JP2016169403A - Low yield ratio high strength thick steel plate for building structure excellent in toughness at super high heat-input heat affected zone and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low yield ratio high strength thick steel plate for a building structure, excellent in toughness at a super high heat-input heat affected zone.SOLUTION: The low yield ratio high strength thick steel plate for a building structure, excellent in toughness at a super high heat-input heat affected zone is provided that contains, by mass%, C:0.05 to 0.15%, Si:0.01 to 0.5%, Mn:1.0 to 1.6%, P≤0.02%, S≤0.003%, 0≤0.003%, Al:0.001 to 0.060%, Ti:0.005 to 0.030%, Cr:0.10 to 0.50%, B:0.0003 to 0.0020%, N:0.0025 to 0.0055% and further contains C, Cr and B so as to satisfy 0.28≤3/2×C+1/2×Cr+100×B≤0.38 and contains Cu, Ni and V each adjusted to be 0.05% or less. The low yield ratio high strength thick steel plate has structure at a position of 1/4 of the steel plate thickness, containing a tempered bainite phase of 40% or more by a volume ratio or further a ferrite phase of 90% or more in total of both phases and the balance perlite of 0 to 10% by a volume ratio.SELECTED DRAWING: None

Description

  The present invention relates to a high-strength thick steel plate suitable for use in welded steel structures, in particular, for building structures, and in particular, high heat input welding applied to construction box columns and the like, for example, submerged arc welding. Alternatively, even if electroslag welding is performed with super-high heat input welding exceeding 400 kJ / cm, heat-affected zone toughness does not deteriorate, super-high heat welding heat-affected zone toughness is excellent, high strength, and low yield ratio The present invention relates to a low yield ratio high strength thick steel plate and a method for producing the same. “High strength” as used herein refers to the case where the yield strength is YS: 385 MPa or more and the tensile strength TS is 550 MPa or more. The “low yield ratio” is the yield ratio YR: 80% or less. It shall mean a certain case. In addition, the “thick steel plate” here refers to a steel plate having a thickness of 60 mm or more, preferably 100 mm or less.

  In recent years, with the increase in size of welded steel structures, high-strength thick steel materials have been demanded as steel materials to be used. In recent years, high strength steel sheets having a tensile strength of TS: 550 MPa or more and a thickness of 60 mm or more have been demanded for building structures. Furthermore, since the Great Hanshin-Awaji Earthquake, there has been a strong demand for improving the earthquake resistance of building structures, and steel materials are required to have a yield ratio of YR: 80% or less in order to ensure the plastic deformability of the steel materials themselves. And since a steel structure is assembled by welding joining, it is calculated | required that steel materials hold | maintain the outstanding toughness including a welding part. Furthermore, recently, from the viewpoint of economy, there has been a demand for reducing the construction cost of welded steel structures.

  In response to such demands, in order to increase welding efficiency and increase construction efficiency, expansion of the application range of large heat input welding is directed. For example, among box columns used in high-rise buildings, super-high heat input welding, such as submerged arc welding and electroslag welding, that exceeds 400 kJ / cm is applied to parts such as corner joints and diaphragm joints. Applicable.

  Generally, in a welded portion subjected to such high heat input welding or super-high heat input welding, deterioration of toughness of a weld heat affected zone (hereinafter also referred to as HAZ) becomes a problem. This is because in the region heated to near the melting point by high heat input welding or super high heat input welding, the cooling is slow, the residence time in the high temperature region becomes long, and the austenite grains tend to coarsen, and further cooling after that. This is because a hard embrittlement phase such as MA (also called island-like martensite) is likely to occur. Such HAZ toughness deterioration becomes more noticeable as the strength of the steel increases, and is particularly noticeable in a high-strength steel having a tensile strength TS of 550 MPa or higher. For this reason, the improvement of the HAZ toughness in high strength steel materials has been desired.

  In response to such demands, conventionally, fine inclusions and precipitates are dispersed in steel to prevent the austenite grains from coarsening, and these inclusions and precipitates are used as nucleation sites for intragranular ferrite. Many techniques have been proposed that function to refine the microstructure of the prior austenite grains.

Recently, for example, in Patent Document 1, Si and / or Mn is added to a molten steel for deoxidation, and the amount of dissolved oxygen is adjusted to 0.0030 to 0.0120% by mass, and then REM is added to reduce the amount of dissolved oxygen to 0.0010. In addition to adjusting to ~ 0.0050 mass%, in mass%, C: 0.03-0.18%, Si: 0.05-0.40%, Mn: 0.5-2.0%, REM: 0.0030-0.0200%, N: 0.0040-0.0070%, Furthermore, it contains one or more of Nb, Cu, Ni, and is made of molten steel with Al and Ti adjusted to 0.004% or less respectively, cast into a steel material, and then heated to 1000 ° C or higher to finish rolling. Excellent super-high heat input welding heat that is hot-rolled at an Ar ₃ transformation point or higher, accelerated at a cooling rate of 600-250 ° C at a cooling rate of 1 ° C / s or higher, and air-cooled. A method for producing a thick steel plate for building structures having an affected zone toughness is described. As a result, CeN with REM oxysulfide and REM sulfide as nuclei and Ce combined with N precipitates during welding cooling, and this CeN acts as ferrite nuclei, producing many fine ferrites. It is said that heat input HAZ toughness is improved.

