JP6112265B2 - High-strength extra heavy steel plate and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength extra-thick steel plate excellent in brittle crack propagation stopping characteristics, having a plate thickness of 70 mm or more, used for large structures such as ships, marine structures, low-temperature storage tanks, and construction / civil engineering structures. It relates to the manufacturing method.

  In large structures such as ships, marine structures, low-temperature storage tanks, and construction / civil engineering structures, if an accident associated with brittle fracture occurs, the impact on the socio-economic environment and the environment is large. For this reason, the large structure is always required to be improved in safety, and the steel sheet used as the material of the large structure is required to have a high level of brittle crack propagation stopping characteristics at the use temperature.

  Ships such as container ships and bulk carriers use high-strength thick steel plates for the hull outer plate due to their structure. Recently, as the hull size increases, higher strength is required and it becomes a material. Thick steel plates are becoming thicker.

  Generally, the brittle crack propagation stopping property of a steel sheet tends to deteriorate as the strength increases or the wall becomes thicker. For this reason, the request | requirement for the brittle crack propagation stop characteristic with respect to the thick steel plate used for a large-sized structure is further advanced.

  Here, as a means for improving the brittle crack propagation stopping characteristic of a steel sheet, a method for increasing the Ni content in the steel has been conventionally known. For example, in liquefied natural gas (LNG) storage tanks, 9% Ni steel is used on a commercial scale.

  However, an increase in the Ni content in the steel necessitates a significant increase in production costs. For this reason, 9% Ni steel is difficult to apply to uses other than the LNG storage tank.

  On the other hand, for a comparatively thin steel plate with a thickness of less than 50 mm used for ships and line pipes, which does not reach extremely low temperatures such as LNG, the fine graining is attempted by the TMCP method. By improving toughness, excellent brittle crack propagation stopping characteristics can be realized.

  Further, in order to improve brittle crack propagation stopping characteristics without increasing the alloy cost, Patent Document 1 proposes a steel sheet in which the structure of the surface layer portion is made ultrafine.

  In the steel sheet having excellent brittle crack propagation stopping characteristics described in the above-mentioned Patent Document 1, when a brittle crack propagates, a shear lip (plastic deformation region) generated in the surface layer portion of the steel sheet improves brittle crack propagation stopping characteristics. It was completed by paying attention to its effectiveness. It is characterized by absorbing propagation energy possessed by a propagating brittle crack by refining the crystal grains of the shear lip portion.

At the same time, as a manufacturing method, the above-mentioned Patent Document 1 discloses that after cooling the surface layer portion to Ar ₃ temperature or less by controlled cooling after hot rolling, the control cooling is stopped and the surface layer portion is reheated to the transformation point or more. During the process of repeating the process one or more times, by applying a reduction to the steel sheet, it is possible to repeatedly transform or to reprocess and recrystallize, thereby generating an ultrafine ferrite structure or bainite structure in the surface layer portion. Have been described.

  In Patent Document 2, in order to improve brittle crack propagation stopping characteristics in a steel sheet mainly composed of ferrite-pearlite, both surface portions of the steel sheet have a circle-equivalent grain size of 5 μm or less and an aspect ratio of 2 It is described that it is important to suppress the variation in the ferrite grain size while constituting the ferrite structure having the above ferrite grains with a layer having an area ratio of 50% or more. In addition, as a method of suppressing this variation, it is described that the local recrystallization phenomenon is suppressed by setting the maximum reduction rate per pass during finish rolling to 12% or less.

  Patent Document 3 discloses a technique on the extension of TMCP that improves brittle crack propagation stopping characteristics by focusing on subgrains formed in ferrite crystal grains as well as refinement of ferrite crystal grains. Is described.

