BR112014028230B1 - high strength structural steel sheet which has excellent brittle fracture interruption capability and method for producing it - Google Patents

Abstract

Abstract: "High strength structural steel sheet which has excellent brittle fracture breakability, and method for producing it". a thick steel plate having a chemical composition of steel including, by weight, c: 0,03 to 0,20%, si: not more than 0,1%, mn: 0,5 to 2,2%, p: no more than 0.008%, s: no more than 0.01%, nb: 0.005 to 0.05%, ti: 0.005 to 0.03%, al: 0.005 to 0.08%, n: no more than 0.0075%, and optionally one, or two or more elements between cu, ni, cr, mo, v, b, ca and rem, the balance being fe and the inevitable impurities. The steel plate has a deformed ferrite microstructure. The steel sheet has a charpy fracture appearance transition temperature of no more than -40 ° C as measured from a portion found at 1/2 the plate thickness (t) + 6 mm. A plate having the above chemical composition is heated to a temperature of 1000 to 1200 ° C, and is laminated such that the cumulative reduction of lamination is not less than 30% in the temperature range.

Description

[001] The present invention relates to thick steel sheets of high resistance with excellent ability to break fragile fractures and method for producing such steel sheets. In particular, the invention relates to techniques suitable for large structures involving thick steel sheets of 50 mm or more such as ships, marine structures, low temperature storage tanks, and construction and civil engineering structures.

Background [002] In large structures such as ships, marine structures, low temperature storage tanks, and construction and civil engineering structures, the economic and environmental impacts of accidents associated with fragile fractures are so great that there is a constant demand for added security. The steels used in such structures must have tenacity and the ability to interrupt the fragile fracture at service temperatures.

[003] For structural reasons, ships such as container ships and bulk carriers involve thick sheets of high-strength steel for the outer plates of ship hulls. Due to the recent increase in hull size, the strength and thickness of steel sheets have also been increased. In general, the ability to break brittle steel sheet fractures tends to deteriorate with increasing strength or thickness. Thus, there was a higher level of demand for the ability to interrupt a fragile fracture.

2/24 [004] A known approach to improve the brittle fracture breaking capacity of steels is to increase the Ni content, and steel with 9% Ni is used on a commercial scale in LNG storage tanks. .

[005] However, increasing the Ni content gives a significant increase in cost and it is difficult to adopt in applications other than LNG storage tanks.

[006] For steels with a plate thickness of less than 50 mm, which are used for ships and pipelines where an ultra low temperature such as that of LNG is not experienced, the reduction of the grain size by TMCP (THERMOMECHANICAL CONTROL PROCESS ) can increase toughness at low temperature to transmit excellent ability to break fragile fractures to steels.

[007] In addition, Patent Literature 1 proposes a steel in which the structure of the surface portions has undergone an ultra-fine crystallization to achieve an increased brittle fracture interruption capacity without increasing the bonding cost.

[008] Patent Literature 1 focuses on the fact that the edges of the fracture (plastically deformed regions) that occur in the surface portions of the steel during the propagation of brittle fractures effectively serve to increase the ability to break brittle fractures. The steel with excellent ability to break fragile fractures described there is characterized by the fact that the crystal grains at the edges of the fractures (plastically deformed regions) are reduced in size and are allowed to absorb the propagation energy possessed by small fractures that are being propagated.

[009] In the production method described, a step is performed one or more times in which the surface portions are cooled to, or below, the Ar3 transformation point by cooling contro3 / 24 side after hot rolling, and then cooling controlled is interrupted and the surface portions are recovered up to, or above, the transformation point. During this stage, lamination is applied to the steel. In this way, cyclic transformation or deformation recrystallization produces an ultrafine ferrite structure or a bainite structure in the surface portions.

[0010] Patent Literature 2 is directed to a steel having a microstructure based on ferrite-perlite, and describes that to improve the brittle fracture breaking capacity of the steel, it is important that the surface portions on both sides of the steel is composed of layers containing 50% or more of a ferrite structure having ferrite grains with an equivalent average circle diameter of no more than 5 pm and a grain aspect ratio of no less than 2. It is also described that a small variation in the grain size of the ferrite is important. In order to control the variation, the maximum reduction of lamination per pass in the finishing lamination is limited to 12% or less in order to suppress the occurrence of a phenomenon of local recrystallization.

[0011] According to Patent Literature 1 and 2, steels with an excellent ability to break fragile fractures have a specific structure obtained by selectively cooling the surface portions of the steel and subsequently recovering the surface portions while applying a deformation during recovery. Such a process is difficult to control on the current production scale. In particular, the process causes high loads on the rolling and cooling equipment when applied to thick steel sheets with a sheet thickness of more than 50 mm.

[0012] On the other hand, Patent Literature 3 describes a technique adapted based on TMCP (THERMOMECHANICAL CONTROL PROCESS) that increases the interruption capacity of

4/24 brittle fracture focusing not only on reducing the size of the ferrite crystal grains, but also on the sub-grains formed in the ferrite crystal grains.

