006% or less, Al: 0.05% or less, N: 0.0060% or less, Ti: 0.005 to 0.30%, the component composition consisting of the balance Fe and inevitable impurities Have. Further, the tempered high-tensile thick steel plate of the present invention includes, as optional components, Ca: 0.0005-0.0050%, REM: 0.0010-0.0050%, Mg: 0.0010-0.0050%. You may include 1 type, or 2 or more types chosen from the inside. Hereinafter, each component will be described. In the description of the component composition, “%” means “% by mass”.

C: 0.04 to 0.30%
C is an element useful for increasing the strength of steel and imparting the necessary strength to steel structures. The “strength necessary for the steel structure” varies depending on the application, but in this specification, it means that the tensile strength (TS) of the tempered high-tensile steel plate is 550 MPa or more. In order to obtain such an effect, the C content needs to be 0.04% or more. Preferably, it is 0.10% or more. On the other hand, if the C content exceeds 0.30%, the weldability and toughness of the steel are significantly reduced. For this reason, content of C shall be 0.30% or less. Preferably, it is 0.20% or less.

Si: 0.50% or less Si acts as a deoxidizer and dissolves in steel to increase the strength of the steel. In order to obtain these effects, the Si content is preferably 0.01% or more. More preferably, it is 0.10% or more. On the other hand, if the Si content exceeds 0.50%, the toughness of the steel decreases. For this reason, the Si content is limited to a range of 0.50% or less. Preferably it is 0.40% or less.

Mn: 2.0% or less Mn is an element that increases the strength of steel by solid solution. Moreover, it combines with S in steel to form MnS and prevent toughness deterioration due to S. In order to obtain these effects, the Mn content is preferably 0.4% or more. More preferably, it is 1.0% or more. On the other hand, if the content of Mn exceeds 2.0%, the toughness of the base metal after welding and the toughness of the weld heat affected zone (HAZ) are significantly reduced. For this reason, the Mn content is limited to 2.0% or less. Preferably it is 1.8% or less.

P: 0.020% or less P is an element that lowers toughness, particularly toughness of a welded portion. In the present invention, it is desirable to reduce the P content as much as possible. Excessive reduction of the cost increases the refining cost and is economically disadvantageous. For this reason, it is preferable to make content of P 0.005% or more from a viewpoint of suppressing the manufacturing cost of a thick steel plate. On the other hand, when the P content exceeds 0.020%, the above-described adverse effects become remarkable, so the P content is limited to 0.020% or less. Preferably it is 0.016% or less.

S: 0.006% or less S is present in the steel as sulfide inclusions such as MnS, and serves as a nucleus of transformation from austenite (γ) to ferrite (α), and improves the toughness of the weld. Have. In order to obtain such an effect, the S content is preferably 0.0010% or more. On the other hand, if the S content exceeds 0.006%, a large amount of MnS is generated in the central segregation portion of the steel and the toughness is lowered. Moreover, inclusion of S exceeding 0.006% facilitates generation of defects in slabs and the like. Therefore, the S content is 0.006% or less. For this reason, it is preferable to make content of S into the range of 0.0010 to 0.006%. In addition, More preferably, it is 0.0010 to 0.0025%.

Al: 0.05% or less Al is an element that acts as a deoxidizer. In the high-strength steel deoxidation process, Al is most commonly used as a deoxidizer. In order to obtain such an effect, the Al content is preferably 0.01% or more. Further, if the Al content exceeds 0.05%, the HAZ toughness after welding is lowered, and Al is mixed into the weld metal during welding, so that the toughness of the weld metal is also lowered. For this reason, the content of Al is limited to 0.05% or less. Preferably it is 0.04% or less.

