JP2008013831A - Thick steel plate with high young's modulus for welded structure, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick steel plate for welded structures which has excellent weldability and low-temperature toughness and is increased in Young's modulus in an L direction and a C direction and also to provide its manufacturing method. <P>SOLUTION: A steel stock, containing 0.06 to 0.2% C and having ≤0.45 carbon equivalent Ceq, is subjected to: heating to a temperature range between 500°C and a temperature below the Ac<SB>3</SB>transformation point and then rolling while regulating, in a temperature range from ((Ac<SB>3</SB>transformation point)-40°C) to 500°C, a draft per pass and a cumulative draft to ≤10% on average and ≥50%, respectively; or heating to a temperature range between the Ac<SB>3</SB>transformation point and 1,180°C, primary rolling while regulating a cumulative draft in a temperature range between 900°C and ((Ar<SB>3</SB>transformation point)-20°C) to ≥30% and then secondary rolling while regulating, in a temperature range from ((Ar<SB>3</SB>transformation point)-20°C) to 500°C, a draft per pass and a cumulative draft to ≤10% on average and ≥50%, respectively. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a thick steel plate suitable for use in welded steel structures such as buildings, bridges, construction machines, industrial machines, and marine structures, and in particular, the rolling direction (L direction) and the direction perpendicular to the rolling direction (sheet width). The direction (C direction) relates to a thick steel plate having a high elastic modulus (Young's modulus). Here, the “thick steel plate” is a steel plate having a plate thickness of 6 mm or more, and is manufactured by performing reverse rolling one by one.

  With the recent increase in size of welded steel structures such as buildings, bridges, construction machines, industrial machines, and marine structures, it is strongly desired to reduce the weight of the structures by reducing the thickness of the steel sheets used. However, there is a problem that the rigidity of the structure is lowered due to the thinned steel plate used. If the shape is constant, the rigidity of the structure is proportional to the elastic modulus (Young's modulus) of the steel sheet used. For this reason, a steel sheet having a high Young's modulus is desired.

As a method of obtaining a high Young's modulus steel sheet,
(1) A method of using a texture,
(2) A method for dispersing high Young's modulus particles is known.
For example, Patent Documents 1 to 6 describe a technique for obtaining a high Young's modulus steel sheet by controlling the texture. In Patent Document 1, a steel slab whose temperature is not lower than Ar ₃ transformation point and not higher than 1250 ° C is hot-rolled at a reduction ratio of 20% or higher in a temperature range not higher than the recrystallization end temperature and not lower than Ar ₃ transformation point, and immediately 5 It describes a method for producing a high Young's modulus structural steel sheet excellent in low temperature toughness, which is cooled at a cooling rate of at least ° C / s and performs two-phase rolling at a reduction ratio of less than 50% below the Ar ₃ transformation point. . According to the technique described in Patent Document 1, a structural steel plate having excellent low-temperature toughness and improved rigidity (Young's modulus) in a specific direction (C direction) by about 10% is obtained. In Patent Document 2, a steel slab whose temperature is not lower than the Ar ₃ transformation point and not higher than 1250 ° C. is hot-rolled at a reduction rate of 20% or higher in a temperature range not higher than the recrystallization end temperature and not lower than the Ar ₃ transformation point. Immediately cool at a cooling rate of 5 ° C / s or more, perform two-phase rolling at a reduction ratio of 50% or more below the Ar ₃ transformation point, quench, and then temper, high Young excellent in low temperature toughness A method of manufacturing a rate structural steel sheet is described. According to the technique described in Patent Document 2, a structural steel plate having excellent low-temperature toughness and improved rigidity (Young's modulus) in a specific direction (C direction) by about 20% or more is obtained.

In the techniques described in Patent Documents 1 and 2, it is possible to develop crystal grains having the {211} <110> orientation as the main orientation, thereby increasing the Young's modulus in the direction perpendicular to the rolling direction (C direction). However, the techniques described in Patent Documents 1 and 2 have a problem that the Young's modulus in the rolling direction (L direction) cannot be increased.
In Patent Document 3, C: 0.02 to 0.15%, Cr: 0.60% or less, Cu: 1.5% or less, Ni: 3.0% or less, V: 0.10% or less, Ti: 0.10% or less, Ca: 0.0050% or less In addition, a steel piece containing two or more of Nb, Mo, and B so as to satisfy a predetermined specific relationship has a cumulative reduction ratio of 50% in the temperature range from 950 ° C. to Ar ₃ transformation point. above, subjected to hot rolling to a cumulative reduction rate of Ar less than ₃ transformation point and less than 5%, the method of producing a high Young's modulus steel sheet have been proposed. In the technique described in Patent Document 3, the {211} <110> orientation crystal grains are developed, thereby increasing the Young's modulus in the direction perpendicular to the rolling direction (C direction). There was a problem that the Young's modulus in the direction (L direction) could not be increased.

Further, Patent Document 4, Ac ₃ starts rolling from the above transformation point, Ar ₃ the two-phase region rolling reduction rate of 50% or more in a temperature range of less than transformation point, steel sheet average temperature 1200 ° C. to Ar ₁ transformation point The ratio between the cooling time and the total rolling time is over 0.2, the surface cooling rate is 2 ° C / s or more while cooling, the structure has excellent scale adhesion and high Young's modulus A method of manufacturing a heavy steel plate has been proposed. According to the technique described in Patent Document 4, the Young's modulus in the C direction is improved by 10% or more. However, there has been a problem that the Young's modulus in the L direction is at most about 220 GPa even when the increase in the Young's modulus in the L direction is small and the reduction rate in the two-phase temperature range is 80%.