Specifically, in a steel sheet having a thickness of 30 to 40 mm, without requiring complicated temperature control such as cooling and recuperation of the steel sheet surface layer,
(A) rolling conditions to ensure fine ferrite crystal grains,
(B) rolling conditions for producing a fine ferrite structure in a portion of 5% or more of the steel plate thickness;
(C) Rolling conditions for developing a texture in fine ferrite and rearranging dislocations introduced by processing (rolling) by thermal energy to form subgrains; and (d) Fine ferrite grains formed and fine Cooling conditions to suppress coarsening of subgrain grains,
Describes a technique for improving the brittle crack propagation stopping characteristics.

  In addition, in controlled rolling, a method of improving the brittle crack propagation stopping property by developing a texture by reducing the transformed ferrite is also known. Separation occurs on the fracture surface of the steel plate in the direction parallel to the plate surface, and the stress at the brittle crack tip is relaxed, thereby increasing the resistance to brittle fracture.

  For example, in Patent Document 4, the resistance to brittle fracture is improved by controlling the (110) plane X-ray intensity ratio to 2 or more and controlling coarse grains having an equivalent circle diameter of 20 μm or more to 10% or less by controlled rolling. Is described.

  Patent Document 5 is characterized in that, as a welded structural steel excellent in brittle crack propagation stopping characteristics of a joint portion, the (100) plane X-ray plane strength ratio in a predetermined rolled surface has a value of 1.5 or more. Steel sheet is disclosed, and it is described that it has excellent brittle crack propagation stoppage characteristics due to the deviation of the stress load direction due to the texture development and the angle of crack propagation direction.

Japanese Patent Publication No. 7-100814 JP 2002-256375 A Japanese Patent No. 3467767 Japanese Patent No. 3548349 Japanese Patent No. 2659661

Propagation behavior of long brittle cracks in thick shipbuilding steel, Proceedings of the Japan Society of Marine Science and Technology No. 3, 2006, pp 359-362

  In the invention of the steel sheet excellent in the brittle crack propagation stop characteristic described in the above-mentioned Patent Documents 1 and 2, only the surface layer portion of the steel sheet is once cooled, reheated, and then subjected to processing during recuperation, thereby having a specific structure. Get. For this reason, the techniques described in Patent Documents 1 and 2 are processes that are not easy to control on the actual production scale, and are particularly heavy in the production of thick materials having a plate thickness of 70 mm or more, and are heavy on the rolling and cooling equipment.

  Further, in a large container ship exceeding the recent 6,000 TEU, a steel plate having a plate thickness of 70 mm or more is used. Non-Patent Document 1 evaluates the brittle crack propagation stopping characteristics of a steel sheet having a thickness of 65 mm, and reports a result that the brittle crack does not stop in a large brittle crack propagation stopping test of the base material.

Furthermore, in Non-Patent Document 1, the ESSO test of the specimen shows a result that the Kca value at the use temperature −10 ° C. is less than 3000 N / mm ^3/2. This suggests that in the case of a hull structure using a steel plate with a thickness greater than that, safety cannot be ensured enough.

  Moreover, as for the steel plates described in Patent Documents 1 to 5 described above, from the manufacturing conditions and the disclosed experimental data, the plate thickness: about 50 mm is the main target, and the thick material is 70 mm or more. As for the application, it is unclear whether a predetermined characteristic can be obtained, and the crack propagation characteristic in the plate thickness direction necessary for the hull structure has not been verified at all.

  The present invention advantageously solves the above-described problems, and in particular, an object of the present invention is to provide a high-strength extra-thick steel plate having a plate thickness of 70 mm or more and excellent brittle crack propagation stopping characteristics, and a method for producing the same.

  In order to solve the above-mentioned problems, the inventors have conducted extensive research on a high-strength extra-thick steel plate having excellent brittle crack propagation stopping characteristics even at a thickness of 70 mm or more and a manufacturing method for stably obtaining the steel plate. . As a result, the (211) plane integration degree at the rolling surface at the center of the plate thickness is set to 1.7 or more, and the (200) plane integration degree at the rolling surface on the steel sheet surface (sometimes simply referred to as “surface”) is set. It has been found that when the texture is 1.3 or more and the toughness (vTrs) is −60 ° C. or less, extremely excellent brittle crack propagation stopping characteristics can be obtained.

