JP6536514B2 - High strength steel plate for structure excellent in brittle crack propagation arresting property and method of manufacturing the same - Google Patents

Description

  The present invention is a structural height excellent in brittle crack propagation arresting properties suitable for use as a thick steel plate exceeding 50 mm in thickness for large structures such as ships, marine structures, low temperature storage tanks, construction and civil engineering structures, etc. The present invention relates to a strong thick steel plate and a method of manufacturing the same.

  In large structures such as ships, offshore structures, low-temperature storage tanks, and construction and civil engineering structures, safety is improved because accidents resulting from brittle fractures have a large impact on socioeconomics and the environment. Always sought. The steel materials used are required to have high levels of toughness at use temperatures and brittle crack propagation arresting properties from the viewpoint of preventing the occurrence of fracture even if a crack occurs.

  For example, vessels such as container vessels and bulk carriers use high-strength thick-walled materials for the outer panel due to their structure, but recently, with the increase of the size of the hull, further increase in thickness and thickness increase. It is. In general, since the brittle crack propagation arresting properties of steel sheets tend to deteriorate as the strength or thickness of the steel increases, the requirements for the brittle crack propagation arresting properties are further advanced.

  As a means of improving the brittle crack propagation arresting properties of steel materials, conventionally, a method of increasing the Ni content in steel is known. For example, in a storage tank of liquefied natural gas (LNG), 9% Ni steel Is used on a commercial scale. However, the increase in the amount of Ni in the steel requires a significant increase in manufacturing costs, and thus is difficult to apply to applications other than the LNG storage tank.

  On the other hand, relatively thin steel materials with a thickness of less than 50 mm, which are used in ships and line pipes, do not reach cryogenic temperatures like LNG, for example, are refined by the TMCP method to achieve low temperatures. By improving the toughness, excellent brittle crack propagation arresting properties can be imparted.

  Further, Patent Document 1 discloses a steel material in which the structure of the surface layer portion is ultrafine in order to improve the brittle crack propagation arresting property without increasing the alloy cost. Patent Document 1 focuses on the fact that a shear lip (plastic deformation region) generated in the surface layer of a steel material is effective in improving the brittle crack propagation arresting property when a brittle crack propagates, and refines crystal grains in the shear lip portion. It is disclosed that absorbing the propagation energy which the propagating brittle crack has by carrying out.

Further, in Patent Document 1, as a manufacturing method, a step of cooling the surface layer portion below the Ar ₃ transformation point by controlled cooling after hot rolling, and then stopping the controlled cooling to recover the surface layer portion above the transformation point It is disclosed that, by applying a pressure to the steel material to cause repeated transformation or process recrystallization during one or more repetitions of the steel material, an ultrafine ferrite structure or bainite structure is formed in the surface layer portion. ing.

  In Patent Document 2, in order to improve the brittle crack propagation arresting property in a steel material having a microstructure of ferrite-pearlite as a main component, both surface portions of the steel material have an equivalent circle particle diameter of 5 μm or less and an aspect ratio of 2 It is important to suppress the variation in ferrite grain size while forming the ferrite structure having the above ferrite grains in a layer having 50 area% or more, and as a method of suppressing this variation, the maximum reduction per pass during finish rolling It is disclosed that the local recrystallization phenomenon is suppressed by setting the rate to 12% or less.

  Patent Document 3 discloses a technique on the extension of TMCP, which improves the brittle crack propagation arresting property by focusing not only on refining of ferrite crystal grains but also on subgrains formed in ferrite crystal grains. It is done.

  Specifically, at a plate thickness of 30 to 40 mm, (a) rolling conditions for securing fine ferrite crystal grains without requiring complicated temperature control such as cooling and recuperation of the steel sheet surface layer, (b) steel material Under rolling conditions that produce a fine ferrite structure in 5% or more of the plate thickness, (c) develop a texture in fine ferrite and rearrange dislocations introduced by processing (rolling) with thermal energy to form subgrains There is disclosed a technique for improving the brittle crack propagation arresting property by rolling conditions (d) and cooling conditions which suppress coarsening of formed fine ferrite crystal grains and fine subgrains.

  In addition, in controlled rolling, a method is also known which improves the brittle crack propagation arresting property by applying a pressure to the transformed ferrite to develop a texture. By providing separation on the fracture surface of the steel material in a direction parallel to the plate surface and relieving stress at the brittle crack tip, resistance to brittle fracture is enhanced.

  For example, in Patent Document 4, the brittle fracture resistance property is obtained by controlling the (110) plane X-ray intensity ratio to 2 or more by controlled rolling and setting coarse particles having a circle equivalent diameter of 20 μm or more to 10% or less in area ratio. It is disclosed to improve.

