JP2012102393A - Non-heat-treated, low-yield-ratio, high-tensile thick steel plate and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-heat-treated, low-yield ratio, high-tensile thick steel plate having a tensile strength of ≥550 MPa.SOLUTION: The steel plate has a composition comprising, by mass%, 0.05-0.16% C, 1.2-1.8% Mn, 0.005-0.025% Nb, 0.005-0.020% Ti and ≥2.5 Ti/N, has a microstructure comprising ferrite and a hard phase at least on a surface, has an average particle size of ferrite at the surface of 4.0-18.0 μm, has average hardness at the surface of ≤225 HV and has a hardness distribution in the plate thickness direction where the difference of hardness between the surface and a central part of the plate thickness is ≤60 HV. The non-heat-treated, low-yield ratio, high-tensile thick steel plate is excellent in surface ductility and toughness after cold machining.

Description

  The present invention relates to a low-yield-ratio high-tensile steel plate suitable for a building structure member that requires earthquake resistance and a method for producing the same, and in particular, cold bending work such as a circular steel pipe column or a square steel pipe column is performed. The present invention relates to a low-yield-ratio high-tensile steel plate suitable for the intended use and a method for producing the same.

In recent years, from the viewpoint of ensuring safety during an earthquake, it is required to use a steel plate (steel material) having excellent earthquake resistance as a material in a building structure or the like. In addition, research results so far have shown that steel plates with lower yield ratios have better earthquake resistance. For this reason, building structures are obliged to use low yield ratio steel with a yield ratio (YR) of 80% or less. Furthermore, recently, with the increase in the height and the span of building structures, there are an increasing number of cases where 550 MPa class high-strength steel having higher strength than before is applied to building structures.
Conventionally, a high-tensile steel material of 550 MPa class or higher having a low yield ratio has been generally manufactured by performing a heat treatment such as a two-phase region heat treatment or a tempering treatment. However, the heat treatment has a problem in that the process becomes complicated, the manufacturing period is prolonged, and the manufacturing cost is increased. For this reason, the examination of the non-tempered low yield ratio high-tensile steel material in which the above-described two-phase region heat treatment and tempering treatment are omitted has been advanced.

  For example, Patent Document 1 includes C: 0.02 to 0.04%, solute B: 0.0002 to 0.002%, and a composition satisfying the formula CEN related to the alloy element content in the range of 0.21 to 0.30%, and bainite. 590MPa class non-tempered type low yield ratio high tensile strength steel sheet having a structure in which 0.8 to 2.5% by volume of island martensite is dispersed has been proposed. In the technique described in Patent Document 1, the production is performed only by controlled rolling. However, in the technique described in Patent Document 1, the carbon content of the steel sheet is reduced to 0.02 to 0.04%, so that it is necessary to further contain a large amount of alloying elements in order to ensure the desired high strength. There is a problem that the manufacturing cost increases.

  Patent Document 2 includes C: 0.045 to 0.08%, Si: 0.05 to 0.50%, Mn: 0.6 to 2.0%, containing P, S, Al, and N in an adjusted manner, and Mo and / or An island martensite (MA) containing W so as to satisfy a specific relational expression and having a Pcm of 0.22% or less and a structure in the central part of the plate thickness of ferrite as a main phase and 20% by volume or less. A high-tensile thick steel plate having a low yield ratio, which is a composite structure including a hard phase mainly comprising a phase), is described. With such a structure, a desired low yield ratio can be realized. In order to obtain such a structure, in the technique described in Patent Document 2, a steel material having the above composition is subjected to hot rolling with a rolling end temperature of 800 to 950 ° C. at a surface temperature of 0.5 to 50. It is said that it is preferable to sequentially perform a cooling process of accelerating cooling to a temperature range of 580 to 670 ° C. at an average cooling rate of ° C./s. However, the technique described in Patent Document 2 has a problem in that it requires expensive Mo and W to be contained, resulting in an increase in manufacturing cost.

  In Patent Document 3, steel containing C: 0.03 to 0.30%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.5%, Al: 0.005 to 0.1% is heated, and the rolling end temperature is 900 ° C to The temperature is in the range of the Ar3 transformation point and the hot rolling to reduce the cumulative reduction in the temperature range to less than 30%, and air cooling after hot rolling, the surface temperature is (Ar3 transformation point -20 ° C) to (Ar3 transformation point) A method for producing a low yield ratio non-heat treated steel is described in which water cooling is started after reaching a temperature in the range of −80 ° C. and accelerated cooling is performed to stop cooling between 350 and 600 ° C.

  In Patent Document 4, steel containing C: 0.03 to 0.30%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.5%, Al: 0.005 to 0.1% is heated, and the rolling end temperature is 900 ° C to The temperature is within the range of the Ar3 transformation point, the hot rolling is carried out so that the cumulative reduction rate in the temperature range is less than 30%, and air cooling is performed after the hot rolling. A method for producing a low yield ratio non-heat treated steel is described in which water cooling is started after reaching a temperature in the range of −80 ° C., accelerated cooling is performed until the temperature reaches 250 ° C. or less, and then tempering heat treatment is performed.

  In Patent Document 5, C: 0.01 to 0.20%, Si: 0.6% or less, Mn: 0.50 to 2.2%, Al: 0.001 to 0.1%, Nb: 0.003 to 0.030%, Ti: 0.005 to 0.020%, N : A steel slab containing 0.006% or less is hot-rolled so that the cumulative reduction at 900 ° C or less is 30% or more and the finishing temperature is Ar3 + 100 ° C or less Ar3 or more, and the steel sheet is (Ar3-20 ° C) to (Ar3- A method for producing a low yield ratio non-tempered steel is described in which air cooling to 100 ° C. is started, water cooling is started from this temperature, and cooling is stopped in the range of 400 to 550 ° C.

  As described above, in the techniques described in Patent Documents 3 to 5, in order to reduce the addition amount of alloy elements, high strength is achieved by utilizing accelerated cooling, and both high strength and low yield ratio are achieved. In these technologies, the steel slab is hot-rolled to complete the rolling above the Ar3 transformation point, and then is cooled to the austenite + ferrite two-phase temperature before starting accelerated cooling to produce proeutectoid ferrite. This is intended to reduce the yield ratio. However, in these techniques, it is difficult to refine the pro-eutectoid ferrite and hard second phase that are generated during air cooling, and there is a problem that the toughness of the surface layer portion where a large amount of pro-eutectoid ferrite is generated tends to decrease. . In addition, since the ferrite generation rate varies depending on a slight difference in cooling start temperature, there is a problem that material variations vary from steel plate to steel plate, making it difficult to produce a stable steel plate.

  In Patent Document 6, steel pieces containing C: 0.01 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.1 to 2.0%, Al: 0.001 to 0.1%, N: 0.001 to 0.010% are heated. Rough rolling with cumulative rolling reduction of 10-80% in the range up to 900 ° C and accelerated cooling of 2-40 ° C / s after rough rolling to (Ar3 transformation point + 50 ° C) to (Ar3 transformation point-50 ° C) The austenite (γ) phase is supercooled, finish rolling with a cumulative reduction of 30 to 90% is completed at 650 ° C or higher, and accelerated cooling at 5 to 40 ° C / s is further performed to 250 to 450 ° C. A method for producing a high yield strength steel is described. In the technique described in Patent Document 6, accelerated cooling is performed after rough rolling, the γ phase is subcooled to near Ar3 temperature, and then finish rolling is performed, so that fine ferrite is obtained from the supercooled γ phase. (Α) is generated, and accelerated cooling is performed after finish rolling, so that the ferrite (α), which is a soft phase, is refined, and the ratio of the soft phase to the hard phase is appropriately controlled to achieve high toughness and a low yield ratio. It is said that it will be possible to improve the productivity and at the same time. According to this technology, it is said that a high yield steel material with a low yield ratio can be produced without requiring a large amount of expensive alloy elements or complicated heat treatment with low productivity.

In Patent Document 7, a steel piece or steel plate having a temperature of Ac3 or higher is cooled at least 2 ° C./s in a region (surface layer portion) of 1 mm to 30% of the product thickness in the thickness direction from the surface layer at least. Rapid cooling to the Ar1 point or less at a speed, rolling starts or resumes after the surface layer reaches a temperature of the Ar3 point or less, finishes the rolling in the range of (Ac3-50 ° C) to Ac3, and then reaches the Ac3 point or more. A method of manufacturing a surface low yield strength steel sheet is described in which the surface layer portion is cooled to 1 ° C / s or more to the Ar1 point without reheating, and further retained for 3 minutes or more in the range of (Ac1-100 ° C) to Ac1. Yes. As a result, the structure of the front and back layers from 1 mm to 30% of the thickness of the steel sheet has a grain size more than three times the ferrite grain size inside the thickness, and the yield strength is greater than the yield strength of the inner thickness layer. It is said to be a low-yield-strength steel sheet with a surface layer lower by 5 kg / mm ² or more.