Patent Document 2 includes mass%, C: 0.04 to 0.20%, Si: 0.05 to 0.35%, Mn: 0.50 to 2.50%, Al: 0.003 to 0.03%, Ti: 0.001 to 0.02%, and B: 0.0002. -0.003%, Ca: 0.0003-0.0025% included, N: 0.006% or less, C + 100B value is 0.1-0.32%, and steel contains 0.005-2.0μm Ca, Al oxide the 100 to 3000 pieces / mm ² dispersed so steel plates particles, after the end of hot rolling, water cooling quenching starting from Ar ₃ transformation point or higher, water cooling to 0.99 ° C. or less, in thickness direction, until under the plate surface 5mm A method for producing a low-YR high-tensile steel sheet is described in which the bainite fraction has a structure of 50% or more in the region excluding. As a result, even if the plate thickness is 70 mm or more, it is said that it can be made into a steel plate having excellent large heat input joint toughness with as-quenched, proof stress 440 MPa or more, tensile strength 590 MPa or more, yield ratio 80% or less. .

  Further, in Patent Document 3, Al is added so that the Al content in the molten steel is in the range of 0.005 to 0.08 mass%, deoxidation is performed, and further, the degassing apparatus is used for 15 minutes or more, and then the molten steel temperature is set. Ca is added and cast in a state maintained at 1600 ± 70 ° C, and in terms of mass%, C: 0.01 to 0.2%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.0%, Al: more than 0.005%, 0.08% Hereinafter, a slab containing Ti: 0.0005 to 0.02%, Ca: 0.0003 to 0.02%, N: 0.002 to 0.009%, O: 0.001 to 0.0035%, and after hot rolling the slab, from a temperature of 750 ° C. or less A method for producing a steel material that starts water cooling and has excellent high heat input weld heat affected zone toughness is described. As a result, the spheroidized Ca oxysulfide is dispersed in the steel, and after hot rolling, water cooling from a temperature of 750 ° C. or lower results in a ferrite area ratio of 15% or higher, a low yield ratio, and a large heat input. It is said that steel materials with excellent weld heat affected zone toughness can be obtained.

  In Patent Document 4, the mass% is C: 0.05 to 0.12%, Si: 0.3% or less, Mn: 1 to 2%, B: 0.0003 to 0.003%, V: 0.03 to 0.15%, Al: 0.001 to Continuous cast slab containing 0.1%, Ti: 0.005-0.02%, carbon equivalent Ceq of 0.32-0.45%, effective boron content of 0% or less, and effective titanium content of 0.005% or more, more than 1100 ° C and less than 1300 ° C The steel surface temperature is 850 ° C or higher and the rolling reduction is 50% or higher, and then the steel surface temperature is 800 ° C or higher and accelerated cooling is applied to cool to 500 ° C or lower. A method for producing a thick high-strength steel sheet excellent in heat-affected zone toughness is described. In the technique described in Patent Document 4, the presence state of B and V in γ is optimized by compounding V and B and adjusting the amount of N, and the transformation structure of the base material and the high heat input welding HAZ. , Charpy impact test absorption energy at -20 ° C even in HAZ with a thickness of 50-100mm, yield strength 400-650MPa, tensile strength 490-720MPa, thick high strength and welding heat input 20kJ / mm or more It is said that a steel sheet having good high heat input welded HAZ toughness with a J of 70 J or more can be obtained.

JP 2003-277828 A JP 2005-336541 A JP 2010-180424 A Japanese Patent No. 4681690

  However, in the techniques described in Patent Documents 1 to 4, in a thick steel plate having a thickness of 60 mm or more, in order to ensure a tensile strength TS of 550 MPa or more with a base material and a welded joint, addition of Cu and Ni, , N amount needs to be increased. Addition of Cu and Ni and an increase in the amount of N deteriorate the surface properties of the continuous cast slab, increase the care load on the slab surface and decrease the yield, leading to an increase in manufacturing costs. There was a problem.

  The present invention solves the above-mentioned problems of the prior art and is suitable for use in building structures without reducing or containing alloy elements that reduce surface properties or alloy elements that increase manufacturing costs. YS: 385MPa or more, tensile strength TS: 550MPa or more, yield ratio YR: 80% or less, and a weld formed by super high heat input welding with heat input exceeding 400kJ / cm (hereinafter, super high heat input welding) It is an object of the present invention to provide a high-strength thick steel plate having excellent welding heat affected zone (HAZ) toughness (hereinafter also referred to as super high heat input welding HAZ toughness) and an inexpensive manufacturing method thereof. The “excellent super high heat input welding HAZ toughness” as used herein is HAZ of super high heat input welding (heat input: 1000 kJ / cm), and the average absorbed energy at a test temperature of 0 ° C. in the Charpy impact test. The case of 70J or more shall be said.

  In order to achieve the above-described object, the present inventors diligently studied various factors affecting the productivity, strength, and HAZ toughness of the steel sheet. As a result, Cu, Ni, and V as impurities are adjusted to a predetermined value or less, N content is adjusted to a low range, Cr and B are combined, and C, Cr, and B satisfy a specific relationship. It was newly found out that a steel sheet having excellent super high heat input welding HAZ toughness can be stably and inexpensively produced by adjusting and containing.