  The present invention has been completed by further studying the above knowledge, and the gist of the present invention is as follows.

[1] Thickness: A high-strength ultra-thick steel plate of 70 mm or more, in mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.2%, P: 0.01% or less, S: 0.005% or less, Ti: 0.005-0.03%, Al: 0.005-0.080%, Ni: 0.1-1 .5% and N: 0.0050% or less, Ceq defined by the following formula (1): 0.39 or more, and the remaining component composition of Fe and inevitable impurities, A (211) plane integration degree on the rolled surface is 1.7 or more, and a (200) plane integration degree on the rolled surface on the surface is 1.3 or more, and has a thickness of 1/4 The toughness measured using the impact test piece of JIS No. 4 sampled from the position is vTrs: −40 ° C. or less, and JIS sampled from the surface A high-strength heavy steel plate excellent in brittle crack propagation stopping properties, wherein the toughness measured using No. 4 impact test piece is vTrs ≦ −60 ° C.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
(Here, C, Mn, Cu, Ni, Cr, Mo and V in the formula (1) mean the content (% by mass) of each element, and 0 when not contained.)
[2] The component composition is further in mass%, and one or two of Nb: 0.005 to 0.05%, Cu: 0.1 to 1.0%, and Cr: 0.01 to 0.5%. The high-strength extra heavy steel plate having excellent brittle crack propagation stopping characteristics according to [1], which contains at least a seed.

  [3] The component composition is further in mass%, Mo: 0.01 to 0.5%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less. REM: A high-strength extra-thick steel plate having excellent brittle crack propagation stopping properties according to [1] or [2], which contains one or more of 0.0100% or less.

[4] Thickness: A method for producing a high-strength extra-thick steel plate excellent in brittle crack propagation stopping characteristics of 70 mm or more, comprising a steel material having the component composition according to any one of [1] to [3] , After being heated to a temperature of 1000 to 1200 ° C., the cumulative reduction ratio when the sheet thickness center is in the austenite recrystallization temperature range: 10% or more, the cumulative reduction ratio when the sheet thickness center is in the austenite non-recrystallization temperature range: 50 % After hot rolling under conditions of a cumulative reduction ratio of 10% or more when the surface is in a temperature range where the surface temperature is Ar ₃ temperature or less and the center thickness is Ar ₃ temperature or more, 0.5 ° C./s or more Cool to a cooling stop temperature of 500 ° C. or less at a cooling rate, or cool to a cooling stop temperature of 400 ° C. or less at a cooling rate of 0.5 ° C./s or more, and then baked to Ac ₁ temperature or less after cooling Manufacture of high-strength ultra-thick steel plates with excellent brittle crack propagation stopping properties Law.

  According to the present invention, the texture in the plate thickness direction is appropriately controlled. Therefore, the high-strength extra heavy steel plate of the present invention is excellent in brittle crack propagation stopping characteristics even when the plate thickness is 70 mm or more. For example, in the shipbuilding field, the high-strength extra-thick steel plate according to the present invention is applied to a deck member joined to hatch side combing in a strong deck structure of a container ship or a bulk carrier, thereby contributing to an improvement in ship safety. Thus, the present invention is extremely useful in industry.

  Moreover, according to the manufacturing method of this invention, said useful high intensity | strength extra heavy steel plate can be manufactured by optimizing rolling conditions. The high-strength extra-thick steel plate of the present invention can be stably produced by an industrially very simple process.

  Moreover, as described above, the high-strength extra-thick steel plate of the present invention is excellent in brittle crack propagation stopping characteristics, is excellent in toughness, and has high strength.