N: 0.0060% or less N, when dissolved in steel, causes strain aging after cold working and deteriorates toughness. For this reason, it is preferable to reduce solute nitrogen as much as possible by adding a nitride-forming element such as Ti and fixing it as a nitride. Nitrides such as TiN prevent grain coarsening by pinning grain boundaries, or act as ferrite transformation nuclei and contribute to the improvement of HAZ toughness. For this reason, N is preferably 0.0010% or more. On the other hand, when the N content exceeds 0.0060%, even if the nitride is fixed as a nitride by a nitride-forming element such as Ti, the nitride becomes coarse and the deterioration of toughness becomes remarkable. For this reason, the N content is limited to 0.0060% or less. Preferably it is 0.0050% or less.

Ti: 0.005 to 0.30%
Ti is an element having a strong affinity for N, and precipitates as TiN during solidification, thereby reducing solid solution N in the steel and reducing the toughness deterioration due to strain aging of N after cold working. Ti also contributes to the improvement of HAZ toughness through the improvement of the HAZ structure. In order to obtain such an effect, the Ti content is set to 0.005% or more. On the other hand, if the Ti content exceeds 0.30%, the TiN particles become coarse and the above-described effects cannot be expected. For this reason, the Ti content is limited to a range of 0.005 to 0.30%.

  The above-described components are basic components. In addition to these basic components, if necessary, as optional elements, Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, Mg : One or two selected from 0.0010 to 0.0050% can be contained.

  Ca, Mg, and REM all contribute to the improvement of the ductility of steel and the toughness of the base material after welding through the control of sulfide morphology. Further, when fine sulfide particles containing these elements are dispersed in steel, the dispersed sulfide particles act as ferrite transformation nuclei, thereby improving the HAZ toughness. In order to obtain these effects, if Ca, the content is preferably 0.0005% or more, and if REM, the content is preferably 0.0010% or more. For example, the content is preferably 0.0010% or more. Moreover, when each content of Ca, Mg, and REM exceeds 0.0050%, an excessive inclusion will produce | generate and toughness will fall. Therefore, the content of Ca, Mg, and REM is preferably in the above range.

  The balance other than the above components is Fe and inevitable impurities. As an inevitable impurity, for example, O: 0.005% or less is acceptable.

Steel structure Next, the steel structure of the tempered high-tensile steel sheet of the present invention will be described. In the tempered high-tensile steel plate of the present invention, a steel plate surface layer portion that is a region from the steel plate surface to t ₁ mm (t ₁ = (thick steel plate thickness t) × 0.1) in the plate thickness direction; The central portion of the thick steel plate which is an area of ± 2 mm in the plate thickness direction from the plate thickness central position has the following characteristics.

  In addition, when there is a region between the thick steel plate surface layer portion and the thick steel plate central portion, the steel structure of the region is the same steel structure as the thick steel plate surface layer portion, the same steel structure as the thick steel plate central portion, other than these Any of steel structures may be used. “A steel structure other than these” can be exemplified by a steel structure having an average ferrite grain size of less than 2.00 times the average ferrite grain size in the central portion of the thick steel plate.

  In the tempered high-tensile thick steel plate of the present invention, the average ferrite particle size of the thick steel plate surface layer portion is 30 μm or more, and the average ferrite particle size of the thick steel plate central portion is 15 μm or less. When the average ferrite grain size of the surface layer portion of the thick steel plate is less than 30 μm, the effect of softening the surface layer is small and sufficient bending workability cannot be obtained. On the other hand, when the average ferrite grain size in the central part of the thick steel plate exceeds 15 μm, the strength of the whole steel plate is reduced due to less refinement strengthening. Furthermore, the average ferrite particle size of the thick steel plate surface layer portion is 2.00 to 5.00 times the average ferrite particle size of the thick steel plate central portion. When the average ferrite grain size of the steel plate surface layer is less than 2.00 times the average ferrite particle size of the thick steel plate center, the difference in strength between the thick steel plate surface and the thick steel plate center is small, and sufficient bending workability is achieved. The strength of the entire steel sheet cannot be achieved. Moreover, when it exceeds 5.00 times, a surface layer will soften too much and the intensity | strength of the whole steel plate will fall. In the case where ferrite is not included in the structure, the ratio of the average ferrite particle size of the thick steel plate central portion and the average ferrite particle size of the thick steel plate surface layer portion is obtained assuming that the ferrite particle size is 0 for convenience. And

  Moreover, in order to make the effect of the present invention higher, it is preferable that the average ferrite particle size of the thick steel plate surface layer portion is 40 to 70 μm, and the average ferrite particle size of the thick steel plate central portion is 3 to 15 μm. It is preferable.