According to further studies by the present inventors, it is contained in the ranges of C: 0.05 to 0.15%, Cr: 0.10 to 0.50%, B: 0.0003 to 0.0020% by mass%, and C, Cr and B are as follows: (1) Formula
0.28 ≦ 3/2 × C + 1/2 × Cr + 100 × B ≦ 0.38 (1)
(Here, C, Cr, B: Content of each element (mass%))
By adjusting and containing so as to satisfy the requirements, especially in super high heat input welding HAZ, the generation of coarse grain boundary ferrite, upper bainite, and ferrite side plate is suppressed, and the generation of intragranular acicular ferrite is promoted. The present inventors have found that a welded HAZ having high strength and excellent toughness can be obtained.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 1.6%, P: 0.02% or less, S: 0.003% or less, Al: 0.001 to 0.060%, Ti: 0.005 ~ 0.030%, Cr: 0.10 ~ 0.50%, B: 0.0003 ~ 0.0020%, N: 0.0025 ~ 0.0055%, O: 0.003% or less, and C, Cr, B
0.28 ≦ 3/2 × C + 1/2 × Cr + 100 × B ≦ 0.38 (1)
(Here, C, Cr, B: Content of each element (mass%))
And Cu, Ni, and V as impurities, respectively, adjusted to 0.05% or less, the composition consisting of the balance Fe and inevitable impurities, and at the position of the thickness 1/4, A tempered bainite phase with a volume fraction of 40% or more, or a ferrite phase with a total of 90% or more of the ferrite phase and the balance of pearlite with a volume fraction of 10% or less (including 0%). Strength: 385MPa or higher, tensile strength: high strength of 550MPa or higher, yield ratio: low yield ratio of 80% or less, and low yield ratio for building structures characterized by excellent heat-affected zone toughness of super high heat input welding High strength thick steel plate.
(2) In (1), in addition to the above composition, the composition further contains, by mass%, one or two selected from Mo: 0.01 to 0.30% and Nb: 0.003 to 0.020%. Low yield ratio high strength thick steel plate for building structures.
(3) In (1) or (2), in addition to the above composition, 1% selected from Ca: 0.0005 to 0.0020%, REM: 0.0010 to 0.0030%, and Mg: 0.0010 to 0.0020% in mass% A low-yield-ratio high-strength thick steel plate for building structures characterized by comprising a seed or a composition containing two or more.
(4) A method for producing a thick steel plate for a building structure, which is obtained by subjecting a steel material to hot rolling and subsequent direct quenching treatment and tempering treatment to obtain a thick steel plate, wherein the steel material comprises mass%. C: 0.05 to 0.15%, Si: 0.01 to 0.5%, Mn: 1.0 to 1.6%, P: 0.02% or less, S: 0.003% or less, Al: 0.001 to 0.060%, Ti: 0.005 to 0.030%, Cr : 0.10 to 0.50%, B: 0.0003 to 0.0020%, N: 0.0025 to 0.0055%, O: 0.003% or less, further C, Cr, B is the following (1) formula
0.28 ≦ 3/2 × C + 1/2 × Cr + 100 × B ≦ 0.38 (1)
(Here, C, Cr, B: Content of each element (mass%))
Is a steel material having a composition comprising the balance Fe and unavoidable impurities, wherein Cu, Ni, and V are each adjusted to 0.05% or less and are adjusted so as to satisfy In rolling, the steel material is heated to an average temperature in the range of 1050 to 1200 ° C., the cumulative rolling reduction in the temperature range of 950 ° C. or less at the surface temperature is 30% or more, and the rolling end temperature is 900 ° C. in the surface temperature. ° C. or less and rolling to Ar ₃ transformation point or higher, the direct quenching process, start cooling from Ar ₃ transformation point or more of the temperature at a surface temperature, 2 ° C. / s or higher sheet thickness 1/4 position 40 ° C. / s the following average cooling rate, a process of cooling to the cooling stop temperature of the temperature range of 200 ° C. or less at a temperature of ¼ of the sheet thickness position, the tempering treatment, the surface temperature of 400 ° C. or higher Ac less than ₁ transformation point Manufacturing of low-yield ratio high-strength steel sheets for building structures, characterized by heating to a temperature range Method.
(5) In (4), in addition to the above composition, the composition further contains one or two selected from Mo: 0.01 to 0.30% and Nb: 0.003 to 0.020% by mass%. A method for producing a low yield ratio high strength thick steel sheet for building structures.
(6) In (4) or (5), in addition to the above-mentioned composition, in mass%, Ca: 0.0005 to 0.0020%, REM: 0.0010 to 0.0030%, Mg: 0.0010 to 0.0020% The manufacturing method of the low yield ratio high strength thick steel plate for building structures characterized by setting it as the composition containing a seed | species or 2 or more types.

  According to the present invention, it is suitable for building structures, with a thickness of 60 mm or more, yield strength: 385 MPa or more, tensile strength TS: high strength of 550 MPa or more, and yield ratio YR: 80% or less. Low yield ratio high-strength thick steel plates with excellent heat-affected zone toughness and super-high heat input can be manufactured at low cost and with high productivity without requiring the maintenance of steel materials (slabs). There is a remarkable effect. Moreover, this invention also has the effect of making a big contribution to the enlargement of a steel structure, the improvement of earthquake resistance, the improvement of construction efficiency, etc.

It is explanatory drawing which shows typically the groove shape of the welded joint used in the Example. It is explanatory drawing which shows typically the sampling procedure of the V notch test piece used by the Charpy impact test implemented in the Example.

  First, the reason for limiting the composition of the low yield ratio high strength thick steel sheet of the present invention will be described. Unless otherwise specified, mass% is simply expressed as%.