  Patent Document 5 discloses a steel plate having an X-ray surface strength ratio of (100) plane of 1.5 or more in a rolling surface inside a plate thickness as a steel for welded structure excellent in brittle crack propagation stopping property of a joint portion. It is done. The brittle crack propagation arresting property is improved by the difference between the stress application direction due to the texture development and the angle of the crack propagation direction.

  Furthermore, Patent Document 6 describes a brittle crack propagation stop characteristic that develops a texture in each part in the thickness direction (1⁄4 of thickness, center of thickness, etc.) by defining the average rolling reduction in controlled rolling. A method of making an improved welded structural steel is described.

Japanese Examined Patent 7-100814 JP 2002-256375 A Patent No. 3467767 gazette Patent No. 3548349 gazette Patent 2659661 gazette JP, 5713135, A

Inoue et al .: Long brittle crack propagation behavior in thick shipbuilding steel, Proceedings of the Japan Ship and Ocean Engineering Conference, 3rd, 2006, pp. 359-362.

  However, the steel materials described in Patent Literatures 1 and 2 are intended to obtain a specific structure by temporarily cooling only the surface layer portion of the steel material and then reheating it and adding processing during the reheating. There is a problem that the control is not easy and the load on the rolling and cooling equipment is large.

  Moreover, the steel plates described in Patent Documents 1 to 6 are mainly intended to have a thickness of about 50 mm from the manufacturing conditions and the experimental data disclosed, and for application to a thick material of about 70 mm, It is unclear whether predetermined characteristics can be obtained, and the crack propagation characteristics in the thickness direction necessary in the hull structure are unknown.

Non-Patent Document 1 evaluates the brittle crack propagation arrest property of a steel plate having a plate thickness of 65 mm, and reports the result that the brittle crack does not cease in the large brittle crack propagation arrest test of the base material. Furthermore, the ESSO test of the test material shows that the value of Kca at operating temperature -10 ° C does not reach 3000 N / mm ^3/2 , and safety is secured in the case of a ship structure using a steel plate with a thickness of more than 50 mm. Suggest that it will be an issue.

  An object of the present invention is, in particular, to provide a high-strength thick steel plate excellent in brittle crack arresting properties even in a thick steel plate having a thickness exceeding 50 mm, and a method for producing the same.

  The inventors achieved the following findings as a result of extensive research on a high strength thick steel plate having excellent brittle crack propagation arresting properties and a method of stably obtaining the steel plate in order to solve the above problems. .

1. When the hot rolling is completed in the austenite temperature range, as the temperature at the time of rolling is lower, strain is introduced and ferrite nuclei are increased. In addition, since nucleation energy is increased, the structure after transformation becomes finer, and a high toughness value can be obtained. However, in the case of a thick steel plate having a thickness exceeding 50 mm, if the rolling temperature is lowered to around the Ar ₃ transformation point, the temperature difference between the front and back of the steel plate and the central portion of the steel plate during rolling becomes large. For this reason, the surface of a steel plate transforms into a ferrite structure, and the problem of deteriorating the toughness of the surface of a steel plate generate | occur | produces. Therefore, during rolling during rolling, that is, after rough rolling or before finish rolling or during rough rolling after finish rolling, the front and back surfaces of the steel plate are heated to reduce the temperature difference between the front and back surfaces of the steel plate and the central portion of the steel plate thickness. . Thus, the problem persists, far less than Ar ₃ transformation point or more and low temperatures close to the Ar ₃ transformation point (hereinafter, simply, a low temperature close to Ar ₃ transformation point, sometimes referred to as.) Rolling it can.

2. Furthermore, the texture is hot with a specific chemical component, a cumulative rolling reduction of 45% or more, and a rolling reduction per pass of 3% or more in a low temperature range close to the Ar ₃ transformation point in the central portion of the plate thickness. It is obtained by combination with rolling conditions.

  3. By using the above-mentioned process, excellent base material toughness can be obtained, and at the same time, the {112} <110> orientation strength parallel to the rolling surface in the central portion of the steel plate is 3.0 or more. A texture having a parallel {112} <110> orientation strength of 2.5 or more is obtained. Thereby, extremely excellent brittle crack propagation arresting properties can be obtained.

  Furthermore, according to the present invention, it has become possible to obtain even higher brittle crack propagation arresting properties under conventional rolling conditions.