Japanese Unexamined Patent Publication No. 2000-219934 JP 2007-177325 A JP 63-219523 A JP 63-223123 A Japanese Unexamined Patent Publication No. 1-301819 JP-A-10-306316 JP-A-6-49596

In a building structure, there are many column-beam joints, column-diaphragm joints, etc., and many T joints and cross joints are formed. In such a T joint part and a cross joint part, when a deformation | transformation arises by the shake by an earthquake, a big distortion concentrates on steel plate surfaces, such as a weld toe part.
Fig. 1 is a schematic illustration of a situation in which a joint between a column using a press column (cold-formed square steel pipe) or circular steel pipe and a diaphragm (cross joint) breaks when subjected to repeated tensile and compression deformation due to an earthquake. Shown in When the joint is subjected to repeated tensile and compression deformations, a ductile crack usually occurs at the weld toe of the weld 3, and the ductile crack propagates (proliferates) toward the thickness center of the column 1 and finally breaks. To. In addition, 2 is a diaphragm and 4 is a money.

For this reason, in order to increase the amount of deformation until breakage, it is important that the material in the vicinity of the surface layer of the column, that is, the ductility and toughness of the steel plate surface layer portion are excellent.
In recent building structures, columns, beams, and the like are often configured using circular steel pipes and press columns formed by cold bending. In circular steel pipes and press columns (steel materials) formed by cold bending, the vicinity of the steel sheet surface layer is significantly hardened by cold bending, and the ductility and toughness near the surface layer are compared to the case where the steel sheet is used without being processed. Is in a lowered state. For this reason, when steel joints formed by such cold bending work are used to form T-joints and cross joints such as column-beam joints and column-diaphragm joints, repeated tensile and compression deformation due to earthquakes, etc. When plastic deformation concentrates on the surface layer, the risk of early breakage is high, and the expected member performance may not be exhibited.

Under such circumstances, there is a demand for a low-yield-ratio high-tensile steel plate having excellent ductility and toughness of the steel sheet surface layer even after being cold worked for a building structure member.
However, all of the techniques described in Patent Documents 1 to 6 are mechanical properties (tensile properties) evaluated by full thickness tensile test pieces or round bar tensile test pieces at a thickness of 1/4 t or 1/2 t. , Ductility, toughness) is a technique for the purpose of obtaining desired characteristics, and the characteristics in the vicinity of the steel sheet surface layer are not considered at all, and there is a problem that the above-mentioned demand cannot be dealt with.

  This is because non-tempered thick steel plates using controlled rolling and accelerated cooling (TMCP technology) as described above have the highest surface hardness, as shown in “Before bending” in FIG. It has a hardness distribution in the thickness direction where the hardness at the center is the smallest. When cold bending is performed on a steel sheet having such a thickness distribution in the plate thickness direction, the hardness in the vicinity of the front and back surfaces is further increased, resulting in a hardness distribution like “after bending” in FIG. The difference in hardness between the thickness center portion and the surface layer portion is further enlarged. The steel material whose surface layer has been subjected to such cold bending and whose surface layer has been markedly work hardened is in a state where the surface layer has reduced ductility and toughness. For this reason, when a steel plate with a press column or a circular steel pipe manufactured by such cold bending is used for a joint such as a T-joint or a cruciform joint such as a column-beam joint where stress is concentrated on the surface layer. Even if the tensile properties of the full-thickness tensile test piece or the round bar tensile test piece at 1 / 4t or 1 / 2t position are good, as described above, cracks occur from the surface layer of the joint, and early breakage occurs. It is expected that the risk of

  For such a problem, for example, according to the technique described in Patent Document 7, the front and back layer portions of the steel sheet can have low yield strength, and the ductility of the steel sheet surface layer portion after cold bending is improved. However, in the steel sheet manufactured by the technique described in Patent Document 7, the ferrite grains in the surface layer of the steel sheet are coarse, so it cannot be said that the toughness is sufficient, and the building structure is used as a member. When incorporated in an object, there is a problem that brittle fracture may occur from the member.

  The present invention solves the above-mentioned problems of the prior art, uses a press column and a circular steel pipe without performing heat treatment such as quenching and tempering and normalizing and minimizing the alloy content. Suitable for building structure members, even after cold bending, there is little increase in the hardness of the steel sheet surface layer, excellent ductility and toughness of the steel sheet surface layer part, yield strength: 385 MPa or more, tensile strength: 550 MPa or more An object of the present invention is to provide a non-tempered low yield ratio high-tensile steel plate having a yield ratio of 80% or less and a method for producing the same. The “thick steel plate” here refers to a steel plate having a thickness of 19 mm or more.

In order to achieve the above-mentioned object, the present inventors are required as a member for a building structure after allowing a non-uniform hardness distribution to some extent in the thickness direction, which is unavoidable with non-tempered steel sheets. In order to ensure the desired performance, we have intensively studied the performance that the steel sheet as the material should have.
The plastic strain due to cold bending is maximized on the front and back surfaces of the steel sheet, and is zero at the neutral point near the center of the sheet thickness. For this reason, the work hardening by cold bending becomes the most remarkable in the steel plate surface layer part. Therefore, in order to ensure the desired ductility and toughness in the surface layer portion after the cold bending process, the present inventors control the thickness direction hardness distribution before the cold bending process, We thought that it was important to reduce the hardness. At that time, if the hardness of the surface layer portion is lowered while the hardness of the central portion of the plate thickness is kept as it is, the strength at the full thickness of the steel plate is lowered. In order to secure a desired high strength as a steel plate, it has been thought that it is necessary to ensure a certain level of hardness (strength) at the center of the plate thickness.

  In order to reduce the hardness in the vicinity of the surface layer portion and achieve a low yield ratio, at least the microstructure of the surface layer portion is precipitated with ferrite (preferably 10 area% or more) as a soft phase, In addition, the ductility and toughness of the surface layer portion can be set within the desired range by adjusting the average grain size of the ferrite in the surface layer portion within the desired appropriate range. I also found.

And if it was a thick steel plate which satisfied following (1)-(4), it discovered that the deformation | transformation performance required as an object for building structure members could be secured even after cold bending.
(1) At least the surface layer portion (region of 1 to 5 mm in the plate thickness direction from the front surface and the back surface) of the steel plate has a multiphase structure composed of ferrite and a hard phase.
(2) The average hardness of the steel sheet surface layer satisfies 225 HV or less.
(3) The hardness difference between the steel plate surface layer and the plate thickness center is 60HV or less.
(4) The average ferrite grain size of the steel sheet surface layer portion satisfies the range of 4.0 to 18.0 μm.

  Here, the hard phase means a phase composed of one or more of pearlite, bainite, and martensite, and the steel sheet surface layer portion means a region of 1 to 5 mm in the thickness direction from the steel sheet front and back surfaces. The plate thickness central portion refers to a plate thickness center of ± 2 mm. In addition, the reason for limiting the structure and hardness of the steel sheet surface layer part was that the influence of the surface layer part, which is a 5 mm region in the sheet thickness direction from the steel sheet front surface or the back surface, was great for the destruction of the welded structure. Based on that. In addition, the outermost layer, which is less than 1 mm in the thickness direction from the front or back surface of the steel plate, is excluded because the outermost layer receives an extremely complicated thermal history due to rolling, accelerated cooling, etc. This is because it is often extremely difficult to control.

The present invention has been completed on the basis of such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.05 to 0.16%, Si: 0.05 to 0.45%, Mn: 1.2 to 1.8%, P: 0.020% or less, S: 0.005% or less, Al: 0.05% or less, Nb: 0.005 to Contains 0.025%, N: 0.0040% or less, Ti: 0.005-0.020%, contains Ti and N so that the ratio of Ti content to N content, Ti / N satisfies 2.5 or more, and the balance Fe And a composition composed of inevitable impurities, at least a ferrite in the surface layer portion of 1 mm to 5 mm from the surface of the steel sheet in the thickness direction, and a microstructure composed of one or more of pearlite, bainite and martensite as a hard phase; The average grain size of the ferrite in the surface layer portion of 1 mm to 5 mm in the plate thickness direction from the steel plate surface is 4.0 to 18.0 μm, and the average hardness of the surface layer portion of 1 mm to 5 mm in the plate thickness direction from the steel plate surface Is less than 225HV, and the hardness difference between the surface layer and the center of the plate thickness is within ± 2mm around the center of the plate thickness is 60HV or less. Yield strength: 385 MPa or more, tensile strength: 550 MPa or more, yield ratio: 80% or less, characterized by a certain thickness direction hardness distribution and excellent surface layer ductility and toughness after cold working Tempered low yield ratio high tensile steel plate.