[0013] In detail, the ability to break the fragile fracture of steel sheets with a thickness of 30 to 40 mm is increased without giving complicated temperature controls such as cooling and recovery of surface portions of the steel sheets, according to ( a) the rolling conditions that guarantee fine ferrite crystal grains, (b) the rolling conditions with which a fine ferrite structure is formed in the portions that represent 5% or more of the thickness of the steel layers, (c) lamination in which the texture is developed in the fine ferrite and a displacement introduced by the deformation (lamination) is rearranged by the thermal energy in order to form sub-grains, and (d) cooling conditions that suppress the hardening of the fine ferrite crystal grains and fine sub-grains formed.

[0014] Other methods are also known that increase the ability to interrupt brittle fracture by applying lamination to the transformed ferrite during controlled lamination in order to develop the texture. Separations are made to occur on the surface of the steel fracture in a direction parallel to the plane of the sheet and the stress on the frontal ends of fragile fractures is reduced, thus increasing the resistance to brittle fracture.

[0015] For example, Patent Literature 4 describes that the resistance to brittle fracture is increased by carrying out a controlled lamination so that the steel has an X-ray intensity ratio in the 110 plane of not less than 2 and containing 10 % or less of raw grains with a diameter equivalent to a circle in crystal grains of not less than 20 μm.

List of Citations

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Patent Literature

PTL 1: Publication of Examined Japanese Patent Application No. 7-100814

PTL 2: Publication of Unexamined Japanese Patent Application No. 2002-256375

PTL 3: Japanese Patent No. 3467767

PTL 4: Japanese Patent No. 3548349

Non-Patent Literature [0016] NPL 1: Inoue et al., Long Brittle Crack Propagation of Heavy-Thick Shipbuilding Steels, Conference proceedings, the Japan Society of Naval Architects and Ocean Engineers, Volume 3, 2006, pgs. 359-362.

Summary of the Invention

Technical Problem [0017] Large current container ships exceeding 6,000 TEU (Twenty-Foot Equivalent Unit) involve thick steel sheets having a sheet thickness of more than 50 mm. In Non-Patent Literature 1, the brittle fracture breaking capacity of 65 mm thick steel sheets is assessed and a brittle fracture breaking ability test showed that the base steel failed to stop the fracture progress fragile. [0018] In addition, an ESSO test (ESSO TEST COMPATIBLE WITH WES 3003) against specimens showed that the Kca value at a service temperature of -10 ° C (hereinafter written as Kca (-10 ° C)) is less than 3000 N / mm ^3/2 , suggesting that ensuring safety is a challenge in the case of hull structures using steel sheets with a plate thickness exceeding 50 mm.

[0019] After describing the production conditions and experimental data, Patent Literatures 1 to 4 are mainly related to improving the ability to interrupt brittle fractures

6/24 of steel sheets that are approximately 50 mm thick. Thus, it is uncertain whether the desired properties can be obtained when these techniques are applied to thick steel sheets with a thickness exceeding 50 mm. In addition, resistance to fracture propagation in the direction of sheet thickness, which is an important feature in ship hull structures, is not studied in the literature above.

[0020] It is, therefore, an objective of the present invention to provide a thick high-strength steel sheet with a layer thickness of 50 mm or more having an excess capacity for brittle fracture interruption that can be produced stably by a very simple industrial process. involving the optimization of rolling conditions in order to control the formation of a texture in the direction of the thickness of the sheet, and provide a method for producing such steel sheets.

Solution to the Problem [0021] The present inventors carried out intense studies to achieve the above objective. As a result, the present inventors have obtained the following discoveries in relation to high-strength thick steel sheets that have excellent brittle fracture breaking capacity despite the great thickness and also in relation to methods capable of stable production of such steel sheets.

1. In thick steel plates having a plate thickness of 50 mm or more, increasing the toughness of a central nut to approximately half the plate thickness is effective in improving the ability to break brittle fractures. Particularly good results are obtained when a portion uncovered at 1/2 the plate thickness (t) + 6 mm has a Charpy fracture appearance transition temperature of no more than -40 ° C.

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2. The tenacity value above is achieved effectively with a specific chemical composition, in particular, reducing the levels of sulfur and phosphorus which are impurity elements.

3. In addition to the chemical composition, lamination conditions are also important. By rolling the steel under specific hot rolling conditions in which the temperature of a central portion at half the thickness of the plate is controlled to a specific temperature, a microstructure based on the deformed ferrite can be obtained, and consequently also an increase in the tenacity can be achieved.

[0022] The present invention was made based on the findings above and other studies. That is, the present invention resides in:

1. A high strength thick steel plate for structural use with excellent brittle fracture interruption capacity, the steel plate having a chemical composition of the steel including, in mass%, C: 0.03 to 0.20%, Si: no more than 0.1%, Mn: 0.5 to 2.2%, P: no more than 0.008%, S: no more than 0.01%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.03%, Al: 0.005 to 0.08%, and N: no more than 0.0075%, the balance being Fe and the inevitable impurities, the steel plate having a microstructure based on deformed ferrite, the steel plate having a Charpy fracture appearance transition temperature of no more than -40 ° C as measured with respect to a portion uncovered 1/2 of the plate thickness (t) + 6 mm.