The present invention has been completed by further studying the above findings, and the gist configuration of the present invention is as follows.
[1] Component composition is mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.2%, P: 0.02% Hereinafter, S: 0.01% or less, Ti: 0.005 to 0.03%, Al: 0.005 to 0.08%, N: 0.0050% or less, and the carbon equivalent represented by the formula (1) (Ceq) satisfies 0.30% or more, and the balance consists of Fe and unavoidable impurities,
A texture in which the {112} <110> orientation strength parallel to the rolling surface at the central portion of the plate thickness is 3.0 or more, and the {112} <110> orientation strength parallel to the rolling surface on the steel sheet surface is 2.5 or more A structural high strength steel plate excellent in brittle crack propagation arresting characteristics characterized by having:
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
However, the element symbol in the formula (1) indicates the content (mass%) of each element, and is 0 when not contained.
[2] The high strength thick steel plate for a structure excellent in brittle crack propagation arresting properties according to [1], wherein the structure in the central portion of thickness is a structure of ferrite-pearlite main component.
[3] The high strength thick steel plate for a structure excellent in brittle crack propagation arresting properties according to [1], wherein the structure in the central portion of thickness is a structure of ferrite-bainite main component.
[4] In addition to the above component compositions, Nb: 0.005 to 0.05%, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.5% and Cr: 0 in mass%. High strength for structural excellent in brittle crack propagation arresting property according to any one of [1] to [3] characterized by containing one or more selected from 01 to 0.5%. Thick steel plate.
[5] In addition to the above component compositions, Mo: 0.01 to 0.5%, V: 0.001 to 0.10%, B: not more than 0.0030%, Ca: not more than 0.0050% by mass. The structural material having excellent brittle crack propagation arresting properties according to any one of [1] to [4], which contains one or more selected from the following and REM: 0.0100% or less High strength thick steel plate.
[6] [1], [4] and [5] heat the steel material having the component composition according to any one of the above to a temperature of 1000 to 1200 ° C.,
Then, after rough rolling, during finish rolling before or finish rolling, the temperature of the steel sheet surface was heated to less than the Ar ₃ point (Ar ₃ point +170) ° C., Ar ₃ or more points (Ar ₃ point +150) ° C. below the temperature Hot rolling at a finish rolling finish temperature of Ar ₃ or more at a cumulative rolling reduction in the range of 45% to 90% and a rolling reduction per pass of 3% to 35%,
Subsequently, it is cooled to 600 ° C. or less at an average cooling rate of 2.0 ° C./s or more and 10.0 ° C./s or less at the central portion of the plate thickness. High strength thickness for structure excellent in brittle crack propagation arresting characteristics Method of manufacturing steel plate.
[7] After the cooling, tempering treatment is further performed to a temperature of 250 ° C. or more and Ac ₁ point, and the high strength thick steel plate for a structure having excellent brittle crack propagation arresting properties according to [6] Production method.

In the present invention, excellent brittle crack propagation arresting property means that Charpy absorbed energy at −40 ° C. is 250 J or more and Kca (−10 ° C.) is 6000 N / mm ^3/2 or more at 1/4 part of plate thickness. It means that there is. Further, in the present invention, a high strength thick steel plate refers to a thick steel plate having a tensile strength (TS) of 490 MPa or more and a yield strength (YS) of 355 MPa or more.

  According to the present invention, even when the thickness of the obtained thick steel plate exceeds 50 mm, the brittle crack propagation arresting property is excellent. The present invention exhibits remarkable superiority and is effective when applied to a steel plate having a thickness of 50 mm or more, preferably 70 mm or more, more preferably 80 mm or more, and still more preferably 90 mm or more. For example, in the shipbuilding field, application to deck members joined to hatch side combing in container decks and heavy deck structures for bulk carriers greatly contributes to ship safety improvement, and is extremely useful in industry.

Hereinafter, the present invention will be described in detail.
1. Texture In the present invention, in order to improve the crack propagation arresting property for a crack propagating parallel to the rolling direction or the rolling perpendicular direction, {112} parallel to the rolling plane on the central portion of the thickness of the steel plate and the steel plate surface. 110> Define azimuth strength. That is, {112} <110> orientation strength parallel to the rolling surface in the thickness central portion of the steel plate and the surface of the steel plate is developed, and {112} <110> orientation strength parallel to the rolling surface in the thickness central portion is 3. It is assumed that there is a texture having 0 or more and {112} <110> orientation strength parallel to the rolling surface on the steel sheet surface of 2.5 or more.

  Here, in the present invention, the plate thickness central portion of the steel plate means a plate thickness 1/2 position (hereinafter, may be referred to simply as a plate thickness 1/2 position). Further, in the present invention, the steel plate surface means within 2 mm in the thickness direction from the front and back surfaces of the steel plate, and not only a simple surface of the product but the steel plate surface is treated to a surface where the degree of accumulation of crystals can be measured. The rear surface, for example, when the outermost surface of the steel plate is covered with scale, refers to the surface from which it has been removed. When the outermost surface of the steel plate is a mirror surface, the degree of accumulation of crystals can be measured as it is. In the measurement of the surface integration degree, a position error of several percent is acceptable. Furthermore, in the present invention, the {112} plane is parallel to the rolling surface, and the <110> orientation is parallel to the rolling direction.

  When {112} <110> orientation strength is developed parallel to the rolling surface in the central portion of the plate thickness and on the surface of the steel sheet, microscopic cracks are generated prior to the crack growth to provide resistance to the crack growth. In order to obtain this effect, the {112} <110> orientation strength parallel to the rolling surface is 3.0 or more at the center of the plate thickness, and the {112} <110> orientation strength parallel to the rolling surface is 2. Assume that there are five or more aggregate regions.