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

<Ingredient composition>
The high-strength extra-thick steel plate of the present invention is mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.2%, P: 0 0.01% or less, S: 0.005% or less, Ti: 0.005 to 0.03%, Al: 0.005 to 0.080%, Ni: 0.1 to 1.5%, and N: 0.0. 0050% or less, and Ceq defined by the following formula (1): 0.39 or more.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
Here, the element symbol in Formula (1) means content (mass%) of each element, and is set to 0 when not containing.

  In addition to the above components, as a selection element, Nb: 0.005-0.05%, Cu: 0.1-1.0% and Cr: 0.01-0.5% Or 2 or more types, Mo: 0.01-0.5%, V: 0.001-0.10%, B: 0.0030% or less, Ca: 0.0050% or less, REM: 0.0100% or less 1 type (s) or 2 or more types may be contained.

  The balance other than the essential components is Fe and inevitable impurities.

  Hereinafter, each component will be described. In the present specification, “%” representing the content of a component means “% by mass”.

C: 0.03-0.20%
C is an element that improves the strength of steel. In the present invention, in order to secure a desired strength, the C content is set to 0.03% or more. On the other hand, if the C content exceeds 0.20%, the weldability is deteriorated and the toughness is also adversely affected. For this reason, C content is taken as 0.03 to 0.20% of range. In addition, C content preferable about a lower limit is 0.05% or more, and C content preferable about an upper limit is 0.15% or less.

Si: 0.03-0.5%
Si is effective as a deoxidizing element and as a strengthening element for steel. If the Si content is less than 0.03%, these effects cannot be obtained. On the other hand, when the Si content exceeds 0.5%, not only the surface properties of the steel are impaired, but also the toughness is extremely deteriorated. Therefore, the content is made 0.03 to 0.5% of range.

Mn: 0.5 to 2.2%
Mn is included as a strengthening element. If the Mn content is less than 0.5%, the effect is not sufficient. On the other hand, if it exceeds 2.2%, the weldability deteriorates and the steel sheet cost also increases. Therefore, the Mn content is 0.5 to 2.2%.

P: 0.01% or less, S: 0.005% or less P and S are inevitable impurities in steel. When these contents increase, toughness deteriorates. Plate thickness: In a steel plate of 70 mm or more, in order to maintain good toughness, the P content is suppressed to 0.01% or less and the S content is suppressed to 0.005% or less. Note that 0.006% or less and 0.003% or less are more desirable ranges, respectively.

Ti: 0.005 to 0.03%
Ti has the effect of forming nitrides, carbides, or carbonitrides by adding a trace amount, and making the crystal grains finer to improve the base material toughness. The effect is acquired by making Ti content 0.005% or more. On the other hand, when the Ti content exceeds 0.03%, the toughness of the base material and the weld heat affected zone is lowered. Therefore, the Ti content is set to 0.005 to 0.03%.

Al: 0.005-0.080%
Al acts as a deoxidizer, and in order to use Al as a deoxidizer, the Al content needs to be 0.005% or more. Moreover, when Al content exceeds 0.080%, while toughness will fall, when welding, the toughness of a weld metal part will fall. For this reason, Al content was prescribed | regulated in the range of 0.005-0.080%. In addition, Al content preferable about a minimum is 0.020 or more, and Al content preferable about an upper limit is 0.040% or less.

Ni: 0.1 to 1.5%
Ni is an element that enhances the hardenability of steel. Ni can be directly added to improve the strength after rolling, and can be contained for improving functions such as toughness, high-temperature strength, and weather resistance. All of these effects are exhibited when the Ni content is 0.1% or more. Moreover, excessive Ni content deteriorates toughness and weldability. Plate thickness: Ni content is 0.1 to 1.5% as a range in which toughness and weldability are not deteriorated while maintaining sufficient strength with a steel plate of 70 mm or more.

N: 0.0050% or less N combines with Al in the steel, adjusts the crystal grain size during rolling, and strengthens the steel. In order to obtain this effect, the N content is preferably 0.0010% or more. Moreover, when N content exceeds 0.0050%, toughness will deteriorate. Therefore, the N content is 0.0050% or less.