(2) In (1), in addition to the above composition, Cu: 0.05 to 0.30%, Ni: 0.05 to 0.30%, Cr: 0.05 to 0.50%, Mo: 0.04 to 0.40%, V: 0.01 A non-tempered low yield ratio high-tensile steel plate characterized by having a composition containing one or more selected from ˜0.06%.
(3) In (1) or (2), in addition to the above composition, 1% further selected from Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, and Mg: 0.0010 to 0.0050% in mass% A non-tempered low yield ratio high-tensile steel plate characterized by comprising a seed or a composition containing two or more.

  (4) A steel material is subjected to hot rolling to form a thick steel plate, and following the rolling step, the steel plate is a two-stage cooling process comprising a first stage cooling and a second stage cooling including an intermediate cooling stop. In the method for producing a non-heat treated thick steel sheet subjected to accelerated cooling, the steel material is, in mass%, C: 0.05 to 0.16%, Si: 0.05 to 0.45%, Mn: 1.2 to 1.8%, P: 0.020% or less, S: 0.005% or less, Al: 0.05% or less, Nb: 0.005-0.025%, N: 0.0040% or less, Ti: 0.005-0.020%, Ti and N, Ti content and N The ratio to the content, Ti / N is contained so as to satisfy 2.5 or more, and a steel material having a composition comprising the balance Fe and inevitable impurities, the heating temperature of the hot rolling is 1050 to 1200 ° C., and the heat The intermediate rolling is a rolling with a cumulative reduction amount of 30% or more in the temperature range of 950 ° C. or less at the surface temperature and a rolling end temperature of 900 ° C. or less at the surface temperature of the Ar3 transformation point or more, The first stage cooling starts from the temperature above the Ar3 transformation point at the surface temperature, accelerates the cooling at an average cooling rate at the thickness of 1/4 t at a cooling rate of 2 ° C / s or more, and the surface temperature becomes (Ar3 transformation point) -100 ° C) Accelerated cooling is stopped when it reaches 400 ° C or less, and after cooling stops, it is reheated and the surface temperature is (Ar3 transformation point + 10 ° C) or less, 650 ° C or more, and the temperature between the surface and the center of the plate thickness. When the difference becomes 60 ° C. or less, the second-stage cooling is started, and the second-stage cooling is performed at an average cooling rate at a thickness of 1/4 t at a cooling rate of 2 ° C./s or more. Yield strength: 385MPa or more, tensile strength: 550MPa, characterized by accelerated cooling to a cooling stop temperature such that the surface temperature becomes 600 ° C or lower and 400 ° C or higher by reheating after stage cooling is stopped The above is a method for producing a non-tempered low yield ratio high-tensile thick steel plate having a yield ratio of 80% or less.

  (5) In (4), instead of the first-stage cooling, the first-stage cooling is started from the surface temperature at a temperature equal to or higher than the Ar3 transformation point. Accelerated cooling where the cooling stop temperature is 400 ° C. or more at the surface temperature at a cooling rate of ℃ / s or more is assumed to be repeated cooling multiple times with the cooling stop and subsequent recuperation sandwiched, A method for producing a non-tempered, low yield ratio, high-tensile steel plate, comprising at least one accelerated cooling at which the cooling stop temperature is 400 ° C. or higher at a surface temperature (Ar3 transformation point−100 ° C.).

  (6) In (4) or (5), instead of the second-stage cooling, the second-stage cooling is accelerated and cooled at a cooling rate of 2 ° C./s or more at an average cooling rate at a thickness of 1/4 t. Cooling is repeated multiple times with the cooling stop and subsequent recuperation sandwiched, and among the multiple accelerated coolings, the surface temperature becomes 600 ° C or lower and 400 ° C or higher due to reheat after cooling stop A method for producing a non-tempered, low yield ratio, high-tensile steel plate, characterized in that accelerated cooling to cool to a cooling stop temperature is final cooling.

(7) In any one of (4) to (6), following the cooling step, a tempering step of tempering at a temperature of 400 ° C. or higher and 700 ° C. or lower is performed. A method for producing a tension thick steel plate.
(8) In any one of (4) to (7), in addition to the above-mentioned composition, Cu: 0.05 to 0.30%, Ni: 0.05 to 0.30%, Cr: 0.05 to 0.50%, Mo: 0.04 A method for producing a non-tempered, low yield ratio, high-tensile steel plate characterized by comprising a composition containing one or more selected from ˜0.40% and V: 0.01-0.06%.

  (9) In any one of (4) to (8), in addition to the above composition, it is further selected by mass% from Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, Mg: 0.0010 to 0.0050% A method for producing a non-tempered, low yield ratio, high-tensile thick steel plate, characterized in that the composition contains one or more types.

  According to the present invention, even after cold bending, there is little increase in the hardness of the steel sheet surface layer part, and the steel sheet surface layer part is excellent in ductility and toughness, suitable for building structure members, yield strength: 385 MPa or more, Tensile strength: high strength of 550 MPa or more and yield ratio: low yield ratio of 80% or less, non-tempered low yield ratio high tensile steel plate without heat treatment or containing a large amount of alloy, It can be manufactured and has a remarkable industrial effect. Moreover, the non-tempered low yield ratio high-tensile thick steel plate according to the present invention also has an effect of greatly contributing to the weight reduction of the steel structure and the improvement of the earthquake resistance of the steel structure.

It is explanatory drawing which shows typically the destruction condition in a column-diaphragm junction part. It is explanatory drawing which shows typically plate | board thickness direction hardness distribution of a non-tempered thick steel plate. It is explanatory drawing which shows typically the outline of the three-point bending test (column bending test) method of a press column-diaphragm junction part. It is explanatory drawing which shows typically the welding part vicinity in a test body. It is explanatory drawing which shows the point of the column bending test typically. It is a graph which shows typically an example of a load-deformation amount hysteresis curve in a column bending test. It is explanatory drawing which shows typically the cooling conditions of the cooling process in this invention.

First, the reasons for limiting the composition of the steel plate of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.05-0.16%
C is an element useful for increasing the strength of steel and ensuring the strength required as a structural steel material. Further, C has an effect of increasing the volume fraction of the hard phase and decreasing the yield ratio. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 0.16%, weldability and toughness are significantly reduced. For this reason, C was limited to the range of 0.05 to 0.16%. In addition, Preferably it is 0.06 to 0.15%.

Si: 0.05-0.45%
Si acts as a deoxidizer and dissolves in the steel to increase the strength of the steel. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 0.45%, the toughness of the base metal is lowered and the weld heat affected zone (HAZ) toughness is markedly lowered. For this reason, Si was limited to the range of 0.05 to 0.45%. In addition, Preferably, it is 0.05 to 0.35%.

Mn: 1.2-1.8%
Mn is an element that has the effect of increasing the strength of steel by solid solution and is inexpensive, and in the present invention that minimizes the inclusion of other expensive alloy elements, the desired high strength (tensile strength: 550 MPa or more) ) Is required to contain 1.2% or more. On the other hand, if the content exceeds 1.8%, the toughness of the base metal and the HAZ toughness are significantly reduced. For this reason, Mn was limited to the range of 1.2 to 1.8%. In addition, Preferably it is 1.2 to 1.6%.

P: 0.020% or less P is an element having an action of increasing the strength of steel, but is an element that lowers toughness, particularly toughness of a welded portion. In the present invention, it is desirable to reduce as much as possible, but excessive reduction However, it is preferable to make the refining cost about 0.005% or more because it raises the refining cost and is economically disadvantageous. On the other hand, if the content exceeds 0.020%, the above-described adverse effects become remarkable, so P is limited to 0.020% or less. In addition, Preferably it is 0.015% or less.

S: 0.005% or less S is present in steel as sulfide inclusions such as MnS, which deteriorates the toughness of the base metal and welds and is unevenly distributed in large amounts in the center segregation part of the slab. Make it easier to generate defects. Such a tendency becomes remarkable when the content exceeds 0.005%. For this reason, S was limited to 0.005% or less. Preferably it is 0.003% or less. In addition, since excessive S reduction raises refining cost and becomes economically disadvantageous, it is desirable to make S into about 0.001% or more.

Al: 0.05% or less
Al is an element that acts as a deoxidizer, and is most commonly used as a deoxidizer in the molten steel deoxidation process of high-strength steel. In order to obtain such an effect, it is desirable to contain 0.01% or more. However, if the content exceeds 0.05%, the toughness of the base metal is lowered and mixed with the weld metal during welding to lower the weld metal toughness. Let For this reason, Al was limited to 0.05% or less. In addition, Preferably it is 0.010 to 0.045%.

Nb: 0.005-0.025%
Nb is an element that increases hardenability and increases the strength of the base metal through the action of increasing the effect of controlled rolling and refining the microstructure, and is a useful element for increasing the strength. In order to obtain such an effect, it is necessary to contain 0.005% or more. On the other hand, the content exceeding 0.025% lowers the toughness of the base material and HAZ. For this reason, Nb was limited to the range of 0.005-0.025%. In addition, Preferably it is 0.007 to 0.020%.