More specifically, Kca (-10), which is a thick-walled material exceeding 50 mm that has come to be used for ship's outer panels such as recent container ships and bulk carriers, is targeted for securing structural safety. In order to obtain brittle crack propagation arrest properties of ° 6000 N / mm ^3/2 , the steel sheet surface has a {112} <110> orientation strength parallel to the rolling surface of 3.0 or more at the center of the thickness of the steel sheet. The {112} <110> orientation strength parallel to the rolling surface in the above needs to be 2.5 or more.

  If the above-described texture includes the central portion of plate thickness and the surface of the steel plate and the area is 5% or more of the plate thickness, the above-described effects can be obtained. Therefore, the upper limit of the area in the thickness direction of the texture is not particularly defined. However, it is needless to say that the above-mentioned effect is exhibited even if the total thickness of the plate thickness is the texture. The {112} <110> orientation strength is a numerical value representing the degree of integration of the {112} crystal plane of the target material, and can be measured by the method described in the examples described later.

The structure at a 1/4 thickness position (hereinafter sometimes referred to simply as a 1/4 thickness part) of the steel plate is preferably mainly composed of a ferrite-pearlite structure or a ferrite-bainite structure. With such a structure, toughness can be secured. In order to obtain {112} <110> orientation strength parallel to the above-mentioned rolling surface, it is preferable that the structure in the central portion of the plate thickness be a ferrite-pearlite main structure or a ferrite-bainite main structure, ferrite- The ratio of the structure based on pearlite or the structure based on ferrite and bainite to the total structure is preferably 60% or more in area ratio. More preferably, it is 70% or more. In addition, a board thickness center part says board thickness 1/2-1/4 position.
In addition, there is no limitation in particular in another structure | tissue, It may be the same as the conventionally well-known steel plate for structures. For example, retained austenite or martensite. In addition, the identification of tissues can also be performed according to conventional methods.
2. Chemical composition of steel In the following description,% in steel plate components means mass%.

C: 0.03 to 0.20%
C is an element that improves the strength of steel, and in the present invention, it is 0.03% or more in order to secure a desired strength. On the other hand, if it exceeds 0.20%, not only the weldability is deteriorated but also the toughness is adversely affected. Therefore, C is set to 0.03 to 0.20%. Preferably, it is 0.05 to 0.15%.

Si: 0.03 to 0.5%
Si is effective as a deoxidizing element and as a strengthening element of steel, but its content can not be obtained at a content of less than 0.03%. On the other hand, if it exceeds 0.5%, not only the surface properties of the steel are impaired, but also the toughness is extremely deteriorated, so the content is made 0.5% or less. Therefore, Si is set to 0.03 to 0.5%.

Mn: 0.5 to 2.2%
Mn is contained as a strengthening element. The effect is obtained by containing 0.5% or more. On the other hand, if the Mn content exceeds 2.2%, the weldability is deteriorated and the steel material cost is also increased. Therefore, Mn is set to 0.5 to 2.2%.

P: 0.02% or less, S: 0.01% or less P and S are unavoidable impurities in steel, and when the content is increased, the toughness is deteriorated. In order to maintain favorable toughness with respect to the steel plate whose plate | board thickness exceeds 50 mm, P shall be 0.02% or less, S shall be 0.01% or less. Preferably, P: 0.01% or less and S: 0.005%. More preferably, P: 0.006% or less and S: 0.003% or less.

Ti: 0.005 to 0.03%
Ti has an effect of forming nitrides, carbides or carbonitrides by containing a slight amount, refining the crystal grains and improving the base material toughness. The effect is obtained by containing 0.005% or more. On the other hand, if the content is more than 0.03%, the toughness of the base material and the heat affected zone of welding is reduced. Therefore, Ti is made 0.005 to 0.03%.

Al: 0.005 to 0.08%
Al acts as a deoxidizing agent, which requires a content of 0.005% or more. On the other hand, if the content is more than 0.08%, the toughness is lowered and, when welded, the toughness of the weld metal portion is lowered. Therefore, Al is made 0.005 to 0.08%. Preferably, it is 0.02 to 0.04%.

N: not more than 0.0050% When N exceeds 0.0050%, the toughness deteriorates. Therefore, N is made 0.0050% or less. N combines with Al in the steel, adjusts the grain size at the time of rolling, and strengthens the steel. Preferably, it is 0.0015% or more.