C: 0.05-0.15%
C is an element useful for increasing the strength of steel and ensuring the strength necessary for steel for building structures. In order to obtain such an effect, in order to further minimize the amount of other alloy elements, C needs to be contained by 0.05% or more. On the other hand, if the content exceeds 0.15%, the weld cracking resistance and HAZ toughness are significantly reduced. For this reason, C was limited to the range of 0.05 to 0.15%. In addition, Preferably, it is 0.08 to 0.12%.

Si: 0.01-0.5%
Si is an element useful for solid solution to increase the strength of steel and to secure the strength required for steel plates for building structures. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, the content exceeding 0.5% promotes the formation of MA (Martensite-Austenite constituent) phase in the super-high heat input welding HAZ, and increases the risk of reducing the HAZ toughness. For these reasons, Si is limited to the range of 0.01 to 0.5%. Si is preferably 0.30% or less from the viewpoint of HAZ toughness.

Mn: 1.0-1.6%
Mn is an element useful for increasing the strength of steel and ensuring the strength required for steel plates for building structures. In order to obtain such an effect and secure a tensile strength of 550 MPa or more, it is necessary to contain 1.0% or more. On the other hand, if the content exceeds 1.6%, the concentration in the central segregation part becomes remarkable during solidification, which causes an increase in slab defects. Moreover, a large amount of Mn content causes a significant decrease in the base material and the HAZ toughness. For this reason, Mn was limited to 1.0 to 1.6%. In addition, Preferably it is 1.2 to 1.6%, More preferably, it is 1.4 to 1.6% of range.

P: 0.02% or less
P is an element inevitably contained in the steel as an impurity, and is preferably reduced as much as possible in order to reduce the toughness of the steel sheet. However, 0.02% or less is acceptable. If the content exceeds 0.02%, the HAZ toughness is particularly lowered. For this reason, P was limited to 0.02% or less. In addition, excessive P reduction increases the refining cost and is economically disadvantageous. Therefore, the P content is preferably 0.005% or more.

S: 0.003% or less
S is present as sulfide inclusions in steel and causes a reduction in the ductility and toughness of the steel sheet, so it is preferably reduced as much as possible. In particular, MnS bonded to Mn is unevenly distributed in the center segregation portion at the center of the plate thickness, and is elongated by hot rolling, and significantly reduces the Charpy impact test absorbed energy particularly in the plate thickness direction (Z direction). In order to avoid such an adverse effect, S needs to be reduced to 0.003% or less. In particular, since the heat affected zone of electroslag welding covers a wide range, the central segregation zone may be included in the heat affected zone. In such a case, the Charpy impact test absorbed energy in the heat affected zone is significantly reduced. . Therefore, S is limited to 0.003% or less. In addition, Preferably it is 0.002% or less. Moreover, excessive S reduction leads to an increase in the refining cost and is economically disadvantageous, so 0.0005% or more is preferable.

Al: 0.001 to 0.060%
Al acts as a deoxidizer and is the most commonly used element in the high-strength steel deoxidation process. Moreover, N in steel is fixed as AlN, and it has the effect of suppressing the toughness fall and crack generation by N. Such an effect is recognized when the content is 0.001% or more. However, when the content exceeds 0.060%, the toughness of the base metal is lowered, and the weld metal is mixed with the weld metal at the time of welding to lower the weld toughness. For this reason, Al is taken as 0.001 to 0.060% of range. In addition, Preferably it is 0.010 to 0.045%, More preferably, it is 0.020 to 0.035%.

Ti: 0.005-0.030%
Ti has a strong affinity for N and precipitates as TiN during solidification, and suppresses the coarsening of austenite grains in the HAZ or acts as a ferrite transformation nucleus and contributes to increasing the toughness of the HAZ. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.030%, the TiN particles become coarse and the above-described effects cannot be expected. For this reason, Ti was limited to 0.005 to 0.030% of range. In addition, Preferably, it is 0.008 to 0.020%.

Cr: 0.10 to 0.50%
Cr is one of the important elements in the present invention. Cr dissolves to increase the strength of the base metal and the high heat input weld HAZ, and suppresses the formation of intergranular ferrite, especially in the high heat input weld HAZ, and promotes the formation of acicular ferrite, thereby reducing the HAZ structure. Has the effect of contributing to improvement in HAZ toughness. In order to obtain such an effect, the content of 0.10% or more is required. On the other hand, if the content exceeds 0.50%, weldability decreases. For these reasons, Cr is limited to the range of 0.10 to 0.50%. In addition, Preferably it is 0.15-0.30%.

B: 0.0003-0.0020%
B is one of the important elements in the present invention. B suppresses the formation of intergranular ferrite in high heat input welding HAZ, and then precipitates as B carbonitride in austenite (γ) and promotes nucleation of ferrite in γ grains as a transformation nucleus It contributes to the refinement of the HAZ structure and improves the high heat input HAZ toughness. In order to acquire such an effect, 0.0003% or more of content is required. On the other hand, if the content exceeds 0.0020, coarse B precipitates are produced and the HAZ toughness is lowered. For this reason, B was limited to the range of 0.0003% to 0.0020%. In addition, Preferably it is 0.0010% or less.