N: 0.0040% or less When N is dissolved in steel, strain aging is caused after cold working to deteriorate toughness. Therefore, it is desirable to reduce N as much as possible in the present invention. If the content exceeds 0.0040%, the toughness deteriorates remarkably. For this reason, N was limited to 0.0040% or less.
Ti: 0.005-0.020%
Ti is an element having a strong affinity with N, and precipitates as TiN during solidification, thereby reducing solute N in the steel and reducing toughness deterioration due to strain aging of N after cold working. Ti also contributes to the improvement of HAZ toughness through the improvement of the HAZ structure. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.020%, the TiN particles become coarse and the above-described effects cannot be expected. For this reason, Ti was limited to the range of 0.005-0.020%. In addition, Preferably it is 0.007 to 0.015%.

Ti / N: 2.5 or more In the present invention, an amount of Ti corresponding to the N content is included, and the solid solution N is fixed as TiN. For this reason, Ti content is adjusted so that ratio of Ti content and N content and Ti / N may satisfy 2.5 or more. When Ti / N is less than 2.5, the Ti content is too small compared to the N content, so that a lot of N remains as solid solution N, HAZ toughness decreases, and member deformation performance is caused by brittle fracture from the weld. May decrease. For this reason, Ti / N was limited to 2.5 or more. In addition, Preferably, it is the range of 3.0-5.0.

  The above components are basic components. In addition to the basic composition, Cu: 0.05 to 0.30%, Ni: 0.05 to 0.30%, Cr: 0.05 to 0.50%, Mo: 0.04 to 0.40% V: One or more selected from 0.01 to 0.06% and / or Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, Mg: 0.0010 to 0.0050% One type or two or more types can be contained.

One or more selected from Cu: 0.05-0.30%, Ni: 0.05-0.30%, Cr: 0.05-0.50%, Mo: 0.04-0.40%, V: 0.01-0.06%
Cu, Ni, Cr, Mo, and V are all elements that have an action of increasing the strength of steel, and can be selected and contained as necessary.
Cu contributes to increasing the strength of thick steel plates by increasing the strength of steel plates through solid solution strengthening and hardenability improvement. In order to obtain such an effect, the content is preferably 0.05% or more, but the content exceeding 0.30% causes an increase in alloy cost and deterioration of surface properties due to hot brittleness. For this reason, when it contains, it is preferable to limit Cu to 0.05 to 0.30% of range. In addition, More preferably, it is 0.05 to 0.20%.

  Ni is an element that increases the strength of the steel sheet with almost no deterioration in toughness, and has little adverse effect on HAZ toughness, and is an element useful for increasing the strength of thick steel sheets. In order to obtain such an effect, the content is preferably 0.05% or more. However, a large content exceeding 0.30% causes an increase in alloy cost because Ni is an expensive element. For this reason, when it contains, it is preferable to limit Ni to 0.05 to 0.30%. In addition, More preferably, it is 0.05 to 0.20%.

  Cr is an element that increases the strength of the base material through improvement in hardenability, and is an element useful for increasing the strength of thick steel plates. In order to acquire such an effect, it is preferable to contain 0.05% or more, but inclusion exceeding 0.50% causes an increase in alloy cost. For this reason, when it contains, it is preferable to limit Cr to 0.05 to 0.50% of range. In addition, More preferably, it is 0.05 to 0.30%.

  Mo is an element that increases the strength of the base metal through improvement in hardenability, and is an element useful for increasing the strength of thick steel plates. In particular, Mo has the effect of simultaneously reducing the yield ratio and increasing strength by increasing the hardness of the second phase (hard phase). In order to acquire such an effect, it is preferable to contain 0.04% or more, but inclusion exceeding 0.40% reduces the toughness of a base material and HAZ. For this reason, when it contains, it is preferable to limit Mo to 0.04 to 0.40% of range. In addition, More preferably, it is 0.04 to 0.20%.

  V is an element that increases the strength of the base metal through precipitation strengthening, and is a useful element for increasing the strength of the thick steel plate. In order to obtain such an effect, the content is preferably 0.01% or more, but the content exceeding 0.06% lowers the toughness of the base material and HAZ. For this reason, when it contains, it is preferable to limit V to 0.01 to 0.06% of range. In addition, More preferably, it is 0.02 to 0.05%.

Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, Mg: One or more selected from 0.0010 to 0.0050%
Ca, REM, and Mg all contribute to the improvement of the toughness and ductility of the base metal through the control of sulfide morphology. In addition, when fine sulfide particles are dispersed in steel, they contribute to the improvement of HAZ toughness by acting as ferrite transformation nuclei. In order to obtain these effects, it is preferable to contain at least 0.0005% for Ca and at least 0.010% for REM and Mg respectively. However, if Ca, REM and Mg are contained in excess of 0.0050%, excessive intervention is required. A product may be generated, and conversely, toughness may decrease. For this reason, when it contains, it is preferable to limit Ca to 0.0005 to 0.0050% and REM and Mg to 0.0010 to 0.0050%, respectively.

The balance other than the components described above is composed of Fe and inevitable impurities.
The steel plate according to the present invention has the above-described composition, and at least a surface layer portion of 1 to 5 mm in the plate thickness direction from the steel plate surface is one of ferrite and pearlite, bainite, martensite as a hard phase. Alternatively, it has a microstructure composed of two or more types, and has a microstructure in which the average crystal grain size of the ferrite in the surface layer portion of 1 mm to 5 mm in the thickness direction from the surface of the steel sheet is 4.0 to 18.0 μm.

In the thick steel plate of the present invention, in order to combine the yield ratio: low yield ratio of 80% or less and the tensile strength: high strength of 550 MPa or more, the outermost layer which is an area less than 1 mm in the thickness direction from the steel sheet surface is excluded. The surface layer portion that is at least a region of 1 to 5 mm in the thickness direction from the surface of the steel plate, preferably the entire region in the thickness direction, has a multiphase structure consisting of ferrite that is a soft phase and a hard phase.
At least the surface layer portion, preferably the entire plate thickness excluding the outermost layer, is made of a multiphase structure combining a soft phase ferrite and a hard phase, so that excellent ductility, desired high strength, and a low yield ratio are obtained. Both can be achieved. In particular, in order to achieve both excellent ductility and a low yield ratio, the ferrite that is a soft phase is preferably 10% or more in terms of area ratio. If the ferrite, which is a soft phase, is less than 10% in area ratio, a low yield ratio cannot be achieved. In order to secure a desired high strength (tensile strength: 550 MPa or more), the ferrite content is desirably 70% or less.

The hard phase is one or more selected from pearlite, bainite, and martensite. High strength and excellent ductility can be achieved by using a multiphase structure combining ferrite and these hard phases. In addition, the kind of hard phase and those fractions can be suitably selected by desired intensity | strength and toughness, and also a chemical component and board thickness.
Furthermore, in the steel plate of the present invention, the average crystal grain size of ferrite in the surface layer portion of 1 mm to 5 mm in the plate thickness direction from the steel plate surface is set to 4.0 to 18.0 μm.

  The average crystal grain size of ferrite in the surface layer portion of the steel plate greatly affects toughness, yield ratio and elongation. If the ferrite grain size of the steel sheet surface layer is less than 4.0 μm, the yield ratio increases rapidly and the uniform elongation decreases. For this reason, the plastic deformability of a steel plate surface layer part falls significantly. The decrease in the ductility of the steel sheet surface layer portion tends to cause a ductile crack from the weld toe portion during deformation due to an earthquake. Further, when the ferrite grain size of the steel sheet surface layer exceeds 18.0 μm and becomes coarse, the toughness is lowered and brittle fracture is likely to occur. For this reason, the average grain size of ferrite in the steel sheet surface layer portion is limited to a range of 4.0 to 18.0 μm. In addition, Preferably it is 7.0-14.0 micrometers.

Furthermore, in the steel plate of the present invention, the average hardness of the surface layer portion of 1 mm to 5 mm in the plate thickness direction from the steel plate surface is 225 HV or less, and the plate thickness center is in the range of ± 2 mm centering on the surface layer portion and the plate thickness center position. The thickness difference in hardness in the plate thickness direction is 60HV or less.
By setting the thickness distribution in the plate thickness direction as described above, necessary performance (for example, plastic deformability, brittle fracture prevention capability, etc.) can be secured for a building structural member subjected to cold bending or the like.

  If the hardness of the steel plate surface layer exceeds 225 HV, after the cold bending process, the hardness of the steel plate surface layer portion will further increase, and the toughness and ductility of the steel plate surface layer portion will be significantly reduced. In members such as beam joints (T joints and cross joints), cracks are likely to occur from the surface layer parts such as weld toes at the time of deformation due to an earthquake. In addition, even if there is a region where the hardness exceeds 225HV other than the surface layer part, in a member such as a column-beam joint part of a building structure, a large strain concentrates on the steel plate surface such as a weld toe at the time of deformation due to an earthquake. Therefore, the deformation performance as a member is not greatly impaired.