Carbon equivalent (Ceq): 0.30% or more Carbon equivalent is an important indicator for predicting the strength of the structure, transformation behavior, and the like. In the steel material according to the present invention, in addition to the fact that the above-described components are contained to satisfy the above composition range, Ceq represented by the formula (1) described later is contained so as to be 0.30 or more Need to be On the other hand, if Ceq is less than 0.30%, it is difficult to strengthen the steel material, so that the required base material strength can not be obtained even when the controlled rolling and accelerated cooling conditions are adjusted. Therefore, in order to maintain the strength and texture strength of a thick steel plate having a thickness of more than 50 mm, Ceq is set to 0.30% or more. Preferably, it is 0.33% or more.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
However, the element symbol in the formula (1) indicates the content (mass%) of each element, and is 0 when not contained.

  The balance is iron and unavoidable impurities.

  With the above essential elements, the steel sheet of the present invention can obtain the intended properties, but in order to further improve the properties, the following elements can be contained as necessary in addition to the above essential elements.

Nb: 0.005 to 0.05%, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.5% and Cr: 0.01 to 0.5% One or two selected from More than Nb: 0.005 to 0.05%
Nb precipitates as NbC at the time of ferrite transformation or reheating, and contributes to high strength. In addition, it has the effect of expanding the non-recrystallized area in rolling in the austenite region, and contributes to the grain refinement of the ferrite, which is effective in improving the toughness. The effect is exhibited by containing 0.005% or more. If the content is more than 0.05%, coarse NbC precipitates, which in turn causes a decrease in toughness. Therefore, when Nb is contained, the upper limit is made 0.05%.

Ni: 0.01 to 1.5%
Ni is an element that enhances the hardenability of the steel. Moreover, while contributing directly to the strength improvement after rolling, it can contain for the function improvement of toughness, high temperature strength, or weather resistance. All of these effects are exhibited by containing 0.01% or more. On the other hand, excessive inclusion of Ni degrades toughness and weldability, and also increases the cost of the alloy. Therefore, when it contains Ni, it is 0.01 to 1.5%.

Cu: 0.01 to 0.5%, Cr: 0.01 to 0.5%
Cu and Cr are elements which enhance the hardenability of steel. While directly contributing to the improvement of strength after rolling, it can be contained to improve the functions such as toughness, high temperature strength, or weather resistance. These effects are all exhibited by containing Cu: 0.01% or more and Cr: 0.01% or more. On the other hand, excessive content of Cu and Cr degrades toughness and weldability. Therefore, when it contains Cu and Cr, respectively, Cu: 0.01 to 0.5% and Cr: 0.01 to 0.5%.

Mo: 0.01 to 0.5%, V: 0.001 to 0.10%, B: not more than 0.0030%, Ca: not more than 0.0050%, and REM: not more than 0.0100% Or 2 or more types of Mo: 0.01 to 0.5%
Mo is an element that enhances the hardenability of the steel. While directly contributing to the improvement of strength after rolling, it can be contained to improve the functions such as toughness, high temperature strength, or weather resistance. These effects are exhibited by containing Mo: 0.01% or more. On the other hand, excessive inclusion of Mo degrades toughness and weldability. Therefore, when it contains Mo, it sets it as Mo: 0.01-0.5%.

V: 0.001 to 0.10%
V is an element which improves the strength of the steel by precipitation strengthening precipitated as V (CN), and this effect is exhibited by containing 0.001% or more. However, if the content exceeds 0.10%, the toughness is reduced. Therefore, when V is contained, the content is made 0.001 to 0.10%.

B: 0.0030% or less B is an element which increases the hardenability of steel by a small amount. However, if the content is more than 0.0030%, the toughness of the welded portion is reduced. Therefore, when B is contained, the content is made 0.0030% or less. A good hardenability can be obtained by setting the lower limit of the content to about 0.0006%. More preferably, it is 0.0006% or more and 0.0030% or less.

Ca: 0.0050% or less, REM: 0.0100% or less Ca and REM refine the structure of the weld heat affected zone and improve the toughness. Therefore, even if it contains Ca and REM, the effect of the present invention is not impaired, so it may be contained as needed. However, if Ca and REM are excessively contained, coarse inclusions are formed to deteriorate the toughness of the base material. Therefore, in the case of containing Ca and REM, the upper limit of the content is made respectively Ca: 0.0050% and REM: 0.0100%.
3. Production conditions The production conditions specify slab heating conditions, hot rolling conditions, and cooling conditions after hot rolling.

  First, after melting the molten steel of the above-mentioned chemical component with a converter etc., it is used as a steel raw material (slab) by continuous casting etc., and is heated to the temperature of 1000-1200 degreeC.

  If the slab heating temperature is less than 1000 ° C., sufficient rolling time in the austenite recrystallization temperature range can not be secured. On the other hand, when the slab heating temperature is higher than 1200 ° C., the austenite grains become coarse and not only the toughness is lowered, but also the oxidation loss becomes remarkable and the yield is lowered. Therefore, the slab heating temperature is set to 1000 to 1200 ° C. The preferable slab heating temperature is 1000 to 1150 ° C. from the viewpoint of improving the toughness of the steel plate. More preferably, it is 1030 to 1130 ° C.