  The surface layer of the thick steel plate of the present invention contains a ferrite phase. The ferrite phase content in the surface layer of the thick steel plate is preferably 40 to 90% in terms of area ratio. More preferably, it is 50 to 80%. Moreover, in order to make the tensile strength of a steel plate 550 Mpa or more, it is preferable that a thick steel plate surface layer part contains a pearlite and a bainite phase other than a ferrite phase. The pearlite and bainite phases are harder than the ferrite phases and contribute to the improvement of tensile strength. The total content (area ratio) of the pearlite and bainite phases is preferably 10 to 60%, more preferably 20 to 50%.

  Moreover, the ferrite phase is contained in the center part of the thick steel plate of the present invention. The ferrite phase content in the central portion of the thick steel plate is preferably 40 to 90% in terms of area ratio. More preferably, it is 50 to 80%. Moreover, in order to make the tensile strength of a steel plate 550 Mpa or more, it is preferable that the center part of a thick steel plate contains a pearlite and a bainite phase other than a ferrite phase. The total content (area ratio) of the pearlite and bainite phases is preferably 10 to 60%, more preferably 20 to 50%.

Next, the manufacturing method of the tempered high-tensile steel plate of the present invention will be described. The tempered high-tensile thick steel plate of the present invention is preferably produced by the following method.

  A preferable manufacturing method includes a hot rolling process in which a steel slab having the above component composition is heated and subjected to hot rolling, and a normalizing process in which a thick steel sheet after the hot rolling process is subjected to a normalizing process.

In general, normalization is a process in which the microstructure is refined by heating to a temperature of _Ac3 or higher to transform from a ferrite phase to an austenite phase, then air-cooling and transforming from an austenite phase to a ferrite phase again. This is a heat treatment for achieving uniformity and uniformity.

  A hot rolling process is a process which heats the said steel slab to the temperature of 1000-1250 degreeC, and performs the hot rolling which makes a rolling finishing temperature 800 degreeC or more.

  When the heating temperature of the steel slab is less than 1000 ° C., the strength of the tempered high strength thick steel plate may be reduced. On the other hand, when it exceeds 1250 ° C., the structure becomes coarse and the toughness of the tempered high strength thick steel plate. In some cases, the desired ferrite grain size may not be obtained even after the subsequent hot rolling and normalizing steps. For this reason, it is preferable to make the heating temperature of a steel slab into the range of 1000 to 1250 degreeC. In addition, More preferably, it is 1080 to 1150 degreeC.

  Moreover, rolling efficiency falls that the rolling finishing temperature in hot rolling is less than 800 degreeC. Therefore, the rolling finishing temperature is set to 800 ° C. or higher.

  The normalizing process is a process of applying a normalizing process to the thick steel plate obtained in the hot rolling process. In the normalizing step of the present invention, after the hot rolling step, the thick steel plate is once cooled to about 400 ° C. or less after the ferrite transformation is completed, and then heated in a heating furnace, and then the thick steel plate is taken out of the heating furnace and air-cooled. This method is preferably performed, and this method will be described below. In addition, a heating furnace is a furnace for normal heat processing, and is not a new equipment required in order to manufacture the tempered high tension thick steel plate of this invention. Thus, the point which can utilize the existing installation is one of the characteristics of this invention.