  The above is the basic component composition of the present invention, and the balance is Fe and inevitable impurities. In the present invention, in order to further improve the characteristics, it is possible to contain one or more of Nb, Cu, Cr, Mo, V, B, Ca, and REM. In addition, in the following optional components, there is a component that specifies the lower limit value, but in the case of less than this lower limit value, since the effect of the present invention is not impaired, this optional element is included when the optional element is included below the lower limit value It is assumed that it is included as an inevitable impurity.

Nb: 0.005 to 0.05%
Nb precipitates as NbC during ferrite transformation or reheating, and contributes to increasing the strength. Nb has an effect of expanding the non-recrystallized region in rolling in the austenite region, and contributes to the refinement of ferrite. For this reason, Nb content is also effective in improving toughness. The effect is exhibited when the Nb content is 0.005% or more. If the Nb content exceeds 0.05%, coarse NbC may precipitate, leading to a decrease in toughness. Therefore, when Nb is contained, the content is preferably 0.05% or less.

Cu: 0.1 to 1.0%
Cu is an element that enhances the hardenability of steel. This element contributes directly to improving the strength after rolling, and can be contained for improving functions such as toughness, high-temperature strength, and weather resistance. Although the above effect due to the inclusion of this element is exhibited when the content is 0.1% or more, excessive inclusion deteriorates toughness and weldability. Plate thickness: It is preferable that the Cu content is 0.1 to 1.0% as a range that does not deteriorate toughness and weldability while maintaining sufficient strength even at 70 mm or more.

Cr: 0.01 to 0.5%
Cr is an element that enhances the hardenability of steel. This element contributes directly to improving the strength after rolling, and can be contained for improving functions such as toughness, high-temperature strength, and weather resistance. Although the above effect due to the inclusion of elements is exhibited by the inclusion of 0.01% or more, excessive inclusion deteriorates toughness and weldability. Plate thickness: Cr content is preferably 0.01 to 0.5% as a range in which toughness and weldability are not deteriorated while maintaining sufficient strength even at 70 mm or more.

Mo: 0.01 to 0.5%
Mo is an element that increases the hardenability of steel. This element contributes directly to improving the strength after rolling, and can be contained for improving functions such as toughness, high-temperature strength, and weather resistance. Although the above effect is exhibited by inclusion of 0.01% or more, excessive inclusion deteriorates toughness and weldability. Plate thickness: The Mo content is preferably 0.01 to 0.5% as a range that does not deteriorate toughness and weldability while maintaining sufficient strength even at 70 mm or more.

V: 0.001 to 0.10%
V is an element that improves the strength of the steel by precipitation strengthening that precipitates as V (CN). This effect is exhibited when the V content is 0.001% or more. However, if the V content exceeds 0.10%, the toughness may decrease. For this reason, when it contains V, it is preferable to make V content into the range of 0.001-0.10%.

B: 0.0030% or less B is an element that enhances the hardenability of steel, and the above effect can be obtained even with a B content of 0.0030% or less. Further, when the B content exceeds 0.0030%, the toughness of the welded portion is lowered. Therefore, when B is contained, the B content is preferably 0.0030% or less. From the viewpoint of obtaining the above effect, the lower limit of the B content is preferably 0.0006%.

Ca: 0.0050% or less, REM: 0.0100% or less Ca and REM refine the structure of the weld heat affected zone and improve toughness. Even if it contains these components, since the effect of this invention is not impaired, you may contain as needed. However, when it contains excessively, a coarse inclusion may be formed and the toughness of a base material may be deteriorated. Therefore, when these components are contained, the upper limit of the content is preferably 0.0050% and 0.0100%, respectively.