  Then, hot rolling is performed.

Here, after rough rolling, during finish rolling before or finish rolling, the temperature of the steel sheet surface is heated below Ar ₃ point or more (Ar ₃ point +170) ° C., Ar ₃ or more points (Ar ₃ point +150) ° C. The following Hot rolling is performed such that the finish rolling finish temperature is Ar ₃ or more at a total rolling reduction of 45% to 90% in a temperature range, and a rolling reduction per pass of 3% to 35%.

The present invention is characterized in that the temperature distribution in the thickness direction is controlled to a low temperature close to the Ar ₃ point in hot rolling in which the temperature in the central portion of the thickness is set to the austenite recrystallization temperature range and austenite non-recrystallization temperature range. I assume.

The hot rolling according to the invention is characterized in that rolling is completed in the austenitic zone in order to obtain high toughness values and the above-mentioned texture. However, to be done at a low temperature region close to the Ar ₃ transformation point, in the course of hot rolling, the front and back surfaces of the steel sheet, i.e., the temperature of the steel sheet surface falls below ₃ points Ar. If the Ar ₃ point or less is lowered, the front and back surfaces of the steel sheet will be transformed to a ferrite structure to form a worked ferrite having a low toughness value, and the toughness of the steel sheet as a whole will be reduced. Therefore, to avoid formation of processed ferrite with low toughness value, the temperature of the surface of the steel sheet from the front and back of the steel sheet during hot rolling, that is, after finish rolling before rough rolling or finish rolling after coarse rolling The heating is performed at an Ar ₃ point or more (Ar ₃ point + 170) ° C. or less to control the temperature distribution in the plate thickness direction. By controlling the temperature distribution in the thickness direction, the temperature difference between the central portion of the thickness of the steel plate and the surface of the steel plate becomes smaller, and rolling can be performed at a lower temperature than before. Here, as for the temperature difference of the plate | board thickness center part of a steel plate, and a steel plate surface, 0 degreeC-20 degreeC is preferable. As a result, it is possible to obtain the strength of the texture in the center portion of the plate thickness and the surface of the steel plate.

Finish rolling, Ar ₃ or more points (Ar ₃ point +150) ° C. 90% or less cumulative rolling reduction of 45% or more at a temperature range, and 35% rolling reduction per pass of 3% or more or less, finish rolling end Hot rolling is performed so that the temperature is Ar ₃ points or more.

If the cumulative draft in this temperature range is less than 45%, the target texture can not be obtained. On the other hand, when it exceeds 90%, the efficiency at the time of rolling will fall. Therefore, the cumulative rolling reduction is set to 45% or more and 90% or less. Preferably, it is 50% or more and 80% or less. In addition, if the rolling reduction per pass is less than 3%, the stress of the shear component becomes large and not only the target texture can not be obtained, but the pressure is not sufficiently reduced to the inside, and an internal defect is present at the center of the plate thickness. There is also a possibility. On the other hand, if it exceeds 35%, the load on the rolling mill will increase, and there is a possibility that it will stop in the middle of rolling. Therefore, the rolling reduction per pass is set to 3% or more and 35%. Preferably, it is 3.4% or more and 17% or less. In addition, cumulative rolling reduction shall add and total the rolling reduction in each rolling mill used as a temperature range which is Ar ₃ point or more (Ar ₃ point +150) degrees C or less in finish rolling. Moreover, the rolling reduction per pass is an average of the rolling reduction per pass in a temperature range of Ar ₃ points or more (Ar ₃ points + 150) ° C. or less.

If the rolling end temperature is less than the Ar ₃ point, transformation occurs during rolling, and the target structure can not be obtained. Therefore, the rolling end temperature is preferably Ar ₃ or more. More preferably, it is Ar ₃ point-(Ar ₃ point + 20) ° C. Here, Ar ₃ point is calculated by the following equation.
Ar ₃ points (° C.) = 910-273C-74Mn-5Cu-16Cr-56Ni-9Mo
However, in the formula, each element symbol is the content in steel (% by mass), and when not contained, it is 0.

  In addition, rolling is performed so that a desired plate thickness can be obtained. By combining with the cooling conditions described later, the {112} <110> orientation strength parallel to the rolling surface at the center of the plate thickness of the steel plate is 3.0 or more and {112} <110 parallel to the rolling surface on the steel plate > A texture with an orientation strength of 2.5 or more can be obtained.

  Then, cooling is performed.

The steel plate after the rolling is cooled to 600 ° C. or less at a temperature of ₃ points or more of Ar at an average cooling rate of 2.0 ° C./s or more. By setting the average cooling rate to 2.0 ° C./s or more, the strength of the texture developed at the time of rolling can be maintained. On the other hand, below 2.0 ° C./s, the intended tissue and texture of the present invention can not be obtained. Therefore, the average cooling rate is set to 2.0 ° C./s or more. Preferably, it is 4.0 ° C./s or more. When the average cooling rate exceeds 10 ° C./s, the tissue changes, and the target tissue of the present invention can not be obtained. Therefore, 10.0 degrees C / s or less is preferable.