In the production of the tempered high-tensile thick steel plate of the present invention, the timing of taking out the thick steel plate from the heating furnace in the normalizing step is important. Specifically, when the thick steel plate is taken out of the heating furnace, the thick steel plate surface temperature (T _s ) calculated by heat transfer calculation from the heating conditions, and the thick steel plate center temperature (T _c ) in the thickness direction are as follows (formula 1) to (Formula 4) are satisfied.
A _c3 + 30 ≦ T _s (° C.) ≦ A _c3 +120 (Formula 1)
V _s (° C./min)≧−0.0036×t+0.54 (Formula 2)
A _c3 ≦ T _c (° C.) ≦ A _c3 +20 (Formula 3)
T _s −T _c ≧ 30 (° C.) (Formula 4)
_However, an A c3 = 937-476.5C + 56Si-19.7Mn + 136.3Ti + 198.4Al ( element symbol means the content of each element (mass%).),
V _s is V _s = (T _s −T _s) , where T _s−5 is the steel plate surface temperature calculated by heat transfer calculation from the heating conditions 5 minutes before the steel plate surface temperature (T _s ). ₋₅ ) / 5,
t is the thickness of the thick steel plate to be subjected to the normalizing process.

In Formula 1, the steel plate surface temperature T _s calculated by heat transfer calculation from the heating conditions is set to A _c3 + 30 ° C. or more for the following reason. When the steel plate surface temperature T _s is less than A _c3 + 30 ° C., austenite crystal grains in the thick steel plate surface layer portion do not grow. As a result, ferrite crystal grains generated by transformation from austenite in cooling after normalization are small, and the average ferrite grain size on the surface of the thick steel plate cannot be made 30 μm or more. For this reason, the steel plate surface temperature T _{s is set} to A _c3 + 30 ° C. or higher. Preferably it is _Ac3 + 40 _degreeC or more.

Further, when T _s exceeds A _c3 +120 (° C.) in the formula 1, coarse austenite is formed, and thus the ferrite generated by transformation from austenite becomes coarse, and the average ferrite grain size in the thick steel plate surface layer portion is large. It exceeds 5.00 times the average ferrite grain size in the central part of the steel sheet, and the steel sheet strength decreases. Therefore, T _s was set to A _c3 +120 (° C.) or less.

For Equation 3, if the thick steel plate center temperature T _c (° C.) in the plate thickness direction calculated by heat transfer calculation from the heating conditions is A _c3 or more and A _c3 +20 (° C.) or less, the ferrite inside the steel plate is made finer. It is possible to give sufficient strength to the tempered high-tension thick steel plate by making it into grains. By heating to _Ac3 or higher, the ferrite is once transformed into austenite, and then transformed into ferrite again in the subsequent cooling, whereby a structure composed of fine ferrite grains can be obtained. On the other hand, when the steel plate center temperature T _c (° C.) in the plate thickness direction exceeds A _c3 +20 (° C.), since austenite grains grow, in the subsequent cooling, even if the austenite is transformed into ferrite again, The ferrite crystal grains become large and a desired structure cannot be obtained. From the viewpoint of making the ferrite finer, the thick steel plate center temperature T _c (° C.) is preferably as low as A _c3 or more, but if it is less than A _c3, it becomes impossible to adjust the crystal grains by normalization. For this reason, it is preferable to control the temperature with A _c3 +10 (° C.) as a target.

  As shown in Equation 4, the temperature difference between the thick steel plate surface temperature and the thick steel plate center temperature is 30 ° C. or more. When the temperature difference between the thick steel plate surface layer and the thick steel plate center is less than 30 ° C., the difference in ferrite crystal grain size between the steel plate surface layer and the thick steel plate center is small. It cannot be 2.00 times the particle size or more. For this reason, the temperature difference between the thick steel plate surface temperature and the thick steel plate center temperature is 30 ° C. or more.