  Further, it is desirable that the hardness difference in the plate thickness direction is as small as possible in order to secure a predetermined strength throughout the plate thickness after reducing the hardness of the surface layer portion as described above. If the difference in hardness between the surface layer and the plate thickness center exceeds 60 HV, the deformation concentrates on the plate thickness center with relatively low strength depending on the shape of the welded joint during deformation due to an earthquake, etc. Destruction may occur. In order to prevent such destruction and ensure the necessary member performance, the hardness difference between the surface layer portion and the plate thickness center portion is limited to 60 HV or less in the thick steel plate of the present invention. The difference in hardness is preferably 55 HV or less, more preferably 50 HV or less. Further, the hardness of the central portion of the plate thickness is preferably 175 to 200 HV in order to ensure a desired high strength.

Next, a preferred method for producing the above-described thick steel plate of the present invention will be described.
In the method for producing a thick steel plate according to the present invention, the steel material having the above composition is subjected to hot rolling to obtain a thick steel plate, and following the rolling step, the first stage cooling including a halfway cooling stop to the thick steel plate, And a cooling step for performing two-stage accelerated cooling consisting of second-stage cooling.
The manufacturing method of the steel material used in the present invention is not particularly limited, and any conventional melting method and casting method can be applied. However, the molten steel having the above composition is converted into a converter, electric furnace, vacuum melting furnace. It is preferable to use a continuous casting method or the like to produce a steel material (slab) through a deoxidation process or a degassing process.

The obtained steel material (slab) is first heated and subjected to a rolling process that is hot-rolled into a thick steel plate. In the rolling process, the steel material is heated to a heating temperature of 1050-1200 ° C, the cumulative 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 or less at the surface temperature Ar3 transformation Hot rolling at a point or more is performed to obtain a thick steel plate having a predetermined thickness.
Heating temperature: 1050 ~ 1200 ℃
If the heating temperature is less than 1050 ° C, the strength of the resulting thick steel plate tends to decrease, while if it exceeds 1200 ° C, the toughness of the thick steel plate obtained by coarsening the structure decreases or the hardenability increases too much. Thus, the surface hardness of the resulting thick steel plate tends to increase. For this reason, the heating temperature of the steel material was limited to the range of 1050 ° C to 1200 ° C. In addition, Preferably it is 1080 to 1150 degreeC.

Cumulative rolling reduction in a temperature range of 950 ° C or less at the surface temperature: 30% or more In the present invention, in order to appropriately refine the microstructure of the resulting thick steel plate, controlled rolling is performed in a temperature range of 950 ° C or less at the surface temperature. Do. If the cumulative reduction in the temperature range is less than 30%, the structure becomes coarse and the hardenability increases too much, so that the desired toughness and surface hardness cannot be secured in the resulting thick steel plate. For this reason, the cumulative reduction amount in the temperature range of 950 ° C. or less at the surface temperature is limited to 30% or more. In addition, Preferably it is 35% or more.

Rolling end temperature: 900 ° C or less at the surface temperature, Ar3 transformation point or more When the rolling end temperature exceeds 900 ° C at the surface temperature, the structure becomes coarse and hardenability increases too much. The surface hardness cannot be secured. On the other hand, if the rolling end temperature is less than the Ar3 transformation point at the surface temperature, ferrite is generated during or immediately after rolling and becomes coarse and the toughness of the surface layer portion decreases. For this reason, the rolling end temperature was limited to the surface temperature of 900 ° C or less and Ar3 temperature or more. In addition, Preferably it is 880-780 degreeC.

As the Ar3 transformation point, a value calculated using the following formula is used.
Ar3 transformation point (℃) = 900−332C + 6Si−77Mn−20Cu−50Ni−18Cr−68Mo
(Here, C, Si, Mn, Cu, Ni, Cr, Mo: content of each element (mass%))
In addition, when the element described by the said formula is not contained, the said element shall be calculated as zero.

Subsequent to the rolling process, the steel plate is subjected to a cooling process. The cooling step includes first-stage cooling, a process of stopping cooling and returning to heat, and second-stage cooling. In the first stage cooling, the surface layer part is supercooled and then reheated, and the time until the start of the second stage cooling (cooling stop time) is used to advance the ferrite transformation of the surface layer part to achieve the desired surface layer microstructure. Get.
In the first stage cooling, the cooling starts from the temperature above the Ar3 transformation point at the surface temperature, accelerated cooling is performed at a cooling rate of 2 ° C / s or more at the average cooling rate in the thickness direction, and the surface temperature is (Ar3 transformation point – 100 ℃) Below 400 ℃ or more, preferably when the temperature difference between the surface and the plate thickness center position is 150 ℃ or more, the cooling is to stop the accelerated cooling.

First stage cooling start temperature: Ar3 transformation point or higher at the surface temperature If the first stage cooling start temperature is less than the Ar3 transformation point, ferrite forms before the accelerated cooling starts and becomes coarse. Cannot be achieved. For this reason, the start temperature of the first stage cooling was limited to the Ar3 transformation point or higher.
Cooling rate of the first stage cooling: 2 ° C / s or more at the average cooling rate at the thickness of 1 / 4t position When the cooling rate is less than 2 ° C / s, the cooling is slow, and coarse and low toughness ferrite grains are generated during cooling There is a case. For this reason, the cooling rate of the first stage cooling was limited to 2 ° C./s or more in terms of the average cooling rate at the position where the thickness is 1/4 t. The upper limit of the cooling rate of the first stage cooling is not particularly limited, and is almost determined by the plate thickness and the capacity of the cooling device, and is about 5 ° C./s or more when the plate thickness is 60 mm. Here, the “average cooling rate at the thickness 1/4 t position” refers to the average cooling rate from the start to the end of the accelerated cooling at the thickness 1/4 t position.

Cooling stop temperature for first stage cooling: (Ar3 transformation point –100 ° C) or less at surface temperature 400 ° C or higher In the first stage cooling in the present invention, cooling is performed so that the temperature difference between the surface layer part and the inside is larger. Then, ferrite is generated in the surface layer portion by recuperation after stopping the first stage cooling. Thereby, the hardness of a surface layer part can be reduced and the difference in hardness with a plate | board thickness center part can be made small. If the temperature at which the cooling is stopped exceeds the surface temperature (Ar3 transformation point −100 ° C.), the subsequent recuperation temperature is too high, and ferrite formation in the surface layer becomes insufficient. On the other hand, if the temperature at which cooling is stopped is less than 400 ° C, the temperature of the surface layer portion becomes too low, and the phase transformation is almost completed during cooling, and the surface layer portion is mainly composed of a hard phase such as bainite or martensite. End up.

For this reason, the cooling stop temperature of the first stage cooling was limited to the surface temperature (Ar3 transformation point−100 ° C.) or lower and 400 ° C. or higher. In addition, Preferably it is 650-500 degreeC.
Further, in the present invention, the first-stage cooling may be the cooling that is repeated a plurality of times with the cooling stop and the subsequent recuperation sandwiched therebetween, instead of the cooling that consists of the single accelerated cooling described above. By dividing the accelerated cooling into a plurality of times, it is possible to cool to the target temperature without excessively increasing the temperature difference between the surface layer and the inside. Thereby, the choice of cooling temperature control can be expanded and the precision of cooling temperature control can be improved.

  Accelerated cooling is repeated multiple times with cooling stop and subsequent recuperation, cooling starts from the surface temperature above the Ar3 transformation point, and the average cooling rate at the thickness of 1 / 4t It is preferable that the cooling at a cooling rate of 2 ° C./s or more and the cooling stop temperature be 400 ° C. or more at the surface temperature is repeated several times with the cooling stop and the subsequent recuperation interposed therebetween. In this cooling, it is assumed that the cooling stop temperature includes accelerated cooling at a surface temperature of (Ar3 transformation point−100 ° C.) or lower and 400 ° C. or higher at least once. An example of the history of the steel plate temperature when such cooling is performed is schematically shown in FIG.

Start temperature of the first accelerated cooling (first cooling) in the first stage cooling: Ar3 transformation point or more at the surface temperature Among the one or more accelerated coolings constituting the first stage cooling, the starting temperature of the first cooling is The surface temperature is preferably not less than the Ar3 transformation point. If the first cooling start temperature is less than the Ar3 transformation point, ferrite is generated and coarsened before the start of accelerated cooling, so that the toughness of the surface layer portion decreases. For this reason, it is preferable to limit the starting temperature of the first accelerated cooling (first cooling) to the Ar3 transformation point or higher at the surface temperature.