2. The high strength thick steel plate for structural use with excellent brittle fracture interruption capacity described in item 1, where the chemical composition of the steel also includes, in mass%, one, or two or more elements between Cu: 0.01 to 0.5%, Ni: 0.01 to 1.0%, Cr: 0.01 to 0.5%, Mo: 0.01 to 0.5%, V: 0.001 to 0.1% , B: no more than 0.003%, Ca: no more than 0.005%, and REM: no more than 0.01%.

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3. The thick steel plate of high resistance for structural use with excellent ability to interrupt the fragile fracture described in item 1 or 2, where the microstructure has, as a second phase, one or two or more phases between perlite, bainite, martensite, constituent martensite-austenite (MA), and deformation-free ferrite after transformation from austenite.

4. The high strength thick steel plate for structural use with excellent brittle fracture interruption capacity described in any of items 1 to 3, where the thickness of the plate is greater than 50 mm.

5. A method for producing high-strength thick steel sheets for structural use with excellent brittle fracture interruption capability. Including heating the plate having the chemical composition described in items 1 or 2 to a temperature of 1000 to 1200 ° C, laminate the plate in such a way that the reduction in cumulative lamination is not less than 30% in the temperature range in which the temperature of a central portion at half the thickness of the plate is in the region of recrystallization of the austenite, then perform the first cooling at a cooling rate of no more than 15 ° C / s until the temperature of the central portion at half the thickness is decreased for, or below, point Ar3, laminate the steel sheet with a cumulative rolling reduction of not less than 40% in the temperature range in which the temperature of the central portion at half the thickness of the sheet is equal to or less than the Ar3 point, and perform the second cooling at a cooling rate of not less than 4 ° C / s to a temperature of 600 ° C or less.

6. The method for producing high strength thick steel plate for structural use with excellent brittle fracture interruption capability described in item 5, also including

9/24 temper the steel sheet to a temperature of no more than point Ac1 after the second cooling.

Advantageous effects of the invention [0023] According to the present invention, high-strength thick steel sheets can be obtained which have a suitable texture adequately controlled in the direction of the thickness of the sheet and have excellent brittle fracture interruption capability. The invention can effectively achieve marked advantages over steels according to conventional techniques when the invention is applied to steel sheets having a thickness of 50 mm or more, preferably a thickness greater than 50 mm, more preferably a thickness of 55 mm or more , and even more preferably a thickness of 60 mm or more. The present invention is extremely useful in industry, for example, in the field of shipbuilding where the technique of the invention is applied to the side hatch brackets of the hold and to the members of the resistance deck of the deck structures of large container ships and vessels bulk carriers and thus contributes to increased safety for ships.

Description of modalities [0024] The present invention specifies: 1. the toughness of the base steel, 2. the chemical composition, and 3. the microstructure.

1. Toughness of the base steel [0025] To suppress the progress of fractures, an important requirement is that the base metal has good toughness characteristics at approximately half the thickness of the sheet. In the steel sheets of the invention, the transition temperature of the Charpy fracture appearance of a bare portion at 1/2 the thickness of the sheet (t) + 6 mm is specified.

[0026] To ensure that thick steel sheets with a special

10/24 thickness of 50 mm or more will have a brittle fracture breaking capacity of Kca (-10 ° C)> 6000 N / mm3 ^{/ 2} , which is the target value to guarantee structural safety, the appearance transition temperature Charpy fracture of a portion uncovered at 1/2 the plate thickness (t) + 6 mm is specified to be no more than 40 ° C.

[0027] Here, the Charpy fracture appearance transition temperature of a portion uncovered at 1/2 the plate thickness (t) + 6 mm refers to the fracture appearance transition temperature obtained by an impact test in relation to to a Charpy impact specimen that is sampled so that the central position of the Charpy impact specimen is 6 mm beyond 1/2 the thickness of the plate (that is, the middle of the plate thickness).

[0028] The central position of the Charpy impact test specimen is shifted 6 mm from 1/2 of the thickness of the plate to avoid the influence of central segregation. Since the full-size Charpy impact test specimen has a 10 mm square cross-section (except the notch), the Charpy impact specimen with the offset above is 1 mm from 1/2 of the thickness the plate. Thus, the toughness of the inner portion of the steel sheet can be assessed without being disturbed by the influence of central segregation.

[0029] The tenacity above can be obtained when production conditions are properly selected. Below, the chemical composition and microstructure of the steels of the invention will be described, and the preferred conditions for the production of the steels of the invention.

2. Chemical composition [0030] In the description below, the unit% refers to% by mass.

C: 0.03 to 0.20%

11/24 [0031] Carbon is an element that increases the strength of steel. In the invention, 0.03% or more of carbon is required to ensure the desired strength. However, adding more than 0.20% carbon deteriorates the weldability and adversely affects toughness. Thus, the C content is limited to the range of 0.03 to 0.20%, and is preferably 0.05 to 0.15%.