  Patent Document 5 includes C: 0.04 to 0.15%, Mn: 0.6 to 2.0%, and after heating a steel slab containing appropriate amounts of Si, P, S, N, and Al to 950 to 1200 ° C. There has been proposed a method for producing a thick steel plate that is rolled at 800 to 900 ° C. with a cumulative reduction amount of 60% or more, and then air-cooled and lightly rolled at 450 to 600 ° C. with a cumulative reduction amount of 5 to 15%. According to the technique described in Patent Document 5, a {111} <110> orientation is developed, and a thick steel plate having a high Young's modulus uniformly in the plate surface can be manufactured. However, the Young's modulus achieved by the technique described in Patent Document 5 is at most about 220 GPa in both the L and C directions, and the effect is not sufficient.

  Patent Document 6 includes C: 0.04 to 0.15%, Mn: 0.6 to 2.0%, and after heating a steel piece containing appropriate amounts of Si, P, S, N, and Al to 950 to 750 ° C. A method of manufacturing a thick steel sheet is proposed in which rolling with a cumulative reduction amount of 10% or more is performed in a direction perpendicular to the final rolling direction, and then rolling in the final rolling direction is finished in a temperature range of 650 to 800 ° C. According to the technique described in Patent Document 6, the Young's modulus in the rolling direction and the direction perpendicular to the rolling direction can be increased to about 240 GPa. However, in the technique described in Patent Document 6, it is necessary to turn the steel material by 90 ° during rolling, the size of the steel material is limited, and the rolling load in the direction perpendicular to the rolling direction (C direction) is reduced. There was a problem that productivity was reduced due to many constraints such as large size.

In Patent Document 7, a slab containing C: 0.0005 to 0.30%, Mn: 2.7 to 5.0%, Mo: 0.15 to 1.5% is heated to a temperature of 950 ° C or higher, and 800 ° C or lower, A method for producing a high Young's modulus steel sheet, in which the friction coefficient with the steel sheet exceeds 0.2 and the total rolling reduction is 50% or more, and the hot rolling is finished at a temperature of not less than Ar ₃ transformation point and not more than 750 ° C. Proposed. The technique described in Patent Document 7 introduces a shear strain by the frictional force between the steel plate and the rolling roll during rolling, and {110} <223> or {110} <111> in the vicinity of the steel plate surface layer. This is a method of increasing the Young's modulus in the L direction by developing a sharp shear texture.

  Patent Document 8 includes C: 0.2 to 1.7%, B: 0.5 to 3.2%, and a molten steel containing appropriate amounts of Si, Mn, Ti, Mo, N, V, Nb, Ta, Hf, and Ca. A steel ingot in which hexagonal boride or cubic carbonitride is crystallized in the casting process, and then hexagonal boride, cubic carbonitride and Fe matrix phase are crystallized from the residual molten steel. A method for producing high-strength, high-rigidity steel has been proposed in which hot working with a total area reduction of 50% or more is performed at a predetermined temperature or lower, and further normalized and tempered.

Patent Document 9 includes C: 0.03 to 1.7%, V: 1 to 12%, B: 0.5 to 3.2%, and further contains appropriate amounts of Si, Mn, and N, or further Ti, and / or Alternatively, a slab containing one or more of Nb and Ta and / or Mo and / or one of Ca, Mg and Nd is heated to 1075 ° C. or higher, and 950 to 1050 ° C. In this range, a method for producing high-rigidity steel has been proposed in which hot working is performed at a strain rate of 10 / s or less and a reduction rate of 5 to 20%. According to the technique described in Patent Document 9, MB type boride having an orthorhombic crystal structure with a high Young's modulus can be dispersed in a steel matrix at a volume ratio of 5% or more, and a high Young of 232 GPa or more. And steel material having good toughness can be easily manufactured.
JP-A-4-141519 JP-A-4-293720 Japanese Patent Laid-Open No. 8-311541 JP-A-6-73504 JP-A-4-147915 Japanese Patent Laid-Open No. 4-147917 JP-A-2005-273001 JP 2004-33948 A JP 2004-353063 A

However, in the technique described in Patent Document 7, since the range in which the target texture can be obtained is limited to the vicinity of the surface layer of the plate thickness, a high Young's modulus can be achieved in a thick steel plate having a plate thickness of 6 mm or more. There was a problem that I could not. Moreover, since there is much content of Mn, Mo, B, etc., there also existed a problem that weldability fell.
In addition, according to the techniques described in Patent Documents 8 and 9, a steel material having a high Young's modulus can be obtained regardless of the direction, but it is necessary to contain high Young's modulus particles in a volume ratio of several percent or more. There have been problems such as reduced toughness and weldability. Further, since a large amount of alloy element is required, there is a problem that the material cost is increased.

  The present invention has an excellent weldability and excellent low temperature toughness in view of the above-described problems of the prior art, and has a Young's modulus in the rolling direction (L direction) of 220 GPa or more and a Young in the direction perpendicular to the rolling direction (C direction). An object of the present invention is to provide an inexpensive thick steel plate for welded structure having a modulus of 230 GPa or higher and a high Young's modulus in both the L direction and the C direction, and a method for producing the same.