Cooling rate of accelerated cooling in the first stage cooling: 2 ° C / s or more as the average cooling rate at the position of 1 / 4t thickness When the cooling rate is less than 2 ° C / s, the cooling is slow, and coarse and low toughness ferrite grains May generate. For this reason, the cooling rate of the first stage cooling was limited to 2 ° C./s or more in terms of the average cooling rate at the position where the thickness is 1/4 t. The upper limit of the cooling rate of the first stage cooling is not particularly limited, and is almost determined by the plate thickness and the capacity of the cooling device, and is about 5 ° C./s or more when the plate thickness is 60 mm. Here, the “average cooling rate at the thickness 1/4 t position” refers to the average cooling rate from the start to the end of the accelerated cooling at the thickness 1/4 t position. If it shows in FIG. 7, it will say the average cooling rate from A point to B point. Point A is the time when the temperature at the plate thickness 1 / 4t position becomes equal to the surface cooling start temperature, and point B is the time when the last accelerated cooling in the first stage cooling is stopped.

Cooling stop temperature of accelerated cooling in the first stage cooling: 400 ° C or more at the surface temperature In multiple times of accelerated cooling in the first stage cooling, if the surface layer becomes less than 400 ° C, bainite and martensite transformation occurs during cooling As a result, the surface layer is hardened. For this reason, the cooling stop temperature of all accelerated cooling was limited to 400 ° C. or higher.
Cooling stop temperature at the surface temperature (Ar3 transformation point –100 ° C) or less 400 ° C or more Accelerated cooling: at least 1 time In the first stage cooling, cooling is performed so that a temperature difference between the surface layer part and the inside occurs to some extent. Ferrite is generated in the surface layer by recuperation after stopping. In all of the multiple times of accelerated cooling in the first stage cooling, if the cooling stop temperature exceeds the surface temperature (Ar3 transformation point-100 ° C), the steel plate temperature becomes too high during the subsequent reheating, and the surface layer part Insufficient ferrite formation occurs. For this reason, at least one of the plurality of accelerated coolings was set to accelerated cooling at which the cooling stop temperature was (Ar3 transformation point−100 ° C.) or lower.

In the present invention, after the first stage cooling is stopped, the second stage cooling is started after the surface temperature of the thick steel plate is reheated to a predetermined temperature. Reheating is carried out until the surface temperature (Ar3 transformation point + 10 ° C) is 650 ° C or higher and the temperature difference between the surface and the center of the plate thickness is 60 ° C or lower. Then, the second stage cooling is started.
In the present invention, in order to generate ferrite during recuperation or after recuperation, particularly after recuperation during the cooling stop between the first stage cooling and the second stage cooling, the steel plate temperature after recuperation, that is, the second stage The cooling start temperature of the cooling is an important factor from the viewpoint of the structure control of the ferrite generation amount.

If the recuperation is less than 650 ° C. at the surface temperature, acicular ferrite and bainite with relatively high strength and yield ratio are generated in the surface layer, resulting in a decrease in the elongation of the surface layer and an increase in the yield ratio. The performance is reduced. On the other hand, if the recuperation exceeds (Ar3 transformation point + 10 ° C.) at the surface temperature, the phase transformation does not proceed after recuperation, and ferrite formation in the surface layer becomes insufficient.
Further, the temperature difference between the surface and the center of the plate thickness causes a difference in the amount of ferrite generated in the plate thickness direction. If the temperature difference between the surface and the plate thickness center at the start of the second stage cooling exceeds 60 ° C., the microstructure of the surface layer portion and the plate thickness center portion may become too large, resulting in a large hardness difference. When the hardness difference in the plate thickness direction is too large, the deformation performance as a member is deteriorated during deformation such as an earthquake.

For this reason, the temperature after recuperation, that is, the cooling start temperature of the second-stage cooling, is 650 ° C or more at the surface temperature (Ar3 transformation point + 10 ° C), and the temperature difference between the surface and the center of the plate thickness is 60 ° C or less. It was limited to the range.
In the second stage cooling of the present invention, the cooling is started after the surface temperature is reheated to (Ar3 transformation point + 10 ° C) or lower and 650 ° C or higher, and the temperature difference between the surface and the center of the plate thickness is 60 ° C or lower. In the second stage cooling, the cooling stops when the surface temperature is 600 ° C or lower and 400 ° C or higher by reheating after stopping the second stage cooling at an average cooling rate of 2 ° C / s or more at the thickness of 1 / 4t. Accelerate cooling to temperature.

The second stage cooling is carried out after the first stage cooling so that the untransformed portion becomes a hard phase such as pearlite, bainite, martensite or the like. By making the untransformed part a hard phase, the final structure can be (ferrite + (bainite and / or martensite)),
The desired high strength and low yield ratio can be realized.
Second-stage cooling rate: 2 ° C / s or more at an average cooling rate at a thickness of 1/4 t, in order to make the untransformed part a hard phase, the second-stage cooling is 2 ° C / s or more, preferably 8 Cool at ℃ / s or higher. If the cooling rate is less than 2 ° C./s, the amount of transformation to the hard phase decreases, and the desired high strength and low yield ratio cannot be realized.

Cooling stop temperature of the second stage cooling: Temperature at which the surface temperature becomes 600 ° C or lower and 400 ° C or higher by recuperation after cooling is stopped. The cooling stop temperature of the second stage cooling is recovered after the cooling stop of the second stage cooling. At a temperature at which the surface temperature exceeds 600 ° C. due to heat, the amount of transformation to the hard phase decreases, or the strength decreases due to self-tempering, making it impossible to ensure the desired high strength. On the other hand, at the cooling stop temperature at which the surface temperature is less than 400 ° C. due to recuperation, the hard phase hardness becomes too high and the toughness is lowered. For this reason, the cooling stop temperature of the second stage cooling is limited to a temperature at which the surface temperature becomes 600 ° C. or lower and 400 ° C. or higher by recuperation after the cooling is stopped. Since the temperature after recuperation depends on the temperature at the plate thickness 1 / 2t position when the accelerated cooling is stopped, it can be predicted from the cooling stop temperature at the 1 / 2t position by various heat transfer calculations.

Further, in the present invention, similarly to the first stage cooling, the second stage cooling is replaced with the cooling consisting of the above-described one-time accelerated cooling, and the accelerated cooling is sandwiched between the cooling stop and the subsequent recuperation, It is good also as cooling repeated several times. By dividing the accelerated cooling into a plurality of times, it is possible to cool to the target temperature without excessively increasing the temperature difference between the surface layer and the inside.
Of the multiple times of accelerated cooling that constitutes the second stage cooling, the first cooling recovers the surface temperature to (Ar3 transformation point + 10 ° C) or less, 650 ° C or more, and the temperature difference between the surface and the center of the plate thickness to 60 ° C or less. It is preferable to start cooling after heating.

  If the starting temperature of the first cooling in the second stage cooling exceeds the surface temperature (Ar3 transformation point + 10 ° C.), the phase transformation does not proceed, and ferrite formation in the surface layer becomes insufficient. Also, if the starting temperature is less than 650 ° C. at the surface temperature, acicular ferrite and bainite are generated in the surface layer portion, resulting in insufficient ferrite generation, resulting in a decrease in the elongation of the surface layer portion and an increase in yield ratio. The performance is reduced. For this reason, it is preferable to limit the starting temperature of the first accelerated cooling (first cooling) in the second stage cooling to 650 ° C. or more (preferably below the Ar 3 transformation point) (preferably below the Ar 3 transformation point) at the surface temperature. .

Further, the temperature difference between the surface and the center of the plate thickness causes a difference in the amount of ferrite generated in the plate thickness direction. If the temperature difference between the surface and the center of the plate thickness exceeds 60 ° C at the start of the second stage cooling, the microstructure of the surface layer portion and the center portion of the plate thickness becomes too large, which may cause a large hardness difference. . When the hardness difference in the plate thickness direction is too large, the deformation performance as a member is deteriorated during deformation such as an earthquake.
For this reason, the start of the first cooling of the second stage cooling is the surface temperature (Ar3 transformation point + 10 ° C) (more preferably Ar3 transformation point) or less, 650 ° C or more, temperature difference between the surface and the center of the plate thickness It is preferable to set the temperature at 60 ° C. or lower.

The cooling rate of the plurality of times of accelerated cooling constituting the second stage cooling is preferably set to 2 ° C./s or more in terms of the average cooling rate at the position where the thickness is 1/4 t.
If the cooling rate is less than 2 ° C / s at an average cooling rate at a thickness of 1/4 ton, the cooling is slow, and coarse and low toughness ferrite grains may be produced during cooling, and hard phases such as bainite and martensite As a result, the desired high strength and low yield ratio cannot be ensured. For this reason, the cooling rate of the second stage cooling was limited to 2 ° C./s or more in terms of the average cooling rate at the plate thickness of 1/4 t. The temperature is preferably 8 ° C./s or more. The upper limit of the cooling rate of the second stage cooling is not particularly limited, and is almost determined by the plate thickness and the capacity of the cooling device. Here, the “average cooling rate at the position where the thickness is 1/4 t” refers to the average cooling rate from the start to the end of the accelerated cooling at the position where the thickness is 1/4 t. If it shows in FIG. 7, it will say the average cooling rate from C point to D point. Point C is the time when the temperature at the plate thickness 1 / 4t position becomes equal to the surface cooling start temperature, and point D is the time when the last accelerated cooling in the second stage cooling is stopped.