Si: no more than 0.1% [0032] Silicon is effective as a deoxidizer and also to reinforce steel. However, an excessively high Si content causes a marked decrease in toughness. Thus, the Si content is limited to no more than 0.1% to avoid a decrease in the toughness of a central portion of the steel sheet.

Mn: 0.5 to 2.2% [0033] Manganese is added as a reinforcing element. At less than 0.5%, the effect is insufficient. Adding more than 2.2% manganese deteriorates the weldability and also increases the cost of steel. Thus, the Mn content is limited to 0.5 to 2.2%.

P: no more than 0.008% [0034] Phosphorus is an inevitable impurity found in steel. An increase in the P content causes the toughness to deteriorate. To ensure that the central portion of the steel plate has good toughness, the upper limit of the P content needs to be 0.008% or less.

S: no more than 0.01% [0035] Similar to phosphorus, sulfur is an unavoidable impurity found in steel. At more than 0.01%, the toughness is deteriorated. Thus, the S content is desirably not more than 0.01% and more desirably not more than 0.005%.

Nb: 0.005 to 0.05% [0036] Niobium is precipitated as NbC during the transformation of ferrite or reheating, contributing to the reinforcement. Also, p

12/24 niobium serves to increase the range of the non-recrystallization region during lamination in the austenite region, contributing to the refining of the ferrite grain. Thus, niobium is also effective in improving toughness. To achieve these effects, 0.005% or more of niobium must be added. However, the addition of more than 0.05% niobium results in the precipitation of crude NbC, causing a decrease in toughness. Thus, the Nb content is limited to 0.005 to 0.05%.

Ti: 0.005 to 0.03% [0037] A number of traces of titanium are effective for refining crystal grains through the formation of nitride, carbide or carbonitride, thus increasing the toughness of the base steel. This effect can be achieved by adding 0.005% or more of titanium. By more than 0.03%, however, the toughness of the base steel and the zone affected by the heat from the weld are decreased. Thus, the Ti content is limited to 0.005 to 0.03%.

Al: 0.005 to 0.08% [0038] Aluminum serves as a deoxidizer. For this purpose, the Al content needs to be 0.005% or more. When, however, more than 0.08% of aluminum is added, the toughness is decreased and also the welding of such steel results in a metallic weld showing poor toughness. Thus, the Al content is limited to 0.005 to 0.08% and is preferably 0.02 to 0.04%.

N: no more than 0.0075% [0039] Nitrogen binds to aluminum in steel to make it possible to adjust the size of the crystal grain during deformation in the rolling, thus reinforcing the steel. However, adding more than 0.0075% nitrogen decreases toughness. Thus, the N content is limited to no more than 0.0075%.

[0040] The precedent is the basic chemical composition of the invention, and the balance is iron and the inevitable impurities. For other

13/24 properties, one, or two or more elements between copper, nickel chromium, molybdenum, vanadium, boron, calcium and rare earth metals can be added.

Cu, Ni, Cr and Mo [0041] Copper, nickel, chromium and molybdenum are elements that increase the hardening capacity of steel. These elements directly contribute to increase the strength after lamination, and can also be added to improve functions such as toughness, high temperature resistance and weather resistance. However, adding excessively large amounts of these elements decreases toughness and weldability. Thus, the upper limits of these elements, when added, are preferably Cu: 0.5%, Ni: 1.0%, Cr: 0.5% and Mo: 0.5%. On the other hand, the above effects are not obtained if the levels of the respective elements are less than 0.01%. Thus, the contents of these elements, when added, are each preferably 0.01% or more.

V: 0.001 to 0.1% [0042] Vanadium is an element that increases the strength of steel by reinforcing precipitation in the form of V (CN), and 0.001% or more of vanadium can be added. At more than 0.1%, however, toughness is decreased. Thus, the vanadium content, when added, is preferably 0.001 to 0.1%.

B: no more than 0.003% [0043] A number of traces of boron can be added to increase the hardening capacity of the steel. However, adding more than 0.003% of boron decreases the toughness of welds. Thus, the boron content, when added, is preferably not more than 0.003%.

Ca: no more than 0.005% and REM: no more than 0.01%

14/24 [0044] Calcium and rare earth metals improve toughness by refining the structure of an area affected by the heat of the weld, and can be added as needed. The addition of these elements does not impair the effects of the invention. However, the addition of excessively large amounts of these elements results in the formation of crude inclusions and a consequent decrease in the toughness of the base steel. Thus, the upper limits of these elements, when added, are preferably 0.005% for calcium and 0.01% for rare earth metals.

[0045] To guarantee an acceptable welding capacity for steels for structural use, the weight of the carbon equivalent (Ceq) represented by the following equation is preferably no more than 0.45%.

Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 [0046] (The chemical symbols on the right side indicate the contents (mass%) of the elements).