N: 0.0025 to 0.0055%
N combines with nitride-forming elements such as Ti and Nb to form nitrides, prevents the austenite grains from coarsening due to the pinning effect, functions as a nucleation site for ferrite and bainite, and refines the HAZ structure It is a contributing element. In order to acquire such an effect, it is necessary to contain 0.0025% or more. In addition, N forms a solid solution and forms island martensite in CGHAZ (Coarse Grain HAZ) for super high heat input welding and ICCGHAZ (Inter Critically reheated Coarse Grain HAZ) for small heat input multi-pass welding. Promotes reduction of toughness. On the other hand, if N exceeds 0.0055% and the amount of nitride is too large, the amount of nitride becomes too large, and the nitride becomes coarse and toughness decreases. For these reasons, N is limited to a range of 0.0025 to 0.0055%. In addition, Preferably it is 0.0030 to 0.0050%.

O: 0.003% or less
O (oxygen) is present as oxide inclusions in steel, but oxide inclusions are preferably reduced as much as possible because they reduce ductility and toughness. When the content exceeds 0.003%, the cleanliness of the steel sheet is remarkably lowered, and the ductility and toughness are remarkably lowered. For this reason, O was limited to 0.003% or less.

Further, in the present invention, C, Cr and B are contained in the above-mentioned range, and the following formula (1)
0.28 ≦ 3/2 × C + 1/2 × Cr + 100 × B ≦ 0.38 (1)
(Here, C, Cr, B: Content of each element (mass%))
The content is adjusted so as to satisfy.

  By adjusting the contents of C, Cr, and B so as to satisfy the formula (1), an increase in intragranular nucleation sites by B carbonitride and an increase in hardenability by solute Cr will result in γ grains. The generation of acicular ferrite in is promoted, and a weld HAZ having high strength and excellent toughness can be obtained. (1) When the median value of the formula exceeds 0.38, upper bainite and ferrite side plates that adversely affect toughness are generated, and toughness decreases. On the other hand, when the median value of the formula (1) is less than 0.28, it becomes impossible to suppress the formation of intragranular polygonal ferrite and coarse grain boundary ferrite which may reduce the weld joint strength. Therefore, in the present invention, the contents of C, Cr and B are adjusted so as to satisfy the expression (1).

  Furthermore, in the present invention, Cu, Ni, and V as impurities are adjusted to be 0.05% or less, respectively.

  Cu and Ni easily cause slab cracking, adversely affecting yield and productivity, and V is an element that is concerned about a decrease in toughness due to precipitation embrittlement, and is reduced as much as possible in the present invention. Limited to 0.05% or less. In addition, Preferably it is 0.03% or less.

  The above is the basic component of the present invention. In the present invention, in addition to the basic composition, as a selection element, one selected from Mo: 0.01 to 0.3% and Nb: 0.003 to 0.020% or You may contain 1 type (s) or 2 or more types chosen from 2 types and / or Ca: 0.0005-0.0020%, REM: 0.0010-0.0030%, Mg: 0.0010-0.0020%.

One or two selected from Mo: 0.01-0.30%, Nb: 0.003-0.020%
Mo and Nb are both elements that contribute to an increase in the strength of the steel, and can be selected to contain one or two as required.

  Mo is an element that contributes effectively to increasing the strength of steel. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, if the content exceeds 0.30%, the high heat input welding HAZ toughness decreases. For this reason, when it contains, it is preferable to limit Mo to 0.01 to 0.30% of range. Mo is an expensive element like Ni, and Mo is likely to promote MA formation in HAZ, so it is preferable to make it as low as possible within the above range. From the viewpoint of improving HAZ toughness, Mo is more preferably 0.10% or less.

  Nb contributes to increasing the strength of the steel by precipitation strengthening, has the effect of suppressing the recrystallization of austenite and expanding the non-recrystallization temperature range, facilitates the processing in the non-recrystallization temperature range, and then It is an element that contributes to the toughening of the base metal by refining the transformation structure. In order to acquire such an effect, Nb needs to contain 0.003% or more. On the other hand, if the content exceeds 0.020%, the base material and the HAZ toughness are significantly reduced. For this reason, when it contains, it is preferable to limit Nb to 0.003 to 0.020% of range. In addition, More preferably, it is 0.003 to 0.010%.

One or more selected from Ca: 0.0005 to 0.0020%, REM: 0.0010 to 0.0030%, Mg: 0.0010 to 0.0020%
Ca, REM, and Mg are all elements that contribute to improving the ductility of steel through the control of sulfide morphology. Furthermore, sulfides or oxide particles of these elements are combined with MnS and act as ferrite transformation nuclei during welding, contributing to the improvement of HAZ toughness. In order to obtain such effects, it is necessary to contain Ca: 0.0005% or more, REM: 0.0010% or more, and Mg: 0.0010% or more. On the other hand, if Ca: 0.0020%, REM: 0.0030%, and Mg: 0.0020% are respectively contained in excess, excessive inclusions may be generated, which may lower the toughness. For this reason, when it contains, it is preferable to limit to Ca: 0.0005-0.0020%, REM: 0.0010-0.0030%, and Mg: 0.0010-0.0020%, respectively.

  The balance other than the components described above consists of Fe and inevitable impurities.

  Next, the reason for limiting the structure of the steel plate of the present invention will be described.

  The steel plate according to the present invention has the above-described composition, and includes a tempered bainite phase of 40% or more by volume ratio, or a ferrite phase in total at 90% or more in the plate thickness 1/4 position, with the balance being the volume ratio. It has a structure composed of pearlite of 10% or less (including 0%).

  The thick steel plate of the present invention is mainly composed of a structure composed of a tempered bainite phase having a volume ratio of 40% or more and further a ferrite phase at a position where the thickness is 1/4. Here, “mainly” means a case where the volume ratio is 90% or more.