Ceq: 0.39 or higher In the high-strength extra heavy steel plate according to the present invention, in addition to the above-described components being in the above-mentioned range, the Ceq represented by the following formula (1) is adjusted to 0.39 or higher. When Ceq <0.39, it is difficult to increase the degree of (211) plane integration on the rolled surface at the center of the plate thickness. Moreover, although the upper limit of Ceq is not specifically limited, in order to ensure weldability, it is preferable that Ceq is 0.51 or less.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
Here, C, Mn, Cu, Ni, Cr, Mo and V in the formula (1) mean the content (% by mass) of each element, and 0 when not contained.

<Group organization>
The high-strength ultra-thick steel sheet of the present invention has a (211) plane integration degree of 1.7 or more at the rolled surface at the center of the plate thickness, and a (200) plane at the rolled surface on the surface (in the range of 1 mm below the surface) It has a texture with an accumulation degree of 1.3 or more. By adopting the above component composition and controlling the texture to satisfy the above range under the manufacturing conditions described later, a high-strength heavy steel plate excellent in brittle crack propagation stopping characteristics can be obtained.

<Toughness>
In the present invention, the toughness measured using a JIS No. 4 impact test piece sampled from a ¼ position of the plate thickness is vTrs: −40 ° C. or less, and the JIS No. 4 impact test piece sampled from the surface is used. The measured toughness is vTrs ≦ −60 ° C. When the toughness at a specific position is within the above range, the brittle crack propagation stopping property is improved.

  Moreover, the high strength extra heavy steel sheet of the present invention satisfies high strength and high toughness (such as brittle crack propagation stopping characteristics) by adjusting the above component composition and controlling the texture and toughness at a specific position.

  Moreover, even if the thickness is 70 mm or more, the high-strength extra-thick steel sheet of the present invention has the above-described effects such as excellent brittle crack propagation stopping characteristics.

<Manufacturing method>
Molten steel having the above component composition is melted in a converter or the like, made into a steel material (slab) by continuous casting or the like, heated to 1000 to 1200 ° C., and then hot-rolled.

  If the heating temperature is less than 1000 ° C., sufficient time for rolling in the austenite recrystallization temperature region cannot be secured. On the other hand, when the heating temperature is higher than 1200 ° C., the austenite grains become coarse, leading to a decrease in toughness, and an oxidation loss becomes remarkable, resulting in a decrease in yield. Therefore, the heating temperature of the steel material is in the range of 1000 to 1200 ° C. A preferable heating temperature range is 1000 to 1150 ° C. from the viewpoint of improving the toughness of the steel sheet. In addition, the temperature of a steel raw material means the board thickness center temperature of a steel plate.

  In the hot rolling, first, rolling is performed so that the cumulative reduction ratio in the austenite recrystallization temperature range is 10% or more at the center of the plate thickness. By setting the cumulative rolling reduction in this temperature range to 10% or more, a Charpy fracture surface transition temperature (vTrs) at a thickness of 1/4 is achieved at −40 ° C. or less. When the cumulative rolling reduction is less than 10%, the austenite is not sufficiently refined and the toughness is not improved, and the Charpy fracture surface transition temperature at the ¼ thickness position: −40 ° C. or lower is not achieved. The upper limit of the cumulative rolling reduction is not particularly limited, but the cumulative rolling reduction is preferably 45% or less because the effect of improving the fine particles becomes small. In addition, in the case of the component composition of this invention, the said conditions, Preferably, the cumulative reduction in the temperature range contained in 1100-950 degreeC in the said hot rolling is 10% or more.

  Further, rolling is performed at a cumulative reduction ratio of 50% or more when the temperature at the center of the plate thickness is in the austenite non-recrystallization temperature range. By setting the cumulative rolling reduction in this temperature range to 50% or more, a texture in which the (211) plane integration degree on the rolled surface at the plate thickness center position is 1.7 or more is obtained. On the contrary, if the cumulative rolling reduction in this temperature range is less than 50%, a texture where the (211) plane integration degree on the rolled surface at the plate thickness center position is 1.7 or more cannot be obtained. The upper limit of the cumulative rolling reduction is not particularly limited, but the cumulative rolling reduction is preferably 75% or less so as not to inhibit the rolling efficiency. In the case of the component composition of the present invention, the above condition is preferably such that the cumulative rolling reduction in the temperature range included in the hot rolling at a temperature of less than 950 ° C. and 700 ° C. or more is 50% or more.