3. Microstructure [0047] The toughness is greatly affected by the microstructure as well as the chemical composition. In the steel sheets of the invention, excellent toughness is obtained by configuring the microstructure so that the main phase is a ferrite structure, in particular, a ferrite structure that has been deformed and planed, that is, a ferrite that has undergone deformation. (hereinafter also written simply as deformed ferrite). With this configuration, the structure can be refined in the direction of the plate thickness to achieve improved toughness.

[0048] In the event that sufficient strength cannot be obtained only with deformed ferrite, a second phase including one, two or more phases between perlite, bainite, martensite, and martensite-austenite (MA) constituent can be dispersed accordingly with the desired level of resistance in order to satisfy both the resistance and the toughness.

[0049] In the invention, the structure based on the deformed ferrite refers to a structure having a deformed ferrite area fraction of 50% or more. The balance is one, two or more phases selected from perlite, bainite, martensite, martensite-austenite (MA) constituent, and deformation-free ferrite after transformation from austenite.

4. Production conditions

Regarding the conditions for the production of the thick steel sheets of the invention, the invention specifies the heating temperature of the plate, the reduction of cumulative rolling in hot rolling in the region of recrystallization of austenite, the rate of cooling after rolling in the region of recrystallization of the austenite up to, or below the point Ar3, the cumulative lamination reduction at, or below the point Ar3, the subsequent cooling rate, the cooling stop temperature, and the tempering temperature. In the following description, the temperature (° C) indicates the temperature of a central portion at half the thickness of the steel plate (a 1/2 t portion (t: plate thickness)). The temperature of a central portion at half the thickness of the sheet can be obtained, for example, by simulating a calculation based on data such as sheet thickness, surface temperature and cooling conditions. For example, the temperature of a central portion at half the thickness of the plate can be determined by calculating the temperature distribution in the direction of the plate thickness based on a difference method.

[0050] For thick steel sheets with a sheet thickness of 50 mm or more that are used for outer ship hull sheets on recent ships such as container ships and bulk carriers, the structural safety guarantee requires that the sheets of steel have a brittle fracture breaking capacity of

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6000 N / mm ^3/2 or more in terms of Kca value at -10 ° C, ie Kca (10 ° C). Initially, a molten steel having the aforementioned chemical composition is melted in an oven such as a converter furnace, and is cast into a slab by a method such as continuous casting. Then the obtained plate is heated to a temperature of 1000 to 1200 ° C and is then cold rolled.

[0051] If the heating temperature is below 1000 ° C, a sufficient length of time is not available for lamination in the austenite recrystallization region. At more than 1200 ° C, austenite grains are hardened to cause a decrease in toughness, and also such heating causes a marked loss of oxidation and the consequent decrease in yield. Thus, the heating temperature is limited to 1000 to 1200 ° C. From the point of view of toughness, the heating temperature is preferably in the range of 1000 to 1150 ° C, and more preferably 1000 to 1050 ° C.

[0052] In hot lamination, initially, the plate is laminated in such a way that the cumulative reduction in lamination is no less than 30% in the temperature range in which the temperature of a central portion at half the thickness in the recrystallization region of austenite. If the cumulative rolling reduction is less than 30%, the austenite grains are not sufficiently refined and the toughness is not improved.

[0053] After lamination in the austenite recrystallization region, initially the cooling is performed until the temperature of the central portion in half the thickness of the plate is decreased to, or below the point Ar3. Here, excessively rapid cooling does not allow recrystallization to proceed to a sufficient extent. Thus, the cooling rate up to, or below, the Ar3 point is limited to no more than 15 ° C / s. In the invention, the Ar ₃ point (° C) is obtained from the

17/24 following equation:

Air ₃ (° C) = 910 - 273C - 74Mn - 57Ni - 16Cr - 9Mo - 5Cu [0054] In the equation, the chemical symbols indicate the respective levels (% by mass) in the steel and are 0 when the elements are absent.

[0055] Performing the first cooling in the above manner, the steel sheet can be subjected to subsequent lamination as long as the temperature of the central portion in half the thickness of the sheet has been lowered to, or below the point Ar3 without incurring the hardening of the refined austenite grains that resulted from the lamination performed above while the temperature of the central portion at half the thickness is in the region of recrystallization of the austenite. The first cooling thus contributes to refining the final structure.

[0056] Next, the steel sheet is rolled with a cumulative rolling reduction of not less than 40% in the temperature range in which the temperature of the central portion at half the thickness of the sheet is equal to or less than the point Ar3 . The structure cannot be refined sufficiently and poor toughness is caused unless the cumulative reduction in rolling in this temperature range is 40% or more.

[0057] For the increase of fracture interruption capacity, lamination at point Ar3 or less is more effective than lamination in the non-recrystallization region. It is therefore necessary for effective lamination to occur in this temperature range as much as possible. Thus, lamination is not carried out in the region of non-recrystallization in the invention.