  If the tempered bainite phase is less than 40% by volume, it is impossible to secure desired high strength (yield strength: 385 MPa or more, tensile strength: 550 MPa or more) for a building structure. For this reason, the tempered bainite phase was limited to 40% or more by volume ratio. By containing 40% or more of the tempered bainite phase by volume, it is possible to combine high strength and high toughness. In order to ensure a desired high strength more reliably, the tempered bainite phase is preferably 50% or more by volume. In addition, since the tempered bainite phase has a low yield ratio, the tempered bainite phase alone can have the desired high strength and yield ratio: low yield ratio of 80% or less, but the yield ratio: 80% more reliably. In order to obtain the following low yield ratio, it is desirable to include a ferrite phase having a volume ratio of 20% or more.

  The balance other than the tempered bainite phase and the ferrite phase described above is pearlite having a volume ratio of 10% or less (including 0%). If the volume ratio exceeds 10%, the desired base material toughness cannot be secured. Since pearlite lowers the base material toughness, the smaller the pearlite, the better.

  For this reason, the structure of the steel plate of the present invention includes a tempered bainite phase having a volume ratio of 40% or more, or a ferrite phase in a total thickness of 90% or more at the 1/4 thickness position, with the balance being the balance. It was limited to a structure composed of pearlite having a volume ratio of 10% or less (including 0%).

  In addition, the structure was defined at the position of the sheet thickness 1/4 because the structure at the position of the sheet thickness 1/4 represents an intermediate structure between the surface layer of the steel sheet and the center of the sheet thickness, and represents the average structure of the entire steel sheet. It is because it is thought that it is doing.

  Below, the manufacturing method of this invention thick steel plate is demonstrated.

  In the present invention, the steel material having the above composition is subjected to hot rolling, followed by direct quenching and tempering to obtain a thick steel plate having a predetermined size.

  The method for producing the steel material is not particularly limited. However, it is possible to melt the molten steel having the above-described composition by a conventional melting method such as a converter, electric furnace, vacuum melting furnace, etc. It is preferable that the steel material (slab) is obtained by an ordinary casting method such as a continuous casting method through an acid treatment or a degassing process. Needless to say, the present invention is not limited to these.

  Next, hot rolling is performed on the obtained steel material (slab).

In hot rolling, the steel material is heated to an average temperature in the range of 1050 to 1200 ° C, the cumulative rolling reduction in the temperature range of 950 ° C or less at the surface temperature is 30% or more, and the rolling end temperature is the surface temperature. Rolling is performed at 900 ° C. or lower and Ar ₃ transformation point or higher.

Heating temperature: 1050 ~ 1200 ℃ at average temperature
When the heating temperature is less than 1050 ° C. as an average temperature, precipitates such as coarse carbides precipitated during solidification cannot be completely dissolved, and a desired high strength cannot be ensured. On the other hand, at a high temperature exceeding 1200 ° C., the structure becomes coarse, the desired base material toughness cannot be secured, and the hardenability increases too much to obtain the desired structure. For this reason, the heating temperature of the steel material is limited to an average temperature in the range of 1050 to 1200 ° C. In addition, Preferably it is 1080-1150 degreeC. Here, the “average temperature” refers to an average value in the cross section in the thickness direction obtained from the temperature distribution of the steel material calculated by heat transfer calculation from the measured surface temperature.

Cumulative rolling reduction in a temperature range of 950 ° C. or lower at the surface temperature: 30% or more In the present invention, controlled rolling is performed in a temperature range of 950 ° C. or lower in the surface temperature in order to appropriately refine the microstructure. If the cumulative rolling reduction in the temperature range of 950 ° C. or lower is less than 30%, the structure becomes coarse, and the desired structure cannot be refined, and the desired high toughness cannot be ensured. Moreover, hardenability increases too much by coarsening of a structure | tissue, and it becomes impossible to ensure a desired structure | tissue. For this reason, the cumulative rolling reduction in the temperature range of 950 ° C. or less at the surface temperature in hot rolling is limited to 30% or more. In addition, Preferably it is 35% or more. If the cumulative rolling reduction at a surface temperature of 950 ° C or less exceeds 60% and is too high, a crystal grain is stretched in the rolling direction, separation occurs in the Charpy impact test, and the base metal toughness decreases. To do. For this reason, it is preferable that the cumulative rolling reduction in the temperature range of 950 ° C. or less at the surface temperature is 60% or less.

Rolling end temperature: 900 ° C or less at surface temperature and above Ar ₃ transformation point If rolling end temperature exceeds 900 ° C at surface temperature, the structure becomes coarse and the desired base metal toughness cannot be secured, and the hardenability increases too much. The desired organization cannot be secured. On the other hand, the rolling end temperature is in the Ar less than ₃ transformation point at a surface temperature, coarsened to produce the ferrite phase immediately during rolling or rolling, toughness of the base material is lowered. For this reason, the rolling end temperature is limited to the surface temperature of 900 ° C. or lower and the Ar ₃ transformation point or higher. In addition, Preferably it is 880-780 degreeC.

The Ar ₃ transformation point is expressed by the following formula: Ar ₃ transformation point (° C.) = 900−332C + 6Si−77Mn−20Cu−50Ni−18Cr−68Mo
(Here, C, Si, Mn, Cu, Ni, Cr, Mo: content of each element (mass%))
The value calculated using is used. In addition, about the element which is not contained among the elements described in a type | formula, the said element shall be calculated as zero.