In Equation 2, V _s (° C./min) is set to −0.0036 × t + 0.54 or more for the following reason. In normal normalization, after the entire thick steel plate reaches the temperature of the heating furnace, the thick steel plate is held in the heating furnace for about 5 minutes and taken out. Therefore, the normalizing normal baked, when a temperature of 5 minutes before reaching the _{T s} and _{T s-5} ° _C., the _{(T s -T s-5)} / 5 0. However, in this step, when the thick steel plate is taken out from the heating furnace, it is necessary to give a temperature difference between the thick steel plate surface layer and the inside of the thick steel plate. The temperature of the heating furnace is such that the temperature of the heating furnace is A _c3 +30 to A so that the center temperature of the thick steel sheet and the surface temperature of the thick steel sheet satisfy the above-mentioned formulas 1, 3, and 4. _{As a} result of heat transfer calculation under the conditions of _c3 + 200 ° C. and a thickness t of the thick steel plate of 15 mm to 60 mm, the surface layer of the thick steel plate can be obtained by setting V _s (° C./min) to −0.0036 × t + 0.54 or more. It was confirmed that the temperature difference inside the thick steel plate was 30 ° C. or more.

  The tempered high-tensile thick steel plate of the present invention is obtained by removing the thick steel plate from the heating furnace at the timing as described above.

  In addition, after taking out from a heating furnace, a thick steel plate is cooled by air cooling. At this time, it is preferable that the cooling rate of the thick steel plate is 0.3 ° C./s or higher and 1.0 ° C./s or lower at the steel thick steel plate center temperature. The cooling rate is an average cooling rate from 800 ° C to 500 ° C. By cooling at this cooling rate, in addition to the ferrite phase, a structure having a total content (area ratio) of pearlite and bainite phase of 10 to 60% can be obtained.

  A steel slab (thickness 250 mm) having the component composition shown in Table 1 was heated and then hot-rolled. The thick steel plate obtained by hot rolling was once cooled to 300 ° C. or less, and then heated to normalize. Specific production conditions are shown in Table 2.

In addition, "surface temperature at the time of steel plate taking out" among the conditions shown in Table 2 is the thick steel plate surface temperature (T _s ) calculated by heat transfer calculation from the heating conditions when the thick steel plate is taken out from the heating furnace. Further, the “center temperature at the time of taking out the steel plate” is the center temperature of the thick steel plate in the plate thickness direction calculated by heat transfer calculation from the heating conditions when the thick steel plate is taken out from the heating furnace.

  The following evaluation was performed about the thick steel plate manufactured as mentioned above.

[Tissue observation]
The steel sheet structure was obtained by taking a sample of a cross section perpendicular to the rolling direction, polishing the cross section to a mirror surface, then corroding with a methanolic nitric acid solution, and t ₁ mm (t ₁ = (thick steel plate thickness) from the steel plate surface to the plate thickness direction. t) × 0.1), and a range of ± 2 mm in the thickness direction from the central portion of the plate thickness is photographed with an optical microscope at a magnification of 400 times with a plurality of continuous screens. Phases were identified and the area fraction of each phase was determined. Table 3 shows the contents of the main phases. Further, from the photograph taken above, the number of ferrite grains n, the area A (mm ² ) of the ferrite was obtained, and the average ferrite particle diameter D (mm) of the ferrite phase at the thick steel plate surface layer portion and the thick steel plate central portion was as follows: It was derived from Equation 5. The measurement results are shown in Table 3.
D = 1 / (n / A) ^-1/2 (Formula 5)
[Tensile properties]
A JIS No. 5 tensile test piece is taken in the 90 ° direction (C direction) with respect to the rolling direction, and subjected to a tensile test at a crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241. Yield stress (YS), tensile Strength (TS) was measured. The measurement results are shown in Table 3. It can be evaluated that the tensile strength (TS) is 550 MPa or more.
[Bending workability]
Based on JIS Z2248 (2006), a steel material sample (width 100 mm × length 300 mm × original thickness of steel plate; tmm) is used, and the bending radius R = plate thickness (t) and 180 ° by the bending method. A bending test was performed. If the sample after the bending test had no tears or other defects, the bending workability was considered good.
[Base material toughness]
V-notch test specimens were collected from the direction perpendicular to the rolling direction at the plate thickness 1/2 position of each steel plate in accordance with JIS Z 2202 (1998), and conformed to JIS Z 2242 (1998). Then, each steel plate was subjected to a Charpy impact test at three temperatures, the absorbed energy at a test temperature of 0 ° C. was determined, and the base material toughness was evaluated. The average value of the three absorbed energy at the test temperature of −40 ° C. (sometimes referred to as “vE- ₄₀ ”) was 200 J or more, indicating excellent base material toughness.