[0058] After finishing the lamination, the steel sheet is subjected to the second cooling to 600 ° C or less to a cooling rate of not less than 4 ° C / s. At a cooling rate below

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4 ° C / s, the structure is stiffened and the toughness is reduced. If the cooling stop temperature is greater than 600 ° C, the recrystallization is allowed to continue even after the cooling has ended, resulting in a failure to obtain the desired texture. Thus, the cooling stop temperature is limited to 600 ° C or less.

[0059] After the cooling is finished, the steel sheet can be hardened. Through hardening, the toughness of the steel sheet can also be increased. The tempering temperature is not higher than the point Ac1 to ensure that the structure formed by the lamination and the cooling remain intact. In the invention, the point Ac1 (° C) is determined by the following equation:

point Ac1 = 751 - 26.6C + 17.6Si - 11.6Mn - 169Al - 23Cu - 23Ni + 24.1Cr + 22.5Mo + 233Nb - 39.7V - 5.7Ti - 895B [0060] In the equation, the symbols chemicals indicate the respective levels (% by mass) in the steel and are 0 when the elements are absent.

EXAMPLES [0061] Cast steels (steels A to P) that had the chemical compositions shown in Table 1 were cast in a converting furnace and were continuously cast into plates (plate thickness: 280 mm). The plates were hot rolled to a plate thickness of 50 to 80 mm while a first cooling was performed between the phases, and were subsequently subjected to the second cooling. Thus, the tests were obtained steels ^Nos 1 to 22. Table 2 describes the conditions of lamination and cooling conditions.

Table 1 (mass%)

Steels Ç Si Mn P s Nb You Al V Ass Ni Cr Mo N B Here REM Ceq Ara (° C) Ac-i (° C) THE 0.11 0.08 1.43 0.005 0.004 0.013 0.015 0.03 - - - - - 0.0032 - - - 0.35 774 731 B 0.07 0.07 1.55 0.006 0.004 0.015 0.011 0.03 0.01 - - 0.02 - 0.0047 0.0012 0.002 - 0.33 776 730 Ç 0.05 0.07 1.82 0.004 0.003 0.013 0.012 0.04 - 0.13 0.22 - - 0.0038 0.0014 0.002 - 0.38 748 717 D 0.06 0.08 1.61 0.008 0.002 0.013 0.013 0.04 - 0.27 0.33 0.02 0.02 0.0057 0.0013 - - 0.38 754 714 AND 0.06 0.05 1.44 0.006 0.002 0.013 0.013 0.03 - 0.35 0.43 0.02 - 0.0032 - 0.003 - 0.36 760 714 F 0.07 0.07 1.52 0.006 0.003 0.013 0.015 0.04 0.02 - 0.20 - 0.02 0.0056 0.0012 0.002 0.006 0.34 767 723 G 0.11 0.05 1.58 0.007 0.003 0.010 0.012 0.04 - - - 0.02 - 0.0028 0.0023 0.002 - 0.38 763 725 H 0.08 0.06 1.61 0.006 0.004 0.013 0.013 0.04 0.03 - - 0.02 - 0.0066 0.0013 - - 0.36 769 726 I 0.05 0.07 1.77 0.006 0.003 0.012 0.012 0.05 0.01 - - - 0.02 0.0034 0.0021 0.002 - 0.35 765 723 J 0.06 0.15 1.63 0.006 0.003 0.013 0.012 0.04 - 0.33 0.23 - 0.02 0.0033 - 0.002 - 0.37 758 717 K 0.23 0.06 0.84 0.006 0.002 0.013 0.013 0.04 - - - 0.02 0.02 0.0061 0.0014 0.003 - 0.38 785 732 L 0.06 0.08 1.53 0.007 0.004 0.013 0.014 0.05 - 0.25 0.51 - - 0.0087 0.0030 0.002 - 0.37 750 707 M 0.05 0.08 1.45 0.021 0.003 0.014 0.015 0.04 - 0.14 0.33 0.02 - 0.0054 0.0011 0.002 - 0.33 769 719 N 0.07 0.22 1.57 0.006 0.002 0.012 0.013 0.04 - 0.10 0.03 0.02 - 0.0051 0.0013 0.003 - 0.34 772 727 O 0.09 0.07 1.43 0.005 0.003 0.013 0.037 0.03 0.03 - - - 0.02 0.0047 0.0015 - - 0.34 779 729 P 0.08 0.04 1.52 0.016 0.003 0.012 0.012 0.04 0.02 - 0.20 - 0.02 0.0042 0.0018 - - 0.35 764 721

Note 1: Underlined values indicate that the data are outside the ranges of the invention.

Note 2: Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (Chemical symbols indicate the contents (% by mass) of the respective elements).

Note 3: Ar3 (° C) = 910-273C-74Mn-57Ni-16Cr-9Mo-5Cu (Chemical symbols indicate the contents (% by mass) of the respective elements) .Note 4: Ac1 (° C) = 75126, 6C + 17.6Si-11.6Mn-169Al-23Cu-23Ni + 24.1Cr + 22.5Mo + 233Nb-39.7V-5.7Ti-895B (Chemical symbols indicate the contents (% by mass) of the respective elements ).