In order to achieve the above-described object, the present inventors diligently studied various factors that affect the improvement of the Young's modulus in the L direction in addition to the C direction.
The inventors first focused on rolling in the (α + γ) two-phase region and the α single-phase region. When a steel material such as a slab is rolled into a (α + γ) two-phase region and / or an α single-phase region to obtain a thick steel plate, the thick steel plates are {100} <110>, {211} <110>, {111} <110>, {111} <110>, {111} <211> and the like exhibit a texture in which the processed texture is strongly developed. For such a thick steel plate, when the structure is observed in the L section (longitudinal section, plane parallel to the rolling direction and perpendicular to the rolling surface), the {100}, {211}, {111} planes are substantially parallel to the rolling surface. It was found that it was composed of a group of ferrite grains (within 5 °) (hereinafter referred to as ferrite grain colonies). The ferrite grain colony is a collection of single or a plurality of ferrite grains elongated in the rolling direction and has a high aspect ratio. Moreover, the inside of the ferrite grain which comprises a ferrite grain colony contains a subgrain and a subcrystal.

  In order to improve the Young's modulus in the L direction, it is important for the inventors to develop a {110} texture different from the center of the plate thickness on the surface layer in addition to such a processed texture. I came up with something. By developing a {110} texture different from the central portion of the plate thickness on the surface layer of the thick steel plate, the L direction / C direction difference in Young's modulus can be reduced and the Young's modulus in the L direction can be improved. . The present inventors have also found that the macroscopic Young's modulus of the thick steel plate is increased by refining the ferrite grain colonies forming the texture. When the ferrite grain colony is refined, the restraining force acting between adjacent ferrite grain colonies increases during deformation, the stress necessary to give the same elastic strain increases, and the macroscopic Young's modulus increases. Conceivable.

  And in order to develop the {110} texture in the surface layer, the present inventors have an average reduction rate per pass of 10% or less in at least (α + γ) two-phase region and α single-phase region, and It was newly found out that rolling with a cumulative reduction ratio of 50% or more is essential. By reducing the average rolling reduction per pass, it is considered that the shear strain concentrates near the steel plate surface, and a texture ({110} texture) different from the central portion of the plate thickness develops. As a result, it was found that ferrite grain colonies were also refined and toughness was improved.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) Thick steel plate for welded structure, and the thick steel plate contains C: 0.06 to 0.2% by mass, and the following (1) formula Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 …… (1)
(Where Ceq: carbon equivalent (%), C, Si, Mn, Ni, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by the formula (1) is 0.45 or less, and the sum of the X-ray diffraction integrated densities of the (200) plane and (211) plane at the center of the plate thickness is 4.5 or more, 110) X-ray diffraction integrated density is 1.5 or more, and any one of {100} plane, {211} plane, {110} plane, {111} plane is within 5 ° with respect to the rolling plane. A thick steel plate for welded structure with high Young's modulus, characterized in that the average major axis length of ferrite grain colonies arranged in a uniform thickness is 60 μm or less at the center of the plate thickness and 30 μm or less at 1 mm below the surface.

(2) In (1), the composition includes, in mass%, C: 0.06 to 0.2%, a carbon equivalent Ceq defined by the formula (1) is 0.45 or less, and Si: 1% or less. A steel plate for high Young's modulus welded structure characterized by having a composition comprising Mn: 2% or less, Al: 0.1% or less, the balance being Fe and inevitable impurities.
(3) In (2), in addition to the above composition, by mass%, Cu: 1% or less, Ni: 1.5% or less, Mo: 1% or less, Cr: 1% or less, B: 0.01% or less A thick steel plate for high Young's modulus welded structure characterized by comprising a composition containing one or more selected from

(4) In (2) or (3), in addition to the above-mentioned composition, in addition, by mass%, one or two selected from V: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less A thick steel plate for welded structures having a high Young's modulus, characterized by comprising a composition containing at least a seed.
(5) In any one of (2) to (4), in addition to the above composition, the composition further contains one or two kinds selected from Ca: 0.01% or less and REM: 0.01% or less by mass%. A thick steel plate for a welded structure with a high Young's modulus, characterized by comprising

(6) A method of manufacturing a thick steel plate for welded structure by rolling a steel material to form a thick steel plate, wherein the steel material is contained in mass% and includes C: 0.06 to 0.2%, and the following formula (1) Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
(Where Ceq: carbon equivalent (%), C, Si, Mn, Ni, Cr, Mo, V: content of each element (mass%))
A steel material having a composition with a carbon equivalent Ceq defined by ≦ 0.45 or less, heating the rolling to a temperature of 500 ° C. or more and less than the Ac ₃ transformation point, and then (Ac ₃ transformation point−40 ° C.) or less 500 ° C. A method for producing a thick steel sheet for a high Young's modulus welded structure, characterized in that rolling is performed such that the rolling reduction per pass is 10% or less on average and the cumulative rolling reduction is 50% or more in the above temperature range.

(7) In (6), instead of rolling, after heating to a temperature not lower than Ac ₃ transformation point and not higher than 1180 ° C., the cumulative reduction ratio in a temperature range not lower than 900 ° C. (Ar ₃ transformation point −20 ° C.) is obtained. The primary rolling is 30% or more, and then the (Ar ₃ transformation point-20 ° C) or less is 500 ° C or more, and the average rolling reduction per pass is 10% or less, and the cumulative reduction in the temperature range. A method for producing a thick steel plate for a welded structure with high Young's modulus, characterized in that the rolling is performed by secondary rolling with a rate of 50% or more.

(8) In (6) or (7), the composition includes mass%, C: 0.06 to 0.2%, a carbon equivalent Ceq defined by the formula (1) is 0.45 or less, and Si: A method for producing a thick steel sheet for a high Young's modulus welded structure, comprising 1% or less, Mn: 2% or less, and Al: 0.1% or less, the balance being Fe and inevitable impurities.
(9) In (8), in addition to the above composition, by mass%, Cu: 1% or less, Ni: 1.5% or less, Mo: 1% or less, Cr: 1% or less, B: 0.01% or less A method for producing a thick steel plate for welded structures having a high Young's modulus, characterized in that the composition contains one or more selected from the above.