After the accelerated cooling is stopped and reheated, the above-described accelerated cooling at the average cooling rate is further performed a plurality of times. Then, the second stage cooling is stopped, with the cooling that is accelerated to the cooling stop temperature at which the surface temperature becomes 600 ° C. or lower and 400 ° C. or higher by the recuperation after the cooling stops as the final cooling.
When the final cooling of the second stage cooling is a recuperation after cooling stops and the surface temperature exceeds 600 ° C, the amount of hard phase generated is small, and the desired high strength can be secured by reducing the strength due to self-tempering. Disappear. On the other hand, when the final cooling is such that the surface temperature becomes less than 400 ° C. by recuperation after stopping the cooling, the hardness of the hard phase becomes too high, and the desired ductility and toughness cannot be ensured. For this reason, it is preferable that the final cooling of the second stage cooling is a cooling that is accelerated and cooled to a cooling stop temperature at which the surface temperature becomes 600 ° C. or lower and 400 ° C. or higher by reheating after the cooling stops.

In addition, after performing the above-mentioned cooling process, you may perform a tempering process for the purpose of intensity | strength and toughness adjustment as needed. Tempering is preferably performed at a temperature of 400 ° C. or higher and 700 ° C. or lower. If the tempering temperature is less than 400 ° C., the desired effect cannot be expected. On the other hand, when the temperature exceeds 700 ° C., the strength is significantly reduced.
Hereinafter, the present invention will be further described based on examples.

Example 1
The steel material having the composition shown in Table 1 was subjected to the rolling step and the cooling step shown in Table 2 to obtain a thick steel plate having a plate thickness of 40 mm. In addition, the cooling process was made into the accelerated cooling which consists of 1st stage cooling and 2nd stage cooling through cooling stop-recuperation. In each process, the steel plate temperature is measured by measuring the surface temperature with an infrared radiation thermometer, and based on this, the temperature at the plate thickness 1 / 4t position and the plate thickness central temperature are calculated using various heat transfer calculation methods. Calculated.

The obtained thick steel plate was subjected to structure observation, hardness test, tensile test, and impact test. The test method was as follows.
(1) Microstructure observation A specimen for microstructural observation is collected from the obtained thick steel plate, the cross section in the L direction is polished and subjected to nital corrosion, and the surface layer portion which is a region of 1 to 5 mm from the surface in the thickness direction, and the thickness Using the optical microscope (magnification: 400 times) or scanning electron microscope (magnification: 2000 times), the microstructure is observed over 3 fields of view and imaged at the center of the plate thickness, which is an area ± 2 mm from the center position. By image analysis, the type of structure and the structure fraction (area ratio) of ferrite were obtained. Moreover, about the surface layer part, the nominal particle diameter of the ferrite was calculated | required. The nominal grain size of the ferrite was obtained by calculating the average area of the crystal grains, and the square root of the average area of the obtained crystal grains was defined as the nominal ferrite grain size (average crystal grain size) of the thick steel plate.

(2) Hardness test Take a specimen for hardness measurement from the obtained thick steel plate, and measure the hardness of the cross section in the thickness direction using a Vickers hardness tester in accordance with the provisions of JIS Z 2244. went. The measurement position is an area of 1 to 5 mm (surface layer part) in the sheet thickness direction from the front and back surfaces of the steel sheet, and an area of ± 2 mm (sheet thickness center part) from the sheet thickness center position, and a 1 mm pitch in the sheet thickness direction in each area. Then, four or more points were measured. The test load (test force) was 1 kg (9.8 kN). The obtained hardness HV was arithmetically averaged to obtain the average hardness HV in that region.

(3) Tensile test JIS No. 5 full thickness tensile test specimens were collected from the obtained thick steel plate in accordance with the provisions of JIS Z 2201 so that the tensile direction would be the L direction, and conformed to the provisions of JIS Z 2241. Then, a tensile test was performed to determine tensile properties (yield strength YS, tensile strength TS). Further, the yield ratio YR (= YS / TS × 100%) was calculated from the obtained measured values.

(4) Impact test V-notch impact test specimens were collected in accordance with JIS Z 2242 from the position of 1 / 4t thickness and 1mm below the surface (the center position of the specimen was 6mm below the surface). Then, a Charpy impact test was carried out to determine the fracture surface transition temperature vTrs (° C.). In addition, the case where vTrs is -40 degrees C or less was considered to be excellent in toughness.

The obtained results are shown in Table 3.
Next, a square steel pipe (press column) was produced by cold pressing using the obtained thick steel plate. In addition, the cross-sectional dimension of the square steel pipe (press column) was 500 × 500 mm, and the seam welding was a submerged arc welding with one layer on each side. Then, as shown in FIG. 3, the obtained square steel pipe (press column) (length 3250 mm) 1a, 1a is welded with a SN490 steel plate through diaphragm (plate thickness 40 mm) 2a by carbon dioxide welding, A four-sided BOX column 3a was placed between the two diaphragms 2a and 2a, and carbon dioxide was welded to obtain a specimen for a column bending test. The strength and rigidity of the four-sided BOX column were made sufficiently higher than those of the press column to prevent plastic deformation other than the press column during the test. FIG. 4 shows an enlarged view of the vicinity of the welded portion of the press column 1a, diaphragm 2a, and 4-sided BOX column 3a in the test body.

A column bending test was performed using these specimens. The column bending test was as follows.
(5) Column bending test A three-point repeated bending test (column bending test) in which both ends of the obtained specimen are supported and positive and negative loads are repeatedly applied in the vertical direction to the central part of the specimen as shown in FIG. ). The load P and the deformation amount (rotation angle) θ were measured, and a load (moment, M) -deformation amount (rotation angle, θ) hysteresis curve as shown in FIG. 6 was created.

When the load (moment) decreases by 5% from the maximum value due to local buckling or brittle fracture, the specimen is considered to be fractured, and the total plastic rotation angle (cumulative plastic rotation angle Σθpl) of the specimen is calculated and tested. The cumulative plastic deformation ratio η was determined as an index of the plastic deformability of the body. Η is calculated from the following equation.
η = Σθpl / θp
θp = (Pp / 2) L ² / (3 · E · I) + Pp / 2 / (G · Aw)
Where Pp: total plastic load (N) = Mp / L,
L: Column cantilever length (distance from diaphragm to column end support point, 3250 mm)
E: Young's modulus 205000 (MPa), G: Shear modulus 79000 (MPa),
Mp: Total plastic moment of the column

I: Column moment of inertia, σy: Yield strength of steel (MPa)

D: Column diameter (mm), t: Column thickness (mm),
r: bending radius of the inner surface of the column corner, R = r + t
Aw: Shear area (mm ² )

When the cumulative plastic deformation magnification η is 30 or more, it is assumed that the structural member is excellent in earthquake resistance (plastic deformation performance).
The obtained results are also shown in Table 3.

  Each of the inventive examples has a yield strength YS: 385 MPa or more, a tensile strength TS: 550 MPa or more, a yield ratio YR: 80% or less, and vTrs at the surface layer portion and 1/4 t position in the plate thickness direction is − It is a high-strength, high-toughness, non-tempered, low yield ratio, high-tensile steel plate that satisfies 40 ° C or lower. Furthermore, each of the inventive examples has a thickness distribution in the thickness direction in which the average hardness of the surface layer portion is 225 HV or less, and the hardness difference between the surface layer portion and the plate thickness center portion is 60 HV or less, and cold bending is performed. When a press column-diaphragm joint structural member is constructed by processing into a press column, the cumulative plastic deformation ratio in the three-point bending test of the press column-diaphragm joint is 30 or more, and the seismic performance (plastic deformation performance) is improved. It can be said that it is an excellent non-tempered low yield ratio high-tensile thick steel plate that can be used as a structural member. On the other hand, in comparative examples that are outside the scope of the present invention, the strength, yield ratio, toughness is insufficient, or the ductility and toughness of the surface layer part after cold working are reduced, and the cumulative plastic deformation ratio as a structural member is lowered. ing.

(Example 2)
The steel material having the composition of steel No. A to No. E shown in Table 1 was subjected to the rolling process and the cooling process shown in Table 4 to obtain a thick steel sheet having a thickness of 40 mm. Note that the cooling process is accelerated cooling including first-stage cooling, cooling stop-recuperation, and second-stage cooling. Then, the first stage cooling is the cooling consisting of one or more accelerated coolings including the cooling stop and the subsequent recuperation, and the second stage cooling is the cooling stop and the subsequent recuperation. It was set as the cooling which included the accelerated cooling more than once included. In addition, like Example 1, the steel plate temperature in each process measures the surface temperature with an infrared radiation thermometer, and based on this, the temperature at the plate thickness 1 / 4t position and the plate thickness central temperature are set as necessary. It was calculated using various heat transfer calculation methods.