Furthermore, in the present invention, in hot rolling, when the surface is in the temperature range of Ar ₃ temperature or lower and the plate thickness center temperature is higher than Ar ₃ temperature, the rolling is performed at a cumulative reduction ratio of 10% or higher. Under this condition, a (200) plane integration degree on the rolled surface of the surface is 1.3 or more, and a toughness (vTrs) measured using a test piece collected from the surface is −60 ° C. or less. If the surface is in this temperature range and the cumulative rolling reduction is less than 10%, the desired texture and toughness cannot be obtained. Here, Ar ₃ is Ar ₃ temperature = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (the element symbol in the formula means the content (mass%) of each element, and 0 does not include ). Incidentally, a suitable temperature range to the rolling in the temperature is below Ar ₃ temperature of the surface is _Ar 3 _{to Ar} 3 -80 ° C.. Further, in the plate thickness center temperature is above Ar ₃ temperature, the preferred temperature range in the rolling is _{Ar 3 + 80 ℃ ~Ar 3 ℃} .

  In the hot rolling according to the present invention, rolling outside the specified temperature range is not limited, and it is sufficient that the specified cumulative reduction rate is reduced at least in the specified temperature range.

  The rolled steel sheet is cooled to a cooling stop temperature of 500 ° C. or lower at a cooling rate of 0.5 ° C./s or higher. When the cooling rate is less than 0.5 ° C./s, the (211) plane integration degree on the rolled surface at the plate thickness center position cannot be secured to 1.7 or more.

When the tempering process is performed after rolling and cooling, it is necessary to set the cooling stop temperature in the immediately preceding cooling to 400 ° C. or lower and to perform the tempering process at a plate thickness central temperature of Ac ₁ temperature or lower. This is because when the tempering process exceeds the Ac ₁ temperature, the texture developed during rolling is lost. Here, Ac ₁ is Ac ₁ temperature = 751-26.6C + 17.6Si-11.6Mn-169Al-23Cu-23Ni + 24.1Cr + 22.5Mo + 233Nb-39.7V-5.7Ti-895B It means the content (mass%) of the element, and the one not contained is 0).

  In the above description, the plate thickness central temperature is obtained by heat transfer calculation from the steel plate surface temperature measured with a radiation thermometer. Further, the temperature condition in the cooling condition after rolling is the plate thickness central temperature, and the cooling rate is also an average cooling rate calculated based on the plate thickness central temperature.

  Next, examples of the present invention will be described.

  Molten steel (steel symbols: 1 to 19) having each component composition shown in Table 1 is melted in a converter and made into a steel material by a continuous casting method. After hot rolling to a plate thickness of 70 to 120 mm, cooling is performed and No. . 1-27 test steels were obtained. Table 2 shows heating conditions, hot rolling conditions and cooling conditions. Moreover, the tempering temperature was also shown about what tempered.

  About the obtained steel plate, the JIS14A test piece of (PHI) 14 was extract | collected from plate | board thickness 1/4 position, the tensile test was done, and the yield strength (YS) and the tensile strength (TS) were measured. A sample having a YS of 390 MPa or more and a TS of 510 MPa or more was evaluated as good.

  A JIS No. 4 impact test specimen was taken from the 1/4 position of the plate thickness and the steel sheet surface so that the longitudinal axis direction of the test specimen was parallel to the rolling direction, and subjected to a Charpy impact test to determine the fracture surface transition temperature (vTrs ) The case where the vTrs at the 1/4 position was −40 ° C. or lower and the surface vTrs was −60 ° C. or lower was considered good.