(10) In the above (8) or (9), in addition to the above-mentioned composition, by mass%, one or two selected from V: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less The manufacturing method of the thick steel plate for high Young's modulus welded structures characterized by setting it as the composition containing seed | species or more.
(11) In any one of (8) to (10), in addition to the above-described composition, the composition further comprises one or two selected from Ca: 0.01% or less and REM: 0.01% or less by mass%. The manufacturing method of the thick steel plate for high Young's modulus welding structures characterized by the above-mentioned.

  According to the present invention, while having excellent weldability and excellent low temperature toughness, the Young's modulus in the rolling direction (L direction) is 220 GPa or more, and the Young's modulus in the direction perpendicular to the rolling direction (C direction) is 230 GPa or more. A steel plate for welded structure having a high Young's modulus in both the direction and the C direction can be easily manufactured at low cost without a reduction in productivity, and has a remarkable industrial effect.

  The thick steel plate for welded structure of the present invention has different textures in the surface layer and the central portion of the plate thickness. Thereby, the Young's modulus in the L direction and the C direction can be improved. The central portion of the thick steel plate of the present invention has a texture where the sum of the X-ray diffraction integrated density of the (200) plane and the (211) plane is 4.5 or more. By setting the total X-ray diffraction integrated density of the (200) plane and (211) plane to 4.5 or more at the center of the plate thickness, the Young's modulus in the C direction is improved, and 230 GPa or more can be secured. However, this alone only provides a high Young's modulus in the C direction and cannot increase the Young's modulus in the L direction.

  The thick steel plate of the present invention further has a texture in which the X-ray diffraction integrated density of the (110) plane is 1.5 or more at the surface layer portion 1 mm below the surface. By setting the X-ray diffraction integration density on the (110) plane to 1.5 or more at the surface layer 1mm below the surface, the texture changes in the plate thickness direction, the Young's modulus in the L direction is improved, and 220 GPa or more can be secured. The difference between the Young's modulus in the C direction and the Young's modulus in the L direction can be reduced.

  In addition to the above-described texture, the thick steel plate of the present invention has a structure in which ferrite grain colonies are refined over the entire plate thickness. The “ferrite grain colony” referred to in the present invention is a ferrite in which any one of {100} plane, {211} plane, {110} plane, and {111} plane is aligned within 5 ° with respect to the rolling plane. Say a collection of grains. The angle formed between the rolled surface and each surface of the ferrite can be easily measured using, for example, crystal orientation analysis by an EBSP (Electron Back Scattering Pattern) method. FIG. 1 shows and shows an example of the distribution state of ferrite grain colonies based on the crystal orientation analysis by EBSP in the cross section in the rolling direction of the steel plate of the present invention. FIG. 1A shows the case of 1 mm below the surface, and FIG.

  The refinement of the ferrite grain colony referred to in the present invention refers to the case where the average major axis length of the ferrite grain colony is 60 μm or less at the center of the plate thickness and 30 μm or less at 1 mm below the surface. The “average major axis length” of the ferrite grain colony is defined as a ferrite grain colony region that meets the above definition based on the crystal orientation analysis by EBSP at each position of the cross section in the rolling direction of the thick steel plate. The length in the rolling direction (major axis length) of the grain colony is measured, and an average value obtained by averaging these values is used.

By refinement of such ferrite grain colonies, the Young's modulus can be further increased even if the type of texture (azimuth component) and the degree of accumulation do not change. In order to maintain the continuity of stress and deformation between adjacent ferrite grain colonies when macroscopic elastic deformation is applied, a microscopically complicated stress field and elastic strain are introduced. This is to show a stress-strain relationship different from the average characteristic obtained by averaging the characteristics of the orientations. That is, the finer the ferrite grain colonies, the higher the degree of restraint against deformation between adjacent ferrite grain colonies, and it becomes necessary to apply a larger stress to obtain the same macroscopic elastic deformation amount. That is, Young's modulus increases. Note that the refinement of ferrite grain colonies also has the effect of improving toughness. When the average major axis length of the ferrite grain colonies exceeds 60 μm at the center portion of the plate thickness and 30 μm at 1 mm below the surface, the degree of Young's modulus improvement effect and toughness improvement effect are reduced.
The size of the ferrite grain colony depends on the initial structure at the start of rolling and the rolling conditions, but the ferrite grain colony can be improved by reducing the initial structure, increasing the cumulative rolling reduction, and optimizing the time between passes and the rolling temperature. Can be miniaturized. In addition, since {110} texture is generated so as to sever ferrite grain colonies such as {100}, {111}, {211}, it is necessary to develop {110} texture near the steel sheet surface layer. It is also effective in making grain colonies finer.

  The Young's modulus in the L direction and the C direction is not less than a predetermined value for the first time by simultaneously achieving the formation of different textures in the plate thickness direction and further refinement of the ferrite grain colonies at each position of the plate thickness. Can be increased. If any one of texture formation and ferrite grain colony refinement at each position of the plate thickness is outside the scope of the present invention, the Young's modulus in the L direction and the C direction cannot be increased to a predetermined value or more. This situation is shown in FIGS. Formation of the desired texture, represented by the sum of the density of the (110) plane at 1 mm below the surface and the density of the (200) and (211) planes at the center of the plate thickness, and at 1 mm below the surface At least one of the conditions of the refinement of the ferrite grain colony in which the average major axis length of the ferrite grain colony and the average major axis length of the ferrite grain colony at the central portion of the plate thickness are equal to or less than a predetermined value is the present invention. At symbols Δ and ▲ outside the range, the Young's modulus in the L direction and / or the C direction is less than a predetermined value.