  In addition, the thick steel plate obtained by the above hot rolling is directly quenched after the hot rolling.

The direct quenching process is performed at a temperature range of 200 ° C. or less at an average cooling rate of 2 ° C./s or more and 40 ° C./s or less at a temperature at the position of the plate thickness ¼ from the temperature above the Ar ₃ transformation point. It is set as the process which cools to cooling stop temperature.

Cooling start temperature: temperature above the Ar ₃ transformation point at the surface temperature If the cooling start temperature is below the Ar ₃ transformation point at the surface temperature, a ferrite phase is generated and coarsened before the start of the cooling treatment. For this reason, refinement | miniaturization of the ferrite grain of a surface layer part cannot be achieved, and a desired low yield ratio cannot be achieved. For this reason, the starting temperature of cooling was limited to the Ar ₃ transformation point or higher at the surface temperature.

Average cooling rate: 2 ° C / s or more and 40 ° C / s or less at a temperature of 1/4 position of the plate thickness When the average cooling rate at the position of 1/4 thickness of the plate is less than 2 ° C / s, cooling is slow and cooling Coarse and low toughness ferrite grains are formed inside. When the average cooling rate exceeds 40 ° C./s at the temperature of the thickness 1/4 position, a martensite phase is generated and toughness is lowered. For this reason, the cooling in the direct quenching process was limited to the range of 2 to 40 ° C./s at the average cooling rate at the position of the plate thickness ¼. When the plate thickness exceeds 100 mm, it becomes difficult to ensure 2 ° C./s or more at the average cooling rate at the 1/4 position of the plate thickness.

  Here, the “average cooling rate” refers to an average cooling rate from the start of cooling to 500 ° C. The cooling rate at the “plate thickness ¼ position” uses a value calculated from the temperature at the plate thickness ¼ position calculated by heat transfer calculation from the measured surface temperature.

Cooling stop temperature: Temperature in the temperature range of 200 ° C or less at the 1/4 thickness position If the cooling stop temperature exceeds 200 ° C at the 1/4 thickness position, the transformation to bainite phase decreases and the tempered bainite phase. However, a structure having a volume ratio of 40% or more cannot be obtained, and a desired high strength cannot be ensured. For this reason, the cooling stop temperature is limited to 200 ° C. or less at the position of the thickness ¼.

Moreover, after performing a direct quenching process, the steel plate surface temperature is further subjected to a tempering process in which the steel sheet is heated to a temperature of 400 ° C. or higher and lower than the Ac ₁ transformation point.

If the tempering temperature is less than 400 ° C., the desired high strength and high toughness cannot be achieved. On the other hand, at a temperature exceeding the Ac ₁ transformation point at the steel sheet surface temperature, the strength is significantly reduced. For this reason, the tempering process was a process of heating to a temperature of 400 ° C. or higher and lower than the Ac ₁ transformation point at the steel sheet surface temperature. In addition, this tempering process can be achieved by charging a thick steel plate into a heating furnace adjusted to a temperature of 400 to 700 ° C. at an atmospheric temperature (furnace temperature).

Ac ₁ transformation point is the following formula: Ac ₁ transformation point (° C.) = 751−27C + 18Si−12Mn−23Cu−23Ni + 24Cr + 23Mo−40V−6Ti + 233Nb−169Al−895B
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al, B: content of each element (mass%))
The value calculated using is used. In addition, the element which is not contained among the elements described in the formula shall be calculated as zero.

  Next, the present invention will be further described based on examples.

  Using a converter and ladle refining, molten steel having the composition shown in Table 1 was melted and cast by a continuous casting method to obtain a steel material (slab: wall thickness 250 mm). After the obtained steel material was heated to the heating temperature shown in Table 2, it was subjected to hot rolling under the conditions shown in Table 2, followed by direct quenching treatment and tempering treatment under the conditions shown in Table 2, A thick steel plate having a thickness shown in FIG.

Test pieces were collected from the obtained thick steel plates and subjected to structure observation, tensile test, impact test, and welded joint test. The test method is as follows.
(1) Microstructure observation A specimen for microstructural observation is collected from the obtained thick steel plate, polished so that a cross section (C cross section) perpendicular to the rolling direction becomes an observation surface, and corroded (Nital liquid corrosion). The structure in the vicinity of the 1/4 thickness position was observed with an optical microscope and a scanning electron microscope and imaged. The obtained tissue photograph was subjected to image analysis, and tissue identification and tissue fraction were measured.
(2) Tensile test JIS No. 4 tensile test specimens were taken from the position of 1/4 thickness of the obtained steel plate so that the tensile direction was parallel to the rolling direction (L direction). A tensile test was performed in accordance with the regulations, tensile properties (yield strength YS, tensile strength TS) were measured, and yield ratio YR (= (YS / TS) × 100%) was calculated.
(3) Impact test V-notch Charpy impact test specimens were collected from the position of 1/4 thickness of the obtained thick steel plate so that the longitudinal direction of the specimen was in the direction perpendicular to the rolling direction (C direction). In accordance with the provisions of 2242, a Charpy impact test was performed at a test temperature of 0 ° C. to determine the absorbed energy vE ₀ (J). The absorbed energy was an average value of three of each.
(4) Welded joint test From the obtained thick steel plate, a joint test plate (size: 400 x 600 mm) was sampled and assembled into the groove shape shown in Fig. 1, and then electroslag welding (welding-in) Welded joints were produced with a heat quantity of 1000 kJ / cm). The wire used was JIS Z 3353 YES62 equivalent, and the flux was JIS Z 3353 FS-FG3 equivalent.