  Further, in order to evaluate the texture of the steel sheet, the (211) plane integration degree on the rolling surface at the center of the plate thickness and the (200) plane integration degree on the steel sheet surface (range from the extreme surface to 1 mm below the surface) were measured.

  The degree of surface integration was measured using an X-ray diffractometer (manufactured by Rigaku Corporation) and using a Mo ray source.

Next, in order to evaluate the brittle crack propagation stopping characteristics, a temperature gradient type ESSO test was performed to obtain a Kca value at -10 ° C (hereinafter also referred to as Kca (-10 ° C) N / mm ^3/2 ). A material having a Kca (−10 ° C.) of 6000 N / mm ^3/2 or more was considered good.

  Table 3 shows the results of these tests.

From the results shown in Table 3, the test steel No. In the case of 1 to 10 and 27, the (211) plane integration degree on the rolling surface at the center of the plate thickness is 1.7 or more and the (200) plane integration degree on the rolling surface on the steel sheet surface is 1.3 or more. It has a structure, toughness is (vTrs) −60 ° C. or lower, Charpy fracture surface transition temperature at ¼ position of the plate thickness is −40 ° C. or lower, and Kca (−10 ° C.) is 6000 N / mm ^3/2. As described above, excellent brittle crack propagation stopping characteristics were obtained.

On the other hand, the test steel No. In the case of 11 to 26 (excluding No. 18), one of the above-mentioned regulations was not satisfied, and the value of Kca was 5500 N / mm ^3/2 or less. No. 18 was insufficient in strength.

Claims (4)

Plate thickness: A high-strength ultra-thick steel plate excellent in brittle crack propagation stopping characteristics of 70 mm or more,
In mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.2%, P: 0.01% or less, S: 0.005 %: Ti: 0.005 to 0.03%, Al: 0.005 to 0.080%, Ni: 0.1 to 1.5% and N: 0.0050% or less. Ceq defined in 1): 0.39 or more, with the remaining component composition consisting of Fe and inevitable impurities,
A (211) plane integration degree at the rolled surface in the center of the plate thickness is 1.7 or more, and a (200) plane integration degree at the steel sheet surface is 1.3 or more,
The toughness measured using the impact test piece of JIS No. 4 taken from the 1/4 thickness position is vTrs: −40 ° C. or less,
A high-strength heavy steel plate having a toughness of vTrs ≦ −60 ° C. measured using a JIS No. 4 impact test specimen collected from the surface.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
(Here, C, Mn, Cu, Ni, Cr, Mo and V in the formula (1) mean the content (% by mass) of each element, and 0 when not contained.)   The component composition is further in mass%, and one or more of Nb: 0.005 to 0.05%, Cu: 0.1 to 1.0% and Cr: 0.01 to 0.5%. The high-strength extra-thick steel plate according to claim 1 contained.   The component composition is further in mass%, Mo: 0.01 to 0.5%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, REM: The high-strength extra heavy steel plate according to claim 1 or 2, containing 0.0100% or less of one type or two or more types. The plate thickness according to any one of claims 1 to 3 , which is a method for producing a high-strength heavy steel plate excellent in brittle crack propagation stopping characteristics of 70 mm or more,
After heating the steel raw material which has the component composition in any one of Claims 1-3 to the temperature of 1000-1200 degreeC,
Cumulative rolling reduction when the sheet thickness center is in the austenite recrystallization temperature range: 10% or more Cumulative rolling reduction when the sheet thickness center is in the austenite non-recrystallization temperature range: 50% or more, the surface is Ar ₃ temperature or less and the sheet thickness After performing hot rolling under conditions of a cumulative reduction ratio of 10% or more when the central temperature is in the temperature range of Ar ₃ temperature or higher,
After cooling to a cooling stop temperature of 500 ° C. or less at a cooling rate of 0.5 ° C./s or after cooling to a cooling stop temperature of 400 ° C. or less at a cooling rate of 0.5 ° C./s or more Ac A method for producing a high-strength ultra-thick steel plate tempered to a temperature of ₁ point or less.