Next, the reason for limiting the composition of the thick steel plate of the present invention will be described.
In particular, the influence of alloying elements is small on the development of a desired texture in (α + γ) two-phase rolling and α single-phase rolling. For this reason, it is not necessary to contain a special alloy element in order to develop a desired texture. However, because of the refinement of ferrite grain colonies, the content of C is limited, and further, excellent welding for welded structures. In order to ensure the property and the low temperature toughness, at least the limitation of the C content and the limitation of the carbon equivalent are required. Hereinafter, the mass% in the composition is simply expressed as%.

C: 0.06-0.2%
C is an element that increases the strength of the steel by solid solution, and needs to be contained in order to secure a desired strength for the welded structure. By rolling in the (α + γ) two-phase region and / or the α single-phase region, the recrystallization of ferrite grains proceeds and the ferrite grains become finer. However, if C is less than 0.06%, the recrystallization progresses slowly and aggregates. Since the refinement of the ferrite grain colonies forming the structure does not proceed, the ferrite grain colonies become coarse. For this reason, a desired high Young's modulus cannot be achieved. On the other hand, if the content exceeds 0.2%, the weldability decreases rapidly. For these reasons, C is limited to a range of 0.06 to 0.2%. In addition, Preferably it is 0.08 to 0.18%.

Carbon equivalent Ceq: 0.45 or less Carbon equivalent Ceq is an index for evaluating weldability, and the following equation (1): Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
(Where Ceq: carbon equivalent (%), C, Si, Mn, Ni, Cr, Mo, V: content of each element (mass%))
Defined by In addition to the above-described C content, the present invention can further contain a desired amount of alloy element in order to ensure desired characteristics for a welded structure, but the carbon equivalent Ceq increases beyond 0.45. If the alloy element is contained in a large amount, the weldability is remarkably lowered, and a desired weldability for a welded structure cannot be ensured. For this reason, in the present invention, the carbon equivalent Ceq is limited to 0.45 or less within the range of the C content described above. In addition, Preferably it is 0.4% or less.

The thick steel plate of the present invention preferably contains Si: 1% or less, Mn: 2% or less, Al: 0.1% or less in addition to the above-described C and Ceq, and the balance Fe and inevitable impurities.
Si: 1% or less
Si is an element that acts as a deoxidizer and has the effect of increasing the strength of steel by solid solution strengthening. In order to obtain such an effect, the content is preferably 0.1% or more. However, if the content exceeds 1%, the surface properties are impaired and the toughness is extremely lowered. For this reason, it is preferable to limit Si to 1% or less.

Mn: 2% or less
Mn acts as a strengthening element in steel. In order to acquire such an effect, it is desirable to contain 0.2% or more. However, if it contains more than 2%, the weldability is lowered and the material cost is increased. For this reason, it is preferable to limit Mn to 2% or less.
Al: 0.1% or less
Al is an element that acts as a deoxidizer, but in order to obtain such an effect, it is desirable to contain 0.01% or more. On the other hand, the content exceeding 0.1% increases the amount of inclusions and also reduces toughness. For this reason, it is preferable to limit Al to 0.1% or less.

  In addition to the above components, Cu: 1% or less, Ni: 1.5% or less, Mo: 1% or less, Cr: 1% or less, B: 0.01% or less, and / or Alternatively, V: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less, and / or Ca: 0.01% or less, REM: 0.01% or less 1 type or 2 types selected can be contained.

Cu: 1% or less, Ni: 1.5% or less, Mo: 1% or less, Cr: 1% or less, B: 0.01% or less
Cu, Ni, Mo, Cr, and B are all elements that increase the hardenability of steel, contribute directly to improving strength, and also improve toughness. Can be contained. Such effects become significant when Cu: 0.01% or more, Ni: 0.01% or more, Mo: 0.01% or more, Cr: 0.01% or more, B: 0.0005% or more, Cu: 1%, Ni : 1.5%, Mo: 1%, Cr: 1%, B: Excessive content exceeding 0.01% decreases toughness and weldability. For this reason, it is preferable to limit to Cu: 1% or less, Ni: 1.5% or less, Mo: 1% or less, Cr: 1% or less, and B: 0.01% or less.

One or more selected from V: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less V, Nb, and Ti all form nitrides, carbides, or carbonitrides. The element has the effect of refining the crystal grains and strengthening the steel, and can be selected as necessary to contain one or more kinds. In order to acquire such an effect, it is desirable to contain 0.003% or more of V, Nb, and Ti, respectively. On the other hand, if it contains more than V: 0.1%, Nb: 0.05%, Ti: 0.05%, the slab is cracked and the manufacturing cost rises. For this reason, it is preferable to limit to the ranges of V: 0.1% or less, Nb: 0.05% or less, and Ti: 0.05% or less.

One or two selected from Ca: 0.01% or less, REM: 0.01% or less
Both Ca and REM are elements that contribute to the improvement of ductility and toughness through the shape control of inclusions, and can be selected as necessary to contain one or two kinds. In order to obtain such an effect, it is preferable to contain Ca: 0.0005% or more and REM: 0.001% or more, but a large content exceeding Ca: 0.01% and REM: 0.01% reduces toughness. For this reason, it is preferable to limit to Ca: 0.01% or less and REM: 0.01% or less.