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Table 2

^Nos Products Steels Plate thickness (mm) Production conditions Class Heating temperature (° C) Cumulative rolling reduction in the austenite recrystallization region (%) First cooling rate (° C / s) Cumulative rolling reduction in point Ar3 or below (%) Second cooling rate (° C / s) Cooling stop temperature (° C) Tempering temperature (° C) 1 THE 60 1070 36 1.8 67 7.7 420 - Ex. Of the invention 2 B 65 1000 39 0.6 62 7.1 510 - Ex. Of the invention 3 Ç 50 1050 50 1.1 64 9.3 360 - Ex. Of the invention 4 D 60 1150 45 0.5 60 7.5 100 500 Ex. Of the invention 5 AND 80 1050 33 1.5 57 5.9 430 - Ex. Of the invention 6 F 65 1100 42 4.5 60 6.7 520 - Ex. Of the invention 7 G 75 1150 43 0.7 53 6.3 320 - Ex. Of the invention 8 H 70 1000 45 2.6 55 6.5 150 550 Ex. Of the invention 9 I 70 1200 34 0.8 62 6.8 370 Ex. Of the invention 10 G 75 1000 45 2.4 51 6.2 340 Ex. Of the invention 11 Ç 80 1050 38 1.5 54 5.7 350 Ex. Of the invention 12 J 75 1100 46 1.3 50 6.9 420 Comparative Example 13 K 65 1050 40 1.2 61 7.1 360 Comparative Example 14 L 75 1000 46 0.0 50 6.4 380 Comparative Example 15 M 65 1050 34 0.3 65 7.2 460 Comparative Example 16 N 60 1150 46 0.7 60 6.5 470 Comparative Example 17 O 80 1000 38 0.8 54 6.1 120 610 Comparative Example 18 P 70 1200 38 2.7 60 6.8 560 - Comparative Example 19 AND 70 1100 13 0.2 71 7.2 470 - Comparative Example

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^Nos Products Steels Plate thickness (mm) Production conditions Class Heating temperature (° C) Cumulative rolling reduction in the austenite recrystallization region (%) First cooling rate (° C / s) Cumulative rolling reduction in point Ar3 or below (%) Second cooling rate (° C / s) Cooling stop temperature (° C) Tempering temperature (° C) 20 H 75 1000 64 1.3 25 6.5 510 - Comparative Example 21 G 60 1150 45 30.0 61 7.6 360 - Comparative Example 22 B 70 1050 50 0.7 50 Air cooling - - Comparative Example (<0.5)

Note 1: Underlined values indicate that the data are outside the ranges of the invention.

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22/24 [0062] From the thick steel sheets obtained, JIS No. 14A specimens with a diameter of 14 mm were shown from a portion found 1/4 of the thickness of the sheet (t) so that the longitudinal direction of the specimens was perpendicular to the rolling direction. The specimens were submitted to a tensile test to measure the yield limit (YS) and the tensile strength (TS). A cross section 1/4 of the thickness of the plate extending parallel to the lamination direction was observed with an optical microscope at 400 x magnification in relation to three fields of view, thus identifying the phases that make up the microstructure.

[0063] In addition, JIS No. 4 impact specimens were sampled so that the center of the specimen corresponded to 1/2 of the plate thickness (t) + 6 mm. This sampling was carried out in such a way that the direction of the specimen's longitudinal axis was parallel to the lamination direction. The fracture appearance transition temperature (vTrs) was determined by a Charpy impact test. The specimens were evaluated to be in the range of the invention when the transition temperature of Charpy fracture appearance measured in relation to the portion corresponding to 1/2 of the plate thickness (t) + 6 mm was -40 ° C or less.

[0064] Next, to assess the ability to break a fragile fracture, an ESSO gradient and temperature test (ESSO TEST COMPATIBLE WITH WES 3003) was performed to determine Kca (10 ° C).

[0065] The results of these tests are shown in Table 3. The test steel sheets (product ^Nos 1 to 11) that had a tenacity value of approximately half the plate thickness in the range of the invention had Kca (-10 ° C) of not less than 6000 N / mm ^3/2 , indicating excellent ability to break a fragile fracture. All products paragraphs 1 to 11, the plates of the microstructure

23/24 steel from the tests had a deformed ferrite volume fraction of not less than 50%.

[0066] In contrast, the Kca values (-10 ° C) was 3 800 N / mm and ^3/2 or less compared unfavorably to Invention Examples in the case of test steel sheets (products paragraphs 12 to 18) in which steel sheet had a chemical composition outside the range of the invention and in the case of steel sheet (product paragraphs 19 to 22) were produced under conditions outside the ranges of the invention , and thus failed to meet the sheet texture requirements steel of the invention.