  In addition, when the cooling stop temperature exceeds 600 ° C., the texture obtained by rolling is impaired. Therefore, the temperature is set to 600 ° C. or less. Preferably, it is 550 degrees C or less. If the cooling stop temperature is less than 0 ° C., the toughness may be deteriorated. Therefore, 0 ° C. or higher is preferable. More preferably, it is 100 ° C. or higher.

Furthermore, when tempering treatment is performed after cooling, it is preferable to carry out at a temperature of 250 ° C. or more and less than ₁ point of Ac. If the tempering treatment is at Ac ₁ point or more, there is a risk of losing the texture developed at the time of rolling. On the other hand, if the tempering treatment is lower than 250 ° C., there is a possibility that the desired structure and texture of the present invention can not be obtained. There is a risk that the base material toughness is reduced and the brittle crack propagation arresting properties are reduced. Here, Ac ₁ point is calculated by the following equation.
Ac ₁ point (° C.) = 751-26.6C + 17.6Si-11.6Mn-169Al-23Cu-23Ni + 24.1Cr + 22.5Mo + 233Nb-39.7V-5.7Ti-895B
However, in the formula, each element symbol indicates the content in steel (% by mass), and when not contained, it is 0.

  In the steel plate manufactured according to the above-mentioned manufacturing conditions, since the surface unit in the Charpy test is refined, the Charpy fracture surface transition temperature in 1/4 part of plate thickness is -40 degrees C or less, and Charpy absorption in -40 degrees C Energy of 250 J or more is achieved. In addition, the Charpy fracture surface transition temperature in 1/4 part of board thickness and the Charpy absorbed energy in -40 degreeC can be measured by the method as described in the Example mentioned later.

  In the above description, the temperature at the central portion of the plate thickness is determined by heat transfer calculation from the surface temperature of the steel plate measured by a radiation thermometer, or a value calculated based on the center temperature measured in advance. Further, the temperature condition under the cooling condition after rolling is the temperature at the central portion of the plate thickness.

  By the above, the high strength thick steel plate excellent in the brittle crack propagation arresting property of the present invention can be obtained. In the high strength thick steel plate excellent in the brittle crack propagation arresting properties of the present invention and the method for producing the same, the absorbed energy at −40 ° C. is 250 J or more. Since good toughness of the base material is a premise for suppressing the growth of cracks, the steel sheet according to the present invention is a brittle crack propagation arresting property for which the Charpy absorbed energy at the 1/4 thickness position is desired. Define accordingly as appropriate. In the present invention, the absorbed energy at −40 ° C. is 250 J or more. Preferably, the absorbed energy at -50 ° C is 250 J or more.

  Next, examples of the present invention will be described.

  Table 1 shows the chemical composition of the test steel, and Table 2 shows the manufacturing conditions. Molten steel of each composition in Table 1 (Steel symbol: 1 to 17) is melted by a converter and made into a steel material (slab: 300 mm thick) by continuous casting method, then after hot rolling under the production conditions of Table 2 Then, cooling was performed to produce a thick steel plate (steel symbol: 1 to 30) having a thickness of 50 to 130 mm.

Mechanical Properties The obtained thick steel plate was subjected to a tensile test by collecting a JIS 14A No. 14 test piece having a diameter of 1/4 14 from the 1⁄4 position of the plate thickness, and the yield strength (YS) and tensile strength (TS) were measured.

Charpy fractured surface transition temperature of 1/4 part of plate thickness From the 1⁄4 position of plate thickness, the direction of the longitudinal axis of the test piece was taken parallel to the rolling direction and the Charpy impact test was conducted. The Charpy fracture surface transition temperature (vTrs) was determined.

Charpy absorbed energy at -40 ° C From the 1/4 thickness position of each steel plate, the JIS No. 4 impact test specimen is collected so that the direction of the longitudinal axis of the test specimen is perpendicular to the rolling direction, and Charpy impact at a test temperature of -40 ° C. A test was conducted to determine the absorbed energy (vE- ₄₀ ) (average of three). Those having a Charpy absorbed energy of 250 J or more at −40 ° C. are included in the scope of the present invention.