The balance other than the components described above consists of Fe and inevitable impurities. As unavoidable impurities, P: 0.1% or less and S: 0.006% or less are acceptable. If the content exceeds P: 0.1% and S: 0.006%, the toughness decreases.
Below, the preferable manufacturing method of this invention steel plate is demonstrated.
It is preferable that the molten steel having the above composition is melted by a conventional melting method such as a converter and used as a steel material such as a slab by a conventional casting method such as a continuous casting method.

The obtained steel material is then rolled into a thick steel plate.
In the present invention, rolling, a steel material, after heating to a temperature below 500 ° C. or higher Ac ₃ transformation point, in (Ac ₃ transformation point -40 ° C.) or less 500 ° C. or higher temperature range, the rolling reduction per one pass Is preferably a rolling with an average rolling reduction of 10% or less and a cumulative rolling reduction of 50% or more.
In the present invention, the steel material is rolled in the (α + γ) two-phase region and / or the α single-phase region, and {100} <110>, {211} <110>, {111} <110>, {111} A thick steel plate having a structure in which a processed texture such as <110> and {111} <211> is strongly developed. Therefore, the steel material is heated to a temperature of 500 ° C. or higher and lower than the Ac ₃ transformation point. When the heating temperature is less than 500 ° C., the deformation resistance of the steel material becomes too large, the load on the rolling equipment becomes too large, and rolling may become difficult. For this reason, the heating temperature of the steel material is preferably limited to a temperature of 500 ° C. or higher and lower than the Ac ₃ transformation point.

The steel material heated to the above-mentioned heating temperature is then (Ac ₃ transformation point −40 ° C.) or lower and in the temperature range of 500 ° C. or higher, the average rolling reduction per pass is 10% or less, and the cumulative rolling reduction is 50%. The above rolling is performed. As a result, a desired texture can be developed in the ferrite phase.
When the rolling temperature at which the desired reduction is performed exceeds (Ac ₃ transformation point −40 ° C.) and becomes high, since the ferrite phase fraction is low, the desired texture is difficult to develop and the Young's modulus is sufficiently improved. The effect cannot be expected. On the other hand, when the rolling temperature is less than 500 ° C., the development of the {110} texture in the surface layer portion is suppressed, so that a sufficient Young's modulus improvement effect cannot be expected. Further, when the rolling temperature is less than 500 ° C., the {100} texture at the center of the plate thickness develops, but the toughness decreases because the refining of ferrite grains does not proceed. For this reason, the rolling temperature at which the desired reduction is performed is preferably (Ac ₃ transformation point −40 ° C.) or less and a temperature range of 500 ° C. or more.

Further, in the present invention, rolling is performed at the rolling temperature described above with an average rolling reduction per pass of 10% or less and a cumulative rolling reduction of 50% or more.
If the rolling reduction per pass in the above rolling temperature range exceeds 10% on average, the {110} texture is hardly developed. The rolling reduction per pass affects the thickness direction distribution of plastic strain introduced into the steel material by rolling. As the rolling reduction per pass increases, the strain distribution in the thickness direction becomes more uniform, and the texture change in the thickness direction becomes smaller. When the rolling reduction per pass is reduced, plastic strain concentrates near the steel sheet surface layer, and a {110} texture near the surface layer develops. Therefore, in the present invention, the rolling reduction per pass in the rolling temperature range described above is limited to 10% or less on average. In addition, Preferably it is 3 to 8%.

  In addition, if the cumulative rolling reduction is less than 50% in the rolling temperature range described above, a sufficiently strong texture does not develop at the central portion of the plate thickness, and the ferrite grain colonies are not sufficiently refined. The effect of improving Young's modulus cannot be expected, and excellent low temperature toughness cannot be secured. For this reason, the cumulative rolling reduction in the rolling temperature range described above is limited to 50% or more. In addition, Preferably it is 75% or more.

In addition, you may perform quenching and accelerated cooling directly after completion | finish of rolling.
In the present invention, instead of the rolling described above, after heating to a temperature not lower than Ac ₃ transformation point and not higher than 1180 ° C., the cumulative rolling reduction in a temperature range not lower than 900 ° C. (Ar ₃ transformation point −20 ° C.) is 30 It is good also as the rolling which performs primary rolling which is% or more, and then performs secondary rolling.
In the present invention, the steel material may be heated to a temperature not lower than the Ac ₃ transformation point and not higher than 1180 ° C. before rolling in the (α + γ) two-phase region and / or the α single-phase region. However, in this case, it is necessary to make the austenite grains formed by heating as fine as possible before the secondary rolling that rolls in the (α + γ) two-phase region and / or the α single-phase region. For this reason, after the heating, primary rolling is performed in which the cumulative rolling reduction in a temperature range of 900 ° C. or lower (Ar ₃ transformation point −20 ° C.) or higher is 30% or higher. Thereby, austenite grains are refined, and fine ferrite grains are obtained by the subsequent γ → α transformation. When the heating temperature is higher than 1180 ° C., the austenite grains become too coarse and fine ferrite grains cannot be obtained even by subsequent rolling.

When the rolling temperature of primary rolling exceeds 900 ° C., the austenite grains processed by rolling are recrystallized and coarsened, so that the effect of refining the structure is reduced. In addition, when the cumulative rolling reduction in the rolling temperature range described above for primary rolling is less than 30%, the effect of refining austenite grains is small, and fine ferrite grains cannot be obtained by subsequent γ → α transformation. In order to obtain further fine ferrite grains by the γ → α transformation, water cooling treatment may be performed in a temperature range near the Ar ₃ transformation point during the primary rolling.