As shown in Fig. 2, a V-notch test piece with the notch position as the bond part is taken from the welded joint part obtained, and Charpy impact test is performed at a test temperature of 0 ° C in accordance with the provisions of JIS Z 2242. The absorbed energy vE ₀ (J) was obtained, and the toughness of the super-high heat input welding heat-affected zone (super-high heat input weld joint bond portion) was evaluated. The absorbed energy was an average value of three of each.

  The obtained results are shown in Table 3.

In all of the examples of the present invention, the yield strength YS: 385 MPa or more, the tensile strength TS: 550 MPa or more, the yield ratio YR: 80% or less, the low yield ratio high strength, and the absorbed energy at 0 ° C. vE ₀ : 70 J or more And a thick steel plate having high base metal toughness. Furthermore, in all of the examples of the present invention, the super heat input welding heat-affected zone, in which the absorbed energy vE _{0 at} 70 ° C. is 70 J or more at an electroslag weld joint bond portion with a heat input exceeding 1000 kJ / cm and a heat input exceeding 400 kJ / cm. It is a thick steel plate with excellent toughness.

  On the other hand, in the comparative example outside the scope of the present invention, the base metal part strength is out of the desired range, or the weld heat affected zone toughness is out of the desired range.

Claims (6)

% By mass
C: 0.05-0.15%, Si: 0.01-0.5%,
Mn: 1.0 to 1.6%, P: 0.02% or less,
S: 0.003% or less, Al: 0.001 to 0.060%,
Ti: 0.005-0.030%, Cr: 0.10-0.50%,
B: 0.0003-0.0020%, N: 0.0025-0.0055%,
O: Including 0.003% or less, further including C, Cr, B so as to satisfy the following formula (1), and adjusting impurities Cu, Ni, V to 0.05% or less respectively. A composition comprising the balance Fe and inevitable impurities,
The tempered bainite phase with a volume ratio of 40% or more or a ferrite phase with a total volume of 90% or more at the 1/4 thickness position, and the balance is made of pearlite with a volume ratio of 10% or less (including 0%). Low yield for building structures, characterized by excellent yield strength: 385 MPa or more, tensile strength: 550 MPa or more, yield ratio: 80% or less, and super high heat input welding heat-affected zone toughness Specific high strength thick steel plate.
Record
0.28 ≦ 3/2 × C + 1/2 × Cr + 100 × B ≦ 0.38 (1)
Here, C, Cr, B: Content of each element (mass%)   In addition to the said composition, it is set as the composition containing 1 type or 2 types chosen from Mo: 0.01-0.30% and Nb: 0.003-0.020% by the mass% further. Low yield ratio high strength thick steel plate for building structures as described.   In addition to the above composition, the composition further contains one or more selected from Ca: 0.0005 to 0.0020%, REM: 0.0010 to 0.0030%, and Mg: 0.0010 to 0.0020% by mass%. The low yield ratio high-strength thick steel plate for building structures according to claim 1 or 2. A steel material is a method of manufacturing a steel plate for a building structure, which is hot rolled and subsequently subjected to direct quenching treatment and tempering treatment to make a steel plate,
The steel material is mass%,
C: 0.05-0.15%, Si: 0.01-0.5%,
Mn: 1.0 to 1.6%, P: 0.02% or less,
S: 0.003% or less, Al: 0.001 to 0.060%,
Ti: 0.005-0.030%, Cr: 0.10-0.50%,
B: 0.0003-0.0020%, N: 0.0025-0.0055%,
O: Including 0.003% or less, further including C, Cr, B so as to satisfy the following formula (1), and adjusting impurities Cu, Ni, V to 0.05% or less respectively. A steel material having a composition consisting of the balance Fe and inevitable impurities,
In the hot rolling, the steel material is heated to a temperature in the range of 1050 to 1200 ° C. at an average temperature, the cumulative rolling reduction in the temperature range of 950 ° C. or less at the surface temperature is 30% or more, and the rolling end temperature is the surface Rolling at a temperature of 900 ° C. or lower and Ar ₃ transformation point or higher,
In the direct quenching process, the cooling is started from the surface temperature at or above the Ar ₃ transformation point, and the plate thickness is 1/4 at an average cooling rate of 2 ° C./s or more and 40 ° C./s or less at the plate thickness 1/4 position. It is a process to cool to the cooling stop temperature in the temperature range of 200 ° C or less at the temperature of the position,
A method for producing a low-yield ratio high-strength thick steel sheet for a building structure, wherein the tempering treatment is a treatment in which the surface temperature is heated to a temperature range of 400 ° C. or higher and lower than the Ac ₁ transformation point.
Record
0.28 ≦ 3/2 × C + 1/2 × Cr + 100 × B ≦ 0.38 (1)
Here, C, Cr, B: Content of each element (mass%)   In addition to the said composition, it is set as the composition containing 1 type or 2 types chosen from Mo: 0.01-0.30% and Nb: 0.003-0.020% by the mass% further. The manufacturing method of the low yield ratio high-strength thick steel plate for building structures as described.   In addition to the above composition, the composition further contains, by mass%, one or more selected from Ca: 0.0005 to 0.0020%, REM: 0.0010 to 0.0030%, and Mg: 0.0010 to 0.0020%. The manufacturing method of the low yield ratio high-strength thick steel plate for building structures of Claim 4 or 5 characterized by these.

Images