{112} <110> Orientation Strength In order to evaluate the texture of the steel plate, the {112} <110> orientation strength parallel to the rolling plane at the center of the plate thickness and the steel plate surface was measured. The measurement method is as follows. First, a sample with a thickness of 1 mm is taken from the surface of a steel plate or from the center of thickness, and a surface parallel to the plate surface is mechanically polished and electropolished to prepare a test piece for X-ray diffraction. In the case of a steel plate surface, the surface closer to the outermost surface is polished. Then, using this test piece, perform X-ray diffraction measurement using an X-ray diffractometer using a Mo radiation source to obtain (200), (110) and (211) pole figures and obtain The three-dimensional crystal orientation density function is calculated and determined from the positive electrode diagram shown above. In the central part of the plate thickness, the ones with {112} <110> orientation strength parallel to the rolling surface of 3.0 or more and the {112} <110> orientation strength parallel to the rolling surface at the steel sheet surface of 2.5 or more Within the scope of the invention.

Kca value Next, in order to evaluate the brittle crack propagation arrest property, a temperature gradient type ESSO test is performed, and the Kca value at −10 ° C. (hereinafter also referred to as “Kca (−10 ° C.) (N / mm ^3/2 )) I asked. Those having a Kca value of 6,000 N / mm ^3/2 or more at -10 ° C. are considered to be within the scope of the present invention.

  Table 3 shows the results of these tests.

From the results shown in Table 3, in the case of the test steel plates (Production Nos. 1 to 10) according to the present invention, the chemical composition and the production conditions are within the scope of the present invention, and the texture is {112} in the center of thickness The <110> azimuthal strength is 3.0 or more, the {112} <110> azimuthal strength on the steel sheet surface is 2.5 or more, and the Charpy absorbed energy at −40 ° C. at a plate thickness of 1⁄4 is 250 J As mentioned above, the value of Kca (-10 degrees C) obtained the outstanding brittle crack propagation stop characteristic with 6000 N / mm < ³ > / ² or more.

On the other hand, in the case of the test steel plate (manufacturing No. 11 to 30) which deviates from the present invention, any of the chemical components and the manufacturing conditions is out of the scope of the present invention. The Charpy absorbed energy of the above did not satisfy the definition of the present invention, and the value of Kca (−10 ° C.) was as low as 4300 N / mm ^3/2 or less, and did not reach the inventive example.

Claims (7)

Component composition is mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.2%, P: 0.02% or less, S : 0.01% or less, Ti: 0.005 to 0.03%, Al: 0.005 to 0.08%, N: 0.0050% or less, and the carbon equivalent (Ceq) represented by the formula (1) Is 0.30% or more, and the balance consists of Fe and unavoidable impurities,
The {112} <110> orientation strength parallel to the rolling surface at the thickness 1/2 position is 3.0 or more, and the {112} <110> orientation strength parallel to the rolling surface on the steel sheet surface is 2.5 or more A structural high strength thick steel plate excellent in brittle crack propagation arresting characteristics characterized by having a texture.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
However, the element symbol in the formula (1) indicates the content (mass%) of each element, and is 0 when not contained. The structural high strength with excellent brittle crack propagation arresting characteristics according to claim 1, characterized in that the structure at a position of 1/2 to 1/4 of plate thickness is a structure in which ferrite-pearlite has an area ratio of 60% or more. Strength thick steel plate. The structural high strength with excellent brittle crack propagation arresting characteristics according to claim 1, characterized in that the structure at the position of 1/2 to 1/4 of plate thickness is a structure in which ferrite-bainite is 60% or more in area ratio. Strength thick steel plate.   In addition to the above component compositions, Nb: 0.005 to 0.05%, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.5% and Cr: 0.01% by mass. The structural high strength steel plate excellent in the brittle crack arresting property according to any one of claims 1 to 3, which contains one or more selected from 0.5%.   In addition to the above component compositions, Mo: 0.01 to 0.5%, V: 0.001 to 0.10%, B: not more than 0.0030%, Ca: not more than 0.0050%, and REM in mass% The structural high strength steel sheet excellent in brittle crack propagation arresting properties according to any one of claims 1 to 4, characterized in that it contains one or more selected from 0.0100% or less. . It is a manufacturing method of the high strength thick steel plate for structures as described in any one of Claims 1-5,
Heating a steel material having the above composition to a temperature of 1000 to 1200 ° C.,
Then, after rough rolling, during finish rolling before or finish rolling, the temperature of the steel sheet surface was heated to less than the Ar ₃ point (Ar ₃ point +170) ° C., Ar ₃ or more points (Ar ₃ point +150) ° C. below the temperature Hot rolling at a finish rolling finish temperature of Ar ₃ or more at a cumulative rolling reduction in the range of 45% to 90% and a rolling reduction per pass of 3% to 35%,
Subsequently, it is cooled to 600 ° C. or less at an average cooling rate of 2.0 ° C./s or more and 10.0 ° C./s or less at the central portion of the plate thickness. High strength thickness for structure excellent in brittle crack propagation arresting characteristics Method of manufacturing steel plate. After the said cooling, tempering process is further performed to the temperature of 250 degreeC or more and Ac ₁ point, The manufacturing method of the high strength thick steel plate for structures excellent in the brittle crack propagation stop characteristic of Claim 6 characterized by the above-mentioned.