Following the above-described primary rolling, in the present invention, the average reduction rate per pass is 10% or less in the temperature range of (Ar ₃ transformation point −20 ° C.) or less and 500 ° C. or more, and the accumulation in that temperature range is performed. Secondary rolling with a rolling reduction of 50% or more is performed. As a result, the desired texture described above is sufficiently developed, and the finally formed ferrite grain colony is refined.
When the rolling temperature of the secondary rolling exceeds (Ar ₃ transformation point−20 ° C.) and becomes a high temperature, the ferrite phase fraction is small and the desired texture described above cannot be sufficiently developed in the ferrite grains. Further, when the rolling temperature of the secondary rolling is less than 500 ° C., the {100} texture at the center of the plate thickness develops, but the toughness decreases because the refining of ferrite grains does not proceed. For this reason, the rolling temperature of the secondary rolling is preferably (Ac ₃ transformation point −20 ° C.) or lower and 500 ° C. or higher.

Further, when the rolling reduction per pass in the secondary rolling temperature range exceeds 10% on average, the {110} texture is hardly developed. For this reason, the rolling reduction per pass in the rolling temperature range of the secondary rolling described above is limited to 10% or less on average. In addition, Preferably it is 3 to 8%.
In addition, if the cumulative rolling reduction is less than 50% in the rolling temperature range described above, a sufficiently strong texture does not develop at the central portion of the plate thickness, and the ferrite grain colonies are not sufficiently refined. The effect of improving Young's modulus cannot be expected, and excellent low temperature toughness cannot be secured. For this reason, the cumulative rolling reduction in the rolling temperature range described above is limited to 50% or more. In addition, Preferably it is 75% or more.

  In addition, you may perform direct hardening and accelerated cooling after completion | finish of secondary rolling.

Molten steel having the composition shown in Table 1 was melted in a vacuum melting furnace to form a small steel ingot, and then slab (steel material) having a thickness of 120 mm was obtained by rough rolling. The obtained steel material was rolled under the conditions shown in Table 2 to obtain a thick steel plate (plate thickness: 12 mm). All thick steel plates were air-cooled to room temperature after the end of rolling.
The obtained thick steel plate was subjected to a structure observation, a tensile test, a Charpy impact test, and a Young's modulus measurement to evaluate the structure, tensile properties, low temperature toughness, and Young's modulus. Measurement and test methods were as follows.

(1) Structure observation From the obtained thick steel plate, parallel to the rolling surface, a plate-shaped test piece (size: 25 × 25mm), the measurement surface is 1mm below the surface, or the center of the plate thickness (however, the center segregation portion) The sample was collected so that the sample was separated from the sample. Then, the measurement surface of the plate-like test piece was finished to a mirror surface by buffing, and then residual strain on the measurement surface was removed by electrolytic polishing or chemical polishing to obtain a measurement test piece. Using the obtained test specimen for measurement, X-ray diffraction measurement was performed to measure a positive pole figure or a reverse pole figure, and an X-ray diffraction intensity ratio between the (200) plane and the (211) plane was obtained. The X-ray diffraction intensity ratio between the (200) plane and the (211) plane was defined as the integrated density of the (200) plane and (211) plane, which was used as an index of the formation state of the processed texture. The X-ray diffraction intensity ratio is the difference between the X-ray diffraction intensity of each surface obtained using the measurement specimen and the X-ray diffraction intensity of each face obtained using a test piece having a random crystal orientation. Is the ratio.

  Also, using the same test specimen for measurement, crystal orientation analysis was performed by the EBSP method, and any one of {100} plane, {211} plane, {110} plane, and {111} plane was rolled. Ferrite grain colonies, which are regions aligned within 5 ° with respect to the surface, were determined. The length of each ferrite grain colony in the rolling direction (major axis length) was measured by printing an image obtained by enlarging the EBSP analysis result 1000 times on paper. And the length (major axis length) of the rolling direction of the obtained ferrite grain colony was averaged, and the value was made into the average major axis length of the ferrite grain colony in each position of each thick steel plate. The number of ferrite grain colonies measured at each position of each thick steel plate was 20 each.

(2) Tensile test No. 14 tensile test from the thick steel plate obtained in accordance with JIS Z 2201 so that the tensile direction is in the direction (C direction) perpendicular to the rolling direction from the center of the plate thickness. A piece (parallel portion: 6 mmφ) was collected and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to determine tensile properties (yield strength YS, tensile strength TS).

(3) Charpy impact test V-notch test specimens were collected from the obtained thick steel plate in the direction perpendicular to the rolling direction (C direction) from the center of the plate thickness in accordance with JIS Z 2242. An impact test was performed to determine the fracture surface transition temperature vTrs (° C.).
(4) Young's modulus measurement From the 1/4 thickness position of the obtained steel plate, the longitudinal direction of the specimen is parallel to the rolling direction (L direction), and the longitudinal direction of the specimen is perpendicular to the rolling direction (C Direction), a Young's modulus test specimen (thickness 1.0 mm × width 6.4 mm × length 105 mm) was collected, and the Young's modulus was measured using a resonance method at room temperature in accordance with the provisions of JIS Z 2208. In the resonance method, the frequency of the vibration applied to the test piece is gradually changed, the primary resonance frequency is measured, and the following equation E = 0.946 × L ⁴ / h ² × ρ × f ²
(Where E: Young's modulus (GPa), L: vibration part length (m) of the test piece, h: test piece thickness (m), ρ: density (g / mm ³ ), f: primary resonance frequency ( s- ¹ ))
The Young's modulus E is calculated from

 The obtained results are shown in Tables 2 and 3.