  As shown in Table 4, the tempered high-tensile thick steel plate of the present invention has sufficient tensile properties, workability, and base material toughness.

Claims (3)

% By mass
C: 0.04 to 0.30%
Si: 0.50% or less Mn: 2.0% or less P: 0.020% or less S: 0.006% or less Al: 0.05% or less N: 0.0060% or less Ti: 0.005 to 0. Containing 30%, having a component composition consisting of the balance Fe and inevitable impurities,
The average ferrite grain size of the thick steel sheet surface layer portion, which is an area from the surface of the thick steel sheet to t ₁ mm (t ₁ = (thick steel sheet thickness t) × 0.1) in the thickness direction, is 30 μm or more, and the center of the thickness The average ferrite grain size in the central part of the thick steel plate that is ± 2 mm from the position in the thickness direction is 15 μm or less, and the average ferrite grain size in the surface layer part of the thick steel sheet is the average ferrite grain size in the central part of the thick steel plate A tempered high-tensile thick steel plate characterized by being 2.00 to 5.00 times greater than   Furthermore, by mass%, one or more selected from Ca: 0.0005-0.0050%, REM: 0.0010-0.0050%, Mg: 0.0010-0.0050% The tempered high-tensile thick steel plate according to claim 1, which is contained. A steel slab having the component composition according to claim 1 or 2 is heated to a temperature of 1000 to 1250 ° C, and a hot rolling step of performing hot rolling with a rolling finishing temperature of 800 ° C or higher,
A normalizing step of performing a normalizing treatment on the thick steel plate after the hot rolling step,
In the normalizing step, the steel plate surface temperature (T _s ) calculated by heat transfer calculation from the heating conditions when the steel plate is removed from the heating furnace, and the steel plate center temperature (T _c ) in the plate thickness direction are as follows: Thickness satisfying (Equation 1) to (Equation 4), which is a region from the surface of the thick steel plate to t ₁ mm (t ₁ = (thick steel plate thickness t) × 0.1) in the thickness direction The average ferrite particle size of the steel plate surface layer portion is 30 μm or more, the average ferrite particle size of the thick steel plate central portion, which is an area of ± 2 mm in the plate thickness direction from the plate thickness central position, is 15 μm or less, and the thick steel plate surface layer A method for producing a tempered high-tensile steel plate having an average ferrite particle size of 2.00 to 5.00 times the average ferrite particle size of the center portion of the thick steel plate.
A _c3 + 30 ≦ T _s (° C.) ≦ A _c3 +120 (Formula 1)
V _s (° C./min)≧−0.0036×t+0.54 (Formula 2)
A _c3 ≦ T _c (° C.) ≦ A _c3 +20 (Formula 3)
T _s −T _c ≧ 30 (° C.) (Formula 4)
_However, an A c3 = 937-476.5C + 56Si-19.7Mn + 136.3Ti + 198.4Al ( element symbol means the content of each element (mass%).),
V _s, when the surface temperature of the steel plate is the T _s of 5 minutes before a, the steel plate surface temperature calculated by the heat transfer calculations from heating conditions and _{T s-5, V s =} (T s - Ts _-5 ) / 5,
t is the thickness of the thick steel plate to be subjected to the normalizing process.