Table 3

^Nos Products Steel Labels Plate thickness (mm) Microstructure frog YS (MPa) TS (MPa) vTrs (° C) Kca (-10 ° C) (N / ^mm3 / ²⁾ Class 1 THE 60 DF + P 425 578 -84 7700 Example of Invention 2 B 65 DF + F + P 375 487 -75 7800 Example of Invention 3 Ç 50 DF + B + M 487 574 -92 8100 Example of Invention 4 D 60 DF + B + MA 473 582 -78 7300 Example of Invention 5 AND 80 DF + F + P + B 434 511 -67 6400 Example of the Invention 6 F 65 DF + F + P 399 504 -87 7600 Example of Invention 7 G 75 DF + F + P + B 486 578 -76 7200 Example of the Invention 8 H 70 DF + B + MA 424 507 -67 6700 Example of Invention 9 I 70 DF + F + P + B 441 534 -61 6900 Example of Invention 10 G 75 DF + F + P + B 469 561 -58 6700 Example of Invention 11 Ç 80 DF + F + P + B 455 577 -62 6800 Example of Invention 12 J 75 DF + F + P + B 472 586 -4 2200 Comparative Example 13 K 65 DF + F + P + B 454 500 -16 1600 Comparative Example 14 L 75 DF + F + P + B 430 556 -11 2500 Comparative Example 15 M 65 DF + F + P 388 458 -15 3100 Comparative Example

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^Nos Products Steel Labels Plate thickness (mm) Microstructure YS (MPa) TS (MPa) vTrs (° C) Kca (-10 ° C) (N / mm ^3/2 ) Class 16 N 60 DF + F + P 411 527 -18 3800 Comparative Example 17 O 80 DF + B + MA 388 508 -9 1300 Comparative Example 18 P 70 DF + F + P 415 557 -7 2200 Comparative Example 19 AND 70 DF + F + P + B 420 534 -2 2800 Comparative Example 20 H 75 DF + F + P + B 433 547 -20 2700 Comparative Example 21 G 60 DF + F + P 447 575 -17 3800 Comparative Example 22 B 70 DF + F + P 320 376 -29 3700 Comparative Example

Note 1: Underlined values indicate that the data are outside the ranges of the invention.

Note 2: vTrs at 1/2 of the plate thickness + 6 mm

Note 3: DF: deformed ferrite, P: perlite, B: bainite, M: martensite, MA: martensite-austenite constituent, F: deformation-free ferrite after transformation from austenite (except deformed ferrite).

Claims (5)

1. High strength thick steel plate for structural use with excellent brittle fracture interruption capacity, the steel plate characterized by the fact that it presents a chemical composition of steel that consists, in mass%, C: 0.03 to 0.20%, Si: no more than 0.1%, Mn: 0.5 to 2.2%, P: no more than 0.008%, S: no more than 0.01%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.03%, Al: 0.005 to 0.08%, and N: not more than 0.0075%, optionally one, or two or more than Cu: 0.01 to 0.5%, Ni: 0.01 to 1.0%, Cr: 0.01 to 0.5%, Mo: 0.01 to 0.5%, V: 0.001 to 0.1%, B: no more than 0.003%, Ca: not more than 0.005%, and REM: no more than 0.01%, the balance being Fe and the inevitable impurities, the steel sheet having a microstructure based on the deformed ferrite, the steel sheet having a Charpy fracture appearance transition temperature of no more than -40 ° C as measured in relation to the portion found at 1/2 of the plate thickness (t) + 6 mm. 2. High strength thick steel plate for use Petition 870190041837, of 05/03/2019, p. 4/10 2/3 structural with excellent brittle fracture interruption capacity, according to claim 1, characterized by the fact that the microstructure presents, as a second phase, one, or two or more phases between perlite, bainite, martensite, martensiteaustenite constituent ( MA), and deformation-free ferrite after transformation from austenite. 3. High strength thick steel sheet for structural use with excellent brittle fracture interruption capacity, according to claim 1 or 2, characterized by the fact that the thickness of the sheet is greater than 50 mm. 4. Method for the production of high strength thick steel sheets for structural use with excellent brittle fracture interruption capacity, characterized by the fact that it comprises heating a plate having the chemical composition, as defined in claim 1, to a temperature of 1000 at 1200 ° C, laminate the plate in such a way that the cumulative reduction in lamination is no less than 30% in the temperature range in which the temperature of a central portion at half the thickness of the plate is in the region of austenite recrystallization, afterwards perform the first cooling at a cooling rate of no more than 15 ° C / s until the temperature of the central portion at half the thickness of the plate is decreased to, or below the point Ar3, laminate the steel plate with a cumulative reduction lamination of not less than 40% in the temperature range in which the temperature of the central portion at half the thickness of the sheet is equal to or less r than the Ar3 point, and perform the second cooling at a cooling rate of not less than 4 ° C / s to a temperature of 600 ° C or less. 5. Method for producing thick, high-strength steel sheets for structural use with excellent Petition 870190041837, of 05/03/2019, p. 5/10 3/3 breakage of fragile fracture, according to claim 4, characterized by the fact that it also comprises tempering the steel sheet to a temperature of no more than the point Aci after the second cooling.