  In all of the examples of the present invention, the total of X-ray diffraction integration density of (200) plane and (211) plane at the center of the plate thickness is 4.5 or more, and X-ray diffraction integration density of (110) plane at 1 mm below the surface is 1.5 or more And a structure having a refined ferrite grain colony satisfying an average major axis length of the ferrite grain colony of 60 μm or less at the center of the plate thickness and 30 μm or less at 1 mm below the surface. This steel plate has a high Young's modulus of 220 GPa or more in the direction and 230 GPa or more in the C direction and excellent low temperature toughness of vTrs: −60 ° C. or less. On the other hand, the comparative example out of the scope of the present invention does not provide the desired high Young's modulus because the desired texture is not obtained and / or the ferrite grain colonies are not refined. .

It is explanatory drawing which illustrated an example of the distribution condition of a ferrite grain colony based on the crystal orientation analysis result by EBSP method. It is a graph which shows the relationship between the total direction of the (200) plane and (211) plane integration density in the plate | board thickness center part, and the (110) plane integration density in 1 mm below the surface which affects the Young's modulus of L and C direction. . It is a graph which shows the relationship between the average major axis length of the ferrite grain colony in the plate | board thickness center part, and the average major axis length of the ferrite grain colony 1 mm below the surface which has on the Young's modulus of L and C direction.

Claims (11)

A thick steel plate for welded structure, the thick steel plate having a composition in which C: 0.06 to 0.2% in mass% and a carbon equivalent Ceq defined by the following formula (1) is 0.45 or less, In addition, the total X-ray diffraction density of the (200) plane and (211) plane at the center of the plate thickness is 4.5 or more, the X-ray diffraction density of the (110) plane at 1 mm below the surface is 1.5 or more, and {100 } Plane, {211} plane, {110} plane, {111} plane, the average major axis length of the ferrite grain colony aligned within 5 ° with respect to the rolling plane is the thickness center A steel plate for a high Young's modulus welded structure characterized by having a structure of 60 μm or less at the part and 30 μm or less at 1 mm below the surface.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Where Ceq: carbon equivalent (%),
C, Si, Mn, Ni, Cr, Mo, V: Content of each element (% by mass)   The composition contains, by mass%, C: 0.06 to 0.2%, the carbon equivalent Ceq defined by the formula (1) is 0.45 or less, Si: 1% or less, Mn: 2% or less, Al: The thick steel sheet for high Young's modulus welded structure according to claim 1, wherein the steel sheet contains 0.1% or less and the balance is Fe and inevitable impurities.   In addition to the above composition, one or two selected by mass% from Cu: 1% or less, Ni: 1.5% or less, Mo: 1% or less, Cr: 1% or less, B: 0.01% or less The thick steel plate for high Young's modulus welded structure according to claim 2, wherein the steel plate has a composition containing at least a seed.   In addition to the above composition, the composition further contains, by mass%, one or more selected from V: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less. The thick steel plate for high Young's modulus welded structure according to claim 2 or 3.   5. The composition according to claim 2, wherein the composition further comprises one or two selected from Ca: 0.01% or less and REM: 0.01% or less by mass% in addition to the composition. The thick steel plate for high Young's modulus welded structure according to any one of the above. A method for producing a steel plate for welded structure which is rolled into a steel material to form a thick steel plate, wherein the steel material is contained in mass%, C: 0.06 to 0.2%, and a carbon equivalent defined by the following formula (1) Ceq is a steel material having a composition less than or equal to 0.45, the rolling, after heating to a temperature below 500 ° C. or higher Ac ₃ transformation point, in a temperature range of (Ac ₃ transformation point -40 ° C.) or less 500 ° C. or higher, 6. The method for producing a thick steel sheet for a high Young's modulus welded structure according to claim 5, wherein rolling is performed such that the rolling reduction per pass is 10% or less on average and the cumulative rolling reduction is 50% or more.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Where Ceq: carbon equivalent (%),
C, Si, Mn, Ni, Cr, Mo, V: Content of each element (% by mass) In place of the rolling, after heating to a temperature not lower than the Ac ₃ transformation point and not higher than 1180 ° C., primary rolling in which the cumulative rolling reduction in a temperature range not lower than 900 ° C. (Ar ₃ transformation point −20 ° C.) is 30% or higher is performed. Next, the average reduction rate per pass is 10% or less in the temperature range of (Ar ₃ transformation point -20 ° C) or less and 500 ° C or more, and the cumulative reduction rate in the temperature range is 50% or more. 7. The method for producing a thick steel plate for high Young's modulus welded structure according to claim 6, wherein the rolling is performed by next rolling.   The composition contains, by mass%, C: 0.06 to 0.2%, the carbon equivalent Ceq defined by the formula (1) is 0.45 or less, Si: 1% or less, Mn: 2% or less, Al: The method for producing a thick steel sheet for high Young's modulus welded structure according to claim 6 or 7, characterized in that the composition contains 0.1% or less, the balance being Fe and inevitable impurities.   In addition to the above composition, one or two selected by mass% from Cu: 1% or less, Ni: 1.5% or less, Mo: 1% or less, Cr: 1% or less, B: 0.01% or less The method for producing a thick steel sheet for high Young's modulus welded structure according to claim 8, wherein the composition contains a seed or more.   In addition to the above composition, the composition further comprises, in mass%, one or more selected from V: 0.1% or less, Nb: 0.05% or less, and Ti: 0.05% or less. The manufacturing method of the thick steel plate for high Young's modulus welded structures of Claim 8 or 9.   The composition according to any one of claims 8 to 10, wherein the composition further comprises one or two kinds selected from Ca: 0.01% or less and REM: 0.01% or less in mass% in addition to the composition. The manufacturing method of the thick steel plate for high Young's modulus welded structures in any one.

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