The obtained thick steel plate was subjected to structure observation, hardness test, tensile test, and impact test in the same manner as in Example 1.
Table 4 shows the obtained results.
Further, in the same manner as in Example 1, a square steel pipe (press column) was produced by cold pressing using the obtained thick steel plate. In addition, the cross-sectional dimension of the square steel pipe (press column) was 500 × 500 mm, and the seam welding was a submerged arc welding with one layer on each side.

In the same manner as in Example 1, the obtained square steel pipe (press column) (length 3250 mm) 1a and 1a was welded with a SN490 steel plate through diaphragm (plate thickness 40 mm) 2a by carbon dioxide welding, A four-sided BOX column 3a was placed between the two diaphragms 2a and 2a, and carbon dioxide was welded to obtain a specimen for a column bending test.
And like Example 1, the both ends of the obtained test body are supported, and as shown in FIG. 5, a positive and negative load is repeatedly applied to the center part of the test body in the vertical direction (3-point repeated bending test ( Column bending test).

As in Example 1, when the load (moment) decreased by 5% from the maximum value due to local buckling or brittle fracture, the specimen was considered to be fractured, and the total plastic rotation angle of the specimen (accumulated) The plastic rotation angle Σθpl) was determined, and the cumulative plastic deformation ratio η was determined as an index of the plastic deformability of the specimen.
The obtained results are also shown in Table 5.

  Each of the inventive examples has a yield strength YS: 385 MPa or more, a tensile strength TS: 550 MPa or more, a yield ratio YR: 80% or less, and vTrs at the surface layer portion and 1/4 t position in the plate thickness direction is − It is a high-strength, high-toughness, non-tempered, low yield ratio, high-tensile steel plate that satisfies 40 ° C or lower. Furthermore, each of the inventive examples has a thickness distribution in the thickness direction in which the average hardness of the surface layer portion is 225 HV or less, and the hardness difference between the surface layer portion and the plate thickness center portion is 60 HV or less, and cold bending is performed. When a press column-diaphragm joint structural member is constructed by processing into a press column, the cumulative plastic deformation ratio in the three-point bending test of the press column-diaphragm joint is 30 or more, and the seismic performance (plastic deformation performance) is improved. It can be said that it is an excellent non-tempered low yield ratio high-tensile thick steel plate that can be used as a structural member. On the other hand, the comparative example which is out of the scope of the present invention does not ensure the desired tensile strength or yield strength, the desired yield ratio, or the desired thickness direction hardness distribution, and the structural member The cumulative plastic deformation ratio as is low.

1 Pillar 1a Press column 2 Diaphragm 2a Diaphragm (through diaphragm)
3a 4-sided BOX pillar
4a Column / diaphragm weld 5a Diaphragm 4-sided BOX column weld 4

Claims (9)

% By mass
C: 0.05 to 0.16%, Si: 0.05 to 0.45%,
Mn: 1.2 to 1.8%, P: 0.020% or less,
S: 0.005% or less, Al: 0.05% or less,
Nb: 0.005 to 0.025%, N: 0.0040% or less,
Ti: 0.005-0.020%
Including Ti and N, the ratio of Ti content and N content, Ti / N so as to satisfy 2.5 or more, the balance Fe and unavoidable impurities,
At least the surface layer of 1 mm to 5 mm from the steel sheet surface in the thickness direction has ferrite, and the hard phase has a microstructure composed of one or more of pearlite, bainite, martensite, and the thickness from the steel sheet surface. The average grain size of the ferrite in the surface layer portion of 1 mm to 5 mm in the direction is 4.0 to 18.0 μm,
The average hardness of the surface layer part of 1mm to 5mm from the surface of the steel sheet is 225HV or less, and the difference in hardness between the surface layer part and the sheet thickness center part within ± 2mm around the sheet thickness center position is 60HV or less. Shows the thickness distribution in the thickness direction,
Non-tempered low yield ratio high tensile steel plate with yield strength: 385 MPa or more, tensile strength: 550 MPa or more, yield ratio: 80% or less, characterized by excellent surface layer ductility and toughness after cold working.   In addition to the above composition, it is further selected by mass% from Cu: 0.05 to 0.30%, Ni: 0.05 to 0.30%, Cr: 0.05 to 0.50%, Mo: 0.04 to 0.40%, V: 0.01 to 0.06%. The non-tempered low yield ratio high-tensile thick steel plate according to claim 1, wherein the composition contains one or more types.   In addition to the above composition, the composition further contains one or more selected from Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, and Mg: 0.0010 to 0.0050% by mass%. The non-tempered low yield ratio high-tensile thick steel plate according to claim 1 or 2. A steel material is subjected to a hot rolling process to make a thick steel plate, and following the rolling step, the thick steel plate is subjected to two-stage accelerated cooling consisting of first-stage cooling and second-stage cooling including stopping cooling on the way. In the manufacturing method of the non-tempered thick steel sheet,
The steel material in mass%,
C: 0.05 to 0.16%, Si: 0.05 to 0.45%,
Mn: 1.2 to 1.8%, P: 0.020% or less,
S: 0.005% or less, Al: 0.05% or less,
Nb: 0.005 to 0.025%, N: 0.0040% or less,
Ti: 0.005-0.020%
Ti and N, the ratio of Ti content to N content, Ti / N is contained so as to satisfy 2.5 or more, and the steel material of the composition consisting of the balance Fe and inevitable impurities,
The heating temperature of the hot rolling is 1050 to 1200 ° C,
The hot rolling is a rolling with a cumulative reduction amount of 30% or more in a temperature range of 950 ° C. or less at the surface temperature, and a rolling end temperature of 900 ° C. or less at the surface temperature of the Ar3 transformation point or more,
The first stage cooling starts from the temperature above the Ar3 transformation point at the surface temperature, accelerated cooling at a cooling rate of 2 ° C / s or more at the average cooling rate at the thickness 1/4 t, and the surface temperature is ( Ar3 transformation point –100 ° C) When 400 ° C or higher
After stopping the cooling, reheat, when the surface temperature is (Ar3 transformation point + 10 ° C) or less 650 ° C or more and the temperature difference between the surface and the center of the plate thickness is 60 ° C or less, the second stage cooling is started,
In the second stage cooling, the surface temperature is 600 ° C. or lower and 400 ° C. or higher by reheating after stopping the second stage cooling at a cooling rate of 2 ° C./s or higher at an average cooling rate at a thickness of 1/4 t. Cooling is accelerated cooling to a cooling stop temperature that
Yield strength: 385 MPa or more, tensile strength: 550 MPa or more, yield ratio: 80% or less A method for producing a non-tempered low yield ratio high-tensile thick steel plate.   Instead of the first stage cooling, the first stage cooling is started from the temperature above the Ar3 transformation point at the surface temperature, and the average cooling rate at a thickness of 1/4 t is 2 ° C / s or more. The accelerated cooling at which the cooling stop temperature is 400 ° C. or more at the surface temperature is the cooling that is repeated a plurality of times across the cooling stop and the subsequent recuperation, and the plurality of accelerated coolings are performed at the surface temperature. The method for producing a non-tempered low yield ratio high-tensile thick steel plate according to claim 4, comprising accelerated cooling (Ar3 transformation point-100 ° C) to 400 ° C or higher at least once.   Instead of the second stage cooling, the second stage cooling is accelerated cooling at an average cooling rate at a thickness of 1/4 t at a cooling rate of 2 ° C./s, cooling is stopped, and subsequent recuperation is performed. Between the multiple times of accelerated cooling, the final cooling is the accelerated cooling that cools to a cooling stop temperature at which the surface temperature becomes 600 ° C. or lower and 400 ° C. or higher by reheating after the cooling stop. The method for producing a non-tempered low yield ratio high-tensile thick steel plate according to claim 4 or 5.   The tempering step of tempering at a temperature of 400 ° C. or higher and 700 ° C. or lower is performed subsequent to the cooling step. Production method.   In addition to the above composition, it is further selected by mass% from Cu: 0.05 to 0.30%, Ni: 0.05 to 0.30%, Cr: 0.05 to 0.50%, Mo: 0.04 to 0.40%, V: 0.01 to 0.06%. The method for producing a non-tempered low yield ratio high-tensile thick steel plate according to any one of claims 4 to 7, wherein the composition contains one or more types.   In addition to the above composition, the composition further contains one or more selected from Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%, and Mg: 0.0010 to 0.0050% by mass%. The method for producing a non-tempered low yield ratio high-tensile thick steel plate according to any one of claims 4 